Indirect Acting Solenoid Valve

Indirect Acting Solenoid Valve

Indirect acting solenoid valve

Figure 1: Indirect acting solenoid valve

Solenoid valves are widely used in various industries for controlling the flow of liquids and gasses. An indirect acting solenoid valve employs the differential pressure of the fluid to control the valve. This article explores the features, construction, advantages and disadvantages of indirect acting solenoid valves, along with a comparison with other valve types, including:

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

Table of contents


Construction and working

An indirect acting solenoid valve, also called a pilot-operated valve, uses a solenoid to control the flow of fluids through a system. It is called indirect acting because the solenoid does not directly open or close the valve but instead controls the pressure that operates the valve. Figure 2 shows the construction of an indirect acting solenoid valve.


  • The valve consists of a valve body (Figure 2 labeled I), a seal (Figure 2 labeled F), a diaphragm (Figure 2 labeled G), and a solenoid coil (Figure 2 labeled A).
  • The valve body houses the diaphragm and the valve seat, which controls fluid flow.
  • The solenoid coil is mounted on the valve body and energized to activate the valve.


The inlet and outlet ports are separated by a rubber membrane called the diaphragm, which has a lower and upper chamber area. The membrane has a small hole so the medium can flow from the inlet to both chambers, equalizing the pressure between them. Figure 2 on the left shows that there is more surface area pushing on the diaphragm from the upper chamber than on the lower chamber, which causes a downward force and closes the membrane against the valve seat. For a normally closed solenoid valve, the spring also assists in this closure.

As electric current passes through the solenoid coil, it generates a magnetic field that magnetizes the plunger (Figure 2 labeled E) and causes it to move upwards. This upward motion results in the opening of a small pilot port (highlighted in Figure 2 on the right). This causes pressure in the upper chamber to decrease, resulting in an upward pressure on the diagraphm. This opens the valve and connects the upper chamber to the outlet port (Figure 2 right).

Once the electric current stops, the pilot port closes, and the pressure in the upper chamber increases, which causes the valve to close. A normally open valve contains the same components but operates in the opposite manner.

Indirect acting solenoid valve working principle and components: coil (A), armature (B), shading ring (C), spring (D), plunger (E), seal (F), diaphragm (G), inlet port (H), valve body (I), and outlet port (J). This figure shows the valve in the closed (left) and open (right) states. The highlighted portion shows the opening of the pilot port.

Figure 2: Indirect acting solenoid valve working principle and components: coil (A), armature (B), shading ring (C), spring (D), plunger (E), seal (F), diaphragm (G), inlet port (H), valve body (I), and outlet port (J). This figure shows the valve in the closed (left) and open (right) states. The highlighted portion shows the opening of the pilot port.


  • High-pressure and large flow rate applications: Indirect acting solenoid valves are well-suited for applications involving high-pressure and large flow rates. Direct acting solenoid valves may not generate enough force to overcome high fluid pressures without a significantly large solenoid coil.
    • An indirect acting solenoid valve's solenoid controls flow through the small pilot channel, so small solenoids are suitable for indirect valves. This makes indirect solenoid valves suitable for applications with large pipe diameters, sufficient pressure differentials, and high flow rates.
  • Cost: For high flow or high-pressure systems, indirect acting solenoid valves are more economical. However, these valves are less economical than direct acting valves for low-flow and low-pressure systems.
  • Low power consumption: A pilot-operated solenoid valve consumes less power as it only needs to control fluid flow through a small pilot channel.
    • The solenoid can be relatively small, meaning it consumes relatively little power because it only needs to generate enough force to overcome the pressure differential of the pilot channel. The necessary power is typically in the range of 0.1 - 0.2 watts. Indirect acting solenoid valves can be frequently energized or operated for long periods without overheating.


  • Slow response time: Indirect acting valves rely on the pressure of the fluid in the system to operate the valve, which can take longer to build up and actuate the valve.
  • Leaks: Indirect acting solenoid valves may be more prone to leaks and malfunctions due to multiple components and sealing surfaces.
  • Media purity: A pilot-operated solenoid valve typically has a more complex design and includes an additional pilot valve that controls the flow of the main valve. This pilot valve may have smaller orifices or passages more susceptible to blockages or contamination from particles or debris in the media. Therefore, it is important to ensure that the media used in a pilot-operated solenoid valve is as pure as possible to prevent clogging or malfunction.
  • Unidirectional flow control: Indirect acting solenoid valves control the media flow only in one direction due to the specialized design involving the diaphragm and pilot port.

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 consider when selecting between solenoid valve types.

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 low and high pressures No required pressure difference 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

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


Indirect acting solenoid valves are a suitable option for applications that require high flow rates and pressures. However, these valves may take some time for actuation; therefore, they are often utilized in systems where a slower response time is acceptable, such as in water systems or other fluid handling applications where a delay in response will not negatively impact system performance.


What are the common applications of indirect operated solenoid valves?

Indirect operated solenoid valves are commonly used to control fluid in water treatment and distribution systems, agriculture, and industrial automation.