Semi-direct Acting Solenoid Valve

Semi-direct Acting Solenoid Valve

Semi-direct acting solenoid valve

Figure 1: Semi-direct acting solenoid valve

Solenoid valves are widely used in various industries for controlling the flow of liquids and gasses. A semi-direct acting solenoid valve combines features of both direct and indirect acting solenoid valves; these valves can operate from zero bar and handle large flow rates. This article explores the features, construction, advantages and disadvantages of semi-direct acting solenoid valves, along with a comparison with the other valve types:

Read our article on solenoid valves for the construction, working, and applications of solenoid valves.

Table of contents


Construction and working

A semi-direct acting solenoid valve, also known as an assisted lift solenoid valve, combines the features of direct and indirect operated solenoid valves. The valve uses differential pressure to open and close, but unlike indirect acting valves, it can open from a zero bar differential pressure.


A semi-direct acting solenoid valve consists of two chambers separated by a flexible membrane (Figure 2 labeled E). A small hole in the membrane connects both chambers, resulting in equal pressure in both chambers. The surface area of the membrane in the upper chamber is larger than the surface area in the lower chamber. As a result, the membrane is pushed downwards against the valve seat, creating a reliable seal.


  • When the coil (Figure 2 labeled F) is energized, a magnetic field is created that pulls the ferromagnetic plunger (Figure 2 labeled C) toward the center of the coil.
  • The plunger connects to the membrane via a spring (Figure 2 labeled G) and this connected assembly lifts, which opens the valve.
  • In addition, a small pilot port is opened and creates a connection between the upper chamber and the outlet, which lowers the pressure in the upper chamber.
  • The resulting pressure differential on both sides of the membrane assists the membrane in lifting.
  • When the solenoid is de-energized, the plunger moves down, lowering the membrane and closing the pilot port. The pressure in the upper chamber rises, and the valve shuts off. Without the spring, the plunger would remain in the energized position, and the valve would be unable to function properly.
Semi-direct acting solenoid valve working principle and components: coil (A), armature (B), shading ring (C), spring (D), plunger (E), seal (F), valve body (G), and diaphragm or membrane (H). This figure shows the valve in the closed (left) and open (right) states.

Figure 2: Semi-direct acting solenoid valve working principle and components: armature (A), shading ring (B), plunger (C), valve body (D), diaphragm or membrane (E), coil (F), spring (G), and seal (H).


  • Broad range of pressure: Semi-direct acting solenoid valves are suitable for a broad range of pressure applications, from low-pressure (zero or vacuum) to high-pressure systems. This is because these valves use differential pressure to operate rather than relying solely on the force generated by the solenoid coil.
  • Low power consumption: The valve stem is pushed open by the plunger but then held in place by the valve seat. This design requires less energy to open and close the valve, as the magnetic force is only required to overcome the spring force to open the valve.
  • Powerful coils
    • In a semi-direct solenoid valve, the powerful coil generates a strong magnetic field that overcomes the spring force and opens the valve. This design allows for the valve to be operated with shorter pulses of electricity, minimizing coil heating and extending the valve's service life. Additionally, the stronger coil of a semi-direct solenoid valve makes it more resistant to voltage fluctuations, ensuring consistent performance even in varying operating conditions or when voltage stability is a concern.
    • Indirect acting valves rely on the pressure of the media to operate the valve, with the coil only opening a small pilot valve that allows the media to flow through the valve. This design requires less energy to operate but can result in shorter service life due to the lower power of the coil and the wear and tear caused by the media flow.


  • Risk of clogging: Semi-direct acting solenoid valves have a small opening in the diaphragm, which can clog the valve if the working media contains debris. Use a clean fluid or a y strainer to prevent clogging. On the other hand, direct acting solenoid valves are less prone to clogging because of their larger orifice size and simpler construction with fewer moving parts. However, larger particles or impurities in the fluid can still damage the valve seat or other components.

Comparison with other solenoid valve types

Choosing the right type of solenoid valve depends on the specific application requirements, such as the fluid type, flow rate, pressure, temperature, and environmental conditions. Table 1 summarizes the various factors to be considered while selecting between solenoid valve types.

Table 1: Comparison between direct acting, indirect acting, and semi-direct acting solenoid valves

Solenoid valve type Pressure tolerance Pressure Difference Speed Power consumption Coil life Flow capacity Purity of media Cost
Direct acting Suitable for low-pressure, zero, and negative pressures No required pressure difference Fast High

(5-20 W)

Less Low, typically a (Kv < 0.865) Can handle more fluid debris than indirect or semi-direct, but a strainer is still advised. Low initial cost for low flow rate systems, cost increases as the flow rate increases
Indirect acting High-pressure applications. Minimum pressure differential of 0.5 bar (7.3 psi) Slow Low (0.1-0.2W) Medium High, typically a (Kv > 2.6) Debris can clog the diaphragm. Using a strainer can increase back pressure and reduce efficiency. Economical for large flow rate systems
Semidirect acting Suitable for both low and high pressures No pressure difference required Medium Low High High, typically a (Kv > 2.6) Debris can clog the diaphragm. Mounting a strainer before the solenoid valve can avoid clogging. Economical for large flow rate systems


Semi-direct solenoid valves are designed to handle higher flow rates and pressure differentials than direct acting valves. In addition, they typically have faster response times compared to indirect acting solenoid valves. As a result, they are an excellent choice for applications where a fast response time is necessary, or the valve needs to function from zero bar.

Read our humidifier, vacuum, and water solenoid valve articles for more details on the specific applications of solenoid valves.


What is the difference between a semi-direct solenoid valve and a direct-acting solenoid valve?

Direct-acting solenoid valves lift the valve stem directly using magnetic force, while semi-direct solenoid valves use a pilot valve to control the flow of fluid to the main valve.