Key Factors to Consider in Selection of Valves

Valves are the final element in a control loop, and are critical in providing the correct flow. The valve, usually operates in very harsh conditions, and is one of the most costly elements in the process control system. Their selection and correct installation require both knowledge and experience. Understanding system requirements and manufacturers specifications is paramount, before a careful valve selection is made.

The selection of control valves for a particular application depends on various variables such as the corrosive nature of the fluid, temperature of operation, pressure of the fluid, velocity of the flow, volume of the flow and the amount of suspended solids.

Some of the factors affecting the choice of valves include:

• Valve size from flow requirements, avoiding both the oversizing and under sizing.
• Fail safe considerations for two-way and three-way types of valves.
• Materials used in the valve construction, ranging from PVC to brass to steel, based on considerations of pressure, size and corrosion.
• Levels of acceptable pressure drop across the valve.
• Linear or rotary motion of valve e.g. globe, diaphragm, ball, or butterfly valves.
• Tightness of shutoff, as classified by quality of shutoff by leakage at maximum (valves are classified into six classes depending on leakage from 0.5 % of rated capacity to 0.15 mL /min for a 1 – inch diameter valve).

Related: Flow Control Valves

The type of valve employed in a given process depends on the nature of the process. In the case of fast reaction with small load changes, control is only slightly affected by valve characteristics. When the process is slow, with large load changes, valve characteristics are important. If the load change is linear, then a valve with linear characteristics should be used. In the case of a nonlinear load change, a valve with an equal percentage change may be required.

Other considerations in the choice of valves include:

• Maintenance
• Fail-safe features
• Serviceability
• Pneumatic, hydraulic, solenoid or motor control
• The need for feedback

Related articles:

• Actuators as Automation Components
• Pressure Control Valve Operation
• Directional Control Valves: Function & Principle of Operation
• The Parts and Functions of a Valve
• Pressure Regulators: Function & Types
• Electric Solenoid Actuator – Features & Operation