Control Valve Fail Safe Action

There are two primary system considerations centering around an emergency operational situation for control valves.

I. Loss of instrument air supply (IAS) pressure

II. Loss of electrical power

There are four choices as to control valve “response” to the emergency condition :

a.) Fully Close

b.) Fully Open

c.) In Last Position

d.) Continue Throttling

For either fail-safe loss of IAS or loss of electrical power, it is the actuator’s benchset range spring that “drives” the control valve’s plug to its fail-safe open or closed position.

I. Loss of Instrument Air Supply Pressure

a.) Fully Close

The actuator’s benchset range spring “drives” the valve plug “closed” when loading air pressure goes towards or near 0 psig. (ATO-FC “Reverse”)

Because IAS piping systems can be extensive (big), the IAS can decay too slowly causing operational problems.

In such cases it may be desirable to use a 3-way pilot switching valve to “anticipate” the eventual loss of IAS and quickly stroke the control valve to its fail-safe position, eliminating the transitory operational effects of the slow decay.

b. Fully Open

The actuator’s benchset range spring “drives” the valve plug “open” when loading air pressure goes toward or near 0 psig. (ATC-FO “Direct”)

A similar scheme to I.a. previous may be applied for ATCFO arrangements for fail open control valves to eliminate transitory effects.

c.) In Last Position

This is accomplished with a “lockup valve”, which is a 2- way pilot switching valve. If the air pressure is lost from the control loop’s airset, the lockup valve will “close” and “trap” the air within the actuator’s loading chamber.

If no air leaks are present, the control valve will stay at the last throttling position. Such loops are normally “local”. (Control valve action is a non-issue.)

d.) Continue Throttling

This is an “elaborate” system that can be accomplished two different ways

i.) Volume Tank

A volume tank stores a limited supply of IAS to sustain normal operation for a finite time period, i.e. approximately up to a hour.

ii.) Alternate Supply

A high pressure cylinder is on standby with an alternate gas, usually GN2. If IAS pressure falls below 50 psig, the alternate gas will take over to supply pneumatic control. The high pressure will allow for several hours of sustained operation, enough time for the emergency loss of the IAS to be restored.

Such applications are normally for “very important (critical)” services. Such loops are normally “local” and pneumatic only; i.e. no electrical power involved except the low pressure alarm switch.

II. Loss of Electrical Power

Protection for loss of electrical power is indirectly accomplished thru the use of a solenoid valve. In normal operation the solenoid valve is energized electrically; in an emergency loss of electrical power the solenoid valve is de-energized and trips to the “shelf-position”. Solenoid voltage can be 120 VAC, 240 VAC, 125 VDC, 24 VDC, etc.

a.) Fully Close

By blocking the LOAD air to the actuator and venting the air within the actuator’s loading chamber to atmosphere (P = 0 psig), the actuator’s benchset range spring “drives” the valve plug “closed”. The solenoid valve is a 3-way type.

b.) Fully Open

By blocking the air to the actuator and venting the air within the actuator’s loading chamber to atmosphere (P= 0 psig), the actuator’s benchset range spring “drives” the valve plug open. The solenoid valve and tubing interconnection is identical to the II.a. case previous.

c.) In Last Position

This is accomplished with a 2-way solenoid valve. In the “energized” condition, the solenoid valve ports are “open”; loading air passes thru. In the “de-energized” condition, the solenoid valve “closes” and traps the air within the actuator’s loading chamber.

If no air leaks are present, the control valve will stay at the last throttling position. (Control valve action is a non-issue.)

Abbreviations :

• IAS : Instrument Air Supply
• FTC : Fail to Close
• FTO : Fail to Open
• FO : Fail Open
• FC : Fail Close
• S/SV : Solenoid Valve

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