Installation and Operation of Pumping Equipment

7/28/2019 Installation and Operation of Pumping Equipment

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Installation and operation of pumping equipment


IntroductionProcedures prior to installation

Pump set installation - foundation and bedplate

Pump set lining up and final couplingPipe work installation

Electrical installation

Initial start preparationsOperation procedure

Routine running attention

INTRODUCTION

Pumps when properly installed and given reasonable care and maintenance will generally

operate satisfactorily long periods al a time.

As pumps are built to a wide variety of designs for many different services, this chapter

can only provide general information on typical problems. When installing a particularpump, it is essential to consult the manufacturers detailed instructions which will state

any special precautions required.

It is important to realise that in most cases pumping problems on site are caused by bad

installation or operating system problems. It is, therefore, essential that proper care and

attention is taken during installation to ensure successful operation.

PROCEDURES PRIOR TO INSTALLATION

Although various methods of installing pumps can be adopted, the instructions detailed inthe manufacturer's instruction manual and in associated data sheets should be followed.

Any departure from the procedures detailed should be based on good engineering

practices.

Inspection of equipment

On receipt of the equipment, inspect and check it against the advice note. Examine thecrate and wrapping before discarding them since parts and accessories are sometimes

wrapped individually or fastened to the crate.

Report any damage or shortage to me supplier.If the equipment is not installed immediately, it should be stored under conditions that

will prevent deterioration due to damage or corrosion.

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Storage

Short term. When it is necessary to store a pump for a short time before it is installed,place it in a dry location where it cannot be affected by corrosion or dirt. Protective

blanking plates, fitted before despatch from the factory, should not be removed.

Ensure that the bearings and couplings are properly protected against the ingress of sand,

grit and other foreign matter. To prevent rusting-in or seizing, lubricate the unit before

storing and turn the pump set by hand at least once a week.

Long term. More thorough precautions are required if the pump is to be stored for an

extended period of time. Consult the manufacturer for full information on long-term

storage.

Cleaning prior to installation

All parts of the assembly must be thoroughly cleaned before installation begins. Alltraces of rust preventative should be removed from the discharge and suction flange

faces, exposed shafting, and alt coupling surfaces.

Lay-out of pump parts for installation

Avoid damage to components when handling or installing them.

If suitable lifting tackle is not available, skids should be employed to transfer heavy

weights to ground level. Loaded crates or individual components or sub-assemblies must

never be dropped to the ground from a transport vehicle.Lay out individual components on suitable timbers or other clean surfaces in the order in

which they will be fitted. Check the components against the packing list to ensure that all

are available.

Any packing or other protective material shall be removed before starting the installation

procedure.

PUMPSET INSTALLATION FOUNDATION AND BEDPLATE

Since horizontal base plate pump sets are by far the most common equipment supplied,the following description will concentrate mainly on this type of plant.

General

Hydraulic and/or electrical plant is usually assembled on a single bedplate. The bedplate

may, however, be supplied in two or more sections.

To facilitate leveling and grouting, all components should preferably be removed from

the bedplates (refer to 'Note' below).

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To ensure satisfactory operation of the plant, it is essential that the bedplate be properly

leveled, bolted down and grouted in.

Pump set foundation

An adequate foundation is essential. It should consist of a solid block of concretesufficiently massive and rigid to provide continuous support for the bedplate throughout

its whole length, and to maintain this support throughout the operating life of the

equipment.

The foundation level should allow 25 mm for leveling of the bedplate(s), and holes

prepared to accommodate the foundation bolts. Each hole should be either 100 mm

diameter if drilled or 100 mm square if shuttered.

Grouting

The main reasons for grouting are to prevent lateral movement of the bedplate to supportthe channel flanges, which, in turn, carry the machine pads and to reduce vibration.

Grout should comprise one part portland cement and two parts sand (no aggregate) withsufficient water to produce a free flowing heavy cream consistency. The mixture

described should flow easily under the bedplate and, in order to minimise settlement, it is

best to mix the grout and let it stand for two hours, re-mixing it thoroughly before use butadding no more water.

