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Types of Solenoid Valve

Every day there seems to be a new type of solenoid valve, some have been developed to cope with new types of media, some employ advances in material sciences to reduce physical size or increased performance, others reliant on new more cost effective production techniques. One of the biggest factors driving the development of solenoid valves is the automation industry where most are deployed. Advances in control technology are placing ever increasing demands on solenoid valve performance. Below are some of the factors that expand the range of types of solenoid valve available today.

Solenoid Valve Operation

Solenoid valve operation fall into three main categories.

Assisted Lift

Assisted lift - the armature driven by the solenoid magnet or coil is attached to a diaphragm which in turn controls the opening or closing function of the valve by either lifting the diaphragm or piston away from or pushing against the sealing orifice within the solenoid valve to allow or prevent the flow of liquid of gas media. Typically the piston or diaphragm will have a servo or pilot circuit within them to allow for pressure assistance for higher pressure applications.

Direct Acting

Direct acting - the armature driven by the solenoid electromagnet or solenoid coil is directly controlling the opening or closing function of the valve by exposing or covering an internal orifice within the solenoid valve to allow or prevent the flow of liquid or gas media.

Pilot or Servo Assisted

Pilot or servo assisted - the armature driven by the solenoid magnet or coil controls an internal pilot circuit which in turn allows the pressure differential to lift open or close the main sealing diaphragm or piston. The principle is based on a small pilot hole on one side of the main piston or diaphragm and the larger pilot hole controlled by the armature. When the armature lifts and opens the larger pilot hole the pressure is released down stream faster than it can be replenished through the smaller pilot, thus the pressure under the main seal overcomes the pressure above forcing the main seal or piston to lift to allow full flow.

Solenoid Valve Function

There are several different formats of solenoid valve port configurations, for example 2/2 way 2 port 2 position being 2 ports inlet and outlet with 2 positions open or closed, 3/2 way 3 port 2 positions with inlet outlet and exhaust ports with 2 positions inlet closed outlet open to exhaust or inlet open to outlet with exhaust closed, other options for pneumatic applications would include 5/2 way 5 port 2 position and 5/3 way 5 port 3 position valves.

2/2 Way

2-way solenoid valve is a 2 port (inlet and Outlet) 2 position (Open or Closed) solenoid valve for basic ON/OFF or open or closed control of liquid or gas media along a pipe, such as water, air, oil, vacuum, steam, petrol and diesel fuel, bio-gas, drinking and mains water, steam, aggressive and corrosive acids and alkali's, argon, carbon dioxide, nitrogen and many other gas and liquid media.

2 ports (inlet and outlet) with 2 position open or closed, can be 2/2 way normally closed or fail safe closed with electrical power to open or 2-way normally open (fail-safe open) with electrical power signal to close.

3/2 Way

3/2 way solenoid valves offer 3 port (Inlet Outlet and Exhaust) 2 position (Open or Closed) media control for diverting the flow of media across 3 pipes or connections and are more commonly found used as pneumatic solenoid controllers for larger air actuated valves, pneumatic cylinders, vacuum systems and as media diverting valves such as vegetable oil conversions on motor vehicles.

3 ports (inlet, outlet and exhaust) with 2 positions open or closed, can be 3/2 way normally closed with inlet port closed - outlet port open to exhaust, when energised inlet port opens to outlet and exhaust port closes. 3/2 way normally open inlet port open to outlet with exhaust port closed, when energised inlet port closed outlet port open to exhaust port. 3/2 way universal can be piped up as normally closed, normally open or as a diverting valve for flow in ether direction.

5/2 Way

Typical uses of 5 port 2 position pneumatic solenoid valve single acting with one solenoid coil and spring return fail-safe function or double acting with two solenoid coils are commonly used as compressed air control valves for pneumatic cylinders or external controllers for pneumatically actuated valves. The 5/2 spring return function would allow a cylinder or actuated valve to fully open or extended when coil 1 energized, with fail safe full close or full cylinder retract. The 5/2 double acting valve would allow a cylinder or actuated valve to be fully open when coil 1 energized, power loss valve or cylinder would stay put and then when solenoid coil 2 is energized the valve or cylinder would close or fully retract. 

5/3 Way

Typical uses of 5 port 3 position solenoid valve with double coil offers a similar function as the 5/2 way valve but with the added centre position of stay put at partial position. Typical pneumatic applications would be for better compressed air control for partial valve opening and stay in position of actuated valve pneumatic cylinder. The 5 port 3 positions would be energize coil 1 cylinder extends or actuated valve starts to open, solenoid power loss 5/3 valve shuts all ports and actuated valve or cylinder stays put even if in mid position, power to coil 2 actuated valve or cylinder would start the return back process to original position. Fine tuning and reducing the speed of operation with needle valve or flow restrictor from the exhaust ports is helpful for cylinder or actuator speed control. 

