Solenoid Valves are arguably one of the most common types of valves regularly used in controlling fluid flow. Their main function is majorly shutting off fluids, mixing them and even distributing them to succeeding channels.
We have two main types of solenoid valves; the direct-acting type and the pilot-operated type. Let us briefly look at the two before we dig deep into how they relate.
Direct Acting Solenoid Valves
This is an electro-mechanical valve that we use in the fluid flow automation process. Their structure is made in such a way that the moving core has a seal meaning they are normally closed when there is zero pressure.
Their structure is a simple design with very minimal movable mechanical components compared to any other solenoid valve. They are small in size and are mainly used in applications having low fluid flow or high pressure.
Pilot Operated Solenoid Valves
These are valves that can be opened or closed using the differential pressure of the fluid passing through the ports of the valve. We can also refer to them as servo-assisted solenoid valves.
We usually use them in applications having very high pressure and temperature ranges or those with lower consumption. Unlike the directing acting valve, this is normally open when there is zero pressure.
How the Valves Compare
Now let’s go straight to how the two compare. We are going to have a broad look at each and every aspect common to the two.
Parts and Components of the Valves
Direct Acting Solenoid Valve
- Valve body that houses all the valve components
- Closing Spring which is used to press the plunger against the valve seat to shut it
- Plunger made of magnetic steel and operated by the coils magnetic field
- Seal which adapts to the valve pressure and the temperature of the medium. It prevents the seeping of any fluid from one orifice to the next.
- Solenoid coil that activates the valve after getting energized
- Shading Ring which prevents the plunger from dropping using residual magnetism from the AC voltage
Pilot Operated Solenoid Valve
- Valve body (housing the diaphragm and valve seat)
- Chamber is placed above the diaphragm where the pressure is equalized. This is also where the diaphragm recedes when the valve opens.
- Diaphragm is pushed by the pressure build-up in the chamber.
- Seal which is a soft material below the piston that prevents seeping of any fluid from one orifice to the next.
- Valve seat which controls the flow of the fluid
- Solenoid coil that activates the valve after getting energized
Operating Principle of Solenoid Valves
Direct Acting Solenoid Valve
Their operating principle is simple since they don’t have a diaphragm. The moving core is joined to the valve seal and is always in a closed position when there is no pressure applied.
When no current is applied to the coils, the valve remains closed and blocks the path to the outlet port. This means that the closing spring has applied pressure on the plunger pushing it to the valve seat.
When a current is applied to the coil, it pulls both the spring and plunger up thereby drawing up the medium. This results in the opening of the channel therefore the fluid can flow through the valve.
These valves rely on the differential pressure of the medium passing through their ports too open and close. It contains a small chamber located above the diaphragm that facilitates the functioning of the valve.
The chamber has an inlet port containing an orifice where the process fluid enters and compresses the diaphragm. This action pushes it against its seat therefore closing the seal.
When you apply current to the solenoid coil, the result will be an upward pull of the diaphragm going against the pressure of the spring. This in turn pushes the fluid inside the chamber via the orifice placed in the inlet port where it goes back to the main flow.
Comparing Solenoid Valves Benefits
Direct Acting Solenoid Valve
- The operation of the valve is pressure-assisted
- They require very minimal pressure differential to operate effectively
- It is most suitable for very large flows since it is economical
- It has a very low power rating therefore consumes less electricity compared to the pilot-operated valve
- They have minimal mechanical moving parts and, therefore, can be installed in tight spaces
- Their start-up speed is very fast since they don’t need the differential pressure to do so
Pilot Operated Solenoid Valve
- Works extremely well with negative pressure circuits
- They are compact in size and cost-effective
- They can efficiently operate in high-pressure and high-flow rate conditions
- They have a very low power consumption since the fluid-controlled flows through a very small pilot channel
- It has the ability to withstand particle debris from the fluid
Comparing Piloted or Direct Direct-acting Solenoid Valves
Direct Acting Solenoid Valve
- They have a limited coil life since they burn easily when exposed to high-frequency power supply
- They consume a lot of power due to the size of the solenoid
- We cannot use them in applications having high pressures and flow rates.
Pilot Operated Solenoid Valve
- They have a slow response time since they rely on the system’s fluid pressure to open or close the valve. This pressure may have a long build-up time
- Besides, they have many moving components and sealing surfaces making them vulnerable to leaks
- Usually, they are prone to clogging due to their complex design which includes many small orifices in the pilot valve.
- They have a unidirectional flow control due to the precise design of the pilot port and diaphragm
Applications of Solenoid Valves
Direct Acting Solenoid Valve: frequently used in industrial automation for airflow control, vacuum pressure, and many other chemicals and fluids. They are also applied in pneumatic and hydraulic systems and HVAC systems.
Pilot operated Solenoid valves; are widely used in controlling the fluid flow in distribution systems and water treatment plants. They are also very frequently used in industrial automation for the process control flow of different fluids.
Conclusion
While both valves basically serve you the same purpose, pilot-operated valves work best with high-pressure applications. On the other hand, the direct-acting solenoid valve would serve your low-pressure applications efficiently without needing the differential pressure.
More Resources:
Solenoid Valve – Source: Science Direct
Hydraulic Power Bank – Source: Target Hydraulics
