Rules of Thumbs for Sizing Control Valves

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In many applications where control valves are applied, sizing and selection can be quite challenging. In the majority of applications, it is either the control valve is under sized or over-sized. In an undersized control valve, the valve is unable to deliver the required flow for each stage of the valve lift creating control problems.

Over-sized control valves will under all normal operations be confined to small openings of the valve with great risk of variable sensitivity and aggravation of any uneven movement of the valve and actuator combination. Poor accuracy and unstable control are often the result of over-sized control valves.

Given the problems that could arise because of failure to size control valves accurately, the following rules of thumb are presented as a guide for the selection and sizing of control valves:

How a Temperature Control Valve Works

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A temperature control valve is just like any other control valve. The only difference is that the control valve helps to maintain the temperature of a desired process at a specific level. It is often abbreviated as TCV – Temperature Control Valve – in most process drawings and P&IDs.

Temperature Control Schemes
There are two popular temperature control schemes using temperature control valves:
(1) Mixing of a cold process fluid with a hot process fluid to control the temperature of the hot process fluid.
(2) Exchange of heat between the hot process fluid and a cold process fluid as seen in heat exchangers. However, the process fluids do not mix.

Mixing of Cold and Hot Process Fluids
In temperature control valve applications in its most basic form, there are two basic process fluids:

How Temperature Switches Work

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A temperature switch works just like a typical electrical switch for on /off application. In this case, the temperature switch operates to switch on or off at discrete process temperatures. A temperature switch consists of two basic parts that you will find in all designs:
(a) A sensing part immersed in the process whose temperature is required to be controlled. The sensing part can either be a sensing bulb filled with a fluid –liquid, gas or a bimetallic strip that uses the differential expansion of two dissimilar metals.
(b) Snap-action contacts that act to switch on electrical power to the device controlling process temperature.

How a Temperature Switch Works
Liquid filled temperature switches comprises a sensing bulb and a bellows element. The bulb is immersed in the process whose temperature is being controlled. The bellows element senses fluid pressure (liquid or gas)
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