Pump Fundamentals & Components

Pumps are used throughout water and wastewater systems to move liquid, increase pressure, transfer water between elevations, feed treatment processes, and maintain system flow. Operators work with pumps in wells, treatment plants, distribution systems, lift stations, chemical systems, sludge systems, and many other applications.

Understanding basic pump terminology and components helps operators recognize normal operation, identify developing problems, communicate with maintenance staff, and interpret pressure, flow, vibration, and other operating data.

What Does a Pump Do?

A pump adds energy to a liquid.

That added energy may be used to:

• raise water to a higher elevation;
• increase pressure;
• overcome friction loss;
• move liquid through treatment equipment;
• transfer liquid from one tank to another;
• move wastewater through a force main.

A pump does not create water. It transfers energy to the liquid so the liquid can move through the system.

Pump Head

Pump performance is commonly described in terms of head.

Head is hydraulic energy expressed as the height of a column of liquid.

A pump must provide enough head to overcome the system requirements, including:

• elevation difference;
• pressure requirements;
• pipe friction;
• valves and fittings;
• treatment equipment losses.

Flow and Head

Pump flow and pump head are related.

For many centrifugal pumps:

• higher flow is associated with lower available head;
• lower flow is associated with higher available head.

The exact relationship is shown on the pump curve.

Major Pump Categories

Pumps used in water and wastewater systems can generally be grouped into major categories such as:

• centrifugal pumps;
• positive-displacement pumps;
• specialized pumping devices for specific applications.

Operators should understand the operating principle of the pump they are using because different pump types respond differently to changes in system pressure and flow.

Centrifugal Pumps

Centrifugal pumps are common in water and wastewater systems.

They use a rotating impeller to transfer energy to the liquid.

Typical applications include:

• raw-water pumping;
• finished-water pumping;
• booster stations;
• wastewater lift stations;
• return activated sludge;
• process water;
• recirculation.

Positive-Displacement Pumps

Positive-displacement pumps move a defined quantity of liquid during each operating cycle or revolution.

Common types include:

• diaphragm pumps;
• piston pumps;
• progressive-cavity pumps;
• peristaltic pumps;
• gear pumps.

They are often used for:

• chemical feed;
• sludge pumping;
• high-pressure low-flow applications;
• viscous liquids.

Important Difference Between Pump Types

A centrifugal pump generally changes flow significantly as system resistance changes.

A positive-displacement pump tends to continue moving approximately its displacement volume even when discharge resistance increases.

This makes overpressure protection especially important on positive-displacement pump discharge systems.

Suction Side

The suction side is the portion of the pump and piping where liquid enters the pump.

Good suction conditions are critical to pump performance.

Suction problems can result from:

• low source level;
• blocked piping;
• closed or partially closed valve;
• air leaks;
• excessive suction lift;
• high friction loss;
• poor piping arrangement.

Discharge Side

The discharge side is the portion where liquid leaves the pump.

Discharge conditions influence:

• flow;
• pump head;
• system pressure;
• pump operating point.

Restrictions on the discharge side increase system resistance.

Impeller

The impeller is the rotating component in a centrifugal pump that transfers energy to the liquid.

As the impeller rotates, liquid enters near the center and is accelerated outward.

The casing then helps convert part of this velocity energy into pressure.

Impeller Types

Common impeller designs include:

• open impellers;
• semi-open impellers;
• closed impellers.

The appropriate design depends on the liquid and service.

Wastewater pumps may use designs that better tolerate solids and stringy materials.

Impeller Diameter

Impeller diameter affects pump performance.

A larger effective impeller diameter generally allows the pump to develop more head and flow within the pump's design range.

Impellers can sometimes be trimmed to change pump performance.

Operators should use the actual pump curve for the installed impeller diameter.

Pump Casing

The casing surrounds the impeller and directs liquid through the pump.

In a typical centrifugal pump, the casing helps convert velocity into pressure.

Common casing forms include:

• volute casings;
• diffuser arrangements.

Volute

A volute is a gradually expanding passage around the impeller.

