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THESIS FOR DEGREE OF C.E, ||
RUSSELL ALGER MURDOCH |
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THESIS

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ENGINEERING OF
A MUNICIVAL WATER WORKS SYSYEM

 

Thesis for Degree of C.&.
Russell Alger Murdoch

1915
THESE
ENGINEERING OF
A MUNICIPAL WATER WORKS SYSTEM.

This topic may be diwided into the following parts, viz.,
Water Supply, Pumping Equipment, Water Storage, and Distribre
ution System.

WATER SUrrLY. The prime essential of a water works
system is a sufficient supply of pure and wholesome water.
Insufficient investigation into the source of water supply
is the cause of obtaining improper supply either in quality
or quantity leading to wasteful expenditure of money or to
a complete failure of the water works systen.

Water supply is obtained either from natural lakes or
streams, artifical lakes fed by streams, springs or wells,
or direct from wells. These wells are usually what is known
as deep wells. In the case of springs, streams or lakes the
problem is usually comparatively simple, in the case of deep
wells a great deal depends upon the geological character-
istics of the locality. In some localities water can be
obtained within a comparatively small depth, in other regions
it is necessary to go down several hundred feet before pot-
able water is obtained in sufficient quantities.

The following will serve as an illustration of the un-
certainty of obtaining a water supply from wells:-

In one Village where the writer was in charge of the
water works installation a location for the pumping system
was decided upon. After some investigation of the deepest of
the drilled wells near by, in which water had been found at
depths of from seventy five feet to one hundred and forty
feet, an eight inch well was drilled between two of these
wells with the following results:- On the surface was found
twelve feet of sand which was followed up by seventy eight
feet of blue clay, ninety feet of lime rock and twenty five
feet of light shale, making a total depth of two hundred and
five feet, all of which was so dry that water had to be
poured into the hole so the drill could work up the rock.

At two hundred and five feet a stratum of sand rock was
struck, which ran ten feet deep and contained a small amount
of water heavily saturated with salt, and some gas.

This water would flow out of the top of the casing at
the rate of about one gallon in five minutes. Upon drilling
Geeper, sixty one feet of lime rock, one hundred twenty
eight feet of light shale and eight feet of lime rock was
encountered, all of which was practically dry.

This well was therefore abandoned and the drilling

AORZ925
apparatus was moved about three thousand feet north west
and a three inch test well drilled, with the resuit that
after going through sixty five feet of clay and ten feet
of hard pan, eight feet of gravel containing weter was
found. The water raised to a height of six feet below
the surface and pumped twenty gallons per minute, which
lowered the level to twenty five feet below the surface,
the duration of the test being ten hours. The well was
then reamed out to eight inches and again tested with
the result that while it at first gave sixty gallons per
minute,after six hours continuous pumping it was reduced
to twenty gallons per minute.This capacity it would hold
with continued pumping,showing that @ small bed of gravel
with a small inlet had been struck and that, although it
would stand up under a pumping test of twenty gallons per
minute for a considerable time, when pumped faster than
that, the bed of gravel was soon pumped dry and the only
source of supply was the small inlet of twenty gallons
per minute. Z

It is evident that the testing well was an expense
incurred which was of no value in showing the relative
value of the large and small well, but only in showing
that some water could be obtained in that locality.

In well number three a similar formation was encount=-
ered, sixty five feet of clay, fifteen feet of hard pan
but only one foot of gravel, which was dry;then nine feet
of hard pan and one hundred six feet of dark shale inter-
mingled with a pebble formation producing water to the
extent of forty gallons per minute under continued pumping.
Below this was found four feet of light shale. In encount-
ering the one foot of gravel in well number three evidently
the edge of the gravel bed of the second well was struck.

In well number four the formation was as follows:-
sixty five feet of clay, ten feet of hard pan, and eight
feet of gravel which produced a very clear and good
tasting water, but, when pumped would not stand up under
more than ten gallons per minute pumping. This well was
then drilled deeper through nine feet of hard pen, one
hundred and seven feet of dark shale as’ in well number
three,but with less pebble formation,which produced some
water and gas. On drilling deeper, twelve feet of light
shale was encountered, then a sand rock formation was
struck as in well number one. This produced salt water.
As the last twelve feet of this well was drilled with a
six inch drill the two inch reduction formed a taper into
which @ pine plug, about four feet long, was driven and
thus the salt was completely shut off allowing the use of
the fresh water encountered above. This well was then
tested and showed a capacity of about forty gallons per
minute, without effecting well number three which was
about fifty feet distant. It was thus found that wells
could be spaced as near as fifty feet without effecting
each other. Two other wells were drilled on the lot and
similar amounts of water were encountered in the rock and
pumping them did not effect the other wells.

These wells were all eight inch wells which was the
economical size for the rock wells as there was ninety five
feet of casing anda larger size would have cost more in !
proportion to the volume of water obtained, however in the
case Of well number two, a gravel well,it was evident that
a four inch well would have produced all the water avwail-
able from the gravel bed and would have cost mouh less
money.

The records of the several wells above mentioned
show the marked degree in which strata changes, as in the
fifty feet from well number three to well number two the
gravel changed from one foot of dry gravel to eight feet
of gravel producing &@ small supply of very good water,and
from well number one to well number three,which was three
thousand feet distant, the rock formation was completely
changed, the first showing ninety feet of lime rock and
twenty five feet of light shale with no water and the
latter showing one hundred and six feet of dark shale with
a pebble formation and producing a fair supply of water,
and below these formations as shown in wells one and four
was avery similar form of sand rock and in both cases
producing a very strong salt water, and gas.

Wells are generally used for the water supply in the
Villages of Lower Michigan, except those bordering on the
Great Lakes, however it is sometimes very difficult and
expensive to obtain water in this manner and in some cases
surface water supplies, by which is meant lakes, streams
or springs, have been used.

In the case of surface water supplies where potable
water is available it is only necessary to provide a pro-
per intake pipe with suitable strainer on its end. There
are two methods of connecting the strainers to the intake
pipe, (1) If the water is not very deep dig a well in the
pottom of the stream and turn the strainer down into it,
(2) When the water is deep turn the strainer up toward the
surface. In any case, the strainer should be well sub-
merged so as not to get air. A foot valve in the pipe may
be required depending on the suction of the pumps, if the
water flows to the pumps a foot valve is unnecessary. The
_ foot valve should be kept near the strainer so it can be
easily reached in case of any trouble or repairs.

