The chart may be used in the determination of the correct reading of
moisture percentage and the permanent radiation correction for the
instrument used without computation as follows: Assume the same trial
pressure, feed temperature and normal reading as above. If the normal
reading is found to be 303 degrees, the correction for thermometer and
radiation will be the theoretical reading for dry steam as found from
the chart, less this normal reading, or 10 degrees correction. The
correct temperature for the trial in question is, therefore, 305
degrees. The moisture corresponding to this temperature and 180 pounds
gauge pressure will be found from the chart to be 0.45 per cent.
[Illustration: Fig. 16. Compact Throttling Calorimeter]
There are many forms of throttling calorimeter, all of which work upon
the same principle. The simplest one is probably that shown in Fig. 14.
An extremely convenient and compact design is shown in Fig. 16. This
calorimeter consists of two concentric metal cylinders screwed to a cap
containing a thermometer well. The steam pressure is measured by a gauge
placed in the supply pipe or other convenient location. Steam passes
through the orifice A and expands to atmospheric pressure, its
temperature at this pressure being measured by a thermometer placed in
the cup C. To prevent as far as possible radiation losses, the annular
space between the two cylinders is used as a jacket, steam being
supplied to this space through the hole B.
The limits of moisture within which the throttling calorimeter will work
are, at sea level, from 2.88 per cent at 50 pounds gauge pressure and
7.17 per cent moisture at 250 pounds pressure.
Separating Calorimeter--The separating calorimeter mechanically
separates the entrained water from the steam and collects it in a
reservoir, where its amount is either indicated by a gauge glass or is
drained off and weighed. Fig. 17 shows a calorimeter of this type. The
steam passes out of the calorimeter through an orifice of known size so
that its total amount can be calculated or it can be weighed. A gauge is
ordinarily provided with this type of calorimeter, which shows the
pressure in its inner chamber and the flow of steam for a given period,
this latter scale being graduated by trial.
The instrument, like a throttling calorimeter, should be well insulated
to prevent losses from radiation.
While theoretically the separating calorimeter is not limited in
capacity, it is well in cases where the percentage of moisture present
in the steam is known to be high, to attach a throttling calorimeter to
its exhaust. This, in effect, is the using of the separating calorimeter
as a small separator between the sampling nozzle and the throttling
instrument, and is necessary to insure the determination of the full
percentage of moisture in the steam. The sum of the percentages shown by
the two instruments is the moisture content of the steam.
The steam passing through a separating calorimeter may be calculated by
Napier's formula, the size of the orifice being known. There are
objections to such a calculation, however, in that it is difficult to
accurately determine the areas of such small orifices. Further, small
orifices have a tendency to become partly closed by sediment that may be
carried by the steam. The more accurate method of determining the amount
of steam passing through the instrument is as follows:
[Illustration: Fig. 17. Separating Calorimeter]
A hose should be attached to the separator outlet leading to a vessel of
water on a platform scale graduated to 1/100 of a pound. The steam
outlet should be connected to another vessel of water resting on a
second scale. In each case, the weight of each vessel and its contents
should be noted. When ready for an observation, the instrument should be
blown out thoroughly so that there will be no water within the
separator. The separator drip should then be closed and the steam hose
inserted into the vessel of water at the same instant. When the
separator has accumulated a sufficient quantity of water, the valve of
the instrument should be closed and the hose removed from the vessel of
water. The separator should be emptied into the vessel on its scale. The
final weight of each vessel and its contents are to be noted and the
differences between the final and original weights will represent the
weight of moisture collected by the separator and the weight of steam
from which the moisture has been taken. The proportion of moisture can
then be calculated from the following formula:
100 w
x = ----- (7)
W - w
Where x = per cent moisture in steam,
W = weight of steam condensed,
w = weight of moisture as taken out by the separating
calorimeter.
