FAQ Frequently Asked Questions - Glossary


Technical information



1. What is a Solenoid valve?

Solenoid valves are on-off devices for switching fluid systems. They act by moving an armature within an electromagnetic field. The valve can be direct acting or servo assisted.
The solenoid valve is a combination of three main elements:

1. A coil which is made from copper wire wound around a bobbin.
2. Armature and fixed Core.
3. Valve Body with an orifice where the armature is positioned to control the fluid flow.


2. How does it work?

The word “solenoid” does not refer to the valve but to the coil which is installed on the mechanic valve, which is also known as “pilot” or “magnetic operator”. The word “solenoid” comes from the Greek “solen”, that means“channel”.  In fact, the coil “channels” a strong magnetic force in the middle of the winding when the coil is energized, that is, when power runs through the coil.

The coil is composed of a copper wire wound around a bobbin. When the coil is energized, there is a great concentration of magnetic flow lines in the middle of the coil. The magnetic force so produced lifts the armature inside the coil until it touches the fixed core.

The valve body has an orifice through which the liquids or gases pass when the valve is open. When the coil is energized, the armature’s sealing surface lifts from the valve orifice, as it is integral with the armature. When the coil is de-energized, the thrust exercised by athrust spring puts the armature back to its original closing position on the valve orifice to stop the flow.

In the compact solenoid valve, the electromagnet is installed directly on the armature tube which is perfectly sealed and integral with the valve body; the magnetic armature is free to move inside the armature tube and is generally kept in its position by a thrust spring. When the electromagnet is energized, the armature moves,due to the magnetic field, and the seal, which is integral with the armature, opens (or closes) the valve or the valve pilot.

3. Which valves are available?

a) Direct acting solenoid valves
The armature, which is directly connected to the plug and controlled by the electromagnet, opens or closes the orifice when the solenoid is energized or de-energized.
In this case, the electromagnet supplies all the energy the plug needs to operate and therefore operation does not depend on the fluid pressure or flow rate.
The solenoid valve may operate starting from pressure 0 upto the value shown in the corresponding tables.

b) Pilot-operated solenoid valves (servo-controlled –diaphragm solenoid valves):
These solenoid valves are equipped with an electromagnet operated pilot nozzle and a diaphragm that closes the main valve orifice. Fluid pressure assists the operation of the valve. When the electromagnet is energized, the armature opens the pilot nozzle to allow the pressure exercised on the upper part of the diaphragm to be released towards the outlet of the valve body. The pressure unbalance so created on the diaphragm makes it lift and opens the valve. When the electromagnet is de-energized, the pilot nozzle closes and the pressure, which runs through a transfer orifice, is restored on the diaphragm making the valve close. This system of operation of the solenoid valve requires a pressure difference between the inlet and outlet of the solenoid valve. This difference corresponds to the force required for moving and keeping the diaphragm positioned onto the main orifice. This value is known as “Minimum operating pressure” and is shown in the tables.

4. How are the Valves Classified?

Depending on the application, the solenoid valves can be of two types:

a) 2-way solenoid valves (2 positions):
They have two connections (inlet and outlet) and an orifice and are of three types:
- Normally closed (NC): the valve opens when the electromagnetis energized.
- Normally open (NO): the valve closes when the electromagnetis energized.
- Latching: the valve opens when a positive pulse is sent,the valve closes when a negative pulse is sent.

B) 3/2 solenoid valves (3 ways - 2 positions):
They have three connections and two orifices, one always open and one always closed, and are divided in:
- Normally closed:2 = inlet; 1 = user; 0 = discharge 
- Normally open:0 = inlet; 1 = user; 2 = discharge 
- Diverter valves:1 = inlet; 0 = user; 2 = user 
- Shuttle valves:0 = inlet; 2 = inlet; 1 = user 
- Universal valves: they can have the four functions described above 

Response time:
depends on the following factors:
type of current (a.c. – d.c.), fluid used, viscosity, type of operation, size of the solenoid valve.
For direct-acting solenoid valves (2 or 3-way) and small diameters up to 3 mm, the response time is about ten milliseconds.


