Inflection Point Engineering IPE-TM-807 Pressure Relief Systems

Overpressure Protection for Exchangers

IPE-TM-807-04

Table of Contents

1. Table of Contents 1

2. Purpose 2

3. General 2

4. Inherently Safer Design 3

4.1 Process Design Stage 3

4.2 Project Design Stage 4

5. Internal Tube Failure 4

5.1 Containment vs. Relief Protection 5

5.2 Calculating the Rate of Flow through an Internal Tube Failure. 6

5.3 Impact of HP Fluid Entering the LP System 6

6. External Fire 7

6.1 Services Requiring Dedicated PRV Protection for an External Fire 7

6.2 Utilizing the PRV Protecting Nearby Equipment 8

6.3 Services NOT Requiring Relief Protection during an External Fire 8

6.4 Routing the PRV Outlet when Valve Is “BY OTHERS” 9

6.5 Estimating the Relief Load for an Exchanger Due to External Fire 9

7. Thermal Expansion 10

7.1 Thermal Expansion: Cold-Side Fluid is Liquid Only 10

7.2 Thermal Expansion: Cold-Side Fluid is Vapor Only 10

7.3 Routing the Outlet Piping for a Thermal Expansion PRV 11

8. Vaporization of a Confined Liquid 11

9. Blocked Outlet 11

Attachment 1 – Decision Tree for NEW Schedule A Designs 13

Attachment 2 – Decision Tree for REVAMP Designs 15

Attachment 3 - Protecting the LP System during a Tube Failure 17

A3.1 Tube Failure in a Liquid Product Rundown Exchanger 17

A3.2 Tube Failure in a Liquid-Full Feed Preheat Exchanger (Vapor < 1 wt%) 19

A3.3 Tube Failure in a Preheat Exchanger with Partially Vaporized Feed (Vapor ≥ 1 wt%) 21

A3.4 Tube Failure in a Reboiler Receiving Heat from a HP Fluid 22

A3.5 Tube Failure in a Hot Oil Exchanger that Transfers Heat to a HP Process Stream 23

A3.6 Pump-Around (PA) Exchanger Transfers Heat to a HP Fluid 24

A3.7 Water-Cooled Exchangers 26

A3.8 Steam-Heated Exchanger: Steam Pressure Lower than Process Fluid 27

A3.9 Steam Generator: Steam Pressure Lower than Heating Medium 28

Attachment 4 - Calculating the Flow of HP Fluid through a Failed Tube 30

A4.1 Assumptions 30

A4.2 Pressure Differential across the Break 31

A4.3 Flow of Non-Flashing Liquid through the Failed Tube 32

A4.4 Vapor Only Flow through the Failed Tube 33

A4.5 Two-Phase Flow through the Failed Tube 33

A4.6 Consequences of HP Fluid Mixing with LP Fluid 34

Attachment 5 – Effects that Cascade to the LP System 35

A5.1 General 35

A5.2 Hazards that Cascade throughout Systems within Inflection Point Engineering’s Design Scope 35

A5.3 Hazards that Cascade Outside Inflection Point Engineering’s Design Scope 36

Attachment 6 - Thermal Expansion of a Confined Liquid or a Confined Vapor 37

A6.1 General 37

A6.2 Thermal Expansion of a Confined Liquid 37

A6.3 PRV Sizing when Flashing Occurs in the Nozzle 38

A6.4 Thermal Expansion of a Confined Vapor 39

A6.5 Thermal Expansion of a Super-Critical Fluid 40

Attachment 7 - Vaporization of a Confined Liquid 41

A7.1 Test: Will the Cold-Side Fluid Vaporize? 41

A7.2 Rate Exchanger Duty for the Vaporization Case 41

A7.3 Determine the Vaporization Rate of Cold-Side Fluid 43

A7.4 Locate the PRV Close to Exchanger 44

Attachment 8 - Routing the PRV Outlet Piping 45

A8.1 Safe Routing for PRV Outlet Piping 45

A8.2 Free Draining Requirement 46

A8.3 Pressure Rating of PRV Outlet 46

A8.4 PRV Set Pressure Affected by Back Pressure 46

A8.5 Heat Tracing PRV Outlet Line 47

A8.6 Relieving Hydrocarbons to the Atmosphere 47

2. Purpose

This procedure summarizes Inflection Point Engineering’s design practice for protecting shell and tube heat exchangers from high pressure due to mechanical failure, misoperation, or external events. Typical causes of overpressure are described and design guidance is given for various heat exchange configurations. For unusual exchange services or process conditions not addressed in this procedure, consult the PRV specialist.

3. General

ASME Code requires shell and tube heat exchangers to be protected from overpressure due to internal tube failure and external fire. API Standard 521 expands the list to include thermal expansion and blocked outlet, and gives general guidelines for determining the relief load. This procedure incorporates the Code requirements and provides additional guidance regarding overpressure protection. Also, inherently safer design principles are discussed as applied to heat exchange systems.

Note: throughout the document, “HP” and “LP” refer to high- and low-pressure sides of an exchanger.

This document includes a working definition of “clear path”. See section 5.

4. Inherently Safer Design

An inherently safer design reduces risk by applying design strategies such as: minimization, simplification, substitution, and moderation. Inherently safer designs do not rely upon external add-on devices, such as relief valves or safety shutdown systems for safe operation. As applied to heat exchangers, an inherently safer design avoids heat transfer between incompatible fluids, simplifies the heat exchange system, or reduces the potential for loss of containment. Opportunities for inherently safer designs can be found during both the process and project design stages.

4.1 Process Design Stage

Inherent safety concepts can be incorporated in the process design during the heat exchange analysis. The examples given below are intended to stimulate thinking; the list should not be considered prescriptive or exhaustive.

NOTE: An inherently safer design may compete with other design objectives. For example, less heat may be recovered with an alternate “safer” heat exchange scheme. In such cases, the Technology Specialist must approve the less efficient thermal design. Or, simplifying the exchange scheme may complicate plant start-up or affect downstream operations. In such cases, Field Operating Services must approve the change.

4.2 Project Design Stage

During the project phase, the exchanger specification can be upgraded to reduce the likelihood that a tube will fail, or if it does fail, to minimize the consequences. Examples of a “fortified” design are given below:

NOTE: Before specifying additional exchanger requirements, consult the Exchanger Specialist to confirm the feasibility, applicability, and effectiveness of a more robust design.

5. Internal Tube Failure

ASME code requires that "Heat exchangers and similar vessels shall be protected with a relieving device of sufficient capacity to avoid overpressure in case of an internal failure." In addition, API Standard 521 provides extensive guidance for handling the tube failure case.

Where possible, Inflection Point Engineering increases the design pressure of the LP system to contain the HP fluid rather than providing relief protection. If the design pressure of the LP system is set at (or above) ten-thirteenths (10/13) of the design pressure of the HP side, the relief valve (if present) does not have to be sized for a tube failure case.

NOTE: Designing to this ten-thirteenths “rule” does NOT PREVENT a tube failure from occurring. However, it is a cost-effective way to lower the potential that an internal tube failure will lead to loss of containment. Because ASME Code requires the LP side to be tested at 130% of its design pressure, an exchanger designed according to the ten-thirteenths rule will most likely contain the HP fluid when a tube failure occurs.

If the entire LP system cannot economically be designed according to the ten-thirteenths rule, relief protection must be provided to limit the pressure generated by a tube failure. For example, it may be unrealistic to increase the design pressure of every component in the LP system that could be isolated with the exchanger and subjected to a high pressure during a tube failure. Or, available piping components may constrain the maximum design pressure of an exchanger, e.g. flange gaskets in sea water service are limited to CL600.

When a clear path exists between the LP side of an exchanger and a nearby PRV, the design pressure of the LP system may not have to be increased to contain the HP fluid. Instead, the PRV protecting the nearby equipment could also be used to limit pressure in the exchanger, provided it is sized for the tube failure case. The path between the exchanger and its PRV is considered “clear” when there are no intervening valves which could impose a significant flow restriction (e.g. check, control, or other reduced-port valves). Furthermore, all block valves in the path must be locked open (subject to customer acceptance).

