IPE-TM-807 Pressure Relief Systems
Overpressure Protection of Overhead Systems
IPE-TM-807-03
1. Table of Contents
2. Purpose
This procedure discusses overpressure protection for overhead condensing systems. It provides guidance concerning fractionator and receiver PRV location(s), setting receiver design pressure and temperature, and appropriate handling of liquid flow to the relief header.
Technology centers may have their own guidelines for specific applications (for example, heat integrated condenser-reboiler configurations, or steam-generating condensers). While this procedure’s guidance may be applicable, it is not intended to replace guidelines for technology-specific systems.
3. General
This procedure provides specific guidance for overpressure protection of overhead systems for a fractionator. It is intended to supplement the guidance given in Procedure which does not discuss overpressure protection.
The SET PRESSURE of the receiver PRV is normally equal to the receiver design pressure. By exception it may be set lower to open before the fractionator PRV and allow a condensing credit. However, the magnitude of this credit in sizing the fractionator PRV normally does not justify the lower set pressure.
Attachments 1, 2, and 3 provide pictorial representations of condenser systems along with specific design advice.
Attachments 4 and 5 outline procedures for calculating the design temperature of a receiver for special cases.
Attachment 6 describes how to rate the receiver PRV for liquid flow using the 807 Sizing & Selection Tool.
4. Inherently Safer Design
During the design of the overhead condensing system for a fractionator, consider applying four inherent safety principles, i.e. Minimization, Substitution, Moderation or Attenuation, and Simplification. Note that applying these concepts to a design may involve operational, technical, and/or economic trade-offs which have to be evaluated before the making the design “inherently safer”. All proposed changes should be discussed with the Technology Specialist prior to implementation to make sure that the trade-offs are acceptable.
The examples given below do not constitute an exhaustive list. They are given to stimulate thinking about inherently safer designs. Some examples are more easily applied at the process design stage, while other examples are applicable to the project design stage.
4.1 Minimize
Where possible, minimize the inventory of flammable and/or toxic material in the overhead system, especially when processing light hydrocarbons, i.e. C4s or lighter.
- Consider providing less residence time for the receiver level controller when the net overhead product is pumped to storage, or when flow deviations will not upset downstream processing.
- If water is separated from hydrocarbon liquid in the overhead receiver, consider using a coalescing device to minimize the residence time required for water-hydrocarbon separation.
- If the net overhead liquid is routed to another processing unit, consider using the receiver as a surge drum rather than adding a separate feed surge drum in the downstream unit.
4.2 Substitute
Consider using alternate condensing schemes and/or substituting cooling mediums.
- If a cooling water leak will cause severe corrosion in the overhead system, consider using an air-cooled condenser instead of a water-cooled exchanger.
- Consider using a heat pump to link overhead condensing with reboiler heating. If the heat pump compressor stops, reboiler duty automatically falls to zero.
- Consider using a drumless condensing system as described in Procedure
- Consider using a stab-in condenser instead of an external condenser. In this design, reflux overflows a weir, eliminating the reflux pump, and reducing the inventory of overhead liquid. Generally, stab-in condensers are attractive in vacuum service where a low pressure drop is required, the service is relatively clean, and the condensing duty is relatively small.
4.3 Moderate or Attenuate
Where possible, provide operating or relief conditions that are less severe, minimize liquid discharge to the relief system, or specify equipment that is more fault-tolerant.
- Consider using a trim condenser, refrigerated cooling medium, or vent condenser to permit a lower operating pressure due to the lower receiver temperature.
- If the condenser is located at grade, and if the condenser rundown line enters the receiver at the bottom, install a relief valve on the receiver to protect it during an external fire. Since vapor is generated in liquid-containing vessels, exchangers, and piping during the fire event, this vapor should be relieved directly without entraining displaced liquid from the receiver. (Section 7.2)
- If a single relief valve is used to protect the fractionator and the receiver, locate the PRV on the top head of the fractionator. During an external fire, vapor generated in the receiver will push down on the receiver liquid, flood the condenser and push receiver liquid back to the column. Locating the PRV on the column directly provides disengaging space and minimizes liquid discharge to the relief header. (Section 6.3)
- Consider raising the design pressure of the fractionating system to minimize the thermal driving force (LMTD) in the reboiler during a relief event. See Procedure ” more information.
