Hydrotest and Pressure Rating of 110m Tower 1

This is a rather tall vessel to attempt a field hydrotest on. Don’t forget the added cost of a substantially larger foundation due to this company specification requirement. What is the difference in weight (steel plus liquid) for a vessel designed for a horizontal hydro and (just guessing here) 10m liquid level vs your current proposal? What is the difference in weight when it comes time to rig the vessel? How much are you paying for your cranes, and is this requirement forcing you to a larger, more expensive set of cranes?

Time to talk to your company’s vessel subject matter experts about taking an exception to the spec which is requiring the field hydrotest. I know I would grant it – after taking a close look at what the design considerations are on the vessel and determining that the vertical hydro is causing substantial increase in thickness of the shell, nozzles, etc. If this vessel is for a joint venture project, the possibility exists that it will cross my desk.

Now, if this is a field fabricated tower and you didn’t tell us… things will be different.

jt

This has to be field erected.

WHY DOES IT HAVE TO BE FIELD ERECTED?

the tower is bigger than the saturn V rocket

... which was shop fabricated and field erected.

Alternately, this vessel could be field fabricated in the horizontal position, hydrotested, then erected.

'Tis a tall one,though... It will definitely require a good design to check the rigging condition. I worked on one of a similar size which was field fabricated in the vertical position and then hydrotested. Long story short: We had surveyors on site during the hydro to verify that the foundation was not tilting. Lots of nervous folks that morning.

Guess we'll have to wait for more details from sshep...

jt

Thanks for interest. This tower is actually installed, I wrote the post as a "what if" I had any say in the design pressure because of problems with the relief system as a result decisions (including design P) made earlier. This tower has been in operation for a little over a year, and is one of several what I call "ego towers" which we have on the site- each project including a bigger and taller tower so as to set a new record. I expect an even bigger tower next project, and wish for some flexibility in economically setting the tower design pressure.

This tower was shipped to the site in sections, then field assembled, lifted into place by one of the worlds largest cranes during plant construction. It was hydrotested in the vertical position (there may have been a previous hydrotest also). We have hydrotested several of these large towers in place, and it is a major job, but no problems so far. Myself I always wonder "what if the hydrotest failed", do people really consider the stored energy and volume involved at the bottom of the tower. The mechanical engineers always just tell me they are 100% confident it won't fail.

Get this: the tower is not rated for full vacuum, which as a process engineer seems somewhat unbelievable considering the shell thickness at the base. I cannot believe it wouldn't have been economical to give this thing a vacuum rating considering the cost of vacuum breaking and SIL protections, but I am not a mechanical engineer. I worry about this stuff when we (accidently or on purpose) drain the hydrotest water, etc.

Anyway, is a partial pnuematic test possible and allowable? I envision to fill 80% with liquid, and then pressurize the top 20%. I know a pnuematic test is more dangerous, but the company standards allow it with various approvals and access controls.

best wishes,
sshep

Unbelievable as it might seem, it is the process engineers and / or client who say what the design conditions are - including if full vacuum is required.
Mechanical engineers are the mere mortals - we may advise to design for full vacuum - but in the end it is process engineering who define the requirement.
HAZOP springs to mind.....

Thanks Roca,

Of course the process engineer makes the specifications, but because of a process known as "value engineering" the specifications are not simply tossed over the wall to the mechanical engineer with a note that says, "keep your mouth shut and build it like I say regardless of cost". You can readily see that this tower is big, and there may be cost saving opportunities.

The purpose of this post was to find out if there were legitimate suggestions that can be made to the mechanical engineer to make the tower suitable for 6bar, without adding extra metal thickness simply to meet an extraordinarily high base pressure hydrotest requirement.

best wishes,
sshep

Value engineering versus safe design......hmmm intersting place you work at

Hello roca,

FYI, I never said anything was unsafe. It was a design decision to go for a SIL protection rather than higher design pressure. This pressure rating question came from instrumentation related reliability issues that would not exist otherwise: i.e. higher design pressure => higher process boiling point => reduced heat input => smaller relief system and minimal critical instruments.

This is my counter jab (just for fun):
It must be an even more interesting place where YOU work- i.e. that cost is never a consideration in design. Your engineers must have the easiest job on the planet.

best wishes,
sshep

Thanks Sshep
My job is no easier than anyone elses. We value safety as the number 1 priority AND we always HAZOP systems.

