If the boiler does not produce superheated steam the percentage of
moisture in the steam should be determined by the use of a throttling or
separating calorimeter. If the boiler has superheating surface, the
temperature of the steam should be determined by the use of a
thermometer inserted in a thermometer well.
For saturated steam construct a sampling pipe or nozzle made of one-half
inch iron pipe and insert it in the steam main at a point where the
entrained moisture is likely to be most thoroughly mixed. The inner end
of the pipe, which should extend nearly across to the opposite side of
the main, should be closed and interior portion perforated with not less
than twenty one-eighth inch holes equally distributed from end to end
and preferably drilled in irregular or spiral rows, with the first hole
not less than half an inch from the wall of the pipe.
The sampling pipe should not be placed near a point where water
may pocket or where such water may effect the amount of moisture
contained in the sample. Where non-return valves are used, or
there are horizontal connections leading from the boiler to a
vertical outlet, water may collect at the lower end of the
uptake pipe and be blown upward in a spray which will not be
carried away by the steam owing to a lack of velocity. A sample
taken from the lower part of this pipe will show a greater
amount of moisture than a true sample. With goose-neck
connections a small amount of water may collect on the bottom of
the pipe near the upper end where the inclination is such that
the tendency to flow backward is ordinarily counterbalanced by
the flow of steam forward over its surface; but when the
velocity momentarily decreases the water flows back to the lower
end of the goose-neck and increases the moisture at that point,
making it an undesirable location for sampling. In any case it
must be borne in mind that with low velocities the tendency is
for drops of entrained water to settle to the bottom of the
pipe, and to be temporarily broken up into spray whenever an
abrupt bend or other disturbance is met.
If it is necessary to attach the sampling nozzle at a point near the end
of a long horizontal run, a drip pipe should be provided a short
distance in front of the nozzle, preferably at a pocket formed by some
fitting and the water running along the bottom of the main drawn off,
weighed, and added to the moisture shown by the calorimeter; or, better,
a steam separator should be installed at the point noted.
In testing a stationary boiler the sampling pipe should be located as
near as practicable to the boiler, and the same is true as regards the
thermometer well when the steam is superheated. In an engine or turbine
test these locations should be as near as practicable to throttle valve.
In the test of a plant where it is desired to get complete information,
especially where the steam main is unusually long, sampling nozzles or
thermometer wells should be provided at both points, so as to obtain
data at either point as may be required.