Service Tip: Solenoid Valves

Q: What does the MOPD rating on solenoid valves mean?

A: Solenoid valves are rated in terms of Maximum Operating Pressure Differential (MOPD) against which the valve will open. For example, with the valve closed against an inlet pressure of 250 psi and an outlet pressure of 50 psi, the pressure differential across the valve is 250 minus 50, or 200 psi.

The MOPD rating for the valve is the maximum pressure differential against which the valve will reliably operate. If the pressure differential is higher than the rated MOPD for the valve, the valve will not operate. The temperature of the coil winding and the applied voltage has a significant affect on the MOPD rating. The MOPD is reduced as the coil temperature increases or the voltage decreases. For this reason, the MOPD rating is established by operating the valve at 85% of rated voltage after the coil has attained its maximum temperature by operating at full rated voltage.

Q: Why do some valves have a "Min OPD" rating? What does this mean?

A: "Min OPD" stands for Minimum Operating Pressure Differential. All pilot operated valves (like the Emerson 200 and 240 series) require a small amount of pressure differential to enable the piston or diaphragm to raise off the main seat. Typically, 2 to 5 psig differential is needed to accomplish this. If the pressure differential is less than the Min OPD, then the valve will not open when actuated or will fail to remain open. If a valve is greatly oversized for the application, it can suffer from this affect since the pressure drop across the valve with low flow rates can be below the Min OPD.

Direct operating solenoids (like Emerson’s 50RB and 100RB) do not have a Min OPD specification since they do not rely on system pressures for operation.

Here are some common solenoid problems and causes:
Won't open: valve oversized; not within 10% rated voltage; coil burnout; bent enclosing tube; improper wiring.
Won't close: broken internal parts; installed backwards; foreign material in orifice area; manual stem front-seated.
Intermittent operation: transformer undersized; valve oversized for application; (pressure drop below Min OPD).

Copyright 2005 Emerson Climate Technologies, Inc.

All rights reserved. Used by permission.

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