Design Pressure and Temperature

1. Table of Contents

2. Purpose

This procedure describes how Inflection Point Engineering establishes pressure and temperature design conditions for process equipment.

3. General

Specify equipment design conditions to incorporate a margin above the maximum operating conditions. In certain cases, such as low temperature or vacuum applications, specify design conditions to incorporate a margin below the minimum operating conditions. This Procedure along with referenced procedures summarizes the appropriate equipment design codes and general Inflection Point Engineering engineering practices used to determine these design margins. Note this Procedure does not apply to piping which is subject to separate design codes, nor does it apply to atmospheric and low pressure storage tanks. Technology Design Manuals should be reviewed to determine if there are any technology specific guidelines for determining design conditions.

4. Definitions

4.1 Pressure

a. Operating Pressure

The pressure at which a piece of equipment operates under a particular set of circumstances. At the initial or preliminary design stage of a project, either specify or estimate equipment operating pressures.

The Design Engineer specifies operating pressure at controlled points in the process and estimates other operating pressures based on a hydraulic model of the process unit. As such, most operating pressures are estimates.

b. Maximum Operating Pressure

The highest operating pressure a piece of equipment will see under all applicable hydraulic design cases (normal, rated and turndown).

c. Design Pressure

The Design Engineer, in general, sets the design pressure of a piece of equipment by adding a margin above the maximum operating pressure. In vacuum applications, specify the external design pressure as Full Vacuum. Inflection Point Engineering will allow the external pressure to be set at a level less than full vacuum upon Customer request (see BEDQ, Section 2.3, “Vacuum Design”) when appropriate. Mechanical designers use design pressure (internal and external) to determine head and wall thickness, stiffening ring requirements, etc.

4.2 Temperature

a. Operating Temperature

The temperature at which the equipment operates under a particular set of circumstances. The Design Engineer directly specifies or calculates operating temperatures when developing the heat and weight balance.

b. Design Temperature

If the temperature is above ambient, the Design Engineer sets the design temperature of a piece of equipment by adding a certain margin above the normal operating temperature. Special conditions (regeneration, start-up, etc.) may require a maximum operating temperature that exceeds that represented in the heat and weight balance. Mechanical designers use design temperature to determine metallurgy, allowable stress, wall thickness, etc. See Section 7.10 for information on cryogenic services.

5. Design Pressure

In accordance with the ASME Code, Inflection Point Engineering specifies the design pressure at the top of a completely fabricated vessel in the operating position at a designated design temperature. The design pressure at the bottom may be determined by Contractor/Vendor based upon design conditions at top of vessel, the normal height of liquid (static head), the specific gravity, and frictional pressure drop for upflow or downflow through internals (e.g. trays), all of which must be included in the vessel specification. Include "liquid full" on vessel specification for vessels which operate liquid full. It is possible that more than one internal design pressure condition may exist i.e., low pressure with high temperature and high pressure with low temperature. Specify all coincident temperature and pressure combinations if they affect the design. Information on the decision making process which should be considered for determining which guidelines apply when setting design pressures for exchangers, heaters, and vessels can be found in Attachments 1, 2, and 3.

5.1 Minimum Design Pressure

a. Inflection Point Engineering Practice

The minimum design pressure is normally 50 psig (3.5 kg/cm2g). However, for very large vessels with a low maximum operating pressure, the design pressure is occasionally determined by adding a smaller than normal margin (Technology specific) to the maximum operating pressure to minimize vessel cost. Examples include an FCC reactor/regenerator, Styrene reactor, and vacuum distillation column. The minimum design pressure of 50 psig (3.5 kg/cm2g)is consistent with Inflection Point Engineering’s default basis for PRV back pressure. However, if the customer provides a total relief header back pressure, (see BEDQ Section 5.8.i) significantly higher than 20 psig (1.4kg/cm2), the minimum design pressure must be increased to avoid extensive use of pilot-operated relief valves.

b. Background

Sometimes Customers request a design pressure below 50 psig (3.5 kg/cm2g). While this may lower capital costs on vessels and fractionators, there are additional capital costs for relief valve piping and headers.

If a Customer requests a design pressure below 50 psig (3.5 kg/cm2g), consult the Vessel Specialist to confirm that a lower design pressure actually reduces vessel cost. For example, wind or earthquake loadings may govern structural design, wall thickness, and therefore vessel cost.

A PRV which is set below 50 psig (3.5 kg/cm2g) may require a larger diameter tailpipe and relief header to reduce back pressure at the valve outlet during a relief event. Thus, a reduced vessel cost may be more than offset by a higher cost for the pressure relief system. If the back pressure limit requires a larger diameter header, consider specifying a pilot-operated valve which provides stable operation at back pressures up to 70% of set.

A PRV which has a set pressure below 50 psig (3.5 kg/cm2g) may also require a larger inlet line to meet the 3% inlet pressure loss constraint.

