Using a gear-pump as a motor?


by Inventus
Tags: gearpump, motor
Inventus
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#1
Jul10-07, 10:46 PM
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Hi,

I'm looking into the possibility of making a small steam-engine, with focus on simplicity and low water-loss. Since I need high torque at low rpm, I don't think a turbine would be ideal, but would instead prefer something operating on the principle of "positive displacement", only the old-fashioned piston setup is rather complicated.

Thus I am wondering if a gear-pump could be used instead of reciprocating pistons, the former being a much simpler construction than the latter, more so because it doesn't need valves timed to the rotation. Also a gear-pump should be much easier to make steam-tight than any piston/cylinder setup.

I realize that a gear-pump (or rather "gear-motor") might not be very efficient, but at this time I can live with that (later on I might consider a more efficient design). Also it should be easy to make a two-stage engine, simply using two gear-pumps, one larger than the other, which might increase the efficiency a bit. Since the steam-engine will be recirculating the steam, and likely using a heat-exchanger to recover some of the waste heat, I'm hoping that a low efficiency in the motor won't be too detrimental to the overall performance.

Any input on the feasibility of such a "gear-motor" design is welcome.


Thanks in advance,

Inventus.
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Danger
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#2
Jul10-07, 11:34 PM
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Welcome to PF, Inventus. Your posts in the other threads have been very interesting.
In this case, however, I believe that you're barking up the wrong tree. A gear pump would be very difficult to seal properly; it would be very similar to a Wankel motor in that regard.
As for the complexity of valving a piston system, it's ridiculously simple. Just look into the workings of an old locomotive. The 'rocking' piston just covers and uncovers the ports as it oscillates. A simple wear-resistant gasket takes care of the sealing. This limits the operating pressure, but still gives a quite efficient output.
I had one when I was about 10 years old. It burned a little Sterno-type fuel pellet and spun like a bugger.
Inventus
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#3
Jul11-07, 03:58 AM
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Hi Danger,

Thanks for the welcome and your comments about my gear-pump idea. Very nice forum this one, and I'm glad to hear that my posts were of interest.

I too had a small steam-engine when I was a boy, and with the same kind of sliding valve you describe, however:

I think you might have misunderstood my intention, if not my words, when I talked about "sealing" the pump (or making it "steam-tight" as my words were, "sealing" obviously being the correct term).

I was referring to "not loosing steam to the outside", rather than preventing some steam from "leaking through" the pump. While I imagine the former to be easily achieved (seeing as most hydraulic gear-pumps doesn't leak at all), the latter shouldn't be a big problem. Even if it is, I guess the problem could be addressed simply by having the pump (motor?) run faster, thus lessening the effects of "steam slippage".

When I talk about the gear-pump being simpler than a piston, what I mean is that it consists of just two moving parts, both of which are rotating. There is no need for any kind of valve, sliding or otherwise (well, excepting the speed-regulator valve of course, but that would be necessary under any circumstance). Also, I imagine that a rotating axle is easier to seal than a sliding piston-rod.

A piston/cylinder setup on the other hand, would not only require some kind of control-valve (the sliding one you mention probably being the simplest), but also that the cylinder is well sealed (where the piston-rod exits the cylinder), as well as requiring quite a lot of moving and thus precisely machined parts. Notably the whole cylinder would need to move, or alternatively the piston-rod would need a hinge.

While a gear-pump might also need precision machined parts, my hope was that an existing (say, hydraulic) gear-pump could be purchased or scavenged for my purpose, whereas it is my impression that very few steam pistons and cylinders are produced these days, thus requiring me to manufacture them (or have them manufactured).

A further reason for me wanting to use a gear-pump instead of a piston/cylinder setup, is the fact that I would need either three cylinders, or two double-acting ones, in order to make the engine self-starting. With a gear-pump, one would suffice.

All this being said, I would still like to hear any further comments you may have on this topic. Especially I would like to hear further arguments for the pistons as opposed to the gear-pump, if you still feel there are some..?

Regards,

Inventus.

Danger
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#4
Jul11-07, 05:38 AM
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Using a gear-pump as a motor?


I do have a few more ideas on the subject. Unfortunately, I'm just up for a snack (it's 4:30 am here), so I can't do it now. Will get back to you tomorrow.
FredGarvin
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#5
Jul11-07, 05:59 AM
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I don't see why you couldn't use a gear pump. If you have any kind of problems, it's going to be with sealing and corrosion. Other than that, most gear pumps are meant for higher pressure service, so the cases should be able to handle whatever you throw at them (make sure to check though).
Inventus
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#6
Jul11-07, 07:03 AM
P: 10
Sounds good so far Fred, thanks for your input.

I'm sure there are gear-pumps available, capable of handling far more corrosive fluids than steam, so I don't think this will be a major issue. Also, since the engine will be recirculating the water (and hopefully with a minimal loss), it should be possible to use some kind of anti-corrosive additive (in addition to a lubricant, which might be necessary to keep the gear-pump in good condition?)

I'm not so sure I understand the issue with sealing though, unless you are talking about the seal between the gears and the housing, like I suspect Danger was?

This, as mentioned, I consider a minor problem, since any such "leakage" (ie. through the pump, rather than out of it) should only cause a slightly lower efficiency at very low rpm. This I can live with much better than even a slight leakage to the outside, as it is important that the engine can operate without frequent refilling of the water tank (which should also be as small as possible).