However, some valves must be closed during normal operation and cannot be locked open. For example, isolating valves for filters cannot be locked open because they have to be taken off line to replace the filter element. A similar situation is obtained when driers are taken off-line to be regenerated. Protection with a PRV is required for such situations.

5.1 Containment vs. Relief Protection

Early in the project phase, select the design option (containment or relief protection) that provides the best overpressure protection of the LP system during a tube failure event. provides a decision tree to help choose between the two design options.

NOTE: For revamp projects, design criteria may need to be relaxed to maximize use of existing equipment. A decision tree applicable to revamp projects is given in .

If the design pressure of the LP system is raised to contain the HP fluid, the Design Engineer must determine how far the ten-thirteenths rule extends to adjacent equipment and piping. provides guidance for typical exchanger configurations.

If the LP side of an exchanger is liquid-full during a process operation, it must be designed according to the ten-thirteenths rule even when a relief device is used to protect other components in the LP system. A PRV does not respond fast enough to “catch” the transient pressure spike that develops in the liquid-full system as the LP fluid is displaced out of the exchanger.

If your exchanger system is not addressed in Attachments 1-3, consult the PRV Specialist.

5.2 Calculating the Rate of Flow through an Internal Tube Failure.

See and use to determine the flow rate of HP fluid through a failed tube.

5.3 Impact of HP Fluid Entering the LP System

Designing to the ten-thirteenths rule does not eliminate causes for an internal tube failure. Even with the LP side designed to contain the HP fluid, a tube failure could occur due to vibration, erosion, corrosion, or differential expansion. If additional hazards develop when the HP fluid enters the LP system, these hazards must be addressed. For example, HP fluid leaking into the cooling water system will eventually be discharged to the atmosphere via the cooling tower. Or, HP fluid entering a fractionating column via a failed tube in the feed preheat exchanger may vapor blanket the overhead condenser.

The relief model constructed for the tube failure case must consider the “extra” mass flowing through the failed tube, its impact on equipment in the LP system, and any volumetric expansion due to phase change and/or heat transfer between the HP and LP fluids. Since consequences vary with the exchange service, no general guidance can be given here. See for managing hazards that could potentially develop in the LP system.

5.4 Double-pipe Exchangers

See section 4.2. If the Inflection Point Engineering exchanger specification indicates a double-pipe design, as distinct from a multi-tube type, protection for internal pipe failure is not required. Further, if the double-pipe exchanger is associated with a PRV service via a clear path, a “tube rupture” case does not apply, but a note should be added to the PRV specification:

“The Tube Rupture case is not applicable because schedule pipe has been specified for the internals of the exchanger. Contractor to review this case based on the final exchanger construction.”

6. External Fire

The ASME code requires that a vessel be protected "against excessive pressure caused by exposure to fire…” Therefore, external fire must be addressed on both the shell AND tube side of an exchanger. Design options for fire case protection include: installing a dedicated PRV, utilizing a nearby PRV, elevating the exchanger out of the fire zone, or locking open block valve(s) to provide a clear relief path.

6.1 Services Requiring Dedicated PRV Protection for an External Fire

If an exchanger containing liquid can be isolated, dedicated relief protection is required for the external fire case. Heat from the fire will eventually vaporize liquid in the exchanger and cause overpressure. When an external fire is the ONLY cause of overpressure, Inflection Point Engineering indicates the need for PRV protection on our P&ID and we state that the valve is sized “BY OTHERS” (see ), but we do not give it an item number, nor is it included in the 807 project specification. Inflection Point Engineering does not have the design information needed to size the PRV, e.g. actual dimensions, TEMA type, number of shells, and exchanger elevation.

For example, a pumped product rundown system is often designed for pump shut-in pressure, but it requires PRV protection for the fire case because the exchangers can be isolated. Since external fire is the only cause of overpressure, PRV protection for the rundown system is shown on our P&ID, but no calculations are required (Figure 1).

Figure 1. Relief Protection By Others

If overpressure protection is required for other causes such as tube failure or blocked outlet (example: vaporizers), Inflection Point Engineering will size and select the PRV for the non-fire cases. The valve receives an item number, and it is included in the 807 project specification. However, the relieving rate for the external fire case does not have to be calculated. Instead, we include a note on the 807 project specification stating:

“When exchanger dimensions are known, the contractor shall determine the External Fire case rate and re-size PRV as necessary.”

6.2 Utilizing the PRV Protecting Nearby Equipment

If an exchanger cannot be isolated during the fire, dedicated relief protection is not required. Rather, Inflection Point Engineering utilizes a nearby PRV to protect the exchanger PROVIDED a clear path (see Section 5) exists between the exchanger and the PRV.

When an exchanger or group of exchangers is connected via a free path to a PRV that is protecting a vessel, a fire relief contribution from the exchanger should be calculated. See Section 6.5 for guidance.

6.3 Services NOT Requiring Relief Protection during an External Fire

Some exchange services do not require relief protection during an external fire, either because a clear path exists to the atmosphere, administrative procedures are in place to manage the fire hazard, the exchanger is elevated out of the fire zone, or because a PRV does not provide effective protection.

Figure 2. Providing a Clear Path by Locking Open a Block Valve

6.4 Routing the PRV Outlet when Valve is “BY OTHERS”

Even when PRV protection is provided “BY OTHERS”, Inflection Point Engineering should indicate the discharge location on the P&ID. Refer to for guidance.

6.5 Estimating the Relief Load for an Exchanger Due to External Fire

Inflection Point Engineering generally calculates an exchanger’s contribution to the fire case load of a PRV only when a clear path exists between the exchanger and a PRV protecting another equipment item (Section 6.2), or when requested by the customer. Note that exchanger services listed in Section 6.3 are excluded from this procedure.

When it is decided to exclude the associated exchangers from the external fire relief load for a vessel’s PRV, use the following note in the 807 project specification:

“Relief rate listed for the External Fire case is based on vessel(s) [insert item number(s)]. When the dimensions and location of associated exchanger(s) are known, the contractor shall determine the total fire case rate and re-size PRV as necessary.”

7. Thermal Expansion

If the cold side of an exchanger is improperly isolated, expansion of the cold-side fluid will occur as exchanger temperatures equilibrate. Overpressure due to thermal expansion is addressed in one of three ways: locking open (LO) isolation valves to prevent trapping the cold-side fluid, installing a keyed interlock system to ensure proper valve sequencing during isolation, or providing relief protection to limit the pressure rise.

When thermal expansion relief protection is required, Inflection Point Engineering shows a PRV symbol on the P&ID and we label it “BY OTHERS”, but it is not given an item number, nor is it included in the 807 project specification. The detailed design contractor has responsibility for sizing and selecting this relief valve.

If the customer plans to use an administrative procedure to manage the thermal expansion hazard, Inflection Point Engineering shows the isolation valves as locked open (LO) or car sealed open (CSO) on the P&ID. Under these conditions, relief protection is not required for thermal expansion.

If a customer requests a PRV in cooling water service with isolation valves, and neither valve is locked or sealed open, refer to for Inflection Point Engineering requirements regarding these isolation valves.

7.1 Thermal Expansion: Cold-Side Fluid is Liquid Only

Occasionally, Inflection Point Engineering is asked to size and select relief valves for liquid-filled exchangers. In such cases, refer to to determine the thermal expansion rate, assign an item number to the valve, and include it in the 807 project specification.

If the cold-side fluid is all liquid AND if the cold side can be isolated, protection for thermal expansion is required. A confined liquid will generate excessive pressures when heated only a few degrees. Before evaluating this case, perform a test to determine if the cold-side fluid will vaporize if it is heated to the hot-side inlet temperature. Refer to for further information.

7.2 Thermal Expansion: Cold-Side Fluid is Vapor Only

If the cold-side fluid is all vapor AND if the pressure exerted by the cold-side fluid exceeds accumulated pressure when it is heated to the hot-side inlet temperature, a relief valve is required. Show a PRV “BY OTHERS” on the P&ID, but no sizing calculation is required, nor is the valve listed in the 807 project specification. If the maximum pressure exerted by the cold-side fluid is less than accumulated pressure, thermal expansion protection is not required.

