Why not use positive displacement turbines?

[Stormer]

Positive displacement pumps are used a lot in the industry for pumping water or other liquids. So why is it not used as turbines in hydropower plants?

 

[Baluncore]

Maybe because it interrupts the smooth flow of the water.
Flow pulsation and cavitation causes problems.

 

[Lnewqban]

Also, the huge flows involved imply big reciprocating parts, with the associated big accelerations and forces, and the need to counterbalance those.

A higher number of smaller machines could alleviate those problems, but at the cost of more precise parts to manufacture and maintain, with the associated lower reliability.

Slowing those big parts down would be also possible, but electrical generators need to rotate at 3,000 or 3,600 rpm, and that implies that huge velocity multipliers would need to be used for the slow reciprocating machines.

Another reason: It is easier to keep a few central bearings lubricated than several sliding parts in direct contact with water.

 

[anorlunda]

Be assured that every possible way to make better turbines has been investigated. In the case of hydro, there are numerous turbine types in use. Each is presumably optimum for the range of parameters at that installation. But the only reciprocating ones I've seen are meant to extract power from ocean waves.

 

[Stormer]

Also, the huge flows involved imply big reciprocating parts, with the associated big accelerations and forces, and the need to counterbalance those.

I'm not talking about pistons here. More like lobe pumps, gear pumps, gerotor pumps, geroler pumps, screw pumps, and so on. Some of them have very minimal pulsation. And they don't have any reciprocating parts that need counterbalancing. And they can be driven in reverse to function as pumped hydro for intermittent energy like wind and solar to store energy. A Pelton turbine can not do that, but need separate pumps for that function. While a Francis turbine can work in reverse for pumped hydro, but i don't know how efficient it is in this mode?

 

[anorlunda]

Pumped Hydro (for example Blenheim-Gilboa) typically use Frances turbines. Cycle efficiencies are in the range 70-80%.

Where would you place existing designs of positive displacement turbines on this diagram?

 

[Stormer]

Where would you place existing designs of positive displacement turbines on this diagram?

I don't know. I'm just asking a question.

But if i would have i guess i would think relatively high head and low flow.

Maybe it is good for micro hydro turbines with high head because you don't have to optimize the nozzle design for a pelton, and don't need the complex geometry of a francis, and you don't need the pitch mechanism of a kaplan. So a lobe turbine would maybe be simpler and cheaper to make for a small scale personal hydro plant.

 

[anorlunda]

Well as a

But if i would have i guess i would think relatively high head and low flow.

Here's a calibration. The left hand border of that diagram is a flow of 1 m³ per second. That is 15380 gallons per minute. Most positive displacement pumps operate 1-2 decades to the left of that.

 

[Baluncore]

Positive displacement pumps and turbines have fluid chambers with fixed volumes. Therein lies the problem, with the seals between chambers and the surfaces in sliding contact. Gear motors and pumps are positive displacement systems that are almost always lubricated by the fluid they contact, such as hydraulic oil.

Over the last 70 years, positive displacement piston engines for aircraft have been largely replaced by high reliability gas turbines. Contact between solid materials, such as piston ring pressure against the cylinder walls, has been eliminated. The operating fluid now fills the clearance between the moving parts.

Progressive-cavity-pumps, such as worms, screws, and blowers, are often used with incompressible water, or compressed air. Those engines tend to have elastomer seals that are damaged by fluid contamination. Where inverted for use as motors, they would require higher maintenance, and so would rapidly be replaced by the more reliable flow turbines. That is where we are today. Why might anyone want to go backwards?

 

[Stormer]

Positive displacement pumps and turbines have fluid chambers with fixed volumes. Therein lies the problem, with the seals between chambers and the surfaces in sliding contact. Gear motors and pumps are positive displacement systems that are almost always lubricated by the fluid they contact, such as hydraulic oil.

No. There is no sliding contact between the lobes in a lobe pump, or the screws in a screw pump, or between the rotors and the housing in a pump made for liquid applications like water. It is in fact a small clearance between them that is dictated by the viscosity of the fluid it is made to work with. I work as an automation engineer making processing plants for food, farma, water treatment and so on, and we use these positive displacement pumps for water, milk, beer, yoghurt, ketchup, acid, lye, and so on all the time. And the liquid in the pump is not doing any lubricating work. It has a built in clearance made according to the liquid viscosity you specify when the process engineers calculate the pumps needed.

Progressive-cavity-pumps, such as worms, screws, and blowers, are often used with incompressible water, or compressed air. Those engines tend to have elastomer seals that are damaged by fluid contamination. Where inverted for use as motors, they would require higher maintenance, and so would rapidly be replaced by the more reliable flow turbines.

We use lobe and screw pumps for example in CIP return pumps to get the dirty cleaning water out of thanks that you are cleaning (because a centrifugal pump would just get air locked all the time in that kind of application). And as i said there is no seals on the rotors of these pumps, just a specified very small clearance. And these work for many decades without any need for maintenance for the lobes or screws (just change the oil in the gearbox after some years, and the bearings in the motors when they start to make noise). As long as you are carefull to flush out any metal shavings from the system from grinding and cutting when the piping is made the pump will work for a very long time. In a turbine application i suppose you would put a filter in front of it to remove any sand or rocks from entering the turbine.

 

[Baluncore]

In a turbine application i suppose you would put a filter in front of it to remove any sand or rocks from entering the turbine.

Maybe the food industry is clean. I often see worn pumps from the processing industry, going to scrap.

Who would clean your filter and how often? How would an input filter withstand the low flow, high pressure after it blocked with algae or ice? How could you know it was blocked once the filter material was blown and damaged?

The clearance in pumps and motors is pressure dependent. A high pressure positive displacement motor would have no clearance at low pressure, and so would require lubrication by the fluid.

Choice of pump or motor type makes a big difference to lifetime. For a motor driven by high pressure water, I would use a flow turbine, for the same reasons that centrifugal pumps are widely used in industry.