Pump installation: what to get right before the first start

A pump can only deliver what the suction side gives it. The discharge side moves the liquid; the suction side decides whether the pump gets liquid at all. When an installation fails early, the cause is usually here, so this is where the design effort belongs.

Keep the suction line short, straight and at least the size of the pump inlet. Every bend, valve and metre of pipe on the suction side spends pressure the pump needs. Where the pipe reduces to meet the inlet, use an eccentric reducer with the flat side up, so air cannot collect in a pocket against the pump. A concentric reducer on a horizontal suction line leaves a trapped air space that will find its way into the impeller.

The number that matters is the margin between the suction pressure available (NPSHa) and what the pump requires at the duty point (NPSHr). NPSHa must exceed NPSHr, with margin. When it does not, the liquid boils at the impeller eye and you get cavitation, which pits the impeller, fails seals and puts vibration into the pipework. In practice the failure is almost always a suction tank that is too small or a suction head that is too small, not an error in the arithmetic.

A flooded suction, with the liquid level above the pump, is the arrangement to fight for. A suction lift can work, but the pump must be primed and must stay primed, which adds a foot valve or a self-priming design and one more thing to go wrong. If the layout forces a lift, keep it as small as the site allows.

The pump sits on its base; the pipework must not sit on the pump. Support the suction and discharge lines independently, close to the pump, so the flanges meet the pump square and unloaded. Pipework pulled into line by the flange bolts puts a permanent load through the casing and bearings. The result is misalignment, bearing wear and seal failure, and it will read as a pump fault when it is an installation fault.

The base itself should be rigid and level. For a bare-shaft pump on a baseplate with a coupled motor, the coupling must be aligned after the baseplate is bolted down and again after the pipework is connected, because both steps move it. A close-coupled pump, with the impeller on the motor shaft, removes site alignment entirely, which is a real argument for it on smaller duties.

Leave room to work. A pump that cannot be reached for a seal change gets a seal change late. Space around the pump set costs floor area once; the lack of it costs on every intervention for the life of the installation.

Fit isolation valves on both sides of the pump, so it can be removed without draining the system. On the discharge, fit a non-return valve between the pump and its isolation valve to stop reverse flow and reverse rotation when the pump stops. A pressure gauge on the discharge, and ideally one on the suction, turns every later fault call from guesswork into a reading.

Never throttle on the suction side. Flow control belongs on the discharge; a part-closed suction valve starves the pump and causes the cavitation described above. On duty and standby sets, each pump gets its own isolation and non-return valves, so the standby can run while the duty pump is out.

Where valves close quickly, or the delivery main is long, consider water hammer at the design stage. A pressure wave from a fast-closing valve or a stopping pump can burst joints and split pipework, and the fix is cheaper on the drawing than in the ground.

The electrical installation is a job for a competent electrician working to the motor manufacturer's data, and nothing here replaces that. What the pump side of the job needs from it is specific: overload protection set to the motor's full-load current, and a check of rotation direction before the pump runs against liquid. A three-phase motor runs equally happily backwards, and a centrifugal pump running backwards delivers little flow while looking otherwise normal.

Protect the pump against running dry. A mechanical seal needs liquid to lubricate it; take the liquid away and the seal goes. On a tank or sump duty, a float switch cutting the pump out at low level is the simplest protection. On a pressurised system, a pressure switch or a flow switch does the same job. The cost of the switch is trivial against the cost of the seal change it prevents, and on some pump types dry running is worse than a seal: a progressive cavity pump run dry burns through its stator and damages the rotor.

Where the duty varies, a variable speed drive controls the pump against pressure or flow rather than by throttling. It is not worth fitting on a fixed duty, but on variable-demand systems it saves the energy a control valve would burn.

The first run proves the installation, and it proves it in order.

The most common fault we see was made before anything arrived on site. The customer states a flow. The consultant adds margin. The contractor adds margin, and the installer rounds up again. Nobody in that chain is careless, and the pump that comes out of it is far too big for the duty.

An oversized centrifugal pump runs off the end of its performance curve, starved of the back pressure it was selected for, and cavitates. It also costs more to run for as long as it is installed: power rises with flow and head, so a 3 kW application ends up carrying an 11 kW motor. Size the pump for the duty the system actually has, and resist the temptation to add safety through size. If the duty genuinely grows later, that is what a second pump, or a correctly re-selected one, is for.

If the duty is unusual, the medium is difficult, or the suction layout is tight, get the selection checked before the order goes in. Send us the flow, head, medium and the suction arrangement, and we will tell you what the duty needs.