How much reservoir capacity do I really need?

[dstig1]

Posted in my build thread over in Build- it-yourself but I figured here might get more direct answers to this.

Sweepster Rotary Broom - rebuild and modification

I'm going to start this thread a bit ahead of time so I can actually get input along the way rather than just showing finished pics as there are some areas I am less certain of. I have started taking apart bits of it but probably won't get seriously into it until Spring/early summer when it...

www.tractorbynet.com

Last post has the question on this topic...

 

[mikester]

For future reference: Try posting a direct link to the question, better than endlessly scrolling and hoping the last responded to thread is the correct one.

Better yet simply post your question here instead of expecting me to try and figure out what you want. BFN

 

[dstig1]

Ummm? Sorry? Thanks? Or just hit the "jump to newest post button"?

Here was the question in the last post of that thread:
As I start to sort out the engineering details on this, the one point I am a bit stuck on at the moment is how much fluid capacity I really need. I have seem "rules of thumb" that say 1x flow, 2x flow, 3x flow is needed but nothing really definitive. I do NOT want to have to add radiator cooling to this but obviously the bigger the tank, the more weight, space, and complexity it adds. The main plan is for snow removal, so cold temps, which helps reduce the need for a large hydraulic reservoir. It will be used at times during other seasons to do various clean up tasks but those uses will be much shorter duration. Based on my tests and pump I am looking to be running in the 16-20 gpm range for this. Is a 25 gal tank too big? too small? Just right? I see log splitters with 22 gpm pumps and 6-7 gal tanks, but most of the fluid obviously goes into bypass right back itno the tank except for the moments you are actually splitting wood. If I could use 10-15 gal, that would be great. Making a change after the fact will be very painful as it will be all painted and purdy looking...

 

[oldnslo]

Dstig
That is a tough question since there are several variables that have to be considered some of which are:
1) Will this have any flow controls or will sweeper speed be controlled via engine speed? Reason for this question is flow controls create heat.
2) Heat generation from line loss? I.e. pressure loss of pushing oil through lines, valves, fittings, etc.
3) reservoir shape; can you isolate pump inlet from return flow so that you do not create a vortex causing cavitation.
4) Ambient temp which you already covered

If all you are running is this brush with no flow controls I would say 10 - 15 gallon would work

 

[dstig1]

Thanks. Answers to your questions:

1. Speed controlled by engine speed on the tractor, no flow controls BUT there will be a pressure relief valve right after the pump that dumps back into the reservoir as the hyd motor requires 2000 psi max and the gear pump can generate up to 3600psi max. I'm not sure how much heating that bypass will cause. I kind of had the same thought in my head about excess heating from things like that but the relief should be the only thing that does a lot of that. Hmmm - There is a diverter valve that the fluid all flows through which could be another source of heating. It is used to divert to the angle/swing cylinder, but of course all flow goes through it to be able to split to the angle cyl.

2. Line losses shouldn't be too crazy. Max distance from the pump to the motor looks to be under 10' and the lines will be 3/4" hoses, but if that is a red flag to you, I am all ears.

3. Shape and baffling is something I have thought about a lot to avoid that tunneling that can occur from inlet to outlet in the tank. I am also looking at thinner tank shapes to increase surface area for heat management. Kind of my baseline tank is 36wx18hx9d which is 25 gal in a reasonably thin profile. I would put a couple baffles internally also to force the fluid into a long path back to the suction port.


-Dave

 

[oldnslo]

Dave
The relief should only bypass oil when you exceed the pressure it is set at so in normal operating conditions it should generate no heat.

Diverter valve, is this splitting off some flow to run the angle cylinder? If yes this will create a little pressure drop probably 150range.

2. 3/4 inch hose pass 20 GPM at 15 feet per second flow rate so should be adequate if you keep the number of 90 degree elbows to as few as possible

3 reservoir design use your best judgment since I have no experience designing reservoirs.

 

[dstig1]

Thanks. Not too worried about the reservoir design as I have a decent idea what needs to be done for baffling based on some reading I did.

I'm not real sure what is going to happen with the relief as I don't really know what the pump is going to output for pressure in this situation. If it turns out to be supplying 2500 or 3000 psi then the relief will constantly be bleeding off pressure to maintain the 2000 psi set point for the motor. Not sure if that will cause heating or if it is such a small quantity that bypasses back to tank that it does not matter in reality.

So with all that said do you think 15 gal is sufficient? That would make it a lot easier to fit on there plus save some weight.

 

[oldnslo]

Dave
l Presume this is a gear pump so if that is true pressure is determined by the load so you would have to have a load or something restriction to the flow to create pressure exceeding the relief valve setting. If the restriction is the load on the brush motor doing work you generate very little heat. If the restriction is a flow control keeping the pump at relief pressure while work load is only 500 PSI then you create a lot of heat.

does this makes sense?

 

[MossRoad]

Hope this helps you.

I have an all hydraulic machine. It has 3 pumps. One is a variable volume pump that provides machine direction and speed. That is sort of a semi-closed loop system, with the fluid going back or forth through the wheel motors and the pump. The only fluid it uses is for a charge pump to make up bypass oil. That pump is about 4GPM as I recall. I also have an 8GPM fixed displacement pump that runs continuously and either bypasses to the tank or flows through implements like mower, brush cutter, sweepers (I don't have a sweeper), etc. And there's a 4GPM fixed displacement pump that also runs continuously that provides flow for steering, FEL functions, and auxiliary rams like power angle on a snow plow, grapple, etc.

I have a 10 gallon reservoir for the entire machine. And I have a hydraulic cooler with electric fan. That is fed by the return from the variable volume pump.

So I have about 16GPM total of which, sometimes it's under high pressure and sometimes it's not.

If I start the machine up in winter, and let it sit there at 1/3 throttle for about 20 minutes, the cooler fan will come on. So it does generate heat even when the only load is circulation and not even spinning any motors. Just the friction of it flowing through pumps, hoses, valves and fittings is enough to raise temps.

So my guess is that as a general rule of thumb that I've heard here on TBN for about 20 years is that you should plan on about 1 gallon of reservoir for every 1 gallon per minute of flow. So if you have a 15GPM pump, you should have a 15GPM reservoir.

That being said, my machine's reservoir is just a tad undersized, but is made up for with the cooler and fan.

Again, hope that helps.

 

[LD1]

Without a cooler and continuous operation at pressure is typically 3x flow rate.

The reason a logsplitter only has a 6-7 gal reservoir with a 16 gpm pump is because it is a two stage pump. They only run like 650psi at that 16gpm. Then they drop to 3-4 gpm for max PSI. And yes, also because they arent continuous duty.

Whats the operating pressure? Thats the true unanswered variable. Pressure is what is generating the heat. You have a relief of 2000psi.....but in operation is it gonna continuously need 1800psi to run the broom? Or are you gonna find that you never exceed 500.

For lower pressure stuff.....1000psi needs about 1x flow rate. 2000psi needs 2x flow rate.

If you dont anticipate operating over 1000 psi.....then I think 15gallon tank on a 16gpm pump in cold weather will work fine. But that would be the minimum.

If you are gonna be knocking on the door of the 2000psi relief and also plan on using in hot weather occasionally..... Id like to see a 40 gallon tank on a 16-20 gallon pump or a cooler added.

Look at tractors. Look how hot the hydraulics (loader/backhoe/transmission) get on them when using them continuously. They are usually about 1x or better and usually have coolers, and ALOT more surface area to dissipate the heat.