The source of water supply should be thoroughly gone
over to see that there is no polution, and in case of doubt
the water should be thoroughly analized, the latter is
sometimes compelled by State laws.

Where potable water is not available a filtering
system must be installed, this in all cases proves to be a
very expensive system and usually the Village get their
fire system installed on the first attempt and several
years later installs the filtering plant. In this case,
there are usually very few users of water for domestic

 
and sprinkling purposes and the revenue therefore is very
small and does not aid very much in paying for the fire
system.

Where the river or body of water from which the supply
is taken gets very low in dry weather a large reservoir
must be constructed and an extra pump installed to fill
this reservoir during the high water. This water becomes
stagnant to a certain extent and becomes a breeding place
for mosquitoes, and is also a wery expensive arrangement,
and should be avoided where it is possible to obtain artes
sian wells or potable surface water supplies.

The suction pipe should be made of ample size to keep
down the friction to a reasonable amount. In the case of
a small water works system the suction pipe would not be
very large in any case and would probably be of no very
great length. If this pipe is so small that appreciable
work must be done by the pump to draw water through it to
fill the cylinders, there is a constant expenditure of
power delivered by the prime mover simply to overcome this
Pipe friction. It will readily be seen that this expense
is continuous during the operation of the pump, whereas
there would be little additional first cost in increasing
the size of the suction line to save this expense, for the
interest on the investment which would be almost negligable,
would be largely, if not entirely, offset by the reduced .
power consumption. The first cost would then be practically
the only additional expense,

It is safe to say under ordinary conditions that the
cost for the additional power above mentioned would be far
more than the interest on the additional investment due to
the increase in the size of the suction line. It may also
be pointed out that in some cases the pipe friction in the
suction line would necessitate the use of a prime mover of
larger size to drive the pump than would be required if
this pipe friction could be decreased appreciably. This is
especially true in the case of prime movers at determined
speeds and rated horse powers, such as electric motors,gas
engines,etc.,where we are limited in selection to certain
standard commercial sizes.

In this connection the following points should also
be considered, in general prime movers operating at less
than full load are not so efficient as when operating near
the point of its rated capacity, although in the case of
electric motors a reasonable range at good efficiency may
be obtained by the use of well designed motors. It is also
necessary in the case of high suction liftsto run the pump
at a slower speed than would otherwise be selected, thus
requiring a larger pump at a given capacity than would be
the case if the suction lift were less.

—
It 1s evident from the foregoing that high suction
lifts, whether they be due to vertical lift or its equiv-
alent in pipe friction, are likely to prove expensive
features of design as they require larger pumps and more!
powerful prime movers without any corresponding advantage
in increase, either, of discharge pressure, or of capacity:
It is further true that the higher the total suction lift
the more necessary and also the more difficult it is to
make and maintain an absolutely tight suction line, on the
whole then we may say that high suction lifts are to be
avoided whenever possible. It is not usually practical to
use suction pumps where the suction lift is more than
twenty one or twenty two feet, except under such conditions
as are outside the province of this paper.

If possible the water supply should be near the dis-
tributing and storage systems in order to reduce the cost
of piping and pumping the water to the storage and dis- |
tributing systems.

However this is not so essential where a very large
storage capacity is near the business section of the
Village and an auxiliary pumping plant installed to take
the water from this storage and provide a pressure in case
of fire, or where the storage tank is elevated high enough
to produce 4 gravity pressure great enough to take care of
the fire pressure in case some accident should occur
between the supply system and the distributing systen.

-
PUMPING EQUIPMENT. In dealing with this portion of —-
the subject it will be necessary to pass over some feat-
ures rather hastily as being of small importance to the
Municipal Engineer and dwell more at length upon those
portions of the matter which are of more significance in
connection with the subject we are considering.

Pumps, in general, may be divided into three classes,
broadly speaking; these are, (1) Steam Pumps, (2) Power
Pumps, (3) Special Pumps.

Class (1) may again be subdivided into, (A) Direct
Acting Pumps, (B) Pumping Engines.

Class (2) may be, in the same way, divided into,

(A) Reciprocating rumps, (B) Rotary rumps, (C) Genter pares
Pumps.

Class (3) comprises those pumps which are in neithe
Class (1) nor Class (2), such as Hydrolic Rams, Air Lifts,
Water Lifts, “Screw rumps" etc.

We may define a steam pump as one which is actuated|
by steam and in which the driving mechanism (i.e. cylinders,
valve motion, etc.) forms an integral and inseparable part
of the design. —

A steam pump is said to be direct acting when stean
pressure is applied ( in the steam cylinder to a piston
on one end of a rigid rod at the other end of which a
similar piston acts against the water in the water cylinder.
The axes of the steam and water cylinders, then coincide.
Such a pump works by direct steam pressure, the steam be-
ing admitted to the cylinder during the full stroke, no
advantege being taken of its expansive qualities. There
is no "cut off" of steam at any intermediate point in the
stroke.

A pumping engine is an integral combination of a steam
engine and a pump, (i.e. steam and water cylinders have not
coincident axes, and the steam is used expansively, (i.e.
steam is admitted during only a portion of the stroke, cut
off taking place at the proper point as determined by the
setting of the valve).

A force pump is one in which the motive power is .
furnished by a source separate from the pump itself, such
as a steam or gas engine, an electric motor, etc. The
pump and the prime mover, in this case, may be— and often
is— purchased from different manufacturers,

A reciprocating pump is, as the name implies, one in
which the water is displaced by reciprocating pistons, or
Plungers. This necessitates the transformation of a rotary
to a reciprocating motion, since the motion of power
delivery of practically all prime movers is rotary.

A rotary pump is one which works by displacement
produced by the rotation of two intermeshing cams, which,
in the smaller sizes usually take the form of gears, ae

 
We may define a centrifugal pump as one in which the
water is delivered due to the action of centrifugal force,
which is developed b% the action of a rotary part (the im-
peller) causing the water to rotate in the casing and, by
the centrifugal force due to its rotary motion,to be thrown
outward to the spaces provided in the casing, whence it is
delivered through the proper passages to the discharge.

