When designing a system using stepper motor style electric valves, such as CDS or SDR valves and A8-to-CDS conversion kits, in head pressure control applications additional precautions and items should be considered.
Electric valve/controller operation:
Electric valves operate without a feedback loop. It is possible that over the life of an electric valve step loss may occur, and reinitialization of the valve may be needed. This reinitialization occurs when the valve is closed and overdriven to guarantee its position. When driven at 200 steps/second the reinitialization sequence is completed after approximately 40 seconds.
The Kelvin II Series Pressure Controller and most other controllers reinitialize when power is cycled to the controller (at start-up or after power loss). The Kelvin II Pressure Controller has a built-in safety. If condenser pressure spikes during reinitialization; the controller alarms, terminates reinitialization and normal operation (pressure control) begins.
In general, periodic reinitialization during operation is not recommended for the Holdback Valve. Reinitialization of the holdback valve should only occur when the system is offline. Additionally, the Pressure Control has a built-in algorithm that monitors valve position over time. After 24 hours has lapsed and a 0% open valve position is observed, the controller will reinitialize the valve to verify its position. This only affects the Receiver Pressurization Valve since the Holdback Valve never fully closes.
Electric head pressure control recommendations:
- When using electronic valves for holdback and receiver pressurization, successful operation and control has been seen with setpoint differentials as low as 5-10 psid. When electronic holdback is used with mechanical receiver pressurization valves, the pressure differential should be maintained no lower than 15-20 psid.
- Reinitialize controller when rack is offline.
- The compressor high pressure safety limits must be set to rack manufacturer’s recommendations.
- After power loss, compressor restarts should be delayed 90 seconds to allow for the electric valve’s reinitialization to complete. If not possible, there should be a 45 second delay between subsequent compressor starts. These recommendations will serve to stop a compressor for tripping off on high pressure safety.
- During installation pressure transducer calibration should be verified.
- Reinitializing the valve, while the rack is down, as part of an annual preventive maintenance routine is suggested.
- Service valves may be installed around the electric valves and/or before and after the valve to aid in troubleshooting, servicing and/or reinitialization.
- A bypass valve can be installed in parallel with the electric holdback valve if desired (but is not required). This bypass valve can prevent a compressor trip due to high pressure spike if compressor delays are not implemented as recommended above. An A8 valve with a high pressure setting would serve this purpose.
Third party controller recommendations:
- Periodic reinitialization of the Electric Holdback Valve is not recommended. Its reinitialization should only occur when the rack is offline.
- Sporlan Electric Valves should be driven at 200 steps/second.
Troubleshooting excessive step loss:
If excessive step loss is forcing frequent reinitialization, further investigation in the cause is needed. Excessive step loss may be caused by:
- Damaged wiring
- Wiring run in parallel with high voltage lines
- Damaged or worn valve(s)
- Poor controller output signal
All these should be investigated and eliminated to provide reliable operation.
Article contributed by Dustin Searcy, product manager, Electric Valves, Sporlan Division, Parker Hannifin