Thursday, July 30, 2009
Repair of LOX & LN2 tanks affected by SCC
Field erected storage tanks for storing liquid oxygen and liquid nitrogen are high tech storage devices. A typical tank is a double walled tank, the inner tank is made of stainless steel and the outer tank wall is made of carbon steel.The inter space between the inner tank and the outer tank is evacuated and filled with low thermal conductivity perlite powder.The inner tank and the outer tank is held in place by stainless steel wedges. One end of the Stainless steel wedge is welded to the out side of the inner stainless steel tank and the other end is welded to the inside of the carbon steel tank.The tanks were erected close to the sea shore. The atmosphere is humid and salt laden. When the tanks were ready we asked the steel plant authorities to provide raw water for hydraulic testing. Steel plant authorities could not provide the water required and asked us to postpone hydraulic testing. After one year steel plant authorities asked us to carry out hydraulic testing. When we went inside the stainless steel tank we were surprised to find a number of cracks on the surface. Examination of the cracks closely led us to conclude that the cracks are stress corrosion cracks. We will explain how we came to this conclusion later.
Wednesday, July 22, 2009
"The Crack is in Your Head"
This is how a Managing Director Of BHPV reacted when I told him that the liquid oxygen and liquid nitrogen storage tanks erected at a shore based steel plant cannot be hydraulic tested as the tanks have developed STRESS CORROSION CRACKS.
Perhaps he must have heard the word stress corrosion cracking for the first time in his life. Granting that many would not know about stress corrosion cracking he did not want know what is stress corrosion cracking. In order to camouflage his ignorance on many things he always resorted to insult the person who brought any problem to him. I would like to say few things about his style of functioning.
He was an ordinary engineering graduate. I doubt whether he passed his B.E examination in first class.He had powerful political connections which he used effectively to climb up the ladder in Public Sector Undertakings. At a relatively young age he became the CMD of a company which was famous for manufacturing Three Jaw Chucks! The turnover of that company was around Rs.30 crores.He had no experience in heavy fabrication. Before coming to BHPV,I am sure, he did not know what a pressure vessel or a heat exchanger is let alone knowing how they are designed and manufactured. When the post of MD became vacant in BHPV using his connections he jockeyed to get that post. At that time BHPV was a company with a turn over of over Rs.350 crores. BHPV had good a design and manufacturing team staffed by post graduate engineers. The top was full of people who had put in more than 20 years experience in heavy fabrication. He came to such a place ill equipped with necessary skills to run BHPV. What he lacked in managerial skills he made up by insulting people without any regard for age, experience and qualification.He terrorized all including workers. Unfortunately we had to work with him for nearly five years. When he left BHPV he left a team which was demoralized and demotivated. Hence when he told me that" Crack is in your head" I did not get offended because I knew that I was dealing with a mental case. He demanded that I should repair the tanks in two weeks. Then I told him that he can find a person who could repair the tanks in that time as I was not capable of meeting his dead line and I walked out of the meeting. He threatened me that he will post me out of Vizag and harass me. I stood my ground and finally he agreed to my terms and conditions.Probably he learnt from his coterie that no one expect me can solve this problem. I will explain what is stress corrosion cracking and how we repaired the tank later.
Perhaps he must have heard the word stress corrosion cracking for the first time in his life. Granting that many would not know about stress corrosion cracking he did not want know what is stress corrosion cracking. In order to camouflage his ignorance on many things he always resorted to insult the person who brought any problem to him. I would like to say few things about his style of functioning.
He was an ordinary engineering graduate. I doubt whether he passed his B.E examination in first class.He had powerful political connections which he used effectively to climb up the ladder in Public Sector Undertakings. At a relatively young age he became the CMD of a company which was famous for manufacturing Three Jaw Chucks! The turnover of that company was around Rs.30 crores.He had no experience in heavy fabrication. Before coming to BHPV,I am sure, he did not know what a pressure vessel or a heat exchanger is let alone knowing how they are designed and manufactured. When the post of MD became vacant in BHPV using his connections he jockeyed to get that post. At that time BHPV was a company with a turn over of over Rs.350 crores. BHPV had good a design and manufacturing team staffed by post graduate engineers. The top was full of people who had put in more than 20 years experience in heavy fabrication. He came to such a place ill equipped with necessary skills to run BHPV. What he lacked in managerial skills he made up by insulting people without any regard for age, experience and qualification.He terrorized all including workers. Unfortunately we had to work with him for nearly five years. When he left BHPV he left a team which was demoralized and demotivated. Hence when he told me that" Crack is in your head" I did not get offended because I knew that I was dealing with a mental case. He demanded that I should repair the tanks in two weeks. Then I told him that he can find a person who could repair the tanks in that time as I was not capable of meeting his dead line and I walked out of the meeting. He threatened me that he will post me out of Vizag and harass me. I stood my ground and finally he agreed to my terms and conditions.Probably he learnt from his coterie that no one expect me can solve this problem. I will explain what is stress corrosion cracking and how we repaired the tank later.