As liquid moves through the expanding area, velocity is reduced and pressure increases.

This helps convert the energy added by the impeller into useful discharge head.

Pump Shaft

The shaft connects the pump impeller to the driver, usually an electric motor.

The shaft transmits rotational energy from the motor to the impeller.

Problems involving the shaft can include:

• misalignment;
• bending;
• wear;
• excessive vibration.

Bearings

Bearings support the rotating shaft and allow it to turn with controlled friction.

Bearings may fail because of:

• incorrect lubrication;
• contamination;
• misalignment;
• excessive load;
• vibration;
• overheating;
• normal wear.

Operators should recognize unusual bearing temperature, noise, or vibration.

Mechanical Seals

A mechanical seal helps prevent liquid from leaking along the rotating pump shaft.

Mechanical seals use carefully fitted sealing surfaces.

Problems can result from:

• dry running;
• abrasive material;
• misalignment;
• excessive vibration;
• incorrect installation;
• poor flushing or cooling.

Packing

Some pumps use packing instead of a mechanical seal.

Packing consists of rings of sealing material compressed around the shaft or shaft sleeve.

Unlike a properly operating mechanical seal, packed glands may require a controlled amount of leakage for lubrication and cooling.

Overtightening packing can cause:

• overheating;
• shaft or sleeve wear;
• excessive friction.

Shaft Sleeve

A shaft sleeve can protect the pump shaft from wear where packing or sealing components contact the rotating assembly.

Replacing a worn sleeve may be less expensive than replacing the entire shaft.

Coupling

A coupling connects the pump shaft to the motor shaft.

Couplings may be:

• rigid;
• flexible.

Flexible couplings can tolerate limited small movement or misalignment, but they do not eliminate the need for proper alignment.

Alignment

Pump and motor shafts should be aligned within the requirements of the equipment.

Poor alignment can contribute to:

• bearing wear;
• seal failure;
• coupling wear;
• vibration;
• energy loss.

Motor

The motor is the driver for many pumps.

The motor converts electrical energy into mechanical rotation.

Operators may monitor:

• motor current;
• temperature;
• vibration;
• noise;
• run time;
• starter or drive status.

Motor Current

Motor current can provide useful operating information.

A significant change in current may result from:

• change in pump load;
• flow change;
• mechanical resistance;
• electrical problem;
• pump blockage.

Current should be interpreted together with flow, pressure, and equipment condition.

Baseplate

Horizontal pumps and motors are often mounted on a common baseplate.

The baseplate provides structural support and helps maintain alignment.

Loose mounting hardware or poor foundation conditions can contribute to vibration and misalignment.

Foundation

A stable foundation supports the pump and driver.

Foundation problems can contribute to:

• movement;
• vibration;
• misalignment;
• piping stress.

Suction Piping

Suction piping should deliver liquid to the pump with minimal unnecessary loss and disturbance.

Good suction piping generally avoids:

• unnecessary restrictions;
• air pockets;
• excessive fittings;
• poor reducer orientation;
• leaks that allow air entry.

Discharge Piping

Discharge piping carries liquid from the pump into the system.

It may include:

• isolation valve;
• check valve;
• pressure gauge;
• flow meter;
• control valve.

The arrangement depends on the installation.

Check Valve on Pump Discharge

A check valve is commonly installed on pump discharge piping to reduce reverse flow when the pump stops.

Poor check-valve operation can lead to:

• reverse flow;
• water hammer;
• pump reverse rotation;
• pressure surges.

Isolation Valves

Isolation valves allow a pump to be separated from the system for maintenance.

Operators should know the normal valve position and follow approved startup and shutdown procedures.

Pressure Gauges

Pressure gauges may be installed on:

• pump suction;
• pump discharge.

These readings can help operators evaluate:

• pump head;
• suction conditions;
• system resistance;
• abnormal restrictions.

Flow Meters

Flow measurement helps operators verify actual pump output.

Pump flow should be compared with:

• historical performance;
• pump curve;
• system conditions;
• expected process demand.

Prime

Some centrifugal pumps must be filled with liquid before they can pump effectively.

