Control Valve's TOP 3 MOST asked INTERVIEW Question and Answers

Instrumentation and control engineering interview questions and answers.

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Instrumentation interview question and answers help in last minute preparation before giving interviews and also for revising important concepts.

Instrumentation and control engineering is a branch of engineering that deals with measurement of various parameters like flow, pressure, level and flow.

here we would learn important questions such as

How to select packing material of control valve

What is cavitation flashing and chocked flow

What is CV in control valve

Flow coefficient (Cv) is defined as the number of gallons per minute (gpm) at 60°F that will pass through a full open valve with a pressure drop of 1 psi.

Valve Flow Coefficient (Cv) is a valve’s capacity for a liquid or gas to flow through it. It is technically defined as “the volume of water at 60°F (in US gallons) that will flow through a valve per minute with a pressure drop of 1 psi across the valve.”

When selecting valves, choose the closest to the calculated Cv, typically rounding down unless a specified maximum ΔP of a project is exceeded. An oversized valve may cause control issues such as hunting or poor heat transfer (delta T) across a coil due to overflow. Conversely, an undersized valve may not provide enough flow and exceed available ΔP.

The relationship between pressure drop and flow rate through a valve is conveniently expressed by a flow coefficient (Cv)

Cavitation in control valve : In a control valve, if the liquid's pressure falls below its fluid vapor pressure, vapor bubbles will form and these bubbles burst after the pressure is increased beyond the vapor pressure curve is called cavitation in control valve

Flashing and cavitation in control valves : In a control valve, if the liquid's pressure falls below its fluid vapor pressure, and if the downstream pressure remains below the vapor pressure of the fluid, the process condition is called flashing.

Choked flow control valve : Choked flow is the point at which decreasing downstream pressure will not increase the flow through a valve. At this point the velocities are the highest and cavitation at this stage is very severe.

the most widely used packing material is Teflon (PTFE) and Graphite.

As a rule of thumb, PTFE packing materials are suitable up to 200 °C

And Graphite based packing material are suitable up to 600°C.

However, refer individual material datasheet for detailed information.

Fugitive emission requirements must also be checked, and the vendor must be consulted for special environmental protection packing sets.

For example Kalrez Valve Stem Packing (KVSP), Live load packing, Enviro-Seal and Bellows seal have better sealing properties and are less prone to leakage.

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