Solenoid Valve Media

There are many types of gases and liquids that a solenoid valves is used to control and as a result a vast range of types of solenoid valve have evolved to accommodate the specific properties of these gases and liquids. The parts of the valve component that come into contact with the controlled media need to be resilient to the potential corrosive effects of that media. Various valve seals and associated components must also be carefully chosen for this same reason.

Solenoid Valves for Water

Solenoid valves for mains water and WRAS approved and certified solenoid valves for drinking water (potable water) are typically either brass or plastic bodied 2/2 way normally closed. i.e. 2 ports inlet and outlet with 2 positions either open (allowing water flow) or closed (no water flow allowed). When choosing a water solenoid valve you must consider the key control elements such as port connection size 1/4", 3/8", 1/2", 3/4",1",11/4", 11/2", 2" or more and type BSP, NPT of Flange, water minimum and maximum pressure such as mains 3 to 5 bar water pressure to open outlet, mains water pressure in a closed loop pumped system or simply a gravity fed system, control voltage 12vdc, 24vdc, 24vac, 110vac or 230vac and function 2 port (in/out) normally closed (failsafe closed power to open) or normally open (failsafe open power to close) or 3 port diverting valve. If you peak to a good supplier or stockist of water solenoid valves you will need this information to make sure you are supplied with the correct valve for the application.

Brass solenoid valves have the benefit of being a more robust and reliable are better suited to industrial and commercial water applications where the solenoid valve will be under constant use, exposed or just simply in applications where the cost of break down would be inhibitive. Typical applications would be mains water inlet control, water header tank refill systems, gravity fed water systems, water fountains, water process systems, water cooling systems and commercial or industrial washing machine applications to name but a few.

Plastic solenoid valves, on the other hand are typically less expensive and fall into two main categories.

Water appliance valves as used for water inlet control for domestic washing machines, dish washers, water facets, drinking water dispensers. Designed to be energised open for short periods of time typically a maximum of 2 minutes followed with a minimum cooling off period of 3 minutes. The reason for the cooling off period is that the solenoid coil or coil, pack when energised will become hot, there will be some thermal heat transfer into the water and a minimal loss into the ambient air but if powered for more than 3 minutes will start to cook and fill finally fail. Typical sizes from 1/4", 3/8", 1/2" and 3/4" with options for 10mm and 12mm hose barb to fit rubber outlet hose, with options for single, double and triple water outlets. Most voltages are available from 12volts, 24 volts, 110volts and 230 volts AC 50/60 Hz or DC direct current, protection is typically IPXX with exposed electrical faston tags, pressure range 0.2 to 10 Bar, orifice 11.0mm with NBR sealing.

Agricultural nylon bodied water solenoid valves which are more robust than appliance valves but still less expensive than Brass versions. The glass reinforced nylon body is well able to work up to 10 bar water pressure, and in some models is available with a manual flow adjustment knob and a manual over ride facility so the solenoid valve may be manually opened with the need for electrical supply. The quality range of plastic water valve will be fitted with an industrial standard IP65 Class F 100% duty rated solenoid coil which can be powered ON 24/7 without concern for over heating. Most voltages are available from 12vAC, 24vAC, 48vAC, 110vAC 230vAC and 380vAC or 12vDC, 24vDC, 48vDC, 110vDC and 220vDC, pressure range 0.3 to 10 bar, seals typically reinforced NBR, typical port sizes 1/2", 3/4",1",11/2" and 2" BSP thread. 

Solenoid Valves for Steam

Solenoid valves designed for steam and hot water applications have an arduous tasks of not only having to cope with the aggressive nature of pressurised steam but also the high temperature of steam media. Solenoid valves for steam applications will typically have either a Brass or Bronze body with options for stainless steel and in most cases the seals will be PTFE. PTFE has great chemical resistance and can cope with heat very well up to a maximum of +250 degrees centigrade but has the disadvantage of being a hard non pliable material unlike softer elastomer seals and will cause sealing problems if the steam media contains any debris a particles which would become caught between the PTFE seal and the solenoid valve internal sealing face. There is no simple solution around this sealing issue, some specialised elastomer seals such as Kalrez are available but the costs are quite inhibitive and in most cases push the cost of a solenoid valve outside of budget, so fitting a simple Y type filter strainer in-front of the solenoid valve (up-steam) is the best solution.

