. The function of a pressure relief valve is to protect pressure vessels, piping systems, and other equipment from pressur...
Types of Pressure Relief Valves:
The standard design safety relief valve is spring loaded with an adjusting ring for obtaining the proper blowdown and is available with many optional accessories and design features. The bellows and balanced bellows design isolate the process fluid from the bonnet, the spring, the stem, and the stem bushing with a bellows element. Jacketed valve bodies are available for applications requiring steam or heat transfer mediums to maintain viscosity or prevent freezing. Pilot-operated valves are available with the set pressure and blowdown control located in a separate control pilot. This type of valve uses the line pressure through the control pilot to the piston in the main relief valve and thereby maintains a high degree of tightness, especially as the set pressure is being approached. Another feature of the pilot-operated valve is that it will permit a blowdown as low as 2 %. The disadvantage of this type of valve is its vulnerability to contamination from foreign matter in the fluid stream.
Sizing of Pressure Relief Valves:
Correct and comprehensive pressure relief valve sizing is a complex multi-step process that should follow the following stepwise approach:
1. Each piece of equipment in a process should be evaluated for potential overpressure scenarios.
2. An appropriate design basis must be established for each vessel. Choosing a design basis requires
assessing alternative scenarios to find the credible worst case scenario.
3. The design basis is then used to calculate the required pressure relief valve size. If possible, the sizing
calculations should use the most current methodologies incorporating such considerations as two
phase flow and reaction heat sources.In order to properly select and size a pressure relief valve, the following information should be ascertained for each vessel or group of vessels which may be isolated by control or other valves. The data required to perform pressure relief valve sizing calculations is quite extensive, Let me briefly explain the sam. First, the equipment dimensions and physical properties must be assembled. Modeling heat flow across the equipment surface requires knowledge of the vessel material’s heat capacity, thermal conductivity, and density (if vessel mass is determined indirectly from vessel dimensions and wall thickness). The vessel geometry – vertical or horizontal cylinder, spherical, etc. – is a necessary parameter for calculating the wetted surface area, where the vessel contents contact vessel walls. Second, the properties of the vessel contents must be quantified.This includes density, heat capacity, viscosity, and thermal conductivity. Values of each parameter are required for both liquid and vapor phases. Boiling points, vapor pressure, and thermal expansion coefficient values also are required. Ideally, the properties will be expressed as functions of temperature, pressure, and compositions of the fluid.
The most difficult aspect of the design and sizing of pressure relief valves is ascertaining the controlling
cause of overpressure. This is sometimes referred to as the worst case scenario. Overpressure in
equipment may result from a number of causes or combination of causes. Each cause must be
investigated for its magnitude and for the probability of its occurrence with other events. The objective
might be to document why the particular design basis is the correct choice.
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