Petone Borough Sewerage Scheme

Progress, Volume XI, Issue 4, 1 December 1915, Page 493

Petone Borough Sewerage Scheme

The Petone Borough Sewerage Scheme, the plant for which was completed in August 1915, presents several interesting features both in the special difficulties that were met with and in the methods adopted to overcome them;

The town lies at the mouth of the Hutt River where it flows into Wellington Harbour and is a thriving industrial community of over 6,000 inhabitants having several important manufactures including the Government Railway Workshops, the Wellington Woollen Company’s Mills and the Gear Meat Company’s Freezing Works, probably the largest in the Dominion. In addition to these there is a variety of smaller works less important only in that they employ fewer hands. The borough is built almost entirely on flat ground at sea level and on a subsoil of sand and gravel so that in any part of the district, at a depth of four feet from the surface, water is met with. The laying of the pipes has been an expensive item the whole of the trenches having to be close timbered and pumping operations carried on day and night.

UP TO-DATE METHODS

Tt was found too, that to lay a main intercepting sewer along the foreshore and to deal with the whole of the sewage from one main pumping station would have involved excavation to such depths as would, with running sand and water to deal with, have

proved almost impracticable and the borough was therefore divided into five districts each having its own pumping station. The sewers in each section gravitate to the pumps and are laid with self cleansing velocities of the stations deliver to a main pumphouse which lifts the sewage from them and also that from its own district to the septic tank through a cast iron rising main. The Septic Tank, illustrated in Fig. 1, is an uncovered one built of reinforced concrete and is divided into two compartments each capable of containing 250,000 gallons and the arrangement is such that one compartment can be cleaned whilst the other is in use. . The tank is fitted with the necessary valves for drawing off the supernatent water and on either side sludge chambers have been built (not

seen in photo) which connect with the tank by means of cast iron pipes controlled by valves. As the sewage is raised by means of pumps to the septic tanks special provision has been made to check the velocity of the incoming sewage so that an even depth of flow would pass over the weirs at the intake thus ensuring perfect circulation throughout the tank. On the rising main a byepass is provided so that in case of emergency the whole of the sewage can be turned direct into the Hutt Rver.

The pump chambers and sewage tanks are built of reinforced concrete with a neat motor house of

brick with tiled roof immediately over the pump chambers. The sewage chamber is divided into two compartments so that one can be emptied and cleaned out if required whilst the other is in use and the arrangement is such that if one motor fails to start the other chamber will fill up and start the second motor and in case of failure of the electric power an overflow is provided for temporarily turning the sewage into the sea. The floors of the chambers are laid at such a grade that when the pump is stopping all the excreta, etc., is washed into the sumps. The three pumping stations on the foreshore are ten feet below low water mark (foundaton level) and difficulty was experienced in dealing with the great amount of subsoil water that had to be contended with.

On the cast iron delivery mains near each station are byepasses so that in case of accident to the mains the sewage can be temporarily pumped into the harbour. The laying of the earthenware pipes has been a costly job owing to the expense of timbering and continual pumping, but in spite of the difficulties such care has been taken that the seepage has proved to be very small. On a section of four miles of sewers varying from six inches to eighteen inches diameter the seepage was carefully gauged after completion at the pumping station and amounted to eight and a half gallons per minute or 2-125 gallons

per mile of sewer whilst the amount usually allowed by engineers is from twenty-eight to forty-eight gallons per mile of sewer per minute. The pipes are laid on concrete foundations and incased in concrete to just above the spring of the arch of the pipe the thickness of foundation depending on the size of the pipe. All the house services are carried to the building line of the sections whilst the sewers are being laid so that as soon as a street is completed it can be put into proper order. POWER HOUSE.The original Power House contained two thirty-five horse power suction gas engines, each with its own suction gas plant and each driving by belt on to a centrifugal pump lifting sewage to the river level. As these engines were used intermittently and either one was capable . of doing the work it was decided to utilise the spare

