R
Ronald Deepak
What George said earlier is true. The positioner is in fact working in a cascade (inner) loop while the DCS is controlling the process variable (flow/pressure/level,etc.) From what I know, the positioners in fact have integral gains, so the error in the main process variable is not accumulated. However, like I mentioned earlier, there is an integral dead-zone for the valve travel, so the integral action is disabled when travel varies within this zone outside the set-point. So practically, the integral gain is almost non-existent in most positioners because the dead-zone is set high enough so that the valve position does not oscillate.
However, I do not quite agree to the comment about the usage of positioners in fast loops. At present, positioners are used in all kinds of loops and I don't see any problem with using a correctly sized valve and optimally tuned positioner in any kind of flow loop.
To answer your question about pressure drop :
The flowing pressure drop is used for Cv calculation. You might be aware that
Cv (approx.) = Q * Sqrt(SG/dP)
Where, Q is the vol. flow rate, SG is fluid specific gravity and dP is the pressure drop. In case of gases/vapors, the equation is different -dP is replaced by (dP/P1).
The shut-off pressure drop is used in actuator sizing. Like you said, the shut-off pressure drop occurs when the valve is fully closed. So this value is used in calculating the total actuator thrust required to keep the valve closed, because the actuator has to keep the valve closed opposing this pressure drop, in addition to providing the desired leakage class and overcoming friction.
In short, flowing pressure drop is used for Cv calculation and subsequently for sizing the valve. Shut-off pressure drop is used for sizing the actuator.
Hope this helps. If you still have questions, please let me know.
Ronald Deepak
jronalddeepak [at] gmail.com
However, I do not quite agree to the comment about the usage of positioners in fast loops. At present, positioners are used in all kinds of loops and I don't see any problem with using a correctly sized valve and optimally tuned positioner in any kind of flow loop.
To answer your question about pressure drop :
The flowing pressure drop is used for Cv calculation. You might be aware that
Cv (approx.) = Q * Sqrt(SG/dP)
Where, Q is the vol. flow rate, SG is fluid specific gravity and dP is the pressure drop. In case of gases/vapors, the equation is different -dP is replaced by (dP/P1).
The shut-off pressure drop is used in actuator sizing. Like you said, the shut-off pressure drop occurs when the valve is fully closed. So this value is used in calculating the total actuator thrust required to keep the valve closed, because the actuator has to keep the valve closed opposing this pressure drop, in addition to providing the desired leakage class and overcoming friction.
In short, flowing pressure drop is used for Cv calculation and subsequently for sizing the valve. Shut-off pressure drop is used for sizing the actuator.
Hope this helps. If you still have questions, please let me know.
Ronald Deepak
jronalddeepak [at] gmail.com