3 replies to this topic

[axsatr]

Hello and Good Day

 

I would like to request some opinions for an issue that we've been facing here on a centrifugal pump

 

Background;

This pump supplies caustic to an LPG treater at rated flow of 24.1 m3/hr

There are two treating trains, so, approx. 12 m3/hr to each train as per design

 

Currently, we are only achieving 5-6 m3/hr total flow from this pump at FLC (74 A)

 

The pump performance haven't always been good, ever since we've installed it back in 2009, however, it used to be able in delivering at least 15-16 m3/hr (from 24.1 m3/hr at rated).

 

As of few months ago, the performance just gradually diminished to its current state.

This problem impacts both available pump (running and standby).

Operating both pumps in parallel results in one pump to be operating at FLC, while the other pump not reacting to the flow changes

 

The shut-off ampere of the pump is surprisingly high at 60-63 A, as FLC is at 74 A. Is this behavior normal?

 

The head that we tested was slightly higher at shutoff (4-5 meters higher), is this significant?

 

Things we've done so far (not in chronological order);

1) Suspected faulty flowmeters, recalibrated and had readings compared with portable ultrasound flowmeter and flow calculated from control valve opening vs. Cv   :    +/- 5% error, no issues found

 

2) Suspected clogged inlet piping - cleaned strainers (no debris found) and verified inlet pressure piping at two points. Results are normal (i.e. Vessel pressure + static head) - no excessive pressure drop

 

3) Suspected recirculation through standby pump - did IR scanning to verify thermal profile, and replaced check valves on both pumps for good measure - no improvements/issues found

 

4) Suspected eroded impellers/wear ring/any pump internals - disassembled pump, replaced wear ring (only 0.1 mm wear found), checked impeller/casing clearance (only 0.2 mm gap found, but rectified), and verified no abnormal wear to internal pump parts, fretting fatigue marks on shaft however was visible.

 

5) Suspected motor poor performance - Uncoupled motor and performed test run. Amps recorded were only 20-25 amps. Winding resistance was normal.

 

6) Suspected fluid property changed - Density test showed deviation of 65 kg/m3 (1125 design vs. 1190 kg/m3 found) - acceptable. Viscosity test wasn't performed as no lab would accept caustic to be tested due to corrosive nature, however, visually the caustic was water-like in appearance, as it should be

 

7) Suspected wrong rotation - did bump test and observed the spinning direction. no issues found

 

8) Suspected motor overspeed - Monitored the motor rpm for a prolonged timeframe. No abnormalities found

 

One more thing that we've noticed is, the discharge temperature increased by 3 degC if measured by a temperature gun, compared to the inlet (average over 10 measurement points), but this could be associated with the lower operating flow, right?

 

Currently, we are looking at installing a few pressure gauges and recording the pressure drop at discharge piping, plus a radiographic testing of several points along the discharge piping. Based on my understanding, a clogged discharge should result in a lower flow and lower power (opposed to the current low flow but higher power). Is this statement correct?

 

Any other good troubleshooting tests? One last resort is to spend some money and perform a bench test, and produce a new curve for the pump and motor, but as this involves money, I'm making sure we've exhausted all available options before attempting this.

 

I'm sorry for this long problem statement and question, but I really am stumped by this issue. I hope any of you could share your expertise on this issue.

 

Thank You

Edited by axsatr, 28 September 2017 - 11:38 PM.

[breizh]

Hi ,

what about the hydraulic calculation of your system ? Probably good to review the pump curve vs the system !

Hope this helps

Breizh

 

[axsatr]

Hi Mr. Breizh,

 

So I re-calculated the pressure drop across piping (yet to be verified on-site due to pressure gauge point accessibility) and plotted it for a system curve analysis (i'll re do the system head once the PGs are available)

 

Do correct me, but a system head = Elevation (m) + Pressure drop (m) + (Desired head - Suction head)

 

Is this correct?

 

For clean condition, my roughness used was 0.3 mm, and 1.5 mm for fouled piping, is this okay?

The difference in pressure drop for clean and fouled piping was only 1 bar

 

The curve still overlaps at about 20 m3/hr, unless I reduce suction pressure too by 1 bar, which reduces it more significantly to 6-7 m3/hr

 

However, this condition should result in a low amps rather than high amps, right?

I have verified the suction pressure to be around 4-4.2 barg vs. 5.3 barg as per design.

 

However, when I increased the suction head to 5 barg, no impact was observed at the running amps.

 

Thank You.

 

[breizh]

Hi ,

If needed  this spreadsheet may help you .

 

Good luck

Breizh