- Consider elevating the overhead condenser and receiver above the fire zone, i.e. at least thirty-five (35) feet above grade.
- If ASME code requires PRV protection for the overhead receiver, consider raising the design pressure of the overhead receiver to prevent liquid relief during a reflux failure. (Section 7.4)
- In light hydrocarbon service, specify pumps with dual or tandem seals, upgrade metallurgy where embrittlement due to auto-chilling can occur, and provide a shutoff valve upstream of a drain or sampling valve.
4.4 Simplify
Simplify the design of overhead system by combining pump services, or reducing the number of relief valves specified.
- Unless ASME code requires separate overpressure protection for the overhead receiver, use the PRV on the overhead line to protect both the fractionator and the receiver.
- Consider specifying large-bore, pilot-operated PRV’s to reduce the number of valves needed for large relief loads. The large bore valves have ASME certified capacities and have larger orifice areas than the standardized API valves.
- Where practicable, combine reflux and net overhead in a single pump service.
5. Revamp of an Existing Unit
The primary focus of this procedure is new unit design. When existing designs are revamped, the guidance given here may need to be relaxed to accommodate existing equipment. For example, an existing overhead receiver should not be replaced because its design pressure was set without considering static head due to condenser flooding. Likewise, an existing receiver should not be replaced even though its design temperature was set at 250 °F instead of 50 F above the normal overhead temperature as recommended in Procedure .
If an existing vessel is retired because it is severely corroded or it does not provide adequate residence time, etc., the design guidance in this procedure should be applied to replacement vessels, even when this introduces minor inconsistencies to the design.
Generally for revamp projects, PRV sizing and selection is based on “new” valves and our current design practice applies. The 807 project specification will be used by the detailed design contractor to determine whether existing PRV’s are suitable for the revamp service.
6. Pressure Relief Valve Location
The primary relief protection for a fractionator shall be located upstream of the overhead condenser for both total and partial condensing systems. If the entire relieving rate has to pass through the condenser before it reaches the PRV, an excessive pressure drop could develop when condensing duty is lost, causing the fractionator pressure to exceed its allowed accumulation.
Generally, the fractionator PRV is located on the overhead piping, upstream of and near the condenser (Figure 1). This location provides free-draining from the PRV outlet and it permits convenient access for valve maintenance. However, there are occasions when the PRV is located on the top head, or when it is mounted on the side of the fractionator.
Figure 1. Typical Location for Fractionator System PRV(s)
6.1 Receiver PRV
If the receiver can be isolated from the fractionator, a separate PRV is required by ASME code to protect the receiver during an external fire. For total condensing systems, provide a separate nozzle on the overhead receiver for the PRV. For partial condensing systems, the PRV can be located on the net vapor line.
Although the receiver PRV is sized for the fire case, a capacity credit is theoretically available during other failure events. Inflection Point Engineering generally ignores this capacity credit because it is relatively small. However liquid will be displaced through this valve during a reflux failure. Since this liquid rate could affect the design of the liquid knockout facilities, Inflection Point Engineering reports the displaced liquid rate in a note on the 807 project specification (Section 10).
The receiver PRV may also be located on the inlet line to a free-draining condenser to avoid liquid to the relief header, particularly if the auto-chill temperature would require upgraded metallurgy (Attachment 2).
6.2 Fractionator PRV Located on Swaged Section
If the normal vapor rate in the bottom section is more than twice that in the top section, it is recommended to locate the PRV on the swaged section to minimize potential damage to the top trays during loss of feed (new designs only). The nozzle for the PRV should be located at least two feet above the feed distributor to provide adequate vapor-liquid disengagement. If the dimensions of the swaged section are such that two feet are not available, the PRV nozzle can be located in the top section, below the lowest tray (Figure 2).
Figure 2. Side-Mounted PRV for Swaged Column
Since design pressure is set relative to the operating pressure at the PRV location, the design pressure for this installation will be slightly higher than normal because the operating pressure at the swaged section is higher than at the top. Note that when the PRV opens, vapor flow in the top section will be diverted to the PRV, and pressure will equalize in the top and swaged sections of the column.