The exposed surfaces should be covered with hessian to prevent the grout cracking by too

rapid drying. The Hessian should be kept damp until the grout is sufficiently set (about 48hours).

Leveling of bedplate

Set up and level steel packers adjacent to and on each side of the foundation bolt holes to

within 6 mm of the final height of the bottom bedplate pad. Follow this procedure roundthe plinth with packers each side of each hole, embedding them in concrete, preferably

quick setting compound such as Fondu.

Sling and lift the bedplate to insert the foundation bolts before positioning on the plinth.Align and level to correct height using laminated packers on top of the packers already

set, as in previous paragraph.

NOTE:

In the case of smaller size bedplates, providing they have been checked to ensure that no

distortion has occurred during transit or handling, it is acceptable practice to grout thebedplate with pump and motor mounted thereon together with the foundation bolts in one

operation. Certain contracts may require special instructions in this regard in which case

the contract documents should be consulted.

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The foundation bolts are to be grouted at this stage and each pocket filled with grout to

within 25 mm of the top, thus providing a key for the final grouting of the bedplate.

The bedplate is to be left in position until the grout has set hard (normally 72 hours).

After foundation bolts are set, an accurate spirit level and steel straight edge should be

laid on the various machined pads on the bedplate to check parallel and diagonal levels,and the amount of packing beneath the bedplate adjusted until all corresponding pads are

level. A degree of level of 0.1 mm per 1000 mm length of bedplate is acceptable. To

obtain this level, should the bedplate have been distorted to any appreciable extent intransit, it may be necessary to adjust packing under the bedplate and to tighten certain of

the foundation bolt nuts to strain the bedplate down at certain points. It is for this reason

that the foundation bolts are grouted in first. It should be remembered that when an

adjustment is made, the level in another direction will change.

After leveling and alignment is completed and all foundation bolts pulled down tightly,

preparation for grouting the bedplate can commence. Shuttering should be erected all

round the bedplate to a height of at least 76 mm above the lower flange of the channel togive a slight head on the grout; thus the boarding should normally be 102 to 115 mm

high. The shuttering should be positioned such that when it is removed, a rectangularsection of grout 76 x 50 mm wide remains around the outside of the bedplate.

Saturate the top of the rough foundation with water and pour the grout between theshuttering and the bedplate until it passes beneath the lower flanges, so that when

hardened it supports these by rising about 50 mm above the lower flanges. The flow

should be assisted by agitating with a bent wire (or similar) from all sides and from-the

centre of the bedplate.

NOTE:

It is important that the grout fills the cavity between the lower flanges and the foundationand must not shrink from the bedplate while curing. When the grout is set (normally

about 48 hours) the shuttering should be removed and a smooth finish given to grout and

foundation surfaces.

Direct coupling

In the case of the coupling being of a special type, the manufacturer's instructions shouldbe referred to.

Dowel pins, fitted after the machine is correctly aligned and bolted down, are intended to

facilitate correct relocation of the machine if it is removed and replaced at any lime.Dowling is done at the time of installation.

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PUMPSET LINING UP AND FINAL COUPLING

Initial checks

Examine the oil or grease used for lubrication for both the pump and driver bearings prior

to start. Frequently equipment is left lying on site for many months and condensation oringress of dirt and water from the external environment may occur. It is usually best to

clean out the lubricant and replace it. The pump bearings should also be checked at this

time. If they are of the ball bearing type, damage to the bearings can occur if the pumphas been left in one set position for several months or near a source of vibration. Damage

to the contact surface of the balls occurs; this is known as false brinelling and if the pump

is run under this condition, the bearing will fail fairly rapidly.

Pumps with this type of bearing, when required to be stored for several months should

either have the bearings removed from the pump before storage or should be rotated a

quarter of a turn every few weeks to prevent this happening. Plain bearings should be

checked for corrosion and ingress of dirt which can cause damage when the pump is run.

Turn the pump half-coupling to ensure that the pump rotating assembly is quite free andthat no distortion or damage has occurred in transit.

