- This valve switches an instrument air supply between the two outlet ports.
- The two-way valve can be either a normally opened valve (NO) or a normally closed valve (NC).
- These terms refer to their normal states without a power supply being applied.
- Solenoid-operated valves use an electrical coil to control the position of a poppet, plunger, or spool to open or close a valve.
- The typical solenoid control voltages are 12V DC, 24V DC, 120V AC, or 240V AC.
What are the functional units of the solenoid valve?
The solenoid valve is composed of two basic functional units. They are
- The solenoid is an electromagnetic coil with a plunger arrangement.
- A valve with an orifice where the disc or plug is positioned to allow or stop the flow of air in it.
This solenoid valve is opened or closed by the movement of the electromagnetic coil with a plunger arrangement. Which is drawn into the solenoid when an electric current is passed through the coil.
In the above figure “Working of Solenoid”
The energized solenoid A, this solenoid valve opens when an electric current is passed through the coil allowing the instrument air supply to pass through the coil.
The de-energized solenoid B, this valve gets closed when there is no electric current through the solenoid that is the valve gets de-energized preventing instrument air supply from passing through it, and the air is vented out to the atmosphere through the exhaust port of the solenoid valve.
These solenoid valves have a solenoid that is mounted directly on the valve body with the solenoid core attached to the valve stem. This core is enclosed and is made free to move in a permanently sealed tube inside the solenoid coil. The construction provides a compact and leak-tight assembly.
Solenoid valve configurations and ratings:
Solenoid valves are often used in pneumatic control to activate the control valves and actuators.
The valves in their simplest form have three types mainly,
1. Two-way pneumatic valve:
The two-way pneumatic valve typically has two ports one for the instrument air inlet and the other one for the instrument air outlet.
2. Three-way pneumatic valve:
The three-way valve has three ports, two outlet ports, and one exhaust port or vent.
3. Four-way pneumatic valve:
The four-way valve has four ports, two outlet ports, and two exhaust ports or vents.
What is a direct operated (direct acting) solenoid valve?
- The working principle of direct-operated or direct-acting solenoid valves is very simple.
- Here an instrument air is made to flow through the orifice of a small diameter that can be closed off by a plunger with a rubber gasket provided at the bottom.
- A small spring holds the plunger down to close the valve. The ferromagnetic material is used to make this plunger. And this plunger is positioned by an electric coil surrounding it.
- As soon as the coil is electrically energized, the magnetic field is created into it and pulls the plunger in an upward direction toward the center of the coil. This opens the orifice to make the instrument air flow through it. This is referred to as a Normally Closed (NC) valve.
- Normally open (NO) valve has a different construction so that the orifice gets opened when the solenoid is de-energized. The orifice gets closed when the solenoid gets energized.
- The maximum operating pressure and the flow rate of the valve are directly proportional to the diameter of the orifice and the magnetic force of the solenoid valve. This only principle applies to a smaller flow rate.
- The direct operated solenoid valves don’t require minimum operating pressure or pressure difference, but they can be used for pressure of 0 bar up to the maximum allowable pressure. The solenoid valve used is a direct operated, 2/2 way normally closed valve.
What is a direct operated (direct acting) solenoid valve?
- The 3/2-way direct operated solenoid valves consist of three ports and two switching states.
- Two of three ports are connected in each switching state.
- By energizing the solenoid valve, the valve switches the state and establishes different connection paths between the valve ports.
- When the valve is in a de-energized state, the air flows from the right side port to the top port.
- When the valve is in an energized state, the air flows from the left port to the right port.
- This is called a normally closed 3/2-way valve.
What are indirect operated solenoid valves?
- Indirect-operated solenoid valves are also known as servo-operated or pilot-operated solenoid valve.
- For opening and closing operations, these solenoid valves use the differential pressure of the medium called air over the valve ports.
- Generally, these valves require a minimum pressure differential of 0.5 Kg/cm2.
- The inlet and outlet ports are differentiated by a rubber membrane which is referred to as a diaphragm.
