Measuring Vacuum Pressure in Suction Line

[KimWonGun]

I am testing a single-element gerotor pump (0.58 cu in / rev)using Mobil 254 jet oil at 65 deg F. I am using 0.5" stainless steel piping with generous bends (and no fittings)for both delivery and suction and run the drive motor at 3000 RPM. Using a Bourdon vacuum pressure gage tapped into the pump housing at suction, I expected to see practically no vacuum pressure. To my surprise I am reading 10 inches Hg consistently. What am I overlooking?

 

[KiwiMace]

Is this gauge or absolute pressure?

 

[SteveWag]

What is the vertical distance from the gauge centerline to the surface of the liguid being pumped? Is the supply tank well vented?
Steve

 

[Latexman]

A sketch would help us help you.

Good luck,
Latexman

 

[KimWonGun]

This is gauge pressure.

The vertical distance from gauge centerline to liquid surface is 8.5 inches.

The supply tank is well-vented.

 

[KimWonGun]

Attached is a digital image of the pump, discharge and suction pipes, and oil source. The drive motor is not visible.

 

[Compositepro]

I think the simple answer is that your expectation was wrong. your flow is over 7 gpm.

 

[KiwiMace]

You are measuring the static suction head over your fluid level, as implied above by stevewag, plus the dynamic loss of your suction tube. It seems you anticipate the dynamic loss to be small, but you aren't considering the static head?

Raising the oil can above the suction inlet level will help your static suction. Flaring the suction tube end will make a big difference to the dynamic pressure drop.

 

[KimWonGun]

Addressing Compositepro, are you referring to the dynamic pressure component?

I calculated the dynamic contribution to be under 1 psi.

As the digital image shows, I already attempted to minimize the static head by shortening the vertical distance between the fluid level and suction inlet.

Any other suggestions?

 

[Latexman]

Looks more like tubing to me with those type connectors. Makes a big difference in ID and a huge difference in dP.

At 65 F, the viscosity of that oil is ~ 50 cP. You may be in laminar flow.

Check those two things out and re-calc if needed.

Good luck,
Latexman

 

[hydtools]

That is why hydraulic systems should have large suction lines. Your velocity is about 12 fps, it should be down around 5 fps. Increase the suction tube ID to .75 in. or larger. This will allow for other losses in the pump porting and lower temperature/higher viscosity conditions without getting into suction cavitation problems.

Ted

 

[KimWonGun]

Addressing Latexman, your first sentence is unclear to me. Would you please clarify?

 

[Latexman]

Your first post says, "I am using 0.5" stainless steel pipe". It looks more like tubing to me; not pipe. If you are using the inside diameter (ID) of 0.5" pipe in your calculations and you really have tubing, you will underestimate the dP because the ID of pipe is greater than the ID of the same nominal size of tubing.

Good luck,
Latexman