Submersible Pump Glossary
Flow rate, head, dynamic water level, stages, flow sleeves and more. Short, clear definitions of the technical terms you will meet when selecting a submersible pump and submersible motor.
Hydraulic terms
Flow rate (Q)
The volume of water a pump delivers per unit of time. For submersible pumps it is usually expressed in m³/h (cubic metres per hour).
Head (H)
The energy the pump adds to the water, expressed in metres of water column. Pump curves show the relationship between flow rate and head.
Total dynamic head
The total height the pump must actually overcome: dynamic water level + discharge height above ground + pipe friction loss + required outlet pressure. See the total dynamic head calculation guide.
Friction loss
The head lost as water moves through pipes, bends, valves and fittings. It rises steeply with flow velocity.
Duty point
The flow and head at which the pump curve meets the system requirement. The pump is selected for this point.
Pump curve (Q-H curve)
A curve showing the head a pump produces at different flow rates. As flow increases, head decreases.
Best efficiency point
The flow rate at which the pump runs at its highest efficiency. Operating far from this point increases energy consumption and shortens service life.
Stage
A hydraulic unit made up of one impeller and one diffuser. Submersible pumps are multistage; more stages produce more head. See the pump stages guide.
Impeller
The rotating part that transfers energy from the motor shaft to the water.
Diffuser
The stationary part that converts the velocity of the water leaving the impeller into pressure and guides it to the next stage.
Non-return valve
A valve that allows flow in one direction only. It prevents backflow and reverse rotation of the pump when it stops.
Water hammer
A sudden pressure surge in the pipeline caused by flow starting or stopping abruptly. Soft starters, variable frequency drives and non-return valves reduce the risk.
Bar and metres of water column
Units of pressure. One bar is approximately 10.2 metres of water column.
Well terms
Static water level
The distance from ground level to the water surface when the well is at rest and no pump is running.
Dynamic water level
The depth at which the water level settles during continuous pumping. Pumps are sized on the dynamic level, not the static level.
Drawdown
The difference between the static and dynamic water levels, determined by a pumping test at the intended flow rate.
Well yield
The flow a well can sustain continuously at an acceptable drawdown. A pump can never sustainably deliver more than the well produces.
Pumping test
A test that measures how far the water level drops at given flow rates. It is the only reliable source for the dynamic water level.
Well casing
The pipe that supports the borehole wall. Its internal diameter sets the largest pump and motor that will fit.
Well screen
The slotted section of the casing through which water enters the well. The pump position relative to the screen affects motor cooling.
Rising main
The pipe that carries water from the pump to the well head. The pump hangs from it.
Well head
The cover, connections and valves at the top of the well at ground level.
Sand separator
Equipment that removes sand from the pumped water. It extends pump life in sandy wells.
Motor and electrical terms
Submersible motor
An electric motor that runs submerged in the well and drives the pump.
Water-filled (water-cooled) motor
A submersible motor filled with water and cooled by the well water flowing past it. FANPO manufactures water-cooled submersible motors.
Rewindable motor
A motor whose winding can be rewound after a burnout. The cause of the failure must always be found before rewinding.
Flow sleeve
A tube fitted around the motor that forces water to flow along the motor surface. It is needed in wide casings and when the pump is set below the screen.
Annular velocity
The velocity of the water passing between the motor and the casing wall. The minimum required for cooling is stated on the motor datasheet.
Drop cable
The power cable that runs down the well from the control panel to the motor.
Cable cross-section
The conductor area of the cable in mm². It is selected according to motor current, cable length and supply voltage.
Voltage drop
The loss of voltage along a long cable before it reaches the motor. It makes the motor draw more current and run hotter.
Direct-on-line starting
Starting the motor directly at full voltage. Common at lower ratings.
Star-delta starting
A starting method in which the motor starts in star connection and then switches to delta to reduce starting current.
Soft starter
A device that ramps up motor voltage during starting to reduce mechanical and electrical stress.
Variable frequency drive
A device that adjusts motor speed by changing the supply frequency. Motor cooling must be checked at reduced speed.
Dry-run protection
Protection that stops the pump when the well runs out of water. One of the most valuable protection devices on a deep well installation.
Phase failure relay
A relay that stops the motor on phase loss, wrong phase sequence or phase imbalance.
Overload relay
Protection that opens the circuit when the motor draws more than its rated current for a sustained period. It is set to the motor rated current.
Axial thrust
The force along the shaft axis generated by the pump and carried by the thrust bearing of the motor. It should be checked at high stage counts.
HP and kW
Units of power. One HP is approximately 0.746 kW.
50 Hz and 60 Hz
Supply frequencies. A motor runs roughly 20 per cent faster on 60 Hz, which changes pump performance. FANPO motors are supplied for 50 Hz / 380 V and 60 Hz / 460 V.
Material terms
AISI 304 stainless steel
A corrosion-resistant stainless steel grade. The body, impeller and diffuser of FANPO stainless steel submersible pumps are made from it.
Cast iron
A robust, thick-walled and cost-effective body material. FANPO cast iron submersible pumps are produced in 5″, 6″, 7″ and 8″ sizes.
Corrosion
The chemical degradation of metal caused by water chemistry. Chloride, low pH and dissolved gases accelerate it.
Abrasion
Mechanical wear of hydraulic surfaces by abrasive particles such as sand. Material choice alone does not solve it.
Sand-channel rubber bearings
A grooved rubber bearing design that helps flush sand away from the bearing surfaces. In FANPO stainless steel pumps it contributes to safe operation in sandy wells.
Related guides
See the terms in practice
The submersible pump selection guide shows how these terms are used to select a pump, and the motor cooling and cable sizing guide covers the motor and cable terms.
Prepared by the FANPO engineering team · Last updated: 8 November 2026