Orifice plate differential range

[cam72]

Hi

One of the orifice plate specification has following clause:

For Compressible fluids differential range in inches of water shall not exceed the normal upstream static pressure in psi absolute except that for exhaust steam.

My question is - why this is applicable only for compressible fluids and what is the significance of this statement?

Thank you.

 

[itsmoked]

I'm not a dif plate wizard but I believe they're saying you must limit the flow thru the plate to below a certain amount (described by that rule). That's either to keep it's response linear enough to be usable or to prevent damaging it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[cam72]

Thank you for your response. However , my question is why it is applicable only for compressible fluids? why not for liquids?

 

[DRWeig]

Can you ask the manufacturer of the orifice plate?

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[zdas04]

The only thing I can figure is that they are trying to stay well away from the part of the flow curve that approaches sonic velocity (which is an issue in gas flow, but not liquid flow). One inH20=0.0361 psi. If the SP is 100 psia, then the minimum downstream pressure under that rule is 96.4 psia, which is probably a velocity of 0.2 Mach or less. I don't know why you would be that conservative (since you don't see significant density changes until sometime after 0.3 Mach), but sometimes an easy to remember rule is worth more than the rule meaning anything.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[hacksaw]

hardly a mystery in flow meter engineering

a routine requirement for plate type meters from early days to the present
imposed to minimize errors associated with the expansion factor correction

 

[cam72]

Thank you Zdas04 and hacksaw.I have some clue now.I will check with vendor too.

 

[Latexman]

I think it can apply to liquids too. I don't see why not.

Most orifice run dP transmitters I've seen fall into 1 of 3 ranges - 0-50, 0-100, and 0-200" w.c. Rarely have I seen them exceed 200" w.c. because more than that and you get into deflection of standard orifice plates in larger pipe runs. So, above 200 psia, I don't think I'd follow this simple rule.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[ione]

The first time I’ve read the OP I thought myself to an issue related to sonic flow. Then I considered it could be a limitation related to the orifice plate thickness.

R.W. Miller indicates the following formula for orifice plate thickness

tmin ≥ SQRT [(0.681-0.651*BETA)*deltaP/Y] * pipe ID

where:
tmin = min plate thickness [mm]
BETA = ratio of orifice hole diameter to pipe diameter
deltaP = pressure drop [MPa]
Y = plate material yield stress [MPa]
Pipe ID = pipe internal diameter [mm]

Your specific material properties and geometric configuration could give us more details to think about.

Obviously if were on the right track, the limitation above shouldn't be restricted only to compressible fluids (gases) but should be extended to liquids as well.

 

[hacksaw]

suggest you guys leave the flow meters to the instrumentation folk,

have not seen such round robin confusion over the expansion factor and whether it applies to compressible or incompressible flows, this forum is for added value not mucking things up

my two cents

 

[itsmoked]

hacksaw, how about the short explanation of what "expansion factor" physically refers to?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[hacksaw]

this is hardly a class room, right?

consult any handbook that covers flow meter engineering from the 40's on.

The expansion factor is an empirical correction for compressible flow through an edged meter. Venturi's and flow nozzles use a theoretical correction rather than empirical.

good luck on your studies

 

[itsmoked]

Thanks hacksaw.

Keith Cress
kcress -

http://www.flaminsystems.com