one and reserve power thus available as a part of the new and larger scheme. To each of these two thirty-five horse power engines has been direct coupled a twenty-two K.W. compound wound dynamo at five hundred and fifty volts, the connection being through a flexible coupling. The third set consists of a fifty K.W. Mather and Platt compound wound interpolar dynamo direct coupled to a Crossley Suction Gas Engine developing seventy-five B.H.P. as a normal continuous load and eighty-five B.H.P. maximum for one hour. This engine is of Crossley’s latest design, electric lighting type with flywheel weighing six and a half

tons and having an exceedingly sensitive variable admission gear. The engine is practically self lubricating and is designed for running for long periods without attention; inside the flywheel are cast teeth gearing with a small hand driven pinion placed in such a position that one man can easily turn the engine into position for starting; the starting is by compressed air and is governed by a time cam on the side shaft in such a way that the air is automatically admitted at the right point of the stroke and without any attention on the part of the driver. A special feature about the engine is that the fan for generating the gas and starting, is of the suction type instead of the usual pressure type and is connected at the inlet valve on the engine so that the fan draws gas right up to the engine valve.

This engine is also fitted with separate air compressor which (can be either driven from its own shaft or separately by an auxiliary motor and) charges the receiver up to one hundred and eighty pounds per square inch. The engine also has a separate inlet for town gas. GAS PLANT.—The gas plant is of the latest ‘ ‘ Open Hearth Type ’ ’ for using coke and has proved itself easily able to run the large engine for long periods and with no other attention than an occasional supply of coke. The bars are open to the atmosphere and can be cleaned and clinkered without admitting air to the gas chamber. The Coke Hopper

is a special design so that it automatically closes the air inlet before the coke is dropped into the gas generator. There are no pipes in the gas generator at all: the vaporisation is done in a separate chamber to which a constant, supply of water is given, the engine itself regulating the amount of water admitted as steam to the gas generator. The fuel consumption on test over a run of fortyeight hours was 1-162 pounds of coke per B.H.P. per hour or with coke at twenty shillings per ton the cost is 0-193 d. per unit. COOLER.lnstead of the usual cooling tanks, the whole of these have been dispensed with and a patent cooler made by Heenan and Fronde has been installed and is shown on a stand over the doorway in figure 2. This cooler is capable of dealing with

a total of one hundred and fifty B.H.P. so that it is sufficient for all the engines working at full load. The principle of working , consists in bringing the hot water from the engine into intimate contact with a revolving drum, composed of sheet metal made up into a spiral and having air gaps between the convolutions. The drum is revolved slowly in a tank through which the water is circulating and a high speed fan draws air through the drum and ejects it to atmosphere through the shute also shown in the illustration.

Incidentally it may be pointed out that this machine also ventilates the engine room and ejects the bad air through the cooler to the atmosphere.

The cooler is driven by a Mather and Platt motor 6| B.H.P. and this motor also drives a circulating pump forcing water under pressure from the cooler through the engine water jackets. The inlet is by a valve under the breach end and the outlet is in the form of a pipe throwing water into a funnel on the wall immediately behind the engine; this is seen in the general view of the Power House and the amount of circulating water passing at any instant is always in view of the driver. An auxiliary water supply has been installed so that in case of emergency the town supply can be turned through the engine water jackets. SWITCHBOARD.-The switchboard is a fine well finished piece of work in enamelled slate, fourteen feet long, by seven feet high it has been fixed

on a concrete bed standing slightly above the engineroom floor and with a space of three feet behind it so as to allow ample room for working at any time; the three generator panels are fitted with overload circuit breakers on each pole and a reverse current circuit breaker on one pole only. There is also an equaliser switch on each panel for paralleling; on the third generator panel is also fitted a Watt-Meter registering the total number of units generated. The fourth and fifth panels are special panels for the control of the reversible battery booster described subsequently and on one of these there is fitted a recording ammeter registering both the input and the output of the battery.

The sixth panel controls a motor driven feeder booster shown in the far corner of the engine-room. The sixth panel contains a main circuit breaker and ammeter through which the whole of the power for the various pump houses is supplied. Also this panel carries six auxiliary circuits consisting of D.P. v switches, Tandem type and D.P. detachable fuses. STATION PUMPS.In the pit below the switchboard are the two original centifugal pumps formerly driven by belt from the flywheels of the two small engines and one of which has now been converted to an electric drive; the other being for the present out of use. These are to be removed and a new station built containing Stereophagus pumps similar to those described later on.

