Some processes have a relief system that includes a rupture disk below the relief valve. This is done for several reasons, but the two most common reasons are:
- The process is corrosive and can cause issues with the Relief Valve leading to failures and a shortened life span
- The process contains a Category M fluid and any leakage, ever how minor can lead to serious hazards
But whatever the reason is for this design, it comes with some very specific design requirements that MUST be incorporated into the RV Design Basis. Using ASME requirements, I hope to explain some of the items that we need to look for in the design basis to ensure a PROPERLY DESIGNED and FUNCTIONING relief system and to meet the minimums put forth by OSHA’s PSM and EPA’s RMP.
We start with ASME Section VIII, Division 1 code requirements…
Over Pressure Protection
UG-125 General
(e) Pressure relief valves or nonreclosing pressure relief devices may be used to protect against overpressure. Nonreclosing pressure relief devices may be used either alone or, if applicable, in combination with pressure relief valves on vessels.
NOTE: Use of nonreclosing pressure relief devices of some types may be advisable on vessels containing substances that may render a pressure relief valve inoperative, where a loss of valuable material by leakage should be avoided, or where contamination of the atmosphere by leakage of noxious fluids must be avoided. The use of rupture disk devices may also be advisable when very rapid rates of pressure rise may be encountered.
UG-127 Nonreclosing Pressure Relief Devices
(a) Rupture Disk Devices
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(3) Application of Rupture Disks
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(-b) A rupture disk device may be installed BETWEEN A PRESSURE RELIEF VALVE AND THE VESSEL provided:
(-1) the combination of the pressure relief valve and the rupture disk device is ample in capacity to meet the requirements of UG-125(c);
(-2) the marked capacity of a pressure relief valve (nozzle type) when installed with a rupture disk device between the inlet of the valve and the vessel shall be multiplied by a factor of 0.90 of the rated relieving capacity of the valve alone, or alternatively, the capacity of such a combination shall be established in accordance with (-3) below;
(-3) the capacity of the combination of the rupture disk device and the pressure relief valve may be established in accordance with the appropriate paragraphs of UG-132;
(-4) the space between a rupture disk device and a pressure relief valve shall be provided with a pressure gage, a try cock, free vent, or suitable telltale indicator. This arrangement permits detection of disk rupture or leakage.
(-5) the opening provided through the rupture disk, after burst, is sufficient to permit a flow equal to the capacity of the valve [(-2) and (-3) above], AND there is NO CHANCE OF INTERFERENCE WITH PROPER FUNCTIONING OF THE VALVE; but in no case shall this area be less than the area of the inlet of the valve unless the capacity and functioning of the specific combination of rupture disk device and pressure relief valve have been established by test in accordance with UG-132.
Let’s breakdown (-1) through (-5) requirements:
(-1) is just stating that the combination of the disk and valve will in NO WAY reduce the size of the RV design
(-2) is the 90% safety factor that must be incorporated into this design; in other words, the combination is assumed to have a capacity equal to 90% of the relief valve rated capacity.
(-4) Since the rupture disk reacts to the differential pressure between the upstream and downstream side, we MUST avoid any pressure in the space between the rupture disk and relief valve inlet. Any pressure in this space MUST either be recognized and/or vented to the atmosphere. The most common means to comply with this critical requirement is a telltale pressure gauge or indicator is the most common means used. If the material that could pass through the RD/RV is non-hazardous, we can use a “free vent” to prevent any back pressure from building up on top of the RD. In this and age, the newest design incorporates a pressure switch or transmitter that provides an alarm to operations personnel so they may address the deviated condition. If we use the local tell-tale gauge then there MUST be some frequency of inspection of these gauges so as to recognize pressure is present on top of the RD. This frequency is highly debated, but a facility must be able to defend their decision.
(-5) This is where we get into the type/design of the RD. First – the RD must be NON-FRAGMENTING type RD, so as to prevent any parts of the RD from entering into the RV and potentially preventing the RV from closing/ree-sealing. Secondly, when the RD burst it must be far enough away for the RD to open without interfering with the RV nozzle.
In some relief systems, we see an RD on the DISCHARGE side of the RV when it discharges into a common header. This design ALSO COMES with its specific design requirements. They are listed below, but because this design is not common in my experience I will only provide the code requirements below.
(a) Rupture Disk Devices
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(3) Application of Rupture Disks
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(-c) A rupture disk device may be INSTALLED ON THE OUTLET SIDE OF A PRESSURE RELIEF VALVE which is opened by direct action of the pressure in the vessel provided:
(-1) the pressure relief valve will not fail to open at it’s proper pressure setting regardless of any back pressure that can accumulate between the pressure relief valve disk and the rupture disk. The space between the pressure relief valve disk and the rupture disk shall be vented or drained to prevent accumulation of pressure, or suitable means shall be provided to ensure that an accumulation of pressure does not affect the proper operation of the pressure relief valve,
(-2) the pressure relief valve is ample in capacity to meet the requirements of UG-125(c);
(-3) the marked burst pressure of the rupture disk at the specified disk temperature plus any pressure in the outlet piping shall not exceed the design pressure of the outlet portion of the pressure relief valve and any pipe or fitting between the valve and the rupture disk device. However, in no case shall the marked burst pressure of the rupture disk at the specified disk temperature plus any pressure in the outlet piping exceed the maximum allowable working pressure of the vessel or the set pressure of the pressure relief valve.
(-4) the opening provided through the rupture disk device after breakage is sufficient to permit a flow equal to the rated capacity of the attached pressure relief valve without exceeding the allowable overpressure;
(-5) any piping beyond the rupture disk cannot be obstructed by the rupture disk or fragment;
(-6) the system is designed to consider the adverse effects of any leakage through the pressure relief valve or through the outlet side rupture disk device, to ensure system performance and reliability.
(-7) the bonnet of a balancing bellows or diaphragm type pressure relief valve shall be vented to prevent accumulation of pressure in the bonnet