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CIC - Pressure Relief Systems
DEFINITION AND SCOPE
In this document, the term "Pressure Relief Systems" is defined as a combination of:
- One or more pressure relief devices (PRD)
- The inlet piping, i.e., the piping from the protected vessel to the inlet of the pressure relief device
- The outlet or discharge piping, i.e., the piping from the outlet of the pressure relief device to the atmospheric venting point, the flare, the blow down tank or any other system designed to handle the relief flow
Pressure relief devices protect a vessel against overpressure. A pressure relief device can be a pressure relief valve or a rupture disk.
A pressure relief valve is designed to automatically reclose and prevent the flow of fluid when pressure has dropped below the set pressure. There are various types of pressure relief valves: spring-loaded pressure relief valves, safety (relief) valves, balanced pressure relief valves, pilot-operated pressure relief valves. A description of these types of pressure relief valves is beyond the scope of this document and can be found in literature. The criteria in this bulletin apply to all types of pressure relief valves.
A rupture disk device is a non-reclosing differential pressure relief device actuated by inlet static pressure and designed to function by bursting the rupture disk that is installed in a rupture disk holder. The rupture disk device includes the rupture disk and rupture disk holder.
A pressure relief system may consist of only one pressure relief valve or rupture disk device, either with or without discharge pipe, on a single vessel or line. A more complex system may involve many pressure-relieving devices discharging into common headers to terminal disposal equipment.
Flare systems are not included as part of this CIC, even though they are normally regarded as a part of the pressure relief system.
Figure 1. Reactor with a pressure relief system (U.S. Chemical Safety Board, 2018)
THE PURPOSE OF PRESSURE RELIEF SYSTEMS
A pressure relief system is normally installed where it is necessary to avoid that the pressure in a plant, specific installations, or specific equipment exceeds limits for safe operation of the plant and affected equipment. The system prevents escalation of incidents by rapid pressure reduction and ensures that released substances are removed from the process in a safe manner without causing damage to personnel and equipment. Protection against overpressure is recognized as an important part of process design.
Overpressure can occur due to any number of conditions. Inadvertent valve opening, closed or blocked outlets on vessels, utility failure, excess heat, transient pressure surges, and fire are all examples of situations that may cause overpressure. Pressure relief valves are typically associated with boilers and pressure vessels and storage and transport tanks.
IDENTIFICATION & DOCUMENTATION
For each pressure relief valve or rupture disk device the operator should have a specification sheet.
Templates of these specification sheets can be found in literature and typically contain information such as:
- Identification of the components (item number)
- Identification of each relief position (e.g., the tag number on a P&ID)
- The service conditions (liquid, vapour, two phase flow, temperature, back pressure,...)
- Orifice area
- Operating pressures
- Design code
- Construction materials
- Type of pressure relief valve or rupture disk
EFFECTIVENESS, SIZE & CALIBRATION OF THE PRESSURE RELIEF SYSTEM
The operator should be able to demonstrate that design and dimensioning of the pressure relief system is based on a pressure relief analysis. This analysis should be appropriate for the expected fluid phase (liquid, vapour, or two-phase flow).
The overpressure scenarios for which the PRS serves as a risk reducing measure should be listed. For each scenario, the relief flow (to be discharged) should be calculated. The PRS should be sized for the scenario with the largest relief flow.
Using the largest relief flow, the minimal discharge area of the PRD is obtained. The PRD that are installed should have an orifice area equal or larger than the minimal area required (for the 'largest' scenario).
Calculation should take into account phenomena such as the pressure losses in the inlet and outlet piping and any backpressure present in the blow down system (if present).
RISKS INTRODUCED BY THE PRESSURE RELIEF SYSTEM
When a pressure relief system discharges to the atmosphere, a hazardous situation can occur, depending on the properties of the substances released. For instance, a toxic or explosive cloud can be formed. The operator should assess the risks involved with potential substance releases. The analysis typically involves dispersion calculations. If the risks are deemed unacceptable, measures should be taken to reduce the risks, such as, discharging into a blowdown vessel or to a flare, relocating the atmospheric venting point to a safer location, implementing additional measures to reduce the likelihood of the PRS being activated (e.g., via safety instrumented systems), etc.
Another risk typically associated with PRS discharges to the atmosphere may be caused by reaction forces imposed on outlet piping. The calculation of these forces should be a part of the sizing calculations. The operator should be able to demonstrate that the outlet piping (and more specifically, its supports) can withstand the reaction forces.
Discharging gases under high pressure or liquefied gases can lead to a considerable drop in temperature and hence, to brittle fracture in the outlet piping and blow down system. The calculation of the pressure drop should be part of the sizing calculation. The operator should be able to demonstrate that the materials used for the outlet piping and the blowdown system are suited to handle these low temperatures.
PREVENTION OF DAMAGE TO THE PRESSURE RELIEF SYSTEM
The pressure relief system also should be protected from potential damage during normal operations. For example, many process materials often build up and solidify on internal surfaces. This can create a problem for pressure relief systems such that they may become partially or completely blocked.
Corrosion can also compromise the proper operation of safety valve, causing the disk to stick at the seat or causing leaks between disk and seat.