NOTE:

In the case of a multistage pump with a balance disc, ensure that before rotating the shaft,

the balance disc is clear of its mating renewable face to avoid galling.

Check that the drive rotor is free to turn.

Lining up pump and driver

Mount the pump on the bedplate, taking care to ensure that the feet of the pump and thebedplate pads are perfectly clean and free from burrs. Locate the pump by means of the

dowel pins, if supplied, before tightening the holding down bolts.

The faces and periphery of the half couplings should be cleaned thoroughly to ensure that

all rough or ragged edges are removed as these will give false alignment readings.

Place the driver and/or gear box on the bedplate, having checked that the feet areperfectly clean and free from burrs and insert the holding-down bolts but do not tighten

up. Check the height of the pump and driver and/or gear box couplings and insert shims

(which must be clean and free from oil and burrs) under the driver and/or gear box feet toobtain initial approximate coupling alignment.

Final coupling

When running, the half-couplings should be separated by a gap dependent upon the type

being used, and this distance should be confirmed from the general arrangement drawing.

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NOTE:

If the motor is fitted with sleeve type bearings, the coupling gap must be set with motor

rotor in its magnetic centre. Most motors have the magnetic centre marked, but for furtherdetails consult the motor instruction manual.

Where dial gauges are available to check coupling alignment, follow the procedure

shown in Figure l (b). Where these instructions are not available, the procedure inFigure l (a) may be used.

Once final alignment is complete, the driver feet should be tightened down and the

alignment re-checked.Check the coupling alignment after the pump set has been running for a period of four to

six weeks (or until settlement of foundations is completed), then drill and ream the driver

feet for dowel pins.

NOTE:

(a) Where possible, couplings should be aligned within 0.025 mm. i.e. 0.050 mm full

indicator movement on dial gauge.

(b) It must be emphasized that since pin and bush type couplings are designed to reducethe transmission of shock to bearings, etc, and do nut compensate fur misalignment they

require the same accuracy of alignment as rigid couplings.(c) Some couplings notably double engagement gear type laminated metal element spacer

type and rubber type couplings, can operate satisfactorily with a greater degree of

misalignment. The greater flexibility of these couplings is intended to compensate formisalignment which may develop during operation, due to pipe work loads, settlement of

foundations, etc. and does not preclude the need for accurate initial alignment. For

procedure to be followed when aligning these couplings see separate instructions

supplied by manufacturer.(d) Alignment procedures fur electric motors depend on motor construction. Motors with

end shield mounted bearings can be aligned by moving the complete machine. This will

not upset air gaps. Motors with bedplate mounted pedestal bearings are aligned bymoving bearing pedestals. This will alter motor air gaps and stator must then be moved to

equalise air gaps. Check air gaps at 12, 3, 6 and 9 o'clock positions, making

measurements at both ends. Turn rotor through 180 and repeat measurements. Nomeasured air gap should differ from the mean of measured air gaps by more than 5 %.

(e) When checking the directions of motor rotation, ensure that the pump has been

disconnected from its source of power, i.e. disconnect coupling/belt drive as appropriate.

Axial alignment (Fig.1)

Lay a straight edge across the rims or hub tops of the two half couplings and check radial

alignment at quarter positions. Both couplings should be rotated and readings checked atquarter positions. Check the radial gap as shown, again at quarter positions and rotating

both couplings.

Any repositioning of the motor (see Note (d)) to correct for radial misalignment must be

followed by a further check on gap clearance which should be carried out again at four

points, at 90 to each other using either a taper or a parallel gauge in conjunction with

feelers.

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Parallel alignment (Fig. 1)

1. Clamp a dial gauge to the coupling halves as shown in Figure l (e) and zero the gauge.2. Link the coupling halves so that both shafts can be rotated together. If the shafts cannot

be linked together, mark each coupling half with a coincident line.

3. Rotate both shafts and take readings at each quarter revolution. If the shafts are notlinked together, move first one shaft through a quarter revolutions, then move the other

until the lines marked coincide. Take a reading and repeat at each quarter rotation.

