Averaging Pitot Tube Not Creating DP


Thread Starter


Flow Element i.e. Averaging Pitot Tube (Micro Precision Products make) with DP type Flow Transmitter (Emerson make) installed in Fire Water Service to measure the Pumps discharge flow to the Fire Water System in the Plant. Flow Transmitter is showing less flow while Pumps (Jockey, Diesel Engine and main Electric Pumps) are in service. During normal running of Pumps, Pressure was found to be normal i.e. 11 Kg/Cm2 and Flow Transmitter supposed to show min Flow of 150 m3/Hr and normal Flow of 3000 m3/Hr but it is showing only 8 m3/Hr and 14 m3/Hr respectively.

Checked the Flow Transmitter Impulse Tubing, calibrated the Flow Transmitter and found to be normal. Inspected the Averaging Pitot Tube off line and found to be normal (No choke and no worn out).

Hence we request you to advise to resolve the problem.

The last two times I heard this complaint at a power plant, it turned out that there was no virtually flow through the system--that is, no hydrants were open, etc.--when the flow measurement system was being consulted while the pumps were being run in a normal "exercise" mode.

When the jockey pump runs to maintain pressure in the system, there really isn't very much flow--because no hydrants are open/flowing any water. If the Diesel pump and/or the Main pumps are run just to exercise the motors periodically (as they are in most plants--with no open hydrants), then there will be very little flow after the pumps are started.

I have seen some fire systems with check valves and small bypass/recirculation lines, but, the flow through them would also be relatively small if no hydrants were open.

So, the question is: What are the conditions when the flow is being measured? Are there hydrants (nozzles; fire hoses; etc.) flowing water when the flow measurement system is being monitored? If so, is the flow anywhere near rated, or is it just a small flow?

In general, pumps are usually slightly over-rated for their application to provide some margin for high flow conditions. So, I wouldn't necessarily expect to see anywhere near rated flow unless I knew that sufficient hydrants were flowing enough water to approach rated pump flow.

Finally, are you certain, one, that the pitot tube is installed correctly? And, two, that there is nothing in the piping that is blocking the flow through the pitot tube (moss; algae; etc.)?

Hope this helps!
How are you creating the minimum and maximum flows?

Pumps run status alone or by opening discharge valves to allow flows as pointed out by the first responder.

Fire water pumps are set up differently than most pumping systems, to insure 100% flow delivery in emergencies.
Dear CSA,

Main pump started (Rated Flow-1000Nm3/Hr) with full open of re-circulation valve and 4 discharge Hydrants are opened (Each hydrant capacity is 54 Nm3/Hr). DP monitored in Pitot tube was only 14MMWC (Varying between 2-14 MMWC), and flow was 14 NM3/HR max.

We removed the pitot tube from line, and pitot tube cleaning & flushing done. No chock observed in pitot tube.

After pitot tube installed, same response of DP during main/diesel pump started with hydrants opened.

I am suspecting on Pitot tube may be installed reversely. Even though HP/LP marking is available on Pitot tube and installed as per marking but even may be wrong direction installed.

Pls guide to us, any further troubleshooting to be done.
Dear Sir,

Min Flow created through Jockey pump started (Rated Flow of Jockey Pump 150NM3/Hr that is minimum flow) with discharge hydrants opened and re-circulation full opened, but Pitot tube was showing only 5 MMWC DP and flow it was 8 Nm3/Hr.

We started Main pump (Rated flow=1000Nm3/hr) and diesel Pump also (Rated Flow: 1500Nm3/Hr) and 04 hydrants (Each hydrants rated capacity: 54 Nm3/Hr) opened to check flow in pitot Tube, but it was showing only 14MMWC and 14NM3/HR flow respectively.

1st report shows flow is increasing with the square of the measured d/p.

2nd report shows that the fluctuations of 2-14 mmWC exceeds the measured d/p in the first and third report.

3rd report shows the flow increasing with the square root of the measured d/p.

Suggest that you:

A. Check you meter location in the piping layout to insure that your have required meter runs upstream and down.

B. Confirm that the meter is installed per manufacturers drawing. Typically below the pipe centerline with liquids to avoid entrapped air in the impulse lines. Also make sure the impulse lines are properly purged of bubbles.

C. Determine where your square root extraction is being done, either in the transmitter, or in the DCS/Display.

D. Confirm the pipe size in the meter run (Est NPS18 to NPS24 pipe)

E. Confirm the flow and d/p units in the flow element specification match those used in the flow calculation.

gook luck,