The operator should have evaluated whether such harsh operation conditions are present and should have taken the appropriate measures in place to guarantee the reliable operation of the pressure relief device, such as:
- installing heating or tracing systems) to prevent freezing or solidification
- protection of a pressure relief valve by a rupture disk
- installation of pressure relief devices directly on a vessel (eliminating the need for inlet piping)
- Increasing the inspection and maintenance intervals
PRESSURE BUILD UP BETWEEN THE PRESSURE RELIEF VALVE AND RUPTURE DISK
When a pressure relief valve and a rupture disk are installed in sequence, an enclosed space is formed between the (outlet of the) bursting disk and (the inlet of the) pressure relief valve. Small leaks (pin holes, cracks) in the rupture disk can lead to a pressure build up in this enclosed space. The backpressure in this space will render the bursting disk (that is still largely closed) nonoperational. The bursting disk is activated by the pressure differential over the rupture disk. Due to the backpressure, the pressure differential at which the rupture disk should burst will not be attained when the maximum allowable pressure in the vessel safeguarded by the PRS is reached.
This phenomenon requires installation of a pressure indicator that monitors the space between the rupture disk and the pressure relief valve. The pressure indicator should preferably give an alarm when backpressure is detected. If not, the indicator should be regularly inspected (each shift). Operators must be aware that a high pressure reading requires urgent action.
Figure 2. Diagramme showing the enclosed space between the bursting disk and the pressure relief valve
DEACTIVATION OF THE PRS
A vessel or pipe that is protected by a PRS should never be disconnected from the PRS while it is in operation (and the risk of overpressure is present).
To allow maintenance and inspection of PRD some pressure systems are equipped with two or more (redundant) PRD. This configuration allows one PRD to remain in service (connected to the pressure system) while the other PRD can be shut off by a hand valve or 3-way valve.
A system should be in place to guarantee the right position of the valve in the inlet or outlet piping, making sure that at least one PRD is always lined up with the pressure system under protection.
INSPECTION AND MAINTENANCE
The operator should establish a periodic inspection and maintenance system with all activity documented. Each pressure relief valve should undergo:
- a periodic inspection and maintenance (testing and recalibration) of each pressure relief valve
- a visual on-stream inspection
Periodic maintenance of a safety valve involves removing it from the installation and transporting it to a maintenance shop. It is strongly recommended to test the pressure relief devices in the maintenance shop before disassembly. In this way, it can be tested whether the pressure relief valve opens at the set pressure. (This is sometime referred to as the 'pretest' or the 'as received pop test'.) If a pressure relief valve fails to open at a pressure level above the set pressure (e. g., at 110% of the set pressure), the pressure relief valve is considered to have failed the test. An inquiry to the possible causes then should be executed. If the pressure relief valve fails the test repeatedly, it is a strong indication of a structural problem (e. g., corrosion, sticking, etc.).
After maintenance and reassembly of the pressure safety valve, the pressure safety valve is tested in order to verify:
- The opening pressure (the pop test)
- The tightness of the seat of the safety valve (to make sure it does not leak while in service).
Visual on-stream inspection (while the safety relief valve is in service) should ensure that:
- The relief device does not leak
- Bellows vents are open and clear
- Upstream and downstream block valves are sealed, or chained and locked, in the proper position
- Discharge piping is properly supported to avoid breakage or leakage
- Valve body drains and vent stack drains are open
- Any heat tracing or insulation critical to the proper operation of the relief system is intact and operating properly
- Any rupture disk is properly oriented
The pressure indicators and alarms monitoring the space between the rupture disk and the safety valve should also be periodically inspected.
ANNEX OF POTENTIAL INSPECTION QUESTIONS, VERIFICATION ITEMS AND OTHER ADVICE WHEN INSPECTING PRESSURE RELIEF SYSTEMS
Download Annex of potential inspection questions, verification Items and other advice when inspecting Pressure Relief Systems
A. Questions that inspectors can use addressing Pressure Relief Systems
- Describe the plant's pressure relief system and the philosophy behind it
- Describe how the different components of the pressure relief system are covered in the maintenance and test programmes, and how this is documented. (Historic information is important).
- Before maintenance, are pre-tests conducted in order to determine opening pressure (after being in service since the last inspection)?
- Explain the routines for the handling of deviations from test- or maintenance frequencies.
- How do you ensure that changes in the system do not not go beyond the design basis? Routines for this?
- Do all components in the pressure relief system have a unique identification?
- Describe routines and practices regarding temporary disconnection of parts of the Pressure Relief Systems
- What do you consider as the most critical part of the Pressure Relief System and why?
- Describe periodic checks of:
- inlet and outlet piping (fixation, corrosion,...)
- tracing (if present)
- indicators or alarms
- How do you ensure that people working in the plant understand the importance of a well-functioning pressure relief system?
See also section 4 for ideas on what to ask for.
B. Possible verification issues
- Check identification of the equipment on site. Does this align with to what you have been told?
- Select some tag-numbers and check the different documentation for the maintenance and testing of this equipment. If set frequencies are not followed, is this treated as a non-conformity and is the possible negative effect of this change evaluated?
- Look at reports from the verifications and inspections of notified bodies.
- Check out a rupture disc that has been changed at one point in time.
- Is the replacement disc according to the original specification?
- Are the materials suited for the working temperature and the atmosphere in which the rupture disc is located?
- Is the bursting pressure less or equal to the design pressure of the vessel?
- Is the rupture disc is mounted before a safety valve? Is it of a non-fragmenting type and is there pressure monitoring between the rupture disc and the safety valve?
See also section 4 for ideas on what to ask for.
If you have any suggestions to add to these questions and tips, please contact JRC-MINERVA-INFO@ec.europa.eu
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