STORAGE BATTERY. general view of the storage battery is shown in figure 4; it consists of two hundred and ninety cells D.P. type and having a capacity equal to the total demand of the four pump-houses running continuously for three hours, each of the cells complete with acid weighs about two hundredweight. The battery is arranged in a single tier and set out in rows so as to give convenience of access to any or all of the cells. The whole of the battery works in conjunction with an auto-

matic reversible booster shown adjoining the switchboard in figure 2. The object of this booster is to take up the surplus power when the demand of the Pump Houses is less than the output of the engine or engines running and to put this surplus power into the battery. Should the demand at any time be greater than the capacity of the engine running the booster draws upon the battery for the extra power thus wanted. Its action is entirely automatic and it can be set in a few seconds so as to fix the load on the engine to any extent after which it will automatically maintain the load giving to or drawing upon the battery as required.

Provision is made on the switchboard so that at night or at week-ends the battery can be switched direct on to the bus bars and the whole of the running machinery in the Power House shut down and, so far, it has been found that one man can run the whole of the station maintaining the supply without difficulty and without working overtime. LIGHTING.— number of lamps have been installed in the Power House, battery-room and over the gas generating plants and this is wired through

out in C.T S. (cab tyred sheathed) cable, owing to its special properties of resisting acid, sulphur fumes and being generally impervious to moisture, steam, oil and unaffected by heat. CABLES.—The whole of the cables between the Power House and Pump Houses are of the Western Electric Co. ’s type, insulated with vulcanised bitumen laid in totara troughing filled up solid with bitumen and covered with a layer of bricks. Very great care was taken in the laying of these cables and they are divided into sections so that from the various Pump Houses any length can be tested by merely withdrawing a fuse. Although there are over five miles

of cable thus laid under ground only six joints have been made in the streets.

PUMP HOUSES.—There are four Pump Houses now equipped and a fifth to be added shortly. Each of these contain two “stereophagus” patent pumps with vertical shafts driven by a motor fixed at ground level and figures 5 and 6 give a good general idea of the arrangements in each of these stations, the general scheme being the same in each, but the details vary according to the area to be

drained and the relative position of the pump house. PUMPS.The pumps are a special feature of this installation and are comparatively new to the Dominion; the term stereophagus means eater of solids. The volumes delivered vary from 1250 gallons per minute at No. 1 Pump House to 350 gallons per minute at No. 4 Pump House and the maximum head is eighty-six feet. The design is due to the Hon. K. C. Parsons and the pump is intended for dealing with crude sewage without any screening or handling of any kind. It

was selected largely because, by. thus dispensing with screening, labour could be reduced. to a minimum. On figure 7 will be seen three spare parts belonging to one of the pumps and these have been placed there so as to show its internal construction. It will be seen that the impeller is a cone-shaped arrangement with special blades; it is formed of cast steel and works in a chilled liner also shown. Provision is made on the spindle so that the clearance between the impeller and the liner can be readily adjusted

and also, as the pump in course of time wears, the impeller can be fed forward so as to reduce the clearance to the original figure and thus maintain its efficiency. Through the liner projects a steel knife which can be seen in figure 7 as also the slot in the liner through which it projects and the action of the impeller on this knife is similar to the slicing motion of a garden mower so that whatever solid passes into the pump is thus cut away into sufficiently small pieces to pass through the pump and there have been put through pumps of this type, such things as:—a

suit of overalls, a handful of cotton waste, a dead rabbit and so on. Its efficiency is also very high. ■ The overall efficiency of the pump illustrated in figure 5 at the first test after being fixed into position, was for the pump and motor combined sixtytwo per cent.

The pumps themselves are set in pits adjoining the chamber into which the sewage collects and thus at the time of starting they are below the level of the sewage and no priming is necessary.