5.2 Margin Above Maximum Operating Pressure to Establish Design Pressure

A margin between operating pressure and design pressure is required to maintain pressure relief valve tightness, prevent premature opening, and to provide sufficient blowdown pressure for the valve to re-close. Spring loaded valves will not reclose until the inlet pressure drops 5-7% below the set pressure.

a. Spring-Loaded PRV

Seating force decreases as the margin between operating and set pressure decreases. The margin required to maintain valve tightness shall be 10% of the design pressure (i.e. divide the maximum operating pressure by 0.9 to establish the minimum design pressure), but not less than 25 psi (1.75 kg/cm2). If the Customer requests a lower margin, select a pilot-operated valve or consult the PRV Specialist. Reference Table 1 for the appropriate margin when using a spring loaded PRV.

b. Pilot-Operated PRV

Although valve tightness is not a problem for a pilot-operated PRV, premature opening may be a problem and a margin is required to accommodate code permitted set pressure tolerances and normal variations in operating pressure. The margin required to prevent premature opening shall be 5% of the design pressure (i.e. divide the maximum operating pressure by 0.95 to establish the minimum design pressure), but not less than 5 psi (0.35 kg/cm2). Since a pilot-operated valve tolerates a lower margin, it is generally used to protect equipment with a design pressure above 1000 psig (70 kg/cm2g).

Pilot-operated PRV's may be considered for reasons other than lowering the equipment design pressures, e.g. remote pressure sensing, unit revamps, PRV location, etc. Conversely, spring-loaded PRV's may be considered above 1000 psig (70 kg/cm2g), provided that the additional costs are evaluated. When using pilot-operated PRV's at operating pressures below 1000 psig (70 kg/cm2g), provide a 10 percent or 25 psi (1.75 kg/cm2) margin. Providing the standard margin allows the Contractor to use either pilot-operated or spring loaded PRV's. Consult the Technology or Process Specialist before applying a 5 percent margin for services below 1000 psig (70 kg/cm2g), e.g. for process reasons, for large equipment, revamp situations, or for equipment fabricated from exotic materials.

Reference Table 2 for the appropriate margin when using a pilot-operated PRV.

c. Spring-Loaded PRV - Steam Service

The default margins used to specify design pressures for steam drums in steam generation service are the same as those used for spring-loaded PRV’s. Refer to Table 1 Spring-Loaded PRV for additional information. The maximum operating pressure in the steam drum should be determined using the maximum steam header operating pressure shown in the BEDQ, plus design flow pressure drops between the steam drum and the header.

Steam generation systems designed to ASME Section I require that boiler feedwater be at sufficient pressure to get into the system during relief. If this is a problem with an existing boiler feedwater source, the above guideline can be eased slightly based on recommendations in ASME Section VII. This document suggests the following minimum margins between operating and design pressures, with the margin expressed as a percentage of design pressure:

• For boiler design pressures over 15 psig (1.0 kg/cm2g)to 70 psig (4.9 kg/cm2g), the minimum margin above maximum operating pressure shall be 7 psi (0.5 kg/cm2).
• For boiler design pressures of 71 psig (5.0 kg/cm2g) to 300 psig (21 kg/cm2g), the minimum margin shall be 10% of the design pressure (i.e., divide the maximum operating pressure by 0.9 to establish the minimum design pressure).
• For boiler design pressures of 301 psig (21.2 kg/cm2g) to 1000 psig (70 kg/cm2g) the minimum margin shall be 7% of the design pressure but not less than 30 psi (2.1 kg/cm2).
• For boiler design pressures of 1000 psig (70 kg/cm2g) to 2000 psig (140 kg/cm2g), the minimum margin shall be 5% of the design pressure but not less than 70 psi (4.9 kg/cm2).

The above shall be used for new steam drums. For revamps, slightly lower minimum margins might be allowed.

The most common cause of a steam safety valve failing to open at the set pressure is corrosive deposits accumulating between the valve disk and seat. This usually happens when the safety valve “weeps” or leaks slightly. Adherence to the above minimum differentials will substantially help overcome this situation.

The design pressure of the superheater coil shall be set at the same design pressure as the steam drum. Set the design pressure of the steam generator coil in a forced circulation boiler equal to the sum of the steam drum design pressure plus the design pressure drop in the steam generation coil and associated piping.

The economizer coil design pressure shall be set equal to the steam drum design pressure plus the economizer coil pressure drop.

Table 1

Spring-Loaded PRV

Design Pressure vs. Operating Pressure [1]	Design Pressure vs. Operating Pressure [1]	Design Pressure vs. Operating Pressure [1]
Max. Operating Pressure, psig	Margin	Min Design Pressure, psig
0 24	min design P	50 50
25 50 100 200 225	+25 psi [2]	50 75 125 225 250
226 300 500 750 999	10 percent of design pressure	255 335 560 835 1110
1000 1250 1500 1750 2000 2250 2500 etc.	10 percent of design pressure [3]	1115 1390 1670 1945 2225 2500 2780 etc.

