American Backflow Prevention Association
Southern California Chapter

How Do the PVB/SVB Work?
Part Seven

Submitted by Jim Purzycki, Orange County Director

In our previous articles we have discussed how the RP and DC operate. It is important to understand how they operate in order to know how they are to be repaired. In this article we will discuss how the Pressure Vacuum Breaker (PVB) and the Spill Resistant Vacuum Breaker (SVB) operate.

The PVB consists of an inlet and outlet shut off two test cocks, a check valve and air inlet component. The normal flow of water goes from the inlet into the check valve. The check valve is designed to hold 1 PSI in the direction of flow similar to the check in a DC. The check valve opens and water travels past the check valve and cause a poppet to travel up an air inlet guide. The poppet will compress an air inlet loading (its not always a spring) which is designed to generate a load of at least 1 PSI. The air inlet is pressurized closed for the normal flow of water and is designed to open when the force from the air inlet (1.0 PSI Minimum) is greater than the water pressure in the area after the check valve. The PVB is designed to prevent backsiphonage only and requires the PVB to be installed 12” above the highest point of use or piping on the downstream of the assembly.

Conditions that can cause the check in a PVB to perform below its optimum level are many. The cause of check failure is due to the failure of the disc to seal with adequate pressure against the check seat. The most common cause of failure is dirt and debris between the disc and seat. Another common problem is disc degradation where the disc will not seal against the check seat. The third common cause of failure has to do with the alignment of the check spring. Many models require the spring to be installed with a spring retainer that if not properly installed will exert a side pressure on the spring causing it not to deliver the proper load to the check valve.

The normal cause of failure of the air inlet happens when the air inlet will not fully unseat itself when the water pressure in the body past the check valve is below 1.0 PSI. One cause of air inlet failure happens when the air inlet disc adheres to the air inlet seat. The disc can adhere to the air inlet seat when temperature conditions or water quality conditions cause a bonding. Many times the canopy that covers the bonnet is missing which can also cause direct sunlight onto the air inlet also causing a problem with deterioration from the ultra violet rays of the sun. On some PVB models, the air inlet spring can easily be removed or inserted in such a way as to lower its loading below the 1.0 PSI minimum requirement. There is one brand of PVB that does not use a mechanical spring in the usual sense but rather a fold of rubber on the poppet generates the load and if you are not familiar with this brand you could erroneously assume the spring is missing.

Sometimes the air inlet poppet will not seal on the air inlet seat completely and will leak. This unwanted discharge from the air inlet can be caused by several reasons. The usual is when some dirt or debris is located between the air inlet poppet disc and the air inlet seat. If the disc becomes damaged from this debris or becomes worn for other reasons, it could

inhibit its ability to seal. Another cause of leakage can happen if the air inlet guide is damaged in such a way as to not allow the air inlet poppet to seat squarely on the air inlet seat.

There is a variation of the PVB called an SVB. The SVB has an inlet and outlet shut off, a check valve and an air inlet valve, a single test cock and a bleed screw. The SVB performs similarly to the PVB except when the SVB is initially pressurized. The normal path of water for a PVB is for water to enter the body, then open the check valve, proceed past the check valve and seal the air inlet. The SVB is a little different. Water enters the SVB and instead of causing the check valve to open first, as in a PVB, the air inlet closes before the check valve opens. This is accomplished by the air inlet having a lighter loading (1.0 PSI minimum) than the check valve. Water does not have to travel past the check valve to pressurize the air inlet as it does in the PVB. For this reason the SVB will not discharge from the air inlet on initial start up. Once the SVB is pressurized, the SVB will perform similar to a PVB. The causes of SVB failure are similar to those of a PVB as we discussed above.

In order to repair any assembly RP, DC, PVB or SVB, it is important that the repair technician first understand how the assemblies are supposed to work so that when they are not working the problem can be properly identified. The purpose of the repair process is to return the assembly back into its original factory specifications.



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Comments to
Revised 06/19/2005 6:52 PM