High orifice plate beta ratio 2

[mjpetrag]

I have an orifice plate with a 0.9 beta ratio. I've never seen one so high and I don't even have flow coefficient data since crane 410 only goes up to 0.75. The problem with this meter is that it reads lower than the actual flow, by about 20% at Mac flow. I'm buying a new meter with a .7 beta ratio. I wanted to know why exactly the flow would read lower than reality. Is it because the beta is so high? Where is the flow coefficient factored into the orifice meter's computation? Does the meter adjust a flow coefficient based off flow or just assume one flow coefficient for all flows?

-Mike

 

[BigInch]

[Β] should be near 0.6 Min 0.4, Max 0.7
0.9 will tends to slice the flow stream around the edges and will not cause enough of a disturbance across the full cross sectional area of flow to get a proportional pressure drop. The central fluid passes relatively undisturbed, so there is no pressure drop contributed by that flow at all. The meter senses the pressure drop and has no idea that any of the fluid in the center of the pipe is passing by it.

[Β] is not variable, it is = d/D, and a constant for any given sharp-edged orifice diameter in a given pipe ID. The meter is calibrated for the increasing pressure drops caused by increasing flows, but at a constant [Β] ratio.

Buy a 0.6 [Β]

Independent events are seldomly independent.

 

[zdas04]

The uncertainty in gas measurement is virtually horizontal between 0.32 and 0.72. It increases exponentially from there. 0.4 to 0.7 is a reasonable range.

Every error I have ever seen to a Square-Edged Orifice meter results in a low reading. BigInch described the high β ratio error. A backwards plate lets the flow ease through the hole without a sudden trip event. Rotating flow tends to "screw" through the hole. Leaking seal ring allows more flow than the dP accounts for. All read low.

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"

 

[mjpetrag]

Thanks for the responses. I also was wondering about a variable flow coefficient since the flow coefficient changes with reynolds number and thus flow from the graphs I see. Is this taken into account in the meter?

-Mike

 

[zdas04]

The equations are published in AGA-3. You don't have to guess.

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"