Selection of Valves: Valve Selection Procedure

Valves are one of the important mechanical (sometimes electro-mechanical) device that helps in controlling the fluid flow through pipes or tubes. Selecting the right kind of economic valve for a specified fluid service requires many considerations. This article provides guidelines for the selection of normal type valves. ( Special type Valves are not covered in this article).

It should be recognized that Valves Selection is a very extensive and complex subject and it is not possible to select only one type based on this article. The Valves selected to work under the same conditions will differ from one client to another. It is necessary to compare the cost and come to an agreement with the client.

Valve Selection Based on Valve Function

There are two basic principles on which valves are constructed.

Criteria for Valve Selection

The selection of valve type requires an understanding of the following.

Valves Function

Service Characteristics

Cost of Valves

Valve Terms

Breaking Pressure:

The minimum pressure required to produce flow through a Valves.

Duty Cycle:

100% duty cycle is defined as continuous operation without any damage occurring. For intermittent duty cycle (<100%), alternate energized and de-energized state at regular intervals to allow the Valves to completely cool down to room temperature. F = active concentration / total concentration

Flow Patterns:

A diagram showing how flow can be directed using a particular Valves. (See the “Flow Patterns” box below for further explanation.)

Normally Closed:

Valve stays closed in de-energized state; opens when energized.

Normally Open:

Valve stays open in de-energized state; closes when energized.

Pressure Differential or Pressure Drop:

The difference between the inlet and the outlet pressure through a valve. The outlet pressure is lower than the inlet pressure due to the restriction caused by the Valves.

Three-Way Valves:

Has three ports. Depending on the particular valve, all three ports may be open, two ports may be open, or all ports may be closed.

Two-Way Valves:

Has a single inlet port and a single outlet port.

Valve Selection Based on Valves Function

There are two basic principles on which valves are constructed.

(1) The first principle is developed in three ways:

Moving the stopper by direct thrust on to the orifice seating.  This obturating movement is the basis of globe type valves. Rotating the stopper. This is the basis for plug or ball-type valves. Sliding the stopper across the face of the orifice seating.  This is the basis of gate type valves.

(2) The second principle, squeezing action, is the basis of all diaphragm type valves. These are shown in the following Figure (Fig. 2)

Squeezing Action for Diaphragm Type Valves

Fig. 2: Squeezing Action for Diaphragm Type Valves

(3) Each of the four-valve motivations has its own advantages and disadvantages, as listed in the following table (Fig. 3).

Table comparing the valve features

Fig. 3: Table comparing the Valves features

To summarize, there are four basic valve types, each type having its own individual characteristics and offering its own particular advantages or disadvantages.

For this reason, many modifications and variations of the basic type are made by valve manufacturers. An appraisal of the chief characteristics of the above four basic Valves types is given in the following paragraphs