Orifice Flange Tap Routing and Orientation 1

[macmet]

Hello Everyone,

I've done a search of the internet and this site regarding the routing of the piping to flange taps around an orifice plate. I've found a number of references to the orientation of the taps on the flange, but I'm looking for information on routing of the piping from the ports to the transmitter.

Our orifice plates are in a feedwater line and are measuring the flow feeding a boiler. A colleague and I are having a friendly disagreement as to how the piping to the pressure ports should be routed. We are both of the belief that the ports should be 45 degrees "below" horizontal. Can these ports then be routed up vertically or should they be routed down directly to the transmitter.

Also in my searches I noticed that the pressure ports are often mounted on opposing "sides" (i.e. if I look down the direction of the flow at the flange, I see one connection at about 4 O'clock and one at 8 O'clock). Our installation here has them so they are both at the same spot. (I.e. If I look down the direction of the flow at the flange, both connections are at 4 O'clock (or 8 O'clock).

Any feedback or direction to a good site would be appreciated.

Cheers

 

[zdas04]

Start with the underlying arithmetic to convert dP to flow--it is Bernoulli's equation at its root. Bernoulli's equation requires constant density. If you line is not full, then the density of the total fluid in the control volume changes from second to second and all the calcs are wildly invalid.

Now, if you have taken steps to ensure that the line is always full (I use a significant vertical section downstream of the plate, but there are other ways), then you should put the taps in the top of the pipe to minimize the hydrostatic head that is not related to the flow (and to keep debris out of them). I also want the secondary elements close coupled if there is any way to do it, in any case I want the impulse lines as short as humanly possible. The dP and GP lines need to be adjacent to each other, putting them on opposite sides of the pipe introduces a significant uncertainty.

On the other hand, if you haven't taken steps to ensure that the line is full then it just doesn't matter what orientation you put the taps in, you will always have a moderately expensive random number generator that puts out numbers that have nothing to do with the total flow.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[macmet]

We have installed flange taps and since we are measuring water flow, I believe our taps should be within 45 degrees below the horizontal.

I noticed in your response you stated how important it was for the fluid to have constant density, which makes me think the piping should go straight down to prevent any air being entrained along any high points in the run, which is what I initially thought.

Any other comments would be appreciated.

 

[zdas04]

Amazing the nonsense you can find on the interwebz. Taps below the horizontal is a lame way to try to still get "measurement" when the pipe is not full. It is absolutely invalid. If the pipe is not full, then the dP is no longer proportional to flow and all the math breaks down. You get numbers. Period.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[hacksaw]

you should not have problems with entrained air in a BFW line, never the less during initial line filling, you may be bubbles so it is customary to use drain/vent holes in the plate,

 

[zdas04]

You're kidding right? You may not have air in the line, but it doesn't take much to get steam which acts pretty much like any other gas.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[hacksaw]

it is not a laughing matter,

air, flashing etc in a BFW line is only possible in cases where the mechanical/process design is at fault and someone is likely having a suit against them for practicing engineering without a license

steam plant are dangerous enough as it is

 

[macmet]

In our situation it would have to be a operations error to get air in the lines once operational as they remain full. I understand what you are saying and will keep it in mind though.