Page 260 - Boiler_Operators_Handbook,_Second_Edition

Water Treatment 245

down piping it must come from a section that’s almost or heat up the cooling water side of the cooler to blow it.
the same pressure as in the boiler. If it’s drawn after the It’s also almost impossible to dilute a water sample with
water pressure drops and some of it flashes to steam cooling water.
you have no assurance that your sample is representa- Close the water supply and sample outlet valves
tive. It could be the water left after the steam flashes off to shut down the cooler. When ready to draw a sample
and contain higher concentrations of solutes (the stuff you first check the cooling water drain to be certain the
dissolved in it, including your treatment chemicals) or cooler isn’t leaking then open the cooling water supply.
it could be condensed flash steam and contain almost Once cooling water is flowing open the sample outlet
none of the solutes. If you’re trying to draw a sample off valve to flush the sample piping and get a fresh sample
the blowoff piping or any other volume where the water up to the cooler. Boiler water and deaerated feedwater
is stagnant you’re not getting a representative sample. should start flashing at the outlet so you know you have
The best point to draw a sample from is the continuous a fresh sample. Throttle the sampling outlet valve until
blowdown piping before the water passes through any you get a reasonable flow of cooled water.
orifice or throttling valve. To ensure there’s no vapor vented off your sample,
When I see someone put on chemical prepara- or condensate from the air getting in, the sample should
tion gear and try grabbing a sample off the blowdown be cooled to the same temperature as the air in that area.
valve at the base of the water column I know it’s not a A thermometer sensing the water temperature leaving
representative sample. Samples of raw water, softened the cooler (Figure 8-1) works well but it has to be able to
water, etc., can be collected by simply draining water take the maximum possible temperature of the sample,
from the systems and making certain the sampling pip- the temperature in the boiler. The thermometer is also
ing is flushed so the sample is fresh and representative suggested to be certain you don’t burn your fingers
of the water flowing through the system. Samples of when drawing the sample.
boiler feedwater, hydronic system water, boiler water, Once a sample is flowing you should rinse all the
and most condensate require cooling to ensure you get a apparatus that will be contacting the sample so previ-
representative sample. ous samples don’t contaminate what you’re analyzing.
Sample coolers can be as simple as a large coil of If you must draw a sample from a location away from
copper tubing, to allow air cooling of a low pressure the test bench always draw enough to rinse the testing
boiler water sample, to units designed for operating
pressures up to 5,000 psi. You should read the instruc-
tions for your sample cooler and follow them, but when
they’re lacking, the following guidelines are suggested.
A sample cooler should be shut down except when
it is used to draw a sample. To ensure there’s no vacuum
created in it to draw air in and corrode it and no way to
over-pressure it through thermal expansion leaving it
under pressure is recommended. That means its cooling
water and sample outlet valves should be closed, right?
Well, that’s fine as long as it isn’t leaking and you’ll
never know when it springs a leak under those condi-
tions until the cooling water system is contaminated
with boiler water. I like to connect a sample cooler so
there’s only a cooling water supply valve and the cool-
ing water outlet is piped to form a loop up above the
cooler, vented, then dropped to a drain. The loop keeps
the cooler under static pressure so air can’t get in and
will allow for expansion of the water and even gener-
ated steam to escape if someone opens the sample line
first. If the cooler leaks the boiler water will go to drain,
not back into the cooling water system. There’s no way
some dummy can close an outlet valve that isn’t there to
force leaking boiler water into the cooling water system Figure 8-1. Water sample cooler