OT: water pressure meas. in dist. system

I guess because I know a little about PLCs...I have been asked to review the setup at some remote potable water distribution pressure monitoring "nodes" for a municipal water system. The pressures present in the system are typically about 65 psi.

The problem I see is that the 10" distribution pipe is about 6 feet below grade and the pressure transducer (an E&H Cerabar T) is about 4 feet above grade. Thus the pressure measurement point is about 9-10' above the pipe's tap point.

If I understand my "hydrostatic principals" correctly, this should result in about a 4.5 psi drop from the actual pressure.(2.31ft H20 = 1.0 psi)

Note: This transducer has no adjustments other than the "Zero" point and that is limited to +/- 5%.

SO... I concluded and wish to inform mgmt. that all the reported (measured) pressures are about 4-5 psi lower than actual.

My response is that if improved accuracy is desired and the transducer must be remotely located from the tap point it should be of a programmable or similar type to allow compensation for the elevation's effect on the measured pressure.

A collegue is telling me that "elevation doesn't matter since it is under pressure". Naturally I disagree.

Can someone confirm my thoughts and/or offer other ways to accurately measure these pressures "remotely"?

Thanks in advance...RWW

You co-worker is correct, you can't have pressure drop unless you have flow.
Delta P = specific gravity * (gpm/Cv)^2,
as you can see with a flow rate of zero, the Delta P also equals zero.

Think of it like this, if you had a 9ft tall balloon (filled with water at 65 psig), the pressure on all sides is the same. It would not be less at the top.


Ken

Ken Moore said:

Think of it like this, if you had a 9ft tall balloon (filled with water at 65 psig), the pressure on all sides is the same. It would not be less at the top.


Ken

Click to expand...

Seems weird that they have to build submarines so strong to resist crushing when the water pressure is the same as the surface. Also seems weird to spend money to put a water tower way up in the air if that won't help the water pressure at the bottom.

I disagree, the pressure would be higher at the bottom. OP is correct.

edit:

We are under pressure anyway on Earth (14.7 PSIA). Any increase in an enclosed system would increase pressure by that much.

Using the ballon as an example; it's easier to envision the syatem if we allow a small air pocket in it. If pressurize the ballon to 100 PSI, the air pocket would have a gauge pressure of 100 PSI, the water at the bottom would have a pressure of 100 PSI + Q*g*h.

Q = specific gravity of fluid.
g = gravitational constant
h = height.

The confusion comes from dealing with gasous system, since gas is so light the gravitational component is usually ignored when measuring pressure, however, enough height difference would still make a difference, think of the climber of Mt. Everst. or this picture http://en.wikipedia.org/wiki/Image:Air_pressure_crushing_a_plastic_bottle_p1180559.jpg

"A collegue is telling me that "elevation doesn't matter since it is under pressure". Naturally I disagree."

I am inclined to agree with you, if the verticle pipe were tall enough, at some point the head pressure would overcome the 65 p.s.i. and at the top of the water in the verticle section would be 0 p.s.i.
I could be wrong in this thinking but it makes sense to me.

Head should always be included.
Think of it this way
I have a vertical standpipe 100 feet tall.
I connect a pump whose shut off head (max output pressure) is 50 feet.
I start pump
It pumps until water height in column is 50 feet.

So if I have an instrument 10 feet above a pressurized line and the 10' of pipe between instrument and the line is filled with the fluid then I MUST subtract the elevation head difference from the reading.
UNLESS the 10 pipe is full of air. For practical purposes.

Now if you want to get really picky then you must calculate the pressure of the air column.

Dan Bentler

reference:
Pressure Gauge Handbook, edited by Philip W. Harland, published by Marcel Dekker, 1985

page 178, section 6.4.2, Correction for Liquid Head

If the gauge is installed above the pressure tap, as shown in Fig 6.4 (below), the head H subtracts from the measured pressure and a positive zero correction must be made.

Von said:

My response is that if improved accuracy is desired and the transducer must be remotely located from the tap point it should be of a programmable or similar type to allow compensation for the elevation's effect on the measured pressure.

Click to expand...

The correction factor you've discovered is a value that needs to be added to the gauge reading to account for the static head. This correction factor is a fixed value for each gauge site. Since that value is a fixed value, why not compensate by adjusting whatever the readout is?