The sewage tank is divided into two compartments one pump dealing with each and there is an overflow from one compartment to the other so that if either of the motors fails to start, the sewage would then flow over into the next tank starting the second motor.

The recording ammeter on the switchboard shows the time at which the pumps have worked and, by the current taken, the attendant can see which pump started at a given time, and he thus knows whether they are working regularly. MOTORS AND CONTROLLERS.—The motors are of the interpolar type, shunt wound, the two in each station being exact duplicates of each other and spare armatures, coils and brushes are kept for each size.. Each of the motors is started by a float switch, actuated by a copper drum in the sewage chamber. This switch then sets into action the corresponding automatic motor starter, one of which in figure 7 has been shown with cover removed for clearness. A speed regulator is fitted on each panel and is shown under the ammeter j the motors are run up

to normal speed in about ten seconds after which the speed regulator resistance is cut in and the arrangement is such that the motor thus starts with its full field. In the illustration shown in figure 7 the motors are about seventeen feet above pump level and the staging had not been fitted when this photo was taken. When complete there will be a platform all round each motor with hand-rail and a sluice valve handle with stand close by.

Over the motors and pumps has been fixed a steel beam for lifting and the whole of the wiring and cabling of the motors and starters is in steel tubing.

SPECIAL SWITCH GEAR.—The contractors introduced into this scheme a special feature in the form of discriminating switch-gear. It was calculat-

ed that when the storage battery was driving the pumps the overall efficiency of the complete scheme could be very materially increased, probably by forty per cent., by equlasing the load instead of allowing any or all of the pumps to work simultaneously.

There are four stations with motors respectively, 35 8.H.P., 22\ 8.H.P., 20 8.H.P., and 12 B.H.P. with a fifth to be added later, a total at present of 90 8.P1.P. and as some of the pumps only run for about ten minutes at once, it has been arranged so that when the 35 H.P. motor starts the supply is temporarily cut off from the rest, also that the 22 H.P. and the 20 H.P. motors cannot work together. This has been done by a very simple electric device designed by the contractors working in the feeder cir-

cuits and entirely automatic and provision has been made so that in case of emergency, these automatic cut outs can be put out of action by merely closing a switch and thus all four pumps will then work together; it will be seen too that by equalising the load in this way the losses in the feeders are reduced and the demand on the gas plants, when the engines are driving the pumps direct and not charging the battery, is very much more even. GENERAL.—Throughout- the whole of this scheme two main objects have been kept to the fore: Cl) The reduction of labour cost in running and maintaining the plant. (2) An alternative means of performing any section of the work should that particular piece of machinery be temporarily out of action. The engineers claimed that the reduction of running cost would well justify a little increased outlay in the first instance. The scheme has been designed and carried out in such a way that it is believed that the absolute minimum of supervision and labour

will be required. It has already been said that results up to the present justify the belief that one man will be able to run the Power House alone and the capacity of the storage battery is such, , that until the demand increases very considerably it will be able to supply the pumps not only at night but also for an hour or two in the daytime whilst the machinery can be cleaned or a visit can be paid to any of the pump houses for inspection.

Practically the whole of the apparatus installed is either duplicated or some alternative method is provided of performing its particular function. Thus the three gas plants are interconnected, the engines and generators work singly or together all in parallel. Town gas supply is laid on in case an engine had to be started before the gas generator could be got ready. The water cooling system also has an alternative town supply. The air compressor for starting the engine and the fan for blowing up the generator can be run from the storage battery before the engine starts. The pumps, motors and switchgear are installed in duplicate and the essential spare parts have been provided in addition.

It has been borne in mind that a sewage system being installed to serve any community cannot be stopped without danger to the general health and

thus all precautions have been taken to maintain regularity of work under all possible conditions. The total cost of the scheme is £68,400 out of which £6,000 is to be used in assisting ratepayers to install the house service.

The design is due to Mr, R. Fox who is also per sonally supervising the work and carrying out the sewerage system with day labour. The mechanical and electrical equipment was let as a complete contract to Arthur D. Riley and Co. Ltd. whose engineer Mr. W. Ormrod has collaborated with Mr. Fox in the design of most of the special features described.