If Inflection Point Engineering is asked to size a thermal expansion valve for all-vapor service, refer to to determine the thermal expansion rate, assign an item number to the valve, and include it in the 807 project specification.

7.3 Routing the Outlet Piping for a Thermal Expansion PRV

See for guidance in routing the PRV discharge.

On rare occasions, the customer requests that all thermal relief valves, including those protecting hydrocarbon services, discharge to the oily water sewer. This routing is acceptable, PROVIDED the hydrocarbon fluid is non-flashing and non-toxic, e.g. it does not contain H2S.

8. Vaporization of a Confined Liquid

If hot-side flow continues while the cold-side liquid is blocked-in AND the hot-side inlet temperature is higher than the bubble point temperature of the cold-side liquid at the allowed accumulation pressure, vaporization will occur in the cold side. If the cold-side fluid is normally two-phase, no test is required since vaporization already occurs.

If vaporization of the cold-side liquid can occur: determine the vaporization rate, size and select a PRV, and include it in the 807 specification. Since significantly different temperatures are obtained during vaporization, see for the exchanger rating calculation.

When a PRV is required for the vaporization case, locate it near the exchanger outlet to minimize the amount of liquid displaced to the relief header while a vapor path is cleared to the PRV. Size the PRV for the vaporization rate, and not for an equal volume displacement of liquid.

When such a liquid vaporization case applies, it supersedes the liquid thermal expansion case; the thermal expansion case should not be reported.

9. Blocked Outlet

Generally, the blocked outlet case is eliminated by designing the exchanger for pump shut–in pressure. Occasionally, a PRV is used to limit pressure, particularly when the shut-in pressure is high, affects too much equipment, or raises the flange class of downstream equipment. In these situations, consider routing the PRV outlet to an upstream process cooler or surge drum rather than discharging liquid to the relief header.

NOTE: When routing the PRV discharge upstream of the pump, confirm that the pressure rating of the PRV outlet and bellows (if present) is not exceeded. If the pressure rating of the PRV outlet is exceeded, route the discharge to the relief header or to another low pressure location. See for additional comments on routing.

For process revamps, an existing pump may be modified or replaced to deliver flow at a higher pressure. In such cases, a PRV may be installed on the pump discharge to minimize changes to the downstream equipment.

A blocked outlet case applies to exchangers heated with steam from a Inflection Point Engineering-provided desuperheater, provided that the maximum water supply pressure exceeds the design pressure of the exchanger(s). The PRV should be located between the water TCV and the check valve at the water inlet nozzle. Specify the design and operating temperatures according to the water supply. The set pressure is based on the lowest design pressure among the exchanger(s) tube side and the desuperheater. The PRV discharge should be to a condensate drum if included in Inflection Point Engineering’s scope, otherwise to “Safe Location” (not to Atmosphere at Safe Location). For more information about this PRV service, see . For the relief stream properties, refer to T-807-10, “Flashing Water Properties for Relief”. Finally, the name of this relief case should be “Blocked in at Shutdown”.

Attachment 1 – Decision Tree for NEW Schedule A Designs

Attachment 1 –Decision Tree Notes

Note 1. A path is considered clear when there are no intervening valves between the exchanger and its PRV which could significantly restrict flow. A line with a locked-open block valve can be considered clear PROVIDED it does not have to be closed during normal operation.

Note 2. The HP fluid entering the LP system may cause high pressure differentials and/or interrupt normal functioning of process equipment. Hydraulic conditions that develop during relief must be determined to properly set the design pressure of the exchanger and intervening equipment.

Note 3. When the HP fluid is toxic, reactive, corrosive, or has a pour point above the winterizing temperature, safe routing for the PRV discharge can be extremely difficult. Consult the PRV Specialist.

Note 4. Consideration of the effects of HP fluid entering the LP system via a failed tube is REQUIRED. See for guidance.

Note 5. At pressure differentials over 1000 psi (70 kg/cm2 ), a relief valve will not respond fast enough to limit the transient pressure spike that develops in the liquid-full exchanger when HP fluid enters. If design of the LP side is less than the 10/13 rule dictates, a rupture disc must be specified to protect the LP side, but the design issues are sufficiently complex to warrant involvement of the PRV and Exchanger Specialists.

Note 6. When the HP design pressure is greater than 1000 psig, locate a dedicated PRV near the exchanger. This should be done even if a PRV exists elsewhere in the LP system.

Note 7. An exchanger specified by Inflection Point Engineering as “multitube” does not qualify as a double-pipe exchanger. To qualify, the heat transfer surface must be specified as schedule pipe.

Attachment 2 – Decision Tree for REVAMP Designs

Attachment 2 – Decision Tree for REVAMP Designs (continued)

Decision Tree Notes:

Note A. Consideration of the effects of HP fluid entering the LP system via a failed tube is REQUIRED. See for guidance.

Note B. The old “two-thirds” rule was applied to the operating pressure of the HP fluid, instead of the design pressure of the exchanger.

Note C. A path is considered clear when there are no intervening valves between the exchanger and its PRV which could significantly restrict flow. A line with a locked-open block valve can be considered clear PROVIDED it does not have to be closed during normal operation...

Note D. When the HP fluid is toxic, reactive, corrosive, or has a pour point above the winterizing temperature, safely routing the PRV discharge can be extremely difficult. Consult the PRV Specialist.

Note E. The HP fluid entering the LP system via a failed tube may cause high pressure differentials and/or interrupt normal functioning of process equipment. Conditions that develop during relief must be considered when setting design pressure of intervening equipment.

Note F. At pressure differentials over 1000 psi (70 kg/cm2), a relief valve is not considered fast enough to limit the transient pressure spike that develops when HP fluid enters the liquid-full LP side. A rupture disc must be specified to protect the LP side, but the design issues are sufficiently complex to warrant involvement of the PRV and Exchanger Specialists.

Note G. An exchanger specified by Inflection Point Engineering as “multitube” does not qualify as a double-pipe exchanger. To qualify, the heat transfer surface must be specified as schedule pipe.

Attachment 3 - Protecting the LP System during a Tube Failure

A3.1 Tube Failure in a Liquid Product Rundown Exchanger

If a HP fluid transfers heat to a liquid product stream, a tube failure will cause a rapid pressure rise in the LP side of the exchanger. In addition, if the product rundown line is isolated when the tube fails, the isolated part of the LP system will be subjected to the higher pressure.

Generally, Inflection Point Engineering increases the design pressure of liquid-full systems to contain the HP fluid because this design is inherently safer than providing relief protection. On occasion, the extent of the LP system that could be isolated and subjected to high pressure is considered so large that designing to the ten-thirteenths rule is impractical. In that case, design the exchanger to contain the transient pressure shock that develops when the tube fails, and install a PRV to protect the rest of the LP system during the tube failure.

NOTE: A tube can fail during start-up, normal operation, or during shutdown. Consider the effect of a tube failure during each operation and address the hazards which may develop as HP fluid discharges into the LP system. See for guidance.

a. Protecting the Rundown System when Liquid Product Is Pumped

Apply the ten-thirteenths rule to all equipment and piping in the rundown line that can be isolated and subjected to high pressure. When net product is pumped to storage, the rundown piping and equipment between the discharge check valve and the unit block valves could potentially be subjected to high pressure during a tube failure (Figure A3.1).

Even when the LP system is designed to contain the HP fluid during a tube failure, the rundown coolers may still require relief protection for an external fire, thermal expansion, or a blocked outlet. Although Inflection Point Engineering does not size this valve, we indicate that a PRV is required and that it is sized “BY OTHERS” when the exchanger dimensions are known.

