Power fails on a fuel skid at 2 a.m. Every spring-return solenoid valve on that skid immediately moves to its de-energized state, and nobody in the building chose the moment. You chose the outcome months earlier, however, when you specced normally open or normally closed. That letter pair on the datasheet for a two-way solenoid valve decides whether each line is flowing or blocked when the coils go dark, so it belongs at the top of the spec, ahead of Cv, voltage, and body material.
In this blog post, we're going to look at normally closed and normally open two-way solenoid valves, focusing on what they do when power is lost, how duty cycles work, and the difference between direct-acting and pilot-operated.
What NO and NC Mean: The De-Energized Default State
A normally closed (NC) solenoid valve stays closed with no power applied and opens when the coil is energized. A normally open (NO) solenoid valve rests open and closes when energized. The word "normal" describes the de-energized resting state and nothing else. A normally closed valve on a batching line might sit energized and open for 90% of its service life, and it's still normally closed, because the spring puts it on the seat the moment the coil loses power. Keep that distinction straight, because it drives the duty-cycle math later in this article.
The basic mechanics are simple. Inside the body of a direct acting solenoid you have a coil, a plunger (the armature), a return spring, and a seat surrounding an orifice. In an NC direct-acting valve, the spring presses the plunger against the seat and blocks the orifice. Energize the coil and the magnetic field pulls the plunger off the seat against spring force, and media flows. Most NO valves basically invert the arrangement: the spring holds the plunger off the seat at rest, and coil current drives it to close. Pilot-operated and other designs also use pilot passages plus a diaphragm, piston, or other main-valve element in addition to what we just described.
NC solenoid valves are very widely used for on/off isolation duties. Most processes want flow to stop when something goes wrong, and for those applications an NC valve delivers that without needing a single line of logic.
The figure below illustrates typical 2-way, direct-acting, spring-return solenoid valve operation where "Normally” describes the valve’s de-energized state.
**Internal construction, permitted flow direction, pressure-differential requirements, and port connections vary by valve model; verify the manufacturer’s data sheet and flow schematic.
Fail-Safe Logic: What Happens When Power Is Lost
The first spec decision is what the line must do when power disappears. That could be a plant-wide outage, a tripped breaker, a cut wire, an E-stop, or a controller fault. In every one of those events, the coil de-energizes, and the return spring takes over. Settle this before you look at pressure ratings or coil voltage, because it dictates the configuration outright.
When a conventional NC solenoid valve is used, any loss of power automatically isolates the line. That's why NC is preferred for safety-critical isolation of fuels, gases, and chemical feeds, where uncontrolled flow is the hazard. A de-energize-to-safe valve function can reduce risk during loss of electrical power, serving as a critical layer of a complete process-safety design.
NO valves are correct where flow must continue on power loss. Cooling water circuits, compressor unloading and venting lines all fall in this category. A furnace jacket that loses cooling water during a blackout is a bigger problem than the blackout itself, so the valve feeding it should fail open.
In short, use NC where loss of power must isolate flow; use NO or fail-open/vent arrangements where loss of flow creates the greater hazard. Write down why you selected the solenoid you did in the project file so the next engineer understands. And always confirm the safe state from the process analysis.
Weighing a borderline fail-safe call on a critical line? Call a ValveMan engineer at 888-825-8800 — real engineers, not a call center — and we'll work through it with you.
Duty Cycle and Coil Energy: A Worked Example
A conventional monostable solenoid coil draws current the entire time it holds the valve in its non-normal state. The working rule of thumb is this: a valve that flows 10 minutes per hour should probably be NC, and a valve that flows 50 minutes per hour should probably be NO. Match the resting state to the majority state and the coil rests most of its life. But do not let this optimization override the required fail position - safety comes before economy.
Here is a number worth running as an example. Take a 20 W coil held energized around the clock. That coil draws 20 W × 8,760 hours, which is about 175 kWh per year. At an assumed $0.12 per kWh, you pay roughly $21 per valve per year for a 20 W DC coil or an AC coil with an equivalent measured/rated real-power draw. One valve is lunch money. Forty valves on a skid is about $840 a year, plus 800 W of continuous heat dumped into enclosures and panels.
Extended energization also degrades winding insulation and can cause premature coil failure, so a coil that spends its life energized dies years before an identical coil that rests. Excess temperature from high ambient temps, overvoltage, inadequate enclosure ventilation, heat from the process, or improper coil selection can all accelerate insulation aging and can shorten service life.
That said, a properly specced and implemented continuous-duty rated coil will hold indefinitely at its rated voltage and ambient temp, but high ambient temp plus self-heating still shortens its life.
If a valve will remain energized for long periods in a hot area, verify its continuous-duty rating and temperature limits. If those environmental conditions exceed the published limits, change the coil, valve design, enclosure, ventilation, or control strategy rather than plan for premature replacement.
Coil behavior also differs by supply. AC coils pull a high inrush current at pickup and then drop to a lower holding current, while DC coils draw a steady current and run more predictably during long holds. We cover the trade-offs in our guide to AC vs DC solenoid coils, and it's worth reading before you size panel power for a bank of held-energized valves. If you're replacing coils that cooked themselves, start with our solenoid coils and match voltage and duty to the service.
Direct-Acting vs Pilot-Operated in Both Configurations
Both NO and NC bodies come in two operating principles, and picking the wrong one strands you with a valve that will never shift.
A direct-acting valve uses coil force alone to move the plunger. It can work from zero pressure differential, which makes it a good choice to consider for gravity drains, vacuum service, and low-pressure lines. The cost is flow: coil force limits how large an orifice the plunger can manage, so Cv stays modest. Browse direct-acting solenoid valves when the line pressure is low or unpredictable.
A pilot-operated (servo-assisted diaphragm) valve uses the media's own differential pressure to open and close the main orifice, so a small coil controls a large port. These designs can provide high flow capacity with a relatively small solenoid operator because the system pressure moves the main diaphragm or piston. The catch is the minimum operating pressure differential: without it, the diaphragm may never shift, in either configuration.
Check the datasheet figure against your worst-case line pressure before ordering from the pilot-operated solenoid valves range. The classic mistake is a pilot-operated valve on a gravity-fed tank drain. The coil clicks, the wiring checks out, and the valve sits there closed, because a few feet of head will never generate the differential the diaphragm needs. When a minimum differential cannot be maintained, you should probably consider direct-acting, assisted-lift, hung-diaphragm, externally piloted, or other valve design specifically rated for zero or low differential pressure.
Seal material follows the media. Want the full walkthrough on operating principle and Cv sizing? Read our solenoid valve selection guide, then call 888-825-8800 to match seat and body materials to your service before you order.
Get the Configuration Right the First Time
NO or NC, direct-acting or pilot-operated, coil duty and seal material — get one wrong and the valve either won't shift or won't survive. Call our team in Exton, PA at 888-825-8800, or reach us through the contact page, and talk to a real engineer who's specced these for the service you're running.
