Calculating Tank Size 3

[spggodd]

Morning all,

I have a choice of 3 lubrication tanks that I can scavenge from a redundant project for use on my own.
However I don't know which one will be most suitable.

How would I calculate the correct tank volume to suit the needs of the lubrication system I am using it with I have the following information:

Required Flow = 240 L/min
Oil Pressure = 1.5 / 2 bar
Oil Temp = 40 deg C

Is this enough information to determine the volume of the tank needed?

Thanks
Steve

 

[hydroman247]

At such low pressure, I am assuming this is just a lube oil tank.

In which case, you could get away with 100L tank but it depends mainly on heat. If you have hot components connected to oil galleries, then the tank needs to be larger. What else is in the system?

 

[LittleInch]

The information you really need is what is the max volume when everything is drained down or has flowed in when the pumps stop and what is the min volume when all the pipes are full. Too small and you could overflow when you shut down or just run out of oil when you start up before it starts flowing back in. Your tank is the buffer between these two.

100l tank is only about 70 l usable volume, which is only 20 secs of pumping before you need 240l/min coming back in.

Only you know if that's enough.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Oldhydroman]

The usual rule of thumb for "hydraulic" reservoirs is that the tank should be three times the flow rate per minute. So a 240 L/min flow rate would indicate a 720 litre tank. Then add 10% for the air space above the oil and that makes it 800 Litres.

The reasoning behind this rule of thumb is that the oil need time to de-aerate while resting in the tank and three minutes is considered enough. One of the additives hydraulic oil is put in specifically to speed up de-aeration and I can't see why your lube oil wouldn't have a similar additive. It is also the case that a very cold, or high viscosity, oil would take longer to de-aerate (bubbles rise faster in beer than they do in honey). You can speed up the de-aeration time by having a shallow tank and/or making the returning oil pass over a baffle or weir before it can get back to the pump inlet.

Aeration might be caused by fast moving components splashing through a free surface of the oil (entrained air) or by pressurising the fluid in a system that hasn't been completely bled of air (dissolved air). I suspect your pressures are too low for dissolved air to be a problem. If entrained air isn't a problem either then a smaller tank will be fine - as long as it can dissipate the heat input. If you have a separate cooler then you won't be relying on the tank to do any cooling so you can go smaller again.

DOL

 

[spggodd]

Thanks for your advice, the rule of thumb will definitely be one to remember!

 

[hydroman247]

@oldhydroman

The rule of thumb for reservoir size, is that personal experience? I am looking at sizing a tank up right now and everyone has a different opinion. Some say one minute of full flow, another says 4 times that and the rest are somewhere in between.

Is it a case of make a judgement on each case based around heat load mainly? Aeration shouldn't be an issue in a well designed and maintained hydraulic system although maybe that is my inexperience showing.

Thanks

 

[hydtools]

Don't fool yourself sizing a tank for heat load. They are not very good at dissipating much heat. Only the oil in contact with the tank walls has any chance of heat transfer. Oil is a poor heat conductor and the bulk of the oil will give up very little heat.

Ted

 

[HPost]

With the exception of hydrostatic systems that have a different set of design rules, fluid reservoirs capacity is dictated by component size more than anything else.

A large pump will have a large inlet to handle the flow, hence the bosses on the reservoir will be relatively big. To decelerate the oil adequately without excessive turbulence it is necessary to install baffles and diffusers, which also consume space.

Stuffing all of the hardware into a reservoir pushes the capacity up and in general, the ultimate capacity is approximately 2 - 3 times the displacement of the pump.

Heat dissipation is a function of differential heat, oil heat to ambient temperature. So you can't rely on the volume of the reservoir to control the oil temperature.

This system has a low pressure differential, so will not generate much heat. If space is not a concern use the biggest tank possible. If you driven towards smaller tanks, just make sure the return flow is handled correctly to reduce foaming. Either add a baffle or install a diffuser.

 

[RobertHasty]

The rule of thumb for reservoir size, is that personal experience? I am looking at sizing a tank up right now and everyone has a different opinion. Some say one minute of full flow, another says 4 times that and the rest are somewhere in between.

3 to 5 times is what I have been told.

Also, nice link about hydraulic tanks:

Traditionally, recommended tank size for mineral hydraulic oils has been three to five times Q plus a 10 percent air cushion (where Q is pump flow per minute – or mean pump flow per minute where a variable pump is used).