Of the spiral pumps more will be said hereafter,

Given a proper and sufficient water supply, the next
step is the determination of the type of system to be used
and, consequently, of the type of pump.

There are many points to be considered in the selection
of the pump. Usually the first question which arises is
whether a steam pump or a power pump shall be used.

The direct acting steam pump is one of the most unec-
onomical devices for pumping which is in common,— the
writer almost said too common,— use.

Such points as are in favor ere, (1) Low first cost,
(2)Simplicity,(3) The fact that every competant operating
engineer is familiar with it. Its low cost is what recom-
mends it to most purchasers.

“here electric current is available, and economy is
any object, it should never be used. In fact it is a very
good plan to consider such a pump as the last type to be
seriously thought of for a water works plant, and only to
be used in the case of unanswerable objections to the use
of other types.

The pumping engine is far more economical than the
direct acting steam pump, and for very large water works
systems and high suction lifts it is the type of pump to
be prefered, especially for high duty cintinuous service.

Since the scope of this paper is limited more espec-
lally to the requirements of the water works of smaller
municipalities, from the foregoing it is evident that it
is about the power pumps that we shall have most to say.

Their advantages are, (1) Economy of operation, (2)
Flexibility of operation, (3) Reliability,-(4) Simplicity,
(5) Adaptability. It is not difficult to show (See appen-
dix) that the electric motor is, under ordinary conditions,
the most economical and tyre most convenient prime mover to
use for driving a power pump for use in the average smail
municipal water works plant, even if it be combined with
the lighting plant, as is frequently the case. There is one
_ possible exception to this, and that (steam turbine driven
centrifugal) will be considered later.

We must now consider the different types of power pumps
and their suitability for the various conditions of opera-
tion usually to be met with.

Briefly, we may outline the various types as follows:-

(I) Reciprocating pumps,

(1) Single Cylinder,

(A) Horizontal, (a) Single Acting, (b) Double Acting.

(B) Vertical, (a) Single Acting, (b) Double Acting.

 

 

 
(2) Duplex
(A) Horizontal, (a)Single Acting, (b) Double Acting.
(B) Vertical, (a) Single Acting, (b) Double Acting.
(3) Triplex
(A) Horizontal,(a)Single Acting, (b)Double Acting.
(B) Vertical, (a) Single Acting, (b)Double Acting.
(II) Rotary rumps.
(a) Gear Cut Cams, (b) Special Cams.

"Turbine"
_ (IIT f 1 _
( ) Centrifugal Pumps "Volute.
(1) Single Suction Horizontal
(A) Open Impeller, (a) Single Stage — yventical.
(B) Enclosed Impeller, (a) Single Stage — perticais

(o) uutetcstege - feEtiguit!
(2) Double Suction

(A) Single Stage
(B) Multi Stage.
(IV) Special rumps.
(1) Hydraulic Rams
(2) Air Lifts
(3) Water Lifts
(4) Sinking Pumps
(5) Helical or "Screw Pumps.
(V) Auxiliaries.
(1) Air Compressors
(2) Pressure Regulators
(3) Float Switches
(4) Relief Valves.
Taking the foregoing schedule in order, we may briefly
make the following comments:-
Single cylinder, horizontal, single acting pumps are
not in use except for very small domestic water supply
syatems such as residences, small farms,etc. They are usually
made in small capacities only; they are low priced, and, as
made, are not of wery high efficiency.
Double acting pumps of the same type are made in larger
capacities, and are used for domestic water supply, for
farm use, and for commercial purposes, such as pumping water,
chemicals, and oil or gasoline in factories. They are made
in larger capacities than the single acting pumps of this type
and usually run about sixty percent in efficiency. They, also,
are low priced pumps.
The vertical single cylinder pump is rarely seen in a
power pump, éxcept as a small pump for pressure purposes
(i.e. a force pump or small boiler feed pump.) In a modified

a7
form, however, this type of pump plays a wery important in
connection with Municipal Water Supply.

This modification consists of separating the cylinder
from the pump mechanism and placing it at a distance from
the remainder of the apparatus, (i.e. the cylinder is placed
below the water (submerged) and the crank shaft, gears and
frame are set above, the cylinder being actuated by a wood
or metal rod, known as a “sucker rod" which is really a
greatly lengthened piston rod. In this form the pump is
known as a “Deep well working head", which term does not,
in its strict sense, include the cylinder; the latter al-
ways being spoken of separately.

The particular advantage of this type of pump is that
it will raise the water from a depth, below the surface,
that would be impossible to consider were it necessary to
set the entire pump above ground and depend upon suction.

Duplex pumps, both of the horizontal and vertical type,
both single and double acting, are often used for municipal
water works systems. They are lower priced than the triplex
pumps, but have certain disadvantages and are not so gener-
ally useful as the triplex type.

Triplex pumps are, in general to be prefered for the
class of service we are considering. They are made in both
the vertical and horizontal types, and both single and
double acting.

The smaller sizes are made in the vertical type and
with single acting plungers. The larger sizes (say, four
hundred and fifty gallons per minute, or more ) may be had
in the vertical type and either single or double acting,
although the latter is much more expensive in first cost.

Very large sizes are made in the horizontal double
acting type only, and are quite costly, but in many cases
are better suited to the conditions than a steam pumping
engine.

The advantages of a triplex over a duplex pump may
priefly be summed up as follows:-

(1) The cranks are one hundred and twenty degrees
apart, this gives 2 more continuous discharge than the
duplex pump, in which the cranks are ninety degrees apart,
and an evener and less fluctuating load on the prime mover.
While the double acting duplex pump apparently gives a more
even discharge curve than a single acting trivolex pump, the
load on the prime mover and the strains on the bearings are
not so uniform. Needless to say the double acting triplex
pump has a much more uniform discharge than any duplex pump
could possibly have.

It is evident that while, in a horizontal double acting
pump, the design may be of the outside packed type, ina
vertical pump of the double acting type, a plunger cannot be
10

used, but recourse must be had to a piston; hence all vertical
double acting pumps are of the inside packed patern.

In general, triplex pumps are of higher first cost than
duplex.

