Turbine pumps fall between two stalls, sharing characteristics akin to both centrifugal and positive displacement pumps. This definition difficulty has left them as outsiders in both camps – the ‘Billy No Mates’ of the pump world. We cannot even agree on their name, variously describing them as turbine, peripheral turbine, regenerative turbine or even centrifugal.   

The operational principle of the turbine pump is simple; it works in the way that many of us believed a pump functioned before we were initiated into the mysteries of radial flow centrifugal pumps; in the centre of the pump chamber is a turbine containing multiple buckets around its periphery. These buckets carry liquid round the chamber until it has to leave the pump at the shut-off point, making the turbine pump not dissimilar to an enclosed water wheel. The only slight complication occurs as the result of the liquid recirculating within the buckets as the turbine rotates. This constant circular regeneration of velocity produces increasing pressures that surpass that of a radial flow centrifugal pump impeller of the same diameter. 

Importantly, there is no contact between the turbine’s surfaces and the pump chamber. They are separated by a thin film of liquid on their faces and outer edge at the shut-off point. Liquid is not completely trapped and therefore turbine pumps are deemed to be using kinetic energy to shift the liquid and not displacing a fixed volume. Of course the key word here is ‘completely’ as a thin film of liquid is not very much of an interface and consequently nearly all of the pump’s characteristics are the same as those of a positive displacement pump.

Characteristics

Turbine pumps differ from most centrifugal pumps as:

They have straight differential head – flow curves. 

Power increases with pressure not flow.

They can handle viscous liquids better while being good with thin liquids. 

Pump action is relatively low shear.

They can pump liquids with up to 20% entrained gases.

With no axial thrust loads they have lower NPSH requirements as much less damage is caused in such situations.

However, they are not so good with solids and they would not normally be used to deliver large volumes of liquid.

The main use for turbine pumps comes from their low flow high head capability which compares a single stage ISO 2858 centrifugal curve with that of a turbine pump.  The centrifugal pump is fitted with a 5.5kW motor and the turbine a 2.2kW.

But what about multistage centrifugal pumps? If a mass produced multistage centrifugal pump can be used, then it will invariably be less expensive.  However, if there is anything unusual about the application, be it a chemically aggressive liquid, an ATEX zone or high liquid temperature then a turbine pump will be the better option.

Difficult liquids

Taking Aturia turbine pumps as an example; these are magnetically driven and are available in polypropylene, PVDF, 316 SS and Hastelloy which makes them chemically resistant. However, it’s their low shear capabilities and similarity to PD pumps which brings key advantages. Historically one of the biggest uses for turbine pumps was in pubs when thousands of them delivered beer from cellar to bar without thrashing it into undrinkable foam. This was decades before the advent of cheaper gas pumps.  

Turbine pumps will handle viscous liquids to 150 cP and are better suited to viscous liquids as they become more efficient. This makes them good for caustic solutions as fluctuations in the flow are less than a centrifugal pump, particularly if the caustic is subject to varying temperatures. Their ability to cope with 20% entrained gas makes then useful in situations involving turbulent flows and they can even be used to mix separate streams of liquid and gas together.   

All Aturia turbine pumps can be ATEX compliant. STM pumps are available in several types of metals including Duplex, Hastelloy and Titanium, making them capable of withstanding system pressures to 350 bar. High temperatures can be accommodated, with long coupled models able to handle liquids to 350°C. They are available with self-priming chambers, can be configured to API 685, have polished internal surfaces and come with a range of connection types including tri-clamps.

Applications

Although less well known than other pump types, turbine pumps have been in use for decades. The most commonly encountered are vertical multi-stage pumps used in the municipal water and off shore sectors pumping water at very high heads. However, they can be found cooling all manner of equipment from x-ray machines to VSD assemblies, CIP duties, transferring small flows over long distances, as tanker off-loading pumps, vapour recovery systems, for filter systems where higher pressures are encountered, spray bar applications, soda re-circulation in fast food outlets, shower booster pumps and  LPG distribution.

The use of turbine pumps should be more widespread and could be if engineers had a greater appreciation of the technology and were willing to look just outside their comfort zone more often.