8 replies to this topic

[farid.k]

Hye all,

 

This might be a simple question but I need some technical support from you guys.

By referring the Rules Thumb Of Chemical Engineer Handbook (fluid flow section), it specify the recommended velocity for gas for pipe sizing. But it has term dry and wet gas. How do I know the gas is really dry or wet? Because it has different limit. For dry gas, maximum velocity limit is 100 ft/s but for wet gas, velocity limit is just 60 ft/s.

 

Currently I am evaluating the existing pipe size because we plan to add one more new compressor and tie in to existing system.

 

Does it I need to limit velocity 60 ft/s upstream of the air drier and 100 ft/s downstream of the drier? Some other standard didn’t mention dry and wet. It just mention “gas”.

 

thanks

[Zauberberg]

Like the books says, it is just a rule of thumb. You will rarely see continuous service gas lines designed for velocities at the high-end of the recommended ranges. At around 25-30 m/sec, noise becomes substantial and it is normally not tolerated in a continuously manned facility.

 

One reason I can think of for reduced velocities in a wet gas system, is due to (possible) condensation of liquid droplets from gas and carrying of condensed liquids at high velocities towards elbows, bends, and fittings/valves. This can create significant erosion and damage. In some other cases, when wet gas contains corrosive components (H₂S, CO₂), velocities are reduced below 60 ft/sec so that inhibitor injection does not lose its effectiveness. At higher velocities, inhibitor becomes ineffective according to many field and experimental studies.

 

If you want my personal opinion, I would rarely design any continuous service gas line for velocities above 20 m/sec (~65 ft/sec), regardless if the gas is dry or wet (= contains condensable components).

[ankur2061]

Farid,

 

What  are you evaluating currently? Is it compressed air piping? If yes, then velocity limits are much lower for compressed air. The limits for a well designed compressed air piping system is 6-9 m/s (20-30 ft/s). Refer this famous article on compressed air piping:

 

http://www.chemicalp...05/12/?show=all

 

Regards,

Ankur.

[farid.k]

Farid,

 

What  are you evaluating currently? Is it compressed air piping? If yes, then velocity limits are much lower for compressed air. The limits for a well designed compressed air piping system is 6-9 m/s (20-30 ft/s). Refer this famous article on compressed air piping:

 

http://www.chemicalp...05/12/?show=all

 

Regards,

Ankur.

Dear Mr. Ankur,

 

Yes, this is for air compressor. I just question the existing design for the piping (existing design is since 1994) that goes to plant air is 6” and piping that goes to instrument air is 4” (from inlet to drier up to main instrument air header). Based on user demand, plant air is much lesser than instrument air. But how can the pipe size bigger.  That’s why I afraid that 6” has been chosen due to wet air (as plant air didn’t go through drier) and 4” chosen due to dry air.

 

6-9 m/s for me is too stringent. Based on my company technical standard, gas should be 10-20 m/s. based on this figure, most of existing design is ok. But if evaluate using parameter 6-9 m/s, I think existing pipe design is undersize.

[farid.k]

Like the books says, it is just a rule of thumb. You will rarely see continuous service gas lines designed for velocities at the high-end of the recommended ranges. At around 25-30 m/sec, noise becomes substantial and it is normally not tolerated in a continuously manned facility.

 

One reason I can think of for reduced velocities in a wet gas system, is due to (possible) condensation of liquid droplets from gas and carrying of condensed liquids at high velocities towards elbows, bends, and fittings/valves. This can create significant erosion and damage. In some other cases, when wet gas contains corrosive components (H₂S, CO₂), velocities are reduced below 60 ft/sec so that inhibitor injection does not lose its effectiveness. At higher velocities, inhibitor becomes ineffective according to many field and experimental studies.

 

If you want my personal opinion, I would rarely design any continuous service gas line for velocities above 20 m/sec (~65 ft/sec), regardless if the gas is dry or wet (= contains condensable components).

Dear Zauberberg

 

Yea, currently my basis for line sizing is should be within 10-20 m/s. as long as the velocity below 20, regardless plant air and instrument air, should be ok.

 

Some other typical process standard mention to sizing based on sonic velocity. It state that, maximum velocity shall be less than 50% of the sonic velocity. Have you experienced this before?

