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Mass flow through Venturi tube
3

Mass flow through Venturi tube

Mass flow through Venturi tube

(OP)
Hi,

I have a Venturi tube installed on a superheated steam line. In order to calcula the mass flow I have a couple of questions.
1. does the transmitter have to extract the square root or linear and then which formula would need to be applied to read mass flow and not volumetric flow?


thanks

RE: Mass flow through Venturi tube

A Transducer is sensing a physical parameter and transmitting it.  To use a Venturi meter you will have to process static and differential pressure in an equation that happens to include a square root.  You still have to do the math.

The equation that allows you to infer a flow rate from the differential pressure across a known restriction provides volume flow rate.  To convert that to mass flow rate you only have to multiply times density.  Not terribly difficult.

David

RE: Mass flow through Venturi tube

(OP)
Ttanks, The transmitter provides the signal with the square root extracted so that the do is proportional to the flow. This flow needs to be compensated wiith P and T since it is superheated steam. As far as I understand you are saying that all I have to do is multiply the signal from the transmitter by the density which constantly changing. It might sound silly but this is the part where I get confused since I guess I need to somehow introduce the steam tables right?

RE: Mass flow through Venturi tube

You might check out the Contrec Model 415 Gas Flow Computer. It supposedly contains equations for saturated and superheated steam. There may be other flow computers besides Contrec, but I am using this model for gas flow measurement.

RE: Mass flow through Venturi tube

(OP)
The problem is that on our Steam Generator there is a mass flow meter installed which does not coincide with the venturi tube installed on the HP header. For example, when the SG is measuring 199 tn/h the venturi measures 215 tn/h.

The only difference is that the mass steam flow calculation for the Venturi is done with a DCS function attached while the SG has an orifice with a transmitter ehich corrects the steam flow for P and T.

So basically I woul like to make sure that we are using the function correctly.

RE: Mass flow through Venturi tube

go to your steam tables, use the internet, or talk with you mechanical engineer, to give you the steam density used for the original meter calculations, and the steam density for the new case

you may need to consult the plant record books for the current design basis

according to the writeup you posted, the scale factor should be multiplied by sqrt(rnew/rold) or simply replace r by the new but actual steam density in units required by your computation block.

updating meter factors for changed plant conditions is pretty normal stuff

since the venturi is reading high, it sounds like the operating pressure is lower than originally expected, or the steam temperature higher

 

RE: Mass flow through Venturi tube

(OP)
thanks.
I am still not sure whether the fscale value i use is correct, the venturi datasheet says a max flow of 250 tn/h and i use it a fscale, the rest of variables are ok.
I use the datasheet parameters on the formula. The densiy I calculate it using a macro in excel called water97,
http://www.cheresources.com/content/articles/physical-properties/thermodynamic-and-transport-properties-of-water-and-steam, to calculate density at operating point.


Please find attached the venturi datasheet. Could you please let me know whether I am using the datasheet correctly?
Does it really matter what the max flow is s long as the max dp in venturi is changed accrodingly? Should I be using operating values inthe formula or maximum values?

thank you very much.

RE: Mass flow through Venturi tube

The spec sheet is for an orifice plate with flange taps as opposed to a venturi.

I get virtually identical results steam data, discharge coefficient, etc. for the given condtions. What you have to confirm is the current pressure and temperature (at least the average readings). The get a reading of the actual transmitter differential in mmH2O and the indicated flow in t/hr at the same instant.

 




 

RE: Mass flow through Venturi tube

(OP)
sorry you are right, we use an orifice on the HP header and a Venturi on the LP header.

The transmitter gives an output in mmH20, the DCS does the conversion to t/hr.

For example with these values,
T= 376,9 ºC
P= 99 barg
dp= 155 kPa

The DCS function outputs 214 tn/h while te steam generator which has a rosemount Annubar flowmeter outputs 199 tn/h (there are two steam trains each showing 28 kg/s, so approx 199 tn/h)

This difference is what I dont understand.
I also attach the calc for the steam generator annubar.
 