When the ten-thirteenths rule is applied to the LP side of one exchanger in the rundown system, the rule does not cascade to utility streams just because they exchange heat with the product liquid. For example, if the HP side of E-1 is designed for 900 psig, and the cooling water side of E-2 is designed for 150 psig, the product rundown system is designed for 695 psig to contain the HP fluid. However, the design pressure for the cooling water side of E-2 remains at 150 psig and is NOT raised to 535 psig (ten-thirteenths of 695 psig). If a tube fails in E-1 while the rundown system is blocked in, Inflection Point Engineering considers simultaneous tube failure in E-2 highly unlikely because the process side of E-2 is designed to contain the HP fluid.

Figure A3.1 Product Liquid Pumped from a Fractionator

LP10/13 rule = Section of LP system designed to the ten-thirteenths rule

DPstd = Design pressure determined using standard rules

HP = HP fluid

CW = Cooling water

PRV = Relief protection provided for external fire, thermal expansion, or blocked outlet, but NOT for tube failure

b. Protecting the Rundown System when Liquid Product Is Pressured Out

If the product stream is pressured out AND if a clear path exists between the exchanger and the fractionator PRV, that valve can be used to protect the LP rundown system. However, the liquid-full exchanger and adjacent piping must be designed to contain the transient pressure shock that develops when the tube fails, even though the fractionator PRV protects the rest of the LP system. Since the transient pressure spike rapidly dampens outside the exchanger, the rest of the LP system does not have to be designed to the ten-thirteenths rule (Figure A3.2).

Figure A3.2 Liquid Product Pressured from a Fractionator

LP10/13 rule = LP side of exchanger and adjacent piping designed to the ten-thirteenths rule

DPstd = Design pressure of rundown line determined using standard rules

HP feed = HP fluid

CW = Cooling water

PRV = Relief valve used to protect fractionator and rundown coolers

A3.2 Tube Failure in a Liquid-Full Feed Preheat Exchanger (Vapor < 1 wt%)

If a HP fluid transfers heat to an all-liquid feed AND if the exchangers can be isolated from the fractionator PRV, design the feed preheaters to the ten-thirteenths rule (Figure A3.3). Although PRV 1 is not required for the tube failure case, it may be required for an external fire, thermal expansion, or a blocked outlet. PRV 1 is shown on the P&ID, but it is sized “BY OTHERS” when exchanger dimensions are known.

Since a tube failure could occur during normal operation, HP fluid could flow to the fractionator through the feed control valve. The effect of the HP fluid on the fractionator heat and weight balance and on the operation of the overhead condensing system should be evaluated and the fractionator PRV sized accordingly.

Figure A3.3 Liquid-Full Feed Preheat System

LP10/13 rule = Section of LP system designed to the ten-thirteenths rule

DPstd = Design pressure determined using standard rules

HP = HP fluid

NB = Net bottoms liquid

PRV 1 = Relief valve protects feed preheat exchangers during fire case

PRV 2 = Relief valve used to protect the fractionator system during a tube failure

Although the feed side of the feed-bottoms exchanger is designed to the ten-thirteenths rule, the bottom side is not. If a tube fails in E-1, Inflection Point Engineering considers simultaneous tube failure in E-2 to be highly unlikely because the feed side is designed to contain the transient pressure. In this case, the bottoms side would likely be designed for the shut-in pressure of the net bottoms pump.

A3.3 Tube Failure in a Preheat Exchanger with Partially Vaporized Feed (Vapor ≥ 1 wt%)

If the feed is partially vaporized AND there is a clear path from the exchanger to the fractionator PRV, apply the ten-thirteenths rule only to exchangers that have less than 1wt% vapor in the outlet (Figure A3.4). Although E-1 will be subjected to a sharp pressure rise as the HP fluid accelerates LP fluid out of the exchanger, its higher design pressure will contain that spike. In addition, the pressure spike significantly dampens before it reaches the downstream exchanger, E-2. Therefore, its design pressure can be based on the hydraulics obtained during the tube failure.

Figure A3.4 Feed Preheat System with Partially Vaporized Feed

LP10/13 rule = Section of LP system designed to the ten-thirteenths rule

DPstd = Design pressure determined using standard rules

HP = HP fluid

NB = Net bottoms liquid

∆Prelief = Additional pressure drop obtained in downstream system during the tube failure case

PRV = Relief valve used to protect the fractionator and feed preheat exchangers

A3.4 Tube Failure in a Reboiler Receiving Heat from a HP Fluid

If a reboiler tube fails, HP fluid will enter the fractionating column and cause its pressure to rise. Generally, Inflection Point Engineering utilizes the fractionator PRV to protect the column, receiver, and associated exchangers when a reboiler tube failure occurs.

Since the process side of the reboiler contains more than 1wt% vapor, it does not have to be designed to the ten-thirteenths rule, since the transient pressure spike is dampened by the vapor in the exchanger. In the example shown below, the process side of the reboiler is not designed to the ten-thirteenths rule. Rather, its design pressure is set relative to the fractionator, and both are protected during a tube failure by the PRV on the overhead vapor line.

Figure A3.5 Reboiler Receiving Heat from a HP Fluid

DPstd = Design pressure of reboiler loop determined using standard rules

PRV = Relief valve used to protect fractionator system during the tube failure

When HP fluid enters the column, it mixes with the column inventory, transferring heat and potentially affecting equilibrium. For example, if HP steam is the heating medium and if the condensate leaves on flow control, Inflection Point Engineering assumes that the tube break occurs in the flooded section of the tube bundle. Initially, condensate flows through the tube at a rate governed by hydraulics. After a few minutes, condensate in the reboiler is depleted and condensate flow is limited by the amount of steam condensed. At this point, hydraulics will determine the additional steam that will flow with the condensate into the column.

As condensate and steam enter the bottom of the column, it undergoes additional flashing, but a significant amount of condensate accumulates in the bottom sump. The steam lowers partial pressure, which in turn, lowers the bubble point of the bottom material and increases the reboiler LMTD. As a result, reboiler duty increases, but vaporization in the reboiler tends to fall off due to the higher latent heat of vaporization of water.

When the tube fails, steam migrates throughout the column, and eventually leaves with the overhead vapor. This causes the molecular weight of the overhead vapor to decrease, and the amount of overhead vapor condensed falls off due to the high latent heat of steam. In addition, water and hydrocarbon are recirculated back to the fractionator via the reflux.

Because tube rupture causes major operational problems, no general guidance can be given other than that in . Consult the PRV specialist for your specific application.

A3.5 Tube Failure in a Hot Oil Exchanger that Transfers Heat to a HP Process Stream

During a tube failure, HP fluid will enter the hot oil side of the exchanger, causing pressure to rise rapidly as hot oil is accelerated out of the exchanger. To contain the transient pressure spike, Inflection Point Engineering designs the exchanger to the ten-thirteenths rule. However, the HP fluid flowing through the failed tube will eventually reach the Hot Oil Expansion Drum. Therefore, a tube failure case must be considered when sizing the PRV protecting the expansion drum.

If a clear path exists between the exchanger and the expansion drum PRV, the ten-thirteenths rule only applies to the exchanger, adjacent piping, and isolating block valves (Figure A3.6). Since the transient pressure spike dampens significantly once it is outside the exchanger, Inflection Point Engineering does not carry the higher pressure rating to other exchangers in the hot oil system PROVIDED they cannot be isolated with the failed exchanger. Note that the hot oil flow controller must be located at the exchanger inlet to provide an open path to the expansion drum. In addition, the isolating block valves for the exchanger and hot oil return header must be locked open. If it is NOT possible to increase the design pressure of the hot oil side of the exchanger consult the PRV Specialist. If the isolating block valves cannot be locked open, or if the flow control valves must be located downstream of the exchanger, a PRV (BY OTHERS) is required for thermal expansion of hot oil during an external fire.

When the process fluid enters the expansion drum, Inflection Point Engineering assumes that it completely mixes with the hot oil. If the HP fluid vaporizes at the hot oil temperature in the drum, the vaporized HP fluid is relieved directly, assuming complete V/L disengagement occurs.