Typical performance curves of a triplex pump ( vertical,
plunger type,) are given in the appendix,

It is to be noted that being positive displacement
mechanisms, all of the above pumps can be used on suction. .
lifts, excepting certain types of cylinders used in connection
with deep well working heads.

Regarding rotary pumps, little need be herein said, as
they are used in large sizes only in certain localities —
principally the New Englang States — save for factory pur=-
poses. Their efficiency is fairly good, about sixty percent,
but they are noisy in operation and are subject to certain
inherent traits which place them under bann in many cases.
Their chief advantages are —

(1) Compactness

(2) Lack of reciprocating parts

(3) Positive displacement of water discharged

(4) Suction is good on moderate lifts

(5) Low first cost

(6) Simplicity.

The centrifugal pump is very well suited to certain
classes of service. Designs have, within recent years, been
worked out that would have been considered as utterly impos=
sible, ten years ago.

Where, a few years ago, this type of pump was regarded
as a rather interesting but impractical scientific toy, good
only for large wolumes against low heads, and it was even ;
stated by good engineers that its efficiency could never be
over twenty five or thirty percent, public openion has now
rushed to the other extreme and many engineers and technical
men ( not to speak of laymen ) demand a centrifugal pump for
every installation regardless of its suitability for the
conditions of operation.

It is hardly necessary to say that such people have not
the slightest conception of the theory and design of centri-
fugal pumps and it is to be regretted that they are in such
a large majority, as such a course of action is very likely
to bring into disfavor one of the most useful types of vunp-
ing machinery we have.

It 1s also to be regretted that there is probably more
misunderstanding regarding the centrifugal pump, today, than
there is in connection with any other piece of apparatus
which is in equally common use.

Within its limits, the centrifugal pump is extremely
useful and has many advantages over any other type of pump.
If its limitations are to be exceeded, however, it is far
better not to use it at all, as nearly ewery advantage will
be foregone and it is likely to prove a detriment to the
installation.

 

 

 
LL

Its principal advantages are —
(1) Simplicity
(2)Compactness
(3) Low first cest
(4) Lack of reciprocating parts
(5) Good efficiency.
Its principal disadvantages are —
(1) Lack of suction
(2) Sensitiveness to changes in speed and head
(3) General lack of knowledge of its characteristics,
among both consulting and operating engineers.

It is not fair to charge up against the apparatus the
misrepresentation on the part of careless or unscrupulous
manufacturers or salesmen, which are the cause of considerable
dissatisfaction on the part of the purchaser, when the in-
stallation is finally completed, due to the incompatibility
between the pump characteristics and the operating condit-
ions.

While it is not the intent of this paper to go into the
details of pump design, nevertheless it is not out of place
to say a few words regarding the action of centrifugals, as
they are becoming a more and more important feature of water
works engineering as time goes on.

Since a centrifugal has no suction, if it is desired to
actuate it automatically, it is ewident that it must be in-
stalled with "flooded suction" i.s. so that the water flows
through the suetion piping to the pump.

It is preferable to keep the suction Lift of all pumps
as low as pssible — on centrifugals especially so, In the
case of ea plunger or reciprocating pump, high suction lift
may be compensated for in * measure by selecting a large
sized pump and running it at a slow speed, thus allowing
the cylinders to f111 more easily than would be the case
if a smaller pump were run at a higher speed. With a cent-
rifugal pump, however, this cannot be done, as the speed
(with a given number of stages ) has to be suited to the
discharge pressure required.

Neither is it economical to use a large diameter of
impeller at a slower speed, as the impeller is, from
elementary considerations, a water brake, and, since the
brake action waries as the fifth power of the diameter of
the impeller, it is evident that doubling the diameter
increases the brake action thirty two times, due to this
increase. The brake action depends, of course, on the
friction between the impeller and the surrounding water,
and also upon the adhesion of the waterto the impeller
( which may be considered, naturally, in connection with
the friction ). Since the friction between two moving

 
12

bodies in contact is a function of their relative velocity,

it follows that the brake action is a function of the
velocity of the impeller — i.e. its speed. But since the
linear velocity of any point on the impeller depends upon
the semi diameter (radius) it follows that the brake action
must be some function of the diameter. As a matter of fact,
the brake action of the impeller varies only as the square
of the speed. Inasmuch as the brake action of the impeller
is one of the internal losses of a pump, it is wery evident
that a small diameter and high speed impeller will be more
efficient than a large diameter impeller running at a slow-
er speed, both delivering against the same pressure.

This explains why some manufacturers will bid upon a
pump of more stages than others. They save the brake energy
by using smaller impellers, the speed also being kept down
by using more stages, and, consequently, obtaining less
pressure per stage.

This fact explains why the efficiency of a centrifugal
( or turbine pump ) is not dependent entirely upon work-
manship or design, as, by using a larger number of smaller
impellers, the saving made by reducing the brake losses may
offset careless or improper design. Such a piece of appar-
atus, of course, may be cheaper, but embodies more wearing
parts and, while the material may be of the best, and the
cormercial efficiency high, nevertheless it would be a com-
primise to obtain the desired result.

Briefly, the pump which will do a certain work with
the fewest number of stages with equal or less horse power
than another, is of better design — i.e. its design comes
closer to fulfilling the requirements. It is important to
remember that this brake energy is not indicated in any
way by the so-called “characteristic curves" of a
centrifugal.

The following facts may be briefly stated:-

For a given impeller, (1) the capacity varies directly
as the speed, (2) the pressure(™Y“) varies directly as the
square of the speed. (3) The horse power (10° eos

= Sallons per minute X at X head in pounds X 2.3
33000 ~

. Ky* capacity * head in poundse Ko - KE ‘Ky . Ke
7 K. K

3 3 3
= K, * SY) varies as the cube of the speed.
In the above, Kj = s+ is the factor for converting gallons
to pounds (weight)

; ka: = 2.3 *. " " " pounds
to feet (pressure)

 

> S° se

 
13

Kg is the number of foot pounds in a horse power
(33000 per minute) x. Ki * Ke
K, represents
‘ K3

Where K is the combined proportionality factor between
the speed and the capacity and pressure.

These are the same formulae as are used for steel
plate fan computations.