The gauging device in question is an analog transducer. The 4-20mA signal is read by something: digital readout, PLC, recorder. That something has to be ranged to interpret the 4-20mA signal and convert it to understandable units, like PSI or feet of head.

Measure the amount the pipe is below ground and the impulse pipe extends above ground, convert to whatever engineering units are in use (PSI, feet, whatever) and add that 'offset' value to the readout device, either as a bias or offset; or shift the zero span/setup values so that zero isn't zero, but the static head level; and span is the former span plus the static head level.

Von said:

Note: This transducer has no adjustments other than the "Zero" point and that is limited to +/- 5%.

Click to expand...

I couldn't believe an E&H would be a crappy 5%. That accuracy spec is off by an order of magnitude. (You don't mention which model in the T series you have and I used the sanitary model because I found it first, but there is likely no difference between the specs for the sanitary version and an NPT mount version)

The spec sheet does say "Residual ripple max. 5 %"
That's not accuracy, that's ripple.

Accuracy spec is a combination of the following:

- Analogue output: non-linearity ≤ 0.5 %, including hysteresis and non-reproducibility (limit point method as per DIN IEC 60770)
- Long-term drift ≤ 0.15 % per year
- Influence of temperature • Zero: typical 0.2 %/10K, max. 0.5 %/10K. Values are 0.1 %/10 K higher for measuring spans
≤ 6 bar.
• Span: typical 0.2 %/10 K, max. 0.5 %/10 K
• Switch point: typical 0.2 %/10 K, max. 0.5 %/10K

All combined, these might be 0.5% - 1.0%, but nowhere near 5%

Dan

Von said:

Note: This transducer has no adjustments other than the "Zero" point and that is limited to +/- 5%.

Click to expand...

danw said:

I couldn't believe an E&H would be a crappy 5%. That accuracy spec is off by an order of magnitude. (You don't mention which model in the T series you have and I used the sanitary model because I found it first, but there is likely no difference between the specs for the sanitary version and an NPT mount version)

The spec sheet does say "Residual ripple max. 5 %"
That's not accuracy, that's ripple.

All combined, these might be 0.5% - 1.0%, but nowhere near 5%

Dan

Click to expand...

I think what Von is talking about is only the zero adjustment range. That would be +/-5 psi for a 0-100 psi transducer? I have used many a Cerabar T and S units and have found the inexpensive T's to be <1% off cal as Danw notes. Of course one should never adjust zero for an offset. If you assume a system pressure of 65 and adjust the zero for that, then the transducer will be correct only at that point. For a muni water system the pressure doesn’t change that much you may get away with it. But for a system that you need to know a broader range you will have problems. I get too many of our non-instrument trained guys doing that. You have to do the compensation of transducer position as a constant bias or offset in the PLC, display, DCS, whatever. If not when the transducer gets replaced with a properly cal'ed unit and the person isn't aware of the "fudged" transducer things get screwed up. I have seen that a few too many times. Cal your sensor, cal your system, document any compensation bias. That is my rule.

Thanks to All.

The consensus appears to me that my evaluation and recommendations are accurate.

Limiting this exercise to liquids and "dead head" impulse lines:
Pressure is measured at the location (elevation) of the transducer's sensor "gland", NOT at the tap point (if at a different elevation).

Thus the weight of the column of liquid must be added or subtracted to the measurement to compensate.

This compensation or offset can be applied either to the sensor or at the HMI depending on equipment type(s).

As far as the zero adjust...
I had found an old, yellowed, multi-language E&H install manual for the xducer not a spec sheet... maybe the decimal was faded and the ZERO adjust is +/-0.5%.

Thanks for the added info about accuracy though.

I'm closing the "case" on this one.

Thanks again to all...RWW

Thanks JS,

Looks like we were typing at the same time... I agree with your philosophy and I believe my "collegue" is guilty of some of the things you've hinted at.

I'm kinda new to the "controls" job (<2 yrs.) but not test and measurement. I have seen other such practices and because he feels he is so experienced he discounts my somewhat scientific analysis. Since I do respect his experience I came here to seek the "collective experience" of these forums before proceeding to further explain (respectifully argue) with him.

Thanks