Pressure Relief Systems
Pressure Relief System is a primary process system in oil and gas facilities requiring careful attention. Effective pressure relieving systems help risk management targets, compliance requirements and business objectives.
Common Equipment
- Protected Equipment
- Emergency Shut Down Valves
- Depressurization Valves
- Pressure Safety Valves (PSV)
- Pressure Safety Valves Inlet and Discharge Piping
- Flare Header
- Flare Knock Out Drum
- Flare Stack / Tip
Typical Purposes
The primary purpose of the pressure relief system is to ensure that the operation’s personnel and equipment are protected from overpressure conditions that happen during process upsets, power failures, and from external fires. In some locations and facilities, it is accepted practice to vent the pressure safety valves directly to atmosphere provided the process fluid is discharged at sufficient velocity to ensure good dispersion and that the fluids molecular weight is lighter than air.
Common Parts of the System
Pressure Safety Valves
The purpose of a pressure safety valve is to protect equipment and / or piping from any possible overpressure scenario. There are multiple industry recommended practices and standards that govern the sizing, selection and installation of pressure safety valves.
PSV Inlet and Discharge Piping
Another area that requires close attention is the proper design of the inlet and discharge piping of the pressure safety valves. API Recommended Practice 520, 5th Edition, Part 2 [7], and ANSI / API Standard 521 [5] provide guidance on the installation and design of the inlet and discharge piping for pressure safety valves.
For inlet piping to pressure safety valves, the recommended practice is to maintain the inlet hydraulic losses at no more than 3% of the set pressure of the pressure safety valve. This is because the relief valve is designed to normally close at 97% of the set pressure. A PSV with no inlet flow will sense the same pressure as exists in the protected equipment. Once open however, the pressure at the inlet to the relief will be the pressure at the protected equipment minus the friction loss in the inlet line. If this friction loss exceeds 3%, the valve will close and then reopen once the flow stops. This chattering can destroy the valve. Over sizing of a pressure safety valve can also result in “chatter” from essentially the same phenomenon. There is a potential for pressure relief valve or piping failure from prolonged “chattering” due to mechanical fatigue and potentially thermal fatigue.
Flare Header Design
If the pressure safety valve discharges into a flare header the superimposed and built up back pressure is critical and can impact the valves relieving capacity if the actual back pressure is higher than the originally calculated or assumed back pressure. The maximum allowable back pressure at which a pressure safety valve can function properly depends upon the type of the pressure safety valve.
Flare KO Drum and Flare Stack / Tip
Flare KO Drum and Flare Stack sizing is also critical to the safety of the plant. Oil and Gas Industry Flares are designed to destroy vapor streams only and require an adequately sized Flare KO Drum to prevent flammable liquids from raining out of the flare tip. In determining the sizing, it is important that a Flare Study be conducted to determine the worst case scenario for Flare KO Drum and Stack capacity and to select the proper droplet size separation criteria that the selected flare tip can adequately destroy. ANSI / API Standard 521[5] provides guidance on sizing, design and selection of this equipment.
Depressurization Valves
In the gas processing industry, it has become a standard practice to block in the treating facility with Emergency Shut Down (ESD) Valves rather than depressure the entire facility to the flare. One primary reason for this philosophy is that natural gas fires are not equivalent to liquid hydrocarbon pool fires. Natural gas fire protection and mitigation requires different protection methods than for those used for fighting liquid hydrocarbon pool fires, which can be extinguished using a fire water system or a foam system. . It is standard natural gas industry practice to isolate the hydrocarbon gas sources to the facility and evacuate all personnel from the facility. Once the source of the gas is isolated, the feed to the fire is terminated and the fire is quickly extinguished from lack of fuel.
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