Hence, by doubling the speed of a certain impeller
it will deliver twice the capacity against four times the
pressure, but will take eight times the horse power to do
it.

Since the maximum pressure developed by a centrifugal
pump is independent of anything but the speed of the impel=-
ler, it follows —

(1) That it may be used to discharge into a closed
system without fear of any excessive pressure developing.

(2) The discharge valve may be shut off without harn-
ing the pump (provided it is done so;:as to avoid water
hammer).

(3) When the discharge walve is closed, since the
pump is delivering no water, theoretically the horse power
equels zero, because, in the above formula, canrcity equals
zero, hence, the numerator of the fraction being zero, its
walue is zero, rractically, however, we have the brake action
of the impeller and also the friction of the bearings,etc.
This explains why the horse power curve does not start from
zero at zero capacity. (See appendix).

In the above items are covered the principal inherent
differences in action between a centrifugal and a positive
‘displacement pump.

In the latter, any pressure (up to the safe working
limit of the pump) may be pumped against at any desired
speed (and consequent capacity) provided the required horse
power is availeble and provided the speed exceeds the R.P.M.
slip (See appendix). The centrifugal on the other hand, is
extremely sensitive to variations in —

(1) Speed

(2) External resistance. i.e. variations in the discharge
pressure or “counter pressure", because —

) Any variation in speed changes the delivery
(capacity) accordingly and also the pressure against which’
the pump will operate.

Since, at a &iven speed,the maximum pressure is
fixed, it is evident that, if the external resistance be
increased beyond ae certain point, the pump will no longer

 

 
14

deliver water, but will simply churn the water in the casing.
In fact, such a condition may be demonstrated by gradually
closing off a valve in the disgharge line. The discharge
will gradually decrease as the resistance increases, until,
at the point where the counter pressure equals the pressure
developed, no water will be discharged.
Conversely, it is apparent that, if we have a pump
delivering a certain capacity against a certain discharge
head, and this head is decreased the delivery will be increased.
As a matter of fact, the increase will be rapid — more rapid
in proportion than the reductionof head to which it corresponds.
Refering.to the general formula,
H.P. = 68 G.P.M. » 2.3 ° pressure in pounds

 

 

OU
if the G.P.M. increases more rapidly than the pressure decreases,
it is evident that the H.P. will be decreased.
This brings us to the most important development in the
design of the centrifugal pump — i.e. the "non overload"
impeller.

 

By this term, we signify an impeller of such a form
that, if the pump be designed to deliver a certain capacity
against a certain head, and the head should, by accident or
design,re reduced ewen to free discharge at the pump, the
increase of capacity (the speed being constant) will be so
regulated (automatically, by the design) that the load on
the prime mover will not be increased beyond a certain point
(usually about ten percent increase) and may, beyond a certain
point, actually be decreased. In other words, the horse power
curve should be convex upwards, with a well defined maximum
(See characteristic curves in appendix). This feature is
embodied, today, in the design of nearly all manufacturers,

From the foregoing considerations, and from the shape
of the characteristic curwes (See appendix) it is very evi-
dent that it is quite necessary to know as nearly as poss-
ible the exact condition under which 4 centrifugal pump is
to operate in order to be able to select the proper pump
for the service. This is to be born in mind when dealing
with the manufacturer of such apparatus, as a conscientious
and reputable manufacturer will hesibfate to bind himself to
assume any responsibility for the operation of a centri-
fugal pump unless he knows exactly the nature of the service
for which it is intended. |

If this detailed information is not forth coming, he
will make certain assumptions upon which to base his guar-
antées and recommendations. If these assumptions do not ._
happen to be correct, confusion will inevitably result -
either in connection with the tabuletion of the bids, or
else, when it comes to the final test of the apparatus
for acceptance.

In either case it will be plain to all concerned
that the responsibility rests with the Engineer for his
failure to supply the manufacturer with the proper data.

Prime movers for the small and medium sized Municipal
water works are usually —

 

 
(1) Steam Engine

(2) Steam Turbine

(3) Internal Combustion Engine

(4) Electric Motor.

The steam engine is rarely used because it requires,
in addition to its own rather high first cost (if of an
economical type) the additional outley for a boiler plant
and certain auxillaries.

The steam turbine, while in some cases cheaper than
the steam engine, is also, in those types, less economical
in operation than the high class engine. It has the advan-
tage of lacking reciprocating parts.

Furthermore, the requirements of the Insurance Assoc-
iations or Underwriters make it necessary to carry a cere
tain steam pressure day and night and to comply with cer-
tain other necessary but exacting requirements which en-
tail additional expense.

We must also remember that, in the small sizes which
must be considered (i.e. the whole plant taken as a unit)
the apparatus cannot be as economical in operation as that
in a large central power house, nor, as will be later shown,
is such an arrangement as convenient as others hereinafter
considered,

The internal combustion engine (gas, gasoline or oil
engine) will, in its better types, be found more economical
than the steam engine, nos, considering the fact that it
requires no boiler plant or auxiliaries, will it be so exe
pensive as the steam plant.

It is necessary, where the internal combustion engine
is to be used to drive a plunger pump by direct connection,
to provide a clutch between the pump and the engine. Here,
as in other places, such a clutch is a necessary evil and
a possible source of trouble. This is one good reason why
an internal combustion engine should be avoided if possible.
The engines themselves are now reasonably reliable and have,
by improvements in design, been freed from many of the more
or less serious defects which avpeared in their earlier
forms.

As such engines are commonly rated, they have no over-
load capacity; hence a larger size of engine must be select-
ed to provide for this deficiency.

By far the most convenient form of drive for &@ pump
in a small or medium sized water works plant is an electric
motor. The extension of the Central Station transmission
lines has made electric current available in most towns at
avery reasonable figure. This also is a point in its favor.

15

 

 

 
16

The following considerations also recommend it —

(1) No boilers required

(2) No storage of inflamable liquids to be provided for.
(3) Reliability

(4) Simplicity of operation

(5) Adaptability to automatic control

(6) Adaptability to direct drives

(7) Adaptability to variable speeds where required

(8) Owerload capacity

(9) Compactness

(10) High efficiency
(11) Low first cost, #11 things considered.