[1] Design pressures are rounded up according to the rules given in Section  9.1.

[2] A minimum margin of 15 psi (1.0 kg/cm2) or 10% (whichever is greater) may be used for revamps and/or other unusual situations.

[3] For operating pressures 1000 psig (70 kg/cm2g) and above, pilot-operated PRVs may be more economical because they tolerate less margin between design and operating pressure, e.g. 5%.

Table 2

Pilot-Operated PRV

Design Pressure vs. Operating Pressure [1]	Design Pressure vs. Operating Pressure [1]	Design Pressure vs. Operating Pressure [1]
Max Operating Pressure, psig	Margin	Min Design Pressure, psig
0 24	min design P	50 50
25 50 100 200 225	+25 psi [2]	50 75 125 225 250
226 300 500 750 999	10 percent of design pressure [2]	255 335 560 835 1110
1000 1250 1500 1750 2000 2250 2500 etc.	5 percent of design pressure	1055 1320 1580 1845 2110 2370 2635 etc.

[1] Design pressures are rounded up according to the rules given in Section  9.1.

[2] A minimum margin of 5 psi (0.35 kg/cm2) or 5% (whichever is greater) may be used for revamps and/or other unusual situations.

5.3 Vacuum Design Pressure

Sometimes vacuum requirements impact process equipment design. The vacuum design requirement is an external design pressure that is specified as “Full Vacuum”. Inflection Point Engineering specifies “Full Vacuum” to avoid the problem of exactly setting the external design pressure for process units being built at high altitudes. Equipment may be subject to vacuum during its normal operating cycle, (e.g., during reactor circuit evacuation) or on loss of heat to a reboiler. Thus, it may be necessary to specify external design pressure conditions in addition to the internal pressure design conditions.

Customers sometimes request in the Basic Engineering Design Questionnaire (BEDQ) that vacuum design conditions be specified for equipment.

Reference Procedure,

5.4 Utilities Equipment

The Customer lists the design pressures for utility systems in the BEDQ e.g. steam, air, nitrogen and cooling water. Use the utility design pressure provided in the BEDQ; do not round up unless advised by the Project Manager. If the design pressure for the cooling water side of exchangers is not given in the BEDQ, Inflection Point Engineering recommends a water side design pressure of 100 psig (7.0 kg/cm2g), unless the “ten-thirteenths rule” dictates a higher design pressure.

Reference Section 5.7.

5.5 Steamout Design Pressure

Inflection Point Engineering does not design equipment for the maximum supply pressure of steam, since Inflection Point Engineering expects the atmospheric vent to be open during the steamout operation. Similarly, do not design a vessel for the vacuum condition which could develop if the steamout vent is prematurely closed. If the Customer requests vacuum design in the BEDQ, reference Procedure, “IPE-TM-300-05, “Vacuum Design”.

5.6 Effect of Pressure Drop on Design Pressures

a. Static Pressure

Inflection Point Engineering specifies design pressure at the top of a vessel based on its installed orientation in accordance with ASME code. If the vessel contains a liquid and/or a solid (catalyst, adsorbents, etc.), the pressure at the vessel bottom will be higher than the top by the amount of static pressure. List the liquid level, specific gravity, and weight of solids (catalyst, adsorbents, etc.) in the vessel specification. The vessel vendor will use this information to determine the wall thickness required at the vessel bottom.

b. Upflow Pressure Loss

If the process stream enters at the bottom of a vessel containing catalyst, absorbent, trays, or packing and exits at the top, the bottom pressure may be higher than the top. List the upflow pressure loss in the vessel specification. If the vessel contains trays or packing, list the total pressure loss across the trays or packing in the specification for the fractionator. The vessel vendor will use this information to determine the wall thickness required at the vessel bottom. Reference Standard Specification (3-11,3-12, 3-13 3-15 and 3-17) for further explanation.

c. Downflow Pressure Loss

If a process stream enters the top of a vessel containing catalyst or an absorbent and exits at the bottom, the bottom pressure will be lower than the static condition. If a plugging contingency is added, the bottom pressure may even be less than the top pressure at end of run. Do not list the downflow pressure loss in the vessel specification. If flow is lost during a process upset, the bottom pressure will equalize with the top. If the vessel operation is such that the bottom pressure is significantly less than the top pressure, consult the Vessel Specialist before listing a separate design pressure for both top and bottom.

d. Special Situation

When the relief valve for a vessel is located near the bottom of the vessel, both the top and bottom design pressures may be equal e.g. the isostripper in the Inflection Point Engineering HF Alkylation Process Unit. The relief valve set point is 25 psi (1.75 kg/cm2) or determined based upon the maximum operating bottom pressure. Although the top pressure is less than the bottom pressure during normal operation, the top and bottom pressures will equalize during relief conditions because vapor flow in the column is reduced when the PRV opens.

