Control Valve in Q&A – (Part 1)

Will try in this series to sum up the basics of control valve, its types, structure and problems. Will make it - as I prefer - in the form of Questions and Answers .. Just to be focused and briefed.

I depend mainly on the following 2 references:

1 - Lessons in industrial instrumentations by Tony Kuphaldt, which is a critical reference for any I&C or Automation Engineer.

2 - Fisher Control Valve Sourcebook - Power and Severe Service

H?ope this series to be useful ..

Q1. What are the famous types of Final Control Element (FCE)?

FCE as a part of control loop almost has 3 types: Valve (with a broad of types), Motor (as a drive, and driven could be pump, fan, .. etc) or heating element. This series will focus only on the control valve.

Q2. What are the main parts for "Control Valve"?

Mainly we have: "Valve Actuator" and "Valve Body".

Besides there are many other accessories could be attached to the valve like: positioner, SOV, limit switch, I/P, Hand Jack and Air Pressure regulator/filter.

Will discuss each item in details later.

Q3. How can we categorize the valve types?

There are many types of valves, but we can categorize them according to the following 3 considerations:

a. Valve types based on "Valve Stem" motion.

b. Valve types based on Actuator style.

c. Valve types based on valve function.

Will take it one by one with a suitable briefing for each type.

a. Valve types based on "Valve Stem" motion:

Valve Stem motion is considered an important factor to categorize many valve types - as per the following:

Globe Valve: Very common valve design. Used mainly for throttling. It could be "Single Ported" or "Double Ported" which is somehow useful in minimizing the force applied to the stem by "process fluid" pressure across the plugs - As per the following figure:

Globe Valve is vital important with many topics to be discussed. So it will be detailed later in this series.

Gate Valve: Works by inserting a dam “gate” into the path of the flow to restrict it. Usually used for On/Off rather than throttling - The following figure describes it:

Diaphragm Valve: Also called "Pinch" valve. It uses a flexible sheet pressed close to the edge of a solid dam to narrow the flow path for fluid. It is poor throttling due to elasticity of the diaphragm. Usually used with dirty services like slurries - The following figure describes it:

Switching to "Rotary Stem" category we have the following 3 valve types:

Ball Valve: Very famous type, Used efficiently for throttling or on/off services - The following figure describes it:

V-shaped notch bore is better for control/throttling rather than plain-bore ball valve.

Butterfly Valve: Simply it is a disk that rotates perpendicular to the path of fluid flow. Also used efficiently for throttling or on/off services. Tight shut-off could be obtained if the seating area is lined with a soft (elastic) material - The following figure describes it:

Disk Valve: Also known as "eccentric disk valve", or as "high-performance butterfly valve". Simply it is a variation on the butterfly design intended to improve seat shut-off. The disk’s center is offset from the shaft centerline, causing it to approach the seat with a “cam” action that results in high seating pressure. Tight shut-off is possible even when using metal seats and disks - The following figure describes it:

We can sum up what mentioned for the previous valves in the following:

Next post will continue with the rest of valve types ..