The principal disadvantage in their: use is the fact that
a storm may brake the transmission line from the power house,
which under the present conditions,may be many miles away.
However,the up to date Central Station rerely suffers an
interruption in its service of more than a few hours, and
this may be easily provided against by standpipe capacity.

However, the distributing system of a large up to date
Central Station is so complete that it is usually possible
to reach the same point in at least two ways — one direct,
and the other round about by feeding.through cross connect-
ions from some other town which, though reasonably close,
is on another direct line from the Station.

In some cases, where a system contains several gener-
ating plants, all tied together (electrically) it is possi-
ble to feed any town on the system two ways from each plant,
thus reducing the chances for interruption of service to a
minimun,

The small local plant is usually not so reliable as
to service. but an interruption of service, on account of the
simpler existing conditions is usually a simple matter to
take care of — provided the management and operation is in
capable hands.

The motor itself requires, really, less actual attent-
ion than the pump. It needs only to be kept clean and pro-
perly oiled.

It may be automatically started and stopped by a float
switch, pressure regulator or other similar device, depend-
Ang upon the type of the systen.

On direct drives the electric motor is unequaled.

The direct current motor gives flexibility of speed control
which is desirable in many cases — especially for plunger
pumps. If variable speed is desired in the case of alter-
nating current motors, the slip ring (phase wound) type must
be used.

On account of the torque necessary to start plunger

 

 

 
a7

pumps, for alternating current use, the high torque type
of moter is required; where variable speeds are desired
(polyphase circuits) the slip ring type, as has already
been stated, must be used.

In the case of direct current motors, variable speeds
may be obtained by the well known methods — i.e. shunt
field control or armature control or a combination of both,
using the armature control for speeds below rating and
the field control for speeds above rating.

In the case of a plunger pump, there is no economy
in armature control, as the power required (above friction
horse power) varies, theoretically, directly as the speed,
and, practically, very nearly so.

In the case of the centrifugal pump, however, the
power required drops off much more rapidly than the corres-
ponding speed; hence, less current is required to drive
the pump at slower speed, and, consequently, less current
passes through the rheostat — e.g. since the horse power
varies as the cube of the speed, at half the speed, one
eighth normal horse power would be required. Further,
neglecting the lowering in virtual resistance of the
armature, due to the slower speed, one eighth of the normal
E.M.F. would be required at the terminals (actually, it
would be less than one eighth). This would mean seven
eighths of the E.M.F. would be applied to the resistance
(rheostat), but, since, in a series circuit, the current
is the same in all parts, one eighth of the normal current
passes through it.

The loss in the resistance is — W = I°R

Where W equals Watts power lost

I equals current in rheostat

R equals resistance of rheostat.
hence, only one sixty-fourthn of the loss takes place that
would take place if the whole current were passed through
the rheostat and armature.

Now,since the applied E.M.F. is the same (across
motor and rheostat combined) always (neglecting variable
voltage due to "regulation"), there is a greater loss
when the full current (eight times as much) passes through
the armature, whose resistance -° only one eighth of the
combingd resistance, for, the I*°R formula being true,

I” becomes one instead of one sixty-fourth and,

R is ong eighth;
hence, the I°R loss may roughly be compared as being as
one times one-eighth, as apposed to ,one-sixtyfourtyn
times one, or as one-eighth to one~sixtyfourth — i.e.
eight times as much.

The above is only very rough and neglects electrical
losses etc. in the motor; it also assumes that the current
in the field of the motor remains constant while that of
the armature changes. This is not true, as the shunt field
and armature are in parallel across the line. It will
serve, however, to show that armature control of centri-
fugal pumps is far from being as wasteful as might be
supposed.

 
18

Attention may be called to the fact that some menufatt-
urers rate their centrifugal pumps as requiring so much
horse power per ten feet of head. This is not correct.

In the first place, the friction horse power must be
considered. In the second place, the brake horse power
curve is not a straight line, but a curve. The horse power
per ten feet of head must, therefore, be only an approx-
imate figure. The curves given show typical forms and it
will be evident from them wherein the error lies.

A few words must be said in connection with autom-
atic controll.

The Bressure Regulator should be used where the area
of the water storage is small and the pipe is high making
a large variation in pressure, or, when a float switch
would be likely to get out of working order by freezing
or some other cause.

The Float Switch is very good where the area of the

storage reservoir is very large and the change of pressure
very small.

 

 
19 .

WATER STORAGE. Under this heading we must consider,
the Natural or Artificial Reservoir, the Blevated Tank,
Stand Pipe and Direct Pumping system.

By Natural and Artificial Reservoirs is meant natural
reservoirs formed by valleys, into which water flows or is
pumped, lakes in the hills etc., reservoirs partly natural
and partly artificial,such as, reservoirs formed by hills
and dammed on the low side, rivers dammed up to hold a
large body of water behind the dam or valleys fed by springs
and dammed to hold a large body of water in reserve.
Artificial reservoirs are generally large excavations in
the ground banked up arround the sides with the excavated
material, to increase the depth, thus forming a reservoir.

In some cases these reservoirs are covered with a
cement bottom and sides, with or without rubble foundation
(depending on the soil), or where the earth is in a large
degree impervious to water the reservoir is only sodded
from the water line up, the remainder being left earth,

These reservoirs are used mainly where a direct pressure
eystem or a gravity system, is installed, and in localities
where, during the dry seasons the water supply is very small,
necessitating a large storage to be kept in reserve.

The pumps may, where the water flows into the reservoir,
under low head, take their suction from the reservoir,
which then becomes a settling basin as well as a water
storage basin. Under proper conditions, i.e. in the case
of automatic control, where the pump suction is"flooded",
centrifugal pumps may be used. In this case, we would
have the pumps pumping directly into the mains. Power
economy neglected, with centrifugals running at constant
speed, no relief valve is required provided that the piping
system will stand the shut off head of the pumps, for the
maximum pressure obtainable will depend solely upon the
speed at which the pumps are run, and, other things being
constant, cannot increase beyond this point unless the
speed be increased. (See characteristic curvwes in appendix
and section "Pumping Equipment").

Where the reservoir is elevated, as on top of a hill,
the pumps may take from the source of supply and pump into
the reservoir, the supply pipe feeding the distribution
system being taken from a point near the bottom of the
reservoir.

