Solenoid Valves

General Purpose Solenoid Valves: Operating Types, Seal Selection and Uses

General purpose solenoid valves open and close the flow of water, air, light oil and neutral gases automatically on an electrical signal. Learn about operating types, seal selection and how to choose.

ORFA Technical TeamUpdated: Oct 30, 20264 min read

Opening and closing liquid and gas flows remotely, quickly and repeatably is a basic requirement of many machines and installations in automation. A general purpose solenoid valve is an electromechanical valve that converts electrical energy directly into mechanical movement to control the flow of water, air, light oil and neutral gases automatically.

This article explains how general purpose solenoid valves work, the difference between direct-acting and pilot-operated types, seal and body selection, applications and what to consider when choosing a valve.

What Is a General Purpose Solenoid Valve?

A general purpose solenoid valve consists of a coil, a metal plunger (core) moving inside the coil, a return spring and a sealing element that opens and closes the flow path. When the coil is energised, the magnetic field moves the plunger and the valve changes position; when power is removed, the spring returns the valve to its rest position. “General purpose” means the valve is designed for common industrial media rather than special media such as steam, natural gas or aggressive chemicals.

Operating Types

Type	Operation	Suitable for
Direct-acting	The plunger opens and closes the main orifice directly; no differential pressure needed	Small sizes, low flows, operation from zero pressure
Pilot-operated (servo)	The plunger opens a small pilot orifice; line pressure moves the diaphragm or piston to open the main orifice	Large sizes and high flows; minimum differential pressure required
Assisted-lift (semi-direct)	The plunger is mechanically linked to the diaphragm; line pressure assists opening	Large sizes at low or zero differential pressure, e.g. gravity-fed lines

Valves are also classified as normally closed (NC) or normally open (NO) depending on their de-energised position, and as 2-way (2/2) or 3-way (3/2) by the number of ports. The de-energised position should be chosen according to the safe state of the system.

Seal and Body Material Selection

NBR: A common, economical seal for water, compressed air, light mineral oils and neutral gases.
EPDM: Preferred for hot water and drinking water; not compatible with mineral oils.
FKM (Viton): Suitable for oils, fuels, higher temperatures and some chemicals.
Brass body: The most common, economical body for water, air and light oils.
Stainless steel body: Preferred for corrosive environments, hygienic applications and some chemicals.

Advantages

Fast response: Short opening and closing times improve productivity in repetitive tasks such as filling and dosing.
Automation-ready: Controlled directly by PLCs, timers, thermostats or level controllers.
Compact design: Easy to use in machines and tight spaces.
Defined fail-safe position: Returns to a set position by spring when power is lost.
Low energy consumption: Low-power coils keep operating costs down.

Applications

Water treatment and irrigation systems
Compressed air lines and pneumatic systems
HVAC systems and cooling water circuits
Automotive assembly lines and industrial machinery
Food processing and filling machines
Washing machines and automatic water supply systems

Selection Criteria

Medium: The type, temperature and viscosity of the medium determine the seal and body material.
Pressure: Minimum and maximum differential pressure must suit the valve type (direct, pilot, assisted-lift).
Connection size and flow: Chosen according to the required capacity.
Normally open/closed: Determined by the safe position of the system on power failure.
Coil: Supply voltage (e.g. 220 V AC or 24 V DC), protection class (typically IP65) and duty.

Tip: Use steam solenoid valves on steam lines and natural gas solenoid valves complying with the relevant standards on gas lines; general purpose valves are not designed for these duties. A strainer upstream of pilot-operated valves prevents the pilot orifice from clogging.

Frequently Asked Questions

What is the difference between direct-acting and pilot-operated solenoid valves?

A direct-acting valve opens the orifice using coil force alone and works from zero pressure, but is limited to small sizes. A pilot-operated valve uses line pressure to open large sizes with a small coil, but needs a minimum differential pressure.

Why does a solenoid valve not open?

For pilot-operated valves, the most common cause is insufficient differential pressure. A wrong coil voltage, a burnt-out coil or a pilot orifice blocked by dirt can also prevent opening.

Should I choose a normally closed or normally open solenoid valve?

Choose normally closed (NC) where the valve is mostly closed and flow must stop on power failure. Choose normally open (NO) where flow is continuous and must continue during a power cut.

Conclusion

General purpose solenoid valves are a fast, compact and reliable solution for automatic control of water, air, light oil and neutral gases. A valve selected with a compatible seal, the right operating type for the pressure conditions and the correct coil will work trouble-free in automation systems for a long time. Browse the models in our general purpose solenoid valves product group, or contact ORFA at +90 224 444 67 32 or [email protected] for selection support.

Online Store