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Pressure Drop And Velocity On Onshore Piping

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#1 raxza


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Posted 04 March 2005 - 12:20 AM

I want to know , is there any pressure drop limitation for Onshore Gas Piping Plant? I used to apply limitation on API 14E. Because API 14E is dedicated for Offshore/Piping in the platform, I think the limitation for onshore can be reduced. For example
In API 14E, for Pressure above 500 psig the allowable dP is 0.5-1.2 psi/100 ft, thus for onshore can I extend it to 1.5-2 psi/100 ft? Also how about the velocity can I extend the 60 ft/s to 80 ft/s for Onshore ?

Note : That this is design criteria and the plant isn't build yet so I can't determine the Length of the pipe yet.

#2 gvdlans


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Posted 04 March 2005 - 04:29 AM

I could not find the "pressure drop limitation" of 0.5-1.2 psi/100 ft in API RP 14E. Maybe you can tell me where to look?

Normally pressure drop recommendations are based on an economical reasons: When you increase pipe diameter, compression or pump costs decrease but at the same time pipe costs increase. At one point there is an economical optimum. The exact optimum can vary from place to place and from time to time because of different costs for steel and power etc.

The velocity recommendation in API RP 14E is a general recommendation based on noise considerations. On page 20 it reads "however, the velocity of 60 feet / second should not be interpreted as an absolute criteria. Higher velocities are acceptable when pipe routing, valve choice and placement are done to minimize or isolate noise."

#3 raxza


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Posted 06 March 2005 - 09:14 PM

The limitation is on the API RP 14E 1984 edition , it's no longer exist in the 2000 edition but I always take the criteria from the 1984 to limit the dP calculation.

It is true that the dP calc. is set based on the economical reason. When the dP gets over the limit set, usually I ask my piping designer about the length of the pipe if the pipe segment is short than the it's ok to give the dP above the limita as long the dP/100 ft is still in the limitation.

I also want to ask about the formula in 14E and in the GPSA Eng. Databook Section 17 (2000 edition). For gas piping the 14E gives Panhandle and Weymouth , GPSA gives simplified D'arcy Equation for Gas Piping Plant which used as Two Phase Line dP calc. in API 14E. I observed that the D'arcy eq. gives lower dP than Panhandle/Weymouth with the same parameter. What is the different between this 2 formula (in 14E and GPSA), why 14E put the Simp. D'arcy as two phase dP? Is Simp. D'arcy Eq. valid for gas piping calculation? How good is the accuracy?

And for velocity , yes the velocity is affect to the noise...(I usually set 85 dB)

#4 gvdlans


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Posted 07 March 2005 - 07:34 AM


I am a bit surprised by your reply. Since you seem to be well aware that dP considerations are for economical reasons and velocity considerations are related to noise, why did you write "limitations" and "allowable dP" in your original post? And why do you think the criteria should be any different offshore vs. onshore?

I do not know what API's considerations were when they gave simplified Darcy equation for two-phase flow lines (you have to ask API if you really want to know...).

Personally, I would use Darcy equation for gas lines (provided that pressure drop is less than 10% of total pressure). The GPSA Data Book gives methods for gas transmission lines and low pressure gas lines (where dP can be more than 10% of total pressure).
I would use a specific equation for two phase flow (such as Dukler method as described in GPSA Data Book). For two phase flow the "accuracy" of any calculation method is normally not so good (e.g. +/- 20%)

With the computer programs and spreadsheets that are available nowadays, there is no need to use any of the simplified methods as described in API Recommended Practice 14E (note that latest issue is from 1991).

#5 raxza


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Posted 07 March 2005 - 08:49 PM

Thanks, gvdlans....

For the offshore vs onshore 2 years ago when I designed piping in offshore, my Boss restrict the dP/100 ft to 0.5 psi . He said that for offshore design small size is preferable so logically high pressure will give smaller size (for vessel and pipe), and that was my first experience designing offshore production plant, so I just accepted what he said. Later I ask him if there any RP for offshore vs onshore he said that it was only engineering practice.....

But thanks for the recommendation to use D'arcy because the formula is much shorter than my spreadsheet usually used.....

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