Pneumatic Actuators - MoC of Tubing?

[TiCl4]

I work at a plant that using polyethylene tubing everywhere for tie-in between the air header and the final instrument (mainly valve actuators). Personally, I hate this design choice. It seems to fail quite often - all I have to do is take a stroll through the plant to hear the "pssssst" of leaking air left and right. It is susceptible to heat/physical impact (i.e. steam jacket blowdowns have put pinholes in nearby cooling valve air lines). When I asked the maintenance manager about the basis for the choice -there is no documented reason in the piping codes for instrument air lines- he said that they use it instead of something like stainless tubing because stainless would be a pain to replace when it failed. I countered that the stainless tubing wouldn't be failing at nearly the rate of the poly tubing, but he wasn't buying it.

Having worked at a plant prior that using nothing but stainless tubing, I just much prefer the (perceived) longer life of stainless tubing.

Can anyone chime in on what your experience is with connection tubing? Pros/cons of stainless tubing for instrument connections and actuator connections?

P.S. I know poly tubing has its place. The reasons would be:
[ul]
[li]In high-vibration lines, where ferrule fittings typically will crack due to fatigue[/li]
[li]In pool-fire scenarios where certain valves need to fail. A melting poly tube guarantees an additional means to ensure the valve goes to fail state.[/li]
[/ul]

 

[bcd]

And the obvious reason....poly tubing is much less expensive to buy and install. Fittings are much less expensive as well. Have had better luck with goodyear type reinforced rubber tubing versus poly tube.

 

[EdStainless]

What does your compressed air cost? In many plants it is the MOST expensive utility.
Find out what that number is.
Begin demanding immediate repair of every leak because of the huge costs.
In locations/application where there is more frequent failure (is there a maint database) then perhaps selectively replacing those with SS can be shown justifiable.
The last two places that I worked all air lines were SS from the compressor to point of use. It is just a mind set.


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P.E. Metallurgy, consulting work welcomed

 

[hacksaw]

1. area classification
2. ambient temperatures, and instrument air quality
3. how many valves
4. process fluid
5. process operation: batch, semi-batch, continous?

 

[Compositepro]

I worked in a plant plant where much of the machinery was pneumatically operated with air cylinders, valves and regulators. Polyethylene tubing was used with barb fittings or push-in fittings. This tubing is far cheaper to purchase, install, repair, and to modify. Using metal tubing in a small control panel is not very practical. It works well for machines that are always monitored by an operator and can have the air shut-off when unattended. It would not be appropriate where the consequences of tubing failure would be significant.

I also worked in a chemical plant where the environment was fairly corrosive and almost all the instruments and controls were pneumatic (3-15 psi signals). The instrument air lines were PVC jacketed copper tubing. This worked very well. Stainless tubing was only used to contain process fluids.

 

[georgeverghese]

In the oil/gas industry, have yet to see anyone using plastic tubing for IA - it is always SS316L at least, preferably with a min of 3-3.5%Mo in coastal or offshore applications.

 

[TiCl4]

Hacksaw,

I presume those are all questions directed to me.

1. Class 1 Div 2 with Div 1 bubbles around vessel openings in some areas. Other areas are non classified.
2. Ambient temps range from 10 F to 100 F. Plant air is either -40 or -60 C, I can't remember off-hand.
3. More than 500, less than 2,000.
4. I'm not sure how this is relevant - process fluids vary from various flammables to city water.
5. Yes to all three options. Mainly batch/semi-batch.

 

[moltenmetal]

Replace any of the cheap polyethylene tubing which is leaking, with nearly as cheap, but MUCH more heat resistant and durable, black polyurethane tubing. Black, because black has the best UV resistance.

Problem solved, for very little cost.

Sometimes, people pay us extra to run stainless tubing from the header to their instruments- and this costs big money and delays schedule a lot on a plant, because we don't accept tubing that is mangled into shape, we want all of that work done in a neat, workmanlike fashion and that takes time and effort and skill. It becomes hilarious when they realize that we have to install a short piece of plastic tubing in the air line somewhere near the valve for fire safety reasons, so that the tube will melt off and allow the actuator to go to the failure position before the actuator itself melts in a fire.

If you have a lot of hot, abrasive conditions, fabric reinforced rubber hose is the typical solution. Basically the pneumatic version of hydraulic hose, i.e. without the steel wire reinforcement.

 

[Compositepro]

Polyurethane and nylon tubing may have much superior properties to polyethylene initially, but these polymers are prone to hydrolysis and often do not age well.

 

[TiCl4]

Thank you for the replies, all. So far, my general impression is that the polymeric tubings are suggested simply for the cost effectiveness. Are there any notable cons to SS tubing other than previously mentioned (cost, vibration, and fire-safe case)?

 

[EdStainless]

Cost and installation time are the real drawbacks to SS. We had a number of applications on moving/vibrating equipment where we put a couple of small loops in the SS line and they worked fine. We also had a lot of specialty gas (N2, Ar, H2, He) in the plant and followed the same rules everywhere. Where we had sensitive equipment we switched from air to N2, it didn't cost any more and pneumatic controls lasted forever.

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P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

There may be some benefit in getting some feedback on compression fittings used for these applications also - Swagelok vs Parker etc.

 

[danw2]

Are you sure you're hearing leaking tubing and not the valve positioner air bleed?

When we put together valve actuator positioner accessory assemblies, we use the fabric reinforced rubber hose (+bcd/+Moltenmetal).

We use plastic tubing for pneumatics internally in panels where it doesn't see UV, steam or other nasty service conditions.

 

[TiCl4]

Yes. Then tubes either blow off completely or develop pinholes. Most of that valves are actuated block valves with external solenoid valve. They don’t make any sound except when exhausting air while closing (or opening for fail open valves).

 

[jari001]

If the cut isn't clean/square on plastic tubes, sometimes the tubing doesn't seal properly - I've found push-to-connect especially sensitive to having a good cut.

 

[pierreick]

Hi,
You should consider Rislan from Arkema :

https://www.arkema.com/en/products/product-finder/range-viewer/Rilsan-Polyamide-Family/

Note : I've seen different type of materials used from : Rislan or PE tubing, SS tubing , Copper tubing wrapped with black resins. All in chemical plants , majority belongs to the first group.

Pierre