Internal inspection of a LPG Sphere. How to reach to the wall?

[lganga]

Some old guys told me they did the inspection with an inflated boat,reaching to different levels, filling it with water .
I could not believe it.
Does any of you have seen a safety way of doing this job?
is there any special scaffolding?
Regards

 

[unclesyd]

Been there and done that on several spheres. On two where we had to do repairs the scaffolding was built from the normally used scaffolding components except some parts were at weird angles. There was no problem building the scaffold, but it was hell paying the bills.
We have two trains of 6 100,000 gal reactors each that are routinely cleaned as the crews floats their boats. The boats are very similar to the Zodiac boats and inflated deflated when moved from one reactor to another.

 

[lganga]

Uncleysd I really appreciate your comments.
I thought that the Zodiac boat was avoided in this kind of inspection.
I have an story that the Inspector, from an Intenational Inspection Agency, falled in to the water during the inspection , he was saved, pulled from the top of the sphere, using the safety ropes where he was tied.
I am talking of 15 mts diameter sphere.
Of course an scaffolding is a lot of money.
Regards

 

[unclesyd]

I should have stated that we used an orange inflatable life raft instead of a Zodiac inflatable. It was my misuse of the the word Zodiac.
If I'm not mistaken the life rafts we used are compartmental and would be very hard to sink. This feature also makes it a little troublesome to inflate and deflate.
If you happen to be in the market for an inflatable look in the big Marine Supplies Stores for information about whats available.

With the proper gear this method of inspection can be very safe and economical for vessel that can be easily cleaned for entry. Saying that in our case it was the use of inflatables for cleaning the walls of the reactors that led to use of them to inspect the vessel. We have our on site waste disposal system so getting rid of the water is no problem. Also our reactors are cascaded so it takes only about 50,000 gallons of water for each reactor after the first one is filled.
As far as safety goes I think this approach, if possible, is much better than scaffold building and in our case climbing up and down a rope ladder 5' or 6' from the walls.

have you checked out the use of Acoustic Emission for checking for cracking coupled with UT for thickness. Cracking revealed by AE is what required us to do an internal inspection. To repair the cracking we had to build a scaffold. Another point is make sure you inspect the legs especially near the top. After several years of fighting for inspection of same we were able to remove the fireproofing for inspection of the top 4'. Nearly all showed some corrosion and a few had to to be replaced.

http://www.pacndt.com/

 

[lganga]

unclesyd
Thanks a lot.
You did in advance the other questions I had for you.

- About the boat I realize that it is a kind of Zodiac one.

- WE are going to do AE first, as said in another threads. AE gives good information(If you follow a good procedure)on the Spheres and horizontal vessels.
To avoid the use of water it is possible to do AE with the process fluid, and some inert gas,for getting the pressures steps according to the technical procedure.
My recomendation is to test the AE with Physical Acc(pac).


- As you said, we will use the inflatable boat. Before that, a security procedure will be written and approved.

-Other important element that you mention, is the inspection of the legs. I am sure we have to take the Fire Proofing appart to check if there is some CUI.
But, is there any NDT (like Eddy current) for testing the legs?

thanks for your time and comments

 

[unclesyd]

I haven't heard of any NDT procedure for the legs.

You might want to think about something I done about 30 years ago with the fireproofing on the legs. As our area was short on quality guniting services I replaced the fireproofing with clam shells , by the local concrete culvert company. They were applied and sealed with a cementitious mixture formulated by our concrete expert. He also called out the concrete mix for the half shells. If I recall the initial sealer at the top was Neoprene and later changed out to Butyl.

I just recommended the same process for some proposed inspections of sphere legs.

Addenda:
Not pertinent or relevant.

The aforementioned concrete expert (Perry) was extremely knowledgeable but crazy, he wrote several very good books on concrete and published them himself. When I ask to buy one he told me no that if I had one they wouldn't need him anymore. He was always at odds with management so one morning he came to work a little late wearing a trench coat, outside temperature in the 90's. Needless to say there was mad dash for the exits by all except me. I entered the the engineering building through the drafting department and I noticed everyone was gone except for a couple looking around a corner. I thought I had missed a fire alarm so preceding to exit I had to walk by Perry's office and when I saw him standing there in the trench coat I ask Perry what ate you doing in trench coat and the building is probably on fire. He put his finger to his lips, smiled, and said "Shh". He left the company shortly thereafter. I never did get a copy of any of his books.