Figure A3.6 Hot Oil Circulation System

LP10/13 rule = Section of LP system designed to the ten-thirteenths rule

DPstd = Design pressure determined using standard rules

HP = HP fluid

LP = LP fluid

PRV = Depending on location of valves, a relief valve MAY be required for thermal expansion during an external fire, but NOT for tube failure

A3.6 Pump-Around (PA) Exchanger Transfers Heat to a HP Fluid

If one of the exchangers in a pump-around stream transfers heat to a HP fluid, a tube failure will cause pressure in the pump-around circuit to increase rapidly. Since the pump-around circuit will be subjected to higher pressure, Inflection Point Engineering designs all exchangers, piping, and valves located downstream of the circulation pump and upstream of the last valve in the PA circuit to the ten-thirteenths rule. In addition, Inflection Point Engineering provides a PRV (BY OTHERS) to protect the exchanger during an external fire.

Figure A3.7 Heat Exchangers in a Heat Removal Circuit for a Fractionator

LP10/13 rule = Section of LP system designed to the ten-thirteenths rule

DPstd = Design pressure determined using standard rules

HP = HP fluid

LP = LP fluid

LO = Lock open all valves downstream of the 10/13-designed piping

If the tube rupture occurs during normal operation, the HP fluid will eventually enter the fractionator causing its pressure to rise. Size the fractionator PRV for the tube failure case and consider any “domino” effects, such as vapor blanketing the condenser, in the relief calculations for the fractionator.

On rare occasions, the extent of the PA circuit that could be subjected to high pressure is considered so large that designing to the ten-thirteenths rule is impractical. In that case, design the exchanger to the ten-thirteenths rule, and install a PRV to protect the rest of the PA system. Alternatively, move the pump-around flow control upstream of the exchangers to provide a clear path back to the fractionator. In this case, only the exchanger, flow control valve and piping from the valve to the inlet of the next exchanger need be designed to the ten-thirteenths rule.

A3.7 Water-Cooled Exchangers

When HP fluid enters the cooling water side during a tube failure, a transient pressure spike develops as the HP fluid accelerates the water out of the exchanger. Therefore, Inflection Point Engineering designs the cooling water side of an exchanger to the ten-thirteenths rule to withstand the transient pressure shock. If the water side cannot be blocked in, the higher design pressure is carried to the first sub-header for the water return.

If the water side can be blocked-in, design the exchanger, piping, and block valves to contain the HP fluid. Equipment and piping affected by the ten-thirteenths rule is highlighted in Figure A3.8. A relief valve (BY OTHERS) provides thermal expansion protection when the exchanger is blocked in. However, this PRV provides an escape route to the atmosphere for the HP fluid if the water side is blocked in and a failed tube is pre-existing in the exchanger. For the following HP services, if the PRV (BY OTHERS) is present, then mark the return block valve as LO or CSO (per customer preference): toxic material, benzene greater than 1 mole %, and LPG or lighter.

Figure A3.8 Designing a Water-Cooled Exchanger to Contain the HP Fluid

LP10/13 rule = Section of LP system designed to the ten-thirteenths rule

DPstd = Design pressure determined using standard rules

HP = HP fluid

PRV = Relief valve sized (by others) for thermal expansion & external fire, but NOT for tube failure

Since the pressure spike rapidly dampens once it is outside the exchanger, other exchangers in the cooling water system are generally not subjected to the transient pressure that develops inside the failed exchanger. Therefore, the other water-cooled exchangers in the system need NOT be designed to contain the HP fluid, PROVIDED a clear path exists to the cooling tower, i.e. isolating block valves on the return header are locked open.

If it is NOT possible to increase the design pressure of the cooling water side of the exchanger, e.g. a revamp project, consult the PRV Specialist.

With reference to Figure A3.8, a customer may request isolating block valves with the return valve either locked or car-sealed open. Provided that tube failure is addressed by designing to the ten-thirteenths rule as shown, the PRV (BY OTHERS) is not required.

WARNING: If the HP fluid is toxic, e.g. contains H2S, it will eventually be discharged to the atmosphere during a tube failure via the cooling tower. Consult the PRV Specialist to determine how this hazard can be addressed.

A3.8 Steam-Heated Exchanger: Steam Pressure Lower than Process Fluid

If steam supplies heat to a process stream that operates at a higher pressure, HP fluid will flow into the steam and/or condensate systems during a tube failure. If the HP fluid is toxic, e.g. contains H2S, a check valve is installed in the steam supply line to limit the extent of contamination, and the steam side of the exchanger is designed to the ten-thirteenths rule. Piping, valves, and/or steam trap that could be isolated with the exchanger when the tube fails should be also designed to the ten-thirteenths rule (Figure A3.9). A PRV (BY OTHERS) provides overpressure protection during an external fire.

Figure A3.9 Designing the Steam-Heated Exchanger to Contain an HP Fluid

where:

LP10/13 rule = Section of LP system designed to the ten-thirteenths rule

DPstd = Design pressure determined using standard rules

HP = HP fluid

PRV = Relief protection provided for external fire or blocked outlet, but NOT for tube failure

WARNING: If the HP fluid is toxic, e.g. contains H2S, it will eventually be discharged to the atmosphere during a tube failure via the vent on the condensate drum. If the condensate drum is within Inflection Point Engineering’s design scope, route the condensate vent to a safe location. If the drum is outside Inflection Point Engineering’s scope:

“Contractor shall route all condensate system vents to a safe location.”

A similar approach can be used when the process side contains light hydrocarbons. Release to the atmosphere through the “PRV (BY OTHERS)” is also a risk during a tube rupture, even though the PRV is not sized for that event, because the process-side pressure could exceed the set pressure. To manage the risk, consider raising the steam-side design pressure to equal the process side design pressure (i.e. apply a “13/13” or “10/10” approach). This approach can avoid both the need to route the PRV discharge to the relief header and the consequences of steam/water entering the relief header system.

Occasionally a PRV is installed to protect an existing exchanger that was not designed to the ten-thirteenths rule. While this is an acceptable design strategy, consult the PRV Specialist to ensure that all safety issues associated with relief protection are addressed. See also .

A3.9 Steam Generator: Steam Pressure Lower than Heating Medium

When a tube fails, HP fluid will enter the steam generator and may potentially migrate throughout the steam system. If the HP fluid is non-toxic and does not flash when it enters the steam generator, Inflection Point Engineering assumes that the HP liquid will mix with boiler feed water, transfer its heat, and generate more steam than normal. We assume that the HP liquid accumulates in the steam drum, and it is not carried over with the steam when the PRV opens. Inflection Point Engineering sizes the PRV(s) protecting the drum for the “excess” steam and routes its discharge to the atmosphere.

It is not necessary to design the steam generator to the ten-thirteenths rule. The steam will absorb the transient pressure shock, and the PRV is sized to protect the steam system during a tube failure.

If the HP fluid is toxic (e.g. contains H2S) or is flammable and flashes at the steam generator pressure, the PRV protecting the steam system must discharge to a relief header. In addition, the design of the relief valves and associated piping must address the toxic and/or flammability hazard that could develop during a tube failure. In particular, the design engineer should:

“Contractor shall route all vents and drains to a safe location.”

Figure A3.10 HP Fluid is Toxic and/or Vaporizes at Steam Pressure

Attachment 4 - Calculating the Flow of HP Fluid through a Failed Tube

A4.1 Assumptions

The tube failure analysis assumes the complete failure of one tube in accordance with API Standard 521. Although a failure can occur anywhere along the tube, the maximum flow rate occurs if a guillotine break occurs at the tubesheet, and the HP fluid enters the LP side from two directions, i.e. through the “hole” in the tubesheet and through the end of the severed tube.

Inflection Point Engineering assumes that the tubesheet opening acts as a square-edged orifice with a diameter equal to the tube ID. The physical properties of the HP fluid at its normal operating temperature and pressure are used.

The assumptions for the behavior of the fluid are:

We determine the HP flow rate for single-phase flow using the appropriate equation in Crane Technical Paper No. 410 for a square-edged orifice. For two-phase flow entering the break, the two assumptions above are combined in a homogeneous (no slip) non-equilibrium model.