This is called priming.

If air remains in the pump casing or suction line, the pump may fail to develop normal flow.

Self-Priming Pumps

A self-priming pump is designed to handle a certain amount of air and restore pumping after proper initial filling and setup.

Self-priming does not mean the pump can operate indefinitely without liquid.

Flooded Suction

A pump has flooded suction when the liquid source level is above the pump suction inlet.

This generally provides more favorable suction conditions than requiring the pump to lift liquid from below its elevation.

Suction Lift

Suction lift occurs when the pump must draw liquid from a source located below the pump.

Suction lift is limited by:

• atmospheric pressure;
• liquid vapor pressure;
• friction loss;
• elevation;
• pump suction requirements.

These concepts become especially important when evaluating cavitation and NPSH.

Vertical Pumps

Vertical pumps are used in applications such as:

• wells;
• wet wells;
• clearwells;
• intakes.

The motor may be located above the liquid while the pumping element is lower in the structure.

Submersible Pumps

Submersible pumps operate with the pump and motor submerged in the liquid.

They are common in wastewater lift stations.

Operator concerns may include:

• seal condition;
• moisture alarms;
• motor temperature;
• cable condition;
• clogging;
• guide-rail or lifting systems.

Dry-Pit Pumps

Dry-pit pumps are installed in a dry space adjacent to the wet well or process liquid.

This allows easier physical access to the pump and motor but requires reliable separation from the liquid area.

Progressive-Cavity Pumps

A progressive-cavity pump is a positive-displacement pump that uses a rotating helical rotor inside a stator.

It is commonly used for:

• sludge;
• viscous liquids;
• polymer;
• other difficult-to-pump materials.

Running some progressive-cavity pumps dry can rapidly damage the stator.

Diaphragm Pumps

Diaphragm pumps use a flexible diaphragm to move liquid.

They are common in chemical-feed applications.

Important components may include:

• diaphragm;
• suction check valve;
• discharge check valve;
• stroke or speed adjustment.

Peristaltic Pumps

Peristaltic pumps move liquid by compressing flexible tubing with rotating rollers or shoes.

The liquid contacts only the tubing.

They may be used for:

• chemical feed;
• sampling;
• slurries;
• applications where contamination control is useful.

Tubing condition is an important maintenance consideration.

Air-Operated Diaphragm Pumps

Air-operated double-diaphragm pumps use compressed air to move flexible diaphragms.

They can handle:

• slurries;
• chemicals;
• liquids containing solids;
• portable pumping applications.

Air supply quality and diaphragm condition affect performance.

Pump Nameplate

The pump or motor nameplate provides important equipment information.

Depending on the equipment, it may include:

• manufacturer;
• model;
• serial number;
• motor horsepower;
• voltage;
• current;
• speed;
• other ratings.

Nameplate information should be preserved in asset records.

Pump Curve

A pump curve shows pump performance at different operating conditions.

Common information may include:

• flow;
• head;
• efficiency;
• horsepower;
• NPSH required.

The pump curve is one of the most useful tools for understanding centrifugal-pump performance.

Best Efficiency Point

The Best Efficiency Point, or BEP, is the region where a centrifugal pump operates at or near its highest hydraulic efficiency.

Operating far from BEP can contribute to:

• vibration;
• recirculation;
• seal wear;
• bearing stress;
• reduced efficiency.

Pump Operating Point

The actual operating point of a centrifugal pump occurs where the pump curve and system curve intersect.

The operating point changes when:

• system resistance changes;
• pump speed changes;
• impeller diameter changes;
• system configuration changes.

Pump Efficiency

Pump efficiency describes how effectively input mechanical energy is converted into useful hydraulic energy.

Low efficiency can increase operating cost.

Efficiency can be affected by:

• pump selection;
• operating point;
• wear;
• clearances;
• impeller condition;
• system resistance.

Common Signs of Pump Problems

Operators should investigate changes such as:

• reduced flow;
• reduced discharge pressure;
• higher or lower motor current;
• unusual vibration;
• unusual noise;
• seal leakage;
• bearing temperature increase;
• frequent starts and stops;
• failure to prime;
• repeated trips or alarms.

