Selection of Pressure Reducing Valve in Chemical Engineering Design

In the fluid piping system, the valve is a control elemen t, its main function is to open and close, throttle, adjust the flow, isolation equipmen t and piping systems to prevent media backflow, regulation and discharge pressure. Valve, valve, check valve, diaphragm valve, etc., special valves are traps, safety valves and pressure reducing valve, and so on.There are many kinds of valves, such as valve, valve, stop valve, stop valve, With the rapid development of petrochemical industry, most of the media in petrochemical production facilities are toxic, flammable, explosive and corrosive. The operating conditions are more demanding, operating temperature and pressure are higher, the start-up period is long, Once the valve failure, the light lead to media leakage, both pollution and economic losses caused by heavy equipmen t shutdown caused shutdown, or even cause a vicious accident. Select the appropriate valve is essential, the choice of special valves in the pipeline design is particularly important.

 Pressure reducing valve and its selection method

Reducing valve through the opening and closing pieces of the throttling, the inlet pressure down to a required outlet pressure, and in the inlet pressure and flow changes, the use of its own medium to maintain the energy output of the valve basically the same pressure. Its role is to rely on sensitive components, such as diaphragms, springs, etc. to change the position of the valve flap, the media pressure is reduced, to achieve the purpose of decompression. Under normal circumstances, the pressure relief valve outlet pressure should be less than 0.5 times the inlet pressure. Pressure regulator inlet pressure fluctuations should be controlled in the inlet pressure set value of 80% to 105%, if more than the range of pre-decompression performance will be affected. For operation, adjustment and maintenance convenience, pressure relief valve should be installed in the horizontal pipe.

1.1 Classification of pressure reducing valves

Pressure reducing valve can be divided into corrugated pipe type (direct acting type) pressure reducing valve, piston type pressure reducing valve and membrane type pressure reducing valve.

1) bellows type (direct action type) pressure reducing valve. The simplest pressure relief valve, direct acting pressure reducing valve, with a flat diaphragm or bellows. Independent structure, without the need to install external sensing lines downstream. Is the smallest of the three pressure reducing valves, the use of the most economical one, designed for low flow design. The accuracy of the direct-acting pressure relief valve is usually set at the downstream (+ 10% to -10%);

2) piston-type valve. This type of valve sets two kinds of valve - pilot valve and the main valve in one. The design of the pilot valve is similar to that of a direct acting pressure reducing valve. The exhaust pressure from the pilot valve acts on the piston, causing the piston to open the main valve. If the main valve is large and can not be opened directly, this design will use the population pressure to open the main valve. As a result, this type of pressure relief valve has a higher capacity and accuracy (+ 5% to -5%) compared to a direct-acting pressure reducing valve in the same pipe size. The same with the direct-acting pressure reducing valve, the pressure inside the pressure sensing valve, no external installation of sensing lines;

3) membrane type pressure reducing valve. In this type of pressure reducing valve, the double diaphragm replaces the piston in the internal pilot pressure reducing valve. This increased diaphragm area can open a larger main valve and, at the same pipe size, has a larger capacity than the inner pilot piston pressure reducing valve. In addition, the diaphragm is more sensitive to pressure changes, the accuracy of up to (+ 1% -1%). The higher accuracy is due to the positioning of the downstream sensing line (outside the valve), where the gas or liquid is less turbulent. The valve is very flexible, can use different types of pilot valves, such as pressure valves, temperature valves and air loading valve.

2.2 Pressure relief valve selection instructions

Pressure reducing valve is widely used in steam, compressed air, industrial gas, water, oil and other liquid medium equipmen t and pipeline can be used, the specific selection is as follows:

1) corrugated pipe direct action type pressure reducing valve is suitable for low pressure, small and medium caliber steam, air and other media;

2) Direct acting membrane pressure reducing valve is suitable for medium and low pressure, medium and small caliber steam or water medium;

3) Piston-type pilot valve for a variety of pressure, all caliber, a variety of temperature steam, air and water medium, if made of stainless steel acid,

4) medium working temperature higher occasions, the general selection of piston-type valve;

5) high sensitivity requirements, the choice of spring membrane pressure reducing valve;

6) medium for the air or water (liquid) the occasion, the general choice of direct action membrane pressure reducing valve or pilot membrane pressure reducing valve.