DEARATOR EXPLOSION 12

[hyl]

HELLO
IN NAJYBIA POWER STATION UNIT NO/4 CAPACITY 100MW .THE UNIT WAS IN OPERATION AND SUDDENLY SHUTTDOWN BECAUSE OF NET FAILURE AND ALSO THEY LOST THE EMERGENCY POWER .
THE DEARATOR WHICH IS OPERATED AT PRESSURE OF 5KG/CM^2 THE THICKNESS OF THE DEARATOR AND STORGE TANK IS 12.5 MM THE DESIGN PRESSURE IS 8.5KG/CM^2 .
THE PRESSURE SUDDENLY INCREASED TO 17 KG/CM^2 IN THE DEARATOR .THE DEARATOR SUDDENLY EXPLODED IN CASTROPHY WAY THE MATERIAL EJECTED IN AIR AND DESTROYED ALL THE AUXIALERY PIPING AND NEAR EQUIPMENT ASLO THE STORGE TANK IS EXPLODED .IT IS UPNORMAL CASE .
MY QUESTION .IS THAT CASE IS NORMAL AND HOW TO ANLYSIES THE CASE WILE THE WEAK AND STRONGE POINT HAS THE SAME DEFFECT..AND CAN THE EXPERT GIVE US SOME ADVICE PLEASE ..THANK AND GOD BLESS .
JAMIL/BOILER ENGINEER

 

[gr2vessels]

hyl,
I believe that is normal an overpressurized pressure vessel to fail. Perhaps (or most likely) the failure was due to a number of causes, like failure of the overpressure protection system (if any present), failure of safety relief valves, etc...What was the human error component, it's only you to determine.
You obviously are lacking the experience for a proper investigation, hence I suggest hiring an unbiased expert to conduct the investigation (I assume that you want to get to the bottom of it- most of the people involved would try first to cover their backside). He will list what went wrong and suggest the improvements to the plant to prevent this happening again.
cheers,
gr2vessels

 

[jte]

hyl-

As gr2vessels said, find a compentent engineer to conduct the investigation. And please stop SHOUTING at us. The permanent use of the CapsLock key is poor manners.

jt

 

[davefitz]

Youwill need to complete an analysis of the failure mode of all valves and fluid streams feeding into the dearator.

You will need to start with a P+ID that is red-lined or marked up to represent all the field changes to the piping that were made after the unit was erected by the EPC vendor. The calculate the max "caualty flow" that can pass thru each control valve that feeds the DA. Then compare this total caualty flow rate with the flow capacity of bursting discs and relief valves supplied.

Likely source of energy that caused the damage is either an aux steam (pegging steam) supply or perhaps the LP feed from the LP feedwater system. The extent of damage you described suggests a steam overpressurization.

 

[MJCronin]

hyl,

Deaerators can and do fail. Fundamentally, you have a thin walled pressure vessel subject to internal transients and loads which may, or may not be designed to the pressure vessel code.

This particular failure may be due to simple overpressure and possible neglect

There was a spate of US failures in the late 1980s which caused a re-evaluation of the design and rules. There was a spectacular lethal failure that occured in a paper plant that caused the industry regulating agency (TAPPI) to issue warnings and restrictions. In 1999, The Heat Exchange Institute (HEI) issued new guidelines on this matter (Standards and Typical Specifications for Tray Type Deaerators- 7th Edition, latest edition is 2003)

The new recommended guidelines for a more "robust" DA include:

- ASME vessel construction with a minimum design pressure (50 psig...as I recall)

- PWHT of the vessel-head-to-shell joints

Lot of good information here:

http://www.heatexchange.org

Several inspection organizations developed special "deaerator inspection programs" and wrote papers about thier methodologies and findings.

One inspection organization with much powerplant DA related experience is:

http://www.thielsch.com

The DA fabricators also took an active role in this effort. These fine people have always been in the front of technology, IMHO....

http://www.kansascitydeaerator.com

I would appreciate the experience and knowledge of others on this important matter...

Regards

-MJC

 

[CodeRef]

System Design, Specifications, Operation, and Inspection of Deaerators

There are three kinds of people in this world; those who can Google and those who can't.

 

[toothless]

My experience is limited, but in that experience we have operated our D/A's at a much lower pressure, in the 5-15 psig range. This guy is operating at 71 psig with a vessel rated for 120 psig.

With the way our D/A pressure can rise and fall, it is not uncommon for that pressure to swing up to 20 psig. What I am saying is for a D/A, the ultimate strength (pressure) should be several times that of the normal operating pressure. Also, as others have mentioned, the relief valve should have been blowing, unless of course his failure was due to poor construction or faulty material.

 

[metengr]

We have a number of DA heaters and storage tanks that are inspected internally every 3-5 years, where 50% of the long and girth welds on the ID surface of the vessel are cleaned and prepared for wet fluorescent MT. As indications are found, the inspection coverage is increased accordingly. The vessels are subjected to a 100% exam between 6-10 years, depending on indications found and damage mechanism.

From my expierence, the two main damage mechanisms related to DA vessels are flow accelerated corrosion (FAC) damage (especially on units that have switched to oxygenated water treatment, and environmentally assisted cracking (corrosion fatigue or to others stress corrosion cracking) or EAC.

The EAC damage can be controlled by post weld heat treatment of weld repairs at or below the water line in these vessels, regardless of original code of construction requirments. For new vessels, PWHT would be mandated.

For flow accelerated corrosion (FAC) damage, this is more complicated and requires a change in material from carbon steel to low alloy steel containing chromium.

 

[insult2injury]

"Recommended Practice for Prevention, Detection, and Correction of Deaerator Cracking", NACE Standard RP0590.

