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Technical Articles
January 19, 2016

Fail-Safe Valves

Normally-Closed Valves, Normally-Open Valves, and Valves that can be Converted between Normally-Open and Normally-Closed

Actuated or automatic valves that revert to a pre-determined position after the actuating force is removed are referred to as “fail-safe” valves. The most common type is “fail-safe normally-closed.” On the other hand, “fail-safe normally-open” valves are much less common, but are equally important. These are often found in cooling systems, or are used where flow is shut off only for periodic maintenance, etc. Some valves, such as spring-loaded air actuated ball valves, can be converted between types.

failsafe-explanationThe illustration below shows two air-actuated shut-off valves, identical but for the fact that one is normally-open and the other is normally-closed. The only real difference between the valves is the location of the spring in relation to the piston. Because these valves use compressed air to overcome the spring force, the air acts on the opposite side of the piston. If you study the valves, you can see where the air is fed in.

 

Normally-Closed Valves

An example of a normally-closed actuated valve is Plast-O-Matic’s Series EASMT Solenoid Valve. The valve opens when energized, and when that current is turned off, an isolated spring inside the valve forces it closed. The magnetic force of the solenoid coil must overcome the force of the spring to open the valve. Unless acted upon, the spring force keeps the valve closed. In the event of a power failure, the valve will automatically close.

An example of a normally-closed automatic valve is Plast-O-Matic’s Series CKM Check Valve. The valve opens when pressure is present at the inlet (or upstream) side. When that pressure ceases, the diaphragm inside the valve automatically forces it closed. The inlet pressure must overcome the force of the diaphragm to open the valve. Unless acted upon by inlet pressure, the diaphragm force keeps the valve closed. In fact, the valve closes before reverse flow can take place. (note that the above refers exclusively to Series CKM/CKS; most check valves, including ball type, are not normally-closed valves.)

Normally-Open Valves

An example of a normally-open actuated valve is Plast-O-Matic’s Series EASY-NO Solenoid Valve. The valve closes when energized, and when that current is turned off, an isolated spring inside the valve forces it open. The magnetic force of the solenoid coil must overcome the force of the spring to close the valve. Unless acted upon, the spring force keeps the valve open. In the event of a power failure, the valve will automatically open.

An example of a normally-open automatic valve is Plast-O-Matic’s Series PRE Pressure Regulator. The valve is set to remain open at a predetermined pressure, and begins to close automatically when downstream pressure exceeds that predetermined pressure. When pressure downstream drops back to accepted levels, a spring inside the valve forces it back open. The downstream pressure must overcome the force of the spring to close the valve. Unless acted upon by excessive downstream pressure, the spring keeps the valve open.

Valves that can be Converted between Normally-Open and Normally-Closed

Air-Actuated, Spring-Return Ball Valves — Plast-O-Matic Series ABVS, ABRS & ABMS — are examples of valves that can be normally-open or normally-closed. In these valves, a rather large spring is used to force the actuator back to its original position, whether that is open or closed.

One of the advantages of a Plast-O-Matic Air x Spring Actuated Ball Valve is that it can be converted fairly easily between one type or the other. Because the spring forces the actuator one way or the other, it is simply a matter of redirecting what the spring acts upon.

With Series ABVS, this is done by backing off the adjusting cap (so the spring will not be compressed) then unscrewing the spring housing from one side of the actuator and moving it to the opposite side. (Likewise, the compressed air is swapped).

Converting Series ABRS & ABMS is a little more complex. The actuator must be removed from the valve (very easy to do), then a retaining ring on the bottom of the actuator must be removed (requires retaining ring pliers). The pinion shaft is then pushed out from the top, rotated 180º, then re-inserted. The actuator will then act upon the valve opposite the way it did before.