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I am checking adequacy of an existing gas process line on offshore platform. Following the API 14E guideline the line is inadequate as the gas velocity exceeds 60ft/sec and using industry standards the pressure drop exceeds 0.5 psi/100 ft.
However, could any one advice if there is any momemtum (rho x v²) criteria too that needs to be checked. Further, is there any relaxation to the 60ft/sec guideline.
Regards,
Ashutosh
Worley, Abu Dhabi
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Gas Line Sizing
Started by anischal, May 10 2004 09:22 AM
7 replies to this topic
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#2
Guest_Milton Beychok_*
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Posted 10 May 2004 - 04:16 PM
Ashutosh:
What is the actual gas velocity so we can see how much it exceeds 60 ft/sec and what is your calculated pressure drop so we can see how much it exceeds 0.5 psi/100 ft ??
The guidelines you cited are okay, but they are set at that level primarily to limit the noise that the gas line will create. As long as your system can provide the necessary pressure drop, and you can tolerate the noise, and you are not experiencing excessive line corrosion, you can have higher velocities (up to about 100 to 120 ft/sec).
QUOTE (anischal @ May 10 2004, 09:22 AM)
Following the API 14E guideline the line is inadequate as the gas velocity exceeds 60ft/sec and using industry standards the pressure drop exceeds 0.5 psi/100 ft.
What is the actual gas velocity so we can see how much it exceeds 60 ft/sec and what is your calculated pressure drop so we can see how much it exceeds 0.5 psi/100 ft ??
The guidelines you cited are okay, but they are set at that level primarily to limit the noise that the gas line will create. As long as your system can provide the necessary pressure drop, and you can tolerate the noise, and you are not experiencing excessive line corrosion, you can have higher velocities (up to about 100 to 120 ft/sec).
#3
anischal
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Posted 11 May 2004 - 12:10 AM
Thanks Milton.
The line velocity with 24" size is 86.3ft/s and pressure drop 0.56 psi/100 ft. This line has experienced external corrosion and has been clamped at one location. It is a approx. 70m long pipe across two offshore platforms and is the feed line to gas compressor suction. We are working on the noise levels at present.
So is there no momemtum criteria for line sizing. As we are getting in excess of 11000 Pa in the 24" line.
The line velocity with 24" size is 86.3ft/s and pressure drop 0.56 psi/100 ft. This line has experienced external corrosion and has been clamped at one location. It is a approx. 70m long pipe across two offshore platforms and is the feed line to gas compressor suction. We are working on the noise levels at present.
So is there no momemtum criteria for line sizing. As we are getting in excess of 11000 Pa in the 24" line.
#4
mbeychok
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Posted 11 May 2004 - 11:37 AM
Ashutosh:
Sorry, but I don't have an answer for your question about your momentum criterion. Perhaps someone else will respond in this forum ... or perhaps you should contact a pipeline design consultant.
Milton
Sorry, but I don't have an answer for your question about your momentum criterion. Perhaps someone else will respond in this forum ... or perhaps you should contact a pipeline design consultant.
Milton
#5
anischal
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Posted 11 May 2004 - 11:46 AM
I have checked NORSOK and found that it estimates less than 60ft/s for our gas conditions. However, as API does allow higher than 60ft/s velocity provided noise is not exceeded, I need to estimate line noise to prove my point for change in existing line size.
Could anyone help provide a spread sheet or inform a source for calculating noise level in gas lines.
Could anyone help provide a spread sheet or inform a source for calculating noise level in gas lines.
#6
ShaunHill
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Posted 12 May 2004 - 10:01 AM
Going back to your question about momentum criteria.
I've typically used SQRT(rho V^2) <=100 where rho is in lb/ft3 and V is in ft/s.
If you stay below this limit you shouldn't have erosion problems in a gas line.
I've typically used SQRT(rho V^2) <=100 where rho is in lb/ft3 and V is in ft/s.
If you stay below this limit you shouldn't have erosion problems in a gas line.
#7
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Posted 08 July 2004 - 03:59 AM
Shaun,
As far as I know, the momentum criteria is important for nozzles and not for gas / liquid line sizing. The momentum criteria for the feed inlet nozzle of a 2 phase / 3 phase separator would decide whether to go in for any specialised kind of internals (like Schopoentoeter etc.). However, after doing this exercise of estimating the momentum of the flowing fluid for that particular nozzle, the feed inlet line size could be checked for it's appropriateness (whether the line is OK or a reducer is needed just before the subject nozzle etc...)...or am I missing something??.............kindly clarify!!
As far as I know, the momentum criteria is important for nozzles and not for gas / liquid line sizing. The momentum criteria for the feed inlet nozzle of a 2 phase / 3 phase separator would decide whether to go in for any specialised kind of internals (like Schopoentoeter etc.). However, after doing this exercise of estimating the momentum of the flowing fluid for that particular nozzle, the feed inlet line size could be checked for it's appropriateness (whether the line is OK or a reducer is needed just before the subject nozzle etc...)...or am I missing something??.............kindly clarify!!
#8
Guest_Guest_ShaunHill_*
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Posted 12 July 2004 - 03:12 PM
It is true that the momentum criteria is important in sizing nozzles, and some companies (especially Shell with their Schopoentoeter) place more restrictive criteria on their inlet nozzle momentums. There are also slightly different numbers for this criteria for shell and tube exchanger inlet momentums.
However, the momentum criteria should still be considered in a piping system, because you typically have elbows and tees that can experience errosion due to high velocities. I realize that piping is less expensive to replace than vessels and as such it is sometimes considered less critical. However, if the plant is inteded to be around for a long time, it is worth considering this factor when sizing the lines.
Having said that, I have found that for most cases this is only a concern at very high pressures. At lower pressures, pressure drop will result in larger line sizing.
However, the momentum criteria should still be considered in a piping system, because you typically have elbows and tees that can experience errosion due to high velocities. I realize that piping is less expensive to replace than vessels and as such it is sometimes considered less critical. However, if the plant is inteded to be around for a long time, it is worth considering this factor when sizing the lines.
Having said that, I have found that for most cases this is only a concern at very high pressures. At lower pressures, pressure drop will result in larger line sizing.
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