Lifting Station Design Abstract

State of Qatar -Public Works Authority Drainage Affairs

Volume 2 Foul Sewerage Page 39

1st Edition June 2005 - Copyright Ashghal

1.17 Abandonment of Sewers

Disused sewers and drains have the great potential to allow unwanted flows, such as groundwater to enter the system through deteriorating faults in the system fabric. They therefore need to be removed from the system to prevent structural deterioration, unauthorised use, and ingress of groundwater and infestation by rodents.

Disused sewers shall be removed or, where this is impracticable, they shall be filled in accordance with the materials and details contained on the Standard Drawings in Volume 8.

2 Pumping Stations

2.1 Standards The standards and sources of information to be used are listed in sections 1.1 and 1.2.

2.2 Hydraulic Design The overall design philosophy of the pumping system needs to be a balanced design with due consideration of functional, environmental and economic aspects. For pumping systems in the vicinity of sensitive receivers, reliability of the system is of key concern. Bypass or overflow of raw sewage, even in emergency situations, should be avoided where possible.

Particular attention should be paid to the following issues:

Design flow;

Standby power supply or temporary storage;

Standby pumps;

Overflows and emergency bypass;

Twin rising mains;

Availability of QGWEC power supply;

Land area available and proximity to housing or public areas;

Access to the proposed site.

Since the pumping station will probably be serving an area of new development, it is likely that the initial flows to the station will be much smaller than those expected for the full design. Flows will increase in the following years to reach the design capacity of the station. If the inflows are greatly below the pump output, the result will be excessive periods of inactivity of the station, with the potential for premature failure of equipment. Such infrequent operation of pumps will also result in retention of sewage in the rising main, raising problems with septicity, corrosion and effects on the receiving STW.


Page 40 Volume 2 Foul Sewerage


Consideration should therefore be given to the sizing and numbers of pumps to match the likely build-up of incoming flows. Where possible, similar pumps should be installed, on duty and assist basis, with similar standby pump(s). The use of similar pumps will avoid any changes in pumping regime due to the rotation of duty pumps for operational reasons.

Consideration should also be given to installing twin rising mains. One main would be used in the early years of the scheme to achieve satisfactory maximum flow velocities and hence minimise siltation. When flows increase, then the second main would be brought into use.

Although not strictly required for the early years of a scheme, it would not be economic to construct one rising main and then construct the second within a short period, say five years. The additional costs and disruption of digging a second trench, together with operational and safety requirements of working adjacent to a live rising main, would be avoided.

2.2.1 Hydraulic Principles

A pumping system may consist of inlet piping, pumps, valves, outlet piping, fittings, open channels and/or rising mains. When a particular system is being analysed for the purpose of selecting a pump or pumps, the head losses through these various components must be calculated. The station loss (i.e. the loss on the suction and delivery pipework from the sump to the common header) should also be considered. The frictional and minor head losses of these components are approximately proportional to the square of the velocity of flow through the system and are called the variable head.

Friction losses should be determined using the ColebrookWhite Formula.

Losses in fittings at the station, and outside of it should be determined using the formula:

H = kv2/2g

Equation 2.2.1

Where H denotes the fitting headloss (m), k is the loss coefficient, v the velocity (m/s) and g is the gravitational constant, 9.81m/s2.

Indicative values of k are given in Table 2.2.1below.




Table 2.2.1 Indicative Minor Loss Coefficients,

k, for Various Fittings

Fitting Coefficient k

Standard 900 bend 0.75

Long Radius 900 bend 0.4

Standard 450 bend 0.3

Tee - line to branch 1.2

Tee flow in line 0.35

Taper up 0.5

Sharp Entry 0.5

Bellmouth Entry 0.1

Sudden Exit 1.0

Non-return valve* 1.0

Gate Valve, fully open* 0.12

*Note that for valves it is advisable to obtain

manufacturers data on headlosses. System head

calculations would normally be carried out using valve

open figures.

It is also necessary to determine the static head required to raise the liquid from suction level to a higher discharge level. The pressure at the discharge liquid surface may be higher than that at the suction liquid surface, a condition that requires more pumping head. These two heads are fixed system heads, as they do not vary with rate of flow. Fixed system heads can be negative, if the discharge level or the pressure above that level is lower than suction level or pressure. Fixed system heads are called static heads.

The Total Dynamic Head (TDH) for a system is the sum of the major and minor friction losses plus the static head. The duty point for a pump selection will be the required flow at the TDH.

A system head curve is a plot of total system head, variable plus fixed, for various flow rates. It may express the system head in metres and the flow rate in cubic metres per second. Procedures to plot a system-head curve are:

1. Define the pumping system and its length;

2. Calculate the fixed system head;

3. Calculate the variable system head losses for several flow rates;

4. Combine the fixed head and variable heads for several flow rates to obtain a curve of total system head versus flow rate.

The flow delivered by a centrifugal pump varies with system head. Pump manufacturers provide information on the performance of their pumps in the form of characteristic curves of head versus capacity, commonly known as pump curves. By superimposing the characteristic curve of a centrifugal pump on a system-head curve, the duty point of a pump can be determined.

The curves will intersect at the flow rate of the pump, as this is the point at which the pump head is equal to the required system head for the same flow.

The recommended values for coefficient of ColebrookWhite Roughness Factor (Section 1.5.1 above) ks for use in rising mains are contained in Table 2.2.2 below. Note also the values indicated in Table 1.5.1, which refer to gravity sewers.

Table 2.2.2 Recommended Values of

Colebrook-White Roughness Factors

(ks) for use in Rising Mains

Mean Velocity in m/s ks (mm)

Up to 1.1m/s 0.3mm

Between 1.1m/s and 1.8m/s 0.15mm

The discharge capacity for multiple pumps will not be simply the sum of the discharge capacity of individual pumps because the system-head curve for multiple pumps will be different from that of a single pump.

