Vacuum Pressure

[Bill Linne 0]

In a closed pipe, which is the same diameter for it's entire length, will the measured vacuum be the same at all points along the pipe, regardless of elevation? Specifically, I have a 2" stainless pipe leading from the suction of a blower, up the wall (10 or 12 feet), across the structure, through a valve (now open), and down into a closed basin mostly full of water (a couple feet of air space at the top). We are applying ozone to the water in this basin, therefore must trap the off-gas and run it through an ozone destruct system. The blower mentioned above draws the off-gas through the destruct system and discharges to atmosphere. Heretofore, the blowers ran across the line, and I modulated the valve to establish the desired vacuum in the basin (as measured by a transmitter up at the valve). Now, the blowers are VFD driven, and a new pressure transmitter is installed down by the blowers. The output of the new transmitter modulates blower motor speed. But, when the new (lower) pressure transmitter shows about -1" WC, the transmitter up on the deck is reading closer to -3" WC. (Like I said, elevation difference is about ten to twelve feet.) Sound normal? Thanks, Bill

[Ken Moore 23]

I'm no flow specialist, but for all transmitters to read the same you would have to have zero flow. Since you do have flow the difference is caused by the pressure drop of your piping.

[Bill Linne 0]

I'm certainly in no position to argue with you, Ken, but it seems to me that if pipe friction were the issue, the pressure drop would be the other way around. That is, greater vacuum near the source (the blower suction) and less vacuum on the distant end. That's what got me to thinking about the fact that the transmitters are at different elevations (which, to me, should NOT be an issue with air like it is with liquids). That's why I'm posting the question. At least you've offered something to think about, and I thank you for that. Bill

[Joe E. 259]

It may seem obvious, but are both transmitters calibrated properly? I also wouldn't think that 10-12 feet of elevation would cause much change in pressure in this case.

[andybr 5]

Since 1 atmosphere is 34 feet of water the difference between your two readings is less than 0.5% of full scale if the transmitter reads vacuum only or less if it reads positive pressures as well. This seems a pretty good level of accuracy to me. I would try swapping the transmitters over to see if the difference moves with the instruments. Edited 11 May 2009 by andybr

[Bill Linne 0]

Problem resolved! Turns out the brand-new Rosemount transmitter had an offset value of 2.5 programmed into it. Set the offset to zero and viola! Thanks for your thoughts and posts. Bill

[TimWilborne 108]

Thanks for letting us know what you found