Issue
What is pneumatic actuator spring range shift and how can it be eliminated?
Product Line
Field Devices
Environment
Pneumatic Actuator
Cause
Spring range shift is an inherent feature of all spring return pneumatic actuators without positive positioners. The nominal published spring range of a pneumatic actuator without a positive positioner is the air pressure required to fully stroke the actuator WITH NO EXTERNAL LOAD. In the extend direction against an opposing load as provided by a damper or a valve, the supply air pressure must be increased above the spring range of the actuator to overcome the spring and the opposing force. In the retract direction against an opposing load as provided by a damper or a valve, the supply air pressure must be decreased below the spring range of the actuator so that the spring can overcome the air pressure and the opposing force. Under constant load conditions, the range of the actuator is offset by the amount of air pressure required to provide close-off of the damper or the valve.
Resolution
When is "Spring Range Shift" a concern?
It is a concern when two or more pneumatic controlled devices should be sequenced from one pneumatic signal without an overlap. An example would be a normally open heating valve and a normally closed cooling valve being sequenced from a direct acting thermostat, In this example the spring range of the normally open valve will shift upward and the normally closed valve will shift downward which means the two valves may overlap. Another area of concern is that pneumatic control signal provides proper pressure span to operate the control device and provide "tight" close-off of damper or valve. Examples of limited pressure change would be: pneumatic controllers maximum output which is limited by main air pressure; the low pressure output capability of a pneumatic controller; and electronic to pressure transducer which can have a limited output such as 3 to 15 psig.
What can be done to overcome "Spring Range Shift" when sequencing devices?
Sequencing can be accomplished in a pneumatic system by selecting actuators with appropriate spring ranges, utilizing positive positioners (AK-42309-500 or N800-0555) or ratio relays with bias adjustment (2378-501) or controllers with separate outputs for each actuator.
- Positive positioners on damper and valve actuators provide the versatility of start point and span adjustment plus the availability of full pneumatic power for positioning. This is the ideal way to provide precision sequencing, since variable loading on the actuator will not effect the sequencing.
- Biased start ratio relays may be used to provide a method of sequencing by amplifying or shifting the pneumatic signal.
- For applications involving sequencing a pressure to electric (P.E.) switch with a pneumatic actuator, the P.E. can often be adjusted to operate outside the operating range of the actuator.
- Sometimes larger actuators will help the sequencing problem.
- Using full actuator stroke to rotate the damper less than 90 degrees increases the force delivered to the damper and therefore reduces control signal change required to control the actuator. Rotating the damper 60 degrees usually provides almost full flow through the damper.
- If the actuator has an adjustable start point, adjusting the start points can help prevent sequencing problems. Normally open valves or dampers should have start point adjusted to the minimum and normally closed adjusted to the maximum.
- Sequencing problems can be eliminated by providing controllers with separate outputs for each actuator and do the sequencing in the controller. What can be done to increase the close-off of a damper or valve actuator due to limited or insufficient pneumatic control pressure to the control device? Proper control and close-off of the control device can be accomplished by selecting actuators with appropriate spring ranges, utilizing positive positioners (AK-42309-500 or N800-0555) or ratio relays with bias adjustment ( 2378-501).
- Positive positioners on damper and valve actuators provide the versatility of start point and span adjustment plus the availability of full pneumatic power for positioning (0 to main air pressure to the actuator).
- Biased start ratio relay provide a method of amplifying or shifting the pneumatic signal.
- Selecting larger actuators will increase close-off pressure.
- Using full actuator stroke to rotate the damper less than 90 degrees increases the force delivered to the damper. Rotating the damper 60 degrees usually provides almost full flow through the damper.
- If the actuator has an adjustable start point, proper adjustment of the start points will increase close-off. Normally open valves or dampers should have start point adjusted to the minimum and normally closed adjusted to the maximum.
- Provide electronic to pneumatic transducers with sufficient input span to obtain maximum or sufficient pneumatic output.