Flow through the separated end of the tube is conservatively estimated to be half of the rate flowing through the tubesheet. Thus, Inflection Point Engineering multiplies the calculated flow through the tubesheet by 1.5 to obtain the combined flow through both sides of the break. Refer to .

NOTE: The tube failure rate is the same whether the HP fluid is located on the tube side or shell side. If the HP fluid is on the shell side, it will not flow as readily into the failed tube because of higher entrance losses. However, Inflection Point Engineering has simplified the flow calculation through the guillotine break to be a fixed fraction of the tubesheet flow, and we do not explicitly include the entrance loss when the HP fluid is on the shell-side, nor do we include the exit loss when the HP fluid is tube-side.

A4.2 Pressure Differential across the Break

The pressure differential for incompressible flow is a function only of pressures on both sides of the break.

P = PHP – PLP, psi

PHP = normal operating pressure of the high-pressure side, psig

PLP = pressure in the low-pressure side when the LP system is at accumulated pressure, psig

For compressible fluid (including two-phase flow), the pressure differential and therefore the flow is governed by whether critical (i.e. choked) flow is achieved across the break.

P = PHP – PE, psi

PE = pressure at the exit of the break, psig

If the critical flow pressure, PC, is higher than PLP, then PE = PC. Otherwise, PE = PLP.

The critical pressure ratio for vapor flow is a function of the heat capacity ratio, k=Cp/Cv.

The non-flashing assumption for a liquid at its bubble point, whereby the stream does not reach equilibrium in the short tubesheet path, means that choking is not expected. Therefore, PE = PLP, and the flow is calculated with the incompressible liquid equation.

For two-phase inlet flow, the liquid is assumed not to flash until after the break. Assuming homogenous, non-flashing flow, the critical flow pressure is obtained from the following correlation:

α = volume fraction gas/vapor at the inlet, dimensionless

=

x = mass fraction vapor in the flow entering the break, dimensionless

ρL = density of liquid at the inlet, lb/ft3

ρG = density of gas/vapor at the inlet, lb/ft3

A4.3 Flow of Non-Flashing Liquid through the Failed Tube

Determine the flow of HP liquid through the tubesheet and the broken end of the tube by multiplying the rate from the incompressible flow equation for a square-edged orifice by 1.5.

Where:

W = mass flow of HP liquid through the failed tube, lb/h

d = tube inside diameter, inches

C = orifice flow coefficient, dimensionless

P = pressure differential across the tubesheet during relief conditions, psi

= liquid density of HP fluid at its normal operating pressure, lb/ft3

Assuming the flow coefficient = 0.7, the equation for HP liquid simplifies to:

Since the effect of flashing is delayed until after the flow has passed the break, use the non-flashing liquid equation to calculate flow for a flashing liquid (i.e. no vapor or gas entering the break).

A4.4 Vapor Only Flow through the Failed Tube

Determine the combined flow of HP vapor through the tubesheet and the broken end of the tube by multiplying the compressible flow equation for a square-edged orifice by 1.5.

Where:

W = mass flow of HP vapor through the failed tube, lb/h

Y = net expansion factor, dimensionless

d = tube inside diameter, inches

C = orifice flow coefficient, dimensionless

P = pressure differential across the tubesheet during relief conditions, adjusted for critical flow as needed, psi (see Section A4.2)

SV1 = specific volume of HP vapor at its normal operating temperature and pressure, ft3/lb

As the vapor expands through the tubesheet and failed tube, velocity-pressure conversion takes place. The net expansion factor, Y, implicitly “corrects” the incompressible flow equation for the pressure loss due to the density change. Assuming C = 0.7 and Y = 0.8, the flow equation for an HP vapor simplifies to:

A4.5 Two-Phase Flow through the Failed Tube

If both liquid and vapor can enter the break simultaneously, the flow rate is calculated by assuming the mixture is homogeneous (i.e. no slip between phases). The liquid is assumed not to flash and the vapor expands as pressure decreases.

W = mass flow of HP two-phase fluid through the failed tube, lb/h

C = orifice flow coefficient, dimensionless

d = tube inside diameter, inches

x = mass fraction vapor in the flow entering the break

ρL = density of liquid at the inlet, lb/ft3

ρG = density of gas/vapor at the inlet, lb/ft3

k = ratio of specific heats of the inlet vapor (= Cp/Cv)

PHP = normal operating pressure of the high-pressure side, psia

PE = pressure on the downstream side of the break, psia (see section A4.2)

Scenarios involving two-phase flow into the break require careful analysis. If the vapor fraction in the HP side normally changes significantly from inlet to outlet, consider separate scenarios for the inlet and outlet mixtures. Report only the worse case in the 807 specification. For example, a steam-heated reboiler may experience tube failure in the inlet steam region or the outlet condensate region. Two-phase break flow does not apply in this case, only steam and condensate flow cases.

A4.6 Consequences of HP Fluid Mixing with LP Fluid

The consequences of a tube failure depend on what happens to the HP fluid once it enters the LP side, and how it interacts with the LP fluid. See for design issues which affect the tube failure analysis.

Attachment 5 – Effects that Cascade to the LP System

A5.1 General

When an exchanger tube fails, HP fluid will flow into the LP system, potentially causing other hazards, e.g. equipment failure, severe corrosion, or release of toxic and flammable material to the atmosphere. Even when the exchanger is designed to the ten-thirteenths rule, or when a PRV is provided for the tube failure case, hazards that could develop in the LP system must be identified and addressed.

A5.2 Hazards that Cascade throughout Systems within Inflection Point Engineering’s Design Scope

If HP fluid leaks into the LP system, there may be secondary effects that incrementally increase the relieving rate. For example, reactor effluent may be used to preheat stripper feed. If a tube fails in the preheater, reactor effluent will enter the stripper along with the feed. The reactor vapor will vapor blanket the overhead condenser, stopping all condensing duty and eventually causing reflux to stop. The relieving rate for the tube failure case must consider the mass imbalance caused by HP fluid entering the stripper as well as the heat imbalance caused by loss of condenser duty and reflux.

NOTE: If the HP fluid affects LP equipment in another process unit, transmit the tube failure rate, composition of the HP fluid, and its thermal condition to those responsible for the design of the downstream system.

The following questions probe for secondary effects that could affect relieving rates in the LP system:

For steam-heated thermosyphon reboilers (only) see .

A5.3 Hazards that Cascade Outside Inflection Point Engineering’s Design Scope

If a toxic or flammable vapor leaks into a steam, condensate, or cooling water system, product storage facilities, or non-Inflection Point Engineering process units, the HP fluid could eventually be released to the atmosphere via atmospheric discharge of relief valves, vent lines, or at the cooling tower. Although the Customer/Contractor has primary responsibility to manage hazards in these systems, Inflection Point Engineering project specifications may have to be fortified, especially when the HP fluid is toxic, corrosive, or a flammable vapor. (Strategies for reducing the potential for tube leak or rupture include: strength-welding tubes to the tubesheet, providing impingement baffles, using a U-tube design instead of a floating/fixed tubesheet, upgrading tube metallurgy, or specifying heavier gauge tubes. Consult the Exchanger Specialist before making changes to the exchanger specification.)

If a HP fluid containing H2S is used to generate steam, the relief valves protecting the steam system must discharge to a relief header, and these valves must have a closed bonnet and packed lifting lever. If the winterizing temperature is below 32F, the PRV and its inlet/outlet piping must be heat-traced. During normal operation, steam may leak through the valve, freeze and plug the outlet line. In addition, Inflection Point Engineering requires strength-welding the steam generator tubes to minimize the potential for leaks.

If a HP fluid containing toxic or flammable vapor is cooled with water, a tube failure will release this material to the atmosphere via the cooling tower. The Customer/Contractor may address this hazard by increasing exchanger inspection and maintenance, and by monitoring the concentration of the toxic or flammable material at the cooling tower.