2.2.2 Pump Arrangements

The number of pumps to be installed depends on the station capacity and the range of flows. The maximum discharge rate from a pumping station, when all duty pumps and rising mains are in use should be slightly greater than or equal to the maximum incoming flow to the station. Pumps should be selected with head-capacity characteristics that correspond as closely as possible to the overall station requirements.




Standby capacity is required so that should any of the pumps in the station be inoperable due to routine maintenance or mechanical failure, the operation of the station can still be maintained. For instance, in a station where a single duty pump provides the duty output, a second pump of equal capacity is mounted. Where three duty pumps of equal capacity are required to meet the maximum design flow conditions, a fourth pump of similar capacity is provided as standby.

It is not desirable to have pumps of different sizes for operation and maintenance reasons, unless the flow ranges vary widely throughout the day. To cater for slow build-up of flow in the early years of operation, phased installation of pumps, or the use of a smaller diameter impeller should be considered.

2.3 Rising Main Design

2.3.1 Rising Main Diameters

The minimum diameter of pumping mains is controlled by the need to avoid blockage, and therefore should not be less than 100mm. Where sewage is screened or macerated before pumping the minimum diameter should not be less than 80mm.

The maximum and minimum diameters are sized to maintain flow velocities for all stages of pumping within the ranges specified in Section 2.4.

2.3.2 Twin Rising Mains

The use of twin rising mains should be considered on a case by case basis. The main factors for consideration include the design elements, risk assessment and cost benefit analysis.

Considerations for the design elements comprise the rate of build up of flow, the range of flow conditions, the range of velocity in the mains, the availability of land for the twin mains and associated valve chambers as well as the complications in pump operation.

A thorough risk assessment should be carried out which should include the likelihood of mains bursting, the consequence of failure, area affected, sensitive receivers affected (such as beaches), and

the feasibility of temporary diversion or tankering away.

A cost benefit analysis should include all tangible factors (such as cost of pipework, land cost, energy cost, etc) and intangible factors (such as nuisance, closure of beaches, etc).

Twin rising mains should be considered in the following circumstances:

To accommodate a wide range of flow conditions, such that the velocity in the mains can be kept within acceptable limits. For instance, a pumping system serving a new development may have very low initial flows with a slow build up of flow;

To provide continued operation for a major pumping system when one of the mains is damaged and where the failure of the system would have serious consequence;

To minimise adverse environmental impacts to sensitive areas;

To facilitate future inspection and maintenance of major pumping systems, while the normal sewage flow can be maintained.

When twin mains are found to be preferred, it is advisable to use both mains as duty rather than one as duty and the other as standby, from an economical and operational point of view. Should one of the duty mains be taken out of operation, the remaining one would still be able to deliver a higher quantity of flow at a higher velocity. The occurrence of overflow or bypass can be minimised or even eliminated. Septicity in the standby mains would also pose an operational and maintenance problem.

2.3.3 Economic Analysis

As the size of the rising main increases, the velocity and the system head will decrease, with savings in the cost of pumping. The increase in the capital cost of rising mains will be offset by the power cost of pumping. However, it is also important that the velocity in the mains should be within a suitable range to minimise the deposition of solids. Excessive hydraulic head losses are to be avoided.

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The selection of a suitable size for the rising mains should be based on economic analysis of capital cost and recurrent cost of the pumping system including the power cost. A trial and error approach should be adopted in order to arrive at an optimal solution while maintaining the velocity within acceptable limits.

Therefore, combinations of different sizes of rising mains and the system head should be evaluated, taking into account both the capital cost and the energy cost of pumping.

2.3.4 Rising Main Alignment

The alignment of the rising main should discourage surge in its flow conditions. Where possible the rising main should be laid with continuous uphill gradient of not less than 1:500, and with gentle curves in both horizontal and vertical planes. Long flat lengths of rising main should be avoided therefore pipes should be laid with rise and falls of 1:500, rather than flat. Air release valves should be provided at high points and as the profile of the main dictates. Washouts should be installed at low points. The arrangement and locations of valves should be planned together with the alignment of the rising mains.

2.4 Maximum and Minimum Velocities

The maximum velocity in rising mains should not exceed 2.0 m/s, The desirable range of velocity should be 1m/s to 2m/s with due consideration given to the various combinations of number of duty pumps in operation. (This is because lower velocities cause siltation, and higher velocities increase surge problems and power usage).

2.5 Pipe Materials Pipe materials for use in pumping stations should always be Ductile Iron (DI).

Rising mains outside pumping stations may be ductile iron or Glass Reinforced Plastic (GRP) with concrete protection, however DI is preferred.

2.6 Thrust Blocks Thrust blocks are concrete blocks designed to prevent pipes from being moved by forces exerted within the pipe by the flow of water hitting bends, tapers, and closed, or partially closed valves. In the design of pressurised pipelines, thrust blocks are essential on flexibly jointed pipelines where any pipe movement would open up the joints in the line and cause water leakage. Restraint straps may also be required for above-ground pipework.

Thrust blocks are also necessary near valves where a flexible joint is located to facilitate removal of the valve for maintenance purposes. The size of block is dependent upon the angular deflection, flow, size of pipe and the pressure of water inside the pipe. The designer should also refer to the pipe manufacturers literature.

