Troubleshooting Pneumatic Solenoid Valve Faults

Pneumatic solenoid valves, which are electrically energized or de-energized to allow, shut off, or change the direction of flow, are electromechanical components. As such, they have both an electrical and mechanical component and, solenoid valve failures can be attributed to either aspect.

Since the first industrial revolution, solenoid valves have been an integral part of general industry. They have found use in applications ranging from simple process control on production lines, as well comprising as parts of industrial end products. One notable application has been in altitude control of the early satelites in the 1960s. On this note, the Aeronautical Radio Incorporated (ARINC) conducted research and produced a report on the reliability of solenoid valves in 1962. Some of their findings are still relevant to the trouble shooting of modern pneumatic solenoid valves.

Below, you can find some of the common issues and the corresponding solutions.

1. VALVE FAILING TO OPERATE

The plunger or spool within a valve may fail to move at all. (There is no clicking sound audible.) This problem can be rooted in the following:

1.1 POWER LOSS

A solenoid valve may not be getting the required power for its operation. Perform the following actions to locate the cause.

1. Use a magnetic detector to detect the presence of a magnetic field. If a magnetic field is present, it means that the solenoid coil is  being electralised, i.e. power loss is not the problem. 
2. If the magnetic field is absent, use a multimeter to check where the current path is getting disrupted. Connect the multimeter to the electric terminals of the solenoid valve, as shown below. With multimeter as ohmmeter, measure the resistance.

• Case A: If resistance comes out to be 0 ohms, there is a short circuit.
• Case B: If resistance comes out to be infinity or very high, the circuit inside the solenoid valve is damaged.
• Case C: If the resistance is in a range of 20-40 ohm, the valve is fine, and the problem lies in the power source.

1.2  INCORRECT VOLTAGE

It is essential that the voltage supplied is maintained within +/- 15% of the voltage specified by the manufacturer. Higher voltages can burn out the coil, while the lower voltages cannot generate the necessary force to move the plunger. Therefore, measure the voltage and keep it within the workable limits.

1.3 HIGH DIFFERENTIAL PRESSURE

If the pressure difference between the inlet and outlet is higher than manufacturer suggested operating pressure, the plunger or spool will not move. To resolve this, reduce the inlet pressure according to the valve specifications.

1.4 LOW DIFFERENTIAL PRESSURE

Most solenoid valves possess a minimum operating pressure (MOPD). If the differential pressure between inlet and outlet is lower than that value, the plunger will not move. If conditions require, zero-rated valves can be used.

1.5 CONTAMINATION

Impurities can also render a solenoid valve inoperable. Typical contaminants include oil, water, and suspended particles. While suspended particles may be removed with minor downtime, water and corrosive acids are serious offenders and can render a valve permnanetly useless. For more information, consider the table below.

1.6 OVERSIZED VALVE

If a pilot-operated solenoid valve is oversized, it may fail to close due to low pressure drop. A size may be oversized if its capacity coefficient (Cv) is 25% higher than the calculated one by design conditions.

1.7 RESIDUAL POWER

The unintended effect of solenoid valves' high energy efficiency has been their increased sensitivity towards the residual or leakage currents. To detect it, move the coil slightly and feel if there is any resistance. Do not try to remove the coil, though, as an energized coil will burn out.

1.8 INCORRECT VALVE INSTALLATION

Solenoid valves usually have predefined inlets and outlets or markings showing flow direction. For example, if compressed air is entering through the outlet port rather than the inlet port, then the air pressure, required in addition to solenoid actuation, will be in reverse, and the plunger will not move. Therefore, the installation must be done while keeping this in view. If the valve fails to operate, revisit the valve manual and check if you got the direction right.

1.9 MANUAL OVERRIDE

A valve may be failing to operate due to the manual override blocking it. Check the position of manual override and adjust it if required.

1.10 MISSING OR DAMAGED PARTS

Lastly, if the coil or armature tube is damaged or a component is missing after maintenance, the valve will not operate. Therefore, if the prior workarounds fail to resolve the issue, try to replace the parts, and troubleshoot the valve using that.

2. VALVE OPENS ONLY SLIGHTLY

Another common condition amongst solenoid valve faults is that the valve is able to open but only very slightly. It can be most common attributed to the following:

2.1 DAMAGED ARMATURE TUBE

A damaged or bent armature tube is the common problem in this case. Visually inspect the valve and determine if there is any damage. If this is indeed the case, then it may be possible for to replace the armature tube -- if they are sold as spares. Some manufacturers, however, recommend replacing the entire valve, so it is best to consult your supplier

2.2 CONTAMINATION

If the valve is contaminated but not completely blocked, it will allow limited motion of the plunger or spool. Under this scenario, the impurities should be removed immediately to save the valve from further damage. Moreover, if it due to swelling of internal seals, replace the seals.

2.3 LOW DIFFERENTIAL PRESSURE

Check if the pressure difference across the inlet and outlet is according to the valve specifications. If required, opt for zero-rated valves.

2.4 DAMAGED OR MISSING COMPONENTS

The problem might be occurring due to missing or damaged parts. For example, while the body of modern solenoid valves may be metallic, the end caps of solenoid valves are often of different material composition, such as plastic. These end caps are prone to physical trauma and crack, which will lead to a weakness in the port connections, which may present leakage issues in the future. To address this, the solenoid valve or end cap will need to be replaced. Refer to the valve manual and order the spares if that is the case.

3 COIL BURNOUT

Sometimes, a solenoid valve may fail to open or operate due to a burned-out coil. Coil burnout is usually caused by:

3.1 INCORRECT VOLTAGE

The voltage supplied to the solenoid valve must be maintained within +/-10 % of the required voltage on the specification sheet. Higher voltage or overvoltage will overheat the coil and burn it out. On the other hand, the lower voltage may not provide sufficient force, and the stalled plunger will draw excessive current, leading to burning out of the coil.

3.2 SHORT CIRCUIT

Higher current due to a short circuit can burn out a coil. Therefore, check the electrical connections for the short circuit. Also, check the coil for moisture. Higher moisture can also cause the current to increase, leading to a burnt-out coil.

3.3 OVER USAGE

Most solenoid coils are not designed to be used continuously. For example, continually energizing the solenoid to keep a NO closed would be an application mistake, which will result in over usage of the solenoid coil.

Conversely, continuously energizing an NC valve to keep it open will also be an application mistake. In this respect, it is pertinent to choose the right valve for the right purpose. To sum it up, overuse is likely to cause overheating, swelling, and eventually burning out the solenoid coil.

4. LEAKAGE

Air might be leaking through a port, exhaust port usually. To detect it, you may seal a port using your thumb for 10-15 seconds and check if you hear the sound of air leaving when you remove your thumb. Air leakage occurs due to:

4.1 FOREIGN MATTER

Contaminants may be preventing the valve from fully closing. Try to use an air blow to remove the foreign material. If unsolved, replace the valve.

4.2 VALVE NOT COMPLETING ITS MOVEMENT

If the differential pressure is out of range, the valve will not complete its movement as described in previous cases. Change the pilot pressure so that the pressure falls back within the pressure range to rectify it.

4.3 INCORRECT FITTINGS

Different thread tapers may result in a less than ideal fit. Consider the two common thread variants of tapered threads and parallel threads. While the tapered threads are self-sealing and can create a tight seal when supported by a sealant, the parallel threads require a gasket or seal to do the same. Therefore, make sure that you have a seal in place if you are using straight threads.