Industry
Flexible impeller pumps, sometimes called flexible vane pumps, are one of those designs you tend to meet on site rather than in a textbook. They turn up in breweries and wineries, on dairy farms and milk tankers, in engine rooms, and on the back of a workshop trolley heading towards whatever needs emptying that morning.
The appeal is easy to understand once you have used one. It primes itself from dry, it strips a sump or an IBC without a foot valve, it moves thick and lumpy product without turning it into soup, and you can change the impeller in minutes. So here is how the pumping action works, why it behaves so differently to a centrifugal pump, and what the impeller material really decides. To talk through a specific duty, call us on 01332 913500.
A flexible impeller pump is a positive displacement pump. Instead of flinging liquid outwards and relying on velocity the way a centrifugal pump does, it traps a set volume of liquid, carries it round, and squeezes it out the other side.
The whole thing hinges on one component. A rubber impeller with a set of flexible vanes sits inside a casing that is deliberately too small for it. Because the casing is tighter than the impeller, the vanes stay permanently under a bit of bend, and that bend is what creates the seal.
One section of the casing sits closer to the shaft than the rest. Fitters usually call this the cam, and it is the part that does the squeezing.
Because the vanes stay in contact with the casing, the pump can draw air out of the suction line as well as liquid. That is enough to clear the line and pull the liquid up to itself. It is a priming action though, not a licence to run dry, and the impeller still needs product arriving within a few seconds to lubricate and cool it. So you can drop a hose into a drum, hit start and walk away. No foot valve, and no filling the casing with a jug beforehand.
Once you know how it works, the next question is whether it suits the job in front of you. So it helps to compare it against the pump most people reach for first, then weigh up what it does well against what it will not do.
People compare these two a lot, usually because a centrifugal pump is struggling somewhere and someone wants to know whether a flexible impeller pump will fix it. Often it will, but for reasons worth understanding rather than guessing at.
| Flexible Impeller | Centrifugal | |
| Pumping principle | Positive displacement. Set volume per revolution. | Kinetic. The casing converts velocity into pressure. |
| Flow rate | Low to medium, directly proportional to speed. | Low to high, though system pressure losses dictate it. |
| Priming | Self-priming from dry, typically up to around 6 m depending on the fluid. | Needs priming, a foot valve, or a self-priming design. |
| Viscosity | Handles thick product well. Model dependent, since some ranges reach 50,000 cP. | Performance drops off quickly as viscosity climbs. |
| Shear | Low shear. Product structure stays intact. | High shear. Cream thickens, emulsions break. |
| Solids | Handles solids as standard and holds them in suspension. | Not as standard, and centrifugal force separates solids out. |
| Reversible | Yes. Handy for clearing lines back to the tank. | No. |
| Speed | Limited, since the impeller touches the casing constantly. | Runs at full motor speed. |
| Duty cycle | Suits intermittent duty and batch work. | Specify it for continuous running. |
| Running dry | No. Damage follows very quickly. | No, though it usually loses prime first. |
In short, if the job is high flow of thin, clean liquid on continuous duty, a centrifugal pump is the sensible answer. However, if the job is emptying, filling, dosing or transferring something awkward in batches, and you want it to prime itself, a flexible impeller pump gives you the easier life.
| Advantages | Limitations |
| Self-priming from dry, so no foot valve and no manual priming on every start. | Will not tolerate dry running, since the impeller deforms or melts in a short space of time. |
| Gentle, low shear action that leaves product structure intact. | Low to medium pressure only, so not a boosting or high head solution. |
| Handles soft solids and holds them in suspension rather than separating them out. | The impeller is a consumable, so budget for replacements as part of normal servicing. |
| Smooth, near non-pulsing flow, which suits filling and metering with a flowmeter. | Constant impeller-to-casing contact limits speed, so continuous duty is not its strength. |
| Reversible, so it clears lines back to the tank at the end of a transfer. | You have to check elastomer compatibility against the fluid every time. |
| Very few parts. Sanitary models clean in place, and any head strips down for hand cleaning without specialist tools. | Needs a relief valve on the discharge, since it keeps pumping against a closed valve. |
| Mount the pump head in almost any orientation to suit existing pipework. | Runs noisily at times, and starting torque is high compared with a centrifugal. |
Two things decide how long a flexible impeller pump lasts on a given duty. First, the elastomer you specify. Second, how well you know the handful of components you will be handling at service time.
Everything else on a flexible impeller pump is fairly settled. The casing will be bronze or stainless steel, the connections will suit your pipework, and the motor is a motor. The elastomer, though, decides whether the pump lasts a year or a fortnight.
One thing worth flagging, because it catches people out: additive packages in oils have changed over the years, so a material that coped fine in a given oil a decade ago may now harden or swell. Therefore, if an impeller keeps coming out set in a bowed shape well before it should, rule out chemical attack before you blame the operator.
Food grade is not a blanket property of the pump either. It depends on the specific range, the elastomer and the seal specification. So if the duty is dairy or another product going into the food chain, the Jabsco Pureflo range is the sanitary option rather than a general purpose bronze or utility pump.
Flexible impeller pumps are forgiving in most respects and completely unforgiving in one. In fact, almost every failure we see traces back to the same handful of causes.
This is the big one. Without liquid, nothing lubricates the impeller or cools the mechanical seal, so a very short run can damage both. If there is any chance of the sump or tank emptying while the pump is live, fit a float switch, a level sensor or dry run protection. It costs far less than a burnt out motor and a contaminated batch.
Even though it primes from dry, the impeller appreciates a bit of help on its first ever start. Wet the inlet and outlet ports, and if you can get the end cover off, wet the chambers too. It takes a minute, and it saves the impeller a hard first few seconds.
There is no fixed replacement interval. How long an impeller lasts comes down to the duty: hours run, what it is pumping, the temperature and the pressure it works against. Some go years on light intermittent transfer, others need changing in months. Condition tells you far more than the calendar does, so build an inspection into your service routine. Pull the end cover and look for four things:
If impellers keep coming out damaged well before their time, the answer is rarely a better impeller. Instead, look at pressure, compatibility or speed, and investigate it properly.
A flexible impeller pump is a specialist tool that happens to be very easy to live with. It will not give you high pressure, and it will not run around the clock. However, for batch transfer, tank emptying, filling and dosing of product that needs handling gently, very little else does the job as simply. Get the impeller material right, protect it from running dry, and it will keep going with minimal fuss.
We supply the full flexible impeller pump range, including the Jabsco Bronze Series, Industrial Series and Utility pumps, along with replacement impellers, head kits and spares. So if you are not sure which impeller material suits your fluid, or you want a hand sizing for a viscous duty, call the team on 01332 913500 or visit tfpumps.com.
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