Lift Stations Flashcards
How is waste water raised or lifted in a lift or Pump
Wastewater is raised or lift in a lift station by centrifugal pumps or pneumatic injectors.
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What factors influence the location of lift stations?
The location of a lift station depends on economic factors. They are installed at low points in the collection system at the end of gravity sewers where the following conditions exist: *Excavation cost to maintain gravity flow and sufficient velocity become excessive.
*Soil stability is unsuitable for trenching.
*Groundwater table is too high for installing deep sewers. *Present waste water flows are not sufficient to justify extension of large trunk sewers and a lift station ofters an economical short-term solution. Lift station locations are also influenced by the location of other utilities, buildings and transit systems.
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Which two safety items must be considered in operating and maintaining and lift station?
Safety of The Operators involved in operating and maintaining the lift stations must be considered as well as a safety of the public
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What type of pump is often installed in a wet well?
A submersible pump is often installed in a wet well
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What is a limitation of installing a pump inside a wet well?
Limitation of installing a pump inside a wet well is limited access to the pumping equipment which creates a problem when the pump has to be repaired.
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What is the most desirable operational situation for a lift station?
The most desirable operational situation for a lift station is when all the flow and solids that discharge into the wet well from the gravity sewer are lifted to the higher elevation and continue to the wastewater treatment plant without delay.
Page 14
What are the effects of water hammer?
Water hammer causes extremely high pressure shock waves to travel up and down a force main.
Water hammer associated with check valve slamming can produce a force strong enough to damage check valves and piping in the station.
In severe cases, water hammer can brake-force Mains and thereby cause a system failure and flooding.
In some cases negative pressure sufficient to collapse a pipe can develop and can also contribute to the destructive effects of water hammer.
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What is the difference between static heads and dynamic heads?
Static heads are measured when the pump is not operating and are the differences in elevation (in feet) between the surface of the water on the suction side of the pump and the surface of the water on a discharge side of the pump.
Dynamic heads are measured when the pump is operating and depend on the velocity of water in the pump suction and discharge pipes.
Dynamic heads are greater than static heads because they include the static heads plus the friction loss in the suction and discharge pipes.
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Friction losses in lift stations are the results of which factors?
Friction losses in lift stations result from pipe friction and the friction losses due to the water flowing through pipe fittings such as valves, Reducers, and elbow.
The greater the velocity of flow in pipes the greater the friction losses.
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Why should the pumps in a lift station not start and stop frequently?
Pumps in a lift station should not start and stop frequently because frequent motor starts cause excessive wear on the equipment and may cause surges in the waste water flow. Also frequent starting will increase power cost.
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What problems develop in a wet well that is too large?
If a wet well is too large, solids will tend to accumulate on the bottom because the wet well will be acting as a settling tank, similar to what occurs in a treatment plant sedimentation tank or clarifier.
Accumulated solids partially or totally block the pump suction pipes, over a period of time, microorganisms decomposing the organic material will generate hydrogen sulfide (toxic) and Methane (explosive) gas, as well as nuisance odors. Page 28
What is the main advantage of variable speed pumping equipment?
The main advantage of variable speed pumping Equipment is that the pumping speed or rate can be adjusted to match the pump capacity or the flow rate into the wet well. Page 29
A collection systems for only domestic and Industrial Wastewaters, why have bar racks or screens at the lift stations been eliminated by some agencies?
In collection systems for only domestic and Industrial Waste Waters bar racks or screens at the lift station have been eliminated by some agencies because most pumping units are equipped with open impellers or closed two-port impellers sized to pass solids up to the size of a 2.5 inch diameter sphere.
This is the largest size spear that may pass through most home toilets and disposal systems. In addition the necessity of visiting a Station 1 or 2 times a day to clean a screen and struggle with hauling the screenings to a disposal site is eliminated. Page 31
What happens when a seal fails on a submersible pump?
When a seal fails on a submersible pump, the wastewater can leak into the motor compartment and the unit willshort or burn out the motor.
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What are some advantages of solid state motor starters compared to Conventional electro-mechanical starters?
When compared to Conventional electromechanical starters Solid State motor starters offer certain advantages. Typically Solid State motor starters incorporate current sensing devices in each phase of the power being supplied to the motor. This allows much more precise overload protection for the motor. The system can also detect nuisance types control inputs from floats pressure switches or other input devices. Mechanical wear on the contacts is also reduced which prolongs contact life and reduces maintenance. Other features of solid state starters include phase unbalance and phase loss protection as well as ground-fault protection.
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What are the 3 types of float controllers?
The three types of float controls are: * Rod-attached floats. *Steel tape, cable, or rope-attached floats. *Mercury switch float controls Page 47 - 48
Ball floats are usually attached to a steel rod or float rod. The float rod is equipped with brass stops. Explain how these stops are used to select the wet well rise and fall levels.
When the wet well fills the bottom stop of the float ride pushes a micro switch arm up and starts the pump.
