Waste Collection Systems

Transcription

1 Waste Collection Systems Lift Stations Lift Stations Training Objectives To gain an understanding of: Lift Station vocabulary Lift Station components and their function Lift Station operational requirements Lift Station maintenance requirements Cavitation The formation and collapse of a gas pocket or bubble on the blade of an impeller or the gate of a valve. The collapse of this gas pocket or bubble drives water into the impeller or gate with a terrific force that can knock metal particles off and cause pitting on the impeller or gate surface. 1

2 Comminuter A device used to reduce the size of the solid materials in wastewater by shredding (comminution). The shredding action is like many scissors cutting to shreds all the large solids in the wastewater. Dry Well A dry room or compartment in a lift station, near or below the water level, where the pumps are located, usually next to the wet well. Force Main A pipe that carries wastewater under pressure from the discharge side of a pump to a point of gravity flow downstream. 2

3 Static Head When water is not moving, the vertical distance (in feet) from a reference point to the water surface is the static head. Discharge Head The pressure (in pounds per square inch (psi)) measured at the centerline of a pump discharge and very close to the discharge flange. The pressure is measured from the centerline of the pump to the hydraulic grade line of the water in the discharge pipe. Total Dynamic Head (TDH) When a pump is lifting or pumping water, the vertical distance in feet from the elevation of the energy grade line on the suction side of the pump to the elevation of the energy grade line on the discharge side of the pump. The total dynamic head is the static head plus pipe friction losses. 3

4 Impeller A rotating set of vanes in a pump designed to move water. Pneumatic Ejector A device for raising wastewater, sludge, or other liquid by compressed air. The liquid is alternately admitted through an inward swinging check valve into the bottom of an airtight pot. When the pot is filled, compressed air is applied to the top of the liquid. Stilling Well A well or chamber that is connected to the main flow channel by a small inlet. Waves and surges in the main flow stream will not appear in the well due to the small diameter inlet. The liquid level in the well is measured to determine the flow in the main channel. 4

5 Suction Head The positive pressure (in feet of water or pounds per square inch) on the suction side of a pump. The pressure can be measured from the centerline of the pump up to the elevation of the hydraulic grade line on the suction side of the pump. Suction Lift The negative pressure in feet of water or inches of mercury vacuum on the suction side of a pump. The pressure can be measured from the centerline of the pump down to the elevation of the hydraulic grade line on the suction side of the pump. Volute The spiral-shaped casing that surrounds a pump impeller and collects the liquid discharged by the impeller. 5

6 Water Hammer The sound like someone hammering on a pipe that occurs when a valve is opened or closed rapidly. When a valve position is changed quickly, the water pressure in a pipe will increase and decrease back and forth very quickly. This rise and fall in pressures can cause serious damage to the system. Wet Well A compartment or tank in which wastewater is collected. The suction pipe of a pump may be connected to the wet well or a submersible pump may be located in the wet well. Function of Lift and Pump Stations Lift Station Raises wastewater in a gravity collection system to a higher elevation where it can continue to flow by gravity. Pump Station Provide force necessary for wastewater to flow into and through a force main. 6

7 Type of Lift Stations Manhole Type Precast manhole structure combined with a submersible pump on rails. Manhole acts as the wet well. Submersible pump raises wastewater elevation. Used in small communities with low flows. Type of Lift Stations Wet Well Station Used to lift large volumes of wastewater to significantly higher elevations. Two Configurations: Submersible pumps Above ground pumps Type of Lift Stations Dry Well Station Pumps, motors, and other electrical components are isolated from the wet well in a dry well. Range in size from small prefabricate systems to large built in place stations. 7

8 Wet Well Used to provide a collection point for wastewater before being pumped to a higher elevation. Volume of wet well and rate of inflow will determine the cycle frequency of the pumps. Hydrogen sulfide, methane and other toxic gases may be present in the wet well. Dry Wells Dry section of the lift station used to house equipment required for lift station operation. Typically two levels. Pumps and isolation valves on lower level. Electrical and control systems are on upper level. Bar Racks Installed in older systems to catch large debris that was in the collection system. Modern systems have pumps capable of pumping solids as large as 2.5 inches in diameter. Grinder pumps Comminutors 8

9 Electrical Systems s Metering Disconnect Transfer switch (MCC) PLCs Back-up generator Utility Distribution / 480 V Utility Distribution / 480 V 9

10 Utility Distribution / 480 V Utility Distribution / 480 V Utility Distribution / 480 V 10

11 Utility Distribution / 480 V Utility Distribution / 480 V Utility Distribution / 480 V 11

12 Utility Distribution / 480 V Motors 3 phase alternating current squirrel cage induction motor. Pumps Shaft driven or closecoupled centrifugal pumps are the most common type of lift station pumps. Figure 8.40 pg

13 Valves Valves allow sections of the lift station to be isolated for maintenance. Pump suction valve Pump discharge valve Discharge check valve Cross-connect valve Type: Gate valves Plug Valves Swing check valves Automation Systems Pumps are started and stopped automatically. Level indicators give input: Float Pressure Transducers Ultrasonic Transducers Electrode Pressure (Bubbler) Automation Systems Pumps are started and stopped automatically. Level indicators give input: Float Pressure Transducers Ultrasonic Transducers Electrode Pressure (Bubbler) 13

14 Automation Systems Pumps are started and stopped automatically. Level indicators give input: Float Pressure Transducers Ultrasonic Transducers Electrode Pressure (Bubbler) Automation Systems Pumps are started and stopped automatically. Level indicators give input: Float Pressure Transducers Ultrasonic Transducers Electrode Pressure (Bubbler) Automation Systems Pumps are started and stopped automatically. Level indicators give input: Float Pressure Transducers Ultrasonic Transducers Electrode Pressure (Bubbler) 14

15 Ventilation Installed to keep dry well atmosphere dry and free from toxic gases. Also used to exchange bad air in wet wells with fresh air. Dehumidifiers may be installed to reduce moisture content of air. Lift Station Operational Problems Problems Failure Electrical circuit failures Thermal overload Blown fuse Burned out relay Motor failure Lift Station Operational Problems Control Systems Telemetry system failure Level controller Float Bubbler Electrode 15

16 Lift Station Operational Problems Pumping System Impeller plugged Suction blocked Stuck check valve Broken drive shaft Air bound Packing or seal failure Lift Station Operational Problems Structural Problems Force main plugged Grit deposits in wet well Debris and grease in wet well Corrosion of metal and concrete structural material 16