Re: [foxboro] Typical Orifice Flowmeter Compensation Calculations

Bernoulli's equation underlies the original orifice sizing calculation.
The equation given below is based on the ideal gas law, and is used to
calculate the approximate mass flow at conditions other than the design
temperature/pressure.  The closer the gas is to ideal, the better the
approximation.  Small molecules that don't tend to interact with each other
(e.g., methane, nitrogen) are excellent candidates for this method.

One very common gas behaves in a non-ideal manner:  steam.  (The polarity
of the water molecule causes interactions between the molecules that are
not accounted for in the ideal gas model.)  We have done some calculations
and determined that ideal-gas compensation is in many cases virtually no
better than no correction at all for steam.  The further away you are from
design conditions. the worse the error.

For steam you need a steam table.  There are ways to approximate one using
CHARC blocks.  Does Invensys sell a "physical properties" package that
includes a steam table?  Can this package be accessed by a CP?

Greg Hurwitt
BASF Corporation
Freeport, Texas



                                                                           
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I haven't looked this up or derived it, but isn't this applicable for a gas
only?   I think this comes out of Bernoulli's equation and the ideal gas
law, assuming an adiabatic expansion across the orifice.

The equation would be different for a liquid, correct?

Jim

-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx] On
Behalf Of Pablo Lioi
Sent: Monday, January 23, 2006 5:13 AM
To: foxboro@xxxxxxxxxxxxx
Subject: Re: [foxboro] Typical Orifice Flowmeter Compensation Calculations

Kirk,
Hope this helps:

CALCA BLOCK

RI01  FIxxxx.PNT
RI02  PIxxxx.PNT
RI03  TIxxxx.PNT

M01 1.013      (Base Pressure, Bar)
M02 273.15    (Base Temp, Kelvin)
M03 298.15    (Reference Temp, in Kelvin)
M04 18.103    (Reference Pressure, Bara)
M05 350        ( max differential pressure, mBar)
M06 6.39       (Flow @ max dif pressure, Ref. Pressure and Ref. Temp)

ADD RI02 M01
ADD RI03 M02
DIV
MUL M03
DIV M04
MUL RI01
DIV M05
SQRT
MUL M06
OUT RO01
END


The formula is

Qcomp = Qref Sqrt[( FI/FImax) ((PI+1.013)/(Pref + 1.013)) *
((Tref+273.15)/(TI+273.15))]

Pref+1.013 is stored in M04
Tref+273.15 is stored in M03
FImax is stored in M05
Qref is stored in M06

Determine Qref with a flow calculation program, using FI = max differential
pressure, P = work pressure T = work temperature

Pref is the work pressure and Tref the work temperature

Note in the above formula that if (FI=FImax, PI = Pref and TI = Tref) then
Qcomp = Qref, so the formula compensates for the deviations from the
reference pressure and temperature.

You can put this formula in Excel and make a table of Qcomp vs. P and Qcomp
vs. T and compare results against your flow calculation program to see how
it works

I am using SI units. If you need otherwise, just change the values in
M01-M06
As you see, the transmitter is not calculating the square root of the diff
pressure.

If you have square root at the transmitter then the MUL RI01 line has to be
moved after the SQRT, leaving the rest unchanged (you still need the SQRT
for the FI range, pressure and temp).

Good luck
Pablo Lioi
System Engineer
TOTAL AUSTRAL
ARGENTINA

>From: kirk.d.carver@xxxxxxxxxxxxxx
>Reply-To: foxboro@xxxxxxxxxxxxx
>To: foxboro@xxxxxxxxxxxxx
>Subject: [foxboro] Typical Orifice Flowmeter Compensation Calculations
>Date: Sun, 22 Jan 2006 16:32:17 -0600
>
>I am comparing some different configurations on our project and wanted to
>get a quick survey of how others perform flowmeter compensation in I/A.
>
>Assuming two flavors of 4-20mA flowmeter signal:
>a) in H2O (or psi) differential
>b) volumetric flow (square root in the transmitter)
>How do you compensate the flow for pressure and temperature?
>
>Calc block steps would be appreciated.
>
>
>Hook 'Em,
>Kirk
>
>Kirk Carver, PE
>ExxonMobil Development Company
>Facilities -  Instrumentation and Controls
>12450 Greenspoint Drive
>GP6-315
>Houston, TX 77060
>(281) 654-4881 (Greenspoint Office)
>(713) 350-1158 (Worley Parsons)
>(713) 962-2549 (mobile)
>kirk.d.carver@xxxxxxxxxxxxxx (best chance of contacting me)
>
>




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