The difference between direct acting and pilot operated solenoid valves can be divided into three main categories based on their principles:

1) Direct acting solenoid valve:

Direct acting solenoid valves are generally used in small caliber, low-pressure environments. When this type of valve is opened, there is no need to require the pressure of the medium, and it starts at zero pressure. Therefore, compared to pilot operated solenoid valves, the starting speed is faster, especially suitable for places that require quick shut-off.

Principle: When powered on, the electromagnetic coil generates electromagnetic force to lift the closing member from the valve seat, and the valve opens; When the power is cut off, the electromagnetic force disappears, and the spring presses the closing member against the valve seat, causing the valve to close.
Characteristics: It can work normally under vacuum, negative pressure, and zero pressure, but the diameter generally does not exceed 50mm.

2) Stepwise direct acting solenoid valve:

Step by step direct acting solenoid valves are generally used in situations where pressure is unstable. They can be started at zero pressure and have a larger upper pressure limit, ranging from 0-0.6MPa to 0-1.6MPa. The diameter range is also wider than that of direct acting valves, making them particularly suitable for places with larger diameters, zero pressure operation, and pressure capability.

Principle: It is a combination of direct action and pilot operated principle. When there is no pressure difference between the inlet and outlet, after being energized, the electromagnetic force directly lifts the pilot small valve and the main valve closing member upward in sequence, and the valve opens. When the inlet and outlet reach the starting pressure difference, after being energized, the electromagnetic force pilot small valve, the lower chamber pressure of the main valve rises, and the upper chamber pressure drops, thereby using the pressure difference to push the main valve upwards; When the power is cut off, the pilot valve uses spring force or medium pressure to push the closing member downwards, causing the valve to close.
Features: It can also operate under zero pressure difference, vacuum, and high pressure, but with high power, it must be installed horizontally.

3) Pilot operated solenoid valve:

Pilot operated solenoid valves are generally used in large-diameter, high-pressure applications. When this type of valve is opened, the low pressure of the solenoid valve must not be lower than 0.05 MPa, and there must be a pilot pressure, otherwise it cannot be opened. In addition, the flow capacity of pilot operated solenoid valves is greater than that of direct acting solenoid valves, and the CV can generally reach 3 or more. The purity requirements for compressed air are relatively high, while those for direct acting are not as strict.

Principle: When powered on, the electromagnetic force opens the pilot hole, causing the pressure in the upper chamber to rapidly decrease, creating a pressure difference of high and low around the closing element. The fluid pressure pushes the closing element to move upward, and the valve opens; When the power is cut off, the spring force closes the pilot hole, and the inlet pressure quickly forms a lower and higher pressure difference around the closing valve through the bypass hole. The fluid pressure pushes the closing valve to move downward and close the valve.

Features: The upper limit of the fluid pressure range is relatively high, and it can be installed freely (customized), but it must meet the fluid pressure difference conditions.

The difference between direct acting and pilot operated solenoid valves:

1. Medium cleanliness requirements: Pilot operated solenoid valves have higher requirements for the purity of the circulating medium, while direct acting valves are not as strict.

2. Power and loss: Direct acting solenoid valves have greater power and loss than pilot operated ones.

3. Flow capacity: The flow capacity of pilot operated solenoid valves is slightly larger than that of direct acting solenoid valves, with a general CV value of 3 or more, while the general CV value of direct acting solenoid valves is less than 1.

4. Response time: The starting speed of direct acting solenoid valves is faster than that of pilot operated ones, and they are often used in places where quick shut-off is required. Because the pilot operated solenoid valve opens the small valve first and then the main valve, while the direct acting solenoid valve opens the main valve directly.

6. Pressure bearing: Pilot operated solenoid valves can withstand hydraulic pressure, which is greater than that of straight through solenoid valves.

Valves have a wide range of applications and a wide variety of types, each with its own field of application. The difficulty of classifying valves continues to increase, and now I will roughly introduce several classifications of valves.
1、 Classify by structural features

1. Cut off valve shape: such as Y-shaped cut-off valve, manual DC cut-off valve

2. Gate shape: intercepts water flow, controls water level, regulates flow rate, discharges sediment and floating debris, etc.

3. Spherical plug: a valve that uses a ball with a circular through-hole as the opening and closing element, and the ball rotates with the valve stem to achieve the opening and closing action

4. Swing type: a device used to control the direction, pressure, and flow rate of a fluid

5. Disc shaped: abbreviated as butterfly valve, a type of throttle valve

6. Sliding valve shape: It is driven by hydraulic and pneumatic power, with high driving force and good control performance

2、 Classify by driving characteristics

1. Electric: driven by an electric motor or other electrical device

2. Hydraulic: driven by water flow (liquid) and oil

3. Pneumatic: driven by compressed air

4. Manual: With the help of valve stem, handwheel, lever or sprocket, etc., driven by human power

3、 Classified by function

1. Shut off valve: functions to open and close

2. Check valve: used to prevent medium backflow, it opens by its own kinetic energy, and automatically closes for reverse flow

3. Regulating valve: also includes manual regulating valve and automatic regulating valve, used to regulate flow rate

4. Vacuum valve: used to change the direction of airflow and adjust the flow rate

4、 Classified by purpose

1. For disconnection purposes: such as globe valves, gate valves, ball valves, butterfly valves

2. Check valve: such as check valve

3. Regulating purposes: such as regulating valves, pressure reducing valves

4. Distribution purposes: such as three-way plug valves, distribution valves, slide valves, etc

There are many other special purposes, such as safety valves, accident valves, drain valves, sewage valves, etc. And cleaning valves, venting valves, and exhaust valves. Exhaust valves are widely used in boilers, air conditioners, and oil and gas industries to remove excess gas from pipelines.