Chapter LXVI: Power Stations (2)

Balance pistons as at B, C, F are attached to the rotor,
their office being to oppose end thrust upon those blades in
corresponding diameter of the rotor. Communication is established
through the passage V and pipe M between the eduction pipe and the
back of these pistons, thus increasing the efficiency of their
balancing and also taking care of any leakage past them.

A small thrust bearing T prevents end play of the rotor, and is
adjustable to maintain the proper clearance between the rings of
blades; this varies from ⅛ inch at the admission to 1 inch at the
exhaust. This bearing also takes up any extra unbalanced thrust. A
turbine should operate with a high vacuum, because without this it
does not compare favorably with an ordinary reciprocating engine
from the point of economy.

_Separate air pumps are provided to create the vacuum._

Where the ordinary type of vertical air pump is employed, a
booster or _vacuum increaser_ is added, as nothing below 26 inches
is advisable, 28 and 29 inches being always striven for. It is
also preferable to use a certain amount of _super-heat_ with steam
turbines.

To assist in producing the high vacuum, exhaust passages are
made large, the eduction passage E in fig. 2,750 being nearly
twenty-three times the area of the steam pipe.

Among other details, a noteworthy feature is a small oil pump K,
which circulates oil through bearings of the machinery, the oil
being drawn from the tank under the governor shaft and gravitating
there after use. No pressure of oil is employed. Stuffing rings
prevent leakage; these consist of alternate grooves and collars in
shaft and bearing, like the grooves in an indicator piston.

=Ques. Why is a high vacuum desirable?=

Ans. Because the turbine is capable of expanding the steam to a very low terminal pressure, and this is necessary for economy.

=Ques. What may be said of the working pressures for turbines?=

Ans. To meet the varied conditions of service, turbines are designed to operate with: 1, high pressure, 2, low pressure, or 3, mixed pressure.

High pressure turbines operate at about the same initial
pressure as triple expansion engines.

Low pressure, as here applied, means the exhaust pressure of the
reciprocating engine from which the exhaust steam passes through
the turbine before entering the condenser.

Mixed pressure implies that the exhaust steam is supplemented,
for heavy loads, by the admission of live steam.

=Ques. What determines the working pressure?=

Ans. When all the power is furnished by the turbine, it is designed for high pressure; when operated in combination with a reciprocating engine, low pressure is used for constant load, and mixed pressure for variable load.

NOTE.--There are logical engineering reasons for the existence
of the several types of turbine, viz., single flow, double flow,
and semi-double flow. The double flow turbine is not inherently
superior to the single flow design, but is used under conditions
for which the single flow machine is unsuitable. Similarly, the
semi-double flow is recommended only for conditions which it can
meet more satisfactorily than either of the other types.

NOTE.--Low pressure turbines use exhaust steam from
non-condensing engines and are valuable as an adjunct to existing
plants for the purpose of increasing economy and capacity with a
minimum outlay for new equipment.

NOTE.--Bleeder turbines are for use in plants which are required
to furnish, not only power, but also considerable and varying
quantities of low pressure steam for heating purposes. In these
turbines a part of the steam after it has done work in the high
pressure stages may be diverted to the heating system, and the
remainder expanded through the low pressure blading and exhausted
into the condenser. In this way none of the energy of the heating
steam, due to the difference of pressure between the boiler and
the heating system is wasted. On the other hand if no steam is
required for heating purposes, the turbine operates just as
efficiently as though the bleeder feature were absent.

_The De Laval steam turbine_ is termed by its builders a high
speed rotary steam engine. It has but a single wheel, fitted with
vanes or buckets of such curvature as has been found to be best
adapted for receiving the impulse of the steam jet. There are no
stationary or guide blades, the angular position of the nozzles
giving direction to the jet. The nozzles are placed at an angle
of 20 degrees to the plane of motion of the buckets. The best
energy in the steam is practically devoted to the production of
velocity in the expanding or divergent nozzle, and the velocity
thus attained by the issuing jet of steam is about 4,000 feet per
second. To attain the maximum efficiency, the buckets attached to
the periphery of the wheel against which this jet impinges should
have a speed of about 1,900 feet per second, but, owing to the
difficulty of producing a material for the wheel strong enough to
withstand the strains induced by such a high speed, it has been
found necessary to limit the peripheral speed to 1,200 or 1,300
feet per second.

