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;
State of Qatar -Public Works Authority Drainage Affairs
• 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.
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.