IPE-TM-807 Pressure Relief Systems
Pressure Relief Valve Inlet Piping
IPE-TM-807-02
1. Table of Contents
2. Purpose
This procedure describes Inflection Point Engineering’s design practice for sizing relief valve inlet piping. FOR MOST SCHEDULE A DESIGNS, THIS PROCEDURE IS NOT NECESSARY. The detailed design contractor has ultimate responsibility to size the inlet piping once its equivalent length is known. However, this procedure does apply when inlet sizing is requested by the customer, or when Inflection Point Engineering is the detailed design contractor.
3. Limitations
This design practice is based on the inlet flange sizes listed in API Standard 526 for spring-loaded and pilot-operated pressure relief valves. The recommendations in this procedure are limited to services where:
- The relieving fluid is vapor only.
- The valve set pressure is 50 psig or higher.
- One valve is needed for overpressure protection, e.g. services that do NOT require multiple valves to “handle” the relieving rate. If a spare relief valve is provided, Inflection Point Engineering assumes that it is isolated until required for service.
For relief services involving flashing liquid or two-phase flow, consult the PRV Specialist.
4. Background
If the non-recoverable pressure loss becomes excessive in relief valve inlet piping, the valve will rapidly cycle open and closed. This phenomenon, known as chattering, severely restricts PRV capacity and is particularly destructive to the valve and associated piping. To prevent valve chattering, ASME Code limits the inlet pressure loss to 3% of PRV set pressure, based on its rated capacity.
5. Sizing PRV Inlet Piping (When Required)
This design guidance applies ONLY when Inflection Point Engineering is asked to size the PRV inlet line, or when Inflection Point Engineering is the detailed design contractor. For most projects, Inflection Point Engineering does not show the inlet line size on the P&ID. If a relief valve is directly connected to the vessel, the relief nozzle is listed in our vessel specification, but its size is not given.
In addition, the sizes listed in the tables below apply to spring-loaded pressure relief valves, and to pilot operated relief valves when pressure is sensed at the valve inlet flange. If a pilot operated valve has remote pressure sensing, the inlet line could have the same diameter as the PRV inlet flange, provided the inlet pressure loss does not exceed 10% of set pressure.
5.1 ASME Code Requirements
ASME code requires inlet piping, valves and fittings to have a cross-sectional area equal to, or larger than the PRV inlet flange. This requirement sets a lower limit on inlet piping size. In addition, the code limits inlet pressure loss to 3% of PRV set pressure. Knowing the rated capacity of the PRV and the inlet piping configuration, the inlet line size may be increased to meet the 3% criterion.
5.2 Inlet Piping Configurations
Inlet piping should be made as short and as direct as possible. Preferably, the relief valve should be connected to the protected equipment via a dedicated relief nozzle and short inlet line. However, a relief valve is often located on process piping to provide free draining, accessibility for maintenance, or valve support. If multiple relief valves are required, provide separate inlet connections for each valve, i.e. avoid branching with additional tee connections.
Tables in sections 5.5 to 5.10 list the diameter required for various inlet piping configurations as a function of relief valve size. If the diameter listed in the table is greater than the relief valve inlet flange, the larger diameter applies to all inlet piping, fittings and valves, and a pipe reducer is used to swage down to PRV inlet flange.
Generally, Inflection Point Engineering does not provide spare relief valves unless they are specifically requested by the customer. Sparing requires additional piping, fittings, and block valves which incrementally increase the inlet pressure loss. In addition, isolating valves must be either locked open (LO) or car sealed open (CSO), and the opening and closing sequence must be ensured using an administrative protocol or keyed interlock system. If the inlet piping configuration is not specified by the customer, provide separate inlet connections for the operating and spare valves.
5.3 Available Equivalent Length (AEL)
Inlet pressure loss was determined for various piping configurations assuming the inlet piping was the same size as the PRV inlet flange. If the calculated pressure loss was more than 3% of set pressure, the calculations were redone using a larger pipe diameter.
For all configurations, the equivalent length was calculated to exactly meet the 3% inlet pressure loss criterion. The difference between the maximum length allowed by the 3% criterion and the minimum equivalent length required for a particular inlet piping configuration represents the “extra” length that is available for elevating the PRV above the relief header. This “extra” length is listed in the “AEL” column in the tables.
The procedure for using the AEL is provided in Section 5.11.
Attachment 1 shows how the available equivalent length was derived; Attachment 2 shows the equivalent lengths used for elbows and other components in piping systems; and Attachment 3 shows inlet and outlet sizes listed in API Standard 526. All inlet pressure loss calculations assumed a PRV set pressure of 50 psig. This assumption is conservative when compared to higher set pressures. For set pressures less than 50 psig, consult the PRV Specialist.