5. Which connections are available for the valves?

RPE a wide range of connections and solenoid valves.

Connection Type Dimension
Thread BSP (GAS), NPT, GHT, 
Male, Female
1/2", 3/4", 1", 1"1/4, 1"1/2, 2", 3"
Quick connection  J.Guest 1/4", 6mm, 8mm, 10mm, 12mm
Hose tail   10mm, 13mm
Spiggot   10mm






6. Glossary

Solenoid valve main components

Valve body
This is the main part of the solenoid valve where connections, seats, orifices or working orifices are located. Most of RPE solenoid valve bodies are made of thermoplastics but other materials are used for special applications.

Fixed coreIt
is fixed and completes the magnetic circuit. It is generally made of magnetic stainless steel.

Phase displacement ring (or shading ring)
This is generally made of copper and is mounted onto the fixed core surface to prevent oscillations in case of a.c.-fedcoils.

Armature tube
Generally made of nylon, it is a guide for the armature. Itis generally assembled with the fixed core and the thread.

Made of stainless steel, it is magnetic and coil-operated. It slides inside the armature tube.

Armature spring
It is used to keep the armature in position and bring it back after the electromagnet action.

It is generally mounted onto the armature to close the valve or the pilot orifice.

In the servo-controlled solenoid valves, it is used as a servo-control which opens or closes the main orifice of the valve body when operated by an electromagnet pilot and by pressure.

Used as a supporting element for the diaphragm, it balances the opening and closing pressure through the filling and discharge pilot orifices.

Valve body top cover
It is used to close the solenoid valve and keep the diaphragm in position.

This is the electrical part and is composed of a coil, fixed core and C-bracket. When the coil is energised it produces a magnetic field that moves the armature. For technical characteristics, refer to section “electricalcharacteristics” in section “various solenoid valves”.

Pipe fitting:
Depending on the solenoid valve type there are several variations of connections available:
- threaded pipe fitting with threads in inches; Male/Female(BSP, NPT, HAUS GARDEN)
- Bayonet connection
- Quick connection
- Spiggot connection
- Hose connection
- Flanged pipe fitting

Orifice (DN):
Main diameter; orifice – nominal diameter.

Kv flow rate factor:
This is the amount of water (within the temperature range of +5 to +30 °C) passing through the solenoid valve with 1 bar pressure drop (100 KPa - 0,1 MPa) in m³/h (cubic meters per hour).

Minimum operating pressure:
This is the lowest differential pressure required for operation and is expressed in bars. The direct acting valves do not need a minimum pressure, while in the servo-controlled solenoid valves the minimum differential pressure required is shown in the solenoid valves data sheets.

Maximum differential pressure:
This is the highest differential operating pressure with 90% rated voltage (-10% Vn) applied to the coil (for a.c.) and 95% rated voltage (-5% Vn) (for d.c.).

PN – Nominal pressure or static pressure:
This is the maximum static pressure that can be applied to the solenoid valve to check tightness of mechanical seals (threads, welds) and mechanical resistance of materials. We suggest that this pressure is applied simultaneously from all connections to avoid damage to internal components,seals in particular.

Maximum working pressure (S.W.P.):
This is the line or system pressure which can be safely applied to the valve.

This is the electromagnet rated capacity, which is expressed in Watts.

NBR (Nitrile Butadiene Rubber):
Standard quality synthetic elastomer for neutral fluids such as air, water and oils at working temperatures between-10 °C to +90 °C.

EPDM (Ethylene Propylene):
A synthetic elastomer suitable for hot water and steam at working temperatures between -10 °C to +140 °C.

FPM (Viton):
Fluoridated elastomer suitable for oils, combustible gases,fuels, solvents. Working temperatures from
-10°C to + 140 °C.


1. What is a solenoid valve?

2. How does it work?

3. Which valves are available?

4. How are the Valves Classified?

5. Which connections are available for the valves?

6. Glossary




a. See the animation of a working valve

b. Animation of an irrigation valve









































Connessioni idrauliche

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