In
our first five articles we described how an RP operates in a working
and non-working condition. Before you can properly diagnose any
condition, it is important to know how the assembly is supposed
to operate. In our test procedures we must be able to diagnose
a working assembly and also know when it is not working properly.
This next article we will talk about how the Double Check Assembly
(DC) and the Double Check Detector Assembly (DCDA) are supposed
to operate.
A
DC is simply two approved independently operating check valves
that can hold a minimum of 1 PSI in the direction of flow with
the outlet of the check valve open to atmosphere. These checks
must be located between an inlet and outlet shut-off, and have
4 properly located test cocks. The check valves in a DC must
hold a minimum pressure (1.0 PSI minimum) in the direction of
flow. If a check valve is holding less than 1.0 PSI let us use
.5 PSI as an example, testers will incorrectly say, “The
check valve is leaking”. This leads some people to believe
this check valve would not stop a backflow condition because
it is leaking. The check valve is not leaking at .5 PSI because
it is still sealing off the area upstream and downstream of
the check valve with a .5 PSI loading. The check valve is performing
below the minimum criteria as established in the test procedure
(1.0). The minimum criteria in a test procedure are set at a
point that will trigger a repair before the assembly can degrade
to the point where it cannot prevent backflow (0.0). As long
as our check valve has a positive loading it can prevent backflow
but only when it is above 1.0 does it meet the minimum criteria
as established in the test procedure. So once the test procedure
generates data that the check is maintaining less than 1.0 PSI
we must repair the check to its original working specifications.
Conditions
that can cause a check 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.
Many times the check spring is blamed for this lack of pressure
but this is usually not true. The more 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 is a restriction in the
travel of the guide limiting the movement of the check valve
prohibiting it from sealing properly.
There
is a variation of a DC called a (DCDA). This is a double check
created for fire sprinkler applications. A DCDA consists of
an approved DC with a bypass arrangement that consists of a
by-pass water meter and an approved by-pass DC. This by-pass
is piped around the mainline DC. The purpose of this by-pass
arrangement is to detect and register the first 3 gallons per
minute (GPM) of flow across the backflow preventer into the
fire system. Many testers think a DCDA is simply any small by-pass
DC piped around any main line DC with a meter attached and because
the by-pass DC is smaller the first flow will go through it.
This is not true. In order for the by-pass to detect and register
this 3 GPM the two DC’s and the water meter must be engineered
so that the larger assembly will have a slightly higher differential
at the low flow condition (0-3 GPM). This will assure the first
3 GPMs travels through and is registered by the water meter
in the by-pass. Then if the fire system demands more than 3
GPM, the main line assembly will open up and flow to the designed
flow requirements of the system. An unknowing installer may
install a mainline DC and pipe in a by-pass that looks similar
to the DCDA. These unapproved DCDAs cannot guarantee that they
will detect this first 3 GPM because they are not factory-engineered
assemblies but rather a collection of 2 DC’s and a water
meter assembled to look like a DCDA.
The
reason it is important to detect this low flow of water is that
most fire protection systems do not have a mainline water meter
at their point of service from the water purveyor. Because fire
systems are an emergency connection, water purveyors do not
want the expense or extra flow loss of going through a water
meter. Because fire systems are considered an emergency connection
there should be no flow to detect across a mainline water meter
anyway. Water purveyors use this by-pass to assure that water
users with fire systems do not flow water through this emergency
connection and also with the ability of the by-pass to detect
small flows, they can also detect if there are any small leaks
that may be under ground and out of sight.
The
check valves of a DCDA are similar to the DC and the repair
process will be similar. In many cases the spring loading of
the mainline assembly or the by-pass may be different from the
standard DC, but the repair procedures and the test procedures
to diagnose its workings are the same. Before we can repair
any assembly, it is important to have correct data on the workings
of the assembly to be sure we know what we are fixing and just
as importantly, that it really does need to be repaired.
