Page 141 - Boiler_Operators_Handbook,_Second_Edition

126 Boiler Operator’s Handbook

little to return to the design flow.
If you’re going to do it, do it right and use the
measuring device (you may have to rent it) and the flow
sensing taps on the valve and restore the design flow
which should be shown on the piping drawings. While
you’re at it, check some of the other valves in the same
area to be certain you didn’t alter their flow rates, taking
readings on them before and after you make the adjust-
ment on the one. See the chapter on flow.
Sometimes it’s just a matter of blowing sediment
out. Before we had balancing records for systems I
would recommend opening a valve on each loop after
noting its position then counting the quarter turns and
restore its position afterwards. The temporary jump in
flow would flush out that particular loop and may re-
Figure 4-3. Pressure drop curves for heating coil turn its operation to normal.
Hydronic systems need blowdown just like steam
line read 30 psig and 21 psig, I wonder which one was systems. You shouldn’t have a lot of sludge and sediment
right? You’ll usually get the reading off a coil table in in a system. The problem is—there’s always a little bit of
gpm so multiply by 500 (to convert gpm to pph) and the it; water contains solids and we add chemicals to treat it
difference between the inlet and outlet temperatures to so there’s some in the water. It will be swept along in the
get the Btuh. areas of the piping that have higher velocities and settle
Hydronic systems in the US tend to have much out in the areas that have the lowest velocities.
higher flowing pressure requirements than systems in Systems with sections designed for future expan-
Europe. The Germans in particular look down on us sion include piping larger than necessary for current
because we introduce so much unnecessary differential operation so the velocity in those sections will be consid-
in our systems and it wastes a lot of motor horsepower. erably lower than individual unit loops and other parts
1
That’s a matter of initial design. In many systems I’ve of the system. A unit loop is piping from supply headers
found the operators throttle down on a valve here and to return headers that serves one piece of equipment
there to resolve heating complaints until the whole that uses the heat.
system is operating at a fraction of it’s design flow and When you have future service connections they are
in other situations they adjust valves open enough that the ones you should use to blow down occasionally to
flow through some systems prevents flow in others. flush the mud and sediment out because that’s where it
Building owners don’t like to hear that their dis- will settle (in addition to the bottom of the boiler). If you
tribution system is totally upset and they have to bring don’t clear them occasionally the sludge will build until
in a balancing company to put everything back in order, it can be swept up in chunks by the flowing water and
a task that is very expensive relative to building size. jammed into a smaller distribution or unit loop, then
I’m not telling you to leave the darn thing alone, If you you’ll have a real problem to fix.
believe a small adjustment will solve a problem then try As for how frequently you blow down a hydronic
it; just count every turn or partial turn of that valve and system, it depends on how much of what quality water
log it so you can always return and put it back where it you add to the system. I always recommend installation
was. It’s preparing to dig yourself out of a hole. of a meter on the makeup water supply for a plant be-
Sometimes the flow control valves in hydronic cause that will be your guide to how much water you’ve
systems or piping loops themselves accumulate mud added. Then it’s simply a matter of knowing the qual-
and sludge because the flow is slow enough to allow the ity of the water to see how much mud, sludge, etc. you
sediment to drop out. What should happen is the accu- added along with that water.
mulation reduces the size of the flow stream so velocity The mud and sludge which is dirt that entered
increases until a balance is reached where no more mate- with the makeup water and sludge created by the water
rial accumulates. In the initial years of a building system treatment to remove scale forming salts doesn’t leave
that sediment accumulation can reduce the flow through with a water leak unless the leak is a big one. Usually the
the loop so it’s necessary to open a throttling valve a leak is in the form of steam. If you heat water to 220°F a