This would alay your worries "I worry about this stuff when we (accidently or on purpose) drain the hydrotest water, etc.".
If a vacuum could be pulled under any circumstances it should have been defined in the initial design of the tower.

All the best, and now I'm off back to sleep until its time to leave for home.......

:-0))

This document says that "the tallest distillation columns do not exceed 110 m" ...You have ~115m

That was a good article. Thanks, sshep

If you Google tallest "distilation tower" you will get results that indicate that the tallest is 107m, owned by SASOL Ltd in South Africa.

It was engineered and built for SASOL by BASF.

Yours is considerably taller than this world record holder.

Good luck.....

-MJC

sshep,

Interesting problem you posed with your 115m column. The vortex shedding problem must be a b&%$* cause you must paying for it in shell thickness. I would love to see a photo of the vessel to see if it uses spoilers and what kind.

If you want an answer, you need to bring this question to your vessel engineer to respond to. The reason I say this is because no one here can answer your question without reviewing the calculations to determine the MAWP of the vessel and compare with your new design pressure to see if there is any margin to make it happen. Also, there is the registration issue to content with. All vessels have to be registered (I am assuming your vessel is in a country that has a law regarding registering pressure vessels), and if you change the design pressure it requires new calculations to validate thicknesses, and will require a new hydrotest. If I know vessel engineers and fabricators, then I would say "No" to your request because we always try to max out the MAWP with the minimal thickness. You said that there's a lot extra thicknesses. There's a reason for all that extra thicknesses on your tall tower: Clue is in my first paragraph.

As to your contention about partial hydro/pneumatic testing. I do know what you are asking. It might as well be a full pneumatic test. Why bother with both? It is just as dangerious as a full pneumatic test. A hydrotest is 100 times safer than your pneumatic test; a crack during a hydrotest will just leak whereas a crack in a pneumatic test will propagate and take out a good portion of your plant along with the people. I certainly do not want to be around when you do it on this vessel.

For tall columns, it is sometimes requested by the Process engineer to design it for half vac to save some money. Not unusual at all. Just that the people draining the vessel got to know to open the vents/nozzles.

I know a little about "Value Engineering" as I have to participate in these silly activity. It is just another way of saying "project cost cutting" or "trimming the fat" to try to save the blown budget. This is fine as long as you got all the engineering disciplines in the meeting to prevent wild ideas taking root.

So where is this new record hoding tower and who designed it ?

Gentlemen,

Before I give you any details, I must say that our mechanical engineers obviously know what they are doing and have alot of real life experience with these size columns. On the process side there are also alot of innovations (internal condensers, divided walls, heat interation, etc) in these tall towers. The only purpose of the question was to get input on if there was anyway to optimize the process and mechanical aspects of the design pressure rating.

The tallest towers are owned by Sasol LTD, and located in Secunda, South Africa. The google listed record was probably the installed height of a 103.5m T-T tower which was started up in 2004. This new tower is a 114.8m T-T sitting on a 6.8m tall skirt. It was started up in Jun 2008.

With respect to vortex shedding, this lesson was well learned from a 83.2m T-T column on the site which was set in 1999, and had some problems when the right velocity wind came from a certain directions and other conditions. These more recent towers have a spiral arrangement of vortex shedding fins, but as a process engineer I don't really now any technical details of this type stuff.

If you ever watched the movie "Catch a Fire", you will see the old record and similarly tall towers in the background of some shots. Even though it is tough to put these into scale, it is obvious from the distance that they are pretty tall. I think there is some ego (to set a new record) in these designs, but otherwise I think it an pretty good indication that this company knows how to build a tall tower.

best wishes always,
sshep

I guess in India, where else?
All their tall columns are coming off India, is it sshep?
Cheers,
gr2vessels

gr2vessels,

Yes, I think so too: Reliance Industries at the coast???

Sshep,
I hope you have not misinterpreted my respone to your query. I was not even remotely implying your Mech engineers are not capbable. All I'm saying is your question is best reponded to by the vessel engineer in your organization who has access to the original calcs.

Tall towers are only ego towers to the process engineers who dreamed this up. Us mechanical guys are the poor sap who has make it a reality and pull our hairs out to solve the vortex shedding problems. I'm surprised to see your comment about lessons learnt from a tall tower in 1999. This lesson was learned a long long time ago in this industry.