How does a Solenoid Valve work? Operating Principle, Types

We can define a solenoid valve as a device that has the purpose of controlling the flow of liquids or gases that is electrically actuated and can also be installed in remote or difficult-to-access places.

The solenoid valves can be subjected to hostile working conditions. It can be controlled by simple electrical switches. It can also be controlled by devices like thermostatic, float, low-pressure, high-pressure, clock switches, or any other device that opens or closes an electrical circuit.

Now, if we wanted to define the function of a solenoid valve in an even more understandable way, a good option would be to say that it is the same as that of a stopcock in our homes, that is, its function is to control the supply of gases, and liquids when opening or closing a supply line.

Control versus Regulation

Let us differentiate between two concepts that may resemble each other, but have different meanings: “control” and “regulate”.

When we talk about “Control”, we are referring to the ability of solenoid valves to “open” or “close” the passage of a fluid (all or nothing), which is different from “regulating” the percentage of liquid or gas that we want to let pass, (for this we should use a motorized valve). Solenoid valves, therefore, control flow, but do not regulate it, which is why all valves initially have two positions “fully open” or “fully closed”.

In the first case, when the initial position is open, we are allowing the transit of the fluid until it is closed by an action. In the second case, when the initial position is completely closed, there is no fluid transit until we electrically actuate the valve and open the conduit to allow flow.

On the other hand, these valves can be used to control the flow of many different types of fluids, so we must pay special attention to the types of pressures (high or low) to which the valve will be subjected, to the temperatures involved, the viscosity of the fluid and the resistance of the valve materials to the type of fluid and its corrosive capacity.

Operating principle

Solenoid valves are made up of two actuating parts that work together to control  

  1. Valve body and a
  2. Solenoid

The term solenoid does not refer to the valve itself, but rather to the coil mounted on it. When the coil is energized, a strong magnetic force is created (by the action of the coil that attracts a magnetic steel plunger located in the center of the coil. This plunger is attached at its lower part to the stem.

Of the valve body, which closes or opens the conduit through which the flow flows, this stem has a sealing surface called a “seat” which is what allows a safe closure. It is under this operating mechanism, based on magnetism that all solenoid valves operate.

Types of solenoid valves

There is a wide variety of solenoid valves in the market. Their applications are covered right from petrochemical industry to the automotive industry. Given this diversity, there are also several different ways of classifying them.

A first classification would be general

Direct-acting valves

• Pilot operated

Based on its shape

• Two way

• Three-way

• Four-way or reversible

Direct acting valves

These are used in systems with low flow capacity, in them when the coil is energized, the plunger is attracted to get lifted upwards. At the same time raising the needle consequently opens the fluid conduit.

Pilot Operated

The solenoid coil and the line pressure are combined to operate pilot-operated solenoid valves. In these valves, the plunger is attached to a needle stem that covers a pilot hole instead of the main port. Line pressure holds a floating or independent piston closed against the main port, although on some valve models it may be a diaphragm.

There are three basic types of pilot-operated valves

  1. Floating piston,
  2. Floating diaphragm
  3. Captured diaphragm.

Pilot-operated solenoid valves require a minimum differential of opening pressure between the inlet and outlet (approximately 0.5 psi or more), to open the main port and hold the piston or diaphragm in the open position.

Two-way valves

Among the various types of solenoids, the two-way valve is the most common. It has one inlet and one outlet connection and controls the flow of fluid in a single line. It can be direct-acting or pilot-operated, depending on the capability of the system. Direct-acting or pilot-operated two-way solenoid valves can be normally closed (NC) or normally open (NO).

Three-way valves

There is one inlet connection and two outlets on these valves. They are a combination of a normally closed two-way valve and a normally open two-way valve, with only one body and coil.

These valves are predominantly used in commercial refrigeration and air conditioning units, for heat recovery, compressor capacity reduction and hot gas defrost. Their application can also be in high temperatures and pressures that exist in the compressor discharge gas.

Four-way valves

Four-way valves have one common inlet and three different outlets.

They are alternatively also known as reversible valves. Its use is almost exclusively in heat pumps, to select either the cooling or heating cycle, depending on the requirement.

A heat pump is a central air conditioning unit (A/C), with a reversible cycle. During the summer, the refrigerant absorbs heat from inside the house and expels it outside. In the winter season, the cycle is reversed. The refrigerant absorbs heat from outside and releases it inside the house. The condenser and evaporator are forced to switch functions, reversing the flow of refrigerant, and the operation can be performed only with a four-way valve.

Normally closed and normally open valves

Finally, it is necessary to add that the valves usually also have the denomination of Normally Closed or Normally Open:

In those that are normally closed, their normal state (not energized) is to be closed to the flow, so when they are activated (when the coil is energized) the valve will open to allow the flow to pass.

On the contrary, in valves that are normally open, their natural state (un-energized) is to be open to the flow, so that when they are energized or actuated, they are close to it.

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