Solenoid Valves: Which Position Should It Fail To?
Normally closed or normally open is one line in a datasheet, but the answer is a safety decision. When the power goes, the valve spring runs your process.
The shortest line on a solenoid valve order is usually the one with the largest consequences: normally closed, or normally open. This is not a model preference. A solenoid valve holds its position only while the coil is energised; when the power goes, the spring returns it to the position it was built for. At that moment your process is run by the spring, not by you.
Power loss is a design case, not a fault
Supply will be interrupted one day, a fuse will blow, a cable will be cut. The right question is not “what if it fails?” but “which position does less harm when it fails?”
On dosing, filling and fuel supply duties, flow continuing on its own means overflow, overdosing or a hazardous feed. A normally closed valve is the natural choice: no power, no flow.
The opposite case is just as common. On a line feeding cooling water to hot equipment, stopping flow removes cooling exactly when control has been lost; closing a vent or relief path locks the system in. There a normally open valve fails to the safe side.
To decide, finish two sentences: “the worst outcome if this valve shuts by itself is…” and “the worst outcome if it stays open is…”. The lighter outcome is the valve’s de-energised position.
What actually does the opening
The second decision is which force opens the valve, and it is noticed on the first day of commissioning.
In a direct acting valve the coil’s magnetic field lifts the core straight off the seat. Electricity does all the work, so line pressure is irrelevant to its operation: it opens at zero differential. The price is that it stays within small flow paths, because a coil able to lift a large orifice directly is not practical.
In a pilot operated valve the coil only opens a small pilot orifice. The fluid’s own pressure does the real work: pressure above the diaphragm bleeds away through the pilot, and pressure below lifts the diaphragm. Large flow paths become controllable with a modest coil.
A common mistake: a pilot operated valve needs a minimum pressure differential between inlet and outlet to work at all. Fit one on a gravity-fed tank outlet, an unpressurised drain, or a branch whose pressure drops to nothing when the pump stops, and the coil energises, the magnet pulls — and the valve does not open. It looks like a failure; it is correct behaviour by design. Where pressure cannot be guaranteed under every condition, choose the direct acting type.
If the coil stays energised
A coil produces heat for as long as it is energised and passes that heat into the body, the fluid and the surroundings. On a dosing valve that opens for a few seconds a day this has no practical consequence. If the valve sits energised throughout normal operation it does: winding, ambient and fluid temperature stack up, insulation ages and coil life shortens.
So a valve that will be open for nearly all of its service time is better specified normally open, which protects both the coil and the energy bill. Where safety logic and coil life disagree, safety wins — a burnt coil gets replaced, an overflowed tank does not.
Body and wetted parts
General purpose models are built for clean water, compressed air and neutral media, and cover most duties in a plant. Corrosive media and food processes call for a stainless body. The decision does not end there: the diaphragm, spring and seal in contact with the fluid must suit it too. A valve with a stainless body and an unsuitable seal is simply a valve that will leak soon.
From the catalog
Products mentioned in this article
The technical values of the products below are extracted from the company's printed catalog, not from the article text.
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