Definitions

There are several basic terms and symbols used in discussing pumping hydraulics which should be known and understood by those who must operate and maintain plant pumping facilities. These include:

Acceleration Due to Gravity (g)

The rate at which a falling body gains speed. The acceleration due to gravity is 32 feet/second/second. This simply means that a falling body or fluid will increase the speed at which it is falling by 32 feed/second every second that it continues to fall.

 

 

Cross-sectional Area (A)

The area perpendicular to the flow which the liquid in a channel or pipe occupies.

 

 

Energy

The ability to do work.

Potential Energy: Energy due to the liquid's location or condition.

Kinetic Energy: Energy of motion.

 

 

Flow

The volume or amount of a liquid moving through a channel or pipe. It is measured in million gallons/day, gallons/day, cubic feet/second. In most hydraulics calculations, the flow is expressed in cubic feet/second, cfs. To obtain cubic feet per second when flow is given in million gallons per day multiply by 1.55 cfs/MGD.

 

 

Head

The energy a liquid possesses at a given point or a pump must supply to move a liquid to a given location. Head is expressed in feet.

Cut off head: The head at which the energy supplied by a pump and the energy required to move the the liquid to a specified point are equal and no discharge at the desired point will occur.

Friction head: The amount of energy in feed which is necessary to overcome the resistance to flow which occurs in the pipes and fixtures which the liquid is flowing through.

Pressure head: The vertical distance a liquid can be raised by a given pressure. For example, if a liquid has a pressure of 1 pound per square inch (psi), the liquid will rise to a height of 2.31 feet.

Pump head: The energy in feet which a pump supplies to the fluid

Static head: The energy in feet required to move a fluid from the supply tank to the discharge point

Total head: The total energy in feet required to move a liquid from the supply tank to the discharge point, taking into account the velocity head and the friction head.

Velocity head: The energy in feet required to maintain a given speed in the liquid being moved. If the pump inlet nozzle and discharge nozzle are of equal size, then this term is normally zero.

Suction head: The total head in feet on the suction or supply side of the pump when the supply is located above the center of the pump.

Discharge head: The total head in feet on the discharge side of the pump.

Suction lift: The total head in feet on the suction or supply side of the pump, when the supply is located below the center of the pump.

 

 

Power

Use of energy to perform a given amount of work in a specified length of time. In most cases, this is expressed in terms of horse power.

 

 

Velocity (V)

The speed of the fluid moving through a pipe or channel. It is normally expressed in feet per second (fps).

 

 

Work

Using energy to move an object a distance. It is usually expressed in foot-pounds.

 

 


Centrifugal Pump Terminology

Base Plate

The foundation under a pump. It usually extends far enough to support the drive unit. This base plate may also be referred to as the pump frame.

 

 

Bearings

Devices to reduce the friction and to allow the shaft to rotate easily. Bearings may be ball, roller, or sleeve.

Inboard bearing: In a single suction pump, it is the bearing located farthest from the coupling.

Outboard bearing: In a single suction pump, it is the bearing located nearest the coupling.

Thrust bearing: In a single suction pump, it is the bearing located nearest the coupling.

Radial bearing: In a single suction pump, it is the bearing located farthest from the coupling.

Note: In most cases, where pump and motor are constructed on a common shaft (no coupling), the bearings will be part of the motor assembly.

 

 

Casing

The housing surrounding the rotating element of the pump. In the majority of centrifugal pumps, this casing can also be called the volute.

Split casing: A pump casing which is manufactured in two pieces which are fastened together by means of bolts. Split casing pumps may be vertically (perpendicular to the shaft direction) split, or horizontally (parallel to the shaft direction) split.

Coupling: Device to join the pump shaft to the motor shaft. (Close coupled: Pump and motor are constructed on a common shaft).

Extended shaft: Pump is constructed on one shaft and must be connected to the motor by a coupling.

Frame: The housing which supports the pump bearing assemblies. In an end suction pump, it may also be the support for the pump casing and the rotating element.

Impeller: The rotating element in the pump, which actually transfers the energy from the drive unit to the liquid. Depending on the pump application, the impeller may be open, semi-open, or closed. It may also be single or double suction.

