Solenoid Valve Types Explained: 2/2, 3/2, 5/2 and 5/3, and How to Choose One

The short answer

• 2/2 switches a flow on or off, 3/2 drives a single-acting cylinder, 5/2 a double-acting one, and 5/3 adds a centre position.
• NC or NO, single or double solenoid and the 5/3 centre set what happens on a power cut: choose them for safety.
• Direct-acting valves work from 0 bar; pilot-operated ones need a minimum pressure difference.
• Seals: typically NBR for air and oil, EPDM for water, FKM for hot oil, PTFE for steam.
• Coil: match the supply, usually 24 V DC with a PLC. Size by Kv, not port size.

What 2/2, 3/2, 5/2 and 5/3 mean

The first number counts the working ports, the second the positions; in the ISO 1219-1 symbol each position is a square. Ports: 1 supply, 2 and 4 outlets, 3 and 5 exhausts, 12 and 14 pilot signals (or letters P, A, B, R and S).

Normally closed or normally open

With the coil off, a normally closed (NC) valve blocks the supply and a normally open (NO) valve passes flow. Choose the state that is safe on a power cut: NC stops a tank filling, NO keeps cooling water flowing. Where safety allows, pick the one that is mostly off; a coil on all day runs hot.

Matching the valve to the cylinder

• Single-acting cylinder: a 3/2 valve, usually NC, so the spring returns the cylinder when the coil is off.
• Double-acting cylinder: a 5/2 valve, which pressurises one side while venting the other; a 2/2 cannot, as it has no exhaust.
• Stopping part-way, or no force at a stop: a 5/3 valve.

Single or double solenoid

A single solenoid (monostable) valve returns to rest when the coil goes off, by a spring or, on many pilot valves, by air, so check what happens if the air fails too. A double solenoid (bistable) valve normally stays where it was last switched, even on a power cut, so a pulse is enough (one maker asks for at least 0.1 s). Do not rely on that hold for safety: shock, vibration or loss of air can move the spool. Never energise both coils at once, as the result depends on the valve: interlock them in the PLC.

The 5/3 centre is a safety decision

Springs centre the spool whenever both coils are off: at every stop, on a power cut and on an emergency stop that de-energises the valves.

A closed centre is not a safe hold: air is compressible and valves leak. Hold vertical or heavy loads mechanically, for example with a rod lock, chosen by risk assessment. A vented cylinder can lurch when air returns; a soft-start valve helps.

Direct-acting or pilot-operated

In a direct-acting valve the coil lifts the seal off the orifice, so it works from 0 bar, but its pressure rating falls as the orifice grows, and DC versions are often rated lower than AC.

In a pilot-operated (servo) valve the coil opens a small pilot and line pressure moves the main diaphragm or spool, so a small coil switches a big flow. It needs a minimum pressure difference to open and stay open: commonly 0.1–0.5 bar for water valves and 1–3 bar of supply for internally piloted air valves.

For gravity tanks and low pressure, use a direct-acting or assisted-lift valve, as both work from 0 bar. For vacuum, choose a vacuum-rated valve, such as an air valve with an external pilot supply.

Media and seal materials

Body, seals and coil must suit the medium and its temperature. Pneumatic spool valves are for filtered compressed air; water, oil and steam need a valve rated for them, usually a brass or stainless 2/2.

Steam needs a steam-rated valve, often with PTFE seals and a high-temperature coil. For water, one maker recommends a 50 mesh (about 0.3 mm) strainer upstream and switching the valve at least once a day so scale cannot stick it.

Coil voltage, power, connectors and duty

• 24 V DC: usual with PLCs; extra-low voltage at the valve, a steady current, and a regulated supply rides through mains sags.
• 230 V AC: needs no power supply but puts mains voltage at the valve; connect phase to neutral, never across 400 V.
• 110 V AC or 24 V AC: only where a control transformer already provides it.

An AC coil draws an inrush, often about twice its holding current, until the plunger seats; if dirt jams the plunger or low voltage stops it pulling in, the coil can overheat and burn out. Coils commonly accept about ±10 percent (207–253 V for 230 V); AC coils must be rated for 50 Hz.

Coils on pneumatic valves typically draw 0.1 to 3 W, and on 2/2 water valves 8 to 20 W: at 24 V DC, 3 W is 0.125 A and 18 W is 0.75 A, so add them up when sizing the 24 V supply. Larger and AC coils usually need an interface relay, and coils need suppression (relay vs contactor vs SSR).

Many valves take an EN 175301-803 plug (formerly DIN 43650): form A (18 mm pin spacing) is common on water valves, while forms B and C are smaller; their industry-standard versions (11 mm and 9.4 mm) do not fit the EN ones. IP65 (IEC 60529) needs the gasket fitted and the screw tight.

Coils rated for continuous duty (100% ED) still run hot: one maker rates some DC coils to 50 °C ambient against 80 °C for AC, and a closed panel in summer can exceed that.

Manual override and silencers

The manual override shifts the valve without power for setting up; it moves the cylinder, so clear the area and never leave it latched in the operated position. Silencers on exhausts 3 and 5 cut noise and keep dirt out; a clogged one slows the cylinder. Never plug an exhaust: the pilot often vents through it.

Ports, threads and flow: Kv and Cv

Match the thread: G (BSPP, ISO 228-1) is parallel and seals on a washer or O-ring; R/Rc (BSPT, ISO 7-1) and American NPT are tapered and seal on the thread. NPT has a 60° thread, BSP 55°; at 1/2 and 3/4 inch both have 14 threads per inch, so NPT can start in a BSP port but will not seal.

Port size does not set the flow: one maker’s G1/2 water valves range from about Kv 1 to 4. Kv is the water flow in m³/h at a 1 bar drop across the open valve; Cv is US gallons per minute at 1 psi (Cv ≈ 1.156 × Kv). For water, Kv = Q ÷ √ΔP (Q in m³/h, ΔP in bar). Air valves are rated in L/min, by C and b (ISO 6358) or by Cv.

Worked example: a 2/2 valve on a tank fill line must pass 40 L/min of water with no more than a 0.5 bar drop.

1. Flow: 40 L/min × 60 ÷ 1,000 = 2.4 m³/h.
2. Required Kv = 2.4 ÷ √0.5 = 2.4 ÷ 0.707 = 3.39; take the next size up, Kv 4.0 (about Cv 4.6).
3. Real drop: ΔP = (2.4 ÷ 4.0)² = 0.36 bar, inside the 0.5 bar limit.
4. Pilot check: 0.36 bar is above an assumed 0.3 bar minimum differential (check the datasheet), so a pilot-operated valve stays fully open.
5. Oversizing trap: a Kv 7 valve would drop only (2.4 ÷ 7)² = 0.12 bar, below that minimum, so it may not open fully.

Selection checklist

1. Function: 2/2, 3/2, 5/2 or 5/3.
2. Safe state: NC or NO, single or double solenoid, 5/3 centre.
3. Medium: body, seals and coil rated for it and its temperature.
4. Pressure: within the valve’s range; direct-acting or assisted-lift for low pressure.
5. Flow: Kv or Cv, with the pilot differential checked.
6. Coil: voltage, AC or DC, 50 Hz, power, duty, ambient, suppression, connector, IP rating.
7. Ports: thread type and size, silencers, strainer or filter, manual override.

Browse our valves and pneumatics ranges; brands on the site include SMC, Festo and Danfoss. Send us a photo of the old valve’s label, or the medium, pressure, flow and coil voltage, on WhatsApp and our team will help you choose.