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NPSH of hydraulic gear pump

NPSH of hydraulic gear pump

NPSH of hydraulic gear pump

(OP)
Hi guys,

We have two hydraulic external gear pumps of Rexroth company which are coupled with an electromotor. I don't have the NPSHr of these pumps and couldn't find in the datasheet of these pumps. How can I find the NPSHr of these pumps? which one should be criterion design and compare with NPSHa? The serial number of these pumps are: 1PF2G330/038LD07MBK , 1PF2G330/038RN07MRL.
Thank you for your help.

RE: NPSH of hydraulic gear pump

For lube oil, would say NPSHr would be next to zero since it has no volatile components in it at ambient conditions.

RE: NPSH of hydraulic gear pump

(OP)
Dear Ted
Thank you for your answer. Unfortunately, our pumps are the old types of the Rexroth pumps and their datasheets are not available in the Rexroth site. Meanwhile, I couldn't find the NPSHr of the Rexroth available pumps in their datasheets. Can you help?

RE: NPSH of hydraulic gear pump

(OP)
Dear georgeverghese

Thank you for your answer. The ISO VG46 oil is used in the system of sliding bearing and the Rexroth pump circulate and cool the oil. Is it correct that checking the NPSH is not important for the positive displacement pump such as the external gear pump?

RE: NPSH of hydraulic gear pump

NPSHr is also influenced by the type of fluid pumped. Lube oil has very little volatiles in it, and vapor produced at very low absolute pressure is minimal, so NPSHr is negligible in this case. NPSHr is significant only when there is vapor produced to the extent that it begins to decrease the pumping head.

RE: NPSH of hydraulic gear pump

The leading question is, why are you asking / concerned, do you have or anticipate a problem, or is it a question you think needs an answer - for some unknown reason?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: NPSH of hydraulic gear pump

Irrespective of the age of the pumps, it is the vapour pressure of the oil that dictates the NPSHr.

The vapour pressure of mineral oil is as close to zero as makes no difference. The seals in the pump will be drawn into the pump before the oil boils.

Most pumps have a maximum inlet pressure of 0.8 BAR Absolute - 80 kPa

RE: NPSH of hydraulic gear pump

To prevent air from being drawn into the pump through pump shaft seals, try to keep the suction pressure above atmospheric at max flow / min level in the source tank.

RE: NPSH of hydraulic gear pump

(OP)
Dear Artisi,
Thank you for your comment. Unfortunately, there is noise in the pump and then the coupling (between the pump and motor) fracture. So, we think that maybe the cavitation is the main reason for this problem. We need to know the NPSHr of the pump to check the cavitation possibility.

RE: NPSH of hydraulic gear pump

(OP)
Dear Hpost,
Thank you for your comment. The vapour pressure of ISO VG46 oil is 133.5 Pa.

RE: NPSH of hydraulic gear pump

Is there any signs of cavitation within the pump?
I cannot see "cavitation" causing coupling failure - operating on and off prime could result in failure - is air being entrained causing prime loss, is feed aerated?
Assume mechanically everything is 100%, pump /motor alignment etc.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: NPSH of hydraulic gear pump

Entrained air is not cavitation.

For clarification: this post and the following post refer to 2 deleted comments.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: NPSH of hydraulic gear pump

Entrained air / air bubbles do not lead to cavitation, entraining air into a cavitation pump actually reduces and can in some cases illiminate cavitation.
Air bubbles do not implode, vapour bubbles can implode if at a low enough pressure and pressure increase is high enough.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: NPSH of hydraulic gear pump

Pump noise and coupling failure together would tend to point to a misalignment / bearing fault issue much more than any cavitation.

Gear pumps in general have a very low NPSHr so whilst it would be good to eliminate it from you fault finding I think you need to look at other places first.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: NPSH of hydraulic gear pump

Another obvious place to check for air entrainment into the pump suction would be at the source tank - oil level should be adequate to enable degassing, and there should be a vortex breaker on the tank exit line to the pump.

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