Attachment 6 - Thermal Expansion of a Confined Liquid or a Confined Vapor

A6.1 General

Inflection Point Engineering does not size relief valves for thermal expansion unless requested by the customer. Thermal expansion relief valves (BY OTHERS) are shown on our P&IDs, but they are not given an item number nor are they listed in the 807 project specification.

When thermal expansion relief protection falls within our scope of design, the thermal expansion calculation is based on the following assumptions:

NOTE: Whenever the cold-side fluid is all liquid, determine if the temperature of the hot-side inlet is sufficiently high to vaporize the trapped cold-side liquid. If so, evaluate a subsequent vaporization case, and size the relief valve for the larger of the two cases. See for calculating the vaporization rate.

A6.2 Thermal Expansion of a Confined Liquid

NOTE: Before evaluating thermal expansion for a confined liquid, first confirm that vaporization does not occur; see . If vaporization can occur, then evaluate that case instead of liquid thermal expansion.

The thermal expansion rate for a confined liquid calculated using an equation given in API Standard 521.

Where:

q = thermal expansion rate, gpm.

B = cubic expansion coefficient of the cold-side liquid, 1/F (See Table A6.1)

H = total heat transfer rate, Btu/h.

G = specific gravity of trapped liquid at normal outlet temperature.

Cp = specific heat of trapped liquid at normal outlet temperature, Btu/lb/F.

Converting the API equation to give the thermal expansion rate as mass flow, it simplifies to:

where W = mass flow rate of expanded liquid, lb/h.

Since pressure rises rapidly in the cold side when it is heated only a few degrees, Inflection Point Engineering assumes that the exchanger duty is the same as normal. If multiple exchangers can be blocked-in, a single PRV can be used to protect the entire train, PROVIDED there are no intervening valves. In that case, add the normal duty for each exchanger together and use the sum in the equation above.

For hydrocarbon liquids, API Standard 521 lists the expansion factor, B as a function of API gravity. Although this data is referenced to 60 F, it is sufficiently accurate for determining the thermal expansion rate at other temperatures.

Table A6.1 Cubical Expansion Coefficient per degree Fahrenheit for Hydrocarbons and Water at 60 °FTable A6.1 Cubical Expansion Coefficient per degree Fahrenheit for Hydrocarbons and Water at 60 °F
API gravityCoefficient, B
3.0 - 34.90.0004
35.0 - 50.90.0005
51.0 - 63.90.0006
64.0 - 78.90.0007
79.0 - 88.90.0008
89.0 - 93.90.00085
94.0 - 100.0 and lighter0.0009
Water0.0001
API Gravity = (141.5 / SpGr) – 131.5API Gravity = (141.5 / SpGr) – 131.5

Inflection Point Engineering uses the cold-side outlet temperature to determine physical properties of the material at the PRV inlet. Although the temperature of the cold-side fluid inside the exchanger will approach the hot-side inlet temperature, this hotter liquid does not expand enough to displace all of the cooler liquid out of the piping.

A6.3 PRV Sizing when Flashing Occurs in the Nozzle

If the cold-side liquid contains light hydrocarbon, it could flash in the PRV nozzle as it accelerates. Since a flashing liquid requires more relief area than a non-flashing liquid, determine the amount of flashing (if any) as follows. Set the conditions of the liquid stream using the normal temperature and accumulated pressure. (e.g. TF command in P9.8) Then do an adiabatic flash on that stream at relief header pressure. (e.g. QF command in P9.8) If more than 1 wt% flashes to vapor at the header pressure, use the sizing method for flashing liquid. Otherwise, use the sizing method for non-flashing liquids. Both methods are discussed in ”.

A6.4 Thermal Expansion of a Confined Vapor

When a confined vapor is heated, its pressure rise is proportional to the temperature increase. Use the ideal gas law to determine the temperature needed to raise the cold side to accumulated pressure:

Where:

Tacc = temperature required to reach accumulated pressure in the cold side, F

(T CS in )nor = normal temperature at the cold-side inlet, F

Pacc = accumulated pressure in the cold side, psia

Pnor = normal operating pressure at the cold-side inlet, psia

If the cold-side temperature at accumulated pressure is higher than the hot-side inlet temperature, relief protection for thermal expansion is not required. If the calculated cold-side temperature is lower, thermal relief protection is required, and the following equation should be used to estimate the vapor expansion rate.

where:

Wrel = thermal expansion rate for the confined vapor, lb/hr

Qnor = normal exchanger duty, BTU/hr

Tacc = relieving temperature, F

Cp = heat capacity of the vapor at relieving conditions, BTU/lb-oF

(T HS in )nor = normal hot-side inlet temperature, °F

(T CS in )nor = normal temperature at the cold-side inlet, F

For the special case of an electric heater, the relief rate is not dependent on a temperature differential driving force. Use the following equation to calculate the relief rate.

A6.5 Thermal Expansion of a Super-Critical Fluid

If the cold-side fluid is above its critical point at accumulated pressure and the hot-side inlet temperature, use the sizing procedure given above in A6.4. Since the heat capacity function may give unreliable results near the critical point, use the simulator-generated heat capacity only when it is less than 1.4 BTU/lb F.

Attachment 7 - Vaporization of a Confined Liquid

A7.1 Test: Will the Cold-Side Fluid Vaporize?

Relief protection is required when ALL conditions listed below exist:

NOTE: The last condition applies specifically to liquid-full exchangers. If liquid and vapor phases are normally present in the cold side, vaporization will occur when the cold side is blocked in.

A7.2 Rate Exchanger Duty for the Vaporization Case

Since blocked-in conditions are radically different than normal, the exchanger is rated for relief conditions. The calculation approach depends on whether or not there is phase change on the hot side.

a. No phase change on the hot side (sensible heat transfer only)

Use a trial and error procedure. When convergence is obtained, the hot and cold-side energy balances are consistent with the heat transfer equation. The general steps are:

1. Guess the exchanger duty for the vaporization case.

2. Calculate the hot-side outlet temperature based on the guessed duty, assuming that the hot-side flow rate and inlet temperature are the same as normal.

3. Calculate the LMTD assuming that the temperature of the cold-side fluid is uniformly at its bubble point, or at the adiabatic flash temperature of the cold-side outlet at accumulated pressure, whichever is higher.

4. Calculate exchanger duty using a 25% safety factor, i.e. UArel = 1.25 UAnor.

Compare the calculated duty to the guessed duty, and repeat steps 1-4 until the calculated duty equals the guessed duty. The exchanger rating calculations are given below, listing the appropriate equations and highlighting assumptions. If the assumptions embedded in the rating calculation are not valid, consult the Exchanger Specialist to determine rated duty at relief conditions.

The hot-side outlet temperature calculation assumes the heat capacity of the hot-side fluid is constant, and that no phase change occurs.

Where:

Qguess = guessed duty, MMBtu/h

Qnorm = normal duty, MMBtu/h

(T HS in )norm = normal hot-side inlet temperature, °F

(T HS out )norm = normal hot-side outlet temperature, °F

(T HS out )guess = hot-side outlet temperature based on guessed duty, °F

The LMTD calculation at relief conditions assumes a uniform cold-side temperature. If the cold side is liquid full, determine the bubble point of the cold-side fluid at accumulated pressure. If the cold side normally contains vapor and liquid, determine the adiabatic temperature of the cold-side outlet fluid at accumulated pressure.

Where:

LMTD guess = log mean temperature difference based on guessed duty, °F

T HS in = inlet temperature of the hot-side fluid (normal), °F

T HS out = outlet temperature of the hot-side fluid based on guessed duty, °F

T CS = bubble point temperature of the cold-side fluid at Pacc when it is all liquid, or its adiabatic flash temperature at accumulated pressure when it is partially vaporized, °F

Rate the exchanger assuming a constant UA and a constant thermal effectiveness factor. Include a 25% safety factor to account for exchanger design margins and to cover inaccuracies in the assumptions. The LMTD at normal conditions can be obtained from the exchanger sizing tool, or calculated using the normal inlet and outlet temperatures:

Where:

Q calc = calculated duty, MMBtu/h

LMTD norm = log mean temperature difference at normal conditions, F°

Convergence is obtained when the guessed duty equals the calculated duty (= Qrated).

b. Hot Side with Phase Change

If steam or other condensing vapor is used as the heating medium, the procedure is similar to the above but the solution is a direct calculation. Determine the duty provided by superheat (if any), and rate the exchanger using the saturated temperature. Add the superheat duty to the rated duty to obtain the total heat input during relief. Assume that hot-side flow rate during relief is the same as normal.