I2I

 

[rustcity]

I believe the question is not why the deaerator ruptured, but why the pressure increase was not properly relieved.

Items that come to mind:
* Rupture disk sizing and rating?
* Mechanical pressure relief valve frozen in place or failing closed rather than open.
* If not PR valve per se, did the steam pressure control valve fail open on power failure as designed? If not, what prevented it from opening? Actuator failure or perhaps instrument air contamination?
* D/A water level too high? (Easier to overpressure quickly)
* Feedwater surge, perhaps accompanied by unusually high temperature, brought on by the power outage.
* Steam header overpressure in conjunction with a slow response and oversized steam pressure control valve. Unlikely, but with a power outage...
* Plugged/ blocked pressure or level sensor(s). (I have seen this happen a number of times on older installations-- with float controllers and non-diaphram pressure sensors-- when water softener resin ended up in the D/A.)

Just a note: I was involved with specifying what may have been the first post-weld heat-treated deaerator around 1984/85 at Sohio Chemical, Lima, Ohio, USA. SA-285 Grade C plate was requested over A516-70, (for ductility and increased metal thickness), but was no longer available. And the float level controller was replaced by a diaphram pressure/level sensor. Installation was not until 1987 if I remember correctly. I would be interested in knowing if any PWHT D/A’s were installed before this.

To the person who submitted the question: Give us some feedback as to what you found.

 

[MJCronin]

Jamil...

Is there any chance you could post pictures of the failed deaerator ?

What was your final decision about rebuild/replacement ?

Who were the contractors who provided the most expertise ?

-MJC

 

[Sharik]

Several years ago I was involved in an investigation of a deaerator rupture in a pulp & paper mill in a country which will remain nameless. What our investigation found was: there was a power upset and everything including the steam turbogenerators, tripped off; emergency and standby power were also lost and/or not available; the turbine stop valves closed properly and all boiler PSV’s worked properly; the boiler and boiler feedwater pump non-return valves were passing, did I mention very poor maintenance at the plant?

The high pressure water from the boilers flowed back through the feedwater pumps and into the deaerator storage tank. The PSV on the deaerator was working but could not handle the sudden increase in hydraulic pressure. The deaerator storage tank ruptured at two tee intersections (circ to long seams), mostly in the long seams.

A review of the construction data and analysis of the weld seams revealed the deaerator to be constructed of SA-516 70, fabricated prior to PWHT requirements. The weld seam analysis revealed extensive HIC throughout.

It was a combination of sudden high pressure increase and the HIC which resulted in the failure.

It would be interesting to find out if the deaerator in this thread was constructed of SA-516 70 and if it was PWHT’d.

There are really only two sources of sudden pressure increase for the deaerator: steam headers and boiler feedwater. At most facilities there are enough PSV’s on the boilers, steam pressure letdown stations and on the deaerators to handle steam upsets. Boiler feedwater is a little different.

 

[insult2injury]

Typically, there is a fail-open valve in the overflow line which is generally large enough to prevent the buildup of pressure. Was this valve manual or not in operating condition?

I2I

 

[Sharik]

The overflow line was blocked in (previous problems!!). Check valves on the deaerator feed pumps and the condensate feed pumps did work and the steam letdown station control valve closed and did not pass.

 

[MJCronin]

To all...

It has been my experience that the design of the pressure relief device on a DA is set by the "fail open" position of the pegging steam control valve.

Obviously, the RV seting on that basis would not have protected against the scenario described above where multiple components either failed or were not available.

It is my understanding that may older DAs were constructed to the requirements of the ASME Section VIII code, but were not code stamped and do not carry an appropriate ASME certified relief device.(!!??!!??) Do others seem to find this true ?

Because deaerators are unique in power plant design and operation, I also believe that there is enough variation in design such that a detailed system review should be performed for each and every large DA in order to set the RV capacity and setpoint. (In other words, they cannot be treated the same as an ASME VIII tank/vessel)

Something akin to a PHA/HAZOP analysis should be performed for all large deaerators, calculations and records should be checked, certified and made available for regulatory review.

In my opinion, no DA should be designed for less than MAWP of 75 psig...

....just like our brothers in the Chemical Process Industry

-MJC

 

[insult2injury]

I've never specified a pressure vessel (excluding large atmospheric tanks) for less than 50 psig as a general rule. If the DA is fed from the crossover extraction, which is quite common, the design pressure may need to be well upwards of 200 psig. The higher design pressures also reduce the required relieving capacity.

That being said, Sharik's example sounds as if the relief system may have been correctly designed; however, poor O&M practices resulted in the catastrophic failure. It is not reasonable to design for the simultaneous failure of every valve in the system as well as one of the safety features. Most likely, only a minimum of in-service maintenance would have been required to mitigate the failure.

I2I

 

[hyl]

hello members

I want to introduce my thanks to all members whom gives advice in this case .it give us very good conclusions about the failure and put us on the right route .it contain very good analyses about the case .and also some reply give the right point on what happened .My special thanks to MR sharik and MR MJcronin for their advice.

the link here show the dearator failure .

http://www.uploading.com/files/SB1JKTYX/dearator_explostion.doc.html

jamil a alshahed nama
hartha power station

 

[sarmad1010]

Thanks Mr. Jamil for sharing this incident. However, I cannot open the link you have attached. would you please reload it on another site like megashare?

best regards

 

[hyl]

Hello Mr sarmad1010
thank you very much for your cooperation .my dear i attach to you the the picture of the dearater explostion thank you very much and god bless .

jamil a alshahed nama
hartha power station

 

[sarmad1010]

Thanks Mr. Jamil for the prompt reply. Pictures are self explanatory. Would you pls. provide some details about the root cause of the explosion?

regards,