What is the main function of corrosion coupon 5

[tarekdata]

Dear Corrosion specialist.

What is the main function of corrosion coupon?
What is the best location and position to be installed?
What is the main types and options for this coupon?

 

[BigInch]

A corrosion coupon is a sacrificial element that is inserted and held into the pipe flow stream to measure the corrosive effect of the stream on the material of which the coupon is composed. It is left there for a sufficient length of time to get a measureable loss of coupon material from which a characteristic corrosion rate (grams/unit time, etc.) is deduced for the particular flow stream. Typically this is used to predict a pipe's lifetime, or design a corrosion inhibition treatment program for the piping system.

Best location depends on what the stream looks like, so obviously the coupon should be held in a position that would be expected to give a representative example of the corrosive effect of interest. If the flow was gas/liquid and the liquid was more corrosive than the gas, it might be advantageous to hold the coupon in the liquid stream rather than the gas stream, or both?

Normally the coupon is inserted into the pipeline through a special fitting.
See the following for a well known vendor of this equipment,

http://www.cosasco.com/cosasco-corr...on-holders-coupons-probes-c-101_242-l-en.html

you must get smarter than the software you're using.

 

[marty007]

In a similar fashion, I have also seen rigs of corrosion coupons inserted into vessels for product development.

Say for example you're working on a new process that for one reason or another cannot be simulated in a wet-lab. Corrosion data for Nickel200 shows it will be okay, but there is no corrosion data available for cheaper materials. You could then build the first such system as a Nickel200 system with a bank of test coupons inserted with different materials (Hastelloy / Monel / etc...). You run the first system, it works, all is well. Then during the first shut-down you remove the coupons (or what's left of them), and maybe you find that you could have built the plant with a material that is half the cost. Now the next plant you build will be a whole lot cheaper, and more commercially viable.

 

[racookpe1978]

And, as stated above, those various coupons need to be put in both a static region in the process tank (but where they themselves won't block flow, cause turbulence or a "dead spot" or ponding, nor interfere with the reaction, and in a dynamic region of mixed flow if that is a suspicion.

A corrosion-resistant film might be created in one area, but wiped off in other area of different flow or turbulent flow. In testing, sometimes surprises happen.

 

[zdas04]

I am REALLY not a fan of corrosion coupons. I've checked hundreds that said "no problem here" and dozens of those lines had failures within 6 months. The problem is that the function of the coupon is to evaluate general corrosion and lines fail from pitting or cracking. Pitting is most often an electrolytic reaction, and if we put the coupon on the top of the pipe (to allow it to be retrieved from above while not creating a bluff body in the flow) it never sits in a continuous phase liquid (in a gas line). Cracking is associated with stresses that don't exist on a lump of steel hanging from a rod.

The way I would answer your questions is:
[ul]
[li]What is the main function of corrosion coupon? To provide employment to techs, lab workers, and people working for the manufacturer[/li]
[li]What is the best location and position to be installed?On the shelf in the Manufacturer's warehouse[/li]
[li]What is the main types and options for this coupon?For pipelines the only type I've seen are rod-mounted with a lubricator, there are other types for vessels and tanks. None do very much[/li]
[/ul]

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

 

[marty007]

Cracking is associated with stresses that don't exist on a lump of steel hanging from a rod.

This can be somewhat countered by using a bent plate coupon... To be honest I don't have a ton of experience with them.

 

[zdas04]

An unconstrained bent plate will equilibrate stress to a very low value within minutes of cooling after the bending. I've never heard of a coupon showing signs of stress corrosion cracking (or any of the specialized chemical versions).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

 

[marty007]

That's why you constrain the plate. See the rack setups in the attached link for examples of more involved corrosion coupon test rigs: Link. In this case the bent plates are constrained in a bolted setup.

 

[zdas04]

Where is there room for fluid flow in that lash up? The ones I see in pipelines are flat plates with a threaded rod. The structures in the link would never go through a lubricator.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

 

[bimr]

 

[racookpe1978]

But remember, the coupons will block pigs from passing.