5.4 Process Revamps
Depending on the design scope for the revamp, Inflection Point Engineering will either specify all relief valves as new, or we will evaluate existing relief valve specifications for revamp conditions. When relief valves are specified as new, Inflection Point Engineering will select the valve type based on our current design rules, e.g. specifying a spring-loaded valve or a pilot operated valve with remote sensing to meet the 3% criterion.
When Inflection Point Engineering reviews existing valve specifications, our evaluation is limited to valve type, effective orifice area, metallurgy, and pressure rating. If an existing valve is inadequate or incompatible with the new service, we will specify the valve “as new”. In either case, we do NOT evaluate the existing inlet and outlet piping, unless specifically requested by the customer. The contractor is responsible for field inspection of existing relief valves, and for evaluating the pressure loss in the inlet and outlet piping.
NOTE:Flange sizes listed in API Standard 526 may be different from sizes listed in previous editions. In particular, Standard 526 has eliminated valves with 2½" flanges and replaced them with 3” flanges. If an existing specification lists valves with 2½" connections, they are acceptable and need not be re-specified.
5.5 PRV Connected to Vessel Nozzle
When a relief valve is connected to a dedicated nozzle on a vessel, the nozzle and inlet piping are generally the same size as the PRV inlet flange. The table below lists the minimum size for the vessel nozzle and inlet piping as a function of API orifice size.
| API Orifice Designation | PRV Inlet Flange (1) | Nozzle and Inlet Piping Diameter (1) | AEL (ft) |
|---|---|---|---|
| D E F G H J K L M N P Q R T | 1" 1" 1½" 1½" 2" 3" 3" 4" 4" 4" 4" 6" 6" 8" | 1" 1"(2) 1½" 1½"(2) 2" 3" 3" 4" 4" 4"(2) 4"(2) 6"(2) 6"(2) 8"(2) | 23 6 28 8 13 41 19 35 18 9 4 9 4 9 |
Refer to Section 5.11 for Notes (1) and (2).
5.6 PRV Installed on Process Piping
When a PRV is installed on process piping, the nozzle and inlet piping are generally one to two sizes larger than the PRV inlet flange. The table below lists the minimum size for the vessel nozzle, process piping and PRV inlet piping as a function of API orifice size. Although the process piping can be larger than the diameter listed, it cannot be smaller. If the process piping is smaller, the vessel nozzle and process piping must be enlarged to match the diameter listed. Downstream of the PRV inlet connection, the process piping can be reduced to the diameter calculated using our line sizing criteria.
| API Orifice Designation | PRV Inlet Flange (1) | Nozzle and Inlet Piping Diameter (1) | AEL (ft) |
|---|---|---|---|
| D E F G H J K L M N P Q R T | 1" 1" 1½" 1½" 2" 3" 3" 4" 4" 4" 4" 6" 6" 8" | 1" 1½" 1½" 2" 3" 3" 4" 6" 6" 6" 8" 10" (3) (3) | 16 55 17 21 95 20 58 280 140 65 41 125 0 0 |
Refer to Section 5.11 for Notes (1) and (3).
5.7 PRV Connected to Vessel Nozzle with Isolation Valves for Maintenance
When a PRV is connected to a relief nozzle and when isolation valves are provided for maintenance, the vessel nozzle and inlet piping is generally one or two sizes larger than the PRV inlet flange. The table below lists the minimum diameter for the vessel nozzle, inlet piping, and isolation valve as a function of API orifice size.
| API Orifice Designation | PRV Inlet Flange (1) | Nozzle, Valve and Inlet Piping Diameter (1) | AEL (ft) |
|---|---|---|---|
| D E F G H J K L M N P Q R T | 1" 1" 1½" 1½" 2" 3" 3" 4" 4" 4" 4" 6" 6" 8" | 1" 1½" 1½" 2" 2" 3" 3" 4" 4" 6" 6" 8" 10" 10" | 22 64 26 33 11 37 15 30 14 103 24 52 26 43 |
Refer to Section 5.11 for Note (1).
