Equal percent to Linear type valve]

[hazel]

Dear all,
Could I modify a valve from equal percent to linear by replacing cam in the positioner?
Anything else I have to do ?

thanks in advance
Hazel

 

[PYLERH]

Depending on the brand and model, YES you can linearize an equal percentage valve. The cam can be used to modify the input signal/valve travel relationships to any characteristic desired.

 

[w3285]

Try to use a DVC digital controller (I/P) from Fisher. You can then configure this controller to 'quick opening' which should make the response to become linear.

 

[mtmurthy]

Yes it is possible , with appropriate tuning. But pl. note that characterisation is through positioner cam is not the recommended practice particularly in the fast loops. Because the positioner in the valve is like loop within a loop.

 

[4carats]

You also need to consider the hydraulics and the overall effect on the control system when you change the inherent flow charteristic of a control valve. In parallel valves operation, it is even more important to check if the slope of flow vs lift match between parallel valves. You can change the cam or DCS if no adverse effect (exceedingly high trim fluid Kinetic Energy) is experienced by the control valve, otherwise you need to characterize the trim, not the cam nor the DCS.
The installed flow characteristic depends on:
1) the Over-sized factor
2) the Rangeability
3) the fraction of the dynamic pressure taken
by/of the control valve.
I hope this helps.
Often a linear valve (inherent) will behave like quick-opening (installed).
An equal percent valve (ihherent) will behave like a linear valve.