Pump Talk


Centrifugal pump curves show ‘pressure’ as head, which is the equivalent height of water with S.G. = 1.

This makes allowance for specific gravity variations in the pressure to head conversion to cater for higher power requirements. Positive Displacement pumps use pressure (i.e. psi or kPa) and then multiply power requirements by the S.G.


The vertical height difference from surface of water source to centreline of impeller is termed as static suction head or suction lift (‘suction lift’ can also mean total suction head).

The vertical height difference from centreline of impeller to discharge point is termed as discharge static head.

The vertical height difference from surface of water source to discharge point is termed as total static head.


Total height difference (total static head) plus friction losses & ‘demand’ pressure from nozzles etc. i.e. Total Suction Head plus Total Discharge Head = Total Dynamic Head.


Nett positive suction head — related to how much suction lift a pump can achieve by creating a partial vacuum. Atmospheric pressure then pushes liquid into pump. A method of calculating if the pump will work or not. (more)


Specific gravity. Weight of liquid in comparison to water at approx 20 deg c (SG = 1).


A number which is the function of pump flow, head, efficiency etc. Not used in day-to-day pump selection, but very useful as pumps with similar specific speed will have similar shaped curves, similar efficiency / NPSH / solids handling characteristics.


If the vapour pressure of a liquid is greater than the surrounding air pressure, the liquid will boil.


A measure of a liquid’s resistance to flow. For example, how thick it is. The viscosity determines the type of pump used, the speed it can run at, and with gear pumps, the internal clearances required.


The amount of pressure / head required to ‘force’ liquid through pipe and fittings.


The pound per square inch or, more accurately, pound-force per square inch (symbol: psi or lbf/in2 or lbf/in2 or lbf/sq in or lbf/sq in) is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch:

1 psi approximately equals 6,894.757 Pa, where pascal (Pa) is the SI unit of pressure


The pascal (symbol: Pa) is the SI derived unit of pressure, It is a measure of force per unit area, defined as one newton per square metre. In everyday life, the pascal is perhaps best known from meteorological barometric pressure reports, where it occurs in the form of hectopascals (1 hPa ≡ 100 Pa) or kilopascals (1 kPa ≡ 1000 Pa).[1] In other contexts, the kilopascal is commonly used, for example on bicycle tire labels.[2] One hectopascal corresponds to about 0.1% and one kilopascal to about 1% of atmospheric pressure (near sea level). One hectopascal is equivalent to one millibar; one standard atmosphere is exactly equal to 1013.25 hPa.

Centrifugal pump.

A centrifugal pump is a rotodynamic pump that uses a rotating impeller to increase the pressure of a fluid. Centrifugal pumps are commonly used to move liquids through a piping system. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from where it exits into the downstream piping system.[1][2] Centrifugal pumps are used for large discharge through smaller heads.

Centrifugal pumps can further be classified as

  • end suction pumps
  • in-line pumps
  • double suction pumps
  • vertical multistage pumps
  • horizontal multistage pumps
  • submersible pumps
  • self-priming pumps
  • axial-flow pumps
  • regenerative pumps

Positive Displacement Pumps

The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. The positive displacement pump delivers a constant volume of liquid for each cycle against varying discharge pressure or head.

The positive displacement pump can be classified as:

  • Reciprocating pumps — piston, plunger and diaphragm
  • Power pumps
  • Steam pumps
  • Rotary pumps — gear, lobe, screw, vane, regenerative (peripheral) and progressive cavity

Submersible Pumps

A submersible pump (or electric submersible pump (ESP)) is a device which has a hermetically sealed motor close-coupled to the pump body. The whole assembly is submerged in the fluid to be pumped. The main advantage of this type of pump is that it prevents pump cavitation, a problem associated with a high elevation difference between pump and the fluid surface. Submersible pumps push water to the surface as opposed to jet pumps having to pull water. Submersibles are more efficient than jet pumps. Get more information from our online pump shop.


Q1: What air pressure charge should I have in my pressure tank?

A1: The rule of thumb is 2 PSI (15-20 KPA) lower than the pumps cut in pressure.  You must exhaust out all the water out of the tank before apply air to the tank.

Q2: My Pressure tank feels like it is water logged.

A2: The internal bladder of the pressure tank has spilt.  The tank will need to be replaced.

Q3. My water supply pump keeps cycling when no apparent water is being used.

A3: There could be numerous problems associated to this question.

  1. If the pump has a pressure tank, the pressure tank could have a spilt bladder and become water logged or the tank has the incorrect air pressure charge.
  2. The pump’s non-return valve / foot valve has either failed or has debris stuck in it causing the valve not to seat properly.
  3. There could be a leak on the pumps suction line or discharge line.
Q4. My pump is very noisy when running and has a high pitch squeal.

A4.  The motor bearings need replacing ASAP.

Q5. My pump is leaking between the pumps housing and the front plate on the motor and there is a pool of water forming around it

A5. The pumps mechanical seal is damaged or worn and needs replacing due to the pump running dry or general mechanical wear

Q6. My pump will not get up to pressure and switch off

A6. There can be numerous problems here

  1. The pump could be getting staved of water.
  2. The pump impeller/s could be blocked or could have run dry at same stage and melted the vein of the impeller/s closed
  3. The pumps pressure switch / Flow controller could have failed
  4. You have a major water leak somewhere
Q7. My pump is making a humming noise and will not start.

A7.  There are a few problems associated with this

  1. The pumps motor start capacitor could have failed
  2. The pump has run dry and melted the internals of the pump
  3. The motor bearings could have collapsed
Q8. How do I set the correct operating pressure of my pump

A.8 You should always refer back to the pumps specifications which will be available from the wholesaler

Q9. My Pump does not prime or loses it prime after running for a short time

A9. There are a number of factors that can cause your pump not to prime

  1. Suction lift too great for the pump
  2. Suction inlet or strainer is blocked
  3. Suction line has an air leak
  4. Suction hose collapsed
  5. Mechanical seal has failed allowing air to be drawn into pump
  6. Obstruction in pumps impeller or casing
Q10. My pump is vibrating and or overheating

A10. There are a number of factors that can cause your pump to overheat or vibrate

  1. Obstruction in the pumps’ impeller / casing
  2. Impeller is damaged
  3. Cavitation due to excessive suction lift / friction loss
Q11. My submersible pump is not switching on or turning off

A11. The float switch on the pump is faulty

Q12. What is the best brand of submersible pump

A12. There are a lot of pump manufactures in the market today.  They will all say they are the best.  The leading brands are.

  1. Tsurumi
  2. Grundfos
  3. Zenit
  4. Lowara
  5. Ultra Flow
  6. DAB
Q13. How long do motors last?

A13. Motors typically last an average of eight years before needing either rebuilding or replacing. Noisy, screeching front and/ or rear bearings will let you know when you need to do something.

Q14. How do I know what Horsepower/ Kilowatt my pump motor is?

The horsepower should be listed on the nameplate of the pump motor (in very tiny letters — hp). If the motor nameplate is burnt or worn off, sometimes a part number of the impellers can tell us which hp your pump motor is.  

If the existing pump has done you well, it is easiest to plumb and wire with the exact same pump. The heights and lengths are the same, which makes the job a bit simpler

If you have any other question you would like to ask are expert staff please send use an enquiry to info@alphapms.com.au

Contact us

Phone: 07 5534 6355
Fax: 07 5534 6288


33 Currumbin Creek Road,

Currumbin QLD 4223

Electrical Contractor
License Number: 72379

QBCC - 15091588


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