6.3 PRV Located on the Column Top Head
Occasionally, the fractionator PRV is located on the top head of the column to minimize inlet pressure loss to the valve, or to provide vapor-liquid disengagement during an external fire. (If the condenser is not free-draining and the receiver PRV is eliminated, displaced liquid from the receiver and vapor generated in the column will mix in the overhead line and both phases will have to be relieved during the fire case. Two-phase relief can be avoided by relocating the fractionator PRV to the top head, allowing disengagement to occur in the fractionator.)
If the top head is too small to accommodate the PRV nozzle per Procedure , the PRV nozzle can be side-mounted above the top tray, provided additional vapor-liquid disengaging space is included in the tangent length (Figure 3). In this configuration, the PRV nozzle position on the circumference should be selected to minimize potential entrainment of liquid from the reflux return.
Figure 3. Disengaging Space when PRV is Side-Mounted
6.4 PRV Located in Bottom Section
For some corrosive services, the PRV is located at the bottom of the column to minimize contact with the corrosive compounds that concentrate in the overhead vapor (for example, a sour water stripper or an isostripper). In those instances, the PRV nozzle is located in the vapor space below the collector tray or first contacting stage, depending on the reboiler type. Additional tangent length must be added to the column bottom to ensure that the PRV nozzle will not be submerged when the valve opens, and tray liquid “rains” to the bottom because tray vapor flow is reduced (Figure 4).
Figure 4. Disengaging Space when PRV is Located at Bottom
To determine the maximum liquid level in the bottoms, assume the clear liquid depth on each tray equals the weir height plus one inch, and add that liquid height to the maximum operating level in the bottom, i.e. 80% of the level float range. Locate the bottom of the PRV nozzle at least two feet (600 mm) above maximum liquid level to provide space for vapor-liquid disengagement.
When the valve is located at the bottom, the maximum operating pressure in the bottom is used to determine the design pressure of the fractionator. Provide sufficient margin between operating pressure and the PRV set pressure to prevent valve leakage (for example, 10% or 25 psi for spring-loaded PRV’s).
7. Design Pressure of the Overhead Receiver
During relief, pressure in the receiver is hydraulically linked to, and controlled by the fractionator PRV. Consequently, the receiver design pressure is set relative to the set pressure of the fractionator PRV (which is normally equal to the fractionator design pressure). Depending on the condensing system, the design pressure of the receiver may be equal to or greater than the fractionator design pressure. For example, if a static head develops in the overhead system during a failure case, that head should be added to the fractionator design pressure to obtain the design pressure of the receiver.
NOTE: Whenever the fractionator PRV is set to open below design pressure, substitute the value of set pressure wherever the fractionator design pressure appears in the equations of this section.
7.1 Condenser Rundown Line is Free-Draining to the Receiver
If the condenser is free draining to the overhead receiver, determine the receiver design pressure by adding the static head obtained during a reflux failure to the design pressure of the fractionator (Figure 5).
Figure 5. Free-Draining Condenser Flooded during Reflux Failure
Note: The static head is calculated based on the elevation difference between the overhead condenser and the top of the receiver. Once the condenser floods, vapor will no longer condense and liquid will not build up in the overhead line. Typically, the condenser is located 40 feet above grade, and the static head from the receiver to the condenser is approximately 5 psi (0.35 kg/cm2). If the condenser elevation is known to be significantly different than 40 feet, calculate the static head and add it to the design pressure of the fractionator to obtain the receiver design pressure.
7.2 Condenser is NOT Free-Draining to Receiver
If the condenser is NOT free-draining to the receiver, provide a separate PRV on the receiver for the external fire case (Figure 6).
If the only condenser is located at grade, set the design pressure of the receiver equal to that of the fractionator.
If the condensing system consists of an air-cooled exchanger followed by a water-cooled trim at grade, assume that both condensers flood during a reflux failure and set the receiver design pressure accordingly.