Sampling Nipple--The principle source of error in steam calorimeter
determinations is the failure to obtain an average sample of the steam
delivered by the boiler and it is extremely doubtful whether such a
sample is ever obtained. The two governing features in the obtaining of
such a sample are the type of sampling nozzle used and its location.
The American Society of Mechanical Engineers recommends a sampling
nozzle made of one-half inch iron pipe closed at the inner end and the
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 one-half
inch from the wall of the pipe. Many engineers object to the use of a
perforated sampling nipple because it ordinarily indicates a higher
percentage of moisture than is actually present in the steam. This is
due to the fact that if the perforations come close to the inner surface
of the pipe, the moisture, which in many instances clings to this
surface, will flow into the calorimeter and cause a large error. Where a
perforated nipple is used, in general it may be said that the
perforations should be at least one inch from the inner pipe surface.
A sampling nipple, open at the inner end and unperforated, undoubtedly
gives as accurate a measure as can be obtained of the moisture in the
steam passing that end. It would appear that a satisfactory method of
obtaining an average sample of the steam would result from the use of an
open end unperforated nipple passing through a stuffing box which would
allow the end to be placed at any point across the diameter of the steam
pipe.
Incidental to a test of a 15,000 K. W. steam engine turbine unit, Mr.
H. G. Stott and Mr. R. G. S. Pigott, finding no experimental data
bearing on the subject of low pressure steam quality determinations,
made a investigation of the subject and the sampling nozzle illustrated
in Fig. 18 was developed. In speaking of sampling nozzles in the
determination of the moisture content of low pressure steam, Mr. Pigott
says, "the ordinary standard perforated pipe sampler is absolutely
worthless in giving a true sample and it is vital that the sample be
abstracted from the main without changing its direction or velocity
until it is safely within the sample pipe and entirely isolated from the
rest of the steam."
[Illustration: Fig. 18. Stott and Pigott Sampling Nozzle]
It would appear that the nozzle illustrated is undoubtedly the best that
has been developed for use in the determination of the moisture content
of steam, not only in the case of low, but also in high pressure steam.
Location of Sampling Nozzle--The calorimeter should be located as near
as possible to the point from which the steam is taken and the sampling
nipple should be placed in a section of the main pipe near the boiler
and where there is no chance of moisture pocketing in the pipe. The
American Society of Mechanical Engineers recommends that a sampling
nipple, of which a description has been given, should be located in a
vertical main, rising from the boiler with its closed end extending
nearly across the pipe. Where non-return valves are used, or where 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 should 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.
[Illustration: Fig. 19. Illustrating the Manner in which Erroneous
Calorimeter Readings may be Obtained due to Improper Location of Sampling
Nozzle
Case 1--Horizontal pipe. Water flows at bottom. If perforations
in nozzle are too near bottom of pipe, water piles against
nozzle, flows into calorimeter and gives false reading.
Case 2--If nozzle located too near junction of two horizontal
runs, as at a, condensation from vertical pipe which collects at
this point will be thrown against the nozzle by the velocity of
the steam, resulting in a false reading. Nozzle should be
located far enough above junction to be removed from water kept
in motion by the steam velocity, as at b. Case 3--Condensation
in bend will be held by velocity of the steam as shown. When
velocity is diminished during firing intervals and the like
moisture flows back against nozzle, a, and false reading is
obtained. A true reading will be obtained at b provided
condensation is not blown over on nozzle. Case 4--Where
non-return valve is placed before a bend, condensation will
collect on steam line side and water will be swept by steam
velocity against nozzle and false readings result.]
Fig. 19 indicates certain locations of sampling nozzles from which
erroneous results will be obtained, the reasons being obvious from a
study of the cuts.
Before taking any calorimeter reading, steam should be allowed to flow
through the instrument freely until it is thoroughly heated. The method
of using a throttling calorimeter is evident from the description of the
instrument given and the principle upon which it works.
[Illustration: Babcock & Wilcox Superheater]