 

[rustbuster]

Igana,

Pulsed eddy current or "incotest" can be used on the sphere legs provided that carbon steel mesh was not used for the fireproofing.

 

[Duwe6]

IMHO, the best inspection technique for the leg-to-sphere plate joint is to remove the fireproofing for about 8-inches at the weld, and MT - magnetic particle test it. On a sphere, this weld it the most highly stressed, and on the inner side there is a rather 'tight' acute angle. That angle should be the main target of your inspection. additionally, as the stresses are external, all cracking shoule emanate from the surface and be easily identified using external UT.

Since AE is planned already, if the AE does not identify any areas of concern, I would MT the legs as above. Then I would take thickness measurements of every shell plate from the outside, using a boom lift. If no anomolies are noted, the Internal Inspx can be 'written off' based on RBI - Risk Based Inspection. Typical for spheres inspected under API-510.

 

[lganga]

unclesyd
rustbuster
Duwe6
You all gave me valuable information.

For INCOTEST I dont have experience but surely we will make inspection of the critical area of the welds between legs and sphere.

Fire proofing reach close to that area but does not cover it and of course it has a mesh and as rustbuster said,probably the test will give us some uncertanties.
We have some inspection windows in the Fire proofing where we check thickness and external aspect for the leg.

As DUWE 6 said we will talk with an authorized API inspector to discuss about the internal inspection.
We do inspection every five years and we have already done AE to the three ones we have.
This new inspection is about the 30 years of the date of construction and legally we have to do a Fitness for Service for another 10 years.
I am reading API 510, all the test of thickness for the three are OK, we can calculate the rate of corrosion and the remaining life.
This three spheres were fabricated by CB and one of it used at the startup for off spec, we changed the bottom due to corrosion under deposit (that was 20 years ago).

So to talk to avoid the internal inspection considering that we will complete the inspection with AE and UT for thickness, that will save a lot of water (we dont have many now), is not only money it also saving some resources. I will let you know our last decision.

Unclesyd ,where is Perry now?did you use as arid in the FP the sea clamps?, or that was premolded concrete clamped to the legs?. I am still afraid of using the rubber boat
Thanks to all of you .

 

[unclesyd]

When Perry left the company I've seen him one time in 12 years and that was about 6 months after he left. When I saw him I asked about buying a copy of one of his books and got the brush off. Any knowledgeable cement person should be able to give directions.
These were premolded clamshells bedded in and strapped on with SS banding.

The post above about the highly stressed area where the legs are attached and the location of any cracking has to be tempered a little. Two of our spheres were built for storage of Anhydrous Ammonia and all the cracking was from the inside and there was no increased cracking around the around the supports even though all the other cracking was related to stress.

 

[texasmike45]

We use rope access to assist with the internal inspection of our VCM spheres.

 

[lganga]

Texasmike45
That means that you get to the wall using ropes?.
Just like if you were walking along equators lines and close to the wall?.
Can you give me more information?
Was this a safe way to do it, becausse the person is hanged by several ropes?
Did you put a net rope as safety element?.

 

[texasmike45]

Iganga,

Yes, the rope access people can get to the walls using their rigging. Best thing to do is have a reptuable rope access company with an excellent safety record review your drawings and develop their rigging/rescue plans. Our spheres are 80' diameter and have a 6" stilling well in the center which helps with rigging.

Thanks Mike

 

[MRNDTguy]

lganga,

Disclaimer: I work for a large provider of rope access services

To echo texasmike45's post, we use rope access personnel often to perform both internal and external inspections on spheres, tanks, or anywhere where the cost of accessing the inspection point is disproportunately higher than the cost of the inspection.

As Mike mentioned, make sure to use a reputable company. There are two trade associations that serve the rope access industry (IRATA and SPRAT). IRATA is a global association (including North America) and SPRAT is primarily North America only. Major differences between the two are that IRATA requires an independant third party audit of the providers safety / quality / training programs, an independant assessment of all trainees by a third party who signs-off on their practical qualifications before they achieve each certification level, and IRATA requires ~2x the experience before you qualify for a higher level certification.