Pressure relief device monitoring: how to detect releases, leaking and fugitive emissions — Part 2
Monday, 12 March, 2018
How to comply with environmental regulations and detect PRD malfunctions while minimising costs and cutting operating expenses.
As discussed in Part 1 of this article, pressure relief devices (PRDs) become one of the last lines of defence in keeping 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 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. There are various faults that can occur with PRDs that can go undetected if they are not monitored properly. For example:
- PRVs and PSVs can stick open or chatter around their setpoint, releasing fugitive emissions.
- Valves can be damaged by deposits and corrosion, causing them to be ineffective or stick open.
- Rupture discs, often installed upstream of a PRV or PSV to prevent corrosion, can have pinholes form over time, resulting in a pressure equalisation on each side of the disc, preventing the disc from rupturing when it should.
The effective monitoring of PRDs is therefore necessary to:
- detect when over-pressure releases occur
- detect faults that may occur with PRDs to ensure they are fixed.
The requirements for refineries, chemical plants and other industries are similar worldwide, with the main difference being the tolerated amounts for each type of pollutant released. The more stringent rules can be generalised with three simple requirements:
- Provide indication and location where a PRD event occurs through electronic monitoring.
- Measure the time and duration of the PRD event for recording and reporting.
- Notify the operator of the event so corrective action can occur.
Also, it is expected that the flare operates at all times when emissions may be vented to them, so quick identification of a PRD release is imperative.
Several industries are subject to tight regulations, going so far as to issue detailed requirements for specific units in a plant, such as:
- More stringent operating requirements for flare control to ensure good combustion. This is achieved by such measurements as:
- Measuring and monitoring the flow of waste gas going to the flare.
- Measuring and monitoring the content of the waste gas going to the flare.
- Measuring and monitoring any air or steam added into the flare.
- Emission control requirements for storage tanks, flares and delayed coking units at petroleum refineries.
- Pollutant monitoring around the plant fence line as a development in practices for managing emissions of toxic pollutants from fugitive sources.
- Elimination of exemptions during periods of start-up, shutdown and malfunction.
Most importantly, bypasses and discharges through PRDs are considered a violation of the law in many countries, requiring plants to monitor discharges of individual PRDs.
Historically, PRDs have been difficult to monitor because they are simple mechanical devices by design. Monitoring methods typically include manual inspection of telltale signs. For example, on PRDs releasing to the atmosphere, wind socks are often used to monitor releases.
In order to monitor with this method, it is common to use process instrumentation to observe pressure peaks and valleys around the pressure limit, temperature downstream and flow in the discharge header. However, this method cannot be used in enclosed systems. Plants monitor PRDs by observing process pressure, but when the pressure is close to the operating limit, the peaks and valleys make it difficult to determine when the PRD is actually opened or closed.
Unfortunately, these types of measurements are susceptible to false positives and inaccuracies and provide no insight into the health and status of the individual PRDs. Measuring flow in the discharge header does not show which PRD or PRDs were activated. Observing changes in the flare flame is also inaccurate and does not show which unit and which PRV caused the release.
A significant part of the difficulty when designing and installing a comprehensive monitoring system is that a typical plant will have several different PRD makes, models, sizes and operating pressures from various vendors. This can make it difficult to design a standardised monitoring system. The introduction of additional pressure, flow or temperature measurements in an existing plant also disrupts plant operation and the cost of laying new cables can be very high.
An effective way to monitor PRD activation and leakage
A very reliable, effective and economic way to monitor PRDs is to use wireless acoustic transmitters.
Process fluid flowing through valves and orifices generates acoustic waves in a wide and complex range of frequencies and magnitudes. A majority of the acoustic energy is in the ultrasound range, but some is also in the human audible range. Acoustic transmitters are able to detect ultrasound acoustic waves in the pipe wall as well as its temperature. These devices are wireless, small, lightweight and non-intrusive, so they do not require any change in plant installation. They can be easily clamped on the exhaust pipe, as shown in Figure 2.
PRD operating condition can be determined by the following (Figure 3):
- A noise level increase indicates that the PRD has been activated.
- Noise level returning to the previous level indicates that the PRD is no longer discharging.
- Noise level returning to a level above the previous level indicates leakage due to the valve not closing completely.
- Noise level changing continuously indicates that the valve may be simmering or chattering.
- Temperature changes may be used as an additional indication to validate a release.
Figure 3 illustrates the flow discharge followed by a temperature change.
Relief valve monitoring
Acoustic wireless transmitters should be installed downstream of the relief valve (RV), as close as possible to the valve. The wireless acoustic transmitter installed as indicated in Figure 4 monitors not only discharges or leakages of the relief valve, but can also monitor flow through the bypass valve.
Sometimes the wireless acoustic transmitter can measure noise originating in other parts of the process. If the background noise varies too much, it may be difficult to determine when there is a discharge. In this case, it may be necessary to install a second acoustic transmitter downstream or upstream of the first one to measure the background noise and subtract its signal value from the signal being measured by the PRD monitoring transmitter. The calculation is done in the host system.
Rupture disc monitoring
Some types of rupture discs are equipped with a burst detector that generates a discrete signal indicating disc rupture. There are also devices that can be installed on the rupture disc surface that can detect when the disc ruptures and indicate the event through a discrete signal. The discrete signal is usually wired back to a supervisory system or safety system. A wireless discrete transmitter can be used to transmit the discrete signal, eliminating costly and troublesome wiring, as indicated in Figure 5.
A more effective way to monitor rupture discs
Rupture discs can be better monitored with the use of a wireless acoustic transmitter, as indicated in Figure 6. The transmitter can detect when the disc ruptured and the duration of the discharge, as it does for relief valves, but it may also detect even small leaks caused by pinholes.
Monitoring a combination of relief valves and rupture discs
As discussed before, relief valves must be isolated from harsh process conditions by using rupture discs. In normal operation, the relief valve is not in contact with corrosive, gumming or hot process fluids. If the vessel pressure reaches unsafe values, the rupture disc bursts, followed by the RV opening.
If the rupture disc acquires a pinhole leak, the pressure between the two sides of the rupture disc will be the same, so the disc will not burst. Vent lines may be installed to release eventual leakage, but to be safe, standards and regulations ask for remote monitoring of the pressure in that space between the RD and the PRV.
A wireless pressure transmitter can provide accurate and reliable pressure measurement; however, monitoring the pressure between the RD and an RV is not sufficient to reliably determine when the RV has opened or closed, so still needs to be used in conjunction with a wireless acoustic transmitter downstream of the RV, as shown in Figure 7.
It should be noted that the rupture disc does not need to be replaced immediately after bursting, because the wireless acoustic transmitter is still monitoring pressure releases. This allows maintenance personnel to replace or maintain the equipment at the most convenient time, without having to slow or shut down the process.
Pressure relief device monitoring is necessary for environmental protection compliance and can help to avoid expensive fines, as well as possible process unit or plant shutdowns. Monitoring also prevents waste of costly material and energy, avoids bad publicity and helps improve plant personnel and neighbouring communities’ health. Wireless acoustic, pressure and discrete transmitters are a very effective, reliable, and economic way to have a compliant and better performing process.
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