5.7 Heat Exchanger Overpressure Protection

a. General

Heat exchanger design pressure is generally selected to provide a margin between design and operating pressure in accordance with Inflection Point Engineering design practice. However, the design pressure is frequently raised above these margins to provide overpressure protection for equipment failures and/or mal-operation unique to an exchanger. Typical causes of high pressure in an exchanger are:

• Blocked In: thermal expansion of cold side fluid;
• Blocked In: vaporization of cold side fluid;
• Blocked Outlet: hot side or cold side;
• Internal Tube Failure: low pressure (LP) side

Design options providing overpressure protection for exchangers are:

• Install a PRV to limit overpressure.

• Raise the design pressure to eliminate or mitigate one or more overpressure cases. For example, design the exchanger for pump shut-in pressure to eliminate a blocked outlet case, or design the LP side for ten-thirteenths of the design pressure of the HP side to mitigate the tube failure case.

Change the design to eliminate one or more causes of overpressure. For example, lock open or car seal open (CSO) a block valve to eliminate a thermal expansion case. The use of a CSO outlet block valve should be specified when the customer requests both inlet and outlet block valves on water cooled exchangers. The use of a thermal relief to atmosphere should be avoided to prevent release of toxic or hydrocarbon material to the atmosphere. The water side of the exchanger should still be designed as per the 10/13 rule to contain the effects of tube failure, when appropriate (below). In some cases; e.g., (toxic material such as HF), the Technology Specialist may elect to set the design pressure of the low pressure side (water) equal to the design pressure of the high pressure process side.

The choice between raising design pressure vs. installing a PRV is based on technical and economic considerations.

b. Tube Failure

If the design pressure of the LP side of an exchanger is raised to eliminate the PRV sizing case for a tube failure, the design pressure of the entire LP system expected to contain the HP fluid, e.g. associated LP equipment and piping, must also be increased.

If a PRV is used to limit overpressure, set the design pressure of the LP side relative to the operating pressure using standard margin rules and base PRV sizing on the rate of HP fluid entering the LP side via the failed tube. See discussion below for additional design considerations.

• LP side contains liquid only: Even when a PRV is installed to protect LP system during a tube failure, increase the exchanger design pressure to ten-thirteenths of the HP system design pressure to contain the transient pressure spike. A PRV does not respond fast enough to limit the transient pressure rise in the LP side when it is liquid full. LP side of the exchanger. However outside of the exchanger, the transient pressure shock is quickly dissipated and the design pressure of associated equipment and piping does not have to be increased as long as there is an open path to a PRV sized for the steady state flow of HP fluid into the LP system through the failed tube.

• LP side contains liquid and vapor: If the LP side contains at least 1 wt% vapor, the vapor acts as a shock absorber and attenuates the transient pressure rise. Thus, the design pressure of the LP side of the exchanger does not have to be increased as long as there is an open path to a the PRV sized for the steady state flow of HP fluid into the LP system through the failed tube.

Refer to Procedure, for additional guidance.

6. Design Pressure for Special Cases

6.1 Hydraulic Tabulation Report

The guidelines for establishing the design pressure of equipment are set out in terms of a margin above the maximum operating pressure of the equipment. Typically, the maximum operating pressure is obtained from the Hydraulic Tabulation Report which lists the operating pressures at normal, turndown and rated flow rates. Also consider alternative design cases such as plugged reactor that may dictate maximum operating pressures.

6.2 Reactor Circuits

Use the following steps to determine the design pressures of equipment in the reactor section.

a. Calculate hydraulics for the 100%, 110%, and 100% with plugged reactor cases (if applicable) for all feed cases.

b. Start at the vessel with the PRV. Usually this vessel is the separator and is the vessel where the pressure is controlled. Determine the design pressure of this vessel according to Section 5.2.

c. Based upon all the process cases and flow scenarios in the hydraulics determine the highest pressure difference between each piece of equipment and the vessel with the PRV. This will be the “delta” pressure.

d. Add “delta” to the design pressure of the vessel with the PRV. The values thus obtained are the design pressures for the respective pieces of equipment.

Table 3 shows an example calculation for a high pressure process unit. The reactor pressure drop for the plugged case is 80 psi (5.6 kg/cm2), as opposed to 40 psi (2.8 kg/cm2) during normal operation. Maximum operating pressures appear in bold. Design pressures were rounded up to the nearest 5 psi (0.35 kg/cm2). This example assumes that a pilot operated valve protects the reactor circuit permitting a 5% spread between operating and set pressure.