5.8 PRV Installed on Process Piping with Isolation Valves for Maintenance
When a PRV is installed on the process piping and when isolation valves are provided for maintenance, the vessel nozzle and inlet piping is generally one to two sizes larger than the PRV inlet flange. The table below lists the minimum diameter of the vessel nozzle, process piping, inlet piping, and isolation valve as a function of API orifice. Although the process piping can be larger than the diameter listed, it cannot be smaller. If the process piping is smaller, the vessel nozzle and process piping must be enlarged to match the diameter listed. Downstream of the PRV inlet connection, the process piping can be reduced to the diameter calculated using our line sizing criteria.
| API Orifice Designation | PRV Inlet Flange (1) | Nozzle, Valve and Inlet Piping Diameter (1) | AEL (ft) |
|---|---|---|---|
| D E F G H J K L M N P Q R T | 1" 1" 1½" 1½" 2" 3" 3" 4" 4" 4" 4" 6" 6" 8" | 1" 1½" 1½" 2" 3" 3" 4" 6" 6" 6" 8"(2) 10" (3) (3) | 15 53 16 19 95 17 56 276 136 63 32 124 0 0 |
Refer to Section 5.11 for Notes (1), (2) and (3).
5.9 Operating and Spare PRVs Connected to Single Vessel Nozzle
When operating and spare relief valves are connected to a relief nozzle and when isolation valves are provided for maintenance, the vessel nozzle, inlet piping, tee, and valves are generally one to two sizes larger than the PRV inlet flange. The table below lists the minimum diameter of the vessel nozzle, tee, inlet piping, and isolation valves for each API orifice.
| API Orifice Designation | PRV Inlet Flange (1) | Nozzle, Valve and Inlet Piping Diameter (1) | AEL (ft) |
|---|---|---|---|
| D E F G H J K L M N P Q R T | 1" 1" 1½" 1½" 2" 3" 3" 4" 4" 4" 4" 6" 6" 8" | 1" 1½" 1½" 2" 3" 3" 4" 6" 6" 6" 8" 10" (3) (3) | 15 52 15 19 95 17 55 270 130 60 20 115 0 0 |
Refer to Section 5.11 for Notes (1) and (3).
5.10 Operating and Spare PRVs Installed on Process Piping with Isolation Valves
When operating and spare relief valves are installed on process piping and when isolation valves are provided for maintenance, the vessel nozzle, inlet piping, tees and isolation valves are generally one or two sizes larger than the PRV inlet flange. The table below lists the minimum diameter of the vessel nozzle, process piping, tee connections (two), elbows (two), inlet piping, and isolation valve for each API orifice. Although the process piping can be larger than the diameter listed, it cannot be smaller. If the process piping is smaller, the vessel nozzle and process piping must be enlarged to match the diameter listed. Downstream of the PRV inlet connection, the process piping can be reduced to the diameter calculated using our line sizing criteria.
| API Orifice Designation | PRV Inlet Flange (1) | Nozzle, Valve and Inlet Piping Diameter (1) | AEL (ft) |
|---|---|---|---|
| D E F G H J K L M N P Q R T | 1" 1" 1½" 1½" 2" 3" 3" 4" 4" 4" 4" 6" 6" 8" | 1½" 1½" 2" 3" 3" 4" 4" 6" 6" 6" 10"(2) 10" (3) (3) | 209 45 82 241 80 122 36 246 106 32 112 74 0 0 |
Refer to Section 5.11 for Notes (1), (2) and (3).
5.11 Notes for Tables in Sections 5.5 to 5.10
(1) Inlet and outlet sizes listed for pressure relief valves are based on:
a. Effective orifice area, i.e. API lettered orifices,
b. Flange class of PRV inlet, and
c. PRV type (Spring-loaded or Pilot-Operated).
The Relief Valve Sizing and Selection tool incorporates the flange tables given in API Standard 526. When users specify a valve type, set pressure and required orifice, the tool automatically selects the inlet and outlet sizes. If the flange class is manually overridden with a higher flange class, the Sizing and Selection tool will automatically select consistent inlet and outlet diameters.
- Select the appropriate table in Sections 5.5 to 5.10 based on the inlet piping configuration.
- Use the API orifice letter from the 807 specification to enter the table and obtain the minimum diameter required for the nozzle, inlet piping, and components (from column 3).
- Compare PRV inlet size obtained from the 807 specification to the piping diameter listed in the table. The larger diameter should be used for the minimum inlet piping size.
- Check the AEL. (Note: skip the remainder of this procedure if the relief valve is connected to process piping in the rack)
- Determine the difference in elevation between the vessel nozzle and the relief header. (If the header elevation is unknown, assume it is 40 ft)
- Multiply the elevation difference by two and compare it to the AEL listed in the table.
- If the AEL is greater than two times the elevation difference, use the minimum inlet piping diameter (from above).
- If the AEL is less, increase the inlet piping diameter one size larger than the diameter listed in the table.
The diameter of the vessel nozzle and inlet piping must NEVER be smaller than the PRV inlet flange. (See the Sizing and Selection Tool Documentation, , Attachments 5 and 7.)
