control valves in process control

1. VALVE
CHARACTERISTICS

2. Valves vary the flow of fluid trough
it by means of change of pressure to
the valve top.
The relation between the flow
through the valve and the valve stem
position (or lift) is called the valve
characteristic.

4. The flow through a control valve for a specific
fluid at a given temperature can be expressed
as
q = f₁ (L, p₀, p₁)
q= volumetric flow rate
L=valve stem position
p₀=upstream pressure
p₁=down stream pressure
q= f2(L)

5. m = q/qmax and x = LILmax
where qmax is the maximum flow when the
valve stem is at its maximum lift Lmax(valve is
full-open)
x - the fraction of maximum lift
m - the fraction of maximum flow.
m = q/qmax = f(L/Lmax)
m = f(x)

6. • The sensitivity of the valve, which is the
fractional change in flow to the fractional
change in stem position for fixed upstream and
downstream pressures.
sensitivity = dm ldx
1)decreasing sensitivity
2)Linear
3) increasing sensitivity.

7. Linear valve
 sensitivity is constant
 the relation between flow and lift is linear.
dmldx = a
∫dm=∫adx
a=1
m=x

8. Equal percentage valve
• For the equal percentage valve the defining
equation is
dmldx = βm
where β is constant. Integration of this
equation gives
ln(m/m0)=βx
• where mo is the flow at x = 0.

9. • Equation shows that a plot of m versus x on
semi-log paper gives a straight line.
• dmlm = /3dx or Δmlm = βΔx
• In this form it can be seen that an equal
fractional (or percentage) change in flow ( Δ
m/m) occurs for a specified increment of
change in stem position ( Δ x),

10. • an equal percentage valve, the equal percentage
characteristic will not be followed when the valve is
nearly shut. In practice, the control valve serves as a
throttling valve and is not intended to be wide-open
or completely closed during normal operation.
Rangeability
is defined as the ratio of maximum flow to
minimum controllable flow over which the valve
characteristic is followed.
Rangeability = mmax/m min,
controllable

11. Effective Valve Characteristic
When a valve is placed in a line that offers
resistance to flow, the inherent characteristic of
the valve will be altered. The relation between
flow and stem position for installed in a
process line will be called the effective valve
characteristic.

12. BENEFIT OF AN EQUAL PERCENTAGE
VALVE
 its inherent nonlinear characteristic that
compensates for the line loss to give an effective valve
characteristic that is nearly linear.
 an equal percentage valve overcompensates for line
loss and produces an effective characteristic that is not
linear, but is bowed in the opposite direction to that of
the effective characteristic of the linear valve.
 One can show that as the line loss increases, the linear
valve will depart more from the ideal linear relation
and the equal percentage valve will move more closely
toward the linear region.

13. THANK YOU.