8 replies to this topic

[marthin_was]

Dear Forum,
I have concern with flange-tap to measure differential pressure.
From my company standard, I learn that for liquid metering purpose the flange tap connection is located at bottom and for vapor/gas metering purpose the flange tap connection is at top? both nozzles (for tubing connection) are arranged 45 degrees, so from section view, it looked like V shape.
Questions :
1. What is the reason for bottom and top orientation?
2. why the differential pressure tap connection shall be arranged in V shape?
3. how about measurement for 2 phase flow? let say wet steam, which arrangement we should choose, top? bottom? or may be at side ?

thanks,

[djack77494]

marthin,
Differential pressure flow measurements may use relatively small pressure differences to infer flowrate. The designer must, therefore, exercise care to ensure that the differential pressures sensed correspond to the desired dP. Seems like no big problem, but if you get bubbles in an impulse line that you believe is liquid filled, or if you have a "liquid plug" in a vapor sensing line, then you may not be measuring what you want. The easy solution is to have liquid line configured so bubbles will rise and escape, and to make vapor lines self-draining.

As to the 45 degree configurations, I suspect that this is done to allow easy access to the tap area without concerns of the second tap interfering. At least it looks nice. :-)

Wet steam should be treated as a vapor. You really can't measure 2 phase flows. You must treat the stream as either liquid or vapor and design accordingly. If that's not adequate, select a different technology.

[marthin_was]

thanks Djack for your input,
in my case, I have to assume the steam quality to calculate the flow rate. I confused, since in some part of our facilities, dp flange tap is located at side, I believe with this configuration will resulted in eror reading due to liquid blockage in my tubing. Since what I want to measure is only dp for vapor section.

Our current practice is to read diff pressure with portable dp cell. what is the best configuration for the flexible hose from flange-tap to the DP Cell? do we have to make the hose straight and without low point section? Since IMO with this configuration it would not condensed the vapor.

thanks

[marthin_was]

things getting more confusing, since I just finish reading API RP551, and it recommends to install pressure tap at side of orifice-flanges

[ankur2061]

things getting more confusing, since I just finish reading API RP551, and it recommends to install pressure tap at side of orifice-flanges

Hi,

Check the following weblinks for orifice plate flowmeter installation, specially for steam service:

http://www.spiraxsar...s/SB/s66_11.pdf

http://www.spiraxsar...meter.asp#head4


Regards,
Ankur.

[Andrei]

djack77494,

I would like to disagree with your last statement.
The same thing said a lot of guys in 2004 before we designed and commissioned successfully a two phase measurement in a once through steam generator (70 to 80% steam quality), the largest in the world at the time.
Yes, you can measure a two phase flow using an orifice type device.

[djack77494]

Yes, you can measure a two phase flow using an orifice type device.

Sounds like I was labelled as "old school". Actually it's quite accurate, and I wear that badge proudly. I'm still having trouble imaging how a relatively simple orifice plate and dP transmitter can successfully measure 2 phase flow. The need/desire to measure 2 phase flow has been around for a long time. And the orifice plate/dP cell has been "the standard" in flow measurement forever. I can't imagine that with all the very clever instrument engineers running around they were unable to get that to work. I suspect that any such measurements would require a "known" distribution of phases.

[Andrei]

djack,

If my post sounded to you as labeling I apologize, that was not my intent. I do not label anyone in any way. I may not like to be labeled by others. So, I am not doing to others what I do not like for myself.

[djack77494]

Please don't worry, Andrei. That comment about "old school" was said as much in jest as anything. Anyway, I certainly did not take any offense.
Doug