This document provides an in-depth overview of pressure relief valves (PRVs), including types such as conventional, balanced-bellows, and pilot-operated PRVs, as well as the concepts of backpressure and pressure level settings. It details the calculation methodologies according to API standards for sizing and selecting PRVs to ensure safety against overpressure. The document also clarifies definitions of key terms such as set pressure, accumulation, and maximum allowable working pressure (MAWP).

1. Pressure Relief Valve:
Single Phase Relief
Author: Vikram Sharma Date: 12th March 2017

2. Table of Contents
 Introduction
 Type of Pressure Relief Valves
 Concept of backpressure
 Pressure level settings
 Calculation methodology
 Summary
 References

3. Introduction
 Pressure relief valves a.k.a pressure relief devices
(PRD)
 Primary function → protect an equipment frm.
overpressure that may lead to catastrophic incident.
 Common → PRDs safety valve, PRV, safety relief
valve, PORV & rupture disk
 Focus on three types of PRVs:
 Conventional PRV
 Balanced-bellows PRV and
 Pilot Operated PRV
 Calculation as per API Std. 520 Part 1 9th Ed. (2013)

4. Type of Pressure Relief Valves
 Types of PRV:
 Conventional PRV;
 Balanced-Bellows PRV; and
 Pilot Operated PRV
 Conventional PRV:
 Used when the built-up backpressure should not exceed
10% of the set pressure at 10% allowable overpressure.
 Higher allowable overpressure of more than 10% may
allow a higher max. allowable built-up backpressure
provided the built-up backpressure does not exceed the
allowable overpressure.

5. Type of Pressure Relief Valves (cont’d)
 Balanced-bellows PRV:
 Used when the built-up backpressure (superimposed +
built-up) is too high for conventional PRV.
 Used when the superimposed backpressure varies
significantly in comparison to the set pressure
 Used when the total backpressure (superimposed + built-
up) does not exceed approx. 50% of the set pressure
 Pilot Operated PRV:
 Valve lift is not affected by backpressure

6. Type of Pressure Relief Valves (cont’d)

7. Concept of Backpressure
 Backpressure consist of two parts that are:
 Superimposed backpressure
 Built-up backpressure
 Superimposed backpressure:
 Pressure originating frm. other sources when the PRV is
in READY MODE.
 Two parts that are variable and constant
 Variable
 one or more PRVs discharging into a common header.
 Each PRV may have different backpressure at each moment @
each relief cycle
 Balanced & Pilot → used as backpressures vary significantly under
any operation condition
 Direct impact on the set pressure.

8. Concept of Backpressure (cont’d)
 Superimposed backpressure (cont’d):
 Two parts that are variable and constant (cont’d)
 Constant
 Occurs when the outlet of a PRV is connected to a static pressure
source which does not change significantly under any operational
condition.
 Actual set pressure is defined as the sum of bench set pressure &
backpressure
Constant Variable

9. Concept of Backpressure (cont’d)
 Built-up backpressure:
 Occurs when the PRV is in OPEN MODE and flowing due
to the following reasons:
 Rate of fluid flow through the PRV;
 Size and configuration of the PRV discharge piping; and
 Other source of pressure acting into the discharge header
 Affected by the friction and pressure drop through the
discharge piping.
 Built-up backpressure is always variable

10. Concept of Backpressure (cont’d)

11. Pressure Level settings
 Set Pressure
 Pressure (inlet gauge pressure) at which the relief device set to
open under service conditions
 Accumulation
 Expressed as percentage of MAWP
 Defined as the pressure increased above the MAWP
 Overpressure
 Pressure increase over the PRV set pressure
 Expressed in pressure units or percentage of set pressure
 MAWP
 Maximum Allowable Working Pressure
 It’s a term related to the construction of a vessel or item to be
protected

12. Pressure Level settings (cont’d)
 MAWP (cont’d)
 Defined as the max. allowable pressure at the top of a
completed vessel in its normal operating position and at a
designated temperature.

13. Pressure Level settings (cont’d)
 Confusion between accumulation & overpressure?
 Overpressure is referenced to the set pressure which is a
property of a relief valve.
 Accumulation is related to MAWP which is a property of a
vessel or item to be protected.
 Confusion between MAWP & Design Pressure?
 MAWP → defined as the max. allowable pressure at the
top of a completed vessel in its normal operating position
and at a designated temperature.
 Design pressure → pressure with a margin above the
most severe pressure expected during normal operation
at a coincident temp.
 MAWP is normally higher than the design pressure (API
520)

14. Pressure Level settings (cont’d)
 MAWP or Design Pressure for PRV sizing?
 During design stage where MAWP is unavailable,
designer is to rely on some basis for calc. → design
pressure (Para 3.16 API 520 Part 1 9th Ed. (2013))
 MAWP is a property assigned by the fabricator of the
vessel
 ff
MAWP is normally
higher than Design
Pressure
Set Pressure is also the Set
Point of PRV & shall not
exceed the MAWP

15. Pressure Level settings (cont’d)
 Max. accumulation & set pressure of a relief valve is
further divided by its configuration and relief case
category.
 Relief case – Fire or Non-fire case
 Configuration – Single or multiple device installations