Whenever a separate PRV protects the receiver, it will potentially discharge liquid during a reflux failure. If the auto-chill temperature of overhead liquid drops below -20 ºF when flashed at relief header pressure (for liquid containing C3’s or lighter), consider eliminating the receiver PRV and raise the design pressure of the receiver to accommodate the liquid head obtained when condenser liquid is displaced back towards the PRV protecting the fractionator. Add this static head to the fractionator design pressure to obtain the design pressure of the receiver.
In addition, displaced liquid from the condenser during an external fire will mix with vapor generated in the fractionator and both will have to be relieved, i.e. assume no vapor-liquid disengagement occurs in the overhead line. To avoid liquid relief altogether when the receiver PRV has been removed, relocate the fractionator PRV to the top head (Section 6.2).
Figure 6. Partial Condenser with Pocketed Rundown Line Entering at Top
7.3 Condenser Rundown Line Enters the Receiver at the Bottom
If the rundown line enters at the bottom of the receiver, provide a separate PRV on the receiver for the external fire case (Figure 7). The analysis for this configuration is identical to that discussed in Section 7.2.
Figure 7. Total Condenser with Rundown Line Entering at Bottom
7.4 Avoiding Liquid Relief through the Receiver PRV
If the auto-chill temperature of the overhead liquid is below -20 ºF at the superimposed back pressure of the relief header, or if liquid knockout facilities are not provided, the overhead system can be designed to avoid liquid relief.
Design options depend on the type of condenser system:
1) If the overhead condenser is free-draining and there are no intervening valves between the fractionator PRV and the receiver, eliminate the receiver PRV. Add the vapor generated in the receiver during an external fire to vapor from the fractionator and obtain the fire case load. Although this strategy requires receiver vapor to flow back through the condenser, Inflection Point Engineering does not consider backflow to be a significant problem during an external fire.
2) If the overhead condenser is free-draining and the receiver PRV cannot be eliminated because it is required by ASME code, consider relocating the receiver PRV to the inlet of the overhead condenser, downstream of the pressure control valve. Again, vapor from the receiver will flow back through the condenser to reach the PRV, but this is not considered a problem.
3) If the condenser is not free-draining and the PRV is not required by ASME code, eliminate the PRV and raise the design pressure of the receiver to accommodate the static head obtained when liquid is “pushed” back to the fractionator during external fire.
- Since Inflection Point Engineering prefers not to discharge liquid to the relief header, relocate the fractionator PRV to the top head to provide vapor/liquid disengagement. Displaced liquid from the receiver will displace fractionator vapor, and this volumetric rate should be added to the vapor generated in the fractionator to obtain the total fire case load. If the fractionator PRV must be located on the overhead vapor line, add the displaced liquid to the fractionator vapor, flash the mixture at the accumulated pressure, and relieve the resultant stream. (Note: although liquid relief from the receiver is eliminated by this option, significant liquid will be discharged from the fractionator PRV. The PRV will be several times larger for the external fire cases, and the receiver inventory will be displaced in a short time. Therefore, this option should only be used as a last resort.)
4) If the overhead condenser is not free-draining, but the customer requests the receiver PRV, raise the design pressure of the receiver to keep the PRV from opening in all cases other than external fire. The receiver design pressure should be set relative to the allowed accumulation pressure in the fractionator (i.e. 16% above the fractionator design or set pressure, whichever is lower) and according to the condenser location:
- If an air-cooled exchanger is elevated above the receiver, set the design pressure equal to the fractionator maximum accumulated pressure plus the static head between the elevated condenser and the top of the receiver.
- If the only condenser is at grade, set the receiver design pressure equal to the fractionator maximum accumulated pressure plus 5 psi.
8. Design Temperature of the Overhead Receiver
The design temperature of the receiver must accommodate temperatures that arise in the receiver during normal operation and during condenser failure. The temperature obtained in the receiver is affected by a number of factors: total or partial condensing system, the extent of the condenser failure, the type of pressure control, whether or not the receiver is protected by a separate PRV, and whether or not the receiver is vented to the relief header.
