Compressed air piping 1

[KrisB4]

Hi all,

I'm designing compressed air piping (in ring form) for my recent plant in Poland.
It will be DN100 (4" equiv.) ring, ca 800m circumference, system capacity is 60m3/min max, 36m3/h at the begining, 8bar pressure.
I'd like to use ball valves for separating parts of the ring (in case of any installation/maintenance work).
My preferred material is stainless steel.
Our sister company in Germany uses black pipes, which are already red from the surface rust (indoor installation, less than 6 months) and butterfly valves, with the same sizes and capacity (both factory use Kaeser screw compressors with refrigerating dryers).
I'd like to use st. steel to avoid any problems in the future from rust and dirt build-up inside the pipes.
My prevoius factory used hot dip galvanized pipes, but they are only protecetd on the inside, and though we used air preparation equipment, we had some problems with air quality.

What's your opinion on using stainless steel (I know it's a bit more expensive but it should pay back in the future) and ball valves (as they do not restrict air flow and have better sealing longevity)?

Best regards,

Kris

 

[metengr]

If you need very “clean” air quality for your process functions, AISI Type 304L stainless is fine. We require 304L stainless steel tubing/pipe for instrumentation air.

For house service air systems that drive equipment or tools, carbon steel piping is fine with dryers.

 

[KrisB4]

Some amount of air is needed for dense phase powder conveying, so it better be good, some amount for product quality testing (same story). The rest is used for pneumatic equipment (cylinders, valves, ejector vacuum pumps) and there would be a lot of it.
We will have the dryers, though I'd rather be safe than sorry, especially as I would be responsible in the future for keeping it all running.

best regards,

Kris

 

[Ashereng]

KrisB4,

There is another thread in Eng Tips discussing SS piping for air systems. You may wish to look it up.

The summary of that thread was:
1) Yes, SS is good. Expensive too.
2) If there is water, contaminants in the air, SS would only protect the piping. The water and contaminants will still get to your end devices (actuators, powder, etc.) and may affect your process (product quality testing).
3) If you clean up and dry the air, then you don't really need SS piping.

I guess you need to do a cost analysis to see the payback between going SS, versus a better air systems, versus cost of wet/contaminant to your process.

 

[metengr]

Here is the thread;
thread723-142251

 

[Ashereng]

Thanks for looking it up metengr!

 

[KiwiMace]

If the SS pipe works out much more expensive in your area, i'd just go with the Hot Dip Galv steel pipe. I've used HDG in reasonable environments and had no problems. Wrap all the joints with Denso tape. Spend your savings on filters and dryers! Put a filter at each terminal point and no galv pipe after the filter.

 

[DesignerMike]

I've seen some plants with copper tubing...I can't say that I would use it, but it is somewhat common.
The SS will probably be a LOT more expensive, but should be the last time you install it.

 

[katmar]

People see that Stainless Steel is 7 or 8 times the price of Carbon Steel on a weight basis, and automatically reject it. But this is by no means the whole story.

A factor which makes SS more attractive for this sort of duty is that it is available in Sched 10, which is much lighter than Sched 40 Carbon Steel pipe. The lower weight of pipe translates into cheaper erection rates and if you take the surface preparation and painting into account (at installation time only) the stainless can even be cheaper than CS in some instances. If you factor in the long term maintenance cost of painting it makes SS even more attractive.

I have used pressed SS collars with galvanized CS backing flanges for joints. This is much cheaper than solid SS flanges and perfectly adequate for low pressures.

There is a current over-supply of Type 304 SS and in the short term prices are low. Type 304 is quite adequate for air lines. I would definitely get firm quotes for both SS and CS before making a decision in this instance.

Stainless Steel ball valves are a commodity item and surprisingly cheap, but will still be more expensive than a butterfly valve. A butterfly valve with a SS disc and elastomer liner will not rust, so you just have to consider the pressure loss vs cost comparison.

 

[moltenmetal]

A proper analysis of this depends on labour costs in your region. If the piping is small bore and needs to be painted, stainless steel is usually cheaper because the labour to blast and paint the piping can easily overwhelm the labour cost. But at 4" size, my feeling would be to go with carbon steel and use Victaulic end connections. The pipe can be pre-painted and fitting/erection costs are very low.

At 4" size, butterfly valves are far cheaper than ball valves, and for instrument air a butterfly valve is more than adequate.