Despite safety precautions in design and operation, equipment failure, human error, control loop failure, external events etc. can lead to increases in process pressure beyond the design envelope. Pressure excursions above design pressure could potentially result in leaks or rupture and consequential loss of containment.
A typical safeguard against overpressure is to install a mechanical relief device (Pressure Relief Valve (PRV), Pressure Safety Valve (PSV), or rupture disc) that activates at a specific set pressure and relieves the overpressure to a flare or vent system. Depending on the design codes and credible pressure excursions, a mechanical relief device is typically installed as a secondary barrier against overpressure. A process shutdown on high pressure is the primary barrier.
Even though mechanical relief devices are essential safety measures in the process industry, numerous accidents have occurred due to improper sizing, wrong design, management of change (MOC) deviations, errors during installation, or errors during the maintenance of relief device systems.
When designing and sizing a mechanical relief system, it is imperative to remember that the relief system consists of the relief device and the entire piping system including the piping from the process being protected to the relief device (lead pipe), and the piping from the relief device (tail pipe) to the disposal system (flare or vent). Equally important is to size the lead pipe and tailpipe to cater to the relief conditions as the sizing of the relief device itself.
The sizing of the relief device consists of three main steps:
Identification of relief scenarios and requirements for mechanical relief devices.
Establishing relief rates for relevant relief scenarios identified in item 1.
Sizing of the relief device based on established relief rates in item 2.
Step 1 – Identification of Relief Scenarios
The most critical step in relief device sizing is establishing the relevant sizing cases or relief scenarios. API 521 and ISO ISO 23251 provides a good overview of potential overpressure scenario for oil & gas applications and many are also relevant to the general process industry. Documentation of relief scenarios applied for sizing is often deficient. Many relief device sizing files describe only the governing sizing case with a single word such as “Fire” or “Blocked outlet” in the sizing sheet or datasheet. The process of identifying the relief scenarios must be adequately documented to substantiate that all relevant relief scenarios have been identified.
A Hazard and Operability (HAZOP) study is an efficient activity for identifying relief scenarios. Overpressure scenarios that could be dimensioning for relief device sizing are likely to be identified under the prompts “More Flow” or “More Pressure.” On the other hand, a pitfall with HAZOP is to use the HAZOP only to confirm that a PSV has been implemented (existing safeguard), without going into detail on the dimensioning scenario for a relief device.
Another essential aspect when considering relief scenarios is to evaluate if some or all of the relief scenarios could be eliminated by implementing an inherently safer design (ISD). In this case, a mechanical relief device is either not required or can be reduced in size.
Step 2 – Establish Rates for Relevant Relief Scenarios
When Step 1 is completed, the relief rates associated with the identified relief scenarios can be calculated. Credible worst-case conditions must be applied for calculating the relief rates, such as maximum possible upstream pressure, worst-case fire, and worst-case fluid composition, as well as the credible position of other valves in and out of the device to be protected.
Assumptions for calculating the relief rates and the calculations must be documented in detail. The assumptions are typically not adequately documented in existing relief device files.
Step 3 – Sizing of the Relief Device
The last step is the sizing of the relief device itself. The sizing typically follows a standard approach commensurate with API or ISO standards. For multiphase service or foaming systems etc., the sizing becomes more challenging. However, step 3 is regarded as the least complicated step in relief device sizing design. On the other hand, it is also the step typically given the most attention in relief device sizing files as it is delivered by the relief device vendor.
In light of the steps for sizing the relief device outlined above, a question worth asking is: How do you perform and document the sizing of the relief device in line with steps 1 and 2?
