5 replies to this topic

[Steve90]

Hi guys,

my supervisor asked me to size a natural gas field lines by referring to the API 14E .  For the single phase gas pipes I didn't found the formula to calculate the gas maximum velocity.  I read in some forums that as a rule of thumb we take 20 to 30 m/s for gas pipes as a max velocity.  Anyone has an idea about this pbm?  And is there any standard where i can find the rule of thumb cited above?  hx.

[Art Montemayor]

There is no formula for a maximum velocity - unless you are designing for a possible erosion caused by contaminated gas.

I don’t know which edition of API RP14E you are using, but my copy of the 5^th edition reads as follows:

 

"Sizing Criteria for Single-Phase Gas Lines.

Single-phase gas lines should be sized so that the resulting end pressure is high enough to satisfy the requirements of the next piece of equipment.  Also velocity may be a noise problem if it exceeds 60 feet/second; however the velocity of 60 ft/sec 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."

 

Unless your gas is contaminated with liquids and/or solids, I wouldn’t design for erosion effects on the pipe wall and fittings.  The point in RP API 14E, I think, is pretty clear: your priority for selecting the proper pipe size for your job should be the allowable design pressure drop for the piping run in question.  The velocity you select will play a part in determining the ultimate pressure drop.  What velocity you select is up to your criteria with respect to the noise level you want to avoid as well as possible vibration caused by high velocities.  RP 14E was originally written with offshore platforms in mind, so noise level was (and still is) a major concern.  If your application is on shore, then a gas average velocity of 100 ft/sec is usually a maximum value employed - generally.  Not knowing any more about your application than what you have furnished, I would try a velocity below 100 ft/sec and if this meets with the allowable pressure drop, I would use it.  20 to 30 m/sec falls into that criteria.

[Steve90]

Thank you very much Mr Art . Actually i'm sizing a natural gas pipes for an onshore natural gas processing plant (to finally confirm ) a client pipe diameters). I don't have the pressure drop but i think that using 30 m/s as a max for single phase pipes wille be adequate.thx

[Bobby Strain]

Maybe not. I would size based on pressure requirements, but velocity (feet/second) not to exceed 100 / sqrt(p). Where p is the gas density, lb/ft³. And you need to pay attention to other criteria for entrance/exit for equipment. I would recommend, too, that you consult someone knowledgeable in your organization. If you have a client, she is paying you for your experience. And what guarantees your company has offered? You want to be sure that you don't reduce client size, thus taking on unintended liability.

 

Bobby

[breizh]

Hi ,

 

On top of above , consider this resource . To me you should perform calculation , not using "rules"

Use the search engine in this forum for support on calculation .

Good luck,

Breizh

Edited by breizh, 10 November 2015 - 07:12 PM.

[cylai]

Hello Steve,

 

As Mr. Bobby says, pressure requirement will be more governing. I've seen cases when operating at low pressure, say for instant operating pressure 0.3 barg, but process engineers (and they are a group from 3 - 5 years experience!) with a pressure drop exceeding 0.3 barg simply because they stick to velocity criteria and wanting a smaller pipe size. In reality ofcourse, the pressure will start packing backwards and backpressure increases, it will still flow. Then the process system will potentially operate outside design range.

 

API RP 14E, 5th Edition, Oct 1991, [Section 2, subsection 2.4, page 21] states 60 ft/s but not an absolute criteria. And quote "an approximation of Equation 2.6 can be made when the change in pressure is less than 10% of the inlet pressure..." which leads me to believe that Equation 2.6 (General Pressure drop Equation for Gas phase in API 14E, page 21) is limited to 10% pressure drop criteria, else the equation is not applicable.

 

Realistically, if you are working on a project with a client, you might want to check if you should comply to your client's post award specification requirement. Most clients are confident with their data collected from operation sites and would strictly stick to their velocity limitations and it is not easy to get a deviation from specification accepted without solid reasons to do so. If it's not in the post award specification, then you will need to state clearly the criteria in the sizing basis to let your client know what you are assuming and all your assumptions must come with a reference and request acceptance. For this case, if you refer to API 14E, then you need to adhere to 60 ft/s, unless you can convince your client that why it can be exceeded. You can also look at it from commercial standpoint, spending massive engineering hours to prove that a one meter long 2" pipe is sufficient or just provide a 3" pipe and satisfy client requirement.

 

If, this is just an exercise given to you by your supervisor, assume your supervisor is your client and provide sufficient information (never have the misconception that providing more information is better, for example, if momentum criteria is not required by the client, you can suggest for good engineering purpose and allow the client to review and accept, but not force the client to review an extra parameter that they didn't call for. Plus, if it was never a problem, piping meets momentum criteria, meets 10% pressure drop criteria, etc, perhaps it shouldn't be reported in the first place).

 

Cheerio.

Lai