Table 3 - High Pressure Reactor Circuit Hydraulics

Reactor Circuit Hydraulics	100 Percent H&W	100 Percent H&W	110 Percent H&W	110 Percent H&W	100 Percent H&W Plus Plugged Reactor	100 Percent H&W Plus Plugged Reactor	Design Pressure
	p psi	Inlet Press. psig	p psi	Inlet Press. psig	p psi	Inlet Press. psig	psig
Separator	4	2000	5	2000	4	2000	2000/0.95 = 2110
Compressor Suction		1996		1995		1996
Compressor Discharge	4	2154	5	2185	4	2194
Combined Feed Exchanger	30	2150	36	2180	30	2190	2110 + (2190 – 2000) = 2300
Charge Heater	40	2120	48	2144	40	2160	2110 + (2160 -2000) = 2270
Reactor	40	2080	48	2096	80	2120	2110 + (2120 – 2000) = 2230
Combined Feed Exchanger	20	2040	24	2048	20	2040	2110 + (2048 – 2000) = 2160
Condenser	20	2020	24	2024	20	2020	2110 + (2024 – 2000) = 2135
Separator	4	2000	5	2000	4	2000

6.3 Pump Shutoff Pressure

When equipment is installed downstream of a centrifugal pump, it may be subject to the shutoff pressure of the pump during a blocked outlet case. Specify the equipment design pressure to take into account pump shutoff pressure. No margin is required between the shutoff pressure and the design pressure. Alternatively, if it is decided that the design pressure will be lower than the shutoff pressure, install a relief valve.

Estimate the pump shutoff pressure for motor-driven centrifugal pumps using the following guidelines:

a. Determine maximum suction vessel pressure defined as the relief valve set pressure, not accumulated pressure.

b. Calculate the static head in the suction vessel, assuming the vessel, if a receiver, is liquid full and, if a fractionating column, liquid level is at maximum liquid level of level controller.

c. Add the maximum vessel pressure, the tray sections delta P for column, and static head to determine maximum suction pressure.

d. Add 1.25 times the pump rated differential pressure to obtain the pump shutoff pressure used for equipment design. The 1.25 factor is a typical head rise from rated flow to no flow conditions. Check if the rated fluid has the highest specific gravity (SG) for the service. If an alternate pumped fluid has a higher SG than the rated fluid, use this SG for the differential pressure calculation.

Estimated Pump Shutoff Pressure (Motor Drive Centrifugal Pump)

Pump shutoff = Max vessel pressure + Static head + 1.25 times pump rated differential.

Estimated Pump Shutoff Pressure ( Centrifugal Pump)

Pump shutoff = Max vessel pressure + Static head + 1.25 times 1.10 times pump rated differential.

The 1.10 factor represents the 5% speed capability increase associated with a steam turbine. Inflection Point Engineering does not include the 1.21 factor for steam turbine overspeed trip.

Note: A lower shutoff pressure may be required in unusual circumstances, e.g. to avoid a flange class break or a revamp situation where the pump curve is known and existing equipment design pressures would be exceeded if the above approach was taken. In such circumstances, consult with the Rotating Equipment Specialist for a more detailed review of a specific pump service.

The estimated pump shutoff pressure obtained using above procedure is a conservative approach. Equipment need not be designed for pump shutoff - the use of a relief valve at a lower pressure or rearrangement of equipment location are other potential design options. The use of a relief valve is frequently not selected as an option when it results in a large liquid relief.

Review each individual system to determine the best design approach.

6.4 Fractionator Systems

The relieving condition of a fractionator affects the design pressure of the equipment associated with the fractionator. Use the following procedure:

• Calculate fractionator hydraulics for normal, rated, and turndown operation for all feed cases.
• Determine the location of the relief valve(s) needed to protect the fractionator system. Generally, a PRV is located on the fractionator overhead vapor line, but it may also be installed on the top head, at the swaged section, or at the column bottom. If the overhead receiver can be isolated from the fractionator, a separate PRV is required to protect the receiver.
• Establish the design pressure of the fractionator, providing an acceptable margin between the maximum operating pressure and set pressure. Reference Tables 1 and 2.
• Establish the design pressure for all associated equipment relative to the set pressure of the fractionator PRV. Consider frictional and static effects acting on the associated equipment during relief conditions. For example, an overhead condenser located at grade will have a design pressure that is higher than the fractionating column by the amount of static head between the receiver and the condenser. Similarly, if the condenser is free-draining to the receiver, the static head obtained during a reflux failure requires the receiver to have a design pressure somewhat higher than that of the column by the amount of static head that develops when the condenser floods. See Procedure , Attachment 3, Section A3.7 for additional information on determining the design pressure for a steam heated reboiler.
• For additional guidance related to fractionator overhead systems, see Procedure which may override Procedure IPE-TM-100-02 for receivers and overhead condensers.

