Most undersized pumps are bought on engine horsepower alone. Horsepower is the least useful number on the label. What decides whether a pump will do your job is head and discharge, and those two are linked — as head rises, discharge falls.
Here is how to work it out before you buy.
The three numbers that matter
Suction head is the vertical distance from the water surface up to the pump. This is the one with a hard physical limit: no pump can suck water higher than about ten metres in theory, and in practice around seven metres is a realistic maximum, less at altitude or with a long suction pipe. If your water is deeper than that, a surface pump is the wrong machine and you need a submersible.
Delivery head is the vertical distance from the pump up to the highest point the water has to reach.
Total head is suction plus delivery plus friction losses in the pipe. Friction loss is the part people forget: long pipe runs, narrow pipe, bends and fittings all consume head. A pump that is exactly adequate on paper will disappoint once the pipe is laid.
Discharge is how much water the pump moves, usually in litres per minute. Manufacturers quote maximum discharge, which occurs at minimum head. At your actual working head, you will get less.
Work it out in this order
- Measure the vertical drop from the pump to the water surface. That is your suction head. If it is more than about seven metres, stop — you need a different type of pump.
- Measure the vertical rise from the pump to the highest delivery point.
- Note the total pipe length and diameter, and count the bends.
- Add suction, delivery and an allowance for friction to get total head.
- Decide the discharge you need from your irrigation method and the area you want to cover in a session.
- Choose a pump rated to deliver that discharge at that head, not at zero head.
Inlet and outlet size
Pump size is usually quoted by the port diameter — a two-inch or three-inch pump, for example. A larger port moves more water at lower pressure; a smaller port generally produces higher pressure at lower volume.
Match your pipe to the pump ports. Fitting narrow pipe to a larger pump throttles it and wastes the capacity you paid for.
Matching the pump to the irrigation method
- Flood or furrow irrigation — high discharge, low head. A larger port pump suits this.
- Sprinkler — needs pressure at the nozzle as well as volume, so total head must include the operating pressure the sprinklers require.
- Drip — lower discharge but steady pressure, and filtration matters more than raw output.
- Filling a tank — work from the tank height and how quickly you need it full.
Practical points that affect performance
- Keep the suction pipe short and straight. Every bend costs head.
- Use a foot valve and strainer so the pump holds its prime and debris stays out.
- Prime the pump before starting. Running a centrifugal pump dry damages the seal quickly.
- Keep the suction pipe airtight. A pinhole leak on the suction side stops a pump from priming entirely, and it is the most common fault we are asked about.
- Site the pump as close to the water as you can — reducing suction head is the cheapest performance gain available.
Petrol engine care
Check the engine oil before every use, keep the air filter clean, and use fresh fuel. Before storing the pump, drain the pump casing completely so nothing freezes, corrodes or leaves sediment inside, and drain the fuel if it will stand for months.
Two mistakes worth avoiding
Buying on horsepower. Two pumps with the same engine can have very different head and discharge curves depending on the impeller.
Ignoring friction loss. If your pipe run is long, add a genuine allowance. It is the difference between a pump that just manages and one that works comfortably for years.
See the agriculture water pump range and the petrol water pump models, or send us your suction height, delivery height and pipe length and we will tell you which model fits.