Suction lift versus NPSH: deciding what to change before commissioning
At a water treatment plant, a centrifugal pump draws clarified water from an open sump whose surface is 2.5 m below the pump centreline, and delivers 90 m³/h. The suction line is 150 mm NB, 9 m long, with two 90° bends, a foot valve and a strainer; at design flow the friction loss in it is calculated as 1.2 m of water column. In the hottest week the water reaches 30 °C, at which the vapour pressure is 4.24 kPa and the density is 996 kg/m³. Site atmospheric pressure is 101.3 kPa and g = 9.81 m/s². The pump curve at 90 m³/h shows NPSH required = 7.5 m. Commissioning is scheduled for that hottest week. The operator has two proposals: throttle the discharge valve to 'ease the load on the pump', and shift the pump 1 m further from the sump to free space on the platform, which the piping engineer says will add 0.35 m to the suction friction loss.
1. Calculate the NPSH available at 90 m³/h and decide whether the pump will cavitate during commissioning.
Read the worked answer
NPSHa = (p_atm − p_v)/(ρg) − static lift − suction friction.
Pressure head available = (101.3 − 4.24) × 10³ / (996 × 9.81) = 97,060 / 9,771 = 9.93 m.
NPSHa = 9.93 − 2.5 − 1.2 = 6.23 m.
NPSH required is 7.5 m, so the margin is 6.23 − 7.5 = −1.27 m. The pump will cavitate at design flow in the hottest week; the symptoms will be rattling, falling head, vibration and eventually impeller pitting near the suction eye.
Note how the 9.93 m is spent: atmosphere and vapour pressure give nearly 10 m of head, and the 2.5 m lift plus 1.2 m friction consume a third of it. The margin is negative even before the pump is asked to do anything useful, which tells you the lift is the problem, not the impeller.
2. Determine the maximum permissible static lift if a 0.5 m margin is required, and explain what that implies for the installation.
Read the worked answer
Require NPSHa ≥ 7.5 + 0.5 = 8.0 m.
Allowable (static lift + suction friction) = 9.93 − 8.0 = 1.93 m.
Friction already accounts for 1.2 m, so maximum static lift = 1.93 − 1.2 = 0.73 m.
The pump currently sits 2.5 m above the water surface, so it must be lowered by about 1.8 m, or the sump level raised by that amount (or some combination of the two).
This is the decisive judgement in the case: even if suction friction were reduced to zero, the allowance is only 1.93 m against a 2.5 m lift. No amount of pipe rework, larger bends or a better strainer can rescue the duty while the pump stays where it is. Enlarging the suction line is worth doing anyway, but it removes 1.2 m at most where 1.8 m is needed.
3. Assess the operator's two proposals, and state the options you would actually put to the plant, with numbers.
Read the worked answer
Throttling the discharge. This moves the operating point to a lower flow, where NPSHr is smaller and suction friction is lower, so the noise may well disappear. But the plant then delivers less than 90 m³/h, the energy is dissipated across the valve, and if the valve is later opened the cavitation returns. It conceals the fault; it does not correct it.
Moving the pump 1 m further. NPSHa = 9.93 − 2.5 − 1.55 = 5.88 m, worse than 6.23 m. Reject it - the platform should be rearranged around the pump, not the other way round.
Cooling the water is a partial help only: at 20 °C, p_v = 2.34 kPa and ρ = 998, giving 10.11 − 3.7 = 6.41 m, still short of 7.5 m.
The workable options are to lower the pump or raise the sump level by about 1.8 m, or to re-select a pump with NPSHr ≤ 5.5 m at 90 m³/h (leaving roughly 0.7 m margin), and preferably to do both. Commissioning should not be attempted at design flow until one of these is done.