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Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/10/2015 1:31 PM

An unfortunate incident happened at our triple effect caustic evaporator (32w.t% to 51w.t%). The third effect (highlighted red) where major steam (~12-13bar depending on flowrate) assisted evaporation take place, here for some reason Ni tubes get severely collapsed, while the plant was offline

Construction ..generally for falling film internals

P&ID (illustrated)

www.mediafire.com/view/6dumme618wr63o6/Evp2_P-1442_P_I_Diagram_process.jpg

Damage done

http://www.mediafire.com/view/38zlrmb8hm360hk/P_20151210_152624.jpg

http://www.mediafire.com/view/2r1izd5r3doylts/P_20151210_152653.jpg

http://www.mediafire.com/view/xxwqft25j17f5b1/P_20151210_152754.jpg

http://www.mediafire.com/view/93hlv4byr6x6t6h/P_20151210_152826.jpg

So it went like this

@day1 ..we drained the evaporation while cooling it slowly (~2hrs) to ~60celsius for mechanical maintenance of effect_3. During this time level transmitter was dismantled & taken out from span of effect_3 (a dp transmitter) this therefore produced two potholes in our Ni made span which was now open to atmosphere ..& so were Ni tubes (see internal construction fig.)

@day2,3 ..crane to bring down this mammoth (6.3m length ~6000mm Ni 81 tubes) effect_3 didn't arrive, it was idly sitting there ..ambient temperature here in winter did approach ~3celsius. Do recall span of effect_3 is open to atmosphere so vacuum (via condensation) isn't possible

@day4 ..crane arrived ..supports unbolted & effect_3 was on the floor but the mild steel shell appeared to be ~80celsius indicating minor steam passing into shell (diaphragm steam valve is passing). We (process engineers) didn't notice the tubes though. Had this being done we would have known whether the tubes are inwards bulged merely by viewing across them

@day5 ..in mechanical workshop our jaws dropped when we were asked to view Ni tubes of effect_3.

HERE STARTS OUR DISCUSSION

Mechanical department was to replace any leak Ni tubes so they have to wash every tube because it was laced with NaOH 50w.t% ..Is it possible that because of temperature gradient of washing water & skin temperature of Ni tubes (recall ~80celsius) Ni abnormally expanded & while its two extreme ends are welded indirectly to MS shell it may have twisted & collapsed. What other reasons you think of?

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#1

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/10/2015 2:38 PM

I am unable to view the pictures so I cannot be sure what type of collapse occurred.

Factors to consider:

1) Length of time tube shell has been in service?

2) Tube wall thickness?

3) Tube sheet properly vented to atmosphere at top of structure during drain cycle?

I have observed tube sheet collapse due to tube walls being thin from wear & oxidation then suffering damage from the assembly experiencing too much vacuum pressure caused by the draining of circulating fluid without properly venting the top of the structure.

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#2
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Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/10/2015 2:52 PM

At least one of the pix showed something really weird: encroaching tubesheet holes. (Along with other erratic hole spacings.)

It looked like an external pressure collapse, with many tubes collapsed along nearly their full length. The tube walls looked quite thin, but that's hard to judge by mere eyeballing.

Your (3) sounds quite possible.

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#4
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Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/10/2015 3:04 PM

By any chance did you work at the nuclear generating plant back in the 80-90's? If so do you remember Francis (Skip) Gill?

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#5
In reply to #4

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/10/2015 3:25 PM

No, I was living in Seattle most of that time, doing primarily industrial refrigeration design.

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#7
In reply to #2

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/10/2015 3:55 PM

encroaching tubesheet holes

can you explain this term with some image?

There is an interlock at DCS. If the pump feeding NaOH to effect_3 gets stopped/tripped steam auto-valve closes to zero! But as I said it was passing but to an extent that only it created ~70-80 celsius shell temperature!

However, none of the replies so far has hinted whether thermal linear expansion of indifferent metals may or may not be the cause

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#8
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Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/10/2015 4:13 PM

There are supposed to be spaces (ligaments) of solid tubesheet material between adjacent holes. In TEMA standards, I think the ligaments thickness is >= 0.25 x tube diameter, with possibly some +/- tolerance. In the picture, some tube holes are touching (and even overlapping) each other.

Pump stoppage could lead to instant loss of tube-side pressure; but even if the steam valve closes instantly, the shell-side pressure won't drop instantly, and 12-13 bard external pressure is quite a lot for thin-wall tube.

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#6
In reply to #1

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/10/2015 3:48 PM

Top side of effect_3 (steam entry point)

Bottom side (attached with bottom span)

Tube length (I mentioned earlier) 6000mm

Thickness 1.4mm

No.of tubes 81

3) Tube sheet properly vented to atmosphere at top of structure during drain cycle?

Like I said if span was open to atmosphere (removal of dp transmitter) so are the tubes ..because they attached to one another (see internal construction)

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#9
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Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/10/2015 8:45 PM

Wasn't the DP isolated from the system before removing it by using the valve service isolation valves? If so, then the DP would not have vented the tubesheet to atmosphere or it was probably removed after the damage already occurred.

I would think the venting should have taken place as soon as the pump tripped in order to avoid creation of a vacuum from the fluid draining down to the pump level.

I think there should be an automatic mechanical vacuum breaker valve mounted at the highest point to allow purging air from the system before/during start up and to prevent any vacuum from developing during any unexpected sudden loss of pressure condition.