In some total condensing systems, hot vapor is bypassed around the condenser directly to the receiver (Figures 1 and 7), which continuously exposes the top half of the receiver to hot vapor. For these systems, set the receiver design temperature equal to 50°F above the maximum operating temperature of the column overhead vapor stream, and round up to the next 5° increment. A more precise determination of design temperature should be done when this procedure causes the receiver material or flange class to be upgraded, or when the margin between operating and design pressures is more than 30% of design pressure. In such alternative cases, the design temperature should be set at the dew point of the normal overhead vapor at accumulated pressure, rounded up to the next 5° increment. For example: a low-pressure column with naphtha or heavier overhead product and a 50 psig design pressure per Inflection Point Engineering minimum (see ). Another potential situation with larger-than-normal margin is when the column design pressure is raised to pinch the reboiler at accumulated pressure and obtain lower relief load. When setting a receiver design temperature based on one of the alternative cases mentioned above, one should consider the design temperature of the associated column and limit the receiver design temperature to that of the column.
For some types of partial condensing systems, vapor will pass through the condenser before it reaches the receiver. For this type system, the temperature obtained in the receiver is a function of flow and the extent of condenser duty lost during the failure case. Conservatively, the receiver design temperature can be set for complete loss of cooling, i.e. the dew point of overhead vapor at fractionator accumulated pressure. However, if the temperature of material entering the receiver during a condenser failure case must be determined more accurately (which is seldom), a trial and error calculation is required ().
Note that Inflection Point Engineering uses a minimum design temperature of 250°F (120 C) to accommodate the steamout temperature. In addition, the guidance given in Procedure regarding the minimum design metal temperature should be followed when the receiver contains a light hydrocarbon liquid.
9. Design Pressure of the Condenser
During relief, pressure in the condenser system is hydraulically linked to both the fractionator and receiver. Depending on the configuration and PRV locations, the design pressure of the condenser may be equal to, greater, or less than the receiver design pressure.
- If there is a receiver PRV the design pressure of the condenser will generally be controlled by its location. Consequently, the condenser design pressure is set relative to the design pressure of the receiver.
- If there is no receiver PRV, then condenser design pressure is based on the fractionator design pressure plus any static head developed above it during relief.
Condenser design pressure should take into account hydraulic and relative static head effects considering conditions at relief for each of the failure cases for that service. For example, on a totally condensing system with a free-draining condenser and a hot vapor bypass [see Attachment 2], the reflux failure case sets the design pressure of the receiver (due to static head developed between the condenser and receiver during relief) but the fire case sets the design pressure of the condenser (because there is no static head between the condenser and the receiver). Refer to Attachments 1 through 3 for more guidance on setting condenser design pressure in specific configurations.
NOTE: Whenever the receiver PRV is set to open below design pressure, the value of set pressure can be used to establish condenser design pressure.
10. Rating the Receiver PRV for Liquid Flow
Although the receiver PRV (when present) is sized for the external fire case, this valve will open during other overpressure cases (for example: electrical power failure, condenser failure, or reflux failure). During a reflux failure, liquid accumulates in the overhead receiver, eventually overfilling the receiver and flooding the condenser. Both the fractionator and the receiver PRV’s will open when the column overpressures because the receiver pressure is hydraulically linked to column pressure. Since the receiver becomes liquid-full during the reflux failure, liquid will be discharged to the relief system when the receiver PRV opens.
When liquid is discharged to the relief header, its flow rate and temperature should be reported in the 807 project specification. This information is used to design liquid knockout facilities and piping supports. outlines the calculations required to determine the liquid flow rate that is reported.
This section also applies to total condensing systems with a receiver vent line open to the relief header. Since there is no receiver PRV, the rate and conditions shall be reported on the 807 specification for the fractionator PRV.
In addition to flow rate, temperature may affect relief header design. Determine the auto-chill temperature of the overhead liquid (when it contains C3’s or lighter) when it flashes adiabatically to the relief header pressure, i.e. the PRV superimposed back pressure.
If the flash temperature is below -20 F, the relief header could be subjected to brittle failure, and this hazard is addressed in one of two ways:
1) Prevent liquid from discharging to the relief header by (a) raising the receiver design pressure to either eliminate the receiver PRV or prevent it from opening except during an external fire case (section 7.4), or (b) locating the receiver PRV on the inlet line to an elevated condenser.