6.5 Reactor Circuit Recycle Compressor Settle-out Pressure

API STD 521, Annex B suggests setting the Rx system design pressure, as a minimum, equal to 105% of the pressure the system equalizes to when the compressor stops (i.e. the “settle-out” pressure). Inflection Point Engineering does not have adequate information on the Rx circuit piping and volume to do a good calculation. Also, our field experience indicates that there is no relief upon loss of the recycle compressor. This is felt to be due to some condensation in the Rx products condenser which prevents the system from reaching the calculated settle-out pressure. If there are any questions regarding the need to calculate the compressor settle-out pressure, consult your Technology Specialist and/or PRV Specialist.

6.6 Special Conditions

Special conditions for a particular process may dictate the equipment design pressure.

These conditions may arise at start-up or abnormal operating conditions and they may not appear in the heat and weight balance. Consider the situation where one fractionator upstream of another may not separate light ends properly at start-up. Inflection Point Engineering and/or the Customer may wish to avoid lifting relief valves on the downstream fractionator. Relief may occur if the design pressure of the downstream fractionator is specified using the normal rules. Specifying an extra amount of margin between the “normal” operating pressure shown in the heat and weight balance and the design pressure may be justifiable.

Also consider the case where the equipment normal operating temperature is below ambient temperatures, (such as the case with some C3 splitters, C1 splitters, cryogenic separation systems). During startup or shutdown, the equipment can warm up to ambient temperature, causing the contents of the equipment to vaporize, with the material being vented to the relief header via the pressure relief valve if the design pressure is based only on the normal operating pressure and temperature. The design pressure of such equipment shall be increased to the vapor pressure of the material in the equipment at the maximum ambient temperature.

6.7 Fired heater Reboiler

Reference Procedure ” for information on determining design pressure.

6.8 Feed surge drum design pressure is typically based upon upstream pump shut-off pressure. If the pump shut-off pressure is significantly higher then normal operating pressure or if a lower (lower then pump shut-off) design pressure is required due to a revamp situation, a PRV or a SIS system may be considered. Consult with the Technology Specialist.

7. Design Temperature

7.1 General

Set the design temperature of process equipment at least 50F (28C) above the normal operating temperature, or 250F (120C), whichever is higher. The 250F minimum design temperature was selected because it would not be exceeded during a steamout operation. If the 250F minimum results in a flange rating increase, then a lower design temperature may be considered if other design guidelines will be met. When caustic is present, limit design temperature to 150F (65C) to avoid caustic embrittlement of carbon steel vessels. At maximum operating temperatures near or above the creep range, the design margin has a significant impact on the vessel thickness and a lower margin (below 50oF) may be considered when appropriate. Consult with a Mechanical Design Specialist when operating at elevated temperatures near or above creep range.

The vessel code does not define a margin between max operating and design temperature. In principal the vessel could operate at or within several degrees of design temperature. The margin between max operating and design temperature may be decreased below Inflection Point Engineering standard practice of 50°F (28°C) for a revamp or special situation using good engineering judgment. Margin allowable should be dependent on ability to effectively control operating temperature in a fixed range.

Also the piping code generally referenced and used along with the vessel code allows short term temperature excursions above the design temperature. Short term excursions are generally defined as relief or other conditions where the process unit operation can not be sustained. Consult the Technology Specialist for additional guidance.

7.2 Fractionators

In general, the design temperature of a fractionator is specified relative to the bottom maximum operating temperature, because the bottom composition could migrate overhead during relief conditions. When the bottom maximum operating temperature is less than 650F (345C), set the column design temperature 50F (28C) above the reboiler return temperature. For fractionators that have a bottom temperature greater than 650F (345C), the design temperatures are generally graduated from top to bottom; i.e., different design temperature for top and bottom sections. This provides savings due to lower flange ratings and possibly shell thickness. Some fractionators such as rectifiers, strippers, and FCC main column may not have a reboiler. In these cases, the feed temperature may be the maximum operating temperature. If the feed temperature exceeds 650F (345C), consult with Technology Specialist.

Normally, specify fractionator trays for a structural design temperature equivalent to the fractionator design temperature.

7.3 Design Temperature based upon Reactor Circuit Loss of Liquid Feed

If the combined feed at the reactor inlet is two-phase, loss of liquid feed can cause elevated temperatures in the reactor feed/effluent exchange system. Although liquid feed stops, recycle gas continues to “push” hot liquid held in the catalyst bed and reactor internals out of the reactor for about 15 minutes. Since loss of feed to the cold side of the exchangers causes an immediate loss of heat removal capability, the temperatures in the reactor effluent exchangers will climb. The procedure outlined below accounts for this transient effect, and it assures that the design temperatures for the reactor circuit are properly set.

The following procedure should be used to determine the design temperatures in the effluent exchange system during loss of liquid feed:

(1) Draw a temperature-enthalpy curve for the reactor effluent material. Assume a linear temperature profile and draw a line connecting the enthalpy of the effluent material at the outlet of the reactor (at the reactor outlet temperature) with the enthalpy of the effluent entering the Separator. It is assumed that this profile is not affected during the loss of feed.