It is well known that in a correctly designed nozzle the
adiabatic expansion of the steam from maximum to minimum pressure
will convert the entire static energy of the steam into kinetic
energy. Theoretically this is what occurs in the De Laval nozzle.
The expanding steam acquires great velocity, and the energy
of the jet of steam issuing from the nozzle is equal to the
amount of energy that would be developed if an equal volume of
steam were allowed to adiabatically expand behind the piston
of a reciprocating engine, a condition, however, which for
obvious reasons has never yet been attained in practice with the
reciprocating engine. But with the divergent nozzle the conditions
are different.

_The Curtis turbine_ is built by the General Electric Company at
their works in Schenectady, N. Y., and Lynn, Mass. They are of the
horizontal and vertical types. In the vertical type the revolving
parts are set upon a vertical shaft, the diameter of the shaft
corresponding to the size of the machine.

The shaft is supported by and runs upon a step bearing at the
bottom. This step bearing consists of two cylindrical cast iron
plates bearing upon each other and having a central recess between
them into which lubricating oil is forced under pressure by a
steam or electrically driven pump, the oil passing up from beneath.

A weighted accumulator is sometimes installed in connection with
the oil pipe as a convenient device for governing the step bearing
pumps, and also as a safety device in case the pumps should fail,
but it is seldom required for the latter purpose, as the step
bearing pumps have proven after a long service in a number of
cases, to be reliable. The vertical shaft is also held in place
and kept steady by three sleeve bearings one just above the step,
one between the turbine and generator, and the other near the top.

These guide bearings are lubricated by a standard gravity feed
system. It is apparent that the amount of friction in the machine
is very small, and as there is no end thrust caused by the action
of the steam, the relation between the revolving and stationary
blades may be maintained accurately. As a consequence, therefore,
the clearances are reduced to the minimum.

The Curtis turbine is divided into two or more stages, and each
stage has one, two or more sets of revolving blades bolted upon
the peripheries of wheels keyed to the shaft. There are also the
corresponding sets of stationary blades bolted to the inner walls
of the cylinder or casing.

The governing of speed is accomplished in the first set of
nozzles and the control of the admission valves here is effected
by means of a centrifugal governor attached to the top end of the
shaft. This governor, by a very slight movement, imparts motion
to levers, which in turn work the valve mechanism.

The admission of steam to the nozzles is controlled by piston
valves which are actuated by steam from small pilot valves which
are in turn under the control of the governor.

Speed regulation is effected by varying the number of nozzles
in flow, that is, for light loads fewer nozzles are open and a
smaller volume of steam is admitted to the turbine wheel, but the
steam that is admitted impinges against the moving blades with the
same velocity always, no matter whether the volume be large or
small. With a full load and all the nozzle sections in flow, the
steam passes to the wheel in a broad belt and steady flow.

WEIR TABLE
giving cubic feet of water per minute that will flow over a weir
one inch wide and from ⅛ to 20⅞ inches deep.

--------+------+------+------+------+------+------+------+------
Depth | | | | | | | |
inches | | ⅛ | ¼ | ⅜ | ½ | ⅝ | ¾ | ⅞
--------+------+------+------+------+------+------+------+------
=0= | .00 | .01 | .05 | .09 | .14 | .19 | .26 | .32
=1= | .40 | .47 | .55 | .64 | .73 | .82 | .92 | 1.02
=2= | 1.13 | 1.23 | 1.35 | 1.36 | 1.58 | 1.70 | 1.82 | 1.95
=3= | 2.07 | 2.21 | 2.34 | 2.48 | 2.61 | 2.76 | 2.90 | 3.05
=4= | 3.20 | 3.35 | 3.50 | 3.66 | 3.81 | 3.97 | 4.14 | 4.30
=5= | 4.47 | 4.64 | 4.81 | 4.98 | 5.15 | 5.33 | 5.51 | 5.69
=6= | 5.87 | 6.06 | 6.25 | 6.44 | 6.62 | 6.82 | 7.01 | 7.21
=7= | 7.40 | 7.60 | 7.80 | 8.01 | 8.21 | 8.42 | 8.63 | 8.83
=8= | 9.05 | 9.26 | 9.47 | 9.69 | 9.91 |10.13 |10.35 |10.57
=9= |10.80 |11.02 |11.25 |11.48 |11.71 |11.94 |12.17 |12.41
=10= |12.64 |12.88 |13.12 |13.36 |13.60 |13.85 |14.09 |14.34
=11= |14.59 |14.84 |15.09 |15.34 |15.59 |15.85 |16.11 |16.36
=12= |16.62 |16.88 |17.15 |17.41 |17.67 |17.94 |18.21 |18.47
=13= |18.74 |19.01 |19.29 |19.56 |19.84 |20.11 |20.39 |20.67
=14= |20.95 |21.23 |21.51 |21.80 |22.08 |22.37 |22.65 |22.94
=15= |23.23 |23.52 |23.82 |24.11 |24.40 |24.70 |25.00 |25.30
=16= |25.60 |25.90 |26.20 |26.50 |26.80 |27.11 |27.42 |27.72
=17= |28.03 |28.34 |28.65 |28.97 |29.28 |29.59 |29.91 |30.22
=18= |30.54 |30.86 |31.18 |31.50 |31.82 |32.15 |32.47 |32.80
=19= |33.12 |33.45 |33.78 |34.11 |34.44 |34.77 |35.10 |35.44
=20= |35.77 |36.11 |36.45 |36.78 |37.12 |37.46 |37.80 |38.15
--------+------+------+------+------+------+------+------+------