2) Upgrade the metallurgy of the header to avoid low-temperature embrittlement, for example, specify stainless steel when the flash temperature is below -50 °F, and impact-tested killed carbon steel otherwise.
A relief header flash temperature below -20 °F also requires that the material for the PRV body, bonnet, and possibly the spring be specified as a low temperature alloy (for example, stainless steel).
11. Condensing Credit via the Receiver PRV or Vent
See Tool Documentation for the allowable credits when sizing the fractionator PRV. Note: if credit is taken for flow through the PRV or vent line on the receiver, report that flow rate in a note on the 807 specification.
Attachment 1: Partial Condensing Systems
| When the condenser freely drains to the receiver, the PRV on the overhead vapor line protects both the fractionator and the overhead receiver. Set the design pressure of the receiver higher than the fractionator by the amount of static head obtained when the condenser floods. (Section 7.1) Set the design pressure of the condenser equal to the design pressure of the column. |
|---|
Note 1. While Inflection Point Engineering recommends a separate PRV for the receiver, this valve can be deleted upon customer request. If the valve is removed, vapor generated in the receiver during a fire displaces liquid inventory in the condenser back to the fractionator. The fractionator PRV (if located on the overhead line)must be sized to relieve the two-phase mixture consisting of vapor generated in the fractionator and displaced liquid from the condenser. If the receiver PRV is removed and the fractionator PRV is located on the top head ( to avoid liquid relief) the design pressure of the condenser and receiver should be increased above that of the fractionators by the amount of static head between the top of the fractionator and the top of the condenser(s) and receiver respectively. | Provide separate PRV protection for the fractionator and the overhead receiver. Set the design pressure of the receiver higher than the fractionator by the amount of static head obtained when the elevated condenser floods. (Section 7.2) Set the design pressure of the elevated condenser equal to the design pressure of the column. Set the design pressure of the trim condenser above that of the receiver by the amount of static head between the top of the receiver and the trim condenser (or grade). Size and select the receiver PRV based on the external fire case. Determine the liquid capacity of the receiver PRV when it fully opens during the reflux failure case. Compare the liquid capacity of the receiver PRV to the normal overhead liquid rate (net plus reflux) and report the smaller rate in the 807 project specification (Section 10). If the auto-chill temperature of the receiver liquid drops below -20 ºF when it is flashed at relief header pressure, consider raising the receiver design pressure to eliminate the liquid relief case. (Section 7.4) |
|---|
Attachment 2: Total Condensing Systems
Note 1. While Inflection Point Engineering recommends a separate PRV for the receiver, this valve can be deleted upon customer request. If the valve is removed, the receiver design pressure must be set higher than the fractionator by the amount of static head that develops when vapor generated in the receiver during a fire displaces liquid back to the fractionator. In addition, the fractionator PRV (if located on the overhead line) must be sized to relieve the two-phase mixture of vapor generated in the fractionator and the displaced liquid from the condenser. If the receiver PRV is removed and the fractionator PRV is located on the top head ( to avoid liquid relief) the design pressure of the condenser and receiver should be increased above that of the fractionators by the amount of static head between the top of the fractionator and the top of the condenser(s) and receiver respectively. | Provide separate PRV protection for the fractionator and the overhead receiver. If the only condenser service is located below the overhead receiver, set the design pressure of the overhead receiver the same as the fractionator. Set the design pressure of the condenser above that of the receiver by the amount of static head between the top of the receiver and the condenser (or grade). If the condensing system has an air-cooled condenser followed by a water-cooled trim, set the design pressure of the overhead receiver higher than the fractionator by the amount of static head obtained when both condensers flood. (Section 7.2) Set the design pressure of the air-cooled condenser equal to that of the fractionator, and the design pressure of the trim condenser above that of the receiver by the amount of static head between the top of the receiver and the trim condenser (or grade). Size and select the receiver PRV based on the external fire case. Determine the liquid capacity of the receiver PRV when it fully opens during a reflux failure case. Compare the liquid capacity to the normal overhead liquid rate (net plus reflux) and report the smaller rate in the 807 project specification (Section 10). If the auto-chill temperature of the receiver liquid drops below -20 ºF when it is flashed at the relief header pressure, consider raising the receiver design pressure to eliminate the liquid relief case. (Section 7.4). |
