CREDITS/COPYRIGHT

A Guide to Installation & Selectionby John Dnistriansky, B.A., B.Ed., Grad. Dip. Ed Technol, Dip. T.(Tech)

ISBN: 978-0-9872539-7-2

The copyright for this document is held by the author and was first published in 2011. All rightsare reserved and no part of this publication may be reproduced, stored in a retrieval system ortransmitted in any form by any means whatsoever without the prior permission of the copyrightowner. Apply in writing to the author.

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INTRODUCTION

The most positive mode of fluid transfer from one location to another is by pumps and piping.Conveyance is accomplished through piping whilst pumps supply the energy necessary forflow in overcoming various resistances in the pipe circuit, changes in elevation and pressure ofthe fluid.

The pump and piping must therefore be considered as an integral transfer system. Formaximum efficiency the transfer system must be properly designed with considerations madefor such factors as materials, movement, support, fixing, anchorage, arrangement, accessibilityas well as provision for future changes or extensions.

PUMP SELECTION

The large number of pump types produced by the various pump manufacturers often makes itquite a problem for the average user of pumping equipment to select a unit best suited to aparticular application.

Centrifugal pumps for instance, are designed to meet the requirements of a wide variety ofservices and are recommended for use in general industry. Various systems installed,maintained, and repaired today include pumping equipment.

Some services supply water for human and industrial use, provide fire protection, circulatevarious fluids, remove waste materials and maintain tank storages. Others function to enhancethe beauty of buildings in such forms as fountains, pools and planters; whilst special designsare used to extract water from underground sources for rural needs. In fact pumps are usedanywhere normal reticulated mains pressure water is insufficient or unavailable to be pipeddirectly for use by the various services.

A centrifugal pump should be matched to the pumping system both hydraulically andmechanically. A mismatched pump will result in endless problems. It is with this in mind that theobject of this publication first and foremost highlights some of the pitfalls and other factorswhich would impact on the person making a decision when selecting a suitable pump for aparticular pumping application.

THE DILEMMA OF MAKING A CHOICE

Most intending pump purchasers have a reasonably accurate idea of their water requirements.Many of them even know the approximate head against which the pump is likely to operate.Very few, however, are entirely clear as to the most suitable type of pump which would meettheir requirements, or why different manufacturers put forward such a wide variety of pumptypes when asked to submit an offer. The truth is, that for many combinations of “flow” and“head”, there are numerous alternative designs which could be produced, each with theiradvantages and disadvantages.

The unfortunate purchaser is then faced with the awkward task of choosing between them andis likely to be influenced wholly by the attractions of low price and prompt delivery, since otherimportant factors are unknown to him. Often, in desperation, he chooses a pump at random fromthe manufacturers catalogue or in some instances buys one second hand, without knowing itsfull history and performance.

There are therefore a number of questions which the potential pump purchaser should beprepared to answer to enable the best pump selection to be made for a particular application.With this in mind it is therefore the intention of this article to put forward to the purchaser someof the less understood factors which are nevertheless of paramount importance in enabling acorrect choice to be made, and thus obviating subsequent disappointment or expenses.

CENTRIFUGAL PUMP CHARACTERISTICS

In its simplest form a centrifugal pump consists of an impeller and a volute casing (chamber)which, when completely filled with a liquid and when operating (either electric, diesel or petroldriven), is capable of converting velocity to pressure.

A centrifugal pump is capable of generating a certain “head” which varies according to thepump speed. Furthermore, the performance of any centrifugal pump is expressed in terms of itscharacteristics and includes factors such as capacity, head, power, speed and efficiency.

A pump performance curve is the accepted method of indicating graphically the relationshipbetween the aforementioned factors.

In its simplest form a pump performance curve is often represented by a ‘HQ’ curve, where ‘H’is the head in metres (m) and ‘Q’ represents the flow rate in litres per second (l/s). Typicalcharacteristic curves are shown in figure 1.

figure 1. Typical characteristic curve

The information for head and flow requirements as well as sizing a pump can be derived from aquestionnaire similar to that shown on page 7 and figure 2.

figure 2

IMPELLERS

Without doubt the two major elements of a centrifugal pump are a rotating impeller which isfixed onto a shaft fitted with seals and bearings and a surrounding casing.

Centrifugal type pumps are distinguished from one another by their structural features andgeometry shown in figures 3(a), (b), (c) and (d) and by the type of impeller design, that is, theyare categorised according to the direction of fluid flow through the impeller-.

figure 3(a)

figure 3(b)

figure 3(c)

figure 3(d)

Many types are available for a variety of duties. There are, for example, direct coupled andclose-coupled conventional single-duty volute designs fitted with either closed, semi-open orfully open impellers. Closed impellers, for instance, consist of a series of curved vanes attachedto a central hub and extended outwards between two enclosed plates. An open impeller issimilar, with the exception that there are no enclosing plates. Impellers of this type are fitted involute pumps and are used in applications where fluid material contains a large proportion ofentrained solids.