Angular alignment (Fig.1)

1. Clamp two dial gauges to the coupling halves as shown in Figure l (f) and zero the

gauges.

2. Link the coupling halves or mark them as previously described.3. Rotate both shafts thought 180 and note the gauge readings. If both gauge readings are

the same (but not necessarily zero) the angular alignment in the vertical plane is correct

4. Rotate both shafts through a further 90 and note the gauge readings, if the readings are

the same (hill not necessarily zero), the angular alignment in the horizontal plane iscorrect.

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AXIAL ALIGNMENT a & b

PARALLEL ALIGNMENT (c & d) ANGULAR ALIGNMENT (e & f)

Fig 1 Coupling alignment


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PIPEWORK INSTALLATION

General

It is essential that no load be imposed on the pump branches as this is liable to disturb

shaft and bearing alignment resulting in excessive wear and possible seizure. The pipe

work should therefore be independently supported and anchored against pipe thrusts.When erecting the pipe work the pump flanges should be the final closure joints. Check

coupling alignment before and after the pump flanges have been tightened. Any alteration

in alignment is indicative of pipe work loading on the pump branches and steps must betaken to remove the load.

Fig. 2 Do not overstrain the pipe work when connecting to pump

Suction pipe work

Where possible, arrange for the pump to operate on a positive suction.

Eliminate all possibility of air pockets being trapped in the suction pipe work, e.g. ensurepipe work rises continuously towards the pump; where pipe diameter is reduced in

direction of flow use flat topped taper pieces.

The diameter of the suction pipe should be sized according to the flow and allowablehead loss and may not, therefore, be the same diameter as the pump suction branch.

A short straight length of pipe adjacent to the pump suction branch is desirable. Where abend is unavoidable, it should be of the largest possible radius.

The suction pipe should be accessible and not embedded in concrete.When choosing foot valves, strainers, bends, etc. select those which will provide minimal

restriction to flow.

Some care must be taken in the design of the suction inlet. Ideally the pipe end should be

at a depth of at least five pipe diameters below the surface, and at least 0,5 mm from any

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side wall. If the pipe must be at a depth significantly less than this, it is recommended

that it be positioned within 150 mm of the side wall to reduce the possibility of vortex

formation.

Delivery pipe work

It is essential to ensure that delivery pipe work is adequately supported and anchored toresist hydraulic thrust.

When selecting valves for delivery pipe work, the following points should be noted:1. The diameter of the valve is not necessarily based on the diameter of the pipe work and

is dependent on flow.

2. Non-return valves should be sized to give a maximum flow velocity of 3 m/s. Where a

surge vessel is fitted to the pipeline or pumps operate in parallel, consideration should begiven to fitting non-slam type non-return valves.

Where delivery pipes do not rise continuously and air pockets can form, it is important to

provide some form of air release valve.

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Expansion joints

Care must also be taken when expansion or slip joints are used in the discharge and

suction lines. These items are often employed to avoid transmitting any piping strains tothe pump or to facilitate removal of valves or the pump without disturbing pipe work and

to make erection easier by providing some latitude in the piping lengths to allow for any

errors in the civil work or pipe manufacture. The use of these joints can, however, causegreater problems than they alleviate. The joints eliminate piping stresses but introduce a

reaction and torque on the pump at its foundation.

With the horizontal split case pump shown in Figure 3, an expansion joint used in the

suction line results in a twisting moment being applied to the pump. This can produce

casing distortions and bearing problems.

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Fig. 3 The magnitude of the forces involved

can be quite large. A vertical pump with sidedischarges (Figure 4), will have a side forceequal to the discharge pressure times the

effective expansion joint area.

Fig. 4 Precautions must be observed inthe design of the piping and the

positioning of the joints so that the

reactions due to flow and pressureconditions are always absorbed by

suitably placed supports.


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Debris in pipes

Another major source of trouble on site resulting from piping is the debris left inside the

pipe work. It is important that all the pipe lines connecting to the pump on the suctionside be thoroughly cleaned out before the pump is connected. Damage to pumps

frequently occurs on commissioning due to nuts, bolts and tools being drawn through.

