Facts about spring loaded pressure relief valves
By Lavenda Sekwadi, Process Engineer at Energas
The safety of chemical plants to people and environment is of paramount importance. Overpressures can occur in liquid or gas systems of which the causes can be many things either in the system itself or by external factors.
Since there is a wide range of different types of chemicals in the chemical industry, some chemicals may not be discharged into the atmosphere due to their toxicity to the environment or to any form of life in the proximity.
For environment-friendly chemicals such as air, water, among others, the pressure can be relieved directly into the atmosphere, whereas for toxic chemicals, a relieved chemical is directed to a designated location where it can be reused or destructed into other forms before going off to the atmosphere.
The most simple and widely used type of pressure safety valve (PSV) is conventional spring loaded. There are other types such as pilot-operated and burst disc PSVs which are usually used for special relieving cases.
How it works
A spring loaded PSV uses a spring to establish the required set-point. It is mounted directly on the vessel or pipeline it is meant to protect. The PSV is set at a pressure higher than operating pressure. The margin between the set-pressure and operating pressure is sufficient to avoid unnecessary opening of the PSV.
The PSV only opens as soon as the pressure in the system reaches the set-pressure of the valve. As soon as the overpressure is relieved from the system as a result of the medium exiting the system, the PSV recloses and the process continues normally.
Usually, if there is a sufficient back pressure (at least 10% of the set-pressure for gases and 2,5% for liquids) in the location where the PSV relieves, an option of balanced bellows is considered, which happens to cancel the effect of back pressure during relief. Balanced bellows are also used when the medium to be relived is corrosive. This prevents a corrosive medium to reach the internal parts of the PSV which are responsible for its proper operation.
Sizes and selection of the PSV
The standard widely used for PSV sizing is API 520, which also covers selection and installation of PSV’s in the chemical industry. The main objective of the relief sizing calculation is basically to determine the required relief area of the PSV.
Tai Milano’s smallest available size is 3/4” inlet and 1” outlet, with an orifice type D and the biggest size is 8” inlet and 10” outlet, with an orifice type T1. The set-pressure range for Tai Milano PSV’s is from 40 kPa to 400 kPa.
The body material is mostly available carbon steel (A216-WCB) and stainless steel (A351-CF8M). The valve trim (disc, nozzle and adjusting ring) are usually made of more special materials which are compatible to the medium.
In order to ensure a proper operation of the valve, the tests are carried out on each valve:
• Hydrostatic pressure test on the nozzle, body and bonnet at ambient temperature and pressure of 1,5 times design pressure.
• Tightness test with air under water in accordance with API 527 standard.
Energas Technologies contact:
Laetitia Jansen van Vuuren
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