Isolation of relief valveRupture disks can isolate a safety relief valve from process fluids and materials, so that, under normal operating conditions, the safety valve does not encounter the process chemicals. Since the safety valve is isolated, its internal mechanics will not come into contact with any caustic process chemicals or viscous materials that might interfere with the valve’s operation. Because valve internals are not routinely exposed to process materials, they remain in almost new condition, which allows longer periods between major overhauls.
Also, since the valve is isolated, it is not necessary to have the valve constructed in a material designed for continuous contact. For example, if the process fluid requires that Hastelloy be the preferred material of construction for continuous contact, a carbon-steel valve (with Hastelloy trim) combined with a Hastelloy rupture disk can be used. This will save a significant portion of the valve cost.
Leak preventionAnother major advantage of combining rupture disks with relief valves is leak prevention, under normal operating conditions, the rupture-disk barrier prevents process fluids from escaping into the atmosphere. An example described in Ref. 1 illustrates the savings that can be realized by the combined arrangement:
For conventional safety valves, American Petroleum Institute (API) standard 527 (Seat Tightness of Pressure Relief Valves) allows for an orifice size of F or smaller to have a maximum allowable leakage rate of 40 bubbles per minute (approximately 6 ft3 over a 24-h period, or 2,190 ft3/yr). This leakage is either lost, eroding profits and potentially harming the environment, or requires the installation of a system to recover the leakage.
Test-in-placeCombining rupture disks with safety valves allows the safety valve to be tested in place in the field. With a suitable, reverse-buckling rupture disk installed at the valve inlet, the safety relief valve can be field-tested by a single person with a portable pressure source.
To accomplish this without opening any process piping, air (or nitrogen or another fluid) is injected from the pressure source into the chamber between the rupture disk and the safety valve inlet. The test pressure is increased until the valve releases, and should be within the set pressure tolerance of the valve.
What to considerWhat factors should be considered when deciding whether to use rupture disks in combination with pressure safety relief valves or to use a rupture disk alone? There are likely many, depending on the particulars of the application, but here is a set of basic considerations with which to begin.
CostRupture disks are considerably less expensive than safety relief valves, particularly when the valve needs to be constructed from exotic materials.
Process materialsA rupture disk alone is a good choice for overpressure protection in cases where process contents are inexpensive, nonhazardous and environmentally safe. A rupture-disk and relief-valve combination should be the choice when a leak-tight seal of the pressurized system is needed, and when the conservation of product within the pressurized system is
important, because it contains a corrosive, hazardous or expensive substance.
SpeedThe quick-bursting action of a rupture disk makes it a first consideration when the potential for runaway reactions exists. Safety valves alone will not react quickly enough to protect a process system from the pressure of a deflagration or a detonation.
Liquid propertiesSome liquids may freeze or cause icing under rapid depressurization, leading to blockage within a safety valve, and rendering it ineffective.
Also, highly viscous liquids, such as polymers, may not relieve pressure fast enough through a safety relief valve, and can create a danger of plug ging the valve.
SizingWhen sizing a relief valve, engineers need to determine the required fluid-flow capacity, and simultaneously to analyze the possible emergency scenarios, such as fire, loss of process cooling and equipment failure. The capacity requirements are then entered into a sizing equation to determine the relief valve area.
For a rupture-disk-safety-valve combination, the flow capacity of the combination must be confirmed to support the selection of both the valve and the disk. A combination capacity factor (CCF), which is often determined from ASME-certified capacity testing, can be used to support the decision. The CCF is calculated as the ratio between the capacity of the disk-valve combination over the relief valve capacity alone. CCFs should not exceed 1.
Pressure dropThe proper function of a relief valve requires that the pressure drop between the vessel it protects and the valve inlet is not more than 3% of the valve’s set pressure. Relief valves that are isolated by a rupture disk contribute to piping pressure drop, but by selecting rupture disks having low flow-resistance values, the pressuredrop target is usually reached.
Differential pressureIn a rupture-disksafety-valve combination, the differential pressure across the rupture disk must be monitored. The assembly shown in Figures 1 and 2 contains an excess flow valve to maintain atmospheric pressure in the space between the rupture disk and the safety valve, as well as a pressure gage on the relief valve to provide local confirmation of
pressure status.
References1. Brazier, G., Combining rupture disks with safety relief valves, Chem. Eng., March 2009,
pp. 42–44. Editor’s note: This edition of “Facts at your Fingertips” is adapted from the article referenced above.
Editor’s note: This edition of “Facts at your Fingertips” is adapted from the article referenced above.
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