The following design assumptions are to be adopted:

Thrusts developed due to changes in direction of pipeline, dead end or change in diameter should be considered. Force due to change in velocity head can normally be assumed as negligible unless there is a drastic change in pipe diameter;

Thrust blocks should be designed for the condition of no support being available from the backfill, i.e. to be cast against undisturbed ground;

For most cases, thrust blocks will be designed to transfer forces directly onto undisturbed ground using direct bearing, the acceptable bearing pressure being confirmed by geotechnical investigation. If the adjacent ground has insufficient bearing capacity, the block may need to be designed using ground friction or piling to transfer thrusts to a more competent soil layer. Consideration should also be given to the presence of adjacent services and the possibility of future disturbance during maintenance operations. Complex thrust blocks may be required to avoid transfer of forces and consequential damage to adjacent services;

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For pipes with flexible joints such as DI pipes with socket and spigot joints, all the thrust is assumed to be taken up by the blocks.

Static thrusts may be calculated using the formulae as follows:

For blank ends:

Fs = 100 A P

Equation 2.6.1

Where:

Fs = the static thrust (KN)

A = the cross sectional area (m2)

P = the Pressure (bar)

For Bends:

Fs = 100 A P(2 sin /2)

Equation 2.6.2

Where is the angle of deviation at the bend.

Dynamic thrusts for water or sewage may be calculated using following:

Fd = 2A V 2 sin /2

Equation 2.6.3

Where

Fd = the dynamic thrust (KN)

V = the velocity (m/s)

As stated above, this force is negligible in normal cases, but if significant, then the total thrust should be taken as the sum of static and dynamic thrusts.

The above procedures will be satisfactory for most routine applications. For further guidance, please see CIRIA Report R128xxxviii. It is recommended that this reference is used for more complex applications, such as where thrust forces are in excess of 1000KN or loose material is encountered.

2.7 Air Valves and Washout Facilities

These facilities are required to minimise the adverse effects of surge and to facilitate the operation and maintenance of the rising main.

2.7.1 Air Valves

Air-relief valves are installed at locations of minimum pressure. Air is sucked into the air-relief valve when pipeline internal pressure is below atmospheric. Upon subsequent pressure rise, the admitted air is then expelled. Air valves should be installed at all high points., Additional air valves should also be placed at 800m spacings on long sections of straight grade.

Each air valve will operate independently and therefore several valves may be required along the pipeline if there are numerous rises and falls in the vertical profile of the rising main.

2.7.2 Vented Non-return Valves

An air valve combined with a vented non-return valve allows air enter the pipeline freely on separation of the water column, but controls the expulsion of air as the column rejoins. This has the effect of creating an air buffer between the column interfaces, thus reducing the impact velocity of the rejoining column and the surge potential of the system.

2.7.3 Wash Outs

The purpose of the washout system is to drain the rising main for maintenance works. The washout should be installed at low points of the pipeline profile, and needs to be located carefully, taking into account that sewage will be discharged. For long rising mains with few low points, wash-outs should be strategically located at suitable intervals, generally 800m, to reduce the time required for emptying the main in an emergency. Location should be adjacent to a suitably sized gravity sewer for draindown where possible If a direct connection to a suitably sized sewer is not available, the washout chamber should be provided with a sump

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so that the drained contents of the rising main may be tankered away.

2.7.4 Isolating Valves

For long rising mains, isolating valves should be included to allow sections of the rising main to be isolated and emptied within a reasonable time. In-line sluice or gate valves are often used as isolating valves. The isolating valve installation may incorporate washout facilities.

2.8 Flow Meters

2.8.1 Application and Selection

The variety of choices facing the designer confronted with a flow measurement application is vast. For example, types of flow meter using the positive displacement principle include rotary piston, oval gear, sliding vane, and reciprocating piston. Each type has advantages and limitations and no single type combines all the features and all the advantages.

Differential pressure meters have the advantage that they are the most familiar of any meter type. They are suitable for gas and liquid, viscous and corrosive fluids. However their usable flow range is limited and they require a separate transmitter in addition to the sensor.

Some of the most important parameters for flowmeters are accuracy, flow range, and whether the medium is sewage or water. Meter selection should be made in two steps. First by identifying the meters that are technically capable of performing the required measurement and are available in acceptable materials of construction; and second, by selecting the best choice from those available to cover special measurement features such as reverse flow, pulsating flow, response time and so on.

2.8.2 Magnetic Flowmeters

Magnetic-type flowmeters use Faradays law of electromagnetic induction for measurement. When a conductor moves through a magnetic field of given field strength, a voltage level is produced in the

conductor that is dependent on the relative velocity between the conductor and the field. Faraday foresaw the practical application of the principle to flow measurement, because many liquids are adequate electrical conductors. So these meters measure the velocity of an electrically conductive liquid as it cuts the magnetic field produced across the metering tube. The principal advantages include no moving components, no pressure loss, and no wear and tear in components.

Magnetic flowmeters offer the designer the best solution for pumped sewage flow. With nothing protruding into the flow of sewage, the chances of a blockage, if installed correctly, are non-existent. Magnetic flowmeters should always be installed with full-pipe conditions.

Care should be taken during design to provide sufficient straight lengths of pipeline up-stream and down-stream of the flowmeter, in accordance with the manufacturers installation instructions. As a general guideline, 12 pipe diameters of straight pipe on the inlet, and 6 pipe diameters on the outlet will ensure that the flowmeter is able to achieve the specified accuracy. If the amount of space available is restricted then the minimum length usually accepted by manufactures is inlet run of 5 pipe diameters and outlet run of 3 pipe diameters.

The following International and British Standards are a good source of information on flow meter selection and installation, and can be quoted in specifications:

BS EN ISO 6817xxxix, 1997: Measurement of Conductive Liquid Flow in Closed Conduits;

BS 7405xl, 1991: Guide to Selection and Application of Flowmeters for the Measurement

of Fluid Flow in Closed Conduits.