[farid.k]

Farid,

 

What  are you evaluating currently? Is it compressed air piping? If yes, then velocity limits are much lower for compressed air. The limits for a well designed compressed air piping system is 6-9 m/s (20-30 ft/s). Refer this famous article on compressed air piping:

 

http://www.chemicalp...05/12/?show=all

 

Regards,

Ankur.

Dear Mr. Ankur,

 

I did read the article. Thanks a lot for the informative article. The velocity suggested by The British Compressed Air Society for me too conservative. It will cause a lot more question especially current design.

[katmar]

Rules of thumb for pipe sizing are often given in terms of velocity and while these can be useful for estimates they should not be used for actual designs and studies.  The pressure drop, particularly in compressed air systems, can be the deciding factor.  A velocity of 15 m/s in a typical dry 7 barg air system will lead to a pressure drop of about 100 kPa per 100 m  for a 1" pipe, but only 10 kPa per 100 m in a 6" pipe.  This can have a material effect on the end user equipment.

 

The velocity will of course have a direct impact on the noise levels, and as pointed out by Zauberberg, if you stay under 20 m/s you should be OK.  With the water levels typically found in compressed air systems this velocity should also keep you safely away from erosion.

 

If your air lines are long, or exposed to the weather, there can be condensation of water over the length of the pipe.  The velocity in the pipe impacts on the flow pattern of the liquid and gas (see "Baker two phase flow map") and this will determine the effectiveness of your water trapping equipment. 

 

In a detailed design where you are concerned about liquid levels you should be considering things like the Baker map, and the actual pressure drop, rather than relying on rules of thumb.

Edited by katmar, 01 April 2015 - 03:08 AM.

[Zauberberg]

Farid,

 

I'd suggest you to follow advices as received from Harvey and Ankur - these are right on the spot. One additional comment I have is that all rules of thumb might become invalid in brownfield projects where you are forced to engineer a solution that must fit between given constraints. Identifying constraints that can be (and should be) changed, and those that cannot be changed, will likely lead you towards successful design and later, operation.

 

Having excessive pressure drop in Instrument Air piping will always result in two things:

1) Wasted horsepower of Instrument Air compressors (= $$$);

2) Inability to meet Instrument Air demand for remote users, which can ultimately lead to unscheduled shutdowns/trips.

 

If there is 2 bar pressure drop between the air dryer discharge and the most remote IA user, at the maximum IA demand flow, I would change the piping size. You can easily demonstrate how this works by calculating incremental Opex of the IA compressor versus cost of the new piping if you design it for maximum 50 kPa pressure drop, and you do this over the design life of the plant. I am sure that you can easily prove how expensive it is to pump air through undersized network. This simple economical calculation would serve as the basis for replacement of undersized piping, if the case.

 

So, rather than being tied to any specific "rule of thumb", you need to look at the system in question and make all necessary observations required for proper engineering design. If I remember well, your project was about design and installation of additional IA compressor in an existing plant. What if (I am asking this hypothetically) the bottleneck in IA supply is not caused by machines, but by an undersized piping network? Installing additional compressor will not help much in that situation. It is the same as if you are having an orifice in the piping and you keep adding more compressors upstream to push more flow, whereas it could be that you simply need to increase the orifice size and the flow would simply increase by itself.

[Teknas]

Hi Farid,

 

As per the description given in your question, this seems to be augmentation of the Instrument Air capacity and hence an additional compressor is being installed.

 

This new compressor must have come up due to additional users which may be tapping to your network. In this case the common header size definitely needs to be reviewed. You will need to study the network and find where is excessive pressure  drop happening and then go for the line size increase for those sections.

 

As mentioned in above posts 15 ~ 20 m/s velocity for air is reasonable, but always try to be at the lower end of the spectrum because this always helps in case of future expansions.

 

Other than this, I have never come across when the Plant Air being "wet" is sized differently than Instrument Air. The reason should be some intermittent or peak consumption in the Plant Air circuit which will be requiring the higher flowrate and hence higher line size.

 

Is the minimum ambient temperature coming into picture by any chance for designing bigger plant air line sizes?