The advantages of a direct acting solenoid valve

A direct acting solenoid valve controls the flow of gas or liquid purely by electromagnetic force, without depending on system pressure. That fundamental difference from pilot-operated solenoids determines in practice which valve you need and when a traditional indirect valve falls short.

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How does a direct acting solenoid valve work?

A direct acting solenoid valve consists of three core components: a copper coil, a plunger and a valve seal. As soon as electric current flows through the coil, an electromagnetic field is created that pulls the plunger towards it and away from the valve seal. That movement opens the valve and lets the medium flow through. As soon as the current is removed, a spring pushes the plunger back to its starting position and the valve closes.

There is no pilot channel, no diaphragm assembly and no dependence on the inlet pressure. The electromagnetic force of the coil is the only actuation.

The core difference from a pilot-operated valve

An indirect, pilot-operated valve uses the system pressure as its working force to open the main passage. That works very well with sufficient and stable pressure, but always requires a minimum inlet pressure of 1.5 to 3 bar and a pressure drop across the valve in one direction. A direct acting solenoid works completely independently of this, from vacuum up to the maximum working pressure.

The Humphrey design: three ports, four functions

Humphrey does not work with separate valve types for each function. The starting point is a universal 3-port design in which port 2 is always the common port. In every position, port 2 has an open path to either port 1 or port 3. By connecting the valve differently or plugging a port, the same valve can be configured for four different functions.

Need a simple 2-port on/off valve? Then plug port 1 or port 3. Combine two normally open or two normally closed valves and you have the functionality of a 5-port directional valve for double-acting cylinders. This universal design reduces the number of valve variants needed in a system and simplifies inventory management and maintenance.

Fewer part numbers, more flexibility

Because one valve type can be configured as NC, NO, diverter or selector, an engineer or buyer needs fewer different valve variants. In systems with multiple functions, this results in direct savings in purchasing, logistics and spare parts.

The advantages at a glance

1. Works at any pressure, including zero bar and vacuum

The valve opens and closes at any inlet pressure. No minimum working pressure is required. This makes it suitable for systems that start up without pressure, for vacuum lines and for applications in which the pressure varies strongly or is unpredictable. A pilot-operated valve fails in precisely these situations.

2. Bi-directional use

Because the valve is actuated mechanically and does not depend on a pressure drop in one direction, it can pass media in both flow directions. In systems with changing flow direction, this is a requirement that a pilot-operated valve cannot meet without additional provisions.

3. Fast and accurate response

Without a pilot channel, the valve responds immediately to the electrical signal. Humphrey valves are suitable for high switching frequencies, which makes them usable for pulsating applications, time-critical dosing and fast sequences in automated systems. The direct plunger movement ensures repeatable, precise fluid control.

4. Compact and lightweight

Without pilot chambers and diaphragm assemblies, direct acting valves are considerably smaller. Humphrey supplies valves that literally fit in the palm of your hand and yet are fully suitable for industrial use. For OEM integration, manifold mounting or portable equipment, this size is a decisive advantage.

5. Suitable for clean and aggressive media

Humphrey supplies valves with PTFE seals and inert wetted parts made of stainless steel or inert plastics. Because there are no internal pilot channels, there are fewer surfaces where contaminants can accumulate. This makes the valves reliable in medical applications, laboratories and systems with high-purity gases or corrosive liquids.

6. Reliable fail-safe behaviour

A normally closed direct acting valve closes immediately as soon as the voltage is removed, regardless of the system pressure present. With a pilot-operated valve, this can fail if the pressure is too low at that moment. In safety-relevant applications in which the system can be both de-energised and depressurised, this difference is crucial.

Direct acting versus pilot-operated

Direct acting

+ Works from vacuum to maximum pressure

+ Bi-directional use

+ Fast response time, no pilot delay

+ Compact and lightweight

+ Fail-safe without system pressure

+ Suitable for clean and aggressive media

+ Four functions from one valve type

- Higher coil power consumption with large orifices

- Smaller maximum Cv with limited coil force

Pilot-operated (indirect)

+ Large orifices possible with a small coil

+ Lower power consumption at high Cv

- Minimum pressure of 1.5 to 3 bar required

- Not usable with vacuum

- One flow direction required

- Slower due to pilot delay

- More internal volumes, higher risk of contamination

- Fail-safe operation depends on system pressure

When should you choose direct acting?

Choose direct acting

Low or varying system pressure

The system starts up without pressure, operates under vacuum or has pressures below 1.5 bar.

Choose direct acting

Bi-directional flow

Gas or liquid flows through the valve alternately from both sides.

Choose direct acting

Medical or high-purity applications

No internal pilot channels that could introduce or trap contaminants.

Choose direct acting

High switching frequency required

Pulsating processes or time-critical sequences in which pilot delay is unacceptable.

Choose direct acting

OEM integration and miniaturisation

Lack of space or weight restrictions in devices, manifolds or portable systems.

Choose direct acting

Safety-relevant applications

Fail-safe closing required even in the event of power failure and depressurised condition.

Choose indirect acting

Large orifices at stable high pressure

With large Cv values and a constant high system pressure, an indirect valve can be more energy-efficient.

Choose indirect acting

Heavy industrial pneumatics

Large actuators with a stable compressed air supply above 3 bar and no requirements regarding flow direction or vacuum.