Flow meters should be pressure tested and calibrated by the manufacturer, and certified to a traceable international standard. As a minimum, the overall accuracy should be better than 0.5% of the flow range. The repeatability of the result should be within 0.2%.

In addition to the calibration certificate, the flow meter manufacturers should provide the following:

i. Isolated 4-20mA dc and pulse outputs;

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ii. Programmable in-built alarm relays for empty pipe, low and reverse flows;

iii. In-built digital display for flow rate, totals and alarms;

iv. Transmitter enclosure shall be protected to IP67;

v. Calibration and programming kit.

The earthing rings should be included according to the individual manufacturers instructions. The sensor lining should be neoprene or an equivalent material of similar or improved properties, suitable for the application of pumped sewage flow. In below-ground flow meter chamber installations, the installed equipment should be submersible to the maximum chamber depth.

2.8.3 Ultrasonic Flowmeters

Ultrasonic meters are available in two forms: Doppler and transit-time. With Doppler meters, an ultrasonic pulse is beamed into the pipe and reflected by inclusions, such as air or dirt. The Doppler meter is frequently used as a clamp on device which can be fitted to existing pipelines. It detects the velocity only in a small region of the pipe cross section and as such its accuracy is not good. The single or multi-beam transit-time flow meters project an ultrasonic beam right across the pipe at an acute angle, first with the flow, and then opposite to the flow direction. The difference in transit time is proportional to flow rate. This type of ultrasonic meter is considerably more expensive but offers better accuracy. Unlike the Doppler meter, it requires a relatively clean fluid.

The main use of this type of flow meter in pumped sewage flows is in retrospective installation where the pumping main cannot be broken into for operational reasons. A clamp-on ultrasonic flow meter can be used to give reasonably accurate flow measurement.

For new installations, the lower cost of in-pipe ultrasonic flow meters could make them a viable alternative to magnetic flow meters for large diameter pipe installations.

2.9 Surge Protection Measures

Surge (or water hammer) is an oscillating pressure wave generated in a pipeline during changes in the flow conditions.

There are four common causes of surge in a pipeline:

pump starting;

pump stopping/power failure;

valve action;

improper operation of surge control devices.

The most likely one of these is the sudden stopping of pumps caused by a power failure.

A surge analysis should usually be carried out unless the system is simple. This is best carried out using approved software such as Flowmaster.

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An approximate calculation for a simple pipeline is:

P = a x V g

Equation 2.9.1

Where: P = Pressure change (m)

a = pressure wave velocity (m/s)

V = flow velocity change in 1 cycle (m/s)

g = acceleration of gravity (9.81m/s2)

The above equation can be used for calculation of both negative and positive pressures

The simple cycle time can be calculated with the formula:

Cycle time = 2 x pipeline length

Wave velocity

Equation 2.9.2

Table 2.9.1 Indicative Surge Wave Velocity

Values for Selected Pipe Materials

Pipe Material Velocity (m/s) Ductile Iron 10001400

Reinforced Concrete 10001200 Plastic 300500

If the surge pressure approaches zero or the pipeline maximum pressure, a full surge analysis should be carried out. When surge analysis is complete, suitable surge suppression devices should be selected by consultation with the manufacturer.

Surge Suppression Methods

Surge suppression could be achieved using one of the following devices. The most appropriate device will depend on the individual circumstances of the installation:

Flywheel;

Pressure vessel with bladder;

Dip-tube surge vessel;

Surge tower.

Air valves should not be depended upon as a sole method of surge control, but their operation under surge conditions should be carefully considered.

Flywheels

Flywheels absorb energy on start-up, slowing the rate of velocity change in the pipeline. In reverse, when the pump is stopping, the flywheel releases energy again, slowing the rate of velocity change. Together these two actions reduce the peak surge pressure.

As the flywheel must be located on the drive shaft it is not suitable for submersible pumps or close-coupled pumps. However, they are simple devices for wet well/dry well pumps and are preferred where possible.

If submersible pumps have been chosen, a larger pump running at a slower speed may have the effect of a flywheel.

Because the flow continues through the pump after the stop signal, the effect on the stop and start levels should be carefully considered.

Pressure Vessels

Pressure vessels for surge suppression are tanks partially filled with a gas (air or nitrogen). Usually the liquid is contained in a bladder with gas on the outside to prevent the liquid absorbing the gas or coming into contact with the inside of the pressure vessel, and this is the preferred type. The bladder material should be carefully selected for use in the conditions experienced in Qatar.

Refilling is usually from a high-pressure cylinder and care should be taken to avoid over pressurisation of the bladder. Bladders should not lose pressure in normal operation, but they can fail, leading to absorption of the gas into the liquid, and a drop in pressure.

Vessels without a bladder are charged with air pressure from an air compressor, either manually or automatically. There is therefore additional machinery and an additional maintenance requirement. This type of surge vessel is not recommended.




On pump start-up, liquid enters the vessel, compressing the gas until it equals the liquid pressure. When the pump stops, the gas pressure forces liquid back out into the pipe system, both actions slow the rate of pressure change, which reduces the peak surge pressure.

To dampen oscillations, a non-return valve may be fitted to the surge vessel outlet pipe, to allow unrestricted flow into the pipeline, and a bypass around the NRV fitted with an orifice plate to restrict the flow back into the vessel.

Dip Tube Surge Vessels

A dip tube surge vessel is pressure vessel, the top portion forming a compression chamber limited by a dipping tube with a shut off float valve.

This type of vessel is particularly appropriate for use on rising mains with flat profiles.

Surge Towers

A surge tower is a vertical tank or pipe fitted into the pipeline, open to atmosphere and the energy storage is by the static head of the liquid in the tower.

Surge towers are only practical for systems with relatively low heads and surge pressures, but can pose an odour risk.

