Dynamic Volumetric Flow 1

[iqballhud]

Hi,
I’m new in engineering.
I would like to calculate dynamic flow and pressure of natural gas (MW= 19.8; Density (Kg/m3) = 59) with data as below:
Point 1 (Piping): P₁ = 72 barg; T₁ = 41 oC; D₁ = 36 inch.
Point 2 (Ball valve (as of bypass valve): Size = 2 inch
Point 3 (Piping (closed pipe)): P₂ = ? barg; T₂ = ? oC; it will be as calculation, depending how much flow rate will be transfering); D₂= 36 inch; L₂= 30 M.

1. How to calculate the dynamic volumetric gas flow from the Ball Valve (Point 2) if the opening valve will vary (from 0% up to fully open 100%)?
I don’t know exactly which formula I can use because a lot of formula can be used but which one can represent on this condition, please help to solve it.
2. While the ball valve is open then the pressure on the downstream valve will be increasing. When the pressure is increasing then the volumetric gas flow will reduce.
Which formula to calculate the increasing of pressure on the downstream line valve?
3. If the ball valve size 2 inch, it’s mean the opening area of ball valve is equal with A = 0.25*3.14*2^2 inch2 or is there appendix, chart, or calculation to come to know how much the opening are valve, please?
4. Please let me know also which formula that can I apply, if I would like to calculate for liquid, let’s say amine solvent (Point 1 and Point2).

Thanks in advance for your golden hands.

Best Regards,
Iqballhud

 

[EmmanuelTop]

Pressure downstream of the isolation ball valve can be safely assumed equal to the source pressure, unless you are interested in academic calculations. I believe this is the case of plant/equipment pressurization so it really does not matter if you have 0.01 or 0.5 bar less than the source pressure downstream of the isolation valve. If you are interested in pressure losses, you can use equations from Crane TP410 or whatever manual on hydraulics you have with you. For calculating pressure losses use the flows you obtain from the second part of this post.

As for the second part, you are (partially) correct. Flow through the open 2" globe valve will be constant at the beginning, as long as there is critical flow condition through the valve opening and the source pressure (72 barg) remains unchanged. So until you reach the critical pressure

http://www.engineeringtoolbox.com/nozzles-d_1041.html

in the system downstream of the globe valve you can use the equation shown on the same page for calculating mass flow.

Once you enter sub-sonic flow condition, the globe valve equation comes into play, where flow is determined by knowing pressures upstream/downstream, fluid properties, and valve Cv. For calculating these flows, you can refer to Fisher valve handbook at:

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

I don't understand the part 4 of your post, where you say you want to repeat this same calculation but for liquid phase (amine). You can't pressurize the plant with amine so the flow will simply be determined by the valve equation. Downstream pressure will not change from the initial pressure, unless you fill the whole downstream system with liquid.

Dejan IVANOVIC
Process Engineer, MSChE

 

[iqballhud]

Dear Dejan, Thank you so much for your explanation and link sites.
I will calculate dynamic volumetric flow with vary opening ball valve because i will create dynamic flow simulation for this case. I will try to use the formula as your advice from the site (Mass Flow through Nozzles): mc = Ac (n p1 ρ1)^1/2 (2 / (n + 1))^{(n + 1)/2(n - 1)}; where: mc = mass flow at sonic flow (kg/s); Ac = nozzle area (m2); ρ1 = initial density (kg/m3).
When volume on the downstream valve is increasing then downstream line pressure will be increasing too, up to the pressure is equal between down stream and upstream then no more flow. Which formula can I use to calculate increasing pressure on the downstream valve?
Volume on the downstream valve is 31.505 M³ and if the volume achieved on that value, then will the pressure be equal with the upstream valve (P₁=72 barg). If not then in how much gas volume on the downstream valve pressure will be equal with upstream pressure?
On part no . 4. I want to know which formula can I use to calculate volumetric/mass Liquid flow and you are right the liquid can't pressurized the downstream line except the liquid will fill full on that line/equipment.

 

[EmmanuelTop]

For flow equations in liquid service refer to pages 110-118 of the Fisher handbook. You can apply equations for volumetric or mass flow shown on the page 114. Procedures for calculating other parameters in the equations are available as well.

With regards to gas flow: the flow will be constant till you reach critical pressure ratio for Methane (or whatever it is the gas that flows through the valve). For 72 barg inlet pressure and pure Methane this will be approx. 38-39 barg in the system downstream. Starting from this point (38-39 barg downstream) the flow is sub-sonic and you can use the gas flow equations from Fisher's manual (pages 119-125).

For sub-sonic flow regime, I'd suggest you to take several points (steps) in the calculation (e.g. for each 2 bar increase in downstream pressure) and calculate the resultant flow. Then connect all the points on the chart. Of course this assumes you know the Cv of the valve. If you can't obtain the Cv from manual valve datasheet, perhaps you can use some of the generic Cv data provided in Fisher's book.

Dejan IVANOVIC
Process Engineer, MSChE

 

[iqballhud]

Dejan, thank you so much for your guidance and I will come back if I have doubt about the calculation.
Appreciated and all the best for you.

 

[EmmanuelTop]

Also refer to this excellent FAQ prepared by Dave Simpson:

http://www.eng-tips.com/faqs.cfm?fid=1864

In your case, the sonic part of the curve would be flat and horizontal (because there is no change in upstream pressure). It is kind of a reversed chart for pressurization, but not exactly. The variable in your case is downstream pressure, not the upstream.


Dejan IVANOVIC
Process Engineer, MSChE