Tuesday, July 21, 2009
Details of heat exchangers brazed
17 numbers of heat exchangers were to be brazed, some of aluminium and some of stainless steel.
The minimum core weight of the aluminium heat exchanger(Liquid-Air HE) was o.28 Kg and the finished weight of the same heat exchanger was 0.85 Kg.
The maximum core weight of the aluminium heat exchanger(Secondary HE) was 8.78 Kg the finished weight was 12.0 Kg.
The aluminium fins are as thin as 0,0762 mm.
There were a number of stainless steel heat exchangers.
The minimum core weight of the heat exchanger(Precooler) was 8.8 Kg and the finished weight was 12.0 Kg.
The maximum core weight(Primary HE} was 15.6 Kg and the maximum finished weight was 23.0 Kg.
The fin thickness was 0.0762 mm.
If you look at the thickness of the fins the difficulty in brazing can be gauged. CLOSE CONTROL OF THE BRAZING TEMPERATURE WAS ESSENTIAL FOR THE SUCCESS OF THE BRAZING OF THESE HEAT EXCHANGERS.
All these heat exchangers are performing successfully in the Supersonic air craft developed by Aeronautical Development Agency ,LCA TEJAS, and have clocked hundreds of flight hours. India is one of the few countries which have such a sophisticated technology.
My association with this project was in the initial brazing trials as I left BHPV in 1995. Credit should go to R&D team led by Mr. Panigrahi DGM(R&D) who completed the project. Recently ADA has placed an order worth Rs.20 crores for a few sets of these heat exchangers.
The minimum core weight of the aluminium heat exchanger(Liquid-Air HE) was o.28 Kg and the finished weight of the same heat exchanger was 0.85 Kg.
The maximum core weight of the aluminium heat exchanger(Secondary HE) was 8.78 Kg the finished weight was 12.0 Kg.
The aluminium fins are as thin as 0,0762 mm.
There were a number of stainless steel heat exchangers.
The minimum core weight of the heat exchanger(Precooler) was 8.8 Kg and the finished weight was 12.0 Kg.
The maximum core weight(Primary HE} was 15.6 Kg and the maximum finished weight was 23.0 Kg.
The fin thickness was 0.0762 mm.
If you look at the thickness of the fins the difficulty in brazing can be gauged. CLOSE CONTROL OF THE BRAZING TEMPERATURE WAS ESSENTIAL FOR THE SUCCESS OF THE BRAZING OF THESE HEAT EXCHANGERS.
All these heat exchangers are performing successfully in the Supersonic air craft developed by Aeronautical Development Agency ,LCA TEJAS, and have clocked hundreds of flight hours. India is one of the few countries which have such a sophisticated technology.
My association with this project was in the initial brazing trials as I left BHPV in 1995. Credit should go to R&D team led by Mr. Panigrahi DGM(R&D) who completed the project. Recently ADA has placed an order worth Rs.20 crores for a few sets of these heat exchangers.
Friday, July 10, 2009
Building vacuum furnace
Imported vacuum furnaces are very expensive. Hence we decided to build a vacuum furnace in-house.
A vacuum furnace can be divided in to three subsystems:
1. A double walled chamber to create vacuum and maintain vacuum.
2. Heating system consisting of electrical heating elements and shields to reflect the heat inwards.
3. Power supply and control gear to maintain required temperature in the furnace.
We looked at our strengths and weaknesses to build the vacuum furnace.
We had people who are good at building vacuum tight vessels.
We had people who can design the vacuum system as well as size up the required
vacuum pumps
One of our engineers came forward to design the heating elements.
What we lacked was people who can design the control system.
We decided to off-load control system to a professional company.
Taking in to account the brazing requirement we firmed up the size of the vacuum furnace as follows;
External size : 1200mmID x 800mm long.
Hot zone: 450mm x 450mm x 450mm
Vacuum: 5 x 10-4 mbar
Maximum power required: 60kw.
We arrived at this figure to take in to account the rapid rate of of heating needed for aluminium brazing.
Largest size that can be loaded: 250mm x 250mm x 250mm.
The furnace was double walled furnace with the inner shell of stainless steel and the outer shell of carbon steel.Water circulated in the annular space. Several feed throughs were provided for electrical connections,thermocouples and vacuum gauge.
Vacuum chamber with all the feed throughs were subjected helium leak test.
Vacuum system consisted of a fore vacuum pump, roots blower and a diffusion pump.
The control system was designed to take care of starting the pumps, raising the temperature in the furnace at a controlled rate,holding the temperature constant during brazing and shutting down the heating at a controlled rate. In addition temperature was constantly monitored from the thermocouples fixed in the vacuum chamber. We calibrated the furnace and started our brazing activities.
We built one more high temperature vacuum brazing furnace later to vacuum braze stainless steel These furnaces are in no way inferior to imported furnaces.
We will give details of heat exchangers brazed in the next blog.