In addition to the vacuum breaker there should be a steam side safety valve that prevents the applied steam pressure from exceeding the rating of the tubesheet walls and the vessel.

If these devices are in place then one or both of them did not operate correctly. (Either incorrectly calibrated, broken, or intentionally rendered inoperable.)

If there is not any of these in place the system design is flawed and is unsafe.

Of course it could be that the tubesheet is not original equipment and instead has been replaced at some time with an inferior, thinner-walled aftermarket product.

Although a remote possibility I suppose the vessel and operating system could be poorly or incompletely designed by the OEM.

I would carefully review all original (OEM) specifications and all associated drawings of the system to verify every system component meets the original design specifications before I would attempt to do any repair or replacement of anything.

Here are a few items to take into consideration:

1) Original quantity of individual tubes?

2) Original tube wall thickness and inside diameter?

3) Original tube composition and metallurgy?

4) Original system operating steam pressure and temperature?

5) Original operating set-points for fluid pump pressure & temperature?

6) Original safety and protection equipment types, ratings, calibrations, and locations?

7) Is the system being operated within OEM design specifications?

8) Has/is the system been properly maintained?

I surmise you will discover the collapse was caused by the failure or incorrect operation or missing elements or a combination of some or all of these conditions.

Hope this helps.

Good luck and stay safe.

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#10
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Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/11/2015 7:57 AM

Thanks for the comprehensive reply.

http://www.mediafire.com/view/6dumme618wr63o6/Evp2_P-1442_P_I_Diagram_process.jpg

If you look at the duct D203 from effect_3 towards effect_2 it has breath-in valve, relief valve as well as pressure indicator, however, you are right breath-in valve may have malfunctioned. I will look at pressure trends of PI at DCS which may indicate any appreciable -ive pressure!

Wasn't the DP isolated from the system before removing it by using the valve service isolation valves?

No there aren't any isolation valves because pd transmitter is of this type (flanged with span)

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#12
In reply to #10

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/11/2015 10:09 AM

You are most welcome. Good luck!

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#3

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/10/2015 3:02 PM

Looks to me like a vacuum was created, possibly due to thermal shock....

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#11

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/11/2015 8:46 AM

Dear Sir,

The failure of these Nickel tubes is due to corrosion effect of sodium chlorate impurity available in Caustic soda lye produced from Ion exchange membrane cell technology.The corrosion effect is aggravated due to high temperature of liquor also.To reduce the corrosion effect of the above liquor,it suggest to dose little quantity of sugar solution(or)Hydrogen Hydrate to reduce the corrosion effect of sodium chlorate impurity available in caustic.

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#13

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/13/2015 1:25 AM

Dear Nightcrawler,

Normally when plant is in operation the first effect and second effect heat exchangers will be under positive pressure and the third effect heat exchanger will under negative pressure(i.e. vacuum).There is standard operating procedures(SOP's) for the smooth operation of the plant,it seems SOP's not followed strictly and the first effect heat exchanger is subject to vacuum during the plant shut down and causing the damages to the tubes which are already in weak condition due to corrosion.also recommond to start sugar dosing in the caustic lye fed to plant.

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#14

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/14/2015 3:45 PM

<...HERE STARTS...DISCUSSION...> Discussion needs to take place ideally during the HazOp Study that should take place before the plant is built and before the plant is modified.

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#15

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/15/2015 11:17 AM

ANOTHER OBSERVATION

http://www.mediafire.com/view/yd426hz8id5jt9d/EV20300.pdf

Above is the complete specs of our Ni tubes ..points to consider Ni201(cold/annealed) as material with dimensions (all in mm)

Len 6000

Thickness 1.65

Dia 48.3

Now according to this

http://www.mediafire.com/view/wdmmb1dbtwom68b/nickel200And201DataSheet.pdf

&

http://www.mediafire.com/view/qy61d941org6b2a/NAS_Ni201_E.pdf

the corrosion allowance is b/w 0.1-0.5mil/year* from 1st reference

*1mm/yr=39.4mil/yr

the corrosion allowance is b/w 0.02-0.03mm/year from 2nd reference

I can fairly accept 0.02-0.03mm/year because our conditions are ~178C & 51wt% NaOH

The tubes were added in year-2010 initially 1.65mm thickness so now in 2015 theoretically it will be 1.65−0.03*5= ~1.5mm However what we found that the average thickness of all tubes is ~0.78mm Now because the literature doesn't tell me the amount chlorates/chlorites in tested sample NaOH so I can't quantify that effect..

However I need some help in quantifying something else..

From the dimensions I can calculate surface area of a single tube, assuming one side is under total vacuum & the other (external side) at 101.3kPa I can calculate amount of force suffered by each tube, then IS THERE ANY METALLURGICAL PARAMETER that I can compare with the calculated force that somewhat quantifies whether the subjected force is greater than that parameter. Anything mathematical?

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#16

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/15/2015 8:08 PM

The quick shock from the near immediate collapse of the steam volume caused the failure of marginally designed tubes.

Install a functioning vacuum breaker and thicker tubes in the repaired HX. Cool the unit slowly.

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#17

Re: Shell & Tube Falling Film Evaporator Nickel Tubes Collapsed

12/17/2015 12:18 PM

It sounds like you let the steam head inside these tubes collapse, without any provision for venting, perhaps even while some form of pressure was still present on the outer surface of the tubes.

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