|---|
Attachment 2: Total Condensing Systems (continued)
| Provide separate PRV protection for the overhead receiver as required by ASME code. Set the design pressure of the receiver higher than the fractionator by the amount of static head obtained when the condenser floods. (Section 7.1) If the PRV is located on the receiver set the design pressure of the condenser equal to the design pressure of the receiver. Size and select the receiver PRV based on the external fire case. Determine the liquid capacity of the receiver PRV when it fully opens during the reflux failure case. Compare the liquid capacity to the normal overhead liquid rate (net plus reflux) and report the smaller rate in the 807 project specification (Section 10). If the auto-chill temperature of the receiver liquid drops below -20 °F when it is flashed at relief header pressure, consider raising the receiver design pressure to eliminate the liquid relief case (Section 7.4). Alternatively, locate the PRV upstream of the condenser (see diagram at left). If the PRV is located just upstream of the condenser, then the condenser can be designed for the same pressure as the column and the PRV should have the same set pressure. |
|---|
Attachment 3: Total Condensing System with a Receiver Vent
| When the condenser freely drains to the receiver, the PRV on the overhead vapor line protects the fractionator, condenser and overhead receiver. Set the design pressure of the receiver higher than the fractionator by the amount of static head obtained when the condenser floods. (Section 7.1) Condenser design pressure should be equal to that of the fractionator. The receiver vent line is sized to minimize pressure fluctuations due to changes in overhead liquid flow. Although some of the vapor generated during an external fire will flow out the vent line, report the total combined fire case load on the 807 project specification for the PRV on the overhead vapor line, i.e. take no credit for flow through the vent line. Determine the liquid flow through the receiver vent during a reflux failure case. Compare the liquid capacity of the vent line to the normal overhead liquid rate (net plus reflux) and report the smaller rate in the 807 project specification. |
|---|
| When the condenser freely drains to the receiver, the PRV on the overhead vapor line protects the fractionator, condenser and overhead receiver. Set the design pressure of the receiver higher than the fractionator by the amount of static head obtained when the condenser floods. (Section 7.1) Condenser design pressure should be equal to that of the fractionator. Vapor generated in the receiver during an external fire is relieved via the PRV on the overhead vapor line If the “push/pull” pressure control fails, the condenser could become vapor-blanketed, causing complete loss of overhead condensing duty. |
|---|
Attachment 4: Receiver Design Temperature with Hot Vapor Bypass - Example
Service: Heavy Aromatics Column Receiver
| Column design pressure | 50 psig |
|---|---|
| Column operating pressure | 15 psig |
| Overhead temperature (normal) | 381 °F |
| Design temperature (preliminary) | 381 + 50 = 431; 435 °F |
| Margin | (50 – 15) / 50 = 70% |
Margin > 30%, so check overhead T at relief conditions | Margin > 30%, so check overhead T at relief conditions |
| Dew-point T of overhead at accumulated P | 462 °F |
| Since dew point T at relief is higher, Receiver design T = | 465 °F |
NOTE: When taking this approach to determine receiver design temperature, do not set the receiver design temperature higher than that of the associated column.
Attachment 5: Calculating Receiver Design Temperature
If the guidelines in Section 8 cause the receiver flange class or material to be upgraded, a more precise calculation of the temperature at relief may be justified. The following trial-and-error calculation is recommended.
1) Guess the vapor flow rate into the condensing system during the failure case.
2) Determine the condenser duty remaining during the failure event.
a. For a water-cooled condenser, assume complete loss of condensing duty.
b. For air-cooled condensers, assume 20% of the normal duty remains due to natural convection. If the condenser design is such that natural convection can’t occur during the failure case (for example, a condenser with internal air recirculation, or with louvers to adjust air flow), assume complete loss of condensing duty.
c. For an air-cooled condenser followed by a water-cooled trim, assume that an electrical power failure stops both the fan motors and the cooling water flow to the trim condenser. The residual condenser duty during the power failure is 20% of the normal air-cooled condenser duty.