Due to the design of a surge tower, there is no routine maintenance required to ensure the surge tower keeps operating correctly.

It is unlikely that surge towers would be appropriate for use in Qatar.

Air Valves

Air valves are required on the pumping mains to release air, but they should not be used as a surge protection measure.

However, air valves, particularly if fitted with a vented non-return valve or in-flow check valve, may assist in surge control, and their operation must be carefully considered.

Air valves require regular maintenance because if the air valve does not function correctly, large or negative surge pressures could result, with consequent damage to equipment or personnel.

If air is allowed into the rising main on pump stop/trip through an air valve, the pump control system should be designed to prevent a restart until the transient pressures have stabilised.

Control of the pumps is usually by start/stop level signals, but where surge on start-up may have a significant effect, the use of soft starters should be considered.

2.10 Screens Screen Selection

Screens should generally be provided for pump protection, unless they are small (1000l/s;

Fine screening is not required at the pumping station, but is required at the treatment works to remove debris that may affect the sewage treatment process.

Screen Installation

The manual duty and standby screen should be installed in the incoming channel, so that the standby screen can be lowered into position to

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protect the pumps while the duty screen is removed and cleaned.

Mechanically raked screens should be installed in a channel or similar flow-line, which can be completely isolated from the rest of the system and drained for maintenance. A manually raked bypass screen shall be provided.

Mechanical screens shall be housed in ventilated and odour controlled enclosures.

Screens should be provided with actuated penstocks (or valves) before and after each screen for operational and maintenance isolation.

All mechanically raked screens should have an automatic cleaning mechanism, which will clean the screen of accumulated debris and screenings, depositing them in a collection trough or channel above the highest possible water level.

Screenings Handling

Manually removed screenings should be placed in a covered container until removed from site to avoid odour problems.

Mechanically removed screenings should be washed, compacted and deposited into a covered container to avoid odour problems.

2.11 Pumping Station Selection

Sewage pumping station type selection should be carefully considered for each scheme. In general, submersible pumping stations are generally selected for flows up to 100l/s, and wet well/dry well stations for larger flows. However, each station should be treated on its own merits and the following considerations assessed:

Initial and final design flow;

Total head on the pumps;

Rising main profile and the requirements for surge protection (dry well pumps usually have a greater moment of inertia than submersibles);

Requirement for Variable Speed Drive (VSD): (submersible motors are not always adequately rated for use with VSD);

Space available for pumping station (submersible stations usually require less space);

Proximity of housing or public areas (opening submersible pump wells may create odour nuisance).

An alternative to wet well submersible pumps and dry well pumps is the dry well submersible. These should normally be considered only where an existing dry well installation is being uprated and there is insufficient space to install a conventional dry well pump and motor.

Particular attention should be paid to motor cooling and cabling if dry well submersibles are to be considered.

The designer should present three alternative pump suppliers for tender purposes.

Submersible pumping stations

Submersible pumping stations should incorporate the following features:

Minimum of one duty and one standby pump;

Non-return valve and gate valves for isolation of each pump;

Valves to be in a separate, easily accessible chamber adjacent to the pump sump;

Air reaction operation level controls as follows:

- High level alarm (also float);

- Pump start;

- Pump stop;

- Low level pump protection (also float).

Ultrasonic level controls should not be used for sewage;

Air reaction level equipment should include stainless steel dip pipe and duty/standby compressors.

Where the available pumps have unsuitable duties for the full range of flows, the use of variable speed drives should be considered. However, due to the




additional heat generated in the motor, the approval of the pump manufacturer should be obtained before variable speed drives are used.

Submersible Pump Sump Design

The CIRIA guide The hydraulic design of pump sumps and intakes by M. J. Prosserxli should be referred to when designing pump sumps. Some pump manufacturers also provide guidance on the design of sumps for their pumps. Sump design should be in accordance with the following criteria:

Sumps should be designed so that the dimensions satisfy the requirements for the minimum sump volume to ensure the maximum rated pump starts per hour for the motor and switchgear are not exceeded;

Sumps should be designed to provide a uniform steady flow of water into any pump without creating swirl or entraining air. Unsteady flow can lead to fluctuating loads, vibration, noise and premature failure. Swirl can affect the flow capacity, power and efficiency. It can also result in local vortices that introduce air into the pump, also leading to fluctuating loads, vibration, noise and premature failure;

Sumps should be designed to prevent the accumulation of sediment, scum and surface flotsam;

Sump corners should be benched to 45. Minimising the sump floor area and residual volume will increase the velocity into the pumps and improve scouring;

The use of flushing devices to improve scour in pump sumps should be considered;

The velocity in the pump riser pipe at the design duty should be as high as practicable to reduce the risk of solids deposition. However, the velocity should not normally exceed 2.5m/s to avoid significant headloss and risk of pipe erosion;

The water surface in the sump should be as free from waves and turbulence as possible to provide a strong and reliable echo for ultrasonic level controls;

At the designed stop level there should still be sufficient water surface area without obstructions to provide a good echo return.

Submersible Pump Installation

When submersible pumps are installed, the following should be considered:

There should be sufficient space between them to prevent interaction between the pump suctions. This will depend upon the type of pump being used and the manufacturer should be consulted on configurations at draft design stage; A rule of thumb is to use an initial spacing between pump centres of twice the pump diameter. Further guidance is given in table 2.11.1 below.

Table 2.11.1 Approximate Minimum

PumpSpacingsxlii

Flow (l/s) Spacing (mm)

100 700

200 1000

300 1200

400 1350

500 1500

600 1700

700 1800

800 1900

900 2050

1000 2175

There should also be sufficient space for someone to stand beside each pump, should work be required in the sump;

Pump mounting stools and duckfoot bends should be securely bolted to the structural concrete of the sump and not the benching;

Discharge non-return and isolating valves should be located outside the sump in a valve chamber;




Pump guide rails should rise close to the underside of the sump covers above the pumps;

The covers should have a clear opening large enough to allow the removal of the pump while on the guide rails;

Support points for the pump power cables and lifting chain should be provided under the pump covers, which should be easily accessible from the surface.

