Effects of Excessive Backpressure on Shielded Core Type Solenoid Valves

Greg Wakeman, P.E., Plast-O-Matic Valves, Inc.

This article discusses the effects of backpressure (downstream pressure) on normally closed, 2-way solenoid operated valves (SOV’s) of the shielded core type. These are direct acting, poppet type valves. It explains why this type of valve will fail to close if the backpressure rating is exceeded.

The shielded core type of valve utilizes a seal to prevent the process fluid from contacting the metal solenoid core (plunger) and core tube. It is typically used in either corrosive, or ultra pure environments, where fluid contact with the metal parts would cause corrosion of the metal parts, or contamination of the process fluid by the metal parts, respectively. Most plastic-bodied valves used in plastic piping systems are the shielded core type.

Typical means of shielding the core is by using seals such as u-cups, o-rings, diaphragms or bellows. Bellows sealed valves are also used where any external leakage is intolerable. These valves are generally “pressure balanced” to the inlet pressure (not usually balanced to the outlet pressure). This is accomplished by making the cross-sectional area of the seal (or shield), equal to the seat area. The result is that the inlet pressure produces no net force on the disk, thus high inlet pressures can be realized in comparison to “wetted core” valves, where the fluid does contact the core and core tube.

The majority of SOV’s are designed such that the inlet pressure is applied “over the seat,” see illustration 1. The reason is because SOV’s must have a relatively weak return spring or the solenoid would have to be abnormally large to overcome the spring force when energized to open the valve. In AC solenoids, a strong spring also tends to cause buzzing (60 cycle hum) when energized. Because a relatively weak spring is used, the inlet pressure cannot be applied under the seat since it would cause the valve to leak, and even open under low inlet pressures.

The spring serves two basic functions. First, when the solenoid is de-energized, the spring sets the core and disk in motion (toward closing the valve). Second, the spring provides necessary seating force to preclude seat leakage at low inlet pressures. Remember, in pressure balanced valves, the inlet pressure provides no net force on the disk, and therefore does not provide seating force as it does in a wetted core valve.

Therefore, it is extremely important to install any shielded core solenoid operated valve in the proper flow direction or you will have seat leakage and a low inlet pressure rating.

In the closed position, if the outlet pressure becomes higher than the inlet pressure, a net force is applied to the disk. Because this force is directly on the disk and therefore the spring — which is relatively weak — it will tend to open the valve. Recall that the valve is pressure balanced at the inlet, but not at the outlet. At some point, when outlet pressure exceeds the inlet, the valve will leak, and additional pressure will cause the valve to open. You may encounter this situation when filling a tank elevated above the valve, and then closing the valve and shutting off the pump. Now you have a situation where there is head pressure (backpressure) at the outlet, and no inlet pressure.

The condition less understood is that of backpressure in the energized, flowing condition. Backpressure is caused by the piping restrictions downstream of the valve. Bends, components, and particularly spray nozzles, cause backpressure on the outlet of a solenoid valve. If the backpressure is higher than the valve is rated for, the valve will not close when de-energized. Many SOV manufacturers do not give a maximum backpressure rating in their literature, so it is important that the system designer obtain this information.

The valve will not close because the backpressure is acting over the area of the disk, causing a force opposing the spring. Even though the inlet pressure is higher than the backpressure, the valve will not close because the inlet pressure provides no net force on the disk in a pressure balanced valve. Valves generally have a specific backpressure rating, dependent upon the spring force and the seat area, and independent of the inlet pressure. Of course, in order to obey the flow direction, the backpressure in a flowing condition cannot exceed the inlet pressure. As the inlet pressure rises, the backpressure can be allowed to rise until it reaches the maximum rating. Above this rating, the valve will not close when de-energized. The best way to determine backpressure is to specify a pressure gauge at the valve outlet.

Thus the primary difference in performance between a shielded core solenoid operated valve and a wetted core solenoid operated valve is that the shielded core has a higher inlet pressure rating (for the same orifice size) and a specific backpressure rating, and the wetted core has a lower inlet pressure rating and a backpressure rating generally equal to its inlet pressure rating.

Fig. 1; Diagram of Shielded Core Type Solenoid Valves

Fig. 2; Diagram of Wetted Core Type Solenoid Valves

Most applications for thermoplastic valves require the shielded core type solenoid valve, but some applications may safely use a wetted core design. Our Technical Group is on hand to assist you in specifying the right valve for the job; please call 973-256-3000.

Please note that the features listed above apply to many manufacturers valves, but the specifics refer only to Plast-O-Matic products. For the purposes of this comparison, certain specifications inherent to specialized or unusual models are not addressed.