port, and is connected by an internal lever to the bucket in such a manner that when the tank is filled the valve will be opened and the pump will operate, and when the tank is empty the valve will be closed, and the pump will stop.

The pump-governors are frequently set some little distance

from the pump, but attached in every case so as to produce the results described.

136. The Steam-loop.-A device which has been used quite extensively for returning water of condensation to the boiler when the pressure has been reduced only a few pounds is called a steam-loop, the construction and principle. of operation of which, as described by Walter C. Kerr, is as follows:

The figure shows the loop returning the water, from a separator attached to an engine-main, to a boiler above the separator level. "From the separator drain leads the pipe called the 'riser,' which at a suitable height empties into the horizontal. This runs back to the drop-leg, connecting to the boiler anywhere under the water-line. The riser, horizontal, and drop-leg form the loop, and usually consist of pipes varying in size from three quarters of an inch to two inches, and are wholly free from valves, the loop being simply an open pipe,

giving free communication from separator to boiler. (Stopand check-valves are inserted for convenience, but take no part in the loop's action.)" Supposing, for example, the boilerpressure to be 100 pounds and the pressure at the separator reduced to 95. "The pressure of 95 pounds at the separator extends (with even further reduction) back through the loop,

FIG. 202.-THE STEAM-LOOP.

but in the drop-leg meets a column of water (indicated by the broken line) which has risen from the boiler, where the pressure is 100 pounds, to a height of about 19 feet, that is, to the hydrostatic head equivalent to the 5 pounds difference in pressure. Thus the system is placed in equilibrium. Now the steam in the horizontal condenses, lowering slightly the pressure to 94 pounds, and the column in the drop-leg rises two feet to balance it; but meanwhile the riser contains a column of mixed vapor, spray, and water, which also tends to rise to supply the horizontal, as its steam condenses, and being lighter than the solid water of the drop-leg it rises much faster. By this process the riser will empty its contents into the horizontal, whence there is a free run to the drop-leg and thence to the boiler."

137. Reducing-valves.-The reducing-valve is a throttlingvalve arranged to be operated automatically so as to reduce the pressure and also to maintain a constant pressure on the steam-mains. A great many forms of these valves are in common use. In one a diaphragm of metal or rubber is employed, as in Fig. 203. The low-pressure steam acts on one side of the diaphragm, a weight or spring which may be set at any desired pressure on the other side. This diaphragm is

connected with a balanced valve which is moved to or from its seat as less or more steam is required to preserve constant pressure. Since the pressure in the main steam-pipe does not

effect the motion of the valve, its position will depend upon the pressure on the two sides of the diaphragm. The pressure on one side is that due to the steam which has passed through the valve, and that on the other to a weight or spring which can be set at any desired point.

Another form of reducing-valve with differential piston and diaphragm is shown in Fig. 204, and is described as follows:

Steam from the boiler enters at side "steam inlet" and, passing through the auxiliary valve K, which is held open by the tension of the spring S, passes down the port marked "from auxiliary to cylinder," underneath the differential piston D. By raising this piston D the valve C is opened against the initial pressure, since the area of C is only one half of that of D. Steam is thus admitted to the low-pressure side, and also passes up the port XX underneath the phosphor-bronze diaphragm 00. When the low pressure in the system has risen to the required point, which is determined by the ten

INLET

sion of the spring S, the diaphragm is forced upward by the steam in the chamber, the valve K closes, no more steam is admitted under the piston D. The valve C is forced onto its seat by the initial pressure, thus shutting off steam from

the low-pressure side. This action is repeated as often as the low pressure drops below the required amount. The piston D is fitted with a dash-pot E, which prevents chattering or pounding. In another style of construction. a piston acted on by the lowpressure steam serves to open or close a balanced valve an amount sufficient to maintain the steampressure constant.

138. Transmission of Steam Long Distances. It is fre quently necessary to transmit steam long distances underground, and in many cases this method gives better financial returns than the construction and operation of a large number of small and isolated plants. A number of plants, in which steam has been conveyed long distances in pipes laid underground, have been constructed for the purpose of heating portions of cities, and also various buildings belonging to the same public institution.

VALVE.

The system of heating from a common boiler-plant has not generally proved successful financially in cities, as out of a large number which have been erected the writer knows of only one which is now in use. In the heating of various buildings which belong to the same public institution this system of construction has proved a great improvement in many respects over that of separate heating-plants, although it is doubtful if in a single case it has ever resulted in the lessening of expense for fuel.

The three important requisites in the construction of such plants are, first, a removal of all surface-water so that it cannot possibly come in contact with the steam-pipe; second, provision for taking up expansion of pipe and keeping it in proper alignment; and, third, insulation of the pipe from heat losses.

The first condition, which is the most important of all, is also the most likely to be overlooked, and many failures to secure economic transmission have been caused by allowing the surface-water to come in contact with the heated pipes. This water can be removed by the construction of a drain beneath or by the side of the pipe-system, provided with proper outlets. A perfect drainage-system for the soil is in every case an essential requisite for success.

Provision for expansion may be made by the use of expansion-joints, as already described in Article 62, page 105, or by the use of elbows and right-angled offsets arranged to partly turn as the line expands. The writer has had experience with various forms of these joints, and found nothing equal to the straight expansion-joint, Fig. 90, which should, however, be constructed so that it cannot by any possible accident be pulled apart; this may be done either by use of an internal lug or external brace. These joints should be thoroughly anchored, so that they will stay in position, and should be placed sufficiently close together to take up all expansion without strain on the pipe-line. If the ordinary slip-joints are used, they will need to be placed at distances of about 120 feet apart. The pipe between the joints should rest on rollers or connecting hangers which permit its free motion. If elbows and offsets are employed to take up expansion, there will be an abrupt change in grade, and if any part dips below the main steam-line it should be drained by a pipe connecting to a trap or to the return. If bends convex upward are necessary, means must be provided for removing the air.

In general, in systems where the steam is transmited long distances the best results will be possible only when the boilerplant can be located on lower ground than the buildings to be heated, so that the water of condensation may be returned by gravity. This cannot always be done, and in many cases it will only be possible to return the water of condensation by