4 replies to this topic

[ekar]

Dear ChemErs,

Our client is currently facing a problem of CUI on his piping. Some piping is found to "sweat" due to humidity (offshore platform on tropical region). Icing is found in some others, suspected to be due to big pressure drop over upstream control valve, causing surface temperature to drop below water dew point.

My questions:

1. What solutions are available out there? What's the strength & limitation of each? I understand there may be no one-size-fits-all cure for this CUI problem. So far I've got anti-sweat insulation, coating, some even suggested installing a long drip pan underneath the piping. I'm not familiar enough with the topic of corrosion, so any input will be much appreciated.

2. How do I calculate the amount of sweating on pipe? Any specific formula?

3. All in all, should this issue be handled by piping or process discipline? Or a corrosion specialist?

Thanks for your input.

[daryon]

Hi ekar,

Here's my thoughts:

1.I work in offshore oil & gas industry and this problem of corrosion under insulation is common when rockwool insulation is used to lag a pipe, you would normally clad the rockwool insulation with stainless steel or aluminum jacket and seal it to prevent the ingress of rainwater, spray etc. However, inevitably water does creep in eventually and causes the rockwool to become water ladened and soggy. In a tropical saliferous environment this can cause accelerated corrosion, especially to carbon steel piping. We try and avoid rockwool (mineral fibre) insulation if we can especially on cold pipes, but it is the cheapest and if there is a lot of piping to lag then sometimes its the only cost effective option. Cellular glass insulation is preferable but more expensive, it doesn't hold water or go soggy like mineral fibre, and will hopefully prevent the moist air from contacting the pipe. With mineral wool it's unrealistic to assume you can keep moist air away from a cold pipe, but with celluar glass I think you stand a chance. It would be a good idea to consult a insulation specialist with your specific needs. You can't beat a good paint job underneath any insulation to help prevent corrosion.

Out of interest, what's the purpose of the insulation on your line? Are you trying to conserve the cold fluid temperature? If you not then i'd question if you really need to insulate the line. If its personnel protection that's the objective you could put wire caging around the pipe to prevent people touching it. May be you could accept the sweating and ensure the line is regularly painted, inspected and monitored for external corrosion.

2. The rate of condensate of water from the air will depend on the air humidity and the pipe wall temperature. It's a very challenging calculation i would think, I don't think it's even possible(without some fancy software) to determine how much of the heat transfered from the pipe to the air is sensible heat that is cooling the other gases present (N2, O2 etc) and how much is latent heat which is condensing water vapour. I'd suggest you should be looking into psychrometic relationships and charts to estimate the condensation rate.

3. I'd say this was a process issue but no bad thing to get the input of an experienced piping engineer who'll have some suggestions no doubt.

Hope this is helpful, good luck

[ekar]

Daryon,

Thanks for your thoughts. Most of the insulated lines are hot lines, but there are uninsulated lines with icing. Client says it's due to big pressure difference over an upstream control valve.

From your reply & articles on CUI, it seems to focus on prevention / mitigation. Is there any special repair action for this type of corrosion?

[Art Montemayor]

Ekar:

Daryon is right on top of this subject, even though you failed to identify the precise problem in your original post. What you now describe is a typical downstream problem of any adiabatic expansion taking place in a pressurized hydrocarbon line that is throttled. The subsequent Joule-Thomson effect causes downstream cooling and if the piping design has not taken this into consideration, excessive corrosion will take place on an offshore platform operation. I’ve been there, seen it, resolved it.

As Daryon correctly diagnoses it, the problem is one of several factors – the outstanding one being that the platform designer/constructor applied a cheap, low-cost insulation application to a serious problem. When you don’t apply the correct and logical type of insulation, such as FoamGlas, for a cold application, you will suffer the consequences – especially in a saline offshore platform. The best and most practical solution to your problem is to eradicate what is obviously a bad and wrong engineering application at the very outset. Let’s face it: you have a situation on your hands that is potentially very hazardous and the worse news is that you are not free to spot it on a day-to-day basis because the corrosion is in the form of “stealth corrosion” – you don’t see it because of the insulation on top of it. This is all caused by either a stupid, ignorant, cheap, or “don’t care” type of piping installation. In my 50 years of field experience, I’ve found that these situations are often caused by both the contractor AND the client: the contractor wants to make it cheap to maximize profits and the client is lazy or inexperienced and doesn’t checkup or control what is being built. Either way, you have inherited a mess and a potential stealth hazard – the worse there is.

To my knowledge, there is no special repair action available for this type of corrosion. The only practical and correct way to resolve the problem is as I mention above: eradicate what is obviously a bad and wrong engineering application. You do this by either changing out the piping with a stainless alloy or apply FoamGlas insulation exactly as prescribed by the manufacturer, Pittsburgh Corning Corporation and Pittsburgh Corning Europe NV/SA, employing a certified, experienced, and controlled insulation contractor. This application requires careful and correct VAPOR SEALING as well as expansion joints that must remain hermetic. This is a special insulation application and isn’t cheap. There is no other answer for this application as far as insulation is concerned. The only answer is prevention / mitigation.

You can find out more about this type of insulation at: http://www.foamglas.com/industry/en/

I have used this insulation since 1965. I hope this experience helps you out.

[ekar]

Hi Art,

Thank you for your pointers. I see the issue more clearly now. The facility is indeed rather old (15 years or so), and many of the people working there actually are new. Documentation is lacking also. In any case, I'll definitely take a good look inside the link.