Saturday, March 14, 2009

Suction Line Oil Level

I borrowed this topic from the H & P Forum but I have seen some discussion about my posts:

If positive displacement pump is running and stopped , pump is mounted on tank and suction length above oil level is 200 mm and there is no leakage in suction line and suction chamber ,whether the oil will remain full in suction line after pump stoppage . Will it fall down ?

What happens if length is 400 mm, 800 mm, etc.

4 comments:

woodygb said...

Note that I know nothing at all about a "positive displacement pump" other than a quick Google.

But I believe this is another atmospheric pressure / force difference question.

So ...atmospheric pressure at the suction pipe inlet orifice supports the oil column.
This inlet pressure of one atmosphere ...or whatever it might be at different elevations in the world... is the limiting factor for the oil column height.

1 Atmosphere = 14.7 psi
and approx 1 bar.

1 bar pressure will I believe support a column of water 10 metres high.

JDK said...

1 cubic foot of oil weighs approx. 58 lbs.
58lbs/144 in sq = 0.4 PSI per ft. of height.
At 14.7 PSI atmospheric pressure
14.7/0.4 = 36.75 ft.

Spot On - Woodygb. At 14.7 PSI atmospheric pressure and 36.75 ft or less of oil column height the suction line should stay full.
Note: This does not mean this is an ideal suction line condition, just that the column will stay full when the pump stops.

pnachtwey said...

This only skims the surface. What is really needed is a study of NPSH or net postive suction head.
http://www.engineeringtoolbox.com/npsh-net-positive-suction-head-d_634.html
Even this article doesn't cover the whole story. Luciano mentioned on the H&P forum that there is a pressure loss required to accelerate the oil. This is important to me because I thought that using a VFD instead of a pressure compensate pump would be a good idea. The motion controller knows how fast all the actuators are moving and can calculate the total flow at any instant and therefore the controller can tell the pump to instantly respond instead of waiting for the pressure to drop. However, I was told that there may be problems accelerating the oil on the suction side and causing cavitation. The calculations show this to be a possibilty. I found out that the VFD will need to have acceleration limits. Simply put the acceleration of oil must be taken into account too.

JDK, note the pump and system curve in the article and the intersection at the operating point. Bosch seems graph their pump curves with axes turned 90 degrees.

JDK said...

"Simply put the acceleration of oil must be taken into account too."
Each post on Hydraulic Answers can only address one principle of hydraulics. The intention here is to understand that atmospheric pressure will support a static column of fluid of a defined height.

"This only skims the surface."
I agree.