Production Testing of Solenoid Valves

Hydrostatic Testing

Hydrostatic testing is used to detect any points of external leakage in a solenoid valve. Water is used primarily because it is a relatively safe option and readily available. Water is an incompressible fluid, meaning that its density does not change under pressure. If there is a leak or a burst that occurs during testing, there is not nearly as much stored energy as a compressible fluid such as air would have. With water, the pressure will drop immediately and is much less likely to create projectiles from a failed component.

Solenoid valves are typically designed to meet or exceed the requirements of the ASME Boiler and Pressure Vessel Code. General safety requirements and design guidelines are established to help eliminate failures due to overpressure. Industry standard dictates that the burst pressure of the solenoid valve is at least four times greater than the established maximum operating pressure.

Typical points of external leakage found during a hydrostatic test would be any sealing interfaces, such as the bonnet, rotary shaft, or stem seals.

Production Testing of Solenoid Valves — Various examples of potential points of external leakage.
Gasket joint (left), rotary shaft (right), and weld bead (bottom).

Leaks at sealing interfaces are typically associated with imperfections on the sealing surfaces or improperly torqued fasteners. It is important to maintain proper compression on the gasket, o-rings, or packing used to form the seal. Castings and weld beads can reveal holes and cracks from manufacturing defects. These can be repaired by properly welding these defects shut assuming they are small enough.

Operational Testing

Water and air are typically used for operational testing due to them being readily available. Depending on the valve design and application, one may be more suitable for testing than the other.

For testing with water, a pump is required to meet the test pressure. One drawback of this method is that depending on the test pressure required, it may be hard to get both a high enough flow rate and a high enough pressure to accurately simulate an end user application. For instances where flow rate and pressure cannot be maintained, operational testing with water will only determine if the valve opens immediately at the specified pressure, but does not test how the valve performs under constant flow and pressure.

Testing with air can be a little easier to achieve end user flow rate and pressure, but with higher pressure comes greater safety risks. It is generally not feasible to store large volumes of high pressure air. Depending on the test conditions, this test method may also only determine if the valve will open immediately at the test pressure.

During testing, it is important to check the operational safety factors built into the design. This may include testing the operation of the valve at a pressure slightly higher than the maximum allowable pressure, lowering the applied voltage for operating the solenoid, and even heating up the solenoid to decrease the force generated. Heating up the solenoid has a much more profound effect on DC powered solenoids as they do not have an inrush current like their AC counterparts.

Internal Leakage

Internal leakage is the leakage from the inlet to the outlet while the valve is closed. Internal leakage is typically checked with air or water depending on the application of the valve.

Internal leakage should be examined at several pressures. Different pressure can reveal different leaks depending on the design of the valve and the type of damage on the seating surfaces. Leaks can be caused by damage to the sealing surfaces such as scratches, poor surface finish, misalignment, or component deformation. Typically, a valve would be tested for internal leakage at both Maximum Operating Differential Pressure (MOPD) and 10% of the MOPD.

Sealing classes are established in the FCI 91-2 Standard for Solenoid Valve Seat Leakage. The chart below shows allowable leakage for air and water.

Leakage Class	Maximum Seat Leakage		Test Pressure (% of MOPD)
Leakage Class	Water	Air	Test Pressure (% of MOPD)
I	10cc (10ml) per min. per 25 mm (1 inch) of orifice diameter.	2000cc (ml) per min. per 25 mm (1 inch) of orifice diameter.	10%
II	Less than 4 drops (0.4cc) per minute per 25 mm (1 inch) of orifice diameter.	20cc (ml) per min. per 25 mm (1 inch) of orifice diameter. Alternate: for production testing no movement of soap bubble over outlet port in 10 seconds.	10%
III	Test procedure and leakage per ANSI/UL 429 Standard Paragraphs 29.1 through 29.9	Test procedure and leakage per ANSI/UL 429 Standard Paragraphs 29.1 through 29.9	—
IV	Less than one drop (0.1cc) per minute.	2cc (ml) per min. per 25 mm (1 inch) of orifice diameter.	10%
V	Less than one drop (0.1cc) per 10 minutes.	0.2cc (ml) per min. per 25 mm (1 inch) of orifice diameter.	1%
VI	Test procedure and leakage as specified by customer.	Test procedure and leakage as specified by customer.	—

Water is the easiest method to check for leakage. The inlet is pressurized to the desired amount and the outlet can be visually inspected for water leaking through. The number of drops can be counted along with a stop watch to determine the leakage rate.

Air is a little bit trickier to track. A mass air flow meter can be attached to the outlet. It is very important to ensure that there are no air leaks between the outlet and the flow meter. The meter will measure the mass of the air exiting the valve. A much simpler test as indicated by the chart would be to form a soap bubble on the outlet of the valve. If the bubble grows, the valve is leaking air from the inlet to the outlet. The soap bubble method provides a better indication of an internal leak than mass flow meters with a resolution larger than 0.1 cc/min.