Why Selection and Sizing of Pressure Relief Valves is Critical?

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 same. First, the equipment dimensions and physical properties must be assembled.

Why Selection and Sizing of Pressure Relief Valves is Critical?

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The function of a pressure relief valve is to protect pressure vessels, piping systems, and other equipment from pressures exceeding their design pressure by more than a fixed predetermined amount.The aim of safety systems in processing plants is to prevent damage to equipment, avoid injury to personnel and to eliminate any risks of compromising the welfare of the community at large and the environment. Proper sizing, selection, manufacture, assembly, test, installation, and maintenance of a pressure relief valve are critical to obtaining maximum protection. To learn more on the basic guidelines to selection criteria of valves read "5 Questions you must ask before you buy Industrial Valves?" Click Here to read my earlier post on the subject.

The pressure relief valve must open at a predetermined set pressure, flow at rated capacity at a specified overpressure, and close when the system pressure has returned to a safe level. Pressure relief valves must be designed with materials compatible with many process fluids from simple air and water to the most corrosive media. These design parameters lead to the wide array of pressure relief valve products available in the market today.

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.

To Learn more about "Basic guidelines and applications of Control valves" Click Here to read an earlier post. 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 the chart below i have illustrated relief valve size selection based on orifice designation and orifice area. [lock]

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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 same. 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. To learn more on "How Preventive Maintenance in Process plants can result in Increased Revenues" [Click Here]

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.