Solenoid Valves - Types and Specification

Solenoid valves are used extensively in the process, pharmaceutical and oil and gas industires. Whether to allow the flow of a fluid directly e.g. gas to a fired heater, or more commonly, when integrated into pneumatic and hydraulic valve actuators.
Modern solenoid valves offer fast operation, high reliability, long service life, and compact design.
 

What is a Solenoid Valve?

solenoid valve components

A solenoid valve is an electromechanically operated valve which consists of an electromagnetic actuator (the solenoid) and a valve body. A solenoid valve is a two position valve, therefore it can be considered to be a control unit which, when electrically energised or de-energised, either allows or prevents fluid flow.

When the solenoid coil is energised, a magnetic field is created, causing a plunger inside the coil to move. Depending on valve configuration, the plunger will either open or close a port within the valve. When the coil is de-energised, a spring will return the plunger to its initial position.
 

Three Way Solenoid Valves

solenoid valve pid

Three-way solenoid valves have three ports: inlet (supply port), exhaust and outlet (actuator port). These SOVs are commonly used on air operated valve actuators where they allow on/off valves, and some control valves, to fully close or fully open.

They are typically depicted on P&IDs as shown.

solenoid valve pid

When the solenoid is energised the instrument air can pass from the supply port to the valve actuator via the outlet port. The exhaust port is closed.
When the solenoid is de-energised the supply port is blocked, the exhaust port is opened and air from the valve actuator can now reverse its flow, and travel from the acuator to atmosphere via the exhaust port.
 

 

 

Manual Reset Solenoid Valves

The over whelming majority of solenoid valves used in the process industries are of the Auto Reset type. This means that the SOV will change state without operator intervention.
However, there are occasions for example on furnaces where safety considerations require that a SOV cannot be remotely operated by a control system without operator input at the valve, in other words - a Manual Reset. A "latching lever" falls due to gravity when the SOV is de-energised and holds the valve in the open position regardless of whether the coil is energised on not. The lever has to be manually lifted to allow the valve to be reset.
 

Solenoid Valve Design Considerations

When specifying a solenoid valve the following characteristics should be considered:

Solenoid Exhaust Rate Capacity

Solenoid valve venting capacity can be a concern, particularly for big valves or specialist valves like anti surge control valves as the venting rate affects valve response time. Most reputable manufacturers offer models with a range of venting capacities. In environments where insects could build a nest in the exhaust port of the solenoid, a bug screen should be installed, and its impact on venting rates taken into account.

Coil Voltage

Most control system outputs are configured to provide 24 VDC outputs so 24VDC coils are widely available. Interposing relays may be required if you choose a differently rated coil.

Hazardous Area Classification

Will the solenoid valve be located in a potentially hazardous area? If so it must not act as an ignition source therefore hot surfaces on the SOV and ignition sparks created by the actuator valve have to be avoided. This can be achieved through incorporating explosion protection concepts in the design of the solenoid valve, and esuring proper certification of all internal electric and electronic assemblies. See our pages on Hazardous Area Certification and surface temperature classification for more information on hazardous areas and temperature classification.

Ambient Conditions

As always, thought should be given to the plant’s worst-case ambient temperature extremes, humidity etc.

Electric Connections

What size of electrical entry connections are required. Signal cables usually use M20x1.5 ISO. See our page on cable glands for further discussion on cable entries.

Enclosure Ingress Protection

Location of the valve actuator assembly, e.g. in a splash zone, may require a higher IP rating than normally specified for other actuators on the plant. See our page on IP ratings for more information on Ingress Protection.

 

Technical Library

The following pages on Control and Instrumentation.com give more detail on valves and actuators:

 

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Technical Bookshop

The following books, available from Amazon, may be of interest to those wanting to study valves in greater detail: