Pressure relief device monitoring: How to detect releases, leaking and fugitive emissions — Part 1
Monday, 12 February, 2018
This article outlines how to comply with environmental regulations and detect PRD malfunctions while minimising costs and cutting operating expenses.
Every country has regulations and engineering specifications to protect industrial plants and facilities against overpressure in various processes and operations. Insurance companies and government agencies rely on the observance of these regulations and specifications to determine if designs are correct, and if operations are being conducted correctly.
Enforcement is done by local environmental and occupational safety regulatory agencies that were created to protect health and the environment by writing and enforcing regulations. New fugitive emissions regulations worldwide are growing more stringent, requiring rigorous monitoring of pressure relief devices (PRDs) and bypass valves. They also require better control of flares and air concentration monitoring at the plant fenceline.
Pressure relief devices
The purpose of a process plant control system is to keep process variables at the desired operating point and within safety limits. However, control systems may not be able to handle all process upsets, so operator intervention, safety instrumented systems and PRDs become the last lines of defence. One of the main safety concerns is to keep process pressure within the limits tolerated by vessels, pipes and valves.
PRDs can be pressure relief valves (PRVs), pressure safety valves (PSVs) or rupture discs (RD). They activate when the pressure gets too close to the maximum allowable working pressure (MAWP) of the vessel or process component. As per regulations, all PRDs must be mechanically powered by the process itself, so they do not require external power or intervention to function.
Traditionally, PRDs have a simple mechanical design to ensure reliability under all foreseeable conditions. Excessive pressure in a pressurised system is relieved by blowing process fluid (gas or liquid) to the environment, or to a closed recovery system.
Ideally, hazardous materials being relieved by a PRD should be routed to an enclosed recovery system to be treated and properly disposed of, or neutralised through combustion in a flare system. However, this is not always the case, with many PRDs releasing process fluid directly into the environment. Regardless of whether the PRD releases to an enclosed recovery system or to the environment, or is handling hazardous area pollutants (HAP) like H2S or more benign fluids such as steam, it is important to identify the source, time and magnitude of the release. PRDs releasing to the atmosphere can create explosive and toxic emergencies.
Flare systems are the most commonly used method of neutralising hazardous discharges, but are not perfect. Fast transients caused by sudden fluid composition and volume changes can still cause releases of unburned hazardous material. Additionally, in a closed recovery system it can be difficult to locate the source in order to take corrective action.
In addition to potential environmental and safety concerns, process upsets causing overpressures can affect production and uptime, negatively impacting profitability. A PRD is sometimes the only indicator of process upsets, so the sooner a PRD event can be detected, the sooner operators can respond to the root cause.
As previously stated, there are three main types of PRDs: pressure relief valves, pressure safety valves and rupture discs. The term PRV or relief valve (RV) is generically used for both PRVs and PSVs; however, these two devices have different working principles.
PRV basic operating principles
PRVs are safety devices protecting a vessel against overpressure. Figure 1 shows a typical spring-loaded PRV. The disc between the process side (inlet piping) and the discharge side (discharge piping) is pushed against the seat by a compression spring. The spring force determines the PRV set pressure and it is adjusted by the compression nut during calibration and certification.
When the spring force exceeds the force resulting from the process pressure and the pressure in the discharge side (backpressure), the disc blocks the flow from the process side to the discharge. When the process pressure exceeds the valve set pressure, the disc pushes the spring, opening the valve and forcing the process fluid to the discharge pipe. The valve will remain open until the process pressure drops approximately below 95% of set pressure. The 5% deadband, also known as ‘valve blow down’, prevents the valve from chattering when the process pressure varies close to the valve setpoint.
Unaccounted discharges also occur when the valve chatters. This happens when the vessel pressure oscillates around the PRV setpoint with an amplitude larger than the deadband. Chattering occurs when the valve is not specified correctly or the piping was not designed properly.
The valve opens proportionally to the excess pressure, and returns to the closed position when the process pressure returns to normal. There are more sophisticated types of PRVs, but the basic working principle is the same. In the relief valve calculation, it is necessary to take into account the pressure on the discharge side. In enclosed recovery systems, sometimes there is a back pressure build-up caused by relief of other PRDs in the discharge header.