The effect of nuts, bolts, welding rods, etc. entering a pump designed lo operate with

tight running clearances for high efficiency can be imagined. This can be avoided by

putting a mesh screen into the pump suction temporarily during the initial running perioduntil the pipelines are fully cleared.

Hydro-testing

Once the pipe work has been connected to the pump, it is advisable to hydro test thesystem. Leaks, particularly in the suction lines, can seriously reduce the pump's

performance and indeed make it impossible to prime the pump in order to start it.

Ancillary suppliesAfter the main pipe work and valves have been installed and checked, any ancillary

supplies required by a pump are connected. The systems used depend on the particularapplication the pump is used in. Typical examples of these services would be water

flushing for the mechanical seal or stuffing box of the pump, water supply to cool the

bearings or stuffing box and drain lines from the pump or its base plate to carry awayleakage from the gland and priming and venting pipe systems. Detailed requirements for

any ancillary services will be provided by the pump manufacturer prior to installation.

ELECTRICAL INSTALLATION

Once the mechanical work of installing the pump and driver set is completed, the

electrical controls and instrumentation must be installed. The nature of this equipmentvaries enormously and is outside the scope of this book.

This type of work is normally carried out by an electrical subcontractor, specializing in

this type of work.

INITIAL START PREPARATIONS

Drive unit

The drive unit is checked to ensure that it is functioning correctly. In the case of a three-

phase electric motor, it is essential to check the direction of rotation. This should be done

with the shaft coupling disconnected (see under 'Final Coupling'), as some pumps can bedamaged by reverse rotation. The motor is kicked over and the rotation noted. If

incorrect, two of the three electrical supply cables are interchanged to reverse the

rotation.

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Electrical controls

Once the driver has been checked, the next step is to test the electrical controls and

instrumentation. Five types of control transducers are most commonly used. Pressureswitches, flow switches, level probes, temperature detectors and limit switches. Controls

are checked by carrying out trip tests; these consist of shorting each of the control

transducers and checking that the panel reacts correctly. Instrumentation usually has to becalibrated on site. The procedures for this should be obtained from the instrument

manufacturers. Where an electric motor is employed, the starter system is usually tested

for correct operation together with any automatically operated valves or other equipment.It is most important to check the correct functioning of the electrical equipment before

the mechanical equipment is operated. At the very least, the mechanical plant may not

function correctly and at worst severe damage may occur to the equipment. For example,

some types of starter can reverse the direction of an electric motor in mid-sequence. Thiswill produce dangerous results, particularly if a large or high speed machine is being

operated.

Ancillary servicesAny ancillary services required by the pump must now be tested- For example, it is most

important that a pump with an external flush supply to its packed gland or mechanicalseal is not run without this supply, as severe overheating will occur after a short period of

running with consequent damage occurring to the pump.

Water supply

Finally, and very fundamentally, it is important to ensure that a supply of water is

available to the pump. On many occasions, the commissioning engineer has been called

to site to test a pump only to find, on arrival, that no water is available. Needless to say,running a pump without water is not recommended.

Vertical spindle pumpWhen vertical spindle pumps are installed, there are often some additional checks to be

made to those already described. With a vertical spindle pump of an open design

impeller, the impeller clearance to the bowl of the pump has to be set and in someinstances a p re-lubrication tank is provided to flush the bearings with water before start

up. This must be tested for correct operation. Some of these pumps have oil fed bearings

and the supply reservoir must be checked periodically to ensure it is full.

Most pumps are fully assembled and checked at the manufacturer and do not require any

internal adjustments on site. Vertical spindle pumps are an exception to this because,

often due to their length, they are shipped in several pieces and, therefore, have to beassembled on site.

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OPERATING PROCEDURE

The following procedure outlines the most important steps involved in pump operation.Any departure from this procedure must be based on good engineering practice.

