Steam Flow Constants

Hello guys

I have a question I am hoping someone can help me with.

I have a steam application where I am measuring steam flow from a Diff. Pressure orifice plate flowmeter.

I will be adding a flow meter to the system and have some questions about the math that is needed to get the steam flow from the system.

Here is what I know...

To find flow you take (a Constant x Orifice Plate Diff. Pressure x steam pressure / temperature) and then find the Sq. Root of that number.

I have this equation in many other areas so I know the equation works correctly.

Here is what I don't know...

How do I find the constant or what is the constant based on. A few of the other applications have the following numbers as a constant. 54898, 80946.54, 10083.31, 13724.50, and 700229.59.

I do not know if these are related to the Orifice plate itself such as a BETA number (what ever that is, I have just heard that tossed around when talking about orifice plates).

If anyone has experiance with a situation like this please let me know.

Thanks

Hello mrdmrd,

That question is not so easy to answer, because constant you're talking about depends on many factors such as pipe's diameter, type of material, type of orifice plate, orifice material...
The best approach is to use software for such calculations.
I would suggest http://www.controlengineering.se/flowcalc/flowcalceng.htm
you can download free trial versin and play a little.

I'm little confused about your formula. It shoud be
constant * sqrt(dp * press_corr * temp_corr)
where press_corr and temp_corr is dimensionless units and represents ratio between current and designed pressure and temperature. I know this depends on units of "constant", so it could be just extra math. I've just seen formula above more times.
Anyway, flow calc 32 has what you need.

You can also have a look at this.

Best regards,
Asim

mrdmrd said:

Hello guys

I have a question I am hoping someone can help me with.

I have a steam application where I am measuring steam flow from a Diff. Pressure orifice plate flowmeter.

I will be adding a flow meter to the system and have some questions about the math that is needed to get the steam flow from the system.

Here is what I know...

To find flow you take (a Constant x Orifice Plate Diff. Pressure x steam pressure / temperature) and then find the Sq. Root of that number.

I have this equation in many other areas so I know the equation works correctly.

Here is what I don't know...

How do I find the constant or what is the constant based on. A few of the other applications have the following numbers as a constant. 54898, 80946.54, 10083.31, 13724.50, and 700229.59.

I do not know if these are related to the Orifice plate itself such as a BETA number (what ever that is, I have just heard that tossed around when talking about orifice plates).

If anyone has experiance with a situation like this please let me know. Thanks

Click to expand...

I assume you
are measuring flow in mass per unit time?? pounds per minute etc.
have a fairly constant pressure and temperature and percent saturation for the steam

Venturi or orifice plates have been one of the older methods of measuring steam flow. As you say there are several variables that go into the calculation. I believe these will change with pressure up to the point of a critical orifice where flow will be constant regardless of pressure drop across it. I am not sure about the constant and why you are seeing so much variation in that value.

I would
1. make double sure I have the instrument correctly applied
2. go back to the maker of the instrument and get their advice

Dan Bentler

1) any modern DP cell will do the square root extraction for you.

2) any supplier of commercial orifice plates provides a 'sizing' sheet with the orifice plate.

The sizing sheet provides the values used in calculating the bore (hole) size in the orifice plate.

Those values are known as 'design values'.
- a maximum flow rate
- pressure drop at max flow rate
- at a design temperature
- at a design pressure
- in a pipe of certain dimensions

The maximum flow rate will also be associated with a stated pressure drop at that flow rate at the design conditions, ie 100" w.c. @ 20,000 pounds per hour.

In most control/monitoring systems, one can let the transmitter do the square root extraction and then linearly scale the input over the 4-20mA range using the max flow rate as the 20mA value, ie.
max flow rate = max DP drop = max analog output
100" w.c. = 20ma = 20,000 pounds per hour.

Note that without temperature and pressure compensation, you only have a volumetric value, not a true mass value, even if you use the mass units "pounds" (or Kg), with considerable error for deviation from the design temperature or design pressure.

There are transmitters known as multivariable DP transmitters, which measure the absolute pressure on the high side, the line temperature (RTD or T/C) and calculate a compensated mass value. The premium for a multivariable is ~ $1,200 USD. These multivariables also take into account the superheat condition that takes place when steam drops pressure.

Dan

Here, you can see (in general) how multivariable dp transmitters calculate compensation.
Hope this helps

the constant is related to the orifice plate