Wet/Dry Well Pumping Stations

Wet well/dry well pumping stations should incorporate the following features:

Normally, two sumps with 2 duty and 1 standby pump for each sump, for the ultimate flow;

Non-return and two gate valves for each pump isolation;

Where possible, the discharge manifold should be below ground level to minimise additional pipework and friction losses;

Where wet well/dry well pumping stations are being uprated, dry well submersible pumps could be considered;

Operation level controls (air reaction) as follows:

- High level alarm (plus float);

- Pump start;

- Pump stop;

- Low level pump protection (plus float).

Air reaction level equipment should include stainless steel dip pipe and duty/standby compressors.

Where the available pumps have unsuitable duties for the full range of flows the use of variable speed drives should be considered. However due to the additional heat generated in the motor, the approval of the pump manufacturer should be obtained before variable speed drives are used.

Wet Well Design

The CIRIA guide The hydraulic design of pump sumps and intakes by M. J. Prosser should be referred to when designing wet wells, which should incorporate the following features:

Wet wells should be designed to provide a uniform steady flow of water into any pump without creating swirl or entraining air. Unsteady flow can lead to fluctuating loads, vibration, noise and premature failure. Swirl can affect the flow capacity, power and efficiency, it can also result in local vortices that introduce air into the pump also leading to fluctuating loads, vibration, noise and premature failure;

Wet wells should be designed to prevent the accumulation of sediment, scum and surface flotsam;

Wet well corners should be benched to 45. Minimising the sump floor area and residual volume will increase the velocity into the pumps and improve scouring;

The use of flushing devices to improve scour in wet wells should be considered;

The water surface in the wet well should be as free from waves and turbulence as possible to provide a strong and reliable echo for ultrasonic level controls;

At the designed stop level there should still be sufficient water surface area without obstructions to provide a good echo return;

Wet wells should be designed so that the dimensions satisfy the requirements for the minimum sump volume to avoid excessive pump starts;

The pump suction pipes should be installed through the wet/dry well dividing wall with a downward bend and bellmouth to position the pump suction as close to the sump floor as possible to assist in sediment removal;

There should be sufficient space between the bellmouths to prevent interaction between the pump suctions.

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Dry Well Design

Dry well design should incorporate the following features:

The pumps should be installed along the wet/dry well dividing wall with sufficient space between them to allow access for maintenance and repair;

The pump distance from the dividing wall will be set by the length of the protruding stub pipe, suction valve and pump inlet pipe;

Drive shafts should be supported from concrete beams spanning the dry well;

Consideration should also be given to access around the pumps and valves. Platforms and walkways should be installed to provide access to all equipment at a suitable level for safe operation, maintenance and repair;

The general floor level should be higher than the sump level to reduce the size of pump plinths and the need for access platforms;

Careful thought should also be given to the shipping route for removing equipment;

Access to the dry well and machinery should be by staircase so that tools and equipment can be carried in and out safely;

Lifting arrangements for the pumps and valves shall be provided (see also section 2.21 and 2.22);

The dry well floor should slope gently towards the dividing wall and then to one side where a sump pump should be installed to keep the floor as dry as possible;

The sump pump should be installed in a small well, large enough to accommodate the pump and should discharge back through the wall into the wet well. Consideration should be given to the sump pump discharge to avoid backflow from the wet well to the dry well;

A high level alarm should be installed in the dry well to give a warning of flooding before damage to machinery occurs.

Pump Installation

For the most compact arrangement, a close-coupled pump can be mounted horizontally with the discharge upward, however this results in the motor being low in the dry well and at risk from flooding. The most common arrangement is for a vertical pump shaft with the motor above. This will require a bend between the suction valve and the pump suction. The bend should be fitted with a handhole and valve to enable the pump to be drained prior to maintenance. Further bends may be required to direct the pump or manifold discharge upwards. Where space allows, installation of the discharge manifold at the pump level, with the discharge directly through the side wall should be considered.

Pipes should be sized to achieve sensible velocities, and the risk of cavitation through insufficient NPSH should be considered when designing suction pipework. Pumps must be selected to ensure satisfactory operation when only one pump is operation in a new rising main.

2.12 Pumps and Motors Centrifugal Pumps

These are the most common type pumps for foul sewage and are available in a variety of forms. The pump operates by passing the liquid through a spinning impeller where energy is added to increase the pressure and velocity of the liquid. Submersible pumps are centrifugal pumps.

Sewage pumps should have an open type impeller with a minimum passage of 100mm. Impellers with smaller passages are likely to suffer from frequent blockage due to the nature of sewage debris.

Dry well centrifugal pumps should normally have a maximum running speed of 980rpm. Submersible pumps may run at 1450rpm (4 pole motor), but pumps operating at 2900rpm (2 pole motor) will suffer excessive wear and premature failure, and should not be used.

Pump Motors

Motors on submersible pumps should be certified for use in Zone 1 explosive atmospheres unless operating continuously submerged. Pumps operating in dry conditions should have a casing designed to provide adequate cooling in the operating conditions.

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Pump motors should normally be fed from 415 volts, 50 hertz, 3-phase power supply. For larger motors 690V or 3.3KV motors can be used.

Because additional heat is generated in the motor when used with a variable speed drive, the approval of the pump manufacturer should be obtained before VSDs are used.

