Rotary lobe pump curve
A rotary lobe pump curve plots flow against pump speed at a stated differential pressure, not head against flow. Because the pump displaces a fixed volume per revolution, speed sets the flow. The chart adds absorbed power and NPSHr at the same speed, and shows how much flow slip costs you as pressure rises.
In more detail
If you have arrived from a centrifugal, the chart looks wrong because it describes a different machine. A centrifugal pump's flow moves significantly as the pressure it works against changes, and its performance curve exists to tell you by how much. A rotary lobe pump holds its rate to within a few per cent and takes the pressure change on the motor instead, for as long as the motor has the power. There is no head–flow line to read, because there is no meaningful trade between head and flow. What varies is speed, so speed is the axis. The mechanism behind that behaviour is covered in how a rotary lobe pump works, and the centrifugal chart this one replaces in how to read a pump curve.
Speed is the axis, and flow follows it
A rotary lobe pump is specified by its displacement: the volume it sweeps per revolution. Alfa Laval's SRU range runs from 0.053 to 3.53 litres per revolution across its pump heads, and the manufacturer's own technical data states capacity as m³/h at 1000 rpm. There is no head–flow line anywhere in it. Multiply displacement by speed and you have the theoretical flow. Read the chart the same way round: find the flow you need, drop down to the speed that delivers it, then justify that speed against the medium.
The speed ceiling moves across a range, too. Alfa Laval rates the smaller SRU pump heads to 1000 rpm and the largest to 600 and 500 rpm. A bigger pump is a slower pump. Some of the flow you gain from the extra displacement is given straight back in the speed you lose.
Slip is the gap between the calculation and the line
The rotors never touch each other or the casing. That leaves a clearance, and some medium leaks back through it from discharge to suction. The leak-back is slip. It is why measured flow sits below the displacement calculation, and it is the only reason the line on the chart bends at all.
Slip rises as differential pressure rises, because pressure is what drives medium back through the gap. It falls as viscosity rises, because a thick medium seals the clearance. That is what catches people coming off a centrifugal: a rotary lobe pump moves a thin liquid less efficiently than a thick one. You can look the clearances up. Alfa Laval's SRU instruction manual, which sits with the leaflet on our SRU pump page, lists rotor front, back, radial and mesh clearances in millimetres, given separately for each rotor temperature rating, because a rotor built for 200°C is machined to a different clearance from one built for 70°C.
Differential pressure in bar, not head in metres
A centrifugal curve is drawn in metres of head. A rotary lobe pump's data is quoted as differential pressure in bar, the difference between suction and discharge. Each line on the chart belongs to one differential pressure, so check which one you are reading before you take a flow off it.
The pump's maximum differential pressure is a ceiling you stay under, and it is worth checking what sits either side of the pump too. Alfa Laval rates the SRU's mechanical seals at 20 bar and its SMS and RJT connections at 10 bar, whichever pump head is fitted. So the figure that governs your installation is often not the figure on the pump.
| Centrifugal performance curve | Rotary lobe pump chart | |
|---|---|---|
| The axis you read along | Flow | Speed, in rpm |
| What the vertical axis gives | Head, in metres | Flow, in m³/h or l/s |
| What the separate lines are | Impeller diameters | Differential pressures, in bar |
| What sets the flow | The head the system imposes | The speed you drive the pump at |
| What rising pressure does | Flow falls back along the curve | Flow holds; slip and absorbed power rise |
| The left-hand end | Shut-off: head, no flow | No such point; fit a relief path |
Power and NPSHr still have to be read
Two readings carry over from a centrifugal chart, and they are the two people skip. Absorbed power is what the pump takes off the motor at your duty, and here it climbs with differential pressure and with viscosity, not with flow. That is the constant flow being paid for: the pressure change goes into the motor. Size the motor for the worst case the medium reaches, not the typical one. A medium that thickens on a cold morning is a motor load you have already committed to.
NPSHr is read at the speed you have chosen and rises with it, so a suction side that clears the margin at half speed may not clear it at full. Then note what is missing at the left of the chart. A centrifugal curve ends at shut-off, where the pump makes head and no flow. A rotary lobe pump has no such point. Close the discharge valve and pressure climbs until something gives. The relief path is part of the installation, and it is not optional.
The constraint that appears on no chart
Abrasive and shear-sensitive media both want pumping slowly, and that constraint is on no performance curve of any kind. On a rotary lobe pump it bites harder than on a centrifugal, because speed is the axis you are reading. Run a positive displacement pump too fast in an abrasive medium and you will be back inside it long before you planned to be.
So work the chart in this order. Settle the maximum speed the medium will tolerate, read the flow the pump gives at that speed, and go up a pump head if the flow falls short. Taking it the other way round, flow first and speed second, is how you end up with a pump that meets its duty on paper and wears out early in service. That decision is made before anyone opens the chart, which is why we ask what the medium is doing before we ask what the duty is. Displacements and speed ratings differ across the rotary lobe pumps we carry, so the same flow can land on quite different speeds depending on the pump head.
Frequently asked
What is a lobe pump?
A lobe pump is a positive displacement pump with two lobed rotors that counter-rotate inside a casing without touching, carrying a fixed volume of medium round to the discharge on every revolution. It suits viscous, shear-sensitive and solids-laden media, and its performance data is published against speed, not head.
Why doesn't a rotary lobe pump have a head–flow curve?
Because head and flow are not traded off against each other on a positive displacement pump. A centrifugal pump's flow moves significantly as pressure changes, so a head–flow line earns its place. A rotary lobe pump holds its rate to within a few per cent and takes the pressure change on the motor, so its chart plots flow against speed instead.
How do you work out the flow of a rotary lobe pump?
Multiply the pump head's displacement, the volume it sweeps per revolution, by the shaft speed, then subtract slip. Alfa Laval publishes SRU displacements from 0.053 to 3.53 litres per revolution and states capacity in m³/h at 1000 rpm. The chart shows the slip already deducted at a given differential pressure.
What is slip in a rotary lobe pump?
Slip is the medium that leaks back through the running clearances from discharge to suction instead of leaving the pump. It is the difference between calculated and actual flow. Slip rises as differential pressure rises and falls as viscosity rises, so a thin medium slips more than a thick one.
Does viscosity change a rotary lobe pump's performance?
Yes, and in the opposite direction to a centrifugal. A viscous medium seals the running clearances, so slip drops and volumetric efficiency improves. It also raises the power the pump absorbs and the suction head it needs. Size the motor and the suction side for the thickest the medium gets, not the typical figure.
What speed should a rotary lobe pump run at?
Slowly enough for the medium, then as fast as the flow requires. The manufacturer sets a maximum, and it falls as pump size rises. Alfa Laval rates the smaller SRU heads to 1000 rpm and the largest to 500 rpm. Abrasive and shear-sensitive media want less than that, whatever the chart allows.