My solution was to install a smart digital positioner on the valve, which provides faster stroking speed with no deadband. See the article, “Smoothing Out Compressor Control,” Chemical Engineering, Feb. 1999.
Process Control Consultant
Answer: The selection of the boosters to be used is driven by the speed requirement for the open and or close time of the valve. The primary problem in using a booster to feed and/or exhaust an actuator is that instability may be caused in the valve positioning.
In selecting the booster size needed to meet a speed requirement most valve manufacturers would have a computer program to determine the actuator input and output Cv’s. An estimate of the Cv required to achieve a certain stroke speed can be determined by using the volume of the actuator and the time for a full stroke to establish a flow rate, then with the pressure drop estimate, one can ball- park a Cv of the booster needed.
It is rare for the presence of a positioner in combination with the booster to cause an instability that cannot be readily fixed using the booster capacity adjustment, unless the positioner itself has excessive gain relative to the size of the actuator.
Some rules of thumb in selecting a booster follow:
- The booster controlling the input to the actuator should have the largest Cv so that the input flow is helping to push out the exhaust air on the other side of the piston. If high speed in both open and close directions is required, then stability can be quite sensitive, and it may be necessary to trade off some overshoot in order to achieve the required stroke speed.
- Experience in using boosters for high speed operation indicates that just two booster sizes, Cv = 1 and Cv = 4 is enough for a wide range of valve sizes. One or more of the larger boosters would be used on the larger actuators generally associated with valve sizes 6 in. to 8 in. (125 mm to 200 mm) and larger. For unique applications, multiple boosters can be installed; even placing a Cv = 1 and a Cv = 4 boosters in parallel is not uncommon to achieve good small signal response while minimizing overshoot during large signal changes.
- Most instability problems occur when the input booster is installed with a quick exhaust valve on the other side of the actuator piston. The on/off nature of the quick exhaust and the tendency for quick exhaust valves to have excessive Cvs compared to boosters for the same nominal pipe size easily can cause excessive overshooting. You can minimize this by limiting the quick exhaust Cv relative to the booster Cv to no more than a 1.5:1 ratio, and by using a small booster to “pilot” the quick exhaust valve, so that it is shut off by the pilot booster as quickly as possible once the desired valve position is reached.
Herbert L. Miller, PE, Consultant