Heat - steam is very hot by nature and standard solenoid valves will not cope with this extreme heat which is why a steam rated solenoid valve not only have steam compatible sealing but also a high temperature rated solenoid coil fitted, typically either a Class H or Class N coil. A temperature Class H coil is designed to work up to +185 centigrade or steam up to 10 bar, where as a Class N coil is designed to work up to +200 centigrade or 12 bar steam. Steam temperatures typically relate directly to pressure so the higher the steam pressure the higher the steam temperature will be, which can be seen and read from a steam pressure to temperature graph. 

Solenoid Valves for Air

Solenoid valves for air and pneumatic applications are available in a variety of functions and operation, either 2/2 way general purpose, 3/2 diverting for actuated valve and basic air cylinder control, 5/2 and 5/3 way with single or double coil combinations designed for air actuated pneumatic automation systems. Air solenoid valves are typically brass for 2/2 and 3/2 way or die cast aluminium for 3/2, 5/2 and 5/3 way applications and are fitted with NBR elastomer sealing as standard.  

Solenoid Valves for Inert  Gas

Solenoid valves work well for the ON/OFF open and closed flow control for most clean inert or noble gases at ambient temperature i.e. -20C to +60C, such as nitrogen, helium, neon, xenon, krypton, argon, oxygen* and most atmospheric gases and compressed air. Typically brass solenoid valves with standard NBR sealing are fine so long as the maximum permitted pressure ratings are followed and in the case of pressure / pilot assisted solenoid valves that the minimum pressure differential is maintained. Keep in mind that compressed gases will become significantly cold if allowed to expand as this draws heat and for liquid gas applications you will require a cryogenic rated solenoid valve to cope with temperatures down to -200C.

When dealing with oxygen users must we aware that oxygen can be extremely hazardous, valves must be degreased so be completely oil and grease and combustible contaminants. Also rapid opening of a valve can cause generate high temperatures by adiabatic compression and friction caused by metal to metal components can cause hot spots or potential ignition points for one of the materials. Oxygen valve sealing by FKM or NBR sealing is acceptable for normal ambient temperature conditions. For applications for solenoid actuated valves for oxygen over 40 bar, specialist high pressure oxygen rated solenoid valves must be used.

Solenoid Valves for Natural Gas

Solenoid actuated valves are commonly used for the on/off flow control Natural mains gas, town gas, LPG (Liquid Petroleum Gas) and Bio-gas applications, however users need to be careful that they choose the correct gas safe and EN161 certified valve designed for the application in hand. Safety shut off gas solenoid valves to be used on natural gas lines must be EN161 gas approved and installed by a Gas Safe Registered installer which replaces the old original Gorgi registered scheme that now void. 

Natural gas is typically supplied at very low pressures typically 100 to 200 millibar and a gas approved safety shut off valve must be able to offer 100% shut off and sealing at these low pressures, and still provide the required flow with very little pressure drop and loss of gas flow. Port sizes range from 1/8" to 2" BSP thread with flanged PN16 options typically available from DN65 to DN300, beyond this gas valves from DN350 upwards will be actuated by electric motor or pumped hydraulic system just because of the weight / mass of the internal components to be safely controlled.

LPG solenoid valves can have different pressure parameters to natural gas solenoid valves, as LPG is typically supplied from high pressure LPG canisters and pressurised bottles (bottled gas), so users need to be sure what pressure needs to be controlled i.e. what is the gas outlet pressure setting if there is a gas pressure regulator (pressure reducing valve) or if the pressure is unregulated and supplied at high pressure. Typically a brass solenoid valve with NBR sealing will suffice so long as the chosen LPG solenoid valve meets the pressure, flow and function application criteria.

Bio-gas applications require a more specialist solenoid control valve as this biofuel has been produced from the bacterial breakdown of organic matter such as manure, general waste, plant and vegetable material, sewage, green and food waste in the absence of oxygen. There will be a small amount typically 1% of some more challenging gases such as hydrogen sulphide, moisture and siloxanes. Keep in mind that when hydrogen sulphide (h2S) meets moisture (water H20) the combination will generate Sulphuric acid (H2SO4). The main bulk of the gases produced from inside this oxygen free biogas fermentation system are 60% methane and 39% carbon dioxide, which can be combusted or oxidised when mixed with oxygen as a fuel as a renewable energy source for heating or compressed (CNG - compressed natural gas) for motor vehicles.

