HPU Tank Sizing

[macmet]

Hello everyone,

I have a quick question regarding a hydraulic power unit setup we use. A typical unit has a 2HP motor with a pump capacity of arounnd 2.0 usgpm. We generally spec a max pressure of 1100 psi although in the field we have the relief set closer to 800 psi. The unit power cylinders which move a driveshaft.

My question is, does anyone have a rule of thumb about an appropriate tank size? Our vendor just recommended one that is over twice our typical size (we usually have 3 gallons although sometimes less). And no one here seems to know why we use 3 gallons so we're wondering if we have been undersizing them. I am not aware of any problems with our hpu's that might have been caused by this.

Cheers.

 

[budt]

The old Rule of Thumb I grew up with was 2-3* the pump GPM Flow. That was primarily to have enough oil to aid in disiapating heat.

I have seen systems operating Hydraullic Motors that were equal to pump flow or less that worked great since they never had excess energy waste so they would not overheat.

The only other criteria is when using Single Acting Cylinders or Cylinders with Oversize Rods where more oil is leaving the tank than is returning. These circuits can starve the pump due to low oil level at some point in the cycle when all cylinders are extended.

The other condition that needs to be considered is when an Accumulator or Accumulators are in the circuit. They must be filled each startup and require extra oil in the Tank to make sure the pump is not starved for fluid.

Of course the Power Steering pump on most automobiles don't follow the above guide lines. Usually they are in the 4-6 GPM flow range and hold less than 2 quarts of fluid. I have had several of these pumps run over 130,000 miles while in a sales job and usually never did anything to them except check oil level evry time I changed engine oil.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[IceStationZebra]

Bud, you beat me to the punch Anyway....

I have also heard the "rule-of-thumb" that the reservoir should be 2-3 times the pump flow. And I too have been led to believe that it is mainly for heat disapation reasons.

I have personnaly worked on off-road machines that used reservoirs that were only 30-40% of pump flow with no issues. You can get away with the smaller reservoir if you minimize the air in the oil and mind the temperature. Help yourself by:
A. Keep the return submerged and try not to shoot it directly at a wall.
B. Put the return and inlet on opposite sides. Note that the fittings can be on the same side if one of them is plumbed to the other side internally.
C. Use a baffle, if possible, to keep the oil from going directly from the return to the inlet. Time is your friend here - you want the oil to mix and let the air come to the surface.
D. Make sure that the oil level in the reservoir stays at least 1 diameter above the inlet with all cylinders at full extension, otherwise you risk sucking air. Example: if the inlet is a -24 (or 1.5") the oil needs to be at least 1.5" above the inlet.
E. Try to keep your oil temp below 180°F, 220°F max. As for using the reservoir as an oil cooler don't get too worried. If you do the calculations you find that you don't reject that much heat from the reservoir and plumbing - you just don't have enough surface area. Most equipment is banking on the time it takes to heat up all the mass (cylinder, pump, gallons of oil, frame, valve, filter, etc.) and the fact that it is hardly ever run continuously for hours at a time. Every time you stop to talk or get some coffee it gives the oil time to cool off. If you need additional cooling you will be much better off with a small oil cooler, it has a lot more surface area than if you doubled your reservoir size.

ISZ

 

[hydtools]

Right on, ICZ. Reservoirs do not get rid of much heat. They only provide large thermal inertia when they are large.

Our mobile systems have no more than 1x pump flow. An air-oil cooler takes care of rejecting system heat.

Both oil return and pump suction should be below the lowest reservoir level so as not to entrain air in the oil.



Ted

 

[macmet]

Thanks for the input everyone.

I talked to the vendor this morning and he said he recommended the higher tank capacity because he believes you should have about 3x pump capacity. The reason he gave was heat dissapation. Since that seems to be similar to what you all are saying I feel more comfortable buying it now.

Cheers.