MOC for pneumatic actuated valve 2

[molebleeze]

Hello,
We are a chemical process plant in oil gas industry. There are lots of pneumatic on and off actuated valves in the system. (outdoor) The air comes from control cabinet directly. (PLC drives remote solenoid island IO)
Historically, we use SS316 tubings to connect valve actuators to the air source. We are thinking changing to poly tubing (polyurethane with UV protection). One reason is cost. The other is installation time.
What are your thoughts?
We got significant pushbacks from plant maintenance for sure..

 

[Latexman]

Metallic tubing is a gazillion times more robust than plastic tubing.

We only use plastic tubing on fail closed actuated valves we want to close in a fire, and then it's only the last few feet of the tubing at the actuator.

Good Luck,
Latexman

 

[TiCl4]

Coming from having experienced both in widespread usage, I prefer stainless tubing over plastic tubing. You will find that your lower installed cost will be partially eaten up by much more frequent tubing replacement - plastic tubing will usually wear much faster than stainless tubing. I think the life-cycle cost analysis will still lean towards plastic tubing, though, even though it is less robust. This is from a pure cost perspective without consideration of what happens when the tubing fails.

Consider this, however: When plastic fails, it tends to fail catastrophically (though not always), often completely blowing out of the push-connect fitting. Depending on your valve criticality and fail status, this can cause some serious and unexpected valve closure/openings that have safety/uptime/cost implications. In my experience, stainless tubing with ferrules tend to crack at the fitting and leak, but do not interrupt air supply/pressure to the valve.

Plastic tubing does have its place, however, in the following cases:

[ul]
[li]High vibration areas (these are tough on metal tubing and ferrules)[/li]
[li]Flame-responsive tubing that is designed to melt and cut air flow to a valve in the event of a fire, causing a valve to go to fail position. This may be required by your insurance provider.[/li]
[/ul]

 

[Compositepro]

There was an earlier thread on exactly this subject.

 

[thebard3]

Stainless steel is obviously better. Just like a Mercedes Benz is better than a Kia. Do you need the Mercedes, or is the Kia good enough?

Brad Waybright

The more you know, the more you know you don't know.

 

[molebleeze]

The polytubing side argues the following point and I kind of agree with that:
If the stainless steel tubing is run straight continuously nonstop to the air source. SS tubing might have a better reliability.
However, the reality is that the air source is about 30-50 ft away. It would take several bends and interconnects to run the tubing between actuator and air source. Those pneumatic fittings are difficult to install correctly and easy to leak due to thermal cycling. In the mean time, the poly tubing usually just have fittings at the start and end points.
Polytubing is in fact more reliable than SS tubing in reality.

My experience also matches a bit of that. We had a few tubing leaks in the past years. All of them are SS tubing and none of them are for polytubing. (For sure the plant has 90% SS tubing)

 

[TiCl4]

OP, what kind of thermal cycling are you talking about? This is compressed air service - it shouldn't be seeing very high temperature swings. Additionally, ferrule fittings are pretty robust against normal ambient temperature changes (day/night, winter/summer). As far as reliability, my experience is different than yours. I've been in a plant that had 100% plastic fittings and another plant that had 95% stainless. From my experience, the plastic tubing is by far less reliable and more prone to failure.

Ferrule fittings do tend to run into leaking issues if they are repeatedly removed and rejoined, though.

As far as installation goes - you can use single ferrule fittings to help avoid improper installation. Bending tubing is easy, and the metal tubing will not require as many support/tie points to keep the tubing from drooping.

Also, if outside, any kind of plastic tubing will be susceptible to UV-based hydrolysis, even if it is "UV protected". Metal tubing does not have that as a mechanism of failure.

Compositepro - that was my thread! It seems there are a good number of people in both camps, with pros/cons for each side. I don't really think there is a "right" answer, but I still certainly have a preference.

 

[EdStainless]

We bought SS tube in 100' (and sometimes longer) rolls.
Our guys had a small handheld roll straightener that they used to make it look pretty.
All bends were made with a guide and there was a minimum bend radius.
We only used two brands of double ferrule fittings and a gauge had to used on every connection.
There were a few cases like Laytex said, where we used a small section of poly as a fire fuse.
When we had vibration or motion to account for we used loops of SS to allow for it.
We had a manipulator that could move 5' in any direction.
Plastic lasted a couple of weeks (we tried different types of line and connections).
We finally just went to about 50' of SS in large loops (and heavier gage) and the thing has run for years.
We did use a lot of plastic when it was inside enclosures or other protected areas. We also used it for a bunch of air logic stuff, just easier.
I will also note that some of our actuators were run a lot. Many of air lines were actually pure nitrogen to keep things clean.
And all of our air lines were SS, mostly Victaulic PressFit sch5.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[mk3223]

Op,

We got significant pushbacks from plant maintenance for sure

If this is true, then what's the reason the Maintenance pushbacks and why?