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.
