Collection:Solenoid Valves

A solenoid valve is an electrically operated valve for liquids, gases, and compressed air. When voltage is applied to the coil, the valve opens or closes. Typical uses are to fill a tank, to open the cooling water supply of a machine, or to shut off compressed air.

How a solenoid valve works

When current flows through the coil, a magnetic field pulls a plunger up. The plunger opens or closes the valve seat. When the current is switched off, a spring pushes the plunger back.

Types of solenoid valves

• 2/2-way solenoid valves: 2 ports and 2 positions, open and closed. The most common type, for example to shut off water or compressed air.
• 3-way solenoid valves: to switch the flow between 2 pipes.
• Proportional valves: to control the flow rate or the pressure continuously with a control signal, for example 0–10 V or 4–20 mA.
• Coaxial solenoid valves: for high pressure and for viscous or contaminated media.
• Pneumatic solenoid valves (3/2, 5/2): to control pneumatic cylinders and actuators.
• Hydraulic solenoid valves: for high-pressure hydraulic systems.
• Accessories: replacement coils, connectors, and repair kits. A replacement coil must fit the valve series.

NC or NO

• Normally closed (NC): closed without power, open with power. If the power fails, the valve closes. Most applications use NC.
• Normally open (NO): open without power, closed with power. Choose NO when the flow must continue during a power failure. Also choose NO when the valve is almost always open, because then the coil is almost never energized.

Operating principle

The pressure difference across the valve (the pressure before the valve minus the pressure after the valve) shows which operating principle is suitable.

• Direct-acting: the plunger opens the seat directly. The valve works from a pressure difference of zero, but the small orifice gives a low flow rate.
• Indirect-acting (servo-assisted, pilot-operated): the plunger opens a small pilot orifice, and the pressure of the medium opens the diaphragm. Thus a small coil can operate a valve with a large orifice. This gives the highest flow rate, but the valve needs a minimum pressure difference of typically 0.2 to 0.5 bar (3 to 7 psi). Below this value, the valve does not open fully.
• Semi-direct: the plunger is connected to the diaphragm. The valve works from a pressure difference of zero, with a larger orifice than a direct-acting valve. It often needs more electrical power than an indirect-acting valve, or it is suitable only for a lower maximum pressure difference.

Water from a higher tank gives only approximately 0.1 bar (1.4 psi) for each meter (3.3 ft) of height difference. If this is less than the minimum pressure difference of an indirect-acting valve, use a direct-acting or semi-direct valve. Also check the maximum pressure difference: above it, the coil cannot switch the valve.

Voltage

The coil must have the same voltage and current type (AC or DC) as the power supply. For AC, the frequency (50 or 60 Hz) must also be the same. An AC coil on DC gets too hot, and a DC coil on AC can buzz and does not switch reliably. Only a coil with the marking AC/DC works on both.

• 12 V DC: vehicles, and battery and solar systems.
• 24 V DC: the usual control voltage in industrial automation.
• 24 V AC: building automation and HVAC.
• Line voltage (230 V AC or 110/120 V AC): direct connection to the power supply, through a timer, thermostat, or float switch.

For a valve that stays energized for a long time, choose a coil for continuous duty (100% duty cycle). For outdoor use, choose a coil and connector with a degree of protection of IP65 or higher, and install the connector seal correctly.

Seal material

• EPDM: water, hot water, and steam. Not for oil, fuel, or compressed air that contains oil.
• NBR (nitrile rubber): cold water, compressed air, and oil.
• FKM: compressed air, oil, and fuel, also at higher temperatures. Not for hot water or steam.
• PTFE: steam, solvents, and high temperatures.

Port size, Kv value, and Cv value

Choose the port size and the connection type (thread, flange, or solvent-weld socket) of the pipe. When the flow rate is important, also check the Kv or Cv value. Valves with the same port size can have very different flow rates.

• Kv value (metric): the flow rate of water in m³/h at a pressure drop of 1 bar across the valve.
• Cv value (US units): the flow rate of water in US gallons per minute (gpm) at a pressure drop of 1 psi. Cv = 1.156 × Kv.

To calculate the necessary Kv value for water, use Kv = Q / √Δp. Q is the flow rate in m³/h, and Δp is the pressure drop across the valve in bar. To calculate the Cv value, use the same formula, with Q in gpm and Δp in psi. Example: 1.5 m³/h (6.6 gpm) at a pressure drop of 0.5 bar (7.3 psi) needs Kv ≈ 2.1 (Cv ≈ 2.5). This formula is for water only. For other liquids and for gases, the calculation is different: use the Kv and Cv calculator.

When a solenoid valve is not suitable

• Dirty, abrasive, or viscous media: the small clearances make the valve sensitive to particles and wear. For dirty or viscous media, choose a coaxial solenoid valve or an electric ball valve.
• Pipes larger than approximately DN 65 (2½ in): a solenoid valve in this size is almost always indirect-acting, so it needs a minimum pressure difference. Electric ball valves and butterfly valves are usual in this size range.
• Long periods with power in a hot or closed location: the coil gets hot, and this decreases its service life.
• Flow in 2 directions: back pressure can lift the seal off the seat and open the valve without a signal. Use only a valve that is suitable for flow in 2 directions.

Frequently asked questions

Which data do I need to replace a solenoid valve?

Record the manufacturer and the type number of the old valve. Also record the number of ports, NC or NO, the port size, the thread, and the operating principle. Then record the Kv or Cv value, the medium, the housing and seal materials, the pressure, the temperature, and the coil voltage and current type.

Why is my AC solenoid valve buzzing?

A light hum is normal for an AC coil. Loud buzzing shows a loose coil, a dirt particle that stops the plunger from closing fully, or a voltage that is too low. Check the coil mounting and the voltage, then clean the valve.

Why does the pipe bang when the valve closes?

A fast-closing valve stops the water suddenly. This causes a pressure peak: water hammer. A slow-closing solenoid valve (anti-water hammer) decreases the pressure peak.

Solenoid valve or electric ball valve?

A solenoid valve switches fast and is suitable for clean media in small sizes. An electric ball valve switches slowly, but it is better for dirty media and large pipes, and it uses power only while it switches. Choose a ball valve when the valve stays open or closed for a long time.