 

[LittleInch]

Tarek,
BI has the correct scientific answer, but I'm with zdas04 on this one - I haven't seen one on a pipeline in years now and they've been overtaken by use of pigs and UT scanners / detectors.

It's now much easier and simpler to get a UT scan of A/G pipework on a 360 degree basis at multiple points rather than a single point (top / bottom) on a pipeline with all the fuss and bother of retracting something under pressure with all the risks of spillage, inability to remove the coupon and then only having one data point instead of hundreds for the same time and effort.

Usually they were restricted to non piggable portions of a pipeline after one dropped off and jammed a pig - never again.

Corrosion probes seem to have gone the same way - one of my first jobs was going round every 3 months taking readings from about 4 probes - a good day out of the office, but the readings never seemed to change....

Basically old hat and overtaken by new technology providing much better information

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

Can't say that I disagree with anything of the above.
I just answered the question.

you must get smarter than the software you're using.

 

[marty007]

Yeah, I can see all of those issues in the pipeline industry. The place I've seen them was in the chlorine production industry where high concentrations of saturated bromine was going to occur in the liquid chlorine and no corrosion data was available in literature. This chemistry is extremely difficult to simulate in a lab environment, so instead the first system was built with extremely up-alloyed materials to avoid corrosion. They used the coupons to test a number of lower-alloy materials to learn if they could build the system cheaper the second time around.

 

[zdas04]

OK, I take back that they are never useful. Marty007, you've described a situation where they seem to be an excellent tool to learn something. In a natural gas pipeline they don't let you learn mucyh.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

 

[lhwingking]

Dear zdas04

In reference to your statement " I've checked hundreds that said "no problem here" and dozens of those lines had failures within 6 months"

The material of the corrosion coupon is the very same as that of the pipe right. And, the fluid is going to be the same with same process conditions. How can there be a difference in corrosion rates in the coupon and the pipe? And can you also walk us through what caused a difference in corrosion rates in two very similar materials?

Can you please elaborate as even I am new to this.

Thanks.

 

[BigInch]

Could be a lot of reasons,
Coupons were installed after many years of operations.
Well fluid changed to mostly salt water.
Had the coupon up in the gas flow region only.
Heavy truck ran through the field.

 

[zdas04]

Any of those are possible BigInch, but the coupon measures general metal loss and the failures are pitting in areas with standing water.

LhWingKing,
When new to a subject it is not terribly wise to get confrontational with someone who has been in the field for nearly 40 years. Not all gray-haired fogies are as nice as me.

I never see general metal loss in gas gathering lines (which is all a coupon can see). Ever. I've investigated many failures in gas gathering lines for my employers and clients and every time the root cause has been localized pitting in unpigged lines with small pockets of standing water. Usually it is MIC. Coupons on the top of the line (usually at an accessible high point in the line) don't get pitting. And yes, the material of the coupons is identical to the material in the line. Most everyone pulls the coupon back up to the top of the line to minimize von Karmann Streets downstream of the rod because these eddy's are very energetic and can scour the passivation layer off the pipe (also, pulling them to the top allows pigs to pass without having to remember to go out an retract them for a pig run).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

lhwingking,

I'm with zdas04 on this. Corrosion coupons in theory should see the same general corrosion that the rest of the pipeline does. Whilst this sort of system works in tanks and pressure vessels to a certain extent, it's use on pipelines though is very limited as it takes a combination of events to cause corrosion, often pitting type and if you look at an intelligent pig run, you will only find significant corrosion on probably less than 0.1% of the internal surface area. Therefore the chances of a corrosion coupon accurately predicting corrosion is in the realms of lottery winning odds.

The advent of internal inspection pigs and UT scans have rendered this sort of inspection obsolete and in many ways this is a good thing as they could easily create a false feeling of security about a pipeline that was not justified.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[lhwingking]

Thanks for the promot response.

And apologies if I had sounded confronting or rude. I obviously didn't mean to. I just asked the doubt I had in mind. Maybe I could have used polite words but I really don't see what made you presume I was confronting. I wasn't confronting. I was asking.

Thanks again for your response zdas04 and biginch.