A vacuum furnace can be divided in to three subsystems:
1. A double walled chamber to create vacuum and maintain vacuum.
2. Heating system consisting of electrical heating elements and shields to reflect the heat inwards.
3. Power supply and control gear to maintain required temperature in the furnace.
We looked at our strengths and weaknesses to build the vacuum furnace.
We had people who are good at building vacuum tight vessels.
We had people who can design the vacuum system as well as size up the required
vacuum pumps
One of our engineers came forward to design the heating elements.
What we lacked was people who can design the control system.
We decided to off-load control system to a professional company.
Taking in to account the brazing requirement we firmed up the size of the vacuum furnace as follows;
External size : 1200mmID x 800mm long.
Hot zone: 450mm x 450mm x 450mm
Vacuum: 5 x 10-4 mbar
Maximum power required: 60kw.
We arrived at this figure to take in to account the rapid rate of of heating needed for aluminium brazing.
Largest size that can be loaded: 250mm x 250mm x 250mm.
The furnace was double walled furnace with the inner shell of stainless steel and the outer shell of carbon steel.Water circulated in the annular space. Several feed throughs were provided for electrical connections,thermocouples and vacuum gauge.
Vacuum chamber with all the feed throughs were subjected helium leak test.
Vacuum system consisted of a fore vacuum pump, roots blower and a diffusion pump.
The control system was designed to take care of starting the pumps, raising the temperature in the furnace at a controlled rate,holding the temperature constant during brazing and shutting down the heating at a controlled rate. In addition temperature was constantly monitored from the thermocouples fixed in the vacuum chamber. We calibrated the furnace and started our brazing activities.
We built one more high temperature vacuum brazing furnace later to vacuum braze stainless steel These furnaces are in no way inferior to imported furnaces.
We will give details of heat exchangers brazed in the next blog.
Wednesday, July 8, 2009
vacuum brazed aluminium heat exhangers for aero space applications
Aeronautical Development Agency gave R&D of BHPV a development contract for the design of compact heat exchangers for the supersonic air craft which they were designing.The heat exchangers were to be fitted in an envelope volume specified by ADA. The smallest aluminium heat exchanger had dimensions of 130mm x 30mm x 55mm and the largest heat exchanger had dimensions of 250mm x 140mm x 370mm.
The smallest stainless steel heat exchanger had dimensions of 175mm x 118mm x 200mm and the largest heat exchanger had dimensions of 250mm x 140m x 282 mm.
The challenge was to optimise the design with in the space provided so that the heat exchangers meet the heat dissipating requirements. Our design engineers developed computer programmes to optimise the heat exchangers. It took us over an year to fit all the heat exchangers in the given envelope. Our work was finished.
Brazing of the heat exchangers was the responsibility of another metallurgical laboratory in the defence sector. ADA financed procuring a vacuum furnace for aluminum brazing. Even after several trials over a period of one year they were no where near achieving the brazing. The scientists in the laboratory were so secretive that they did not want us to see the brazing furnace. After much pursuation we were allowed to visit the laboratory where brazing was being carried out. Our visit convinced us that the brazing furnace is defective which was the cause for their failure in brazing. Even after several attempts the laboratory was not successful in brazing. At that time we made a proposal to ADA. We proposed that the brazing also should be a part of our design contract. ADA's scope was to import the components for brazing and supply to us.Once we acheved brazing the technology was the property of BHPV. ADA agreed to our proposal. Imported vacuum brazing furnaces are very expensive and hence we decided to manufacture a brazing furnace designed by us. The next blog describes how we went about the design of the vacuum brazing furnace.
The smallest stainless steel heat exchanger had dimensions of 175mm x 118mm x 200mm and the largest heat exchanger had dimensions of 250mm x 140m x 282 mm.
The challenge was to optimise the design with in the space provided so that the heat exchangers meet the heat dissipating requirements. Our design engineers developed computer programmes to optimise the heat exchangers. It took us over an year to fit all the heat exchangers in the given envelope. Our work was finished.
Brazing of the heat exchangers was the responsibility of another metallurgical laboratory in the defence sector. ADA financed procuring a vacuum furnace for aluminum brazing. Even after several trials over a period of one year they were no where near achieving the brazing. The scientists in the laboratory were so secretive that they did not want us to see the brazing furnace. After much pursuation we were allowed to visit the laboratory where brazing was being carried out. Our visit convinced us that the brazing furnace is defective which was the cause for their failure in brazing. Even after several attempts the laboratory was not successful in brazing. At that time we made a proposal to ADA. We proposed that the brazing also should be a part of our design contract. ADA's scope was to import the components for brazing and supply to us.Once we acheved brazing the technology was the property of BHPV. ADA agreed to our proposal. Imported vacuum brazing furnaces are very expensive and hence we decided to manufacture a brazing furnace designed by us. The next blog describes how we went about the design of the vacuum brazing furnace.
Thursday, July 2, 2009
Machinery and tools designed for the manufacture of cryo containers
One of the most difficult operations in the manufacture of cryo containers was the winding of super insulated blanket on the inner vessel.
One roll of aluminum foil and one roll of paper was mounted on a frame 180 degrees apart. This