Pneumatic Valve Characteristic
The force acting to compress the spring is given by pressure P x Area A The force acting against this is given by the spring Constant K x Spring compression distance L
PA = KL
L = P(A/K)
Since the area of the diaphragm and the spring constant are constants for any given set up it can be seen that the spring movement is directly proportional to applied pressure
Inherent or designed characteristic
The characteristic of a valve is its relationship between valve lift and flow across it for a constant pressure drop across it. A typical set up for measuring this is shown below
Quick Opening (poppet) Characteristic
These valves re used in control systesm mainly as isolating valves. Their main use is for relief or safety valves. Full bore is achieved at one quarter of the diameter\ Linear Characteristics
In this design the flow through the valve is proportional to the lift. The normal design is the vee port ( or fluted) type although for smaller sizes the plug type is used Equal Percentage Characteristics
In this design for equal increments of valve movement the flow increases by and equal percentage.e.g. if the valve is 10mm open and the flow 20, if the valve opens anouther 10mm (100%) the flow increases to 40. If the valve opens a further 10mm (50%) then the flow increase by 50% of 40 which is anouther 20. The action may be expressed as L = logeQ / KIt should be noted that for true equal percentage the minimum flow is 1%. Therefore if closing is required some adaption is required
System characteristics
The above graph shows the efects the system has on the flow through the valve. It can be seen that as flow increases the pressure drop across the valve falls thus significantly effecting the characteristic effect of the valve
Fitting a valve with equal percentage trim produces a near linear characteristic. This will be effected by things like varying system pressure drop and maximum flow rates. If the repssure drop across the system is low at the required flow rate then the rpessure drop across the valve will not significantly alter and a linear characteristic shoudl be used.
In practice chracteristics are available which offer a balance between linear and equal percentage ( parabolic).