In
our first article we pressurized an RP and saw what happens to
each of the three components (1st check, 2nd check & relief
valve). In this article we are going to begin to see how an RP
reacts when the two hydraulic conditions of backpressure and backsiphonage
are applied. Let us assume we have a proper working RP and suddenly
apply backpressure to the outlet side of our RP.
In
our illustration, the inlet pressure is 100 PSI and the outlet
pressure is 85 PSI. Let us apply backpressure to our RP. Backpressure
is when a greater pressure is generated on the outlet side of
the assembly than the inlet pressure. This condition can happen
for many reasons, pumps, thermal expansion etc. etc. If the
starting outlet pressure (85 PSI) increase to105 PSI and the
second check is working properly, the 2nd check closes and keeps
the 105 PSI pressure from migrating into the area (90 PSI) between
the 2 check valves.
Even
if we have a working 2nd check and backpressure is applied we
can get discharge from our relief valve. A condition called
disc compression can cause discharge from a properly working
RP. When backpressure occurs, this increase in pressure placed
on the downstream side of the 2nd check causes the 2nd check
disc to embed farther into the 2nd check seat. The volume of
water in the body between the 2 checks is being squeezed as
the 2nd check disc embeds farther into the seat. Water is a
not a compressible fluid in these pressure ranges, so this squeezing
of this water causes an increase in pressure in the area between
the 2 check valves. If this increase in the pressure between
the 2 checks, which started at 90 PSI in our illustration, is
equal to the inlet pressure minus the relief valve loading (100
– 2.0 = 98), the relief valve will open.
In
our field test procedures, when we perform our 2nd check test
of an RP we are simulating a backpressure condition by bringing
the higher inlet pressure (100 PSI) around to test cock 4 (85
PSI). If you remember from your field test procedures, when
an apparent 2nd check failure is observed, you are required
to open your low side bleed valve on your test kit. This will
draw the elevated pressure from the area between the 2 check
valves, while the second check disc stays embedded into the
2nd check seat from the applied backpressure. When the low bleed
is opened, you are reestablishing the pressure in the area between
the2 checks back to its normal pressure of 90 PSI while the
elevated 100 PSI is maintained after the second check. The non-recognition
of disc compression when performing a field test is one of the
most common failures by a backflow assembly tester. Once the
relief valve discharges when testing the second check, you must
open the low bleed one more time to determine if the 2nd check
is actually working or not. A disc compression scenario may
happen and the tester may incorrectly assume the 2nd check is
not working. This error has happened more frequently with the
inline movable design of check valves in relation to the fixed
poppet style.
Let
us see what happens when we apply backpressure to a non-working
2nd check. Once the pressure begins to increase on the outlet
of our assembly, the 2nd check cannot maintain the separation
of pressures between the inlet and outlet of the 2nd check and
the pressure will equalize on both sides. As the pressure increase
begins (from 85 PSI in our illustration) the area between the
2 checks will also increase. Remember the area before the 2nd
check is where our low pressure is applied to the low pressure
side of our relief valve elastic element. As the pressure increases
above our starting pressure of 85 PSI, and goes to the point
equal to the inlet pressure minus the relief valve opening (100
PSI – 2.0 PSI = 98) of 98 PSI, the relief valve will open.
In
our next article we will apply backsiphonage to our RP and see
what happens.
