Safety is the most important factor in designing a process system. Some undesired conditions might happen leading to damage in a system. Control systems might be installed to prevent such conditions, but a second safety device is also needed. One kind of safety device which is commonly used in the processing industry is the relief valve. A relief valve is a type of valve to control or limit the pressure in a system by allowing the pressurised fluid to flow out from the system.

1. Pressure Relief Valve
Hector Nguema Ondo

2. Pressure Relief valves
• Safety is the most important factor in designing a process system.
LOPA

3. Pressure Relief valves
Define Protected System:
Design considerations
Important an understanding of the operation. One relief valve
can protect more than one equipment
Locate Relief Devices:

4. Pressure Relief valves
Define overpressure scenarios
Design considerations
• Double jeopardy design not required
• Latent failures
• Overpressure protection and instrumentation
MEGPL Basis (Z0210-09-PE-RT-00001-00)
• HP Flare system: 900 MMSCFD with back-pressure of 75 Psig
• LP Flare system: 40 MMSCFD with back-pressure of 15 Psig
• Propane Flare system: 20-22 MMSCFD with no back-pressure
• Butane Flare system: 29-36 MMSCFD with no back-pressure
“Alba_Hysysrun.HSC”
(Dated 08/27/2009).
MEGPL Considerations
 Fire - The relieving pressure is 121% of MAWP
 Operating - The relieving pressure is 110% of MAWP

5. Pressure Relief valves
Causes of overpressure
A. Blocked Discharge
Design considerations
Max design pressure:
1350 Psig @150F

6. Pressure Relief valves
B. Fire Exposure
Design considerations
Max design pressure:
1350 Psig @150F

7. Pressure Relief valves
Design considerations
C. Check Valve Failure
Causes:
• Stuck Open
• Broken flapper
• Check valve seat leakage

8. Pressure Relief valves
Design considerations
D. Thermal Expansion

9. Pressure Relief valves
Conventional pressure relief valve
Type of relief valves
Working principal of the conventional relief valve is the inlet pressure to the valve is directly opposed by a spring force
Conventional Pressure Relief Valve
Pro:
 Wide range of materials
 High Temperature version available
 Lowest cost
Contra:
 Prone to leak
 Long simmer > 95% valve opening
 Sensitive to back-pressure, < 10%
 Vulnerable to inlet pressure lost

10. Pressure Relief valves
Type of relief valves
Balanced Relief Valves
Pro:
 Better handling of corrosive fluids.
 Less sensitivity to back-pressure
 Wide range of materials
 High Temperature version available
Contra:
 Prone to leak
 Long simmer > 95% valve opening
 Higher maintenance cost
 Limited bellows life

11. Pressure Relief valves
Type of relief valves
Pilot Operated Relief Valves
• Low pressure applications for tanks
Pro:
 Smaller & Lighter than other valves
 Easily testable on site
 Not affected by back-pressure
 Can address high inlet losses
Contra:
 Dirty service limitations
 When soft-seat or O-ring is used, limited
chemical compatibility

12. Pressure Relief valves
• Sizing for liquid service
• Sizing for gas service
Sizing relief valves

13. Pressure Relief valves
• Designed pressure relieving devices should be certified and approved under Code.
ASME- Boiler and Pressure Vessel Code Section I, Power Boilers, and Section VIII, Pressure Vessels.
ASME- Performance Test Code PTC-25, Safety and Relief Valves.
ANSI B31.3, Code for Petroleum Refinery Piping.
Application of Codes, Standard and Guidelines
• API are recommended practices for the use of Safety Relief Valves in the petroleum
and chemical industries are:
API Recommended Practice 520 Part I - Sizing and selection of components for pressure
relief systems in Refineries.
API Recommended Practice 520 Part II - Installation of pressure relief systems in
Refineries.
API Recommended Practice 521 - Guide for Pressure-Relieving and Depressuring
Systems.
API Recommended Practice 527 - Seat Tightness of Pressure Relief Valves
API Standard 526 - Flanged Steel Pressure Relief Valves
API Standard 2000 - Venting Atmospheric and Low-Pressure Storage Tanks: Nonrefrigerated
and Refrigerated
API Standard 2001- Fire Protection in Refineries.

14. Pressure Relief Valves
Alternative relief path (ARP)

15. Questions?
Hector Nguema Ondo