In this case,the reservoir furnishes a gravity head
on the system, and the pumps discharge against a constant
(or nearly so), the control being by float device operating
the pumps according to predetermined water levels in the

 

 

 

 
20

reservoir. Given an overflow of proper capacity, and, since
the reservoir is an open basin, the discharge head on the
pumps cannot increase beyond that corresponding to high
water in the reservoir. Hence, except to guard against
accidental or (emergency) closing of the valve in the inlet,
no relief valve or by-pass is required in this case.

The Elevated Tank is probably the most generally used
of the storage systems, where a constant supply of water
is available. They are also sometimes used in connection
with reservoirs, as a method of elevating water to a height
great enough for domestic pressure and a light fire pressure,
These elevated tanks are made of capacities varying from
thirty thousand gallons to one hundred and fifty thousand
gealléns.and sometimes greater. However, tanks of less than
fifty thousand gallons capacity are rarely used as, less
than that would be a very small amount of storage in case
of a fire.

Where the elevated tank is used the pumps are usually
run at intervals controlled by automatic regulators so as,
to keep about two thirds of the tank capacity available at
any time in case of fire.

The Pneumatic Pressure Tank is frequently used in con-
nection with small municipal systems and subdivision water
systems. It is more generally used in connection with farm
systems and residence water systems. In a water works system
where a pneumatic pressure tank is usedfor the storage, and
domestic and fire pressures, tanks of sufficient capacity
to be of any use in case of fire are seldom installed on
account of the great first cost and therefore duplicate
pumping machinery and greater water supply is necessary
than with the other systems. Also an auxiliary air com-
pressor is necessary to keep up the pressure. When using
a pneumatic pressure tank all machinery should be in dup-
licate so, at any time a sufficient water supply could be
obtained in case of fire.

The Stand Pipe, although at one time almost universaly
used as the storage tank for municipal water systems, is
almost completely superceded by other devices, on account
of the great cost and the large volume of water near the
bottom of the stand pipe which is not available at any
appreciable pressure. Its chief use at present is in cases
where it can be set on @ natural elevation thus eliminating
to a large extent the above objections. It then really
becomes a closed elevated reservoir."Pressure Regulators"
or their equivalent are used in connection with it, and
relief valves are usually installed.

 
21

It may be mentioned here, that a relief valve or air
chamber may be used to advantage where pumps operate
against very high heads, to absorb the shock of water
hammer which may occur when the pump ceases to operate —

i.e. when it is stopped. Water hammer may also be caused,
especially in the case of centrifugals, either by the
inertia of water in the discharge due to a high head or
to a large volume of water under low head, as it depends
both upon mass and pressure — i.e. the “force behind the
mass" which produces velocity.

For this reason,a check valve should be installed
in the discharge lines, near the pump. This check valve
may in certain cases be omitted (e.g. when, in the case
of a centrifugal, the suction head is low and there is
no foot valve or equivalent. It must be borne in mind
that whenever a foot or check valve is placed in the
suction line leading to a centrifugal pump, a check valve
must be installed in the discharge; otherwise the water
hammer may burst the casing — a not uncommon occurance
under such conditions.

The value of water hammer is uncertain. It is approx-
imated by the use of various émpyrical formulae, and a
value of fourteen or fifteen times the working pressure
is quite often assummed. This is, however, by no means certain.
It depends upon the rapidity with which the force producing
it is checked — the quicker the inertia of the moving water
is overcome, the greater is the water hammer — i.e. closing
a valve instantaneously to check a fiow of water would
produce a very excessive water hammer as a result, while
closing the same valve very slowly might not produce any
appreciable water hammer.

The direct pumping system will not be dealt with at
any length in this paper as it is used mainly in large
city systems. .

This system consists of pumping directly into the
mains, using no storage or pressure tanks, the pressure
being furnished directly by the pump. There are usually
several pumps used in this kind of 4 system, some pump
working continuously, and the others being thrown into
service as the water consumption demands,

 

 

 
22

DISTRIBUTING SYSTEM. Given the necessary water
supply, the first problem that confronts an Engineer
in the design of a water distribution system is to
determine the proper size of pipes to use.

The water mains should be so designed thet the
combined loss in head from friction in pipes, joints,
bends,etc., under fire consumption will not be great
enough to reduce the pressure below that necessary for
fire use. Long stretches of small pipe should never be
placed in a distributing system, neither should too
many branches lead from a small main. In other words,
the (reasonable) capacity of any main: should not be
exceeded,

The system should also be designed so0 extensions
can be made without taking up pipe already in the system
and replecing it with larger sizes of pipe. A large part
of the cost of a distributing system is in digging the °
trenches and laying the pipe, and in the majority of
cases the extra cost necessary to increase the size of the
pipes enough to make an adequate system out of an inade=-
quate one would be a great deal less than the extra ex-
pense of taking up a small pipe a few years later and
replacing it with a larger one.

Fire hydrants should never be placed on less than a
four inch pipe, as it is impossible to get the proper
fire streams from the smaller sizes. Furthermore the fire
insurance inspectors will not recognize smaller pipe as
any fire protection whatever.

It si, in general, advisable to make all primary and
secondary feeders, in small Municipal water systems, of
six inch pipe or larger, and make short cross ties in the
gridiron system of not less than four inch pipe. The
system should be laid out in such a manner as to make
complete circuits of all lines, thus doing away with all
dead ends (barring very unusual cases) and eliminating to
a large extent the water hammer which is a great strain
on the pipe.

In spacing the hydrants in a water system the Engineer
should always keep in mind that fire hydrants are cheaper
and last much longer than fire hose; therefore the hyd-
rants should be spaced close enough together so that a
sufficient number of fire streams can be obtained at any
point without using excessive lengths of hose. Another
reason for the close spacing of hydrants is because the
23

friction loss per hundred feet of fire hose is very great
compared with the loss in pipes and hydrants and, conse-
quently, if the hydrants are spaced too far apart, a
greater pressure is necessary at the pumping station to
overcome this extra friction. Insupance inspectors require
that hydrants be placed not more than two hundred and fifty
to three hundred feet in the business districts, and in the
residence districts not more than double that distance, in
order to get the maximum reduction in insurance rates,

In placing gate walves in a distributing system care
should be taken to have them so situated that long stretches
of pipe would not need to be cut out in case of a break in
any part of the system, also, that in case of a break at
any point, all fire hydrants could be gated in such a man-
ner that they could be fed from another direction. A good
plan to follow in placing valves and hydrants is to place
valves at all pipe intersections on every pipe leading
from that intersection, and place the hydrant on the largest
of these pipes and between the valves. By so doing ina
gridiron system no hydrant would be cut out of service
unless all pipes leading to that hydrant were put out of
service. In some cities and villages, gate valves are put
in the lead from the main to the hydrant, so the hydrant
may be cut out for repairs without in any way effecting the
remainder of the system.