(2) Assume that the recycle gas continues as per normal during the loss of feed.

(3) Take no credit for any exchange that has liquid feed only on the cold side. For combined feed exchange (liquid plus recycle gas), take credit only for the enthalpy increase of the recycle gas as it is heated up. In this case, it can be assumed that the recycle gas will heat up to within 10F (5.5C) of the hot side inlet temperature.

Construct a temperature profile of the exchange circuit using recycle gas on the cold side and Reactor Effluent on the hot side of combined feed exchanger.

(4) The reactor effluent air cooler can be re-rated to take credit for the higher inlet temperature and heat transfer. Do not take credit for the cooling effect of wash water on the air cooler inlet. A power failure that can cause the charge pump to shut down can also shut down the wash water pump.

7.4 Utilities Equipment

The Customer lists the design temperatures for utility systems in the BEDQ and the design temperatures of process equipment shall be consistent with those utilities. In general, Inflection Point Engineering sets the design temperature on the cooling water side of an exchanger at 250F (120C) to cover steamout conditions. If the Customer lists a lower design temperature for water coolers in the BEDQ, use the BEDQ design number.

7.5 Flange Class

When setting the design temperature of equipment, always consider the impact that increasing the temperature has on flange class to minimize equipment costs. Inflection Point Engineering’s practice of specifying a minimum design temperature of 250F (120C) may be waived in favor of a lower number if this reduces the flange class.

Check flange ratings at the top and bottom of fractionators that have many trays and a significant pressure drop. Different flange ratings at the top and bottom of a fractionator are acceptable.

7.6 High Design Temperature

For reactors operating at high temperatures and/or high pressures, where allowable stress decreases rapidly with increasing temperature, the design temperature may be the same as the maximum operating temperature at the end of run. This practice is conservative if the reactions are endothermic. Consult the Technology or Process Specialist for a particular process with an exothermic reaction.

7.7 Cold Wall Design

When operating temperatures are extremely high (above 1000F), cold wall vessels are sometimes used to reduce alloy and thickness requirements. These vessels are insulated internally, permitting a design temperature of the outside shell which is lower than the normal operating temperature. For example, a Inflection Point Engineering Thermal Hydrodealkylation (THDA) Process Unit has a reactor operating temperature that ranges from 1250 to 1300F (677 to 704C) and the reactor has a cold wall design temperature of 600F (315C). Consult the Technology Specialist for the appropriate design temperatures of cold wall vessels.

7.8 Vacuum Design Temperature

Specify a separate design temperature for the external pressure design condition of Full Vacuum in addition to the design temperature for the internal pressure condition. Reference Procedure ."

7.9 Steamout Design Temperature

Inflection Point Engineering specifies a minimum design temperature of 250F (120C) to accommodate the steamout temperature. If caustic is present in the vessel, limit the design temperature to avoid the use of nickel alloy metallurgy. Stress relieving may also be required. Use the following guidelines for vessels containing caustic:

• Do not steam out the equipment, but use a water wash and/or nitrogen to hydrocarbon free equipment.
• For caustic strengths of 15 wt% or less, specify carbon steel metallurgy, set the design temperature at 150F (65C) and do not specify PWHT. If the Customer insists on steamout for this range of strength caustic, specify carbon steel metallurgy, design temperature of 212F (100C), and specify PWHT.
• For caustic strengths of 15-30 wt%, specify carbon steel metallurgy. A design temperature of 200F (95C) requires PWHT.
• For caustic strengths of 30-50 wt%, specify carbon steel. A design temperature of 150F (65C) requires PWHT.

If the customer insists on steamout of equipment containing caustic, or if the customer mandates a higher design temperature (higher than guidelines), or if operating conditions could result in a higher design temperature, see the Chief Metallurgist for recommended metallurgy and/or PWHT requirements

7.10 Minimum Design Metal Temperature

For cryogenic services, set the minimum design metal temperature at a minimum of 25°F (14°C) below the normal operating temperature. Review the design temperature in cryogenic services for its effect on metallurgy, impact testing, etc. For example, cryogenic services using aluminum have a maximum design temperature limit of 150°F (65°C).

Inflection Point Engineering design practice dictates that low process (result of auto chilling) or ambient temperature conditions be specified for equipment. The low ambient design temperature is given in Site Information section of the BEDQ. Reference Procedure ”.

7.11 Overhead Condenser with Hot Vapor Bypass

• Reference procedure .

7.12 Failure or Bypassing of Upstream Heat Exchangers

Consider the failure of upstream heat exchangers and coolers or open exchanger by-pass(es) when determining the design temperature of equipment.

