Line Sizing Standard for n-Butane Vapor Flow. 2

[Pavan Kumar]

Hi All,

I need to size a line for n-Butane vapor flow to our reactor. Is there any code or standard that provides guideline for pressure drop in psi allowed per 100 feet for this service. Your suggestions would be very helpful to me.

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi,
Consider the doc attached .
Pierre

 

[Pavan Kumar]

Hi Pierre,

Is NORSOK standard an industry standard?. Is there any reference to an code / standard on gas or vapor line sizing in petrochemical industry that you can help me with?.

Also there is a Flow Control Valve on this line which I have specified. Is there a standard that governs the specification and sizing of Vapor Control Valves?.

Your suggestions will be of great help to me.

Thanks and Regards,
Pavan Kumar

 

[LittleInch]

Did you bother reading it?

There is reference to an ISO code there.

But you seem to be looking for a design manual. Codes and standards don't do this.

Large operating companies and design companies often have them. I suggest you find them in your company.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

Sizing a line is an economic trade-off. Large diameter/high capital vs. low operating cost and small diameter/low capital vs. high operating cost. There is no Code or standard for this. It's engineering economics. As LI said, a design manual from a large company who has done their due diligence of an economics study may give you a guideline or rule of thumb. A small company will leave it up to you.

There are a few guidelines in the literature. I recall Perry's usually has an economic line sizing article and graph. Use the most recent Edition!

Good Luck,
Latexman

 

[georgeverghese]

The NORSOK guidelines are of some use, but to be sure, you've got to examine the supply line to the reactor and gauge the physical operating pressure limits of the supply. If you need more help, post a sketch with operating pressure ranges of the source and destination, any control valves and control fittings, and approx line lengths.

 

[pierreick]

Hi Pavan,
You should read the previous document supplied and perform a hydraulic calculation of the line with fittings and other valve .
To add to my previous answer another to document for you and others .
Note : Consider to describe your query right the first time to get a better answer .

My view
Pierre

 

[Pavan Kumar]

Per Norsok guidelines which is developed from ISO 13703 for Design of Piping Systems for Offshore platforms. API 14E is also for Design of Piping Systems for Offshore Platforms but it does not specify any pressure drop limits except that the line should be sized to provide the required pressure to the next piece of equipment and the velocity is less than 60 ft/sec.

Per Norsok standard the pressure drop limit for operating pressures between 0-100 psig is between 0.004 - 0.09 psi/ 100 ft.

For my project I sized the Butane line ( operating pressure of 50 psig) with a criteria of 0.5 psi/100ft and got a 4" line. So pressure drop is an based on economics but there is a limitation for velocity at 60 ft/sec as it would create a lot of noise. Also I have used the Isothermal Frictional Flow equation to calculate the pressure drop.

Is there a pressure drop number that you have typically used?

Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

Agreed, if pressure drop / 100ft doesnt constrain line size selection here, then keep line velocity at <20m/sec at min operating pressure / max operating flow. However, per NORSOK, dp of up to 1.5psi/100ft is acceptable at 50psig operating pressure (if pressure drop is not a constraint).

 

[Pavan Kumar]

Hi georgeverghese,

Yes pressure drop of 0.5 psi/100 ft for the maximum flow rate does not affect my selection of 4" line size the destination pressure after the drops in the all the components, including the pipe, is within the required limits.

Per NORSOK standard P-001 Rev 3 , page 16/29 the acceptable pressure drop for operating pressure range of 0-35 bar(g) is 0.001 - 0.11 bar/100m. For pressures it between the acceptable pressure drop should be prorated. So for 50 psig (3.4 barg) the acceptable DP is 0.0115 bar/100m = 0.05 psi/100ft. You mentioned the 1.5 psi/100ft. Can you explain how you got this number?.

Also I used Isothermal frictional frictional flow equation for calculating the pressure drop even though the flow is adiabatic ( line is insulated and heat traced.) I performed the same line sizing calculation using Adiabatic frictional flow equation and I got nearly the same dP. DP using Isothermal equations is 2.25 psi while with adiabatic equation is 2.26 psi.

I read in an article that for the same flow rate isothermal frictional flow equations give conservative pressure drops than the adiabatic frictional flow equation for the same flow rate. I used the Isothermal frictional flow equations for (1) It is is more convenient to work this (2)The pressure drop is more conservative. I have attached the pressure drop calculations done using the both the methods. Please provide your inputs.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Attaching the line sizing calculation using Adiabatic frictional flow equation here.

