Pumps are the backbone of the collection system. A basic knowledge and understanding of how these devices work can help optimize your collection system.
1. They may be classified by the character of the material handled – raw wastewater, grit, effluent, activated sludge, raw sludge, or digested sludge.
2. They may be classified by pumping conditions – high lift, low lift, recirculation, or high capacity.
3. They may be classified by the principle of operation – centrifugal, propeller, reciprocating, incline screw, progressive cavity, or pneumatic ejector.
Determining the proper pump for the proper application can be a very critical part of designing or maintaining proper flow throughout the collection system.
1. Centrifugal pumps: These pumps have an impeller (paddle-wheel type piece) rotating in a casing. The impeller is supported on a shaft which is then supported by bearings. Liquid enters the casing through the eye (at the center) of the impeller. It is then picked up by the curved impeller vanes and by the rotation of the impeller and is thrown out by the centrifugal force into the discharge. Impellers usually have large openings at their center to prevent clogging. A screen should be used at the intake end of the suction piping to prevent clogging. Impellers may be in closed casings or they may be open if the pump is submersible and being used to pump wastewater from lift station wet wells. The motor or drive mechanism can be connected directly to the shaft or connected by a coupling flange depending upon the application. These pumps should be replaced when groves appear on the shaft. Also when these pumps gradually begin pumping at lower flow rates over a period of time, the impellers could be wearing down.
a. Shaft sleeves are used to cover the shaft that supports the impeller to protect the shaft from the corrosive and abrasive effects of the liquid going through the pump. The sleeves are mounted to the shaft on ball or roller bearings.
b. Wearing rings are used to plug the space between the impeller and the casing to prevent internal liquid leakage. These rings are either attached to the casing, the impeller, or both. Wearing rings should be inspected regularly and replaced when serious wear or leakage is observed. Since water is the lubricant between the rings and the impeller, a pump should never be allowed to run dry.
c. Stuffing boxes are used to prevent air from being sucked into the pump. Air affects the efficiency of the pump and could cause it to lose prime. It consists of a casing containing rings of packing and a gland or membrane at the outside end. Water is used in the stuffing box to block out the intake of air and to lubricate the packing. The water is brought into a seal cage in the center of the stuffing box under pressure by connector piping to a point near the impeller rim provided it is clean liquid. If the liquid being pumped contains grit or other solids, it may be necessary to use potable water to provide the seal. To prevent the possibility of a cross-connection, the connection with the potable water supply must include either an air gap separation or an approved backflow preventer to avoid contamination. Back flow preventers should stop the accidental backflow or reverse flow of wastewater into buildings.
d. The end gland or membrane is used to control liquid flow from the stuffing box. The gland should be tightened just enough such that a thin stream of water flows from the stuffing box. Excessive leakage is indicative of the need to replace the packing.
e. Lantern rings provide the water seal connection between the water supply line and the stuffing box. When packing is being replaced, the lantern ring should be completely filled with grease (if grease seals are used) before all the rings of packing are in place.
f. The efficiency of centrifugal non-clog pumps starts at zero at shut-off and increases rapidly until a peak is reached at approximately the mid-point of the overall capacity range of the pump. Therefore, for peak efficiency, best mechanical performance, and quietest operation; a pump should be selected so that the range of operation will be at the mid-point of the total pump curve. They should be operated near their rated heads (pressure). Otherwise, the pump is apt to operate under unsatisfactory and unstable conditions which reduce the efficiency and operating life of the unit.