In
our first three articles we discussed how a properly working RP
reacts when subjected to backpressure and backsiphonage. In the
next few articles, we will concentrate on how an RP reacts when
components of the assembly are not working properly and how we
can diagnose the condition. In this article we will discuss the
relief valve.
Lets
assume we perform a field test on an RP and generate a 1.5 PSID
(Pounds Per Square Inch Differential) relief valve opening point.
Does this mean the assembly is “leaking” or will
not prevent backflow? The answer is probably no. We know from
our tester training that 2.0 PSID is the minimum acceptable
relief valve opening point. If the relief valve opens at 1.5
PSID the relief valve will open and keep the pressure in the
area between the two checks lower than the inlet pressure if
subjected to a backflow condition. For the assembly to perform
optimally, it must operate at or above this minimum standard
in this case 2.0 PSID.
The
cause of a relief valves opening below the 2.0 PSID minimum
can vary greatly between different models. The assumption that
a spring has worn out and that is why the relief valve will
not open is usually incorrect. The most common cause of low
relief valve opening points is a restriction on the travel of
the relief valve stem mechanism. Either the guide of the relief
valve becomes damaged or a scale or corrosion will cause the
guide to not travel optimally leading to a low relief valve
opening point.
What
happens when our field test data presents an excessively high
relief valve opening point, something above 5.0 PSID? High relief
valve opening points can happen for different reasons depending
on the make, model and size. The most common cause for a high
relief valve opening point is when the relief valve disc does
completely embed into the relief valve seat. The usual cause
for this is when the relief valve disc is not traveling its
full length. An example of this scenario can happen if a rolling
diaphragm in the relief valve is pinched or twisted not letting
the relief valve travel its full designed length. If the stem
does not travel its full length, the relief valve disc cannot
fully embed into the relief valve seat. If this disc is just
touching the relief valve seat, and not fully embedded, then
the relief valve opening point will be excessively high.
The
proper relief valve opening point is important for the proper
working of an RP. In our next article, we will be looking at
non-working check valves in our RP.
