All right, well the purpose of this video is to give you a good
example of what disc compression might look like.
And, so, what I have here is I have a large RP assembly.
It's actually an RP detector assembly.
We're going to ignore the detector portion of it for the
time being. And, essentially, we're going to
assume that I have done all the procedures correctly and that I
am now at the point where I want to check the relief valve
opening point on this assembly. So here's my relief valve vent
here. I got my gauge set up.
I've already got an apparent reading, so I'm good there.
I've done all the other steps. So a couple things about the
relief valve opening point on this that you want to be aware
of on this large assembly, is that the relief valve is
actually in here. And it actually takes it a
little bit of time from when it actually starts venting water to
when it actually comes out of the vent.
It's not instantaneous. So check my relief valve opening
point on this assembly I'm going to -- I've already bled my --
well, I'll just do it again. I'm going to bleed my low-side
bleed valve until it's above the apparent.
Close the low-side bleed valve, let it settle.
So my apparent reading is 9.2 psid.
Next, I'm going to open my high-side control valve
approximately a turn. And I'm going to manipulate my
low-side control valve so that I can get the relief valve opening
point. Remember, don't ever open it
more than a quarter a turn. So I'm just going to barely open
it until the gauge starts to drop.
And on these large assemblies, you know, don't get in a big
hurry. The gauge is falling fast enough
for me. You know, you got to have some
patience here. So it's still falling.
Falling a little bit slower now. Falling a little bit slower.
And maybe I don't have the shutoff valve off all the way.
Be patient. Oops.
There it goes. So my relief valve opened at 2.7
psid. And on a large assembly like
this, where it goes very slow, you know, I'm essentially got
this in balance. So that, if you watch my gauge
carefully, the needle is kind of going up and down a little bit.
And what that tells me is the amount of water that I'm
bleeding through this needle valve into the zone is pretty
much the same amount of water that's coming out the vent.
And, so, I'm pretty confident here that I would say the relief
valve opened at 2.7 psid. Now, what you do next, close
your low-control valve. I'm going to purge my vent.
Now, normally, if this was a real installation, I'd stick the
vent down the drain cup here, but I'm going to vent it over
into my bucket. All right.
I'm going to close my vent control valve.
Hook my vent hose up to test cock number four.
I'm going to make sure I open test cock number four.
Now, I'm going to reestablish the zone of differential by, I'm
going to open up my bleed valve -- and I don't want to get
all wet, so I'm going to hang onto the hose.
So I'm going to change the differential pressure so it's
above the apparent. I'm going to close it.
Wait for the gauge to settle. And, then, the next thing I'm
going to do is I'm going to open up my vent control valve.
And when I open up my vent control valve, watch very
carefully what happens to the needle.
It drops like a rock. Okay, it didn't quite go to the
relief valve opening point. On some days, I can get this
assembly to go to the relief valve opening point.
For whatever reason today, I can't.
But some people will see this drop like a rock and think that,
oh, there's something wrong with the check valve.
No, there isn't anything wrong with the check valve.
It has to do with the way the assembly works and the design of
the assembly. So this is a good example of
what is called disc compression. And you notice that, even though
this dropped, the relief valve didn't open.
Now, I can double-check that this is disc compression by,
again, if I open the low-bleed valve, which is why you should
always do this a second time if your valve drop -- or if your
gauge drops when checking the second check, reestablish the
zone, and, then, close my bleed valve, you know, this, I've left
my vent control valve open, all right?
So, essentially, what I've done is I've relieved some pressure
out of the zone. And the gauge goes back to the
apparent reading of 9 and doesn't go down anymore.
So that is a true indication that we have disc compression
going on, or what's called disc compression going on in this
assembly. Now, part of the reason why
these assemblies, especially this particular model of
assembly is kind of notorious for this, is if you actually
look inside this assembly, the check valves are on modules.
They're not fixed in place. The check valve, I can pull the
whole check valve out as a module.
And when you put the check valve in and you put the keepers back
in this assembly that keep that check valve in place, what
you'll notice is there's actually some play.
Now, it's not quite a quarter of an inch of play, but
essentially, this check module in this assembly can move back
and forth as a whole. And, so, what happens is, when
you put high pressure -- let's say this pressure, if this
pressure was 60 pounds, that means the pressure in the zone's
got to be nine pounds lower, then -- we'll just say 10 --
well, it's 51 pounds, but, then, when I bleed high pressure onto
it, I'm putting 60 pounds here. Well, the forces are going to
drive this check valve module this direction.
And, so, the volume in here goes down, increases the pressure in
the zone, which, then, decreases the differential.
So that's what's going on when you have disc compression.
And, again, typically, you won't see disc compression very much
in a small assembly, but you will see it in large assemblies,
especially particular models that have check valves that are
modules and the whole module can move.
All right, so that's -- that's a good demonstration of disc
compression, and why it's important when you're checking
to see whether a check valve holds tight -- the second check
valve holds tight or not, why, if the gauge drops, you need to
open up the low bleed again, reestablish the zone, see what
happens again, all right? And this is what we're talking
about about double-checking for the tightness on check valve
number two. Thank you.