HI fellows;

we have a flow meter and which shows the reading in PPH but i need to convert into CFM (Cubic feet/minute). How can i fix it?
Thanks
Ashraf

 

Do you need Standard cubic feet/minute or Actual cubic feet/minute?
If Standard, just figure out how many pounds are in a Standard cubic foot of the gas (standard conditions are 60 degF at 14.7 psia) and convert:

SCFM = PPH / (Pounds/SCF) / (60 min/hr)

If Actual, then it depends on what you are using for a flow meter.

 

Calculate the density and adjust.

Regards
cww

 

Hi fellows;
I calculate the ACFM and how i calculate the density?
Fluid is natural gas.
Thanks

 

Density is calculated from the gas properties and varies with temperature and pressure.

You still haven't said what type of flowmeter you have.
If it is a differential pressure instrument across an orifice, flow nozzle or venturi, then you should also have pressure and temperature instruments for the gas. If it is a smart instrument, with the calculation done at the instrument, then the differential pressure, gas pressure and gas temperature sensors are all connected to the local instrument. Dig out the instruction manual for this instrument - there should be instructions for changing from mass flow to volumetric flow.

If the calculation is done within a DCS or PLC, you will have to change the equation. Current equation is likely of the form:
Mass Flow = K1*SQR((delta-P)*T/P)
You will need:
Vol Flow = K2*SQR((delta-P)*P/T)
K1 and K2 are combination of the orifice constants and units conversion, P and T are in absolute units.

If your flowmeter is another type (like turbine), check its instruction manual.

Try reading about flow measurement, there are many books available - it is basic thermodynamics. Look in Section 4 of the Marks' Standard Handbook for Mechanical Engineers, for example.

 

Hi;

Thanks for share a knowledge.Can u give me a example which u write the formula. How i calculate the K1 and k2.

For example a orifice is installed in 2" dia line the diameter of orifice is 1.8". Pressure is about 180 PSig and the maximum temperature of gas is 110 deg Fahrenheit. i connect the differential pressure transmitter across the orifice. the transmitter show 2.5" water column.

how i calculate the K1 and K2 also for absolute value of pressure is 180+14.7=194.7 and absolute temperature?

thanks

 

Googling natural gas density, gives about .05 lbs/cu.ft. Therefore drawing on my higher math skills, 100 ACFM would be about 5 lbs/minute. (.05 X 100) or about 300PPH. (5 X 60) Density varies with temp, pressure, content, phase of the moon etc., which would have to be accounted for in both the CFM and the conversion from volume to weight. But that is how it's done. Kinda scary.

Regards
cww

 

The easiest way to calculate the K1 or K2 constant is as follows:

1. Get a copy of the orifice data sheet provided by the orifice supplier. Look at the design point in the data sheet. There will you will have defined flow (in either mass units or volumetric units or both if you are lucky), at the defined design pressure, temperature and associated differential pressure. Since you seem to be using English units, make sure to add 459.6 to the temperature in Fahrenheit to convert to Rankine.

2. Make any necessary units conversions if the units used on the data sheet differ from the units of your measuring instruments.

3. Plug those numbers into the equations I gave you and solve for K1
or K2.

4. If the data sheet only gives you design flow as mass flow, look on the data sheet for the design density. Divide the mass flow by the density and you get volumetric flow.

Hope this helps.

 

Hi fellows;

I want to calculate the K factor of the orifice but i have not the data from the manufacture of the orifice at desired pressure and temperature.

The orifice diameter is 1.250 inch and it is installed at 2 inch pipe dia. The maximum pressure is 200 PSI, but the operating pressure is near about 155 PSI, and the maximum temperature is about 150 deg F. Fluid type is Natural gas.The orifice is flanged type.

How can i calculate the K factor for calculation of flow. The pressure on the both side means upstream and downstream side. I connect the differential pressure transmitter 0 to 50 inches of water col across the orifice.

Plz help me to calculate the constant of K which is used in mass flow calculation or volumetric flow calculation.
Thanking You i will remain

Bye

 

You need the orifice manufacturer's calibration data. You can use whatever data point is provided, so long as the fluid (gas composition) has not changed. All you need is one point where you have the pressure, temperature, flow, density and differential pressure. It is best if that point is your design point, but not absolutely necessary (unless you are doing revenue metering from this flow element).

If you don't have the calibration data, go back to the manufacturer and request a copy. If it is so old they no longer have the data, then the orifice is probably due for replacement anyway, in which case you specify your current conditions with a current gas analysis and the vendor will do all the work for you for the cost of the new orifice.

If you are bound and determined to calculate everything yourself from the orifice and pipe dimensions, then you need to hit the books. The Mark's Handbook I referred to earlier is as good as any - any decent engineering library would have a copy. There are many other sources - ISA, manufacturers, thermodynamics and fluid mechanics text books. Just make sure you use a consistent set of units.

If your gas composition has changed, you will probably have to go back to the manufacturer and ask for an updated calibration sheet. (They will probably charge you for this.)

 

Be careful when using design density.

The density of natural gas will change as the constituents change (% methane versus % ethane, hexane, hydrogen, nitrogen, etc.).

To get an accurate value, you will need to get a sample of the natural gas to a lab for analysis. The lab can provide the standard density for you (pounds per standard cubic foot or lb/scf).

You may be able to get a fairly good value from the gas supplier - i.e. the same group that bills you for the fuel energy content. They might provide the density as specific gravity. A value of around 0.58 is typical for natural gas.

To get density, multiply by the standard density of air: 0.59 x 0.076328 = 0.045 lb/scf

If you are OK with a +/- 5% value, you can use 0.045 lb/scf for natural gas. This is normally a good value for US standard pipeline nautral gas.

 

Be careful using this - note the volume unit is "scf" = standard cubic foot - which is at 60degF and14.696 psia.

You wanted "acf" = actual cubic foot - which is at actual temperature and pressure.