Q rated = rated duty of exchanger at relief conditions, MMBtu/h

THC = Hot side condensing temperature at normal pressure, °F

QSH = Superheat duty between THS in and THC, MMBtu/h

A7.3 Determine the Vaporization Rate of Cold-Side Fluid

Determine the vaporization rate using the rated duty of the exchanger and the latent heat of the cold-side fluid. The enthalpy data used in this calculation must be consistent with the composition of the cold-side fluid and its vaporization temperature. Obtain this data from the bubble point search, or from the adiabatic temperature search.

Where:

W rel = vaporization rate of cold-side fluid that must be relieved, lb/h

Q rated = rated duty of exchanger at relief conditions, MMBtu/h

HVAPZN = latent heat of vaporization at accumulated pressure (per IPE-TM-807-01)

= hCS vap - hCS liq for a one-component material

h CS vap = specific enthalpy of cold-side vapor, Btu/lb

h CS liq = specific enthalpy of cold-side liquid, Btu/lb

When the cold side is liquid full, vapor generated in the exchanger will initially “push” liquid out the PRV. However, Inflection Point Engineering neglects this liquid displacement when sizing the PRV, even though volumetric displacement of liquid usually requires more relief area than vapor only. The conservative assumptions used to determine the vaporization rate justify this design approach.

A7.4 Locate the PRV Close to Exchanger

When a PRV is sized for the vaporization case, it can be installed on the process piping, PROVIDED the nominal diameter of the piping is larger than the PRV inlet flange. Otherwise, specify a larger exchanger outlet flange to accommodate larger diameter piping for the relief valve.

NOTE: The PRV will be elevated to ensure free draining of the discharge line to the relief header. If the exchanger is normally liquid-full, add a note to the 807 project specification instructing the contractor to adjust valve set pressure when the exact PRV elevation is known. Use the following note, which is standard in TZ-807-01:

“Free-draining discharge piping requires elevated PRV. Contractor to adjust PRV set pressure for liquid static head as necessary.”

Attachment 8 - Routing the PRV Outlet Piping

A8.1 Safe Routing for PRV Outlet Piping

Depending on the heat exchange service, PRV outlet piping may be routed to the relief header, another process location, the oily water sewer, or to the atmosphere. Inflection Point Engineering indicates the discharge location on the P&ID, even when PRV protection is provided “BY OTHERS”.

A8.2 Free Draining Requirement

Inflection Point Engineering requires PRV outlet piping to be free-draining to prevent accumulation of liquid in the discharge line. Generally, the PRV is elevated above the relief header to provide free-draining. Similar reasoning applies to valves that discharge to another process location. If the valve cannot be sufficiently elevated, it must discharge into a liquid KO drum that is vented to the relief header. Consult the PRV Specialist for the design of the KO system.

If a PRV discharges to the atmosphere, e.g. a kettle steam generator, its tailpipe is provided with a 1/2 inch drain to keep rain from accumulating in the outlet piping. The drain also prevents condensate build-up in the outlet if and when the valve leaks.

If the discharge line from a thermal expansion relief valve is routed downstream of an exchanger isolation block valve, it must be free-draining from the relief valve outlet to the process piping downstream of the valve. The relieving rate due to thermal expansion is relatively small and as such, large reaction forces are not expected when the valve opens. However, the pressure rating of the relief valve outlet must be compatible with the pressure rating of the process piping. Refer to sections A8.3 and A8.4 for additional comments.

A8.3 Pressure Rating of PRV Outlet

The pressure rating of the relief valve outlet may be constrained by the pressure rating of the PRV outlet flange, valve body, or bellows (balanced type). Pressure ratings for the valve outlet cannot be specified independent of the valve inlet because commercially-available PRV’s are limited to specific combinations. If the pressure rating of any component is exceeded, the relief valve discharge must be routed to the relief header or another location that operates at a lower pressure.

A8.4 PRV Set Pressure Affected by Back Pressure

The set pressure of a conventional (non-balanced) PRV is directly affected by the outlet pressure. As such, a conventional PRV should only discharge to a system that operates at relatively constant pressure. If discharge pressure falls off during normal operation, the “effective” set pressure of the PRV will be lowered, and the valve may leak or open prematurely. Special consideration should be given to the relative pressures obtained at the valve inlet and outlet during start-up and shut-down operations. If pressure at the outlet is not established before pressure at the inlet is raised to normal, the valve may open prematurely, delaying start-up.

If the PRV discharge pressure cannot be maintained constant, consider using a balanced bellows, or a pilot-operated PRV. The set pressure for either of these alternative valves is not affected by the superimposed back pressure up to about 50% of set pressure. Alternately, consider routing the discharge piping to a location that operates at a constant pressure.

A8.5 Heat Tracing PRV Outlet Line

Assume that every PRV will leak at some time during its life cycle, and that the leaking fluid is cooled to the winterizing temperature. If the leaking fluid will freeze, or become excessively viscous at the winterizing temperature, the valve body and discharge line must be heat traced to prevent plugging.

A8.6 Relieving Hydrocarbons to the Atmosphere

Inflection Point Engineering strongly objects to relieving hydrocarbons to the atmosphere, regardless of molecular weight. Hydrocarbon fluids are flammable and many process streams contain toxic components. Depending on relieving temperature, hydrocarbon may auto-ignite upon release to atmosphere. Even worse, hydrocarbon vapor may accumulate at grade until it “finds” an ignition source, resulting in a flash fire or vapor cloud explosion. Inflection Point Engineering does not consider dispersion models sufficiently accurate to predict ground level concentrations during a relief event. The actual relieving rate and composition may be far different than that based on the conservative assumptions regarding instrumentation and/or operator response. If the relieving rate is lower than expected, the exit velocity will also be lower, and mixing efficiency will fall off. In addition, the hydrocarbon stream may be cooled below its dew point upon mixing with air, and hydrocarbon liquid could “rain” on the process unit.

If, in spite of our objections and concern, the customer insists on atmospheric discharge, they must take full responsibility for that deviation. In such cases, the relief valve specification and P&I drawings should show relief valves discharging to a “location yet to be determined by the customer or contractor”.

Attachment 9 – Tube Rupture Relief Analysis for Steam-heated Reboilers

This procedure applies to the leak of steam (not condensate) into the hydrocarbon side of a thermosyphon or kettle reboiler. The incoming steam’s latent heat is used to vaporize bottoms material, which then displaces overhead vapor to the column PRV.

Where:

NOTE: This procedure applies specifically to steam-heated reboilers in fractionators, but a similar approach may be considered in services where there is sufficient vapor-liquid disengaging space below the PRV.

info@inflectionpointengineering.com

© 2026 Inflection Point Engineering, LLC. All rights reserved. The content of this page — including calculation methods, reference data, written analysis, interactive tools, and source code — is the intellectual property of Inflection Point Engineering, LLC and is protected under applicable copyright, trademark, and trade secret laws. Unauthorized reproduction, redistribution, modification, or derivative use in whole or in part is prohibited without prior written consent.

Disclaimer. This material is provided for informational and educational purposes only and does not constitute professional engineering advice. Calculations, reference data, and methodologies are based on published standards and accepted engineering practice but are not a substitute for engineering judgment, site-specific analysis, or review by a licensed Professional Engineer. Inflection Point Engineering, LLC makes no warranties, express or implied, regarding the accuracy, completeness, or fitness for a particular purpose of any content presented here, and shall not be liable for any direct, indirect, incidental, or consequential damages arising from its use. Users assume all risk associated with applying this content to real-world design, operations, or decisions.

© 2026 Inflection Point Engineering, LLC. All rights reserved.