VALVE CHARACTERISTICS

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.

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 positionp₀=upstream pressurep₁=down stream

pressure q= f2(L)

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 liftm - the fraction of maximum flow.

m = q/qmax = f(L/Lmax)

m = f(x)

• 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 sensitivity2)Linear3) increasing sensitivity.

Linear valve

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

Equal percentage valve • For the equal percentage valve the defining

equation is dmldx = βm where β is constant. Integration of this

equation givesln(m/m0)=βx

• where mo is the flow at x = 0.

• 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),

• 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

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.

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.

THANK YOU.