NOTE.--The weir table on this page contains
figures 1, 2, 3, etc., in the first vertical
column which indicates the inches depth of water
running over weir board notches. Frequently the
depths measured represent also fractional inches,
between 1 and 2, 2 and 3, etc. The horizontal
line of fraction at the top represents these
fractional parts, and can be applied between any
of the numbers of inches depth, from 1 to 25. The
body of the table shows the cubic feet, and the
fractional parts of a cubic foot, which will pass
each minute for each inch in depth, and for each
fractional part of an inch by eighths for all
depths from 1 to 25 inches. Each of these results
is for only one inch width of weir. To estimate
for any width of weir the result obtained for one
inch width must be multiplied by the number of
inches constituting the whole horizontal length
of weir.

=Hydro-Electric Plants.=--The economy with which electricity can be transmitted long distances by high tension alternating currents, has led to the development of a large number of water powers in more or less remote regions.

This economy is possible by the facility with which alternating
current can be transformed up and down. Thus at the hydro-electro
plant, the current generated by the water wheel driven alternator
is transformed to very high pressure and transmitted with economy
a long distance to the distributing point where it is transformed
down to the proper pressure for distribution.

A water wheel or turbine is a machine in which a rotary motion
is obtained by transference of the momentum of water; broadly
speaking, the fluid is guided by fixed blades, attached with a
casing, and impinging on other blades mounted on a drum or shaft,
causing the latter to revolve.

There are two general classes of turbine:
1. Impulse turbines;
2. Reaction turbines.

=Ques. What is an impulse turbine?=

Ans. One in which the fluid is directed by means of a series of nozzles against vanes which it drives.

=Ques. What is a reaction turbine?=

Ans. One in which the pressure or head of the water is employed rather than its velocity. The current is deflected upon the wheel by the action of suitably disposed guide blades, the passages being full of water. Rotary motion is obtained by the change in the direction and momentum of the fluid.

=Ques. Name three classes of reaction turbines.=

Ans. Parallel flow, inward flow, and outward flow.

Parallel flow turbines have an efficiency of about 70% and are
suited for low falls not over 30 feet. Inward and outward flow
turbines have an efficiency of about 85%. Impulse turbines are
suitable for high heads.

=Isolated Plants.=--When electric power transmission from central stations first came into commercial use, the distance from the station at which current could be obtained at a reasonable cost was exceedingly limited.

Consequently, persons desiring electrical power were in the
majority of cases forced to install their own apparatus for
producing it, this being the origin of isolated plants.

From the nature of the case it is evident that an isolated
plant is as a rule smaller and more simple in construction than
a central station, and in consequence much more readily operated
and managed. It is generally owned by a private individual or a
corporation and operated in conjunction with other affairs of a
similar character. A basement or other portion of a building is
usually set aside in which the necessary apparatus is installed.

Although electricity is now transmitted economically to great
distances from central stations, there is still a field for the
isolated plant.

The average type of isolated plant has enlarged from a small
dynamo driven by a little slide valve engine located in an out
of the way corner to direct connected generators and engines of
hundreds and even thousands of horse power assembled in a large
room specially adapted to the purpose.

In the more modern of these, the electrical outputs are each
frequently equal to that of a town central station of respectable
size, and the auxiliary equipments are similar in every
particular. As a matter of fact, in certain modern isolated plants
the only feature that distinguishes them from central stations
is that in the former case the owner of the plant represents the
sole consumer and conducts other business in connection with it,
whereas in the latter case there are a large number of consumers
uninterested financially in the enterprise, which is itself
generally owned and operated by a company conducting no other
business.

