In
our last article we discussed what happens to an RP when the relief
valve is not working properly. In this article we will see what
happens when the check valves are not working properly.
Let
us talk a little about the first check. Look below to our diagram
of an RP. In this example we show an inlet pressure of 100 PSI.
The pressure after the first check shows us 90 PSI which means
we have a 10 PSID. This is the load the first check is generating
on a properly working first check. If the first check was completely
fouled and there was no differential produced that means we
would have 100 PSI before and after the first check (0 PSID),
then the relief valve spring would cause the relief valve to
stay open. The first check rarely fails where there is no differential.
The usual case is that instead of a 10 PSID as shown in our
example the differential begins to fall as the first check begins
to wear out. Let us assume we know our relief valve has a 2.1
PSID opening point. Let’s add further that our first check
is starting to degrade and it can only generate a 2 PSID. In
other words our inlet pressure is still 100 PSI and the pressure
after the first check is 98 PSI and we know we have a 2.1 relief
valve opening point, what would happen to our relief valve?
The answer is that the relief valve would open up and begin
to discharge. If we have a 100 PSI inlet pressure and a pressure
of 98 PSI after the first check you can see where the 98 PSI
along with the 2.1 PSI from the relief valve spring loading
would cause the diaphragm to move causing the relief valve to
open because there is a greater pressure on the downstream side
of the relief valve diaphragm (98 +2.1 =100.1 PSI) than on the
upstream side (100 PSI).
Some
administrative authorities require the loading on the first
check to have a minimum of 3.0 PSID higher value than the relief
valve opening. By having a buffer greater than 3.0 PSID, this
would help minimize relief valve discharge from a small pressure
fluctuation in a static condition. This would mean that if our
relief valve opening point is 2.1 PSID than we would have to
have a first check loading of at least 5.1 PSID to pass the
field test. If a 3.0 PSID buffer was not required in your area,
then any first check value greater (above 2.1 PSID) than the
relief valve opening point would keep the relief valve closed
and would be a passing check value.
The
cause of check failure tends to be due to the failure of the
disc to seal against the check seat easily. Many times the check
spring is blamed for a check failure but this is usually not
true. The more common causes are dirt and debris on the disc,
disc degradation where the disc will not seal, or a check guide
restricting the travel of the check component.
The
criteria for the workings of the second check, like the first
check, must maintain a higher pressure upstream of the check
than the downstream pressure. This differential is established
by the spring loading of the second check spring which is designed
to be a minimum of 1.0 PSID. Our test procedure for the second
check is different than the first check because it is a backpressure
test. In our field test of the 2nd check, we take the higher
inlet pressure form test cock #2 upstream of the first check
(100 PSI) and with needle valves and hoses place it into our
number four test cock (85 PSI) causing the pressure on the downstream
side of the second check to rise until it is higher than the
upstream side of the second check. When the second check fails,
the higher pressure would go past the leaking second check into
the area between the two checks. As the pressure in this area
increases, the relief valve senses the differential. When the
pressure in the area between the two checks increased to 98.0
PSI (relief valve opening point 2.1 PSID) then the diaphragm
would move causing the relief valve to open. The causes of failure
on a second check are similar to the first check.
In
conclusion, the field test is the way we generate the data needed
to determine which part of the assembly is performing below
the accepted minimal standard. When the numbers fall below the
minimum standards established by the accepted test procedure,
a repair must be facilitated to bring the working condition
of the assembly above the minimum standards. The generation
of accurate data is very important and this means using an accurate
test kit and proper test procedures and techniques to assure
the data we generate properly reflects the working condition
of the assembly.
