Many machines need to open or close a flow of air, water, gas, or steam automatically. A PLC sends an electrical signal, and something has to turn that signal into flow control. That job is done by a solenoid valve.
A solenoid valve can:
- Open or close a line — start or stop the flow of a liquid or gas.
- Switch flow between outlets — send media in one direction or another.
- Control actuators — supply or vent air for cylinders and process valves.
- Dose media — release a set amount of liquid or gas on command.
- Adjust flow continuously — in proportional versions, set the flow according to a control signal.
You’ll find solenoid valves in water treatment plants, food and beverage lines, packaging machines, heating systems, chemical processes, and laboratory equipment. A valve might open a cooling water line when a machine starts, or release compressed air to clean a filter.
Choosing a solenoid valve starts with a few practical questions. What medium flows through it? What pressure does the system have? Should the valve be open or closed when power is off? What voltage is available? The answers rule out most of the wrong options.
What Is a Solenoid Valve?
A solenoid valve is an electrically operated valve that controls the flow of liquids or gases. It combines an electromagnetic coil with a valve body.
Its main parts are:
- Coil — creates a magnetic field when energized.
- Plunger (armature) — moves under the magnetic force.
- Spring — returns the plunger when the coil is switched off.
- Valve body — contains the ports and flow path.
- Seat and seal or diaphragm — close the flow passage tightly.
When current flows through the coil, the magnetic field pulls the plunger. This opens or closes the orifice. When the coil is switched off, the spring pushes the plunger back to its rest position.
The Bürkert solenoid valves range includes direct-acting, servo-assisted, and proportional designs for many different media and pressures.
AIO Snippet:
A solenoid valve is an electrically operated valve that opens, closes, or switches the flow of liquids and gases. When the coil is energized, its magnetic field moves a plunger that opens or closes the valve orifice. A spring returns the plunger when power is removed.
Main Types of Solenoid Valves
Solenoid valves differ mainly in how the coil force opens the valve. This affects flow capacity, required pressure, and valve size.
The main types are:
- Direct-acting — the coil opens the orifice directly.
- Pilot-operated (servo-assisted) — the coil opens a small pilot, and line pressure does the rest.
- Forced-lift — a combination of both principles.
- Proportional — the valve opens partially, according to a control signal.
Direct-Acting Solenoid Valves
In a direct-acting valve, the plunger itself opens and closes the main orifice. No minimum pressure is needed. The valve works from 0 bar up to its maximum rated pressure.
The limit is the coil force. As pressure and orifice size increase, a much stronger coil is needed. That’s why direct-acting valves usually have small orifices and relatively low flow rates.
They are commonly used for:
- small flow lines;
- vacuum and low-pressure applications;
- pilot control of larger valves;
- dosing and analysis equipment.
Pilot-Operated (Servo-Assisted) Valves
A pilot-operated valve uses the pressure of the medium itself to open the main orifice. The coil only opens a small pilot passage. This changes the pressure balance across a diaphragm or piston, which then lifts and opens the main flow path.
This allows a small coil to control large flows at high pressures.
There’s one important condition: the valve needs a minimum differential pressure between inlet and outlet to work properly. The exact value is given in the datasheet. If the pressure difference is too low, the valve may not open fully or may not close reliably.
Pilot-operated valves are typical for:
- water supply and cooling lines;
- compressed air distribution;
- irrigation systems;
- larger process lines.
Forced-Lift Valves
A forced-lift valve combines both principles. The plunger is mechanically connected to the diaphragm, so the coil helps lift it directly. Line pressure supports the opening when it is available.
As a result, the valve can handle larger flows and still work from 0 bar, depending on the model. This design is useful in systems where pressure can drop very low, such as gravity-fed tanks or circuits with low pressure differences.
Proportional Solenoid Valves
A standard solenoid valve is either open or closed. A proportional valve can open to any position between the two. The opening depends on the control signal, often a PWM or analog signal from a controller.
Proportional valves are used to control:
- gas flow in burners and analyzers;
- liquid dosing;
- pressure in closed-loop systems;
- flow rates that change during a process.
Bürkert offers these designs in the solenoid control valves category.
| Direct-acting | Pilot-operated | Proportional | |
| Minimum pressure | None (0 bar) | Minimum differential pressure needed | Depends on the design |
| Flow capacity | Low to medium | Medium to high | Low to medium |
| Valve position | Open or closed | Open or closed | Any position between |
| Typical use | Small lines, vacuum, pilot control | Water, air, larger lines | Dosing, gas control, closed-loop control |
Valve Functions: 2/2, 3/2, NC and NO
The function of a solenoid valve is written as two numbers. The first is the number of ports. The second is the number of switching positions.
2/2-Way Valves
A 2/2-way valve has one inlet, one outlet, and two positions: open or closed. It works like an electrically controlled on/off tap.
Typical uses include:
- opening and closing water or air lines;
- filling tanks;
- shutting off gas supply;
- cleaning and rinsing cycles.
See the range of 2/2-way solenoid valves.
3/2-Way Valves
A 3/2-way valve has three ports and two positions. It can connect the supply to an outlet in one position and vent that outlet in the other.
