Process Relief Requirements
The ASME code requires every pressure vessel that can be blocked In to have a relief valve to alleviate pressure build up due to thermal expansion of trapped gases or liquids. In addition, the American Petroleum Institute Recommended Practice (API RP) 14C, “Analysis, Design, Installation and Testing of Basic Surface Safety Systems on Offshore Production Platforms,” recommends that relief valves be installed at various locations in the production system; and API RP 520, “Design and Installation of Pressure Relieving Systems in Refineries,” recommends various conditions for sizing relief valves.
In production facility design, the most common relieving conditions are ( I ) blocked discharge, (2) gas blowby, (3) regulator failure, (4) fire, (5) thermal, and (6) heat exchanger tube rupture. Relief valve design flow rates are commonly determined as follows.
1. Blocked Discharge. It is assumed that all outlets from the vessel are shut in and the total inlet flow stream (gas and liquids) must flow out through the relief valve. This condition could occur, for example, if the equipment has been shut in and isolated and the operator opens the inlet before opening the outlet valves.
2. Gas Blowby. A gas blowby condition is the most critical and sometimes overlooked condition in production facility design. It assumes that there is a failure of an upstream control valve feeding the pressure vessel and that the relief valve must handle the maximum gas flow rate into the system during this upset condition. For example, if the liquid control valve on a high pressure separator were to fail open, all the liquid would dump to the downstream lower-pressure vessel. Then the gas from the high pressure separator would start to flow to the downstream vessel. The lower pressure vessel’s relief valve must be sized to handle the total gas flow rate that will fit through the liquid dump valve in a full open position. We normally assume (conservatively) that the upstream vessel pressure is the PSV set point (or less conservatively at its operating pressure) and the downstream vessel is at its PSV set point. The resulting gas flow rate may be larger than the design gas flow rate to the high-pressure separator inlet. The rate can be reduced by a choke or other restrictions in the line. In that case, the rate would be the maximum rate that would fit through the choke at the appropriate vessel pressures, Most accidents involving overpressuring of low pressure separators are a result of relief valves not being adequately sized to handle the gas blowby condition. Note: Liquid dump valves are normally fail closed to prevent gas blowby. That means that in case of loss of instrument gas or air pressure, the spring will drive the valve to the closed position. However, the valve can mechanically fail because the level controller malfunctions or the seat cuts out due to solids erosion, which would cause it to fail in an open configuration.
3. Regulator Failure. It is assumed that a pressure control valve or regulator fails in the full open position. Regulator failure could occur where a regulator is used to feed gas from a high pressure line to a fuel gas scrubber. Normally, the regulator only opens enough to keep the pressure in the scrubber constant. If the valve fails open, the users can’t take the excess gas, so the pressure in the scrubber goes up until the relief valve opens. The feed to the regulator is assumed to be (conservatively) at the upstream relief valve set point, and the downstream vessel will pressurize to its relief valve set point.
4. Fire, The relief valve must be sized to handle the gases evolving from liquids if the equipment is exposed to an external fire. A procedure for calculating this is presented in API RP 520. This condition may be critical for large, low-pressure vessels and tanks but does not normally govern for other pressure vessels.
5. Thermal Thermal relief is needed in a vessel or piping run that is liquid-packed and can be isolated, for example pig launchers and meter provers. Liquid is subject to thermal expansion if it is heated. It is also incompressible. The thermal expansion due to heating by the sun from a nighttime temperature of 80°F to a sun-heated temperature of 120°F can be enough to rupture piping or a vessel. The required capacity of thermal relief valves is very small.
6. Tube Rupture. It is common for a heat exhanger to have a high-pressure fluid in the tubes and a lower-pressure rated shell. If there is a break in one of the tubes, the higher pressure fluid will leak to the shell, resulting in overpressure. It is conservative to assume a tube is completely split with choked flow from both sides of the break.
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