A pressure-retaining valve will maintain pressure in the pipeline upstream of itself, and the valve is designed to open with a rising upstream pressure. It js constructed with a reverse action of the pressure-reducing valve.
Safety valves must be incorporated into every air or steam line and vessel which is subject to pressure to ensure that the pressure will never exceed the safe upper limit recommended by the manufacturer or a code of practice and to satisfy government legislation and insurance companies. They must also be of the correct type and size to suit the operating conditions and be in sufficient numbers to protect the plant. The reliability of such valves is crucial.
In the simplest valves (Fig. 7.38), a spindle is lifted from its seating against the pressure of gas, steam or
The purpose of the check valve is to prevent accidental reversal of flow of liquid or gas in a pipe due to breakdown in some part of the equipment. There are three basic types of valve: swing check, lift check and combined stop and check with a number of variants. The swing check valve (Fig. 7.37) is most commonly used in fermenter designs. The functional part is a hinged disc which closes against a seat ring when the intended direction of flow is accidentally reversed.
When planning the design of a plant for a specific process, the water, steam and air should be at different, but specified pressures and flow rates in different parts of the equipment. For this reason it is essential to control pressures precisely and this can be done using reduction or retaining valves.
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