Safety gas valves are also available with manual reset devises, as opposed to standard solenoid valves or auto reset gas shut off valves that simply open or close according to the electrical power supplied. In this case a manual reset gas safety shut off valve it will have a manual lever or mechanical control device that has to be used to mechanically open the valve to allow gas flow, the solenoid when energised is only used to keep the valve in the open position. In large gas applications such as industrial and commercial gas boilers and gas powered furnaces there will be two gas solenoid valves, a small pilot flame solenoid valve and the larger manual reset gas solenoid valve. If electrical power is lost and regained or during the initial start-up process only the small pilot flame shut off solenoid valve can open to allow a small amount of gas to be ignited. When this small pilot flame has been confirmed the main large gas valve can be manually lifted into the open position by an engineer. In this way no large quantities of raw unburnt gas be allowed to flow and generate a very hazardous potentially explosive environment.

Solenoid Valves for High Viscosity Fluids

Solenoid valves are available for high viscosity fluids or think glutanous liquids such as most mineral and vegatable oils, olive oil, castor oil, sour cream, liquid fats, thick hydrocarbons, syrup, corn oil, honey, coconut oil, ink, sesame seed oil, palm oil, linseed oil, molten chocolate, sauces and peanut butter. In the solenoid valve world a high viscosity liquid would be any liquid with a measured viscosity over 50 centistokes. Solenoid controlled valves for liquids exceeding 50 CST are available such as direct acting solenoid valves without internal pilot circuits or diaphragms, coaxial solenoid valves that operate a stainless steel tube laterally and air pneumatically operated valves such as angle seat piston valves and coaxial valves. Another solution for the control of thick liquids are motorised ball valves but unlike solenoid valves will require several seconds to actuate into position.

Solenoid Valves for Low Viscosity Fluids

Most general purpose or mainline solenoid valve will cope with any chemically pressure or temperature compatible liquid under 50 CST as this liquid is thin enough not to clogg or block any small pilot pressure control holes or circuits within the solenoid controlled valve. This would include most gases such as compressed air, vacuum, nitrogen, argon, helium, carbon dioxide and oxygen etc but also low viscosity fluids such as sea water, salt water, ro water, pure water, demin water, mains water, potable drinking water, vinegar, acetic acid, alcohols, acetone, bromine, crude oil, diesel fuel, formic acid, fuel oil, gasoline, glycol, petrol, aviation fuels, kerosine and final effluents.

Solenoid Valve Pressure

In addition to the consideration of media, the working pressure, or pressure differential that will be applied to the solenoid valve must also be taken into account. This working pressure will also impact on the requirements of the control electronics (the solenoid) as well as the internal components of the valve that ultimately control the flow of the media

Vacuum Solenoid Valves

Solenoid valves designed for vacuum applications must be zero rated i.e. must be able to work with vacuum or from 0 pressure and can typically be either direct acting or assisted lift and do not reply on positive pressure differentials. Vacuum itself is not aggressive so either a brass, stainless steel, aluminium or on some instances plastic bodied solenoid valve can be used, but the elastomer seal will need to be NBR, as EPDM or FKM have a tendency to stick and PTFE offers poor sealing qualities. Vacuum solenoid valves can be 2/2 way or 3/2 way and are available in a range of port sizes from threaded BSP or NPT from 1/8" to 2" or flanged PN16 or ANSI150Lb from DN10 through to DN300 in size. When choosing a solenoid valve to control vacuum please be sure to check it is suitable for a vacuum application, in some instances a vacuum or suction rated solenoid valve will offer degrees of applied vacuum measured in Torr, Pa or mm Hg  that they can control. Typical applications for vacuum solenoid valves would be vacuum lifting equipment, vacuum handling equipment, suction systems, vacuum packing systems or vacuum pick and place machines.

Low Pressure Solenoid Valves

Solenoid valves designed for low pressure, gravity fed and closed circuit pumped systems will need to be what we call zero rated and can be either assisted lift (hung diaphragm / kick pilot) or direct acting (spool valve), in this way the low pressure solenoid valve does not rely or require any guaranteed pressure difference to operate as the opening and closing function of the valve is directly or assisted from the valve armature movement caused by the electro magnetic coil. Low pressure solenoid valves are available in brass, stainless steel, aluminium and most plastics to suit most low pressure applications. 

High Pressure Solenoid Valves

High pressure solenoid valves are specifically designed for high pressure applications, with a far more robust body, core tube, seal and usually a more powerful electro magnet. High pressure applications need to be dealt with care and careful consideration to the maximum and minimum pressures, required flow, materials of construction, chemical comparability, function, size and control voltage. Pressure is the key element, as the forces generated by high pressures can be considerable as the applied forces generated push against all the components of the valve and body. Even at a reasonably high 50 bar pressure the solenoid valve is having to control 50 pounds of force (22.7 Kg) for every square inch, or 50Kg for every cm2. In the case of a 100 bar rated solenoid valve this equates to 100 Kg force to every square cm, 200 bar 200kg force to every cm2 and so on. Typically high pressure solenoid valves are made from strong stainless steel but in some cases such as high pressure miniature solenoid valves they can be brass bodied with stainless steel internals. 