When the wet well is pumped down the floats drops and the ride travels downward.
When the top stop on the float rod pushes the micro switch arm down it turns the pump off.
The wet well rise and fall levels are selected or Changed by moving the rods stops to new positions on the float rod.
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What are the limitations of rope or cable attached float control systems?
Limitations of rope or cable attached float Control Systems include the following:
- Grease and debris may enter the stilling well and hinder or stop the up-and-down movement of the float. (These must be removed)
- The float attachment line or counterbalance line may break and unwind from the drum sheave thus allowing the counterbalance to fall in all or none of the pumps should be called on to operate.
- Floats develop please which may change or stop pump operation.
- Cables or ribbons attached to the flow can become twisted.
How does an ultrasonic level detector measure water levels?
A transducer is mounted above the highest water level in the wet well. The transducer generates ultra Sonic pulses which hit the water surface and bounce back to the transducer. The system measures the pulse’s travel time and converts this information into an electrical signal in the control system.
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What safety considerations are necessary when using electrode controllers?
Electrical safety regulations must be complied with when using electrode controllers. Be sure that power to the electrodes is turned off and properly tagged before performing any maintenance on the equipment.
Operators should be cautioned not to touch electrodes with bar screen
rakes or other tools.
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What are the limitations of electrode controllers?
Limitations of electrode controllers include the following:
*Rags and the debris can a wrap around electrodes and alter desired pump start and stop water levels and sequences of pumps.
*Grease or slime can cover and electrode, thus preventing good conductivity and causing intermittent or unreliable operation.
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What are the limitations of enclosed-electrode controllers(Sealtrode units)?
Limitations of enclosed Electro controllers (sealtrode units)include the following:
*When the bulb breaks, the electrolyte solution is lost. *Shorts can occur in the electrode wiring.
Bulb life is three to five years.
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How is the depth of water in a wet well measured using a pressure-sensing controller?
Pressure sensing controllers measure the depth of water in a wet well on the basis of air pressure or back pressure in the air bubbler line.
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How are pressure changes detached in a pressure-sensing controller?
Pressure sensing controllers detect pressure changes by measuring the back pressure required to overcome the pressure created by the level of the water above the outlet of the bubbler.
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What are the limitations of pressure-sensing controllers?
Limitations of pressure sensing controllers include The Following:
* Air compressor failure. *Bubbler blockage (the line must be purged regularly) tube breakage or Outlet elevation shifts.
*Complex equipment provides more opportunity for failures and requires a higher level of training for operators to troubleshoot, repair, and keep instruments calibrated.
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What are the two major types of pumps?
The two major types of pumps are positive displacement pumps and kinetic (or dynamic) pumps
page 58
Which type of pump is most commonly used in raw wastewaterlift stations?
Kinetic (or dynamic) pumps, usually of the centrigugal type, are most commonly used in raw Wastewater lift stations.
Page 58
Why should lift stations be equipped with at least two pumps?
What station should be equipped with at least two pumps because they must be capable of operating even peak flow periods with the possibility of one pump been out of service for maintenance. A station designed for capacity at peak flows will normally have one additional pump size to handle Peak flow pumping requirements. If two pumps are required under Peak flow circumstances, the station should have three pumps and so on.
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Why must pump seal water from a domestic water supply have an air gap?
Pump seal water from a domestic water supply must have an air gap to prevent contamination of the public water supply from backflows.
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Under what conditions is cavitation likely to occur?
Cavitation is likely to occur if modifications over time change the basic balance of the hydraulic system if the system was improperly designed or if the wrong pump has been used for a particular application. Page 68
List the major valves found in wastewater lift stations
The major valves found in Wastewater lift stations are:
Pump suction and discharge isolation valves (gate, plug, knife)
Discharge check valves (swing or ball check, wafer).
Cross-connect or pump discharge pipe manifold isolation valves (gate, plug)
Page 75.
Where are pump suction and discharge isolation gate valves located and why?
Suction and discharge isolation gate valves are found immediately before and after the pump to allow isolation of the pump from the wet well and the Force main during pump or check valve maintenance.
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Where are check valves installed and why?
Check valves are installed in the discharge of the pump to provide a positive shut-off from Force main pressure when the pump is shut off and to prevent the forcemain from draining back into the wet well.
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Why should operation and maintenance operators be given an opportunity to review the prints and specfications of a new lift station before the award of a Construction contract.
Operation and maintenance operators should review prints and specifications of a new lift station before construction to be sure adequate Provisions have been made for the station to be easily and properly operated and maintained.
Page 81
Why is accessibility to a lift station important?
Accessibility to a lift station is important not only for equipment, but for operators to get to the station and to easily operate and maintain the station.
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When reviewing lift station specifications for the accessibility of equipment what basic items should be determined?
When reviewing lift station specifications for the acceptability of the equipment determine if the equipment is familiar to your agency and if its reliability has been proven. Find out what warranties, guarantees, and operation and maintenance aids will be provided with the equipment and the lift stations.
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