Pilot-Operated Solenoid Valve Working Principle

Introduction

Pilot-operated solenoid valves are essential components in modern agricultural irrigation systems, providing efficient and automated control over water flow. These valves work by using a small solenoid—a coil of wire that acts as an electromagnet when energized—to control the movement of a pilot mechanism. The pilot mechanism, in turn, regulates the opening and closing of the main valve. When the solenoid is activated, it creates a small pressure imbalance, allowing water to flow through the main valve. This design enables the valve to handle large water flows with minimal energy consumption, making it ideal for agricultural applications where reliability, water conservation, and efficiency are critical.

Internal Components

Solenoid chamber seal and drain orifice
Main diaphragm (bonnet) seal
Balance orifice and metering rod
Bleed port
Metering rod control knob
Upper control chamber
Valve inlet
Valve outlet

Notes

A solenoid (not shown here) will be screwed into the chamber above (1).
The solenoid drain orifice (1) is connected and will drain into the valve outlet (8).
The metering rod controls (3)(5) how far up the bonnet seal (2) can travel, thus limiting the maximum flow rate through the valve.
Typically, there will be a spring (not modeled here) between the bonnet seal (2) and the top of the control chamber to provide a little extra force against the seal, helping the valve seal and close.
The bleed port (4) ensures no air is trapped in the control chamber (6).
(1) and (3) are designed so that when the solenoid is open, more fluid flows through (1) than can be replenished by (3).

Working Principle

Valve Fully Closed

The valve is closed when the solenoid is fully extended and pushing the pilot seal closed. In this closed state (no water flowing through the pipe), the pressure in the lower inlet chamber and the upper control chamber will be the same as they are connected. Because the formula for pressure is force multiplied by area, the same fluid pressure will cause a larger force to act on the top side of the bonnet, pressing it downwards against the sealing surface, preventing fluid from flowing.

Valve Opening

In order to open the valve, the solenoid will retract and allow the pilot seal to open. The port in the solenoid chamber is connected to the outlet of the valve (where there is only atmospheric pressure), which allows pressurized fluid in the upper chamber to vent out of the pilot port. The upper chamber now has fluid under lower pressure than the lower chamber, allowing the bonnet seal to move upwards, allowing the fluid to flow.

Valve Fully Open

When the valve is fully open, some fluid may move its way to the upper chamber through the balance orifice, but the valve will be designed so that the pilot port will bleed the fluid quicker than the balance orifice can supply.

Valve Closing

When the valve needs to be closed, the solenoid will extend, closing the pilot port. Fluid will make its way to the upper chamber through the balance orifice and the valve will close when enough fluid pressure is built up in the top chamber to overcome the force the fluid in the bottom chamber is exerting on the bonnet seal. Typically, a weak spring is used to help force the bonnet seal downwards, decreasing the time it takes to close the valve.

Common Issues with Pilot-Operated Solenoid Valves

The valve is not opening or closing properly. This can be caused by dirt, debris, or damaged seals in the valve components, or insufficient pressure differences.
Slow response times. The valve may take longer to open or close, especially in low-flow scenarios or when the system is not correctly sized.
Leaking valve. Worn or damaged seals or debris in the valve can lead to leaks.
Noisy operation. Vibration or chatter can occur if the valve is not functioning smoothly, often due to debris or improper installation.
Solenoid failure. Electrical issues, such as a burnt-out solenoid coil or loose wiring, can prevent the valve from operating.

FAQ

Why is my valve not closing?

The most common causes include:

A clogged balance orifice or solenoid drain. Clean these components to ensure proper operation.
Damaged or improperly seated seals. Inspect and replace the bonnet and solenoid seals if needed.
Low flow rates cause slow pressure buildup in the upper chamber. Adjust the metering control knob to limit maximum flow, which can help in this scenario.

First, check to make sure all the components are clean and working properly. The solenoid is the part that controls the opening and closing of the valve, and it is not unheard of for them to fail. Open/Close the valve using the Verdi Block Controller and verify that the solenoid is clicking. Dirt and debris can clog the ports inside the valve, so check the balance orifice and solenoid drain and clean if dirty. Finally, burst seals can also cause the valve not to close, so verify both seals (bonnet and solenoid) are in good condition and seated properly.

Due to the operating principle of the valve (water has to fill up the upper chamber to create the sealing force), the response time of the valve is not instantaneous. Under some conditions, such as low-flow scenarios, there is not a high difference between the pressure in the upper and bottom chamber. This causes the valve to close slowly, as the fluid will take a longer time to refill the upper chamber compared to a scenario where there is a large amount of pressure difference.

Why is my valve not opening?

Check the following:

Verify that the solenoid wire or coil is not damaged, and listen for a "click" sound when the open signal is sent.
Inspect the pilot port and balance orifice for clogs caused by dirt or debris. Clean them thoroughly.
Check the inlet pressure; insufficient pressure may prevent the valve from operating.

Why is my valve opening or closing slowly and what can I do to get a faster response time?

Many valves are designed to provide optimum flow and low resistance at the top end of their flow range, so making sure your irrigation system is correctly specified for your usage is important (i.e. the pipe size is not way over what your flow requirements are). This way, the low-flow scenario will not happen with regular usage.

You can also use the metering control knob to control the maximum flow through the valve, this works by limiting how much the bonnet seal opens. Lowering the maximum flow as low as possible without choking the flow will improve the response time of the valve, at a potential cost of a higher pressure drop across the valve.

Switching to a valve of a smaller chamber (typically smaller diameter) will also lead to faster response times, also this will limit the maximum flow through the system. If more flow is expected in the future, putting two smaller valves in parallel and keeping one of them closed until more flow is needed is an option.

Switching to a valve design that does not require fluid pressure to close, this issue is primarily found in pilot-actuated valves; direct-acting valves, such as electronic ball valves operate differently, and will not have this issue.