Hydraulic Pumps - Gear and Vane Designs

Are the two vessels at the same pressure?

Yes, the pressures in both vessels are the same.

And the fluid -

• viscosity?
• density?
• temperature?
• intended flowrate?

Oh, and what would be a first stab at the pipe size?

Conditions are:

• Viscosity & density: hydrulic fluid recommend by pump manufacturer except for
• Temperature: range from -40degF to 212degF
• Flowrate: around 1 gal/min
• Pipe size: 1/2"

Any small gear or lobe pump should cope with it.

As said, any little gear pump will do, but your problem is that this is a tiny little thing and most of these little gear pumps are designed for a very low suction pressure, which is the pressure that the shaft seal will see. You would need a seal that will cost you many times the cost of the pump to seal your 70 odd bar suction pressure.

If you can submerge the pump in one of the chambers the problem of shaft sealing and pressure ratings all go away and a resin enclosed fish tank/fountain pump will do the job quite easily (materials compatible to the oil used).

Just an aside, only by your third post was there sufficient information to even begin to answer your question. Getting the question right will get you the best possible answers. (Gallons = 3.785 L or 4.546 L?? - not that it matters in this case).

Can you put a gearbox in the system to increase your pump speed? I can not recommend using standard gear/vane for this application. Possibly a high pressure diaphragm pump? Gearbox to increase pump speed to above 600 rpm is best solution.

Actually I have found that most good quality vane pumps are designed with springs to position them outward. But not always, check the specifications of the pumps readily avalible in your area.

Have you thought of a hydralic piston pump. It would do your slow rpm and you can set your preasure and flow with it.

This is an unusual pump application. Any little gear pump or vane or maybe even centrifugal pump (derated for viscosity) could do the job of pumping at a few psi differential pressure from one chamber to the other as long as:

Its outer casing could stand a 1500 psi hydrotest (includes a safety factor of typically 50% in the pump world).

If the driving motor is outside then the shaft seal on the pump would have to be able to hold 1000 psi. This suggests a balanced mechanical seal. This is usually a custom installation on industrial pumps.

If a submersible motor inside the pump casing is used shaft sealing is not a requirement.

Is something like this an off the shelf product? I doubt it; but I may be wrong. What's your budget? If this is a serious money making operation then you can probably go to one of the major pump manufacturers and see if they can offer you a special version of a production design to fit this application.

If on the other hand if this is a low budget somewhat experimental effort it becomes a bit of a DIY project unless you can find the off the shelf setup. For DIY I'd look at putting a pump and submersible motor inside a length of 6 or 8 inch high pressure pipe with pressure rated flanges and caps on each end. The electrical connections and inlet and outlet lines would go through pressure tight fittings in the caps.

Note that refrigerator and air conditioning compressors have motors that run in oil. It may even be practical to modify the refrigerant compressor parts to pump 1 gpm of oil at low pressure. If that's not practical keep in mind that any electric motor should be able to run submerged in light viscosity oil as long as it does not rotate so fast that the churning effect and the resistance heating from current flow causes the oil to overheat.

Ed Weldon

I said ".....any electric motor should be able to run submerged in light viscosity oil....."

Well, not quite. I know brush types will likely not work and I have doubts about single phase motors with internal starting switches. If anyone likes this approach would you guys who know electric motors pretty well please comment.

Ed Weldon

These ideas have merit. But hydraulic gear pumps are plentyfull and low cost, and rated for high pressure. Part of the problem finding one is that I need bi-directional pumping at low speed but I can't find OEM information with regard to this.

Questions:

1. Can small gear pumps rated for CW rotation be run CCW and still pump. Or is the efficiency so low that it can't
2. Do small gear pumps have internal check valves that prevent both CW & CCW pumping. If so can these check valves be removed.
3. Are there any gear pumps that don't have pressurized wear plates or case drains.

Timco--

My ability to help you here is limited. I don't have working knowledge in the field of hydraulics. My experience comes from the world of low pressure (under 500psi) industrial pumps. With respect to gear or other positive displacement pumps the engineering fundamentals are the same; but the application details differ.