This is useful for:
- controlling single-acting cylinders;
- operating pneumatic process valves;
- mixing two media;
- diverting flow between two lines.
See the range of 3/2-way solenoid valves.
Normally Closed vs Normally Open
This describes what the valve does when the coil has no power:
- Normally closed (NC) — the valve is closed without power and opens when energized.
- Normally open (NO) — the valve is open without power and closes when energized.
The choice is often a safety question. A fuel gas line should normally close when power fails. A cooling water line may need to stay open so the equipment doesn’t overheat.
The coil also matters. If a valve stays energized most of the time, an NO version can reduce energy use and coil heating, or the other way round.
Media and Materials
A solenoid valve must suit the medium flowing through it. The same valve that works well with compressed air may fail quickly with hot water or chemicals.
Common media include:
- compressed air and neutral gases;
- water and hot water;
- oils;
- steam;
- fuel gases;
- aggressive chemicals.
Body Materials
- Brass — common for air, water, and neutral media.
- Stainless steel — used for aggressive media, food applications, and higher hygiene requirements.
- Plastic (such as PVC or PVDF) — suitable for many corrosive liquids.
Seal Materials
- NBR — used for air, water, and oils at moderate temperatures.
- FKM — suitable for higher temperatures, oils, and many chemicals.
- EPDM — suitable for hot water and some steam applications, but not for mineral oils.
- PTFE — very wide chemical resistance.
The exact temperature and media limits depend on the specific valve. Check both the body and the seal against the actual medium.
For aggressive or very clean media, media-separated valves are used. In these designs, the medium doesn’t contact the plunger or the coil area.
Where Solenoid Valves Are Used
Water Treatment and Irrigation
- opening and closing supply lines;
- filter backwashing;
- dosing chemicals;
- controlling irrigation zones.
Food and Beverage
- cleaning-in-place (CIP) systems;
- filling and dosing;
- controlling water and steam supply.
Here, stainless steel bodies and food-grade seals are often required.
Pneumatics and Machine Control
- supplying air to cylinders;
- operating pneumatic process valves;
- blow-off and cleaning functions.
Steam and Heating Systems
- controlling steam supply to heat exchangers;
- hot water circulation;
- condensate drainage.
Steam requires valves and seals rated for the actual temperature and pressure.
Process and Chemical Industry
- dosing additives;
- shutting off process lines;
- controlling gases and aggressive liquids.
In hazardous areas, valves with the appropriate explosion protection approval may be required.
Laboratory and Analysis Equipment
- small-volume dosing;
- gas switching in analyzers;
- precise flow control with proportional valves.
How to Choose a Solenoid Valve
Start with the medium and the system conditions. Then check the electrical side and the installation.
Check:
- Medium — air, water, oil, steam, gas, or chemical?
- Operating pressure — minimum and maximum pressure in the system?
- Differential pressure — is it enough for a pilot-operated valve?
- Flow rate — what Kv value is needed?
- Function — 2/2 or 3/2-way?
- Rest position — normally closed or normally open?
- Supply voltage — 24 V DC, 230 V AC, or another?
- Duty cycle — will the coil be energized continuously?
- Temperature — of the medium and the surroundings?
- Materials — body and seal compatible with the medium?
- Connection — thread type and size, or flange?
- Protection — IP rating, and explosion protection where required?
- Response time — how quickly must the valve switch?
Flow capacity is described by the Kv value: the flow of water in m³/h at a pressure drop of 1 bar. For water-like liquids, the flow can be estimated as:
Q = Kv × √(ΔP / SG)
where:
- Q — flow rate, m³/h;
- Kv — valve flow coefficient;
- ΔP — pressure drop across the valve, bar;
- SG — specific gravity of the liquid (1 for water).
For example, a valve with Kv = 2 and a pressure drop of 4 bar passes about 4 m³/h of water.
When replacing an existing valve, the type number is a good starting point, but not the only check. Valves of the same type can differ in orifice size, seal material, voltage, and function.
Common Selection Mistakes
Many solenoid valve problems come from a valve that fits the pipe but doesn’t fit the system.
Common mistakes include:
- Using a pilot-operated valve without enough differential pressure — the valve may not open fully or may close slowly.
- Choosing by port size only — two valves with the same thread can have very different Kv values.
- Selecting the wrong rest position — an NC valve in a line that must stay open during a power failure can cause equipment damage.
- Ignoring seal compatibility — the wrong seal material can swell, harden, or leak.
- Using the wrong coil voltage — an AC coil on a DC supply, or the other way round, won’t work correctly.
- Ignoring duty cycle — a coil that is energized continuously gets hot. It must be rated for continuous operation.
- Skipping filtration — dirt in the medium can block the pilot passage or damage the seat.
- Ignoring flow direction — most valves only work correctly in the direction marked on the body.
Expert Insight:
“When selecting a solenoid valve, start with the medium and the pressure conditions. Check whether there is enough differential pressure for a pilot-operated valve. Then confirm the flow, the rest position, the voltage, and the seal material before choosing a specific model.”