For dry well and screw pumps where the motors are installed vertically or at a steep angle, they should be specifically designed for that purpose, with adequately rated end thrust bearings.

Where flywheels are installed, the motor rating shall be suitably uprated.

2.13 Sump Design The CIRIA guide The hydraulic design of pump sumps and intakes by M.J. Prosserxli should be referred to when designing sumps or wet wells.

Sumps should be designed to provide a uniform steady flow of water into any pump without creating swirl or entraining air. Unsteady flow can lead to fluctuating loads, vibration, noise and premature failure. Swirl can affect the flow capacity, power and efficiency. It can also result in local vortices that introduce air into the pump also leading to fluctuating loads, vibration, noise and premature failure.

Sumps should also be designed to prevent the accumulation of sediment and surface scum.

Most sumps and wet wells at standard pumping stations will probably be uniform in section and can be designed to avoid turbulent flows.

Modelling

For non-standard pumping stations, which may have high flows, multiple pumps or complex shapes, or where turbulent flows, vortices, swirl or air entrainment are more likely to occur, modelling should be considered.

For pumping stations, a physical model built to scale can be very effective in identifying flow problems and in some cases modelling by computational fluid dynamics (CFD) methodology may have benefits. Modelling is the process of replicating the hydraulic

performance of drainage, pumping and treatment systems by constructing models of the intended installations. These models need to be verified before use to provide confidence that they adequately represent the actual performance of the system.

The verified model is then used to test system performance under its proposed use. The model must be capable of modification to test various physical configurations and operating regimes for the installation, to produce the optimum solution for actual construction.

Traditionally, physical models were favoured, especially for coastal/estuary/river systems and complex pumping installations. In recent years mathematical models have superseded physical models. Mathematical models are exploiting increased computer hardware and software capability, and are more efficient than physical models in time and effort.

Physical Models

Physical modelling consists of constructing a reduced scale, geometrically similar model of a proposed system, and operating the model to simulate full-scale flow conditions. Model tests can provide the designer with the assurance that the proposed scheme operates satisfactorily, or allows him to improve the flow conditions and achieve a better design.

Changes in the model can be made by trial and error, and are usually based on the experience and intuitive understanding of the engineer conducting the tests. The amount of modification which can be undertaken on a physical model is limited, and therefore the initial model should be as accurate as possible.

Factors to be considered in deciding on the need for physical models include:

The similarity of the proposed scheme to existing satisfactory designs. As well as the designers own experience, much information is available from manufacturers published reports and design guides. However, it should be recognised that most large scale and/or complex designs will be unique, and hence modelling will be needed;

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The size and cost of the proposed scheme. Bearing in mind that physical modelling can take many months with corresponding high costs, then designers of small schemes should seek to adopt standard and well-proven designs for small schemes. Large schemes, such as terminal pumping stations with multiple pumps and complex inlet arrangements would merit modelling;

The time available for modelling. In some cases the scheme can be well under way to completion before the possible need for modelling is realised. Even at such late stages, modelling can save much time and cost in modifying construction works.

For pumping stations, all of the intake should be modelled, including the approach works, the inlets and the sump itself. Upstream pipelines may need to be included.

All hydraulically significant details such as screens, penstocks, support channels and benching, should be included in the model. No components above maximum water level need be modelled.

Model construction should be in durable and waterproof materials, with clear perspex being the best for viewing purposes. Model size should be as large as costs allow. Scales can vary from perhaps 1:4 for very small sumps, up to 1:50 for large intakes to reservoirs or tanks. For sump models, 1:25 would be the smallest desirable scale.

Physical testing could typically take between one and six months for construction, testing and reporting.

Sump Volume

Pump sumps should have a minimum sump volume calculated to ensure that in the worst flow conditions any pump installed does not exceed the maximum allowable starts per hour. The CIRIA guide The hydraulic design of pump sumps and intakes by M.J. Prosserxli should be referred to when designing sumps or wet wells.

The minimum sump volume is the volume between the start and stop levels of the duty pump and for a single pump the worst case occurs when the inflow is exactly half of the pumping rate.

To calculate the minimum sump volume for a specific pump the formula contained in the above CIRIA guide is:

T = 4V/Qp

Equation 2.13.1

Where:

T is the cycle time for the pump, e.g. if the recommended maximum starts per hour for a pump is 10, then the cycle time will be 6 minutes (60/10 = 6)

V is the volume of sump between the start and stop levels in m3

Qp is the pumping rate in m3/minute

Therefore if Qp is 1.2m3/min (20l/s) and the maximum number of starts is 10/hour, the volume required will be:

V (m3) = 6(min) x 1.2(m3/min) / 4

V = 1.8m3

For 10 starts per hour this could also be expressed as:

V = 1.5 x Qp

The sump volume when multiple pumps are installed is calculated as for a single pump, where the minimum sump volume is the capacity between the start and stop level for each pump. However, additional capacity is required to allow a vertical distance of 150mm between the start or stop levels of consecutive pumps.

With sewage there is a possibility of septicity, therefore there are restraints on the maximum volume of the sump related to the retention time of the liquid in that sump.

Maximum and minimum start / stop levels

The minimum stop level should be the level at which the pump can be stopped and restarted without losing suction or as specified by the pump manufacturer.

To avoid turbulence and odour release at foul sewage pumping stations, the lowest pump stop

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level is usually set at the invert of the incoming sewer, the last section of which is laid to a steep fall to avoid the sewer being used as the sump.

The minimum start level should be the required distance above the stop level to provide the minimum sump volume.

Allowable pump starts per hour

The maximum allowable starts per hour should be as specified by the pump or motor manufacturer. In the absence of any specified figure the following are suitable guidance figures:

Less than 100kW - 15 starts/hour

100kW < 200kw - 10 starts/hour

>200kW - 8 starts/hour

Stop / start levels for single and multiple pump operation

The start and stop levels for single pump operation should be set within the maximum and minimum start / stop levels defined in the previous section, provided that the minimum sump volume is attainable.