The number of different sources and types of hydraulic pumps available today is huge. Looking at this in a Google search leaves one wondering where to start to be able to penetrate the legions of sales drones that specialize in reading from catalogs and find someone who knows something. With all due respect to the sales guys, they are likely terrified of giving out information that is liable to leave someone injured from a high pressure oil leak.

I'll try my best to answer your 3 questions from my viewpoint:

#1 Yes. Small low pressure oil pumps don't have the movable pressure loaded sideplates you find in some hydraulic gear pumps to maintain pumping efficiency. They simply aren't needed below around 1000 psi differential pressure. This should answer your question #3 also. See my comment below about shaft sealing.

#2 No internal check valves that I know of. (but take note of my area of expertise). I'd think check valves as required would be outside the pump in the system. Pressure relief valves are a different story. They are usually there to prevent damage due to excess pressure such as may occur if the discharge line becomes restricted. In small industrial pumps such internal relief valves when present are usually reversible with some minor disassembly to allow for opposite shaft rotation and also are adjustable for different maximum pressure settings. Some may be adjustable to the point where they will not open under any level of excess pressure. This would be the same as removing them. Below their set pressure for opening a bypass line they are transparent to the operation of the pump.

#3. See #1 answer above.

Shaft sealing is the real limiter in your application. A major design challenge for pumps, often unnoticed by the user, is how to get a moving shaft which is the main path to get power into the high pressure pump body without liquid leaking out through that path. The historical answer to this problem is some kind of close clearance seal that allows the shaft to rotate. Close clearances in the real world mean friction and resultant heat and energy loss. Higher pressure means closer clearances are needed to prevent excess leakage and the result is still more heat. This is a real problem at typical induction motor speeds.

In most small pump applications we see two types of shaft seals. One type is an elastomer ring around the shaft. An o-ring with somewhat looser fit than the common gasket type o-ring is the simplest. More sophisticated lip seals are often used.

The other type is a rotating face seal often referred to as a "mechanical shaft seal". Two very flat surfaces pushed together to minimum clearance (actually millionths of an inch) are separated when rotating by a comparably thin film of liquid which produces tiny and virtually invisible leakage. This is the approach seen in modern day small water pumps like in automobile engines and swimming pool pumps.

Both types of seals above have the characteristic of sealing tighter as the pressure acting on them increases. A useful property; but it has limits. Like 25 to 100 or so psi depending on the application. So how to get around this in higher pressure pumps?

Simple. The higher pressure is usually present on just one side of the pump, the discharge side. The other side is the "low" pressure suction side. The answer is simple. A short passageway in the pump connects the suction side to the shaft seal cavity.

Now most pumps run in only one direction because of the way they are designed. Common centrifugal water pumps are like that. But some, like gear pumps, can run backwards. Such pumps at higher quality and pressure rating levels have internal porting that can be adjusted, often with simple screw plugs to direct suction pressure to the shaft seal area. This could be simply automated by external controlled valves.

The problem comes when the suction side or low pressure side of the pump is at a pressure too high for a simple seal. There are two basic choices in the pump world for this. One is a special high price seal that can easily cost more than the pump. Balanced seals and double seals come to mind here. The other choice is to transmit power into the high pressure area by some means other than a rotating shaft.

There are two approaches to this. One is the "canned rotor" design in which the motor stator and rotor are separated by a thin stainless steel "can" which must be thin enough to minimize the gap for magnetic fields but thick enough to hold the pressure. Not the choice for this high pressure application without a big sacrifice in motor performance. The other is a fully submersible motor to drive the pump elements.

There is a third choice outside the realm of rotating pumps that perhaps you have already considered and rejected. That is a reciprocating pump. What I'm thinking of here is not the ordinary piston pump driven by some kind of electric motor; but something you can probably build yourself if a commercial type is not available in a nicely engineered "plug and play" package. This would be a double acting oil cylinder for pumping driven by an oil or air driven smaller double acting cylinder for the power source. All that would be needed would be a simple coupling or linkage (the latter would allow for some multiplication factor for different stroke lengths or parallel physical mounting arrangement) as well as the control hardware and valving.