The following are examples of how this should be applied:

a. On loss of feed to a feed-bottoms exchanger the bottoms flow will continue until the bottoms level goes low. Downstream equipment should be designed for the normal bottoms temperature to account for the loss of heat exchange.

b. When there are several heat exchangers in an exchange train (such as in a reactor effluent circuit) it is necessary for the design engineer to consider the effect of full bypass of each exchanger individually on all of the downstream equipment, not just the immediate downstream equipment. Unless there is a specific operational requirement, assume only one exchanger is by-passed at a time.

c. For a typical air-cooled exchanger the power failure duty is assumed to be 20% of the normal duty. If an air-cooled exchanger is provided with a chimney up to 50% of the normal duty can be assumed. However, if louvers are specified for the air-cooled exchanger, then no credit can be taken.

d. Assume zero cooling duty for the loss of cooling water (or process cooling media) to a cooler (or process exchanger) upstream of other process equipment. Review the impact on piping.

The design temperature based upon failure or bypassing of upstream heat exchangers, should, in general, be set equal to the temperature resulting from a failure or exchanger bypass. Some engineering judgment should be applied, however, in general design temperature for these situations is not based upon our normal practice of maximum operating plus 50°F (28°C).

8. and Maximum Operating Temperature

Use the maximum normal operating temperature or the maximum temperature during alternate operation to determine the design temperature.

8.1 Operating Temperature

Obtain the normal operating temperature from the heat and weight balance. Reference Section 4.2, if necessary.

8.2 Special Conditions

A process may have special operating conditions that cause a higher than normal maximum operating temperature. For example, during start-up, a Inflection Point Engineering Hydrotreating Process Unit Stripper may be used to process stripped sweet heavy naphtha as feed to a Platforming Process Unit. The operating temperatures in the stripper overhead system may then exceed those shown in the heat and weight balance.

Other special conditions which impact design temperatures are loss of wash water downstream of a Combined Feed Exchanger and/or loss of feed to a Unicracking unit where recycle gas and feed are heated separately. Review Project Design manuals for specific Technologies for additional information.

8.3 Vessels

Usually, the reboiler return temperature plus a 50F (28°C) margin is used to determine the design temperature of a fractionator. Consider the possible failure of feed heat removal exchange upstream of a vessel which may require an increment of more than 50F (28C) above the normal reboiler return operating temperature.

8.4 Heat Exchangers

Use the higher normal operating temperature (inlet vs. outlet) to determine the design temperature for each side of a heat exchanger. Check all multiple design cases to determine the maximum operating temperature. Consider the failure of other exchangers, or an open by-pass upstream of an exchanger (conditions that would still allow sustained operation) when determining the design temperature.

9. Special Considerations

a. Some vessels and exchangers which have alternate operations such as regeneration may see significantly different operating conditions such that it is not apparent which set of of operating conditions should be used to determine design conditions. In this situation determine both sets of design conditions and list multiple design conditions under “Notes” for example

(1) Normal Design Conditions : 650 psig @ 400°F

(2) Regeneration Design Conditions: 160 psig @ 650°F

b. Inflection Point Engineering has guidelines for setting design temperatures however, the code allows operation up to and at the design temperature (not above). Maintain Inflection Point Engineering guidelines when setting design temperatures for new units, however for revamps and/or reuse of existing equipment in a new unit some engineering judgment involving a margin less than 50°F (28°C) may be used.

c. The Design Temperature and Design Pressure shown on each PRV specification sheet is used for the design of the pressure-retaining parts of the PRV i.e., to establish the inlet flange class. The relieving temperatures shown may sometimes be higher than the vessel design temperature. It is Inflection Point Engineering’s opinion that short-term temperature excursion occurring under relieving cases can be tolerated and need not, in general, impact the design temperature of the PRV, or of process equipment downstream of the PRV location. If the relieving temperature is above 650°F (343°C), consult with a vessel Mechanical Design Specialist for specific guidance.

10, Rounding Up Recommendations

Calculate the design conditions for metric jobs in English units, then convert to metric units and round up.

10.1 Design Pressure

Since operating and design pressures can only be estimated to a certain degree of accuracy, the following rounding up guidelines are recommended for the various units of measurement:

Units	Recommended Rounding Up Procedure	Examples
psig	to next higher 5 psi	56.4 to 60.0, 246 to 250
kg/cm2g & barg	to next higher 0.5 unit	13.13 to 13.5
kPag	to next higher 50 kPa	1511 to 1550,

10.2 Design Temperature

Round up F and C to the nearest five degree increment (401 to 405).

10.3 Exception

When the customer provides utility design conditions in the BEDQ, use the numbers provided without any round up unless indicated otherwise by the Project Manager.

Attachment 1 - Exchanger Design Pressure Decision Tree

Attachment 2 - Vessels Design Pressure Decision Tree

Attachment 3 Heater Design Pressure Decision Tree