Other design variations are single and double entry impellers, impellers for radial, axial and

mixed flow, according to the flow of liquid within the impeller. Multi-stage pumps consist of anumber of impellers which are assembled onto a common shaft and the pumped fluid isdelivered to each impeller in turn to achieve the required head.

Multi-stage pumps may be of a horizontal or vertical design, the latter having the advantage ofrequiring appreciably less floor space.

Information that will help:

Determine the correct pump for your application by answering the following questions beforemaking contact:

1. What will the pump be used for?(Household pressure system, stock watering, swimming pool. Etc.)

2. How much water per hour do you require?

3. Do you require electric drive or engine drive?

4. lf electric drive, how far from the pump is the power outlet?

5. What is the water source?(Tank, creek, river, dam, bore, well)

6. lf a bore or well, what is the tested quantity of water available

7. Is there any existing pipe work? If so:What type is itSuction side: Size; Length?Delivery side: Size; Length?How old is it

8. Fill in the following measurements:Vertical heightWater level to pump suctionLength suction pipeSize suction pipeVertical heightPump delivery to highest pointLength delivery pipeSize delivery pipe

CHOICE

The choice of which type of pump should be used is governed by a number of factors.

The type of flow distribution, for instance where some are produced to deliver a wide range offlow rates, ranging from fractional outputs through to thousands of litres per second andoperating against enormous discharge pressures are limited only by the strength of thematerials involved.

The service to be performed will have an important influence on selection. A pump required forcontinuous service will be subjected to different conditions of wear and tear than that which isrequired for standby service.

The density and viscosity of a fluid influence power requirements and ultimately running costs.

The corrosive properties of a fluid will determine the acceptable materials of pump componentsand construction and solids in suspension will eliminate certain types of pumps. All theseconditions which the pump may operate under must be considered.

There are many different types of pumps needed to perform these various tasks andmanufacturers modify pump design accordingly. The large number of pump types availabletoday therefore makes it quite a problem for the average user of pumping equipment to selectthe most suitable pumping unit for a particular application.

From the forgoing it is obvious that pump selection represents a specialised area of work. Itmust be emphasised that although the plumber may be responsible for pump connections,selection should always remain the domain of the pump specialist.

INSTALLATION

Correct installation procedures are essential if you are to gain maximum efficiency fromcentrifugal pumps. If they are correctly installed and given reasonable care and maintenance,these pumps should give trouble free service even in peak operating conditions.

There are a number of important aspects of correct installation which must be considered.These include adequate space, head room, ventilation and lighting in order that maintenanceand repair work can be easily carried out; correct alignment procedures for the pump and motorshafts in the case of direct coupled units; adequate provision for adjustment and proper pipesupport to selected flanged fittings to allow ease of dismantling.

Obvious problems such as suction lift from a liquid surface below the pump installation shouldbe avoided or reduced to a practical minimum and the pump set should be protected againstflooding.

When installing a centrifugal pump, it is best to locate it as near as possible to the liquid sourcebeing pumped. In order to ensure maximum performance, the pump site should have a shortdirect suction pipe with a minimum of lift. For practical purposes, it is recommended that thepump be located within 4.5 metres of the lowest liquid level. As suction is related toatmospheric air pressure (AAP) the total suction lift should not exceed 8 metres.

Levelling and alignment is best achieved by the use of wedges and shims under the pumpbase plate and on both sides of the mounting bolts. This will level the pump unit on thesupporting base.

In most cases, the alignment of the pump with the prime mover (whether it is electrical, diesel,or petrol) is carried out in the factory workshop. Movement to the site can produce alterationshowever, so the unit should be checked for angular and parallel alignment when placed in situ.(refer to fig. 4(a), (b) and (c)).

Figures 4 (a), (b) and (c) Alignment

The alignment of the pulleys for a vee-belt drive is equally as important as the alignmentrequirements for a flexible coupling. Ensure that both the shafts of the pump and prime moverare level and parallel. Check that the pulleys are correctly fitted to the shafts and that the beltgrooves are directly opposite each other.

Final alignment must be checked after the unit has been connected to the piping and the wholeinstallation must be rechecked periodically.

The pump unit should be set on a foundation which will absorb any vibration and provide apermanent and rigid support for the baseplate. This is important in maintaining the alignment ofa direct coupled unit. Grouting of the pump base is usual and consists of pouring a thin mortarunder the base in order to prevent lateral movement of the baseplate and to reduce vibration.