Ed Weldon

Timco -- After my last post I had one other idea that may be the most workable one. This follow my line of thinking for the need to expose the seal cavity of a gear to only a low pressure that the sealing element can handle.

How about venting the seal cavity of an ordinary high pressure gear pump to some low pressure point else ware in your hydraulic system and opening a valve in that line only a few seconds before and after the pumping cycle? The potential problems here are:

1. The amount of restriction between the pumping elements and the seal cavity to minimize flow and the resultant energy and system efficiency loss while pumping.

2. The ability of the seal to withstand full tank pressure when the pumping and shaft rotation stops.

3. Whether you can practically modify (if really needed) a hydraulic gear pump with pressure loaded wear plates to pump at low differential pressures.

4. Whether your situation is like an OEM application where the cost of modifying a purchased pump may not be cost effective.

5. If industrial or product safety considerations get in the way.

This may require you to buy several small cheap pumps to find the right one to modify.

Ed Weldon

1) Yes - you can use an absolutely symmetrical gear pump that will pump equally well in either direction by reversing direction of rotation.

2) No they are not normally fitted with internal check valves, but they may be fitted with an internal pressure relief valve. This will only function when pumping in one direction, when you change direction this will become ineffective. If you supply some other method of relief (if required) this can be blanked off.

3) Yes - many have no such devices and anyway these would not be required in your case as your differential head is so small that slippage would be negligible.

Your suction pressure is your problem.

A 100 bar pump form someone like Kracht has a maximum suction pressure of around 2 bar because they nearly all use lip seals. At the sort of PV value that you have here, sealing will be difficult but not impossible using a mechanical seal.

Magnetic drive would be an elegant solution. I do not know a supplier with this sort of pressure rating, but I would contact Viking Pumps to see if they could do it for you.

Two vessels at same pressure (before and after )

That means you are already having a pump at the inlet of the first tank.

If you want to move some fluid from tank 1 to 2 why dont you go ahead with some sort of orifice rather than pump ? may be controlled orifice ?

As far as the gear pump is concerend - do.t look for ger pump, go for a geared flow divider - these can typically work at almost zero head and are used in hydraulics for splitting the flow into branches. And a few of them are self driven (ie the in;et flow is used to drive the gears) check on this aspect, but then your outlet pressure will be a bit less than the inlet pressure.

Why bother with a pump? Just lower the pressure in the vessel you need to have the fluid transferred to. If its sort of a tapping system, same thing. A 1000 psi with that temperature a little difficult to control try the differential pressure first.

Use a bi-derectional gear motor, and drive it as a pump. Many are availible. look for one with a seperate drain line so the shaft seal does not see the pressure. should be easy to do.

Thanks from this end, Mike. I'm not into hydraulics so I wasn't aware of the bidirectional gear motor being an available product. But it certainly makes sense to have a hydraulic motor that is reversible for many applications and this is the obvious answer to that one.

From your experience can you answer my question about just how much flow (and resultant efficiency loss) such a setup will produce in the drain line with the maximum seal cavity pressure that the seals can hold?

Do such pump/motors come with mechanical shaft seals or the common low pressure lip seals? Mechanical seals do take up more space; so that might require a special longer section where the driving shaft connects to an electric motor.

Ed Weldon

case drain flow---- we limit our tests to about 10% of rated flow after which we feel the internal leakage (wear) is going to to start to affect the system. Running units can exceed that but rebuilt units are expected to run for some time so that is our limit on used units. New units can be less than this.

The extra line (drain) is connected to the shaft/seal area and needs to be routed to a low pressure area. Small gear motors (which can be operated as pumps) typically do not have high pressure seals (25-35 psi normal) but I have adapted a few with 5000 psi rod seals that I expect to hold 100 -250 psi for a longer time. Some special high pressure shaft seals are capible of 150 psi max. so...? best to have the slight leak age led to a low pressure area. NOTE: do not use a Charlynne type motor as it has internal gearing (thru a wobble "dog bone" shaft) and no extra drain line!

Hydraulic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement cannot be adjusted, or variable displacement pumps, which have a more complicated construction that allows the displacement to be adjusted.