RE: Mass flow through Venturi tube

You might need to confirm that your XMTR span is 0-22000 mm and not 0-25000 mm



        FLOW METER SIZING CALCULATION

        Tag no                                
        Service                               
        Fluid                            Steam
        Meter       Square Edge Orifice Plate
        Connections                Flange Taps


        Pressure                       99.000  Barg(100.013 Bar(A))
        Temperature                   378.0    Deg C
        Ambient Pressure                1.013  Bar(A)   (    0  Meters Above msl)
        Base Conditions (STP)           1.013  Bar(A) ,    0.0 Deg C
        Specific Volume (Vf)            0.0248 M3/kg
        Isentropic Exponent             1.271
        Viscosity                       0.025  cP

                                     normal |  maximum
        Flow                         196000 |  250000   kg/hr
        Pipe Reynolds No.   (Rd)   12115205 | 15453067
        Mean Fluid Velocity (Pipe)   32.4   |  41.4     m/s
        Line Loss                     1.27  |   2.07    (Bar)/100 m
        Friction Factor     (f)       0.0138|   0.0138
        Meter Loss(Installed)         0.61  |   1.00    Bar
        Meter Velocity               58.3   |  74.4     m/s


        **** METER CONSTANTS ****

        Meter Bore       (d)             6.7587  @  20 Deg C
        Pipe i.d.        (D)             9.0618  Inches
        Beta Ratio       (d/D)           0.7458  Inches/Inch     Caution
        Metal Expansion  (Fa)            1.0126 (316 Stainless Steel)
        Expansion Factor (Y1)            0.9946
        Reynolds Factor  (Fc)            0.9593
        Drain/Vent Corr. (Fh)            1.0000
        Discharge Coef.  (Cd)            0.59547
        Spinks Factor     (S)            0.41553


        **** TRANSMITTER DATA ****

                                     normal |  maximum
        Meter Differential (h)      13448.0 |   21878.9  mm w.c.
        Flow Rate                    196000 |  250000   kg/hr


        W (kg/hr) =  8.121 * S * D^2 * Fa * Fc * Fh * Y1 * sqrt( h / Vf )
 

RE: Mass flow through Venturi tube

(OP)
The span is correct and set to 22000 mm w.c.

With a value of 15809 mm w.c. and your formula I obtain 215 tn/h so it seems to coincide with my calculation. I guess maybe the error is in the pitot on the Steam generator?

What is the difference between the value in  **** TRANSMITTER DATA **** of 21878.9  mm w.c. and the span of 22000 mm w.c.? I guess none.

I need if possible the formula for the LP flowmeter, this is the next strange thing, is that the flow in HP is 215 tn/h and the LP is 183 tn/h, how can there be so much loss in steam between Hp and lp.

Op T: 325 ºC
Op P: 16,4 barg
Attached is the Venturi data sheet.

What SW are you using for the calculation and to derive the formula? Is it Flowcalc32?

btw: Remind me to include you in my Christmas cards lists!! you are awesome, thank you.

                                    
 

RE: Mass flow through Venturi tube

Meter reconciliation is always tough, you have to check all the details, including the actual installation

You haven't lost steam, unless you've an extraction turbine, usually it is the changed process conditions. Often the LP steam will have wildly varying temps depending on the turbine load, with the higher temps at reduced load

the sw is custom developed, but used in lots of meter surveys to sort out the non-sense and keep suppliers in line. Uses the equation of state for steam/condensate.

hopefully with attachments

RE: Mass flow through Venturi tube

        FLOW METER SIZING CALCULATION


        Tag no                                
        Service                               
        Fluid                            Steam
        Meter    Venturi W / 15 Deg Exit Cone


