Compressible Flow Nozzle Pressure Drop Equation 2

[Pavan Kumar]

Hi All,

I determined the Vacuum ( Manway) nozzle size for the Steam-Out scenario using the calculation methodology provided by Art Montemayor in this thread, link copied below. I am however concerned about the pressure drop in the nozzle that is 6" long and want to make sure that the tank pressure does not drop below its design vacuum. Is there a formula to calculate the pressure drop through a nozzle on at atmospheric
(API 650) tank.

https://www.eng-tips.com/viewthread.cfm?qid=155185

Thanks and Regards,
Pavan Kumar

 

[Latexman]

Sounds like a sudden contraction, 6” of pipe, and a sudden expansion. Or, K = 0.5 + fL/D + 1.0. Use this in the Darcy–Weisbach equation.

I doubt flow will be close to compressible on a nozzle into an atmospheric tank. If it is, the tank will probably collapse.

Good Luck,
Latexman

 

[casflo]

I agree with the recommendation of Latexman. I have used this method to calculate the vent size of atmospheric tanks in nuclear power plants, with excellent results.

 

[Pavan Kumar]

Thanks Latexman and casflo for your inputs. I will get back with my results.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Hi Latexman, Casflo,

One of my inbreathing scenarios for the API 650 storage tank in question is Uncontrolled drain down through a 6" drain nozzle. I wanted to know if I should add the thermal inbreathing rate to it as it is possible for the ambient temperature to drop when the tank is draining through the 6" nozzle. Is this too conservative to consider for Conservation Vent sizing?.

Also if the design vacuum of the tank is not known what could be a conservative assumption?.

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi,
Consider this resource about compressible flow :

http://www.engsoft.co.kr/download_e/steam_flow_e.htm

Good luck
Pierre

 

[Pavan Kumar]

Hi All,

I calculated the nozzle size for the Steam Out scenario and per my calculations based on Art Montemayor's spreadsheet the size came out to be 20" and I selected 30" nozzle size ( please see tab "Steam Condensation" in the attached spreadsheet.) I used Darcy-Weisbach equation to calculate the pressure drop with the fittings as mentioned for the 30" nozzle.

1. Pipe Entrance
2. Pipe/ Nozzle of 6" length
3. Pipe exit

Based on 30" Sch 80 nozzle I got the pressure drop through the nozzle as 3.5 psi ( 96.3 in WC). This means the tank pressure during the relief event reaches to -96.33 in WC(g). Per API 650 Section 5.2.5 the tank can subjected to a partial vacuum of 25 kpa or 1" WC. The 96.33" WC vacuum far exceeds the API 650 specification. The tank vacuum reaches 14 in WC even if the nozzle size is increased to 48" which itself it ridiculously large for a vacuum nozzle for Steam-Out scenario. I need help to corect this issue.

Besides this I want to make sure I am interpreting API 650 Section 5.2.5 correctly, the text of which is copied below. Please let me know what you interpret the statement below.

"5.2.5 External Pressure: See Appendix V for the provisions for the design of tanks subject to partial internal vacuum exceeding 0.25 kPa (1 in. of water).Tanks that meet the requirements of this Standard may be subjected to a partial vacuum of 0.25 kPa (l in. of water), without the need to provide any additional supporting calculations."

Thanks and Regards,
Pavan Kumar

 

[Latexman]

It’s crystal clear. With no additional vacuum support(s), API 650 tanks are limited to a partial internal vacuum of 0.25 kPa (1 in. of water).

Good Luck,
Latexman

 

[Pavan Kumar]

Hi All,

As you can see in my calculation that even with 48" nozzle size the vacuum relief for steam out scenario is not satisfied. Does anyone have any inputs on this?.

Thanks and Regards,
Pavan Kumar

 

[Latexman]

We use administrative controls plus open manway(s) to mitigate this scenario.

Good Luck,
Latexman

 

[Pavan Kumar]

Hi Latexman,

The size of the open man way I calculated is more than 48" and that is the concern I was expressing. Clearly the steam condensation rate that I am calculating is too conservative. Even I reduce the overall heat transfer coefficient from 500 to 250 Btu/ft2-hr-Deg F the 48" nozzle is not acceptable.

Thanks and Regards,
Pavan Kumar

 

[don1980]

Pavan - when the vapor space in an uninsulated tank is filled with a condensable vapor (e.g. steam), and the tank is suddenly cooled (e.g. cool rain shower), then there's a very high chance that that tank will be destroyed due to vacuum collapse. And sudden condensation (sudden vacuum) occurs extremely quickly - almost always too quickly for the vacuum relief device(s) installed on the tank. As stated above by Latexman, the best defense is the use of administrative controls and open manways (top and bottom). Administrative controls can include thinks such as injection of air (or N2) into the tank during the stream out, ensuring that the task is not performed when there's a chance of rain, and ensuring that the manways are not closed until the tank has cooled (at which time the tank is inherently filled with a high concentration of air rather than condensable steam).

It's very difficult to accurately calculate the vacuum relief flow and open area needed to protect against a sudden vapor collapse scenario. That's why the best plan is to prevent this scenario from happening, and to provide as much open area as possible/practical during this steam out task.

 

[Latexman]

Most of the tanks we steam are finished product tanks. In cold climates they are in heated buildings, so cold rain is always mitigated. In tropical climates they are outside and we use FRP, which provides an insulation value. We also avoid rainy days. In all cases we have air blowers running during the steaming and two manways open, one lower side and one top head. We’ve been doing this about 60 years and haven’t sucked in a tank yet.

Our record when unloading tank trucks is not nearly as good.

Good Luck,
Latexman

 

[Pavan Kumar]

Hi don1980 and Latexman,

Thank you very much for the guidance. I will keep this in mind when specifying Steam-Out scenario when specifying Vacuum Nozzle sizes, Conservation and Emergency vents for atmospheric storage tank. I am still wondering on the efficacy of the calculation methodology proposed by Art Montemayor. It would be great if I was able to contact him somehow.

Thanks and Regards,
Pavan Kumar

 

[Latexman]

Art has not replied to anyone in ~ 1.5 years. We do not know, but we fear the worst.

Good Luck,
Latexman

 

[Pavan Kumar]

I pray to God that he is doing fine.

Thanks and Regards,
Pavan Kumar