control valve

Pressure Drop System across a Control Valve

Control valves are crucial components in every process industry.  In process industries, these control valves are used for controlling the various types of fluids like water, gas, and steam in the process.

As fluid passes through a control valve, the pressure systems across the control valve especially a pressure drop varies with fluid flow through that pipe or line.

During the process of proper valve selection, and valve sizing, the basic concept of the pressure systems across a control valve must be known.

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The pressure drop data is used in the control valve sizing and is critical.  for that, we must predict the actual pressure drop across the control valve systems from minimum flow to normal and to maximum flow rates.

If the pressure drop across the control valve is zero. Then the controller would have zero ability to change the value of the controlled variable and thus would have no effect.

The control valve flow is the function of both pressure drop across the valve and the valve travel.

In selecting a control valve for a particular application in the process industry the pressure drop plays an important role in determining the valve, trim style, and also the materials of construction.

Higher pressure drops generally require a highly engineered control valve for good performance.

The maximum pressure drop across the control valve can resist a turnoff or partially or fully open.

Generally, the globe valves are superior because of the rugged nature of their moving parts.

Generally, the flow rate in the control valve is affected by a pressure drop in the control valve.

The pressure drop across the valve is directly proportional to fluid flow through that valve.

  • When the flow rate through the control valve increases the pressure drop across the valve will reduce. Until it reaches the minimum allowable pressure drop across the valve at maximum fluid flow.
  • As the flow rate through the valve decreases, the pressure drop across the valve will get raised and increase correspondingly.

The pressure drop variation in the control valve is shown below diagram:

pressure drop variation in the control valve
PRESSURE DROP VARIATION IN THE CONTROL VALVE

From the above diagram we can conclude that:

1. As flow increases across the control valve, the upstream pressure P1 gets reduced. And downstream pressure P2 gets raised.

2. As flow increases ΔP pressure drop across the valve decreases.

3. At zero or no flow, the ΔP is maximum and the downstream pressure P2 = 0.

4. The ΔP is minimum at maximum flow across the control valve.

What is a pressure drop in a control valve?  How it is measured?

The pressure drop across the control valve is the difference in the pressure system between two points in a pipeline with flowing fluid. It is caused by resistance to the fluid flowthrough the line or tube.

The pressure drop is the difference between the inlet pressure and outlet pressure.

This differential pressure or Delta P (ΔP) is the inlet pressure minus the outlet pressure.

Delta P (ΔP) = P1 – P2

A pressure drop in the valve occurs due to frictional forces,

P1 is measured as 2 pipe diameters upstream or inlet of the valve and

P2 is measured as 6 pipe diameters downstream or outlet of the valve.

PRESSURE DROP
PRESSURE DROP

What is the importance of pressure drop in valve sizing?

Pressure drop is a crucial element in the sizing and selection of the control valve.

In designing a system the design engineer must have a relevant idea about the pressure drop to ensure proper valve selection.

How do we calculate the pressure drop across the control valve?

To calculate the pressure drop across a control valve by using the following formula.

∆p = 0.5Kρv2

where
∆p = Pressure drop, in Pa
K = Valve K factor
ρ = Density of water, 997 kg/m3
v = Velocity of water, in  m/s

Pressure drop across a control valve is about 0.5 times the K factor time’s density of water times velocity of water squared.

The unit is on Pascal (Pa) scale.   But we usually divide it by 1000 to get in kilo Pascal (kPa).

Which valve has the highest pressure drop?

The globe valve has the highest pressure drop in a fully open position compared to other valves.

This is due to the change in the direction of the fluid flow inside the control valve.

The L/D ratio for the globe valve is approximately 340.

Which valve has the least pressure drop?

  • The gate valves have the least pressure drop.
  • This type of valve is most often used in industrial piping.
  • The significant feature of the gate valve has less obstruction to fluid flow,
  • It has less turbulence within the valve and very little pressure drop

How to calculate the pressure drop in the control valve knowing the Cv and flow rate?

To calculate the pressure drop, flow rate, or the flow coefficient across the valve the basic valve sizing equation is given as

     Q = Cv* √∆P

For the fluids rather than water, a correction factor for a difference in specific gravity of the solution is necessary.

This revised formula would be

Q = Cv * [√∆P/S]

Where

S = specific gravity of the liquid used.

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