The start level for each additional pump should be set a suitable height above the previous pump to prevent accidental pump starts caused by surface waves or level sensor errors.

The stop level for each additional pump should be set at the required distance below the start level to provide the minimum sump volume for that particular pump. The stop level will normally be just above the previous duty pump stop level.

The effect of flywheels should be considered in determining stop/start levels because the flywheel increases the pump start-up and stop times.

Pump duty level

The pump duty level for a single pump should be the midpoint between the pump start and stop levels. For multiple pump installations it should be the midpoint between the top water level (last duty pump start level) and the bottom water level (first duty pump stop level).

Pumps should also operate within their performance curve at both top and bottom water levels under single or multiple pump operation.

2.14 Suction/Delivery Pipework, and Valves

Pipework

Only superior materials are acceptable for use in pumping station pipework. The pipework installation should incorporate the following features:

Sufficient bends and flange adapters to allow easy dismantling and removal of pumps, non-return valves or other major items of equipment;

Each dry well pump should be installed with suction and discharge isolation valves to permit isolation of the pump from the wet sump and discharge pipework for maintenance;

Each submersible pump should be installed with a discharge isolation valve to permit isolation of the pump from the discharge pipework for maintenance;

Each pump should also be fitted with a non-return valve to prevent reverse flow back through the pump when stopped;

Valves should be positioned to permit the removal of each pump and non return valve without draining either the wet well or discharge manifold, and allow the other pumps to continue operating normally;

Suction isolating valves for dry well pumps should be bolted directly to a flanged pipe securely fixed through the sump wall;

Discharge isolation valves should be bolted directly to a flange on the discharge pipe or manifold;

Discharge non-return valves should be bolted directly to the discharge isolation valve. They should be installed in horizontal pipework with a short length of pipe and a flange adapter on the pump side to allow dismantling;

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Where the pump delivery pipework joins the pumping station discharge manifold, the entry should be horizontal;

At the opposite end of the pumping station discharge manifold, a valved connection back to the sump should be provided for draining the discharge pipework, or flushing the sump;

Consideration should be given to providing an isolating valve on the pumping main before it leaves the pumping station/chamber and before any over pumping connection, to allow the pumping station to be fully isolated and the fixed pipework drained for repair;

All flexible couplings should be restrained on both sides by securely fixed equipment, thrust blocks or tie straps across the coupling to prevent displacement of the coupling under pressure.

Valves

Valves should incorporate the following features:

Isolation valves for sewage should be of the double-flanged wedge-gate type with a bolt-on bonnet. When fully open, the gate should be withdrawn completely from the flow. The valve should have an outside screw rising stem and the handwheel direction of operation should be clockwise to close. Station valves should have metal seats;

Valves greater than 350mm diameter should be fitted with actuators. Where installed in chambers they could be fitted with non-rising stems to limit the headroom required;

Reflux valves for sewage should be of the double flanged, quick action single door type, designed to minimise slam on closure by means of heavy doors, weighted as necessary. The door hinge pin/shaft should extend through the side of the body and be fitted with an external lever to permit back flushing;

Reflux valves should be provided with covers for cleaning and maintenance without the need to remove the valve from the pipeline. The covers should be large enough so that the flap can be removed and the valve can be cleaned;

The non-return valves should have proximity switches to prevent dry running and allow a change of duty (standby on high level will then start);

All reflux valves should be installed in the horizontal plane;

Butterfly valves should not be used with sewage.

2.15 Pumping System Characteristics

NPSH, Vibration, Cavitation and Noise

Net Positive Suction Head (NPSH) is used to check the pumping installation for the risk of cavitation.

Cavitation is the formation and collapse of vapour bubbles in a liquid. Vapour bubbles are formed when the static pressure at a point within a liquid falls below the pressure at which the liquid will vaporise. When the bubbles are subjected to a higher pressure they collapse causing local shock waves, if this happens near a surface, erosion can occur.

Cavitation will typically occur in the impeller of a centrifugal pump, where it can cause noise and vibration as well as affecting the pump efficiency. If allowed to persist it can lead to damage to the pump or even breaking away of foundations.

NPSH is the minimum total pressure head required in a pump at a particular flow/head duty. It is normally shown as a curve on the pump performance sheet.

NPSH = Pa Vp + Hs Fs

Equation 2.15.1

Where:

Pa = atmospheric pressure at liquid free surface

Vp = vapour pressure of liquid

Hs = height of supply liquid free surface, above eye of pump impeller

Fs = suction entry and friction losses

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In order to avoid cavitation, the NPSH available should be at least 1m greater than the NPSH required by the selected pump at all operating conditions.

When calculating NPSH, absolute values for atmospheric and liquid vapour pressures are used.

Pump Duty Point

Each pump has a performance curve where the flow is plotted against head.

Each pipework system has a friction curve where the friction head is plotted against flow.

The system curve is obtained by adding the static head to the friction losses and plotting the total head against the flow.

The pump duty point is where the pump performance curve and the system curve cross. It shows the flow that a particular pump will deliver through the pipework system at a particular total head at the pump duty level.

In multiple pump installations, it is essential that the operating conditions of a single pump running are carefully checked to ensure that the pump will operate at maximum and minimum static heads satisfactorily, and without risk of cavitation.

The duty point should be used when considering the suitability of alternative pumps for a particular duty by comparing the efficiency and power requirements for each pump at the duty point.

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Figure 2.15.1 Characteristic Curve for Multiple Pumps

Characteristic curve for new pipe

Lifting Station Design Abstract

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