- This rubber membrane has a very small hole so that the air can flow only to the upper compartment.
- The chamber above the rubber membrane is connected by a small channel to the low-pressure port. The solenoid blocks the connection in the closed position
- The diameter of the hole in the membrane is smaller than the diameter of this pilot orifice.
- The pilot orifice gets opened when the solenoid is energized, it drops the pressure which is present above the membrane.
- The membrane will be lifted and the air is allowed to flow from the inlet port to the outlet port due to the pressure difference on both sides of the membrane.
- An extra pressure chamber that is located above the membrane will act as an amplifier, so a higher flow rate can be controlled by the smaller solenoid.
- Indirect solenoid valves are unidirectional, that is the flow is in one direction.
- These servo or pilot-operated solenoid valves are used for high desired flow rates.in irrigation systems and car wash systems.
- Indirect valves are also referred to as servo-controlled valves.
Semi-direct operated solenoid valves:
- These are the type of valves that are a combination of both direct and indirect valves.
- These valves can handle higher flow rates.
- These valves look like indirect valves
- These valves feature a smaller orifice with a movable membrane along with pressure chambers on both sides of the valve.
- In this type of valve, the solenoid plunger is connected directly to the membrane.
- It lifts the membrane as the plunger lifts to open the valve directly.
- The plunger opens the second orifice simultaneously.
- The diameter of the second orifice is larger than the first orifice within the membrane.
- The diameter difference between these orifices causes the pressure to drop above the membrane.
- The pressure difference also makes the membrane lift.
- These combinations also result in a valve operating from zero bar and have the ability to control relatively larger flow rates.
- Most often, the coils in semi-direct operated valves are more powerful than indirect operated valves.
- Semi-direct operated valves are also known as assisted lift solenoid valves.
- These Semi-direct operated valves allow working from zero bar.
Solenoid Valve Symbols:
- The solenoid valves can have two or more ports to control the flow of the air between those ports.
- The different switching states are defined by the circuit function of the valve.
- The designations Normally Closed (NC) or Normally Open (NO) determine the state of the valve is either open or closed in the de-energized state.
- Valves are identified with two digits, for example, a 2/2-way valve, 3/2-way valve, or 5/2-way valve.
- The first digit indicates the total number of connection ports available in it. The second digit indicates the total number of switching states available in it.
- The 2/2-way valve has two port connections namely the inlet port and outlet port and two switching states called open state and closed state.
- The 3/2-way valve has three port connections namely the inlet port, outlet port, and exhaust port. And two switching states called open state and closed state.
- A single square is represented for each state of the valve.
- A 2/2 valve has two states called open state and closed state.
- It is represented by two adjacent squares. Each square represents the flow of medium between the ports.
- The medium flow is indicated by an arrow mark.
- This indicates which ports are connected and the flow direction of the medium.
- Closed ports are indicated by a letter T to indicate which square is active when the solenoid coil is electrically energized.
- The little actuator symbol is used on both sides.
- The solenoid symbol at the left indicates that the left square is energized.
- The spring symbol at the right indicates the rest state.
What is the function of a solenoid?
The solenoid is the specific term for a coil of the wire which is used as an electromagnet. It also refers to any device that converts electrical energy to mechanical energy using a solenoid. The solenoid uses the magnetic field to create a linear motion that is obtained from electric current.
Where does a solenoid valve work?
- The solenoid-operated valve is used to replace the manual valves or for remote control.
- The function of the solenoid valve is either closing or opening an orifice in a valve body
- The solenoid valve either allows or prevents the flow of air through the valve. When the coil is energized the plunger in the valve will open or close the orifice by raising or lowering within a sleeve tube.
How does a pilot-operated solenoid valve work?
- The pilot-operated solenoid valve works by using the differential pressure of the medium over the valve ports for opening and closing.
- This solenoid valve provides higher flow rates, it will also operate for higher ranges of pressure and temperature.
- These are also known as servo-supported solenoids because these solenoids consume lesser power.