16. Calculation Methodology
 Simplified P&ID of Fuel Gas (FG) Knock Out (KO)
Drum
 (Source: MOHIB, 2016)

17. Calculation Methodology (cont’d)
 Important facts
 Assume the KO drum is at design stage.
 Set Pressure (SP) = Design Pressure of the drum
 Backpressure ≤ 50% of the SP → Balanced-Bellows PRV
 Design data:
 Gas density (ρG): 4.1 kg/m3
 Ratio of specific heats, (Cp/Cv) = k: 1.55
 Compressibility factor (Z) = 0.95
 Molecular Weight of FG (MW): 20.0g/gmol
 Relieving Temp. (T): 20°C
 Set Pressure (SP): 4.5 barg
 Accumulation: 10%
 Backpressure @ relief valve discharge: 2.1 barg

18. Calculation Methodology (cont’d)
 Check if the PRV conforms to critical or sub-critical
flow condition
 What is critical flow condition?
 Expansion process seen when a compressible fluid (gas) flows
across a nozzle at constant U/S condition
 Results to increased gas vel. & specific volume with decreasing
D/S pressure
 At constant U/S condition, the mass flow ↑ to a point where
further ↓ in D/S pressure will not see ↑ in gas flow: Critical
flow rate

19. Calculation Methodology (cont’d)
 Check if the PRV conforms to critical or sub-critical
flow condition
 What is critical flow condition? (cont’d)
 Determine the Critical flow pressure (Pcf)
 Require info. : upstream relieving pressure & Cp/Cv @ ideal
condition @ relieving temp.
 P1 is a f(SP, Allowable overpressure, Patm)

20. Calculation Methodology (cont’d)
 Check if the PRV conforms to critical or sub-critical
flow condition
 Determine the Critical flow pressure (Pcf) (cont’d)
 Critical flow: Downstream pressure (P2) ≤ Pcf
 Sub-critical flow: Downstream pressure (P2) or backpressure Pcf
 Backpressure > Pcf → SUB-CRITICAL FLOW

21. Calculation Methodology (cont’d)
 Check if the PRV conforms to critical or sub-critical
flow condition
 Calculate the relief discharge area (A)
 PRV is w/o a rupture disk; Kd = 0.975 for PRV installed
with / w/o rupture disk, Kd = 0.62 when PRV is not
installed
 Kc = 1.0 for PRV is not installed with rupture disk, Kc = 0.9
for PRV installed in combination with rupture disk

22. Calculation Methodology (cont’d)
 Calculate the relief discharge area (A) (cont’d)
 Kc = 1.0 for PRV is not installed with rupture disk, Kc = 0.9
for PRV installed in combination with rupture disk (cont’d)
 Sizing eq. for PRD for vap. & gas service based on the
following assumptions:
 Pressure-specific volume relationship conforms along the
isentropic path
 Assumption may not be valid for the following conditions:
 At very high pressures; and
 Gas or vapours approaching the thermodynamic critical locus
 Compressibility factor, Z, provides an indication whether the gas
or vap. may be in the above conditions, i.e. Z < 0.8 or Z > 1.1.
Refer to Annex B of API Std. 520 Part 1 9th Ed. (2013)

23. Calculation Methodology (cont’d)
 Calculate the relief discharge area (A) (cont’d)

24. Summary

25. References
 "Pressure Relief Valve Sizing Calculations". (2017). Pressure Relief Valve Sizing Calculations – Subcritical Gas
Flow Service. Retrieved January 12, 2017, from Engcyclopedia: http://www.enggcyclopedia.com/2011/11/pressure-
relief-valve-sizing-calculations-subcritical-gas-flow/
 API. (2013, December). API Standard 520 Part 1. Sizing, Selection, and Installation of Pressure-relieving Devices,
9th. Washington, D.C: American Petroleum Institute.
 Coker, A. K. (2006). Process Safety and Pressure-Relieving Devices. In Applied Process Design for Chemicals and
Petrochemical Plants (4th ed., pp. 575-578). Oxford, 1: Gulf Professional Publishing.
 Coker, A. K. (2006). Process Safety and Pressure-Relieving Devices. In Applied Process Design for Chemical and
Petrochemicals (4th ed., p. 580). Oxford: Gulf Professional Publishing.
 Crowl, D. A., & Tipler, S. A. (2013, October). Sizing Pressure-Relief Devices. Chemical Engineering Progress, pp.
68-76.
 Gas flow through nozzles - sonic chokes. (n.d.). The Engineering ToolBox. Retrieved January 18, 2017, from
Nozzles: Gas flow through nozzles - sonic chokes: http://www.engineeringtoolbox.com/nozzles-d_1041.html
 Hellemans, M. (2009). Terminology. In The Safety Relief Valve handbook: Design and Use of Process Safety
Valves to ASME and International Codes and Standards (1st ed., p. 44). Burlington: Butterworth-Heinemann.
 Hellemans, M. (2009). Terminology. In The Safety Relief Valve Handbook: Design and Use of Process Safety
Valves to ASME and International Codes and Standards (1st ed., pp. 34-35). Burlington: Butterworth-Heinemann.
 MOHIB. (2016, October 9). CHEMEWORK. Retrieved January 13, 2017, from FRESH WATER AND FUEL GAS
SYSTEM [Blog post]: http://www.chemework.com/2016/10/09/fresh-water-and-fuel-gas-system/
 Triyanto SR. (n.d.). Process Engineer. Retrieved December 26, 2016, from Built Up and Superimpossed Back
Pressure [Blog post]: http://process-eng.blogspot.my/2011/02/built-up-and-superimposed-back-pressure_2304.html
 Triyanto SR. (n.d.). Process Engineer. Retrieved January 1, 2017, from Accumulation and Overpressure [Blog
post]: http://process-eng.blogspot.my/2012/03/accumulation-and-overpressure.html
 Whitesides, R. W. (2008). PDH Course M112: Selection and Sizing of Pressure Relief Valves. PDHOnline.