Hydraulic press aeration

Hi guys just got done putting together a press and plumbed it today, left it running a few minutes and next thing foamy oil is erupting out of the filler neck so I'm getting a heap of air in my circuit somehow, watching the return line it behaves like a garden hose coughing it's air out when first turned on (big bubbles, pockets, not micro bubbles).

I'm just wondering what the heck could cause this, if I don't have any leaks? My best guess is I have reduced the intake size by quite a bit by converting from a European flange mount style to a BSP thread as noone in NZ really stocks fittings for flange style. So I have reduced a ~19mm hole to whatever the ID is of a 1/2" BSP nipple. Is that enough to cause this behaviour? I would've expected if it was cavitating it wouldn't be pumping big air pockets around the circuit but rather dragging a tiny amount of dissolved air out of the oil, possibly...? Is it this simple?

Circuit is 1/2" hose from bottom of tank to pump, flooded inlet (tank above), and for simplicity sake let's say pump to filter via 3/8 hose (problem still shows with this simpler circuit), filter screwed directly into top of tank then flows down internal spout to just above the floor of the tank. There's a baffle in the tank but not blocking flow whatsoever. I can see waves in the tank, not well but what looks more like air bubbles than general turbulence.

It's a casappa pump, my best guess is a Polaris 20D, nothing written on it so don't know specs for sure. Spinning it at ~1400rpm. Might not be quite up the right isle for this forum but I suspect the knowledge will see me right...

Thanks in advance as always

I would think the largest possible intake would be required for any hydraulic pump.......the depth of fluid above the intake may be insufficient to prevent pulling air down through the fluid......the oil may also be too thick for the conditions......you may find the problem disappears as the oil heats and becomes much less viscous.......You may also have an air leak somewhere ,including the drive shaft of the pump.

The oil level must be 20" or so above the inlet, there won't be air at the pump intake I am certain of that. But yes I wondered Cavitation or pump Seal leaking, how do I determine between the two? I thought if the shaft seal was leaking the pump would leak oil when sitting?

I could turn and mill a flange style fitting with a big nipple and join it to the tank with some 1" hose, but I'm reluctant to get into this if I've got something else going on

How big is the tank ?

Rule of thumb I was taught (but we doo use less sometimes) is 3 minutes capacity.

Just looking at the aspect ratio between the suction line and the pump size your suction line is undersized. Flow rate on the pump would be required for better than spitball accuracy. What oil viscosity are you using?

A rotary pump seal is very good at containing oil under pressure.

Not as good at excluding air if your pump case is at negative pressure (vacuum).

You need a larger intake line and adequate "net positive suction head"

petersen

I'll get another seal anyway I imagine they're pretty reasonable.

Will fab a big flange nipple and find some heavier hose, see if it helps, Will let you know how I go.

Here's a photo of the mess hah, also showing tank size it's about 25L. Might be a bit small but I only plan on using it for short periods, if I need to fab a larger tank I can later.

Did you put baffles in?, perforated plates,
Mark

My suggestion, Don't use pressure hose on suction lines. Go to a larger hose barb and use wire reinforced suction hose. My usual guessing method is a 3/4" output gets a 1" input. The input should always be bigger than the output. And for double acting cylinders the lines need to be double the pump volume because in reverse nearly double the volume is coming back the return.. I have a large ram on one press that is 4 to 1 on retraction.. undersizing lines wastes energy and slows cycle time.

I did ask the hose shop about an alternative for the suction line and return line, was still given that. At as an exorbitant price as the rest of them. I'll turn a big 1" barb flange fitting tomorrow.

Have you burped it a few times with time to settle air inbetween cycles? I'm assuming so, but these systems benefit from a few flushes if they foam on the first run.

what i learned recently is that a hydraulic pump can suck in air on the high pressue side (without any oil leaking).

Sooo.. Any chance of water in the tank or in the fluid? Hydraulic fluid should have near zero vapor pressure, so the NPSHr should be very low (in case you really want to eliminate that as an issue, elevate the tank a few feet). But if you have water in the tank it can cavitate or even volatilze when released from the press. The tank and system should be scrupulously dry.

Alternately check the intake pipe and pipe fittings. You could have a leak and be sucking air into the intake.

I'm no expert but I thought the tank on #7 seemed too small.
I installed a hydraulic centrifuge in Singapore and had to get two local experts to help me out. First thing they said was: Get a much larger tank! We bought one from them, and it worked. Can't explain why but it seems those guys had a lot of experience.
Also, I think you should close the filling entrance air tight.
Just my 2 cents.
fusker

Are you filling the tank higher than your return line? If the return is above the fill level it will spray into the tank and that can be the source of your aeration. The return should probably be buried under the surface of the fluid so you don't get spray aeration or surface turbulence aeration. Might be as easy as filling that tank almost full and running it.

The return line enters the tank at the top where the filter screws in but there is a a straw inside it that takes it to about 15mm from the tank floor.

I ended up machining a flange fitting for the pump then drilled the bsp thread out of the tank and turned a larger nipple for that. Using 1" 1wire and hose clamps, it has completely resolved the problem. Nice opaque clean round jet of oil blasting back through the return line now, so that's all it was.

Maybe someone in the next hundred years will think of trying this and I can finally contribute by saying it won't work. By the time you tap your port with the largest size you can fit inside your four port bolt holes, you still reduce the ID too much by putting a nipple in there.

Most likely the smaller intake hose combined with the flow rate caused enough negitive pressure to leak air past the pump shaft seal. Quick way to check that is to squirt some heavy oil on the shaft and seal while the pump is running and note the change in noise. Some types of pumps are worse than others. Pump shaft seals are not subject to out put pressure.

It's been a tiring project this one nothing went simply but I'm finally here. All's left to do is add an interface to the spear end to change dies, wedges or pins etc. What have you seen on your presses to achieve a quick change type fitting?

It stalls out at 3200psi which I'm pretty happy with, just over 37ton. I think it's the relief valve in the spool letting go so it'd probably do more but the motor is working pretty darn hard at that point anyway. I may also gusset the cylinder somehow yet.