Do Not Assume Every Pump Problem Is Inside the Pump

Abnormal pump performance can be caused by system conditions.

Examples include:

• closed valve;
• blocked suction;
• dirty strainer;
• low source level;
• increased system resistance;
• air in the suction line;
• incorrect valve configuration.

Operators should evaluate the pump and the connected hydraulic system together.

Pump Rotation

Centrifugal pumps are designed for a specified direction of rotation.

Incorrect rotation can cause:

• low flow;
• low head;
• poor efficiency.

Rotation should be verified after motor wiring or maintenance when required by procedure.

Dry Running

Many pumps rely on the pumped liquid for cooling, lubrication, or sealing.

Dry running can damage:

• mechanical seals;
• progressive-cavity stators;
• pump components;
• bearings in certain designs.

Operators should understand whether the installed pump can tolerate dry operation.

Pump Cycling

Frequent pump starts and stops can increase wear on:

• motors;
• starters;
• couplings;
• check valves;
• mechanical components.

Excessive cycling may indicate:

• incorrect level-control settings;
• insufficient storage volume;
• control problems;
• pump sizing issues.

Standby Pumps

Critical systems often include standby pumps.

A standby pump should be:

• available;
• tested;
• maintained;
• capable of starting when needed.

A standby pump that has not been exercised or maintained may fail during an emergency.

Lead and Lag Pumps

Multiple-pump stations may use lead and lag control.

The lead pump starts first.

A lag pump may start when:

• demand increases;
• level rises;
• pressure falls;
• additional capacity is needed.

Control systems may rotate lead duty to balance equipment run time.

Common Pump Terminology Mistakes

• Confusing pump head with pressure only.
• Assuming a pump always produces its rated flow regardless of system conditions.
• Ignoring suction conditions when troubleshooting low flow.
• Assuming a flexible coupling eliminates the need for alignment.
• Overtightening packing to stop all leakage.
• Running pumps dry when the design does not permit it.
• Ignoring motor current, vibration, and bearing temperature trends.
• Assuming all pump types respond the same way to a closed discharge valve.
• Ignoring check-valve condition.
• Assuming standby equipment is reliable without testing it.

A Practical Pump Inspection Sequence

1. Confirm the correct pump is operating.
2. Check suction and discharge valve positions.
3. Review suction and discharge pressure.
4. Review flow.
5. Listen for abnormal noise.
6. Observe vibration.
7. Check seal or packing condition.
8. Check bearing temperature where appropriate.
9. Review motor current and alarms.
10. Compare current operation with normal historical conditions.
11. Document abnormal findings.

What to Remember for the Exam

• A pump adds energy to liquid so it can overcome elevation, pressure, and system resistance.
• Centrifugal pumps use a rotating impeller to transfer energy to the liquid.
• Positive-displacement pumps move a defined volume during each cycle or revolution.
• The suction side is where liquid enters the pump.
• The discharge side is where liquid leaves the pump.
• The impeller is the primary rotating hydraulic component of a centrifugal pump.
• The casing directs flow and helps convert velocity into pressure.
• The shaft transmits rotational energy from the driver to the impeller.
• Bearings support the rotating shaft.
• Mechanical seals and packing control leakage around the shaft.
• Packing may require controlled leakage for lubrication and cooling.
• The coupling connects the motor and pump shafts but does not replace proper alignment.
• Suction restrictions can reduce pump performance and contribute to cavitation.
• A pump curve relates pump head to flow and may also show efficiency, horsepower, and NPSH required.
• The centrifugal-pump operating point occurs near the intersection of the pump curve and system curve.
• BEP means Best Efficiency Point.
• Submersible pumps are common in wastewater lift stations.
• Positive-displacement pumps generally require protection against excessive discharge pressure.
• Abnormal pump performance may result from the connected system rather than the pump itself.
• Operators should monitor flow, pressure, vibration, noise, leakage, temperature, current, and alarms for changes.