Selecting the right solenoid valve

In this ValveExperts guest article, Tom Rozek of Farmington Engineering offers advice on selecting the right solenoid valve to meet the requirements of a given application.

Solenoid valves are used in many common industrial applications such as automatic taps and automatic flush toilets, refrigeration machines, dental and medical equipment as well as commercial washing machines.

But how do you go about selecting the right solenoid valve to meet the requirements of your application? Here are some brief guidelines.

Key determinants in the selection process are: flow rates; inlet and outlet pressures; types of valves (pinch valves, diaphragm valves, brass valves and piston valves); available electrical power; temperature requirements; and the overall system design.

Flow rates are important in determining the size of the valve needed.

Inlet and outlet port sises typically vary from 0.125 to 3in and are usually threaded.

The higher the flow rate needed, the larger the valve.

The fine adjustment is the orifice inside the valve.

A typical valve comes with a number of orifice sise options depending on the required flow rates and working pressures as well as the purity of the fluid being delivered.

Both the flow rate and the resulting pressure drop are interrelated.

The flow rate increases with the square root of the increase in pressure differential.

Each valve has a defined flow curve and the flow coefficient (Cv) can be calculated to measure the flow rate of specific fluids.

Inlet and outlet pressures are also critical to performance.

Most solenoid valves will operate at pressures up to 200lb/in2.

Direct acting valves have a fast response time but a limited to a 0.5in maximum flow orifice diameter.

This is because the force to open and close the valve equals the orifice area times the pressure.

The greater the surface area or pressure, the greater the force required.

For larger orifice valves, a piloting mechanism is needed.

In this case, the solenoid only opens and closes the pilot orifice, which bleeds fluid behind the seal of the primary orifice, causing it to open.

Usually a 5lb/in2 pressure differential between inlet and outlet is needed for effective operation.

The type of fluid is critical in determining a valves construction.

The valve body can be brass, nickel plated brass, stainless steel or plastic.

The seal can be various elastomers: NBR, EPDM, PTFE or FPM.

Corrosive fluids require stainless steel or plastic bodies as well as PTFE or FPM seals.

If no direct contact with the fluid is desired, a pinch valve can be used.

Pinch valves will squeeze 0.125-0.375in OD silicone or similar hardness tubing.

The tubing should be approximately Shore A 55 hardness.

The pinch force determines the power (wattage) of the solenoid.

The valves are panel mountable and can be latching.

Latching means the valve can be held open or closed for a preset time by a permanent magnet.

Reversing polarity terminates the latch.

Diaphragm valves (also known as isolation or dry valves) are used where limited contact with the fluid is desired.

These valves contain a diaphragm that can be made from EPDM, silicone or Viton rubber.

The valve bodies are either plastic or stainless steel.

The fluids wet only the valve body and the diaphragm.

These valves are typically lever assisted direct acting.

The valves contain a low dead volume and are cleanly swept by fluids.

A three way version provides three ports: inlet, outlet and common.

Maximum working pressures are 40lb/in2.

The response time is about one to two seconds, which can prevent the water hammer effect in plumbing systems.

When the fluid is a non corrosive water, steam, refrigerant, oil or gas, brass is recommended.

These valves can be direct acting or internally piloted depending on pressures and flow rates.

They can also be two way and three way.

Brass can be nickel plated to provide increased corrosion resistance with lower cost than a stainless steel valve.

For high pressures (up to 290lb/in2) and high temperatures (up to 180C), a piston valve can be used.

These valves are operated by using compressed air to actuate a piston connected to the main seal.

They are typically used in industrial applications because of the broad flow ranges, robust construction and seal integrity.

Available electrical power is a key determinant in valve selection.

Some applications require low power consumption DC (AA batteries) and others high voltage AC.

Voltage ranges from 24 to 240V AC and 12 to 48V DC.

Typically medical or laboratory instruments need minimal heat build up so lower power consumption is desirable.

Vending machines operate on low voltage due to the safety requirements of the field service technicians.

Most coils have connector pins but also can have flying leads.

Temperature requirements of a solenoid valve are determined by the seal material.

NBR and neoprene are operable at media temperatures from -20 to +90C, PTFE from -45 to +200C, EPDM from -30 to +155C and FPM from -10 to +150C.

The overall system design is very important in solenoid valve selection.

A normally closed (NC) valve is opened when energised.

A normally open (NO) valve is open and flowing without power and closed when energised.

Some valves have a manual override.

Others can have a flange design or can be mountable on a manifold.

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ValveExperts also provides a venue for valve manufacturers to submit articles and case studies which may be of interest to process engineers.

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