5.11 (continued) Notes for Tables in Sections 5.5 to 5.10
(2) If the pressure loss in the inlet pipe configuration exceeds 3% of set pressure, the vessel nozzle may be rounded to reduce the pressure loss due to sudden contraction as an alternative to increasing the piping size.
(3) Spring-loaded "R" or "T" orifice valves cannot be used with this configuration. Provide separate relief nozzle connections on the vessel, consider using a pilot operated relief valve with remote sensing, or specify multiple spring-loaded relief valves, each with a smaller effective orifice area, e.g. two “Q” orifice valves instead of one “R” orifice.
Attachment 1
Calculation of Available Equivalent Length (AEL)
The available equivalent length listed in the tables of Section 5 for each inlet piping configuration was determined as follows:
(1) Using an inlet pressure loss equal to 3% of set pressure, the maximum equivalent length for each configuration was determined for relief valves having a 50 psig set pressure. (At higher set pressures, the available equivalent length is slightly higher than that reported in the tables. For set pressures less than 50 psig, consult the PRV Specialist.)
(2) The total equivalent length for components in each inlet piping system was based on component type and number required for a particular inlet piping configuration:
| Component Counts | Section No. | Section No. | Section No. | Section No. | Section No. | Section No. |
|---|---|---|---|---|---|---|
| 5.5 | 5.6 | 5.7 | 5.8 | 5.9 | 5.10 | |
| Sudden Contraction (a) | 1 | 1 | 1 | 1 | 1 | 1 |
| Elbows | 0 | 1 | 0 | 1 | 1 | 2 |
| Gate Valve | 0 | 0 | 1 | 1 | 1 | 1 |
| Tee Connections | 0 | 1 | 0 | 1 | 1 | 2 |
| Reducer | (b) | (b) | (b) | (b) | (b) | (b) |
(a) A sudden contraction occurs at the relief nozzle or when entering the process piping.
(b) If the inlet piping diameter was larger than the PRV inlet flange, an equivalent length for the reducer was added.
(3) The available equivalent length was determined by subtracting the equivalent length of piping components (isolation valves and fittings) in a specific inlet piping configuration from the maximum equivalent length that still meets the 3% inlet pressure loss constraint, i.e.:
(4) The available equivalent length allows for additional elbows and all horizontal and vertical piping necessary to locate the relief valve where it is free draining or readily accessible.
(5) When the relief valve is connected to a vessel nozzle, the Design Engineer should confirm that sufficient AEL is available to ensure free draining to the relief header. If the PRV elevation (relative to the vessel) is such that AEL is exceeded, increase the inlet piping one line size larger than the value listed in the table.
Attachment 2
Equivalent Lengths for Components in a Piping System
This table was derived from Crane Technical Paper No. 410 and equivalent lengths are given in feet.
| Size | Contraction | L.R. Elbow | Gate Valve | Tee Connection |
|---|---|---|---|---|
| 1" 1½" 2" 3" 4" 6" 8" 10" 12" | 2 3 4 7 10 17 23 30 35 | 3 4 5 8 10 15 20 25 30 | 1 2 2 3 4 6 9 11 13 | 5 8 10 15 20 30 40 50 60 |
Reducer equivalent length is calculated using an equation given in Crane Technical Paper No. 410.
Where:
K = resistance coefficient for the reducer (based on D)
f = friction factor
L = equivalent length of the reducer, ft. (Knowing D and f, solve for L.)
D = inlet piping diameter, ft
β = ratio of reducer’s outlet and inlet diameters. (β < 1.0)
θ = transition angle between inlet and outlet. (Assume θ = 90°.)
Attachment 3
Inlet and Outlet Sizes Listed in API Standard 526
| API Orifice Designation | Orifice Area (in2) | Minimum PRV Flange Sizes (a) | Maximum PRV Flange Sizes (b) |
|---|---|---|---|
| D E F G H J K L M N P Q R T | 0.110 0.196 0.307 0.503 0.785 1.287 1.838 2.853 3.600 4.340 6.380 11.05 16.00 26.00 | 1 D 2 1 E 2 1½ F 2 1½ G 3 1½ H 3 2 J 3 3 K 4 3 L 4 4 M 6 4 N 6 4 P 6 6 Q 8 6 R 8 8 T 10 | 1½ D 3 1½ E 3 1½ F 3 2 G 3 2 H 3 3 J 4 3 K 6 4 L 6 4 M 6 4 N 6 4 P 6 6 Q 8 6 R 10 8 T 10 |
(a) Minimum PRV sizes apply to spring-loaded PRVs with lower ASME flange classes.
(b) Maximum PRV sizes apply to spring-loaded PRVs with higher ASME flange classes, and/or Pilot-Operated PRVs.
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