When things don’t work as expected
Sometimes, when the process pressure returns to normal conditions, the PRV does not close completely. There are several reasons for this:
- Pressure increase on the discharge side
- Valve seat damaged after repeated actuations
- Deposition or formation of solids between the disc and the seat
- Altered process fluid
- Mechanical malfunction
Even a small leakage (0.1% from the PRV flow area) can cause losses of tens of thousands of dollars per year. Additionally, the leakage can cause significant emissions violations, resulting in expensive fines and even required shutdowns.
Pressure safety valves
This device is commonly known as a ‘pop valve’ because it opens completely and rapidly when the pressure exceeds the setpoint. The valve will remain open until the process pressure drops to approximately 95% of set pressure. These valves are mostly used for gas and steam.
PSVs are slightly different than PRVs. The disc blocking the nozzle has a smaller area and is contained in a larger diameter chamber. When the pressure exceeds the setpoint, the stem starts to lift, allowing the process fluid to flow to the chamber.
As the chamber area is much larger than the one exposed by the disc, the uplifting force is much larger than the spring force and the valve opens completely, as compared with a PRV in which the amount of opening is proportional to the pressure differential. With the discharge, the pressure reduces in the chamber and the valve closes. If the process pressure is still above the setpoint, the valve keeps popping open until the pressure returns to normal levels.
When the process pressure fluctuates around the PSV setpoint value, the blocking disc will lift to allow the chamber to fill and lift the stem. The process fluid vents to the discharge pipe, reducing the pressure but not opening the valve completely. This process is called simmering and occurs frequently. Simmering can also cause material build-up on the disc seating and stem misalignment, which prevents the valve from closing completely. The discharge caused by simmering and its side effects are not usually detectable by conventional methods.
PSVs (Figure 2) are commonly equipped with a lever so an operator can initiate a manual release. This is useful to test the valve, clean possible scale or solids deposited on the seat surface, and deal with special process conditions during startup or during shutdowns.
Rupture discs (Figure 3) are safety devices for one-time use. They consist of a membrane that bursts when the differential pressure between its two sides exceeds a set value. These devices are used alone or in combination with a PRV, providing a physical isolation layer between the process and the relief valve, especially on processes containing highly corrosive fluid. Some models are equipped with a sensor that indicates when the diaphragm is broken.
Rupture discs are very simple devices, with no moving parts. Unlike pressure relief or safety valves, the rupture disc will remain open until the ruptured diaphragm is replaced. Diaphragms are less susceptible to causing fugitive emissions, but there is always the possibility of pitting corrosion which creates pinholes, leading to undetectable leakage.
Safety relief devices require shutoff valves and a bypass valve as shown in Figure 4. These valves are used for device maintenance and special process conditions. If a rupture disc diaphragm has to be replaced, for example, the device has to be isolated using these valves. In some cases such as during startup, shutdowns, tests or load changes, it may be necessary to bypass the PRD.
It is not uncommon for plant personnel to forget and leave these valves in the open position or not close them properly, causing process fluid losses and emissions that can go undetected for a long time. Monitoring bypass valve position enables quick response to human error or defective equipment.
RV with rupture disc
In some applications, it is necessary to use a rupture disc installed upstream from the RV (Figure 5). The main reasons for this are:
- The rupture disc can prevent fugitive emissions through the RV.
- The rupture disc protects the RV against corrosive process fluids. The RV may not be available with materials required for long-term resistance to the process fluids, or it may be too expensive to provide one that is resistant. The rupture disc diaphragm works as a shield between the process and the relief valve.
- The rupture disc protects the RV against solid particles. These particles can damage or prevent the RV from working properly, failing to open or remaining open after a release.
- The rupture disc protects the RV against frozen vapours, material polymerisation, hydrate formation or other problems that may prevent it from working properly.
It is important to note that if the rupture disc diaphragm has a pinhole leak caused by corrosion or other adverse conditions, the pressure between the rupture disc and the RV will be equal to the process pressure. Therefore, the pressure differential on the rupture disc will be always zero — it will never blow, even if the process pressure exceeds its limit. Therefore, the leakage caused by the pinhole goes to the discharge line and can go undetected for a long period of time.
To avoid this problem, a vent line is often installed (L1 in Figure 5) to keep the pressure between the disc and the valve equal to the discharge line pressure.
In Part 2
In Part 2 of this article, we will examine the most effective ways to monitor relief devices for releases, fugitive emissions and failures.
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