PSV Sizing: A Lifecycle Approach to Asset Integrity

In this article:

1. Background

2. The Role of API RP 521 in PSV Sizing

3. PSV Sizing as a Lifecycle Discipline

4. Conclusion

   
   
   
   

   

   
   

Background

In industrial processing environments, despite robust engineering controls and operational procedures, pressure excursions beyond the design envelope can and do occur. These can be as a result of equipment failure, control loop malfunctions, utility loss, or human error. If not adequately managed, such excursions can lead to leaks, rupture, or loss of containment, posing significant risks to personnel safety, environmental protection, and asset integrity. To guard against these scenarios, the process and safety design phases typically incorporate multiple layers of protection, including pressure control systems, high-pressure shutdown logic, alarms, and interlocks. However, if any of these designed pressure protection systems fail, Pressure Safety Valves (PSVs) act as the final mechanical safeguard, providing a controlled and automatic release of excess pressure to prevent catastrophic failure.

The Role of API RP 521 in PSV Sizing

API RP 521 provides a structured approach to PSV sizing and relief system design. It details the key steps required to ensure the valve and its relief system are suitably sized and maintained to respond effectively during upset conditions throughout the asset’s lifecycle.

The three main steps involved in PSV sizing are:

1. Identification of Overpressure Scenarios

   
   

Identify all credible sources of overpressure, e.g. blocked outlets, fire, gas blowby, thermal expansion. Scenario identification may have been discussed during a HAZOP or LOPA study, typically under ‘More Flow’ or ‘More Pressure’ guidewords. However, a dedicated review using the guidance provided in API RP 521 is recommended as a standalone activity to understand the system and consider all applicable overpressure scenarios, using the latest as-built documentation. This should evaluate other phases of operation (e.g. start-up, shutdown and non-routine operations) where process conditions such as flow rate, temperature or pressure can vary considerably from normal operations.   

2. Relief Load Calculation  

   
   

For the scenario(s) identified, calculate the relief rate using conservative ‘worst-case’ assumptions. These can include maximum upstream pressures, worst-case temperatures or compositions, credible worst-case valve positions, and actual fluid properties generated at the specified fluid conditions. The calculation basis and assumptions used should be clearly documented to ensure transparency and assist with future updates during Management of Change (MOC).

3. Relief Valve Sizing

   
   

The relief valve is sized to handle the calculated (required) flow, established in Step 2. For the scenario this corresponds to, the calculation is carried out for the fluid at relieving conditions, considering parameters such as fluid phase and backpressure, along side API-standard coefficients. Preliminary selection of an orifice area (greater than the required orifice area) typically uses the API RP 520 Part 1 methodology and letter-designated data table. For final selection and sizing, ASME-certified coefficients and valve capacities are used, with certification in mind.

The use of suitable process simulation software can enable accurate system modelling using the latest process data, and offer in-built relief valve sizing calculation tools based on API or ISO standards. This can also be used to analyse the overall relief and blowdown system, enabling assessment it’s effectiveness in a variety of relief or blowdown scenarios, and provide inputs for thermal radiation and dispersion analysis.

PSV Sizing as a Lifecycle Discipline

The sizing of a PSV should be considered as an ongoing engineering responsibility and such, requires quality control throughout the lifecycle of the device – from initial design to maintenance:

• Design: Validate overpressure scenarios and relief loads, check against API RP 521, and consider dynamic conditions such as multi-phase flow or foaming;

• Procurement: Ensure correct set pressures, materials of construction, and certifications per service conditions;

• Installation & Commissioning: Confirm installation details match design, i.e. orientation, accessibility, and connection to the correct relief system;

• Operations & Maintenance: Regular functional testing and inspection to safeguard against fouling, corrosion, or drift in set pressure;

• Management of Change (MOC): Any process modification (e.g. capacity increase, fluid change, control logic updates) should be accompanied by a PSV review to ensure continued adequacy.

Conclusion

PSV sizing plays an essential role in protecting process assets from overpressure events and ensuring long-term reliability of pressure systems. While well-designed control logic and shutdown systems act as primary safeguards, the PSV provides a last-resort mechanical protection if those measures fail.

The methodology provided in API RP 521 provides guidance on credible overpressure scenarios and conservative assumptions to help determine relief loads. However, PSV sizing should be managed throughout the valve lifecycle to ensure protection continues to be optimal for changing process conditions.

For operators and engineers, the key question isn’t just “Is the PSV sized correctly?” — but “How confident are we that it still is?”