        Pressure                       17.000  Barg(18.013 Bar(A))
        Temperature                   378.0    Deg C
        Ambient Pressure                1.013  Bar(A)   (    0  Meters Above msl)
        Base Conditions (STP)           1.013  Bar(A) ,    0.0 Deg C
        Specific Volume (Vf)            0.1623 M3/kg
        Isentropic Exponent             1.289
        Viscosity                       0.024  cP

                                     normal |  maximum
        Flow                         183000 |  200000   kg/hr
        Pipe Reynolds No.   (Rd)    5230769 |  5716687
        Mean Fluid Velocity (Pipe)   39.7   |  43.3     m/s
        Line Loss                     0.11  |   0.14    (Bar)/100 m
        Friction Factor     (f)       0.0121|   0.0121
        Meter Loss(Installed)         0.02  |   0.03    Bar
        Meter Velocity               87.5   |  95.6     m/s


        **** METER CONSTANTS ****

        Meter Bore       (d)            13.6450  @  20 Deg C
        Pipe i.d.        (D)            20.2618  Inches
        Beta Ratio       (d/D)           0.6734  Inches/Inch
        Metal Expansion  (Fa)            1.0126 (316 Stainless Steel)
        Expansion Factor (Y1)            0.9918
        Reynolds Factor  (Fc)            1.0000
        Drain/Vent Corr. (Fh)            0.0000
        Discharge Coef.  (Cd)            0.99349
        Spinks Factor     (S)            0.50554


        **** TRANSMITTER DATA ****

                                     normal |  maximum
        Meter Differential (h)       1920.3 |   2293.6  mm w.c.
        Flow Rate                    183000 |  200000   kg/hr


        W (kg/hr) =  8.121 * S * D^2 * Fa * Fc * Fh * Y1 * sqrt( h / Vf )
 

RE: Mass flow through Venturi tube





        FLOW METER SIZING CALCULATION


        Tag no                                
        Service                               
        Fluid                            Steam
        Meter    Venturi W / 15 Deg Exit Cone


        Pressure                       16.400  Barg(17.413 Bar(A))
        Temperature                   325.0    Deg C
        Ambient Pressure                1.013  Bar(A)   (    0  Meters Above msl)
        Base Conditions (STP)           1.013  Bar(A) ,    0.0 Deg C
        Specific Volume (Vf)            0.1527 M3/kg
        Isentropic Exponent             1.293
        Viscosity                       0.022  cP

                                     normal |  maximum
        Flow                         183000 |  200000   kg/hr
        Pipe Reynolds No.   (Rd)    5740138 |  6273375
        Mean Fluid Velocity (Pipe)   37.3   |  40.8     m/s
        Line Loss                     0.11  |   0.13    (Bar)/100 m
        Friction Factor     (f)       0.0121|   0.0121
        Meter Loss(Installed)         0.02  |   0.03    Bar
        Meter Velocity               82.3   |  89.9     m/s


        **** METER CONSTANTS ****

        Meter Bore       (d)            13.6450  @  20 Deg C
        Pipe i.d.        (D)            20.2618  Inches
        Beta Ratio       (d/D)           0.6734  Inches/Inch
        Metal Expansion  (Fa)            1.0106 (316 Stainless Steel)
        Expansion Factor (Y1)            0.9920
        Reynolds Factor  (Fc)            1.0000
        Drain/Vent Corr. (Fh)            0.0000
        Discharge Coef.  (Cd)            0.99354
        Spinks Factor     (S)            0.50557


        **** TRANSMITTER DATA ****

                                     normal |  maximum
        Meter Differential (h)       1813.1 |   2165.6  mm w.c.
        Flow Rate                    183000 |  200000   kg/hr


        W (kg/hr) =  8.121 * S * D^2 * Fa * Fc * Fh * Y1 * sqrt( h / Vf )
 

RE: Mass flow through Venturi tube

(OP)
It is a 50 MW Siemens extraction turbine. Do you sell that sw by any chance? Its amazing.

