positioning of orifice plate
positioning of orifice plate
(OP)
I have some queries regarding the positioning of an orifice plate in a vertical gas main approx 30 inches in diameter.
Ideally it should be positioned with 10 straight pipe diameters upstream and 5 downstream.
Assuming I only have 10 straight pipe diameters in total (a 90 degree bend at one end and an isolation valve at the other) where would be the best place to position it? I assume that keeping things in the same ratio, it
would be best with 6.6 diameters upstream and 3.3 downstream?
With regards to straight pipe either side of the orifice plate, which would cause the biggest error to the d.p. produced, not enough upstream or downstream straight pipe diameters?
Also how critical is the smoothness of the pipe bore? If the main had at some time in the past been cut through, separated by 6 inches and had a band welded on the outside of the main to join it back together – would this affect the d.p. if it was within a couple of pipe diameters of the orifice plate? Also there is an inspection hatch close by, I assume this would be detrimental too?
I look forward to your replies.
omby
Ideally it should be positioned with 10 straight pipe diameters upstream and 5 downstream.
Assuming I only have 10 straight pipe diameters in total (a 90 degree bend at one end and an isolation valve at the other) where would be the best place to position it? I assume that keeping things in the same ratio, it
would be best with 6.6 diameters upstream and 3.3 downstream?
With regards to straight pipe either side of the orifice plate, which would cause the biggest error to the d.p. produced, not enough upstream or downstream straight pipe diameters?
Also how critical is the smoothness of the pipe bore? If the main had at some time in the past been cut through, separated by 6 inches and had a band welded on the outside of the main to join it back together – would this affect the d.p. if it was within a couple of pipe diameters of the orifice plate? Also there is an inspection hatch close by, I assume this would be detrimental too?
I look forward to your replies.
omby





RE: positioning of orifice plate
even if you are using plate flanges, a 30" plate will need special support, etc. especially in thin wall ducting.
RE: positioning of orifice plate
Installation of new isolation valves downstream have meant altering the main as they are bigger than those used previously.
The orifice plate installation wasn't ideal to start with, and since I joined the current project (which is not a new installation) half way through and the deadline is rapidly looming, I'm trying to weigh up my options.
I know how it should be installed in an ideal world, but I'm unsure as to the expected size of any errors due to what I have to work with (as in the original post).
RE: positioning of orifice plate
the error and uncertainty are in the 10% range.
RE: positioning of orifice plate
RE: positioning of orifice plate
Take a look at an averaging pitot tube. Dieterich Standard/ Annubar is one tradename. Meriam Instrument/ Accutube is another. These sample the flow at 4 points across the pipe diameter, and internally average the reading. So you can theoretically read even if the flow profile is distorted. Also, an averaging pitot is easy to install: just a couple of holes opposing each other, with weldolets attached.
RE: positioning of orifice plate
Some head type flow elements such as the McCrometer V-Cone or Fluidic Techniques Flow Pak are available for about 3 diameters upstream.
RE: positioning of orifice plate
In 30-inch pipe with vertical flow (the OP didn't say if the flow was up or down), unknown pipe roughness, a varying diameter in the upstream pipe, short pipe runs, no flow-conditioner, going through an "isolation valve" (didn't say if that was upstream or downstream or what technology the valve was, it could be a butterfly valve for all we know) or a 90 elbow. The OP also didn't say if he was using an OFU or a cast plate holder (different uncertainties for each). If you run all of these into the uncertainty calculation (I haven't) you would probably get about the same uncertainty as a random number generator and a dead-band so wide that the numbers are simply worthless either for engineering analysis or (god forbid) for custody transfer.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
RE: positioning of orifice plate
RE: positioning of orifice plate
RE: positioning of orifice plate
Have a go at it, there's over 20 papers available for download . . .
http://www.ceesi.com/pubs_uncertainty.aspx
Dan
RE: positioning of orifice plate
http://www.ceesi.com/pubs_orifice.aspx
RE: positioning of orifice plate
Jim, I have had experience of averaging pitot tubes and have already considered them but the gas mix in the main can be prone to condensate and I think this may be an issue with such a device.
I know the current installations are not ideal, but they work. I'm aware of how they should be, and there is plenty of info about explaining how they should be, but finding facts to properly explain the consequenses of all the things which 'aren't quite right' is more difficult.
The fact that the installation does work makes it all the more harder to persuade management to adopt a more suitable primary measuring element or even make modifications to the pipework, so for the time being I have to make the best of what I have.
Thanks everyone
omby
RE: positioning of orifice plate
Have you ever done a material balance using this inadequate equipment? I've gone into some operations where 30% more mass was coming out the back end than went in the front. They wouldn't let me patent the pipe that was making gas. In every case it was lousy measurement.
David
RE: positioning of orifice plate
Knowing the estimated errors from the papers will just confirm that the meter is doing what it can do and if you want to use it as a trending device you can't claim a system problem as long as the error is more than the estimated error.
RE: positioning of orifice plate
R.Thiyagarajan
RE: positioning of orifice plate
RE: positioning of orifice plate
Measurement of air, gas or temperature doesn't have to be far off before poor combustion occurs or a probem is flagged from the supervisory system.
From the initial research I have done I favour using a v-cone meter, though this would be a job for the future, not this project.....
RE: positioning of orifice plate
http://www