150HP motor & pump issues ( Pump looses pressure )

Davebones

Senior Member
We have a hydraulic test stand that the operators are trying to get up to 6000 psi on it . They tell me that it gets up to 3700 psi and then drops down to 2000 psi . They claim they've replaced the pump and still have this issue . They say they've checked the test stand pressure relief valves , piping for obstruction and can't find any issues . They now are wondering if there is something wrong with the motor to cause this . ( continuous duty , 150 hp , 480 V , no VFD , across the line starting ) I don't see this as the problem but would like any input on what to look for in this situation ...
 

PaulMmn

Senior Member
Location
Union, KY, USA
Do you have a tachometer you can use to 'watch' the motor fail/slow down? I'm assuming that would be the reason the pressure would drop...

Voltage fluctuations-- can you watch those as well?

Are there seals/valves in the pump itself that are failing?? Although you did say that they replaced the pump...
 

Besoeker3

Senior Member
Location
UK
Do you have a tachometer you can use to 'watch' the motor fail/slow down? I'm assuming that would be the reason the pressure would drop...
If the motor slowed down significantly, the current rise, probably to tripping levels.
I'd be inclined to think it is a hydraulic issue.
 

Barbqranch

Senior Member
Location
Arcata, CA
Occupation
Plant maintenance electrician
Is the pump dead-headed, or is there plumbing that simply lets too much fluid flow (bad relief valve?) so it can't build to a higher pressure? If it is a positive displacement pump, I don't think the rpm (within reasonable limits) would matter unless the fluid is just flowing.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Anything happening electrically will be reflected in the current draw and protection scheme of the motor. Hydraulic pumps are usually positive displacement so the only way for it to not pump at its maximum pressure is to bypass the fluid or stall the motor.

Beside leaks, the other thing I would look for is some sort of high pressure automatic safety bypass that dumps hydraulic fluid back to the receiver. It could be factory set for 3700PSI and the user is unaware it even exists, ie the OEM designed this for 3700PSI max but the user is trying to get it to do something it is not designed to do. I’ve seen that before.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
And there may well be hysteresis in the valve so pressure will drop to 2000 and not rise again until the pump is stopped.

Sent from my XT1585 using Tapatalk
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
What makes them think they have a pump that is capable of delivering 6000 PSIG? At what flow rate are they expecting to achieve that pressure? Do they have the pump curves to show you? This may start out being a mis-application.

That said, the issue of reaching one pressure level and then dropping off sounds to me to be very much a mechanical issue. Presuming this is a centrifugal pump, the pressure at its discharge point will drop off as the flow rate increases, and the pump curves will describe how that is supposed to happen. So where is the fluid going, and what is happening to the fluid flow rate as the test progresses?
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
We have a hydraulic test stand that the operators are trying to get up to 6000 psi on it . They tell me that it gets up to 3700 psi and then drops down to 2000 psi . They claim they've replaced the pump and still have this issue . They say they've checked the test stand pressure relief valves , piping for obstruction and can't find any issues . They now are wondering if there is something wrong with the motor to cause this . ( continuous duty , 150 hp , 480 V , no VFD , across the line starting ) I don't see this as the problem but would like any input on what to look for in this situation ...
Hydraulic pumps may incorporate an internal pressure relief that limits output pressure. If you could grab a model number that would be very helpful.
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
We have a hydraulic test stand that the operators are trying to ...
Electro-hydraulics was my business until (and some after) retirement. If you can share a schematic, ideally with the BOM, I'd be glad to look at it. I'll PM you my email address.

That 3700 to 2000 is too much pressure drop for any relief valve hysteresis of which I'm aware. One possibility is significant overheating and severe efficiency loss ...

It's ALMOST for sure a positive displacement pump, although it may be a variable displacement.

I've done some constant horsepower designs which allow temporary (several seconds) overload of the electric motor before reducing pressure ... but more commonly flow ... to stay within the electric motor capacity.

Ohm's law applies ... pressure (voltage) * flow (current) is power.

Rule of thumb is 1 gpm @ 1500 psi is 1 HP (100% efficient, 1 gpm @ 1714 psi) maintaining 6000 psi with 150HP you'd be able to create flow to 35-40 gpm.
 

oldsparky52

Senior Member
Rule of thumb is 1 gpm @ 1500 psi is 1 HP (100% efficient, 1 gpm @ 1714 psi) maintaining 6000 psi with 150HP you'd be able to create flow to 35-40 gpm.
I'm not understanding something here. I know I can get more than 40 gpm from a 2 HP submerged pump in a diesel tank, a 2" line 500' long (working at around 50 psi). I would think a pump that could develop 6000 psi could pump a lot more than 40 gpm. What am I not understanding?
 
Look at it another way:
- The pump creates a volumetric flowrate.
(determined by the pump speed and the volumetric displacement per turn)
- Resistance in the hydraulic circuit creates pressure.

... What am I not understanding?
I can't see the inside of your mind from here, but I'm guessing you're getting lost in unit conversions and an inconsistent system of measurement units.

Ohm's Law for fluids:
(volumetric flowrate) x (pressure) = power

In metric units,
1 [cubic meter per second] x 1 [newton per square meter] = 1 [Newton*meter per second]

also known as
1 [cubic meter per second] x 1 [Pascal] = 1 [Watt]
 
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rambojoe

Wireman
Location
phoenix az
Occupation
Wireman
We had a problem with our welders old iron man machine (a huge metal punch/shear multi tool) -everything worked fine until he punched or sheared the maximum thickness of steel, then it would go very slow and even fail to complete the action. The pump motor was spinning the wrong way. Just fyi...
 

Besoeker3

Senior Member
Location
UK
Look at it another way:
- The pump creates a volumetric flowrate.
(determined by the pump speed and the volumetric displacement per turn)
- Resistance in the hydraulic circuit creates pressure.


I can't see the inside of your mind from here, but I'm guessing you're getting lost in unit conversions and an inconsistent system of measurement units.

Ohm's Law for fluids:
(volumetric flowrate) x (pressure) = power

In metric units,
1 [cubic meter per second] x 1 [newton per square meter] = 1 [Newton*meter per second]

also known as
1 [cubic meter per second] x 1 [Pascal] = 1 [Watt]
OK, pedant strikes again - and will probably get shot down for it again......:D

These are not metric units, they are SI (Système international).
You can't measure Watts by the metre.
 
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