Measuring Flow Through Adjustable Choke Valves 1

[robertw619]

I am looking for a orifice table relating to natural gas. I need to measure the flow rate of gas wells with ranges from 100mcf/d to 600mcf/d. The plan is to purchase an adjustable choke valve and adjust the choke valve until the flow stabilizes upon which the choke valve opening can be compared to the table. Anyone know of a table or calculation for determing the flow or is there a better way. The measurement accuracy is not critical but somewhat important therefore a +/- 5-7% is ok.

Thanks,
Rob Curtis

 

[Zapster]

Contact the choke manufacture and get the Cv range for a given trim design. Ask if they have the correlation between the Cv and stem position.

 

[robertw619]

I contacted the manufacturer and all they could give me was the Cv for three positions. I am still not sure how this will help me to determine the flow. Any further assistance would be appreciated

 

[ChokeEng]

Contact your choke supplier and provide them with the range of flows, operating temperature, upstream and downstream pressures and let them "size" their product for you.

don f.

 

[Ashereng]

Hi Rob,

I'm sorry. I don't quite understand what you are looking for.

Your statement "The plan is to purchase an adjustable choke valve and adjust the choke valve until the flow stabilizes upon which the choke valve opening can be compared to the table."

Are you trying to measure flow in your gas well using a choke valve, instead of a flow measuring device?

How will you determine that the flow has stabilised? And what do you mean by "until the flow stabilizes"?

Also, at a given valve position, the flow through the valve will depend on pressure drop, up to "choked flow" (beyond which down stream pressure doesn't matter). Are you controlling pressure, or is pressure variable?

Or am I missing the whole thing altogether?

 

[robertw619]

Ashereng,

Thaks for your post. I am trying to figure out a way to measure the flow rate of natural gas wells that are not connected to a gathering system. These wells typically have a Xmas tree and I can connect to the tree with my adjustable choke valve. I would prefer to do this rather than a flow measuring device. We should be able to determine if the flow has stabilized with the pressure gauge on the well head. The valve manufacturer has provided the Cv but they cannot provide me the equation that goes along with the Cv. I was hoping to locate, create, or find a table/equation that I could use to determine flow. Since we will be dealing with 500-1200 psi wells I agree that we do not need to be concerned with the downstream pressure. The downstream pressure will be atmospheric since we are venting to atmosphere.

 

[Ashereng]

Hi Rob,

I see. Okay, here goes.

All you valve and fluid guys out there, please take a look at this - cause I may be off of my rocker.

Before I begin, are you allowed to vent natural gas to atmosphere?

Anyway, here goes.

Assuming that all of your wells are pretty much the same, and that you don't have a lot of water coming up with your gas (like, almost none at all), the flow rate can be calculated from knowing the Cv of the valve at a particular opening, pressure drop (which is from your pressure gauge to atmosphere), the density (or specific gravity) of the gas (you need to analyze the gas, and why I said assuming the wells are pretty much the same), by plugging them into the Cv equation (available in most handbooks).

The tricky part is the +/- 5-7%.

I am not sure how you would determine exact valve position to correspond with the vendor's Cv values at 3 positions. If you are even a few millimeters off on the valve position, depending on where you are on the curve, the Cv change may be a % or more.

The density of the gas would also be difficult. It would vary in the same well.

Have you though about a portable flow meter?

If you are indeed allowed to vent to atmosphere, a pipe with flanges and flow meter (clamp-on ultrasonic flow meters, turbine, vortex) would be a practical, portable, with good turndown, and be a fairly accurate way to measure flow. Of course, you would need power. Hey, is this the driving concern - you don't have power at these wells? If so, I guess you would need to buy some batteries and step up the voltage to power the flow meter's electronics.

Like I said. I may be waaaaaayyyyyyyy off my rocker. It's getting late in the afternoon.

 

[jsummerfield]

Buy a flow element. If using an adjustable choke my guess is that the line is small, perhaps not over 3-inch. Consider a simple orifice plate and flanges. If the line is large and upstream diameters are an issue, consider a V-Cone or the other element from Veris that requires no upstream or downstream run.

John

 

[ChokeEng]

Although adjustable chokes are frequently "calibrated" in 64ths equivalent bean size, it is unlikely that you'll be able to cross-reference the opening to an orifice plate. Additionally, the Fl and Xt values would have to be known across the entire flow characteristic of the adjustable choke. To my knowledge, this is not generally published information by choke manufacturers who have only recently (reluctantly) even acknowledged that Cv exists. Given the pressure differentials that you describe, equation inaccuracies and the choke's indicator calibration error could easily consume the +/- 5+% error.

don f.

 

[Ashereng]

Hi Rob,

Just a further note. I suggested the ultrasonic and turbine meters because they have better turn downs than say an orifice or similar DP elements.

I assumed (rightly or wrongly) that you probably intended to do this at multiple well sites, and that there would probably be a variety of flowrates.

If you only need it on a single well site, I would go with the cheapest solution. If you need to do this with multiple sites, a large turndown would accommodate a wider range of flow.