Control Valve Selection Guide

Valves are manual or automatic fluid controlling elements in a piping system; in other words, valves isolate, switch, and control fluid flow in piping systems. They are constructed to withstand specific range of temperature, pressure, corrosion, and mechanical stress. Valves can be operated manually using levers or gear operators or remotely using electric, pneumatic, electro-pneumatic and electro-hydraulic powered actuators. Manual valves are utilized only if they will be operated infrequently or no power source is available. There are different types of valves, for instance: Isolation valves, Control valves, Pressure-relief valves, Steam traps, Thermostatic expansion valves, and so on. In this article, we focus on the requirements for the selection of control valves.

Figure 1.0: Pneumatic control valve (without a positioner)

Requirements for Control Valve Selection

The selection of a control valve requires specific considerations for various factors for example capacity requirements, system operating pressure ranges, etc. To make the appropriate selection of a control valve, at least the following details should be provided:

Type of fluid to be controlled.
Temperature of fluid.
Specific gravity of fluid.
Viscosity of fluid.
Flow capacity required (maximum and minimum).
Inlet pressure at valve (maximum and minimum).
Outlet pressure (maximum and minimum).
Pressure drop during the normal flowing conditions.
Pressure drop at shutoff.
Maximum permissible noise level, if relevant and the measurement reference point.
Degrees of superheat or existence of flashing, if known.
Inlet and outlet pipeline size and schedule.
Special tagging information required.
Body material – body material selection is normally based on the pressure, temperature, corrosive properties and erosive properties of the flow media. Cast carbon steel is the most commonly utilized valve body material for relatively non-corrosive fluids at sensible temperatures and pressures however some service conditions may require the use of exotic alloys and metals to with stand specific corrosive properties of the flowing fluids.
End connections: threaded/screwed ends (they are inexpensive but can be stripped and leak), welded ends (provide no leaks and are cheap initially, but if there are problems the valve must be cut off), flanged ends (are the most expensive but are the best from an installation and removal standpoint).
Instrument air supply available.
Instrument signal (3-15 psig, 4-20 mA, HART, and so on).
Action desired on air failure (valve to open, close or retain last controlled position).

Figure 1.1: Pneumatic control valve (Includes a positioner – grey section with gauges)

The above are the primary requirements but depending on the engineering practices that the end user may be adhering to, further information may be provided as follows:

Valve type number.
Valve size.
Valve body construction (angle, double-port, butterfly, and so forth).
Valve plug guiding (cage-style, port-guided, and so on).
Valve plug action (push-down-to-close or push-down-to-open).
Port size (full or restricted).
Valve trim materials required.
Flow action (flow tens to open valve or flow tends to close valve).
Actuator size required.
Bonnet style (plain, extension, bellows seal, etc.).
Packing material (PTFE V-ring, laminated graphite, environmental sealing systems, and so on).
Valve accessories required e.g. positioner, handwheel, and so forth

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John Mulindi

John Mulindi is an Industrial Instrumentation and Control Professional with a wide range of experience in electrical and electronics, process measurement, control systems and automation. In free time he spends time reading, taking adventure walks and watching football.