High pressure solenoid valves are available in both 2/2 and 3/2 way format, with either direct acting function (zero rated) as found with most small compact miniature valves however in the case of standard high pressure solenoid actuated valves these will typically be pressure or servo pilot assisted that utilise and simply control the pressure differential to lift the valve into the open of closed position, thus requiring a smaller less energy consuming electromagnet and are found in sizes 1/2" to 2" and upwards.  There are some direct acting high pressure solenoid valve available but these solenoid coils can consume upwards of 100 to 200 watts of electrical power.

Solenoid Valve Terminology

The world of solenoid valves has developed its own dialogue and terminology to deal with the ever increasing permutations of functionality and layout of the solenoid valve as its range grows to meet the demands of the industries it serves. For those new to solenoid valves and even those who work with them everyday it is often useful to have a reference to terms commonly used when describing a solenoid valve, its function or specification. Below are explanations of the most common terms used when referencing a solenoid valve and its functionality.

Solenoid Valve Port Size

Solenoid valve port or connection size are most commonly measured by thread or flange connection.

Threaded solenoid valves:

Typically an internal thread measured by size and thread type, for example 1/4" BSP thread would be 1/4" British standard pipe thread as found in most of Europe and sometimes referred to a "G" or in this case G1/4", in most parts of America NPT (National Pipe Taper) is the common thread and in most parts of Asia PT is the common thread.

Flanged solenoid valves.

Flanges are designated by flange type, pressure rating and size for example PN16 DN50 flange (diameter nominal) 50mm flange type PN with pressure rating 16 bar, alternatively an ANSI150Lb DN100 flange would be a flange type ANSI with internal diameter 100mm or 4" and a pressure rating of 150 psi.


The function of a solenoid valve is given as the number of ports, the number of positions and the failsafe positions. For example of 2/2 way normally closed solenoid valve will be 2 ports (inlet and outlet) with 2 positions (open or closed) with failsafe position closed i.e. shut preventing flow of media without electrical supply, a 3/2 way normally closed solenoid valve will have 3 ports and 2 positions with failsafe inlet port closed, 5/2 way 5 ports 2 positions or 5/3 way 5 ports 3 positions.


Solenoid valve operation can be direct acting, assisted lift or servo assisted.

Direct acting - the armature moved by the electromagnetic coil is directly controlling the position of the valve.

Assisted lift - also known as hung diaphragm - hung piston - kick pilot - the armature is connected to a piston or diaphragm to assisted the valve open or closed position.

Servo assisted - also known as pressure assist - pilot assist or floating diaphragm or piston - the armature is not connected to anything and simply controls the piston or diaphragm position by controlling the valve pressure differential. It is this pressure difference that moves the piston or diaphragm into the open or closed position.

Working Pressure

The working pressure of a solenoid valve is measured in terms of units of force against area, for example 15 PSI or 15 pounds force for every square inch. Another measurement of pressure commonly used is Bar, 1 bar = 1 atmosphere or 14.71 PSI so 10 bar would equate to 147.1 psi, another term kg/cm2 or kilograms force per square centimetre. Pressures and vacuum can be easily converted from Torr, mmHGa, PSI, Hgv, Atm, H2o, Kg/cm2, kPa, Bar and mBar. 

Flow Rate ( K Value)

Kv values or flow coefficients define the flow through a solenoid valve by the metric measurement of water in cubic meters an hour with a pressure differential of 1 bar (difference between inlet and outlet pressure across the solenoid valve) between 5 and 40 degrees Celsius.

Cv values or flow coefficients define the flow through a solenoid valve by the imperial measurement of water in US gallons per minute with a pressure differential of 1 PSI at 60 degree Fahrenheit.

Qn values or flow coefficients define the flow through a pneumatic solenoid valve by the metric measurement of air in litres per minute at 6 bar with 1 bar differential at 20 degree Celsius.

Flow Rate Conversions.

Kv, CV and Qn flow coefficient flow rates can be converted by the following formulae.

Kv = 0.86 x Cv or Kv = Qn/1078

Cv = Kv/0.86 or Cv = Qnx0.0008

Qn = Kv x 1078 or Qn = Cv/0.0008