With large pumps it is usual to provide a concrete base. The pump unit is then mountedseparately and secured to steel bolts set into the concrete. (refer to fig. 5)

figure 5

PIPING

Suction and delivery lines must be supported independently of the pump and fitted withvibration eliminators especially with larger pumps as shown in figures 6 (a) and (b).

figure 6 (a)

figure (b)

Inlet and outlet pipes must match up with their respective flanges without imposingunnecessary strain on the pump casing. Forced alignment has a distorting effect on the pumphousing. The distortion can also interfere with the alignment of the pump unit, causingexcessive heat and wear on the pump bearings.

Suction Piping: Suction piping must be installed with extreme care, as an incorrectly installedsuction line will cause endless trouble and inconvenience.

Air leaks are the greatest enemy of the centrifugal pump and air must be prevented fromentering the unit at all costs.

One potential source of danger can be eliminated by ensuring that the pump will operate withpositive or flooded suction, as suction Iift conditions always create a situation where any slightfault in the suction piping may result in air being drawn into the pump.

Careful consideration should therefore be given to the layout and dimensions of the suctionpiping. The suction pipe should be as short and direct as possible and must never be smallerthan the pump suction nozzle. Bends should be used in preference to short radius elbows asthe latter increases entrance losses. In the case of pumps operating on suction lift, the suctionline should slope gradually upwards to the pump from the liquid intake.

To prevent air pockets from forming, the pipe reducer at the pump suction nozzle shouldpreferably be of the eccentric type shown in figure 7(b). If reducing sockets are required, theyshould be fitted in the vertical line as shown in Figure 7(a).

Figure 7(a)

Figure 7(b)

Suction Details: With suction lift installations, a quality footvalve with a water openingequivalent to at least the area of the suction pipe is essential.

Formation of vortex action or whirlpools in the liquid source can result in air entering the suctionpipe, so care should be taken to ensure the footvalve is installed approximately 1 metre belowthe lowest level of the liquid to prevent this problem occurring.

In new installations, care should be taken to prevent impurities eg. pipe scale, welding beads,pebbles, from entering the pump. Before operation, the suction system and pump should beflushed out, and if the liquids to be pumped contain foreign matter, a strainer should be installedon the suction tail pipe.

Discharge piping: The discharge piping should be large enough to allow the pump to operateat maximum capacity without creating excessive friction head. Always check the friction head ofthe installation before deciding on the diameter of the pipe to be used. As a general rule,choosing a pipe diameter which is one size larger than the pump discharge outlet isrecommended. Under no circumstances should the size of discharge piping be smaller than thepump discharge outlet.

The discharge line should always be fitted with an appropriate regulating valve. In some casesa conventional gate valve is satisfactory, provided the gate valve is in a completely open orclosed position. If throttling is required, however, a globe valve should be fitted to regulate flow.

Reflux Valve: It is also general practice to have a reflux valve fitted on the discharge pipe,particularly when liquids are pumped to overhead storage. The function of this valve is toprevent backflow into the pump. The correct valve arrangement is shown in Figure 8.

Figure 8.

Pump Nozzle Positions: The pump discharge nozzle may be turned to a number of alternativedischarge angles as shown in Figure 9.

Figure 9 Discharge angles

Adjustment to the required angle is made by undoing the setscrews attaching the nozzle to thebody and turning the nozzle to the required angle.

Tapping : Pressure gauge tappings are situated on the side of the pump body. These aretapped with a BSP thread, and should be plugged when the gauges are not used. Always try toview a pumping installation as a whole, rather than just the pump unit. This is particularlyimportant with centrifugal pumps whose performance may be directly altered and evensubstantially reduced by the system around them.

IMPORTANT RULES FOR SUCTION PIPING

Always:

Make suction piping as short and straight as possible.

Keep suction lift as low as possible.

If possible, use larger diameter suction piping than that for which the pump is screwed (orflanged).

Install suction piping to connect pump without strain.

Make certain suction piping is free from air leaks.

Never:

Use sharp angle bends or elbows.

Use smaller diameter suction piping than recommended.

Allow suction piping to rise above the pump unless the pump is set below the lowest source ofsupply.

Operate the pump with suction lift greater than shown on manufacture’s performance curve.

EARTH BANK INSTALLATION

Relevant pump details, suction conditions and requirements:

Pump Make/Model

Pump Size

Flow Rate

Pump Speed

“A” - Maximum vertical height from lowest water level to centre of impeller

“B” - Diameter of suction pipe

Route length of suction pipe from pump to strainer

Size of footvalve and strainer

Table of ContentsCREDITS/COPYRIGHTINTRODUCTIONPUMP SELECTIONTHE DILEMMA OF MAKING A CHOICECENTRIFUGAL PUMP CHARACTERISTICSIMPELLERSCHOICEINSTALLATIONPIPINGIMPORTANT RULES FOR SUCTION PIPINGEARTH BANK INSTALLATION