=Sub-Stations.=--According to the usual meaning of the term, a sub-station is a building provided with apparatus for changing high pressure alternating current received from the central station into direct current of the requisite pressure, which in the case of railways is 550 to 600 volts.

Where traffic is heavy and the railway system of considerable distance, sub-stations are provided at intervals along the line, each receiving high pressure current from one large central station and converting it into moderate pressure direct current for their districts.

=Ques. Upon what does the arrangement of the sub-station depend?=

Ans. Upon the character of the work and the type of apparatus employed for converting the high pressure alternating current into direct current.

In general it should be substantial, convenient to install or
replace the heavy machines, and the layout arranged so that the
apparatus can be readily operated by those in attendance.

An overhead traveling crane is the most convenient method of
handling the heavy machinery, and is frequently used in large
sub-stations.

Fig. 2,776 shows a sectional view, and fig. 2,777, a plan for
a small sub-station containing two rotary converters and two
banks of three single phase static transformers operating on a
three phase system at 11,000 or 13,200 volts, together with the
auxiliary apparatus.

=Ques. For three phase installations, what are the merits of separate and combined transformers?=

Ans. With separate transformer for each phase, repairs are more readily made in case of accident or burnouts in the coils. The three phase units have the advantage of low first cost.

Sub-station transformers produce considerable heat, due to the
hysteresis and eddy currents, and it is necessary to get rid of it.

Small transformers radiate the heat from the shell and the
medium sizes have corrugated shells which increase the surface and
provide more rapid radiation.

Large transformers are cooled by an air blast supplied by motor
driven blowers or by water pumped through a coil of pipe which is
immersed in the insulating oil of the transformer. The large size
oil insulated, water cooled transformers are used on circuits of
33,000 volts or more. In water turbine plants, the water may be
piped to the transformer under pressure and the pump omitted which
cuts down the cost of operating. Air blast transformers usually
have a damper or shutter for air control.

=Ques. Explain the use of reactance coils in sub-stations.=

Ans. In order that the direct current voltage of the ordinary rotary may be regulated by a field rheostat, which calls for a corresponding change in the alternating current voltage, a reactance coil is provided between the low tension winding and the converter.

Without such a reactance the maintenance of the same voltage at
full load as at no load involves excessive leading and lagging
currents and consequently excessive heating in the armature
inductors, unless the resistance drop from the source of constant
pressure is small, or the natural reactance of the circuit high.

=Ques. What is the effect of weakening the converter field?=

Ans. A lagging current is set up which causes a drop in the reactance coil.

=Ques. State the effect of strengthening converter field.=

Ans. A leading current is set up which gives a rise of voltage in the reactance coil.

Hence when a heavy current passes through the series coil of a
compound wound converter and tends to produce a leading current,
the reactance coil will balance it, and improve the power factor
of the whole line.

=Portable Sub-Stations.=--A portable sub-station constitutes a spare equipment for practically any number of permanent sub-stations and renders unnecessary the installation of spare equipment in each.

It can be used to increase the capacity of a permanent
sub-station when the load is unusually heavy, or to provide
service while a permanent sub-station is being overhauled or
rebuilt.

The transformer can be used for emergency lighting, the primary
being connected to a high pressure line and the secondary to the
load, if special provision be made at the time the transformer is
built to adapt it for these applications.

When an electric railway has a portable sub-station, direct current can be provided at any point on the system where there is track at the high pressure line. The direct current can be made available very quickly as its production involves only the transferring of the sub-station, and its connection to the high pressure line.

Portable sub-stations range in capacity from 200 to 500 kw., and
for all alternating current voltages up to 66,000, and frequencies
of 25 and 60 cycles.

Although portable sub-stations usually must be of more or
less special design to adapt them to the conditions under which
they must operate, there are certain general features that are
common to all. All members are readily accessible and there are
no unnecessary parts. The weight and dimensions are a minimum
insuring ease of transportation. Live parts are so protected that
the danger of accidental contact with them is minimized.

=Ques. What are the advantages of using outdoor transformers on portable sub-stations?=

Ans. All high pressure wiring is kept out of the car. The transformer is more effectively cooled and the heat dissipated by the transformer does not warm the interior of the cab. The transformer is much more accessible. The car can be run under a crane and the transformer coils pulled out with a hoist.

Taps for different high and low pressure voltages can be readily
provided at the time the transformer is being built.