5 replies to this topic

[keby9vam]

Hi all,

I am an intern working in an oil and gas firm. I would like ask if there is any other formula used to calculate the Noise limit velocity for a gas line, apart from V= 175(1/rho)^0.43 ?

I came across a formula; V= K(rho)^-0.506, where is a constant, in the template used in the company I am attached with. But my problem is, I cant seem to find any sources that state the use of this equation. Thank you in advance for your advice.

Regards

keby9vam

[ankur2061]

keby9vam,

The formula mentioned by you is from Norsok Standard P-001 - "Process Design" Edition 5. Please note that Norsok standards were specifically developed for off-shore Oil & Gas installations. Offshore installations are designed considering that space is at a premium cost, which means higher flows are required through smaller piping, which thus translates into higher velcoities through piping on the off-shore platform.

If your company has a formula for max velocity than obviously there has to be a source for it. You need to check it with some of the veteran engineers in your company.

Some engineeering companies like Bechtel restrict maximum gas velocities to 120 ft/s for any gas piping applications. The reason provided is not only noise but a combination of factors such as:

1. Excessive Pressure Drops
2. Economic Pipe Operation
3. Life Cycle Cost
4 Noise
5. Vibration
6. Erosion / Corrosion
7. Extensive stress analysis
8. Expensive Pipe Support systems in high velocity gas lines to mitigate vibrations
9. Requirement of periodic fatigue analysis to ascertain pipe integrity in high velocity gas lines.

All these factors should be evaluated to arrive at optimum velocities for gas piping.

Hope this helps.

Regards,
Ankur.

[Shivshankar]

Keby9vam,

If you come to know the source of these formulas, please let me know.

V = 175 X (1/ρ)^0.43.

V = K*(ρ)^-0.506.

Regards
Shivshankar

[kkala]

1. Recommended velocities in sales (natural) gas pipelines have been discussed in http://www.cheresources.com/invision/topic/15547-natural-gas-pipe-line-sizing-calculations/. At least in high pressure range (e.g. 60 Barg), such velocities mentioned are lower than those dictated by acceptable noise criteria. The query asks for gas velocities to result in acceptable noise levels. It may concern long distance lines and / or in plant lines.
2.1 According to local practices applied inside refineries, gas velocities in PSV discharge lines and flare header (during an emergency discharge to flare) shall be limited as below, to reduce noise to acceptable levels:
α. u (m/s) = 183*ρ^-0.5 for pipe size <8" and u (m/s) = 244*ρ^-0.5 for greater pipe sizes,
where ρ=actual gas density expressed in kg/m3.
β. alternatively keep velocity less than 0.7 of sonic, at point of lowest pressure of sized line.
2.2 Mentioned refinery practices determine max allowable gas velocity u (m/s) = 122*ρ^-0.5 for hydrocarbon vapor lines (ρ= actual gas density in kg/m3), without clarifying reason of limiting to this value. Criticism was received because max allowable velocity for two phase lines is expressed by same formula (but ρ=density of two phase mixture, kg/m3), yet that is what mentioned practice recommends for either gas or two phase flow. See http://www.cheresour...n-control-valve '> http://www.cheresour...n-control-valve , post No 10 and on.
2.3 It is pointed out that formulas mentioned in 2.1, 2.2 comply fairly well with the u (m/s) = K*ρ^-0.506 (ρ in kg/m3) of keby9vam's employer, for different values of K. More explanations on the latter (on what lines is it applicable? values of K?) would be welcomed (post No 4 by Shivshankar).
2.4 It is reasonable to assume higher noise in case 2.1 (short time flow) compared to case 2.2 (more or less continuous flow), hence higher allowable velocities. Velocity criteria for sales gas pipelines to produce an acceptable noise effect would be welcomed.
3. There is also a relevant topic from http://www.cheresour...g-calculations/ '> http://www.cheresour...g-calculations/ , post No 17, which will be hopefully answered:
Economics affect allowable (frictional) ΔP per unit length and may dictate a lower velocity. Allowable max velocity (initially specified) does not seem to depend on allowable ΔP per unit length. Advice on this point appreciated.
That is allowable velocity (max or min) is specified for technical reasons (here for noise reduction, other reasons may dictate additional allowable velocities), while pressure drop per unit length is affected by economic reasons. All of them are "conservatively" combined to determine line diameter. Here we consider allowable velocity and allowable ΔP/unit length, both independently taken from practices.
Comments on it would be welcomed.

[ankur2061]

Local Practices.................???

[kkala]

Local Practices.................???

Point extensively discussed in http://www.cheresources.com/invision/topic/13563-supersonic-velocity-in-control-valve/. posts No 16 to 20, and briefly in http://www.cheresources.com/invision/blog/4/entry-191-the-art-of-responding-to-queries-on-an-engineering-forum/. More discussion would be useless. Of course anyone can comment on what local practices express, or report a substitute judged better. But let us concentrate to main queries of the topic, instead of formalities diverting from it.