There are several kinds of pipe used in distributing
systems, the chief of these are, cast iron bell end spigot
pipe, cast iron pipe with patented "Universal Joint", and
wood pipe. Cast iron flanged pipe are sometimes used,
especially, in high pressure lines, and, wrought iron and
steel pipe are also sometimes used. The latter three will
not be taken up at any length in this paper as they are
not in general use in this vicinity. They are, however,
frequently used in the West, where exceptionally high heads
are sometimes obtained. In this case, spiral riveted steel
pipe is commonly selected.

Cast iron bell and spigot pipe, probably the most wide-
ly used in municipal systems, is made in four classes — A,
B, C, and D, weights and thicknesses of which, depend upon
the pressures which they are to withstand, class"A" being
the lightest and class "D" the heaviest. The weights, dimen-
tions etc., of these classes of pipe are given in a set of
specifications adopted by the American Water Works Associa-
ticn, and are recognized by all foundrymen casting bell end
spigot pipe.

The standard length of these pipe, is twelve feet.
Hewever, some foundries have recently started casting lengths
up to sixteen feet in the smaller diameters, and claiming
that there is a large saving in lebor and joint material
and also leaving less liability for joint leaks by reducing
the number of joints. There has been a great amount of talk,
on
24

among those not casting the longer pipe, about the inability
of casting pipe symetrical in the added length, and state-~
ments have been made that the pipes are liable to be crooked
and uneven in wall thickness. The writer has used both
twelve and sixteen foot pipe in the same job and has found
upon cutting pipe of both lengths that the wall thickness

is as even in one as the other, and that pipe that can be
handled without the use of a derrick lays much faster in the
longer lengths than in the twelve foot lengths.

fhe classes of pipe usually used in municipal systems
are class "B" and class "C", class "C" being used chiefly
where the village is not equipped with a fire engine but
develops all the pressure at the pumping station and attaches
the fire hose directly to the hydrants. It is also used to
“some extent in the larger of the village systems in the
vicinity of the pumping station or, where an extra pressure
is liable to occur.

In laying bell and spigot pipe great care should be
exercised in making the jointaé, the bells and spigots should
be thoroughly cleaned before making the joint, end suffic-
ient lead should be put into the joint to allow calking it
firmly to make it water tight. Care should be taken in calk-
ing tyne joint not to split the bell by driving the lead too
hard,which is frequently the case.

It is well in putting in any kind of distributing pipe
to have all pipe tested in the open trench, as often small
leaks will show up that would not show for some time after
the trench had been filled in, also, in sandy soil, the
water leaking out would run away in the sand and the leak
would have to be very lerge in order to show on the surface
of a closed trench. When leaks are found in an open trench
it is a very simple matter to repair them, while, in a
closed trench the water may appear, if it appear on the
surface at all, some distance from the leak and considerable
dirt must be removed before the leak is found. The only
indication of such ea leak may be a more rapid settling of the
back filling in the general vicinity of the trouble.

Some of the chief objections to bell and spigot pipe
are, inexperienced calkers frequently split the bells in
making the joints, or, make leaky joints which are hard to
repair, (2) Liability of expansion and contraction meking
leaky joints, and,(3) the high cost of leying the pipe and
making the joints.

Cast iron pipe with patented "Universal Joint", is cast
in six foot lengths making it very easy to handle and with
machined joints, one slipping inside of the other and form-
ing a tight joint, metal to metal, the joint is painted

 

 
25

with white lead before being made and is then drawn together
by bolts through two lugs on each pipe, én the larger sizes
three lugs are cast. These pipe are very easily put together,
as all that is needed is a rachet wrench, and a pail of paint,
eliminating the high cest of making joints and the liability
of leaks on account of inexperienced calkers. The "Universal
Joint" pipe is comparatively new pipe on the market and
Engineers hesitate to recommend its use in municipal water
systems, although some Engineers prefer it to bell and spigot
pipe.

Some of the chief objections to this kind of pipe are —

(1) The liability of breaking the lugs at the joints by
drawing the pipe together too tight.

(2) High cost of pipe on account of the machined joint.

(3) Liability of the wrought iron bolts rusting in two
and loosing the joint, causing a leak.

(4) The machined surface of the joints,being exposed to
the water on one side and the earth on the other, béing sub-
ject to corrosion quicker than a coated pipe.

(5) A smaller weight and thickness of pipe of the same
class. The makers claim the lack of weight is compensated
by using a better grade of iron on account of the necessity
of machining the joints.

The wood pipes used in distributing systems at the present
time are made of staves bound together by a spiral of strap
steel and, rolled in a composition of pitch and sawdust, to
preserve the wood from the soil. On account of the difficulty
of obtaining good wood for staves, a good wood pipe is hard
to get, and,,consequently, its use is not so general as in
former years. Wood pipe, unless made of one or two kinds of
wood especially adapted to use in water, is liable to impart
a taste of wood to the water.

Under large pressures, wood pipe is very apt to leak
and often burst causing great inconvenience to the people,
on the lines bursting. Wood pipe is lower in cost than cast
iron; howewer, this difference is getting smaller contin-
ually as wood becomes more expensive and cast iron pipe

becomes cheaper, and it is probable that this {s to a large
extent the reason for the difficulty in obtaining good wood
pipe in recent years.

Wood pipe is light to handle and easy to lay, as the
joints are made by a sleave on one pipe slipping over the
end of the other, and swelling tight when soaked in water.

These pipe properly made last a great number of years
under ground, and always being smooth inside so that the
pipe friction is a minimum. One of the best woods to be
used for making wood pipe is number one white pine.
,

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