Re: [foxboro] Typical Orifice Flowmeter Compensation Calculations

Greetings,

Invensys sells a very good flow compensation package if you need very =
accurate calculated flows.  You can also use it to update C' type values =
in the equations already discussed.

...regards


Robert Brennen
Shell Canada Limited


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> -----Original Message-----
> From: foxboro-bounce@xxxxxxxxxxxxx
> [mailto:foxboro-bounce@xxxxxxxxxxxxx]On Behalf Of Gregory A Hurwitt
> Sent: January 26, 2006 7:44 AM
> To: foxboro@xxxxxxxxxxxxx
> Subject: Re: [foxboro] Typical Orifice Flowmeter Compensation
> Calculations
>=20
>=20
> Bernoulli's equation underlies the original orifice sizing=20
> 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=20
> the design
> temperature/pressure.  The closer the gas is to ideal, the better the
> approximation.  Small molecules that don't tend to interact=20
> with each other
> (e.g., methane, nitrogen) are excellent candidates for this method.
>=20
> One very common gas behaves in a non-ideal manner:  steam. =20
> (The polarity
> of the water molecule causes interactions between the=20
> molecules that are
> not accounted for in the ideal gas model.)  We have done some=20
> calculations
> and determined that ideal-gas compensation is in many cases=20
> virtually no
> better than no correction at all for steam.  The further away=20
> you are from
> design conditions. the worse the error.
>=20
> For steam you need a steam table.  There are ways to=20
> 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?
>=20
> Greg Hurwitt
> BASF Corporation
> Freeport, Texas
>=20
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>                          "Jim                                =20
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>           cc=20
>                          Sent by:                            =20
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>                          foxboro-                            =20
>      Subject=20
>                          bounce@f         Re: [foxboro]=20
> Typical Orifice   =20
>                          reelists         Flowmeter=20
> Compensation          =20
>                          .org             Calculations       =20
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>                          freelist                            =20
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>                           s.org                              =20
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> I haven't looked this up or derived it, but isn't this=20
> applicable for a gas
> only?   I think this comes out of Bernoulli's equation and=20
> the ideal gas
> law, assuming an adiabatic expansion across the orifice.
>=20
> The equation would be different for a liquid, correct?
>=20
> Jim
>=20
> -----Original Message-----
> From: foxboro-bounce@xxxxxxxxxxxxx=20
> [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=20
> Calculations
>=20
> Kirk,
> Hope this helps:
>=20
> CALCA BLOCK
>=20
> RI01  FIxxxx.PNT
> RI02  PIxxxx.PNT
> RI03  TIxxxx.PNT
>=20
> 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)
>=20
> ADD RI02 M01
> ADD RI03 M02
> DIV
> MUL M03
> DIV M04
> MUL RI01
> DIV M05
> SQRT
> MUL M06
> OUT RO01
> END
>=20
>=20
> The formula is
>=20
> Qcomp =3D Qref Sqrt[( FI/FImax) ((PI+1.013)/(Pref + 1.013)) *
> ((Tref+273.15)/(TI+273.15))]
>=20
> Pref+1.013 is stored in M04
> Tref+273.15 is stored in M03
> FImax is stored in M05
> Qref is stored in M06
>=20
> Determine Qref with a flow calculation program, using FI =3D=20
> max differential
> pressure, P =3D work pressure T =3D work temperature
>=20
> Pref is the work pressure and Tref the work temperature
>=20
> Note in the above formula that if (FI=3DFImax, PI =3D Pref and TI=20
> =3D Tref) then
> Qcomp =3D Qref, so the formula compensates for the deviations from the
> reference pressure and temperature.
>=20
> You can put this formula in Excel and make a table of Qcomp=20
> vs. P and Qcomp
> vs. T and compare results against your flow calculation=20
> program to see how
> it works
>=20
> 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=20
> root of the diff
> pressure.
>=20
> If you have square root at the transmitter then the MUL RI01=20
> line has to be
> moved after the SQRT, leaving the rest unchanged (you still=20
> need the SQRT
> for the FI range, pressure and temp).
>=20
> Good luck
> Pablo Lioi
> System Engineer
> TOTAL AUSTRAL
> ARGENTINA
>=20
> >From: kirk.d.carver@xxxxxxxxxxxxxx
> >Reply-To: foxboro@xxxxxxxxxxxxx
> >To: foxboro@xxxxxxxxxxxxx
> >Subject: [foxboro] Typical Orifice Flowmeter Compensation=20
> Calculations
> >Date: Sun, 22 Jan 2006 16:32:17 -0600
> >
> >I am comparing some different configurations on our project=20
> and wanted to
> >get a quick survey of how others perform flowmeter=20
> 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)
> >
> >
>=20
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> ______________________________________________________________
> _________
>=20
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> =20
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>=20

 
 
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