Basic Control Valve Principles

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Basic Control Valve Principles

The basic purpose of a control valve is to control the flow of a medium in a pipe  —
  •   turning it on or off, or
  •  varying it continuously.  
However, a control valve designed primarily to throttle energy is not necessarily designed for shut-off purposes. These two requirements often have to be balanced or realised in separate systems.
Research studies indicate that the final control element is responsible for 60 to 70% of poor-functioning control systems. The problems lie not just with the valve itself but also with:
  •  the valve actuators, 
  •  I/P converters and 
  •  positioners.
However, probably the majority of problems can be attributed to oversized valves and undersized actuators.
basic principle of control valve
If the PD is subject to a step change,  by how much will the PV change?
This is determined by what is called the Process Gain (Kp) and is given by:
Thus, for example, if we make a step change of 30% to the PD and the PV also changes by 30% then:
Process Gain (KP) = 1.
However, if the PV only changes by 15% then:
Process Gain (KP) = 0.5.
Alternatively, if the PV changes by 60% then:
Process Gain (KP) =  2.
Generally, the process gain should lie between the 0.5 and 2.0.  If it is less than 0.5 then, typically, the transmitter span is too wide for good control.  If the process gain is greater that 2.0, this is usually an indication that the control valve is oversized.
vena contracta
Bernoulli’s equation.

where:
Q = flow rate;
k = constant;
Cd = discharge coefficient;
ΔP = differential pressure  (P1 – P2); and
ρ = density of fluid.
where:
Q = flow rate;
Cv = valve flow coefficient;
ΔP = differential pressure  (P1 – P2); and
SG = specific gravity of fluid (water at 60°F =1.0).
Valve flow coefficient.

The  valve flow coefficient, Cv,  is an index used to measure the capacity of a control valve.

Cv is determined experimentally, using water as the test fluid, for each style and size of valve with the valve either fully open or at a given valve opening — usually stated as a percentage of maximum travel.
Numerically, Cv  is defined as:
  •  the  number of US gallons per minute 
  • of water at 60°F 
  • which will pass through a given flow restriction
  •  with a pressure drop of 1 psi across the valve

CV values

Choked flow

  • Flow is only proportional to √ΔP within the sub-critical flow region.
  • If the differential pressure is further increased, a point is reached where no further flow increase occurs —despite increasing the differential pressure.
  • Called ‘choked flow’ (critical flow) and is the maximum flow rate possible through that valve.
choked flow

High vs. low recovery valves

  • A streamlined valve will dissipate less energy through the restriction and will have more energy downstream for recovery to a higher pressure.
  • In a less streamlined valve larger amounts of energy are dissipated through the restriction and less energy will

be available downstream  for recovery to a higher pressure.

  • Streamlined valves produce relatively higher velocities through their restriction than do less streamlined, restrictive valves.
  • Velocity, being inversely proportional to pressure, suggests lower pressure at the vena contracta with high recovery streamlined valves .

Flashing

flushing

Solution?

Solution?

Or…

Lower the temperature of the medium!

 

Even worse…


Cavitation in Valve

This two-stage phenomenon, vapour bubble formation and their subsequent collapse, is known as cavitation. If the bubbles collapse on or near solid surfaces, material is chipped away. The amount of damage in a short period of time can be extensive and eventually prevents the control valve from performing its intended function.


cavitation

 

cavitation

Trim selection in Valve

trim selection

Noise in Valve

Noise
Please read also:  All Fluid Control Valves Types- Instrumentation
Please read also: Basic Control Valve Principles
Please read also: CALIBRATION OF VALVES WITH ACTUATOR & ACCESSORIES
Please read also: Pressure drop calculation in valve (Instrumentationl)
Please read also: Measurement and calculation the position of control valve

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  1. August 9, 2018

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    […] Please read also: Basic Control Valve Principles […]

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    […] Please read also: Basic Control Valve Principles […]

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