A Question About Hydraulic Pressure

[Scorpion]

A general question on hyraulic pressure here. If a pump has an maximum recommended working level of 275 bar how can it generate a greater pressure?

I only ask as this happened at work last week, I checked the pressure on a machine after a breakdown and the 400 bar rated pressure gauge went off the scale, the only thing that stopped it going any further was the pin at zero.

I looked up the pump rating and it was 275 bar, so how could it reach over 400 bar?

Pressure = resistance to flow so I'm assuming that if the outlet port of the pump is say 40mm then the 275 bar rating would be if it were pumping through piping of the same diameter? If the pipe reduced to say 25mm then the fluid moving through it would encounter greater resistance to flow and the pressure would increase, wouldn't it? Or am I barking up the wrong tree?

I know that this is a really wide ranging question but I know some of you may have a definitive answer. (There are some clever buggas on here).

(The engineers at work couldn't explain it by the way!)

[gwh200]

Was the gauge uncle franked?

If not there's a multitude of reasons a pump can fail...but it depends on what sort of pump it is and what it does.

[Scorpion]

Gauge was fine Graham, once I adjusted the high pressure valve I got it to go back down to 250 bar, which was the systems normal operating pressure.

The pump powers 2 cylinders which drive a ram that compresses waste paper. The ram had jammed on the forward stroke and it looks like some clever sod thought it would be okay to just keep turning the HP valve up until it could go no further!

[gwh200]

This 275 bar...is that a safe working pressure, a max working pressure or does the pump achieve that pressure on each stroke. My guess us that the 275 bar is a safe working pressure. There will be a high pressure relief valve somewhere in the system...and I reckon you've answered your own question. If some lunatic bypasses/changes the rating by messing with the hp side of the pump...leg it!

When I re read your pist jim, I noticed the 250 bar normal working pressure.
Is the 275 bar a supposed max pressure on each pump stroke?

[Scorpion]

The pump was rated at 275 bar working pressure, I assume this isn't the maximum achievable? The normal working pressure of the rams should be 250 bar, this should be enough to shear the paper between the main ram and a fixed 'knife'.

However the operators often overfill the compression chamber and the main ram jams. In automatic mode it will try to retract and push again 6 times and then cut out.

Of course some wise guys think that if they keep turning the HP valve up the ram will eventually force the paper through - wrong! To cut a long and complicated story short the hydraulic cylinders burst the end caps free due to the extremely high pressure!

[BUSHWHACKER]

Where's WikkiDave when you need him?

[Scorpion]

Where's WikkiDave when you need him?

Exactly! I've searched for answers but hydraulic pressure is such a generic question that there is so much info to take in.

Did you get my PM Vince?

[BUSHWHACKER]

Did you get my PM Vince?

I didn't, but now i have.

Just had a look on that site, plenty of info on there, surprised it's not been mentioned on here before.

Thanks Jim.

[g3po]

If the pressure relief valve was faulty or someone had adjusted it, the pressure could go higher than the rated pump pressure.

[NiftyNev]

(The engineers at work couldn't explain it by the way!)

You need engineers that actually know about hydraulics.

Why is anyone (other that qualified personell) able to adjust pressure on the pump. That's crazy. They should not have access to it at all.

Nev

[Bogus]

A positive displacement pump will go as high as it likes until something breaks or its driver can no longer take the strain. The pressure which it discharges at will be controlled by downstream pressure, possibly by a control valve which opens at a certain pressure and releases the fluid to whatever pipework or system is downstream. It will also be protected by a PSV ( safety valve) or releif valve which could be internal or external and will open to release pressure at a point below MAWP. ( Max allowable working pressure) It could also be fitted with safety cutouts which will trip the driver motor. Any positive displacement pump which has no safety valve, and which is pumping against a closed discharge and has no auto cutouts,will as said already, carry on developing pressure ad infinitum until the weakest link gives. This could be something like a seal failure, a drive motor which just cant take it any more, or it could be a catastrophic failure of the pump, ( Like a bomb going off with shrapnel flying everywhere).

Bogus

[Bogus]

You need engineers that actually know about hydraulics.

Why is anyone (other that qualified personell) able to adjust pressure on the pump. That's crazy. They should not have access to it at all.

Nev

It depends exactly what they adjusted. If they were adjusting the setting of a safety / relief valve, thats coming close to a hanging offence. Such devices should be tagged with their set point , the date it was last tested and sealed. If they were adjusting a legitimate discharge pressure controller, thats OK. and we have here a failure of a safety system, which of course is not OK

Bogus

[dieselboy]

A positive displacement pump will go as high as it likes until something breaks or its driver can no longer take the strain. The pressure which it discharges at will be controlled by downstream pressure, possibly by a control valve which opens at a certain pressure and releases the fluid to whatever pipework or system is downstream. It will also be protected by a PSV ( safety valve) or releif valve which could be internal or external and will open to release pressure at a point below MAWP. ( Max allowable working pressure) It could also be fitted with safety cutouts which will trip the driver motor. Any positive displacement pump which has no safety valve, and which is pumping against a closed discharge and has no auto cutouts,will as said already, carry on developing pressure ad infinitum until the weakest link gives. This could be something like a seal failure, a drive motor which just cant take it any more, or it could be a catastrophic failure of the pump, ( Like a bomb going off with shrapnel flying everywhere).

Bogus

Not that I know anything about hydraulics. Just thought I would put my bit in however I used to use a Huth hydraulic bending machine for bending stainless pipework (exhaust pipes). The smaller of the rams on this machine was used for flaring the ends of pipes or shaping them down. One day the rear of the ram broke off whilst under load. The ram shot across the workshop and hit the post of a 4 poster ramp putting a reasonably sized dent. The end of the ram probably weighed around 5 or 10kg as well. It was lucky for us that no one was standing at that part and the ram hit the post and not the car that was on the ramp.