 

[georgeverghese]

No need to interpolate to get pressure drop at 50psig. The pressure drop range is for all pressures within the range. 1.5psi/100ft is the max permissible in this pressure range.
Line size seems acceptable since total drop is only 15kpa for 600ft total length. I got 17kpa with simpler incompressible flow calc, okay. Line velocity of 10m/sec is acceptable. 3inch would have given 62ft/sec, pressure drop of 1.7psi/100ft, so 4inch is better.
Check that this flow of 6600lb/hr at 52psig is the lowest operating pressure for this max flow. But even at a lower pressure, this line size of 4inch would still be acceptable. Also ask for a free draining line for this supply to the reactor, else condensate pools will accumulate at piping low points, invalidating this pressure drop calc.

 

[pierreick]

hi,
Dh = pipe Diameter thus Rh= d/2
Based on my calculation with a different formulae,
delta P= 145 mbar / 182.88 m or 0.35 PSI / 100 Ft , adiabatic model , D = 4 inches.
Pierre

 

[Pavan Kumar]

Hi georgeverghese,

Thanks for your reviewing my calculations and for your replies.

1. I still do not understand how you are getting the 1.5 psi / 100ft limit if I convert the range for
0-35 barg operating pressure (0.001 - 0.11 bar/100m) it is 0.044 - 0.48 psi /100 ft. The upper limit is 0.48 psi/100ft.

2. The vapor line is heat traced and insulated. The chances of condensate forming are remote. Having a condensate drain would be hazardous as you would be venting Butane to atmosphere if the drain valve is opened accidentally.

3. Regarding the equations I used for the line sizing calculation, is isothermal frictional flow equation good enough. I want to know when are Weymouth and Panhandle equations used?.

4. Are there guidelines, codes and standards for specifying control valves. I calculated the upstream and downstream pressure across the control valve for min, normal and max flow rates specified the control valve already. I want to make sure that I am meeting the applicable codes and standards if there are any.

As always your replies will be very helpful to me.

Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

1. My error on units conversion, 0.5psi/100ft is correct.
2. So the line is pocketed, which is why you have applied heat tracing and insulation to stop condensation? So what will you do if heat tracing temporarily fails?
3. Isothermal friction flow equations are the modern calc procedures. The Weymouth equation was good for the "slide rule days", but it is rarely used these days, same for Panhandle. But they give the same answers nevertheless at high Nre.
4. Good that you have accounted for min flow also, since turndown capability of the control valve is also important. You could read up the GPSA chapter on control valves.

 

[Pavan Kumar]

Hi georgeverghese,

Regarding point 2, I agree if the Heat tracing fails there is good chance for condensation and slug formation. In our case the line is not pocketed as the line continuously slopes from the Butane vaporizer to the reactor inlet. Also we have a Superheater that makes sure that the Butane liquid droplets if any formed due to condensation are vaporized before they enter the reactor.

I will read the section on Control valves from GPSA as you suggested.

Thanks and Regards,
Pavan Kumar

 

[Latexman]

Using incompressible, ΔP[sub]100[/sub] = 0.37 psi and ΔP[sub]TOTAL[/sub] = 1.76 psi.

1.76/66.7 x 100 = 2.6% < 10%, so okay per Crane TP410.


Good Luck,
Latexman

 

[georgeverghese]

Since this supply line for superheated butane vapor to the reactor is pocketed over this 300ft length, would suggest a guard knockout pot just upstream of the reactor with a high level alarm if there are safety or product quality concerns with condensate slugs getting into the reactor due to some failure to maintain superheat.

 

[pierreick]

Hi,
To add to the discussion a reprint from Chemical engineering magazine year 2000:
Pierre

 

[Pavan Kumar]

Hi pierreick,

Thanks for sending this article. I will go through it.

I have read an article which says that Isothermal frictional flow equations give more conservative pressure drop over adiabatic friction flow equations. I wanted from your experience if that is true. I have attached the article with this note. Please refer to the Conclusion 1 on page 10/10 of the article.

"Conclusions
Based on the derivations in this article, some important conclusions can be made:

1. For pipe sizing with compressible fluids, the isothermal flow equation is preferable since it gives a more conservative pressure drop estimate in the scope of practical engineering
design."

What tool(s) does the industry use, I mean the equations. I tend to use the isothermal frictional equations as they are simple and easy to use. However I feel it may not be appropriate to use for all cases say for example if I want to calculate the pressure drop for inlet/outlet piping for a PSV.

Thanks and Regards,
Pavan Kumar.