How Do You Troubleshoot A Malfunctioning Pneumatic Valve?

Post By: Holly Shaw On: 08-05-2024 Read Time: 5 minutes - Guides - Pneumatics

Pneumatic systems depend on a large collection of components, with valves performing a priority function in directing the airflow. Learning how to troubleshoot a malfunctioning pneumatic valve will prevent it from causing you major disruption. We’ll be looking at the most common causes of malfunction and how to prevent future problems so that your pneumatic system runs optimally. Other types of valve, such as pneumatic solenoid valves, may be more complicated.

Pneumatic Control Valves

You’ll find a variety of valves in a pneumatic system, of which pneumatic control valves are perhaps the most important. These are mechanical devices that direct and control the airflow and are usually controlled by a PLC. When you begin troubleshooting pneumatic valves, you’ll find you can detect some problems visually, although many other issues will require a deeper analysis. Some of the most common issues include leaks, valves stuck open, stiction, crawling, hysteresis and total loss of operation.

Leaks

You might find leaks in pneumatic control valves both internally and externally, but both will result in a pressure drop. This can create inefficiencies and have a serious impact on the whole system. If there’s a bit of leakage when a spring-return or double-acting pneumatic valve is first commissioned, you might need to adjust the actuator stops to close the valve completely. The valve seat may also need stripping, cleaning and re-testing.

Most control valves aren’t designed for a 100% shut-off, which means they usually come specified with a maximum allowable leakage limit.  If the leakage in your valve isn’t within the specified range, it could mean its internal components are worn or damaged. Alternatively, the actuator may not be properly adjusted or the positioner not correctly calibrated. Possible causes of external leakage are typically related to valve packing issues, such as incompatible materials, improper maintenance, thermal cycling, excessive temperatures or vibration.

Valves Stuck Open Or Closed

Valves sometimes get stuck, leading to immediate issues with the process or equipment. Often it’s a case of debris in the supply line and may mean you have to clean or replace your filters. If it’s a double-acting or spring-return pneumatic valve, check that the air pilot pressure in the system is set above the minimum operating pressure for the actuator – usually 90 psi. Also make sure that butterfly valves are not binding and correctly aligned with the flanges. 

Use a multimeter to check that the coil isn’t an ‘open circuit’. Check the specs for correct voltages and the LED display to make sure power is applied. If you have an air pilot solenoid valve in your system, you’ll need to manually override it to check its function. You may have to remove the actuator and check whether the valve is binding or jammed with a spanner.

Stiction

Stiction is a linguistic compression of the physics term ‘static friction’ and refers to the friction that exists between two surfaces at rest. This static force resists the two surfaces when they should begin to move smoothly over one another. If you find a valve sticking in one particular position, this means it’s experiencing stiction and needs more force to push it out of that position. But this can also be an issue, as applying too much additional force might make the valve overshoot and get stuck in another position. You’ll also probably notice that it’s taking longer than normal to respond. 

Stiction is sometimes an effect of valve packing, or over-torquing a valve body to help prevent leaks. Other possible causes might relate to issues with the positioner or solenoid, sticky internals or worn or undersized valve actuators.

Crawling

Crawling refers to a sluggish piloted control valve, which will negatively impact the pneumatic system. The control valve might be moving slowly due to insufficient supply pressure, dirt or blockages in the pilot air supply passages, or an over-tight valve assembly or spool. Pneumatic systems run on compressed air and keeping that air clean is important. Air containing dirt, oil or moisture could cause corrosion or other problems if it comes into contact with pneumatic valves. A well-set-up pneumatic system will have a good filtration unit and perhaps an air dryer to eradicate such contaminants before they enter the system.

How Do You Troubleshoot A Malfunctioning Pneumatic Valve

Hysteresis

Hysteresis is another term used in physics and defines a time lapse between something causing an effect and the effect actually happening. In mechanically-operated valves it relates to the pressure differential between the level when the valve opens and the level when it resets to closed. Normal hysteresis is about 1%-3% – any greater differential than that can cause increased control variance, jerky motion or slow responses. This makes control of the pneumatic system unpredictable and unreliable. This problem is often caused by backlash, or a defective mechanical linkage connecting the valve to the actuator.

Valve Won’t Work

Sometimes pneumatic valves just stop working. Troubleshooting a non-functional valve means exploring common causes like it being worn out and needing replacement. You might also look for contamination or blockage in the air lines. If the problem’s not obvious, you’ll need to check that the valve or other parts of the assembly are properly specified, calibrated and sized. Sometimes it’s just the wrong type of valve altogether for the application.

How To Avoid Issues With Pneumatic Valves

As Industry 4.0 progresses, experts are working on harnessing the power of AI and ML to prevent unnecessary downtime altogether with preventive maintenance. Pneumatic system programs for preventive maintenance are designed to look for minor issues in areas like control valves and fix them before they become serious. This can help eliminate potential issues, but you can also do some preventive maintenance yourself with regular inspections. 

This should include a visual inspection of pneumatic valves for wear and corrosion, repairing any leaks, calibrating the positioner and replacing any worn-out components with properly-sized new parts. You should also ensure that the mechanical assembly isn’t exhibiting any signs of vibration and that pneumatic hoses or tubing have no bends or discolouration.

Any deterioration in pneumatic valves will impact equipment performance, so regular inspection and maintenance is essential to keep the whole system running at its best.