Large line mixed gas flow

[Chinook82]

I've looked at most everything I can think of for this application.A mixed gas by volume % of SO2:11.73, O2:11.73, CO2:0.14, N2:76.4. The pipe is 86" diameter, 10mm wall CS. Norm flow 78,000 Cu ft/hr, 150 deg F., upstream press -2PGIG. minimal up/downstream diameters too as per the attached.

I've looked at averaging pitot tubes, thermal mass, ABB Torbar and Meriam Accutube. Trying to sort out if just taking taps inside/outside the elbow might work or a transit time gas meter?

All suggestion gladly taken.

Thanks, Dave

 

[danw2]

Look real carefully at the spec for minimum pressure requirement for transit time ultrasonic. It's my recollection that the clamp-ons need quite a high pressure, probably to achieve a density sufficient to carry the signal. 100psig comes to mind.

 

[Niwot]

Chinook, can you check the flow rate and/or units again (SCFM vs. SCFH for example). The velocity at 78,000 SCFH in a 86" line seems extremely low (just 1-2 FPS). For example, at 78,000 SCFM (not H), the velocity would be roughly 40-45 FPS. Is there a rating on the main blower shown in your diagram?

 

[Chinook82]

Thanks Niwot, I discovered this earlier and should have updated this site. The norm flow is 86,500 CFM, other parameters remain unchanged.

I'm still stuck on this one if anybody can help.

Thanks

 

[Niwot]

Chinook, given the revised flow rate I don't see why this can't be done with an avg. pitot. I ran a flow calculation (using a mole wt. of 32 based on the gas composition) and get roughly .72" H2O DP (with a good draft range DP transmitter, this is reasonable). Of course, I don't know what turndown you are expecting (no min. flow rate was given). You didn't have the straight-run identified from elbow to elbow in the vertical section, but based on the elevation details, I estimated roughly 5d. Options:

1) Install an avg. pitot in plane with the top elbow. Install it 1.5d in front of the bottom elbow in the vertical section (with limited straight-run, better to cheat on the downstream side). No reason you shouldn't get good repeatability (accuracy would have to be "estimated").

2) To improve accuracy, cross two avg. pitots in the same location and a) manifold them to one common DP transmitter or b) install a DP transmitter on each avg. pitot and average the combined signals in the DCS. I prefer option 2 for redundancy.

3) For ultimate accuracy, perform a dual axis pitot traverse while the avg. pitot(s) are in line (installed and operating). Calibrate the avg. pitots based on the traverse data.

Obviously others may have differing opinions, but I've seen many piping/duct scenarios in coal fired power plants (primary air, secondary air, air to pulverizers) that have less straight-run. It's tough measurement with limited options (especially in the large duct sizes) and you have to live with a degree of inaccuracy, although repeatability should be very good if the meter is installed correctly and all variables are considered.

I assume the SO2 is dry?

If the application is "blessed" by the manufacturer, you could always ask for a trial (delayed billing) based on performance.

Hope this helps.