Impeller eye: The center of the impeller, the area which is subject to lower pressures due to the rapid movement of the liquid to the outer edge of the casing.

Prime: Filling the casing and impeller with liquid. If this area is not completely full of liquid, the centrifugal pump will not pump efficiently.

Seals: Devices used to stop the leakage of air into the inside of the casing around the shaft. (Packing: Material which is placed around the pump shaft to seal the shaft opening in the casing and prevent air leakage into the casing).

Stuffing box: The assembly located around the shaft at the rear of the casing. It holds the packing and lantern ring.

Lantern ring: Also known as the seal cage, it is positioned between the rings of packing in the stuffing box to allow the introduction of a lubricant (water, oil, or grease) onto the surface of the shaft to reduce the friction between the packing and the rotating shaft.

Gland: Also known as the packing gland, it is a metal assembly which is designed to apply even pressure to the packing to compress it tightly around the shaft.

Mechanical seal: A device consisting of a stationary element, a rotating element, and a spring to supply force to hold the two elements together. Mechanical seals may be either single or double units.

Shaft: The rigid steel rod which transmits the energy from the motor to the pump impeller. Shafts may be either vertical or horizontal.

Shaft sleeve: A piece of metal tubing placed over the shaft to protect the shaft as it passes through the packing or seal area. In some cases, the sleeve may also help to position the impeller on the shaft.

Shut-off head: The head or pressure at which the centrifugal pump will stop discharging. It is also the pressure developed by the pump when it is operated against a closed discharge valve. This is also known as a cut off head.

Shroud: The metal plate which is used to either support the impeller vanes (open or semi-open impeller) or to enclose the vanes of the impeller (closed impeller).

Slinger: A device to prevent pumped liquids from traveling along the shaft and entering the bearing assembly. Also called a deflector or slinger ring.

Wearing rings: Devices which are installed on stationary or moving parts within the pump casing to protect the casing and/or the impeller from wear due to the movement of liquid through points of small clearances.

Impeller ring: A wearing ring installed directly on the impeller.

Casing ring: A wearing ring installed in the casing of the pump. Also known as the suction head ring.

Stuffing box cover ring: A wearing ring installed at the rear of the impeller in an end suction pump to maintain the impeller clearances and to prevent casing wear.

 

 

Large Volume Pumping

Generally low speed, moderate head, vertically shafted, centrifugal pumps are used for high volume capacity.

 

 

Non-clog Pumping

Specially designed centrifugal pumps using closed impellers with, at most, two to three vanes. Usually designed to pass solids or trash up to 3" in diameter.

 

 

Dry Pit Pump

Depending on the application may be either a large volume pump or a non-clog pump. Located in a dry pit which shares a common wall with the wet well. This pump is normally placed in such a position as to insure that the liquid level in the wet well is sufficient to maintain the pump's prime.

 

 

Wet Pit or Submersible Pump

Usually a non-clog type pump which can be submerged, together with its motor, directly in the wet well. In a few instances, the pump may be submerged in the wet well while the motor remains above the water level. In these cases, the pump is connected to the motor by an extended shaft.

 

 

Underground Pump Stations

Using the wet well-dry well design, the pumps are located in an underground facility. Wastes are collected in a separate wet well, then pumped upward and discharged into another collector line or manhole. This system normally uses a non-clog type pump and is designed to add sufficient head to the waste flow to allow gravity flow to the plant or the next pump station.

 

 

Recycle or Recirculation Pumps

Since the liquids being transferred by the recycle or recirculation pump normally do not contain any large solids, the use of the non-clog type centrifugal pump is not always required. A standard centrifugal pump may be used to recycle trickling filter effluent, return activated sludge, or digester supernatant.

 

 

Service Water Pumps

The plant effluent may be used for many purposes. It can be used to clean tanks, water lawns, provide the water to operate the chlorination system, and to backwash filters. Since the plant effluent used for these purposes is normally clean, the centrifugal pump used closely parallel the units used for potable water. In many cases, the double suction, closed impeller or turbine type pump will be used.