RE: Mass flow through Venturi tube

there you have the disappearing steam, i'll give the notion some thought

by the way, good luck on sorting the meters out, it is always a lot of hard work and fun

RE: Mass flow through Venturi tube

(OP)
Thanks, please if you ever consider selling your program, let me know.

RE: Mass flow through Venturi tube

(OP)
btw you mention that         Drain/Vent Corr. (Fh)            0.0000, if this is true then the equation is 0.

RE: Mass flow through Venturi tube

thanks, should be shown as 1 minus the correction

RE: Mass flow through Venturi tube

(OP)
Btw, in case you are interested this has to do with 3 50MW CSP solar power plants.

RE: Mass flow through Venturi tube

fixed the sw, was in the report section, thanks

good luck, btw

RE: Mass flow through Venturi tube

when the calc is run for the pitot using the mfg coefficient:  C=0.5403, I get considerably different numbers for the transmitter calibration, enough of a difference to need clarification from the supplier.

also a bit puzzled by the change in flow units from t/h with your other meters to kg/s

 

RE: Mass flow through Venturi tube

(OP)
to convert from t/h to kg/s I just multiply by 3,6, What do you mean?
The pitot from the steam generator uses kg/s and we use t/h.

RE: Mass flow through Venturi tube

(OP)
what do you mean by fixed the sw?

RE: Mass flow through Venturi tube

the old formula listed in the report(not used in the calc) included an fh factor. This was removed.

Sorted out the differences in the annubar meter d/p calculations. The calculation at the given flow and pipe size is extremely sensitive to fluid density. I am using an older equation of state with a fluid density that differs from the supplier calc by 0.14%. The accepted equations of state for steam have been improved since the calc was developed.

 

RE: Mass flow through Venturi tube

(OP)
I have opened a warranty claim to our EPC contractor so that they check their Pitot. There cant be 205 tn/h on the pitot and 220 tn/h on our orifice! Its impossible

RE: Mass flow through Venturi tube

remember it only instrumentation, good luck with the project.
no drain hole was assumed


        FLOW METER SIZING CALCULATION


        Tag no                                
        Service                               
        Fluid                            Water
        Meter    ASME Long Radius Flow Nozzle


        Pressure                       115.000  Barg(116.013 Bar(A))
        Temperature                   179.0    Deg C
        Ambient Pressure                1.013  Bar(A)   (    0  Meters Above msl)
        Liquid Gravity (Gf)             0.889
        Viscosity                       0.160  cP

                                     normal |  maximum
        Flow                         235000 |  250000   kg/hr
        Pipe Reynolds No.   (Rd)    3936751 |  4188033
        Mean Fluid Velocity (Pipe)    5.4   |   5.7     m/s
        Line Loss                     1.52  |   1.73    (Bar)/100 m
        Friction Factor     (f)       0.0156|   0.0156
        Meter Loss(Installed)         0.46  |   0.52    Bar
        Meter Velocity               15.0   |  16.0     m/s


        **** METER CONSTANTS ****

        Meter Bore       (d)             3.1070  @  20 Deg C
        Pipe i.d.        (D)             5.1870  Inches
        Beta Ratio       (d/D)           0.5990  Inches/Inch
        Metal Expansion  (Fa)            1.0053 (316 Stainless Steel)
        Expansion Factor (Y1)            1.0000
        Reynolds Factor  (Fc)            1.0000
        Drain/Vent Corr. (Fh)            1.0000
        Discharge Coef.  (Cd)            0.99320
        Spinks Factor     (S)            0.38178


        **** TRANSMITTER DATA ****

                                     normal |  maximum
        Meter Differential (h)       8952.7 |   10132.1  mm w.c.
        Flow Rate                    235000 |  250000   kg/hr


        W (kg/hr) =  255.2 * S * D^2 * Fa * Fc * Fh * Y1 * sqrt( Gf * h )
 

RE: Mass flow through Venturi tube

(OP)
thanks again.

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