3) Calculate the phase condition of the material leaving the last condenser
a. Subtract the residual condenser duty from the enthalpy of the vapor entering the condenser corresponding to the guessed flow rate. Flash this stream adiabatically to the accumulated pressure in the receiver. The rate and properties of the vapor entering the receiver are obtained from the flash.
4) Calculate the net vapor flow rate through the pressure control valve at its wide- open position, or the vapor flow rate through the receiver vent line at accumulated pressure, as applicable. The inlet condition for rating the vent control device (vapor temperature and molecular weight) is per step 3.
5) Compare the vapor flow rate entering the receiver to the vapor flow leaving the receiver via the pressure control valve or the vent line.
6) Repeat steps 1-5 until the vapor rate entering the receiver equals the rate leaving. The adiabatic flash temperature obtained for the final guess is the maximum receiver temperature. Set the receiver design temperature at or above this temperature, rounded up to the next 5° increment.
Attachment 6: Liquid Relief through Receiver PRV
If the receiver PRV will open during a reflux failure, rate the receiver PRV for liquid flow, assuming 110% overpressure and that the liquid does NOT flash. [The non-flashing assumption assures using the maximum mass flow for design of the Knockout (KO) Drum.]
1) Complete the sizing of the receiver PRV for the external fire case in TZ-807-01, “Relief Valve Sizing and Selection”.
2) Create a dummy ‘Blocked Outlet’ case and characterize the fluid as a non-flashing liquid.
3) Input the receiver liquid temperature and fluid properties based on normal operating conditions. Input the normal overhead liquid rate (net plus reflux) as the liquid relief rate.
4) Go to the Sizing Summary screen under Main Menu button 3, and note the required orifice area for the dummy case.
a. If this required area is greater than the total installed relief area (AACTUAL), pro-rate the overhead liquid flow by the ratio of the actual orifice area to the required area for the dummy case:
b. If the required area is less than or equal to the total installed area, then the rated flow equals the normal net-plus-reflux liquid rate.
5) Go back under Main Menu button 2 and reset the Cause of the dummy ‘Blocked Outlet’ case to ‘None’. [But leave the other input values in case you want to re-activate them to adjust them or check results.]
6) Although we assume the liquid is non-flashing when determining the rated capacity of the receiver PRV, this assumption is not appropriate when determining the auto-chill temperature. Use P9.8 or another process simulator and set the flow of the receiver liquid stream equal to the rated flow from step 4) above. Based on normal receiver operating temperature and pressure, flash the adjusted stream to the superimposed back-pressure. [This is the lowest pressure in the system, and combined with starting at the normal operating temperature, will result in the lowest potential auto-chill temperature.]
7) Publish the vapor and liquid flow rates for the flashed stream in a note on the 807 Project Specification using one of the two formats below, replacing the information in brackets with the appropriate numbers and units. Note: these template notes are provided in TZ-807-01.
a. If the flash results in more than 0.001 mass fraction vapor at the relief header pressure:
During a reflux failure, the discharge from the PRV will be at [?? ° temp unit] and will consist of [??? Mass/time] of vapor with a molecular weight of [???], and [??? Vol/times] of liquid with a specific gravity of [???].
b. If the liquid does not flash (i.e., the vapor mass fraction is 0.001 or less):
Liquid discharge to relief header at [??? vols/time] during reflux failure case.
8) Specify material upgrades per the following.
a. If the auto-chill temperature is less than -20 °F, specify stainless steel body and bonnet on the receiver PRV 807 specification.
b. If the auto-chill temperature is less than -20 °F but not below -50 °F, impact-tested killed carbon steel should be specified for the tailpipe and relief header.
c. If the auto-chill temperature is less than -50 °F, a stainless steel tailpipe and header is required.
The above procedure generally applies for rating a receiver vent line routed to a relief header. An estimate of the vent line rating can be obtained in TZ-807-03, “Fractionator Relief Load”. For systems that are not suitable for modeling in TZ-807-03, consult the PRV Specialist.
© 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.