If, on the other hand, you are indeed talking about the seal between the pump (housing) and the outside world, I'm afraid I don't quite see the problem?

I do realize that oil, including hydraulic, is more viscous than steam. Still I should think both the axle- and housing-seals, typically made for much higher pressures, should be able to keep the steam inside the pump. Certainly the housing-seal can be made arbitrarily tight. The axle seal is another matter, I just assumed that this problem would have been long since solved, since there are so many applications where a rotating axle has to be sealed against steam under (even much higher) pressure.

I will be looking forward to hearing more from both you and Danger (when the sun shines once again upon his dwelling)...

On a general note: I would like some confirmation that a gear-pump will at all function as a "motor" (ie. be driven by the fluid, rather than pumping it). I'm rather sure this is the case, but would like to have this positively verified, ideally by someone who have actually seen or heard of a gear-pump being used in this manner, or who knows for certain that this is the case.

As always, thanks a lot for your answers.


Regards,

Inventus.
Danger
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#7
Jul11-07, 09:52 AM
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I'm back. Time is still somewhat limited, since I'm getting ready for work.
It was indeed the leakage between the gears and the case that I was referring to; there shouldn't be a problem with the steam escaping to atmosphere. My only other concern is that you ensure that the pump you use won't have trouble with the temperature involved. Right off, I can't think of any that would, but it's best to be sure.
Lubrication would probably be best achieved the same way that it is in pneumatic tools. Atomized oil is injected into the airstream before it gets to the working bits. In your case, that might involve an additional stage of processing, since you'll probably want to filter the oil out of the returning steam/water before it gets back to the boiler and recycle it as well. Condensing the steam and skimming the oil off the top of the water is one way to do that, but I suspect that Fred or someone else knows of something more efficient. One thing that I don't know is how much parastic loss you will incur driving the oil injection pump, or if you even need one. A venturi inlet might work.
A gear pump will indeed work as a motor, so no worries there.
Just two points about the cylinder. I was referring to a rocking valve rather than a sliding one, which actually has more leakage to the air. The small advantage to it is that you can use a solid connecting rod rather than a pivoting one. The other point is that, although I've never seen it done, I think that I've devised a way for a single-cylinder single-action piston engine to be self-starting. It involves a spring-loaded locking wedge that engages a detent in the driveshaft or flywheel when the steam valve is closed. That ensures that the piston always parks just after starting a power stroke.
Q_Goest
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#8
Jul11-07, 10:03 AM
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Hi Inventus, Regarding whether or not a gear pump will run in reverse and output work, I'm sure it can. Most any expansion type motor can. Turbine expanders for example, are notorious for running backwards and must have protection built into the systems to ensure they don't run backwards when shut down and discharge pressure may exceed inlet pressure. Gear pumps will be the same way. There will be a differential pressure on the teeth that will make it run backwards.

I'd agree with the others above though, that the use of steam in a hydraulic type gear pump may result in considerable internal leakage. You're right that external leakage should be a minimum and potentially none at all but internally I suspect they'll leak like a proverbial sieve.

The other problem with gear pump inefficiency is that the volume being displaced (or expanded) is not changing. That volume is the volume trapped between two gear teeth and the housing. That's fine for pumping liquids but lousy for expanding steam.

Another problem will be shaft seals. Most gear pumps are made with shaft seals that are only good for a few psi. Those seals are only exposed to source pressure which is generally ambient. Only the body casing (static) seals and the discharge port is good to the thousands of psi normally generated by a hydraulic gear pump. I had to have one modified with special seals just to handle 150 psi. It can be done, but a gear pump is a poor choice for an expander.

I'd suggest looking into vane motors, also called air motors. The vane's seal the internal leakage much better than a gear tooth would, and the volume increases to improve efficiency. They're also readily available and wouldn't need much modification. I'm not sure about the shaft seals on these types of machines, but I suspect they'll have the same issue as the gear pump, so you may need to make some minor modifications to the shaft seals if discharge pressure exceeds a few psi. Check with the manufacturer.
geff
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#9
Sep21-07, 08:31 AM
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hi,
i have an idea of using a gear pump for driving. can v run a gear pump with the pressure of a fluid stored in an accumulator? the pressurized fluid from the container may be sent to the pump to drive it, in other words a simple turbine. i would request anyone who could advice me on this...

thanking u,
geff
Danger
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#10
Sep21-07, 09:06 AM
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Welcome to PF, Geff.
Your idea certainly will work, but for a very brief time. Accumulators are generally just used for emergency reserve pressure, to smooth out and absorb shock-loads, or to provide steady flow from an intermittent pressure source. The volume capacity is extremely limited. I can't imagine that you'd get more than a few seconds of use out of even the largest ones. There's also the matter of how you would 'recharge' it.
geff
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#11
Sep21-07, 05:35 PM
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Thank you so much. And i wanted to know abt this particularly. Can the gear pump be driven with the pressure in the accumulator? cos my intention is to make use of the stored energy.
Danger
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#12
Sep21-07, 08:08 PM
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I suppose that there might be a pump or two that are made to be non-reversing, but I've never heard of one. Any regular one should work just fine.


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