PrimingPriming a centrifugal pump involves the removal of air, gas or vapour from the suction

pipe work and pump casing. As centrifugal pumps are generally not self-priming, they

must be primed prior to start up. If this is not done, the pump will not operate and, inaddition, serious damage to water lubrication bushes could result.

The simplest method of priming is to arrange the pump so that it has a positive suction

pressure. The pump can then be fitted by opening the air cock provided.

Where a positive suction is not possible, it is normal practice to fit a foot valve and

strainer to the suction pipe inlet and use one of the following methods of priming:

1. Back priming from the delivery using a bypass round the non-return valve. It isimportant to ensure that the suction pipe work and valves are rated for the delivery

pressure.2. Using a tundish fitted at the highest point on the pump casing, fill the pump and

suction pipe work using an external water supply.

3. Vacuum priming equipment, such as an air or water operated ejector (jet pump) or arotary exhauster, can be used to draw water from the suction into the pump casing.

Ventingis an operation carried out to remove any trapped air pockets in the system and

may be necessary on the suction pipeline as well as the pump.

Many modem designs of pump are inherently self-venting. The volute of the pump is

arranged so that air pockets cannot form. However, some types of pumas, particularlymultistage and side or bottom discharge pumps do require venting. Normally suitable

venting cocks or automatic vent valves are fitted. Pipe lines also often have air release

valves fitted at high points to vent out trapped air pockets.

In a new system, the pump should stay fully primed from shut-down to start-up, making

further action unnecessary. However, foot valves are liable to wear or jam and may lose

the prime after a short period. It is advisable, therefore, to check the state of prime beforestart-up.

CAUTION:

It is important that air does not leak into the pump during operation, as even if the pump

operates under such conditions, serious damage may result.

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Starting

1. Before initial start-up, disconnect the drive coupling and check the rotation of thedriver. An arrow on the pump casing indicates the correct direction of rotation.

2. Check the pump shaft for freedom of rotation.

3. Where external scaling water is to be provided, check that the supply is available at thespecified pressure (normally 65 to 100 kPa above pump suction pressure).

4. Ensure pump and suction pipe are correctly primed.

5. Ensure suction isolating valve (if fitted) is fully open.6. Ensure delivery isolating valve is closed.

NOTE:

Where starting conditions have been checked, the pump may be started with an opendelivery valve.

7. Where a surge vessel is fitted in the pipeline, check that the air volume is correct.

8. Start the driving unit according to the manufacturer's instructions.

9. Open the pump delivery valve slowly until the required pressure or flow is obtained.

CAUTION:

a. Prolonged running with a closed delivery valve can damage pump internal components

and must be avoided.

b. If the delivery pressure does not build up, stop the pump and repeat priming operation.

NOTE:

Ideally, a pump should have a non-overloading power absorbed characteristic (Fig. 5).

The electrical equipment is then sized for this maximum power and the pump can then beoperated over its whole range without danger of overloading.

However, in many cases, it is not possible to achieve this type of characteristic. Pumps oflow specific speed (i.e. centrifugal pumps) often have rising power characteristics (Fig.6) and are thus usually started against a closed discharge valve. This reduces the total

power demand required at start up and thus reduces the peak current drawn during thestarting cycle. As soon as the pump is up to speed, the discharge valve is then opened

fully, Pumps with a high specific speed, i.e. axial flow units, usually have a drooping

power characteristic (Fig. 6). These pumps are never started against a closed discharge

valve; to do so would require a much larger motor than necessary for normal operationand additionally higher rated switchgear would be needed. This entails unnecessary cost.

If it is essential for the system to have closed valve on start up, a bypass tine can be

installed.

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Stopping the pumping equipment

This procedure may seem very simple and in many cases it is. A typical sequence ofoperation for shutting down the pump set might be:

1. First shut down the driver (as the pump stops the discharge non-return valve shuts andprevents reverse flow of liquid through the pump).

2. If the pump is not required for some time, the suction and discharge isolating valves

can be closed and any ancillary services such as cooling or flushing lines can be shutdown.

3. Finally, in some instances, it may be desirable to drain the pump set, i.e. if

maintenance is required.

This simple procedure, however, can become rather more complicated. When pumps

have to be stopped and started at frequent intervals, automatic control equipment is often

used this might consist of actuators fur valves or systems to prevent the pump losing

rime or systems to automatically re-prime the pump. Additionally, a fair amount ofelectrical control and instrumentation may also be used so that the pump sets can be

monitored and operated from remote locations. Where an axial flow pump is used andoperates against an open discharge, it may be found that, after switching the pump off, a

syphon is created which will result in the reverse flow of water through the system. In

this case, a syphon breaker system is obviously necessary.

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ROUTINE RUNNING ATTENTION

As soon as the pump has run up to a speed, a number of routine checks must immediatelybe carried out.

1. Soft packed glands must be checked to ensure that there is a slight leakage from thegland. This lubricates and cools the gland. Excessively tight glands cause wear to the

shaft or sleeve and the pump uses excessive power.

2. The bearings of the pump and driver must be checked to ensure that there is nooverheating. As a general rule, it should be possible to hold a hand on a bearing housing

without discomfort, although some designs do run at a greater temperature. A common

cause of overheating with anti-friction bearings is over or under packing with grease.

3. Where auxiliary services are required by the pump, such as flushing or coolingsupplies, these must be checked to ensure they are operating satisfactorily.

If possible, pressure gauge readings should be taken to ensure that the pump is operating

at the correct duty. On the suction side, an unduly low reading may indicate a blockage inthe line or a valve not fully open. Similarly on the discharge side, an unduly high reading

would indicate the same. It is important that the pump runs within the correct band ofdifferential head that it was designed for. When a differential pressure reading is too low,

the pump may start to cavitate which can cause damage if allowed to continue for a long

period. A very low discharge pressure may be the result of a valve left open in thesystem, or the system characteristics not as originally calculated.

It is equally important when monitoring the operation of a pump that the pump is not run

for long periods at too high a differential pressure and hence at too low a flow rate. As a

general indication, a pump should not be run at less than 15 % of its best efficiency pointflow rate. In many types of pump operating at reduced flows can cause overheating in the

pump (Fig.7). Since no pump is 100% efficient, the inefficiency in the pump's conversion

of power into hydraulic energy results in heat, which is transferred to the liquid passingthrough the pump.

The severity of the temperature rise in the pump depends on the ratio of the quantity ofliquid passing through the pump to the amount of heat generated by the pump per unit

time. When liquids containing abrasive particles are pumped, low flow rate operation also

greatly increases the internal wear in the pump the abrasive particles recirculate in the

pump instead of passing straight through. If closed valve or low flow rate operation isunavoidable, a bypass system should be used and adjusted when the pump is running to

maintain a minimum of 15% flow through the pump. Low flow operation also imposes

unacceptably high thrusts on volute casing designs. This ultimately results in shaftfailures.

Most pump installations with electric motor drivers have an ammeter installed. Thisinstrument is a good indication of running problems. If the ammeter is running at a higher

value than it should, then the pump may not be running at the correct duty. This can be

checked with pressure gauges. Also, if the pump is not running freely, a packed gland

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may be too tight or solids in the liquid may have built up behind the impeller or in the eye

ring areas.

Other problems to look for when the pump is running are excessive vibration and noise

from the pump. Vibration can be caused by misalignment of the pump and driver or

distortion of the pump by the pipe work or distortion of the base plate due to foundationsshifting. This can be checked and corrected if necessary. Vibration can also be caused by

hydraulic problems, either in the system or in the pump. One of the simplest examples of

this is cavitation which can occur under the conditions already mentioned. This problemwill result in the pump running roughly and making loud cracking noises, which are

caused by vapour bubbles collapsing in the pump water passages. Vibration can also be

set up in the pipe work system; this may be due to resonance set up by throttling or other

restrictions in the system, the use of short radius bends at too high a flow rate or possiblewater hammer in the system on start up. All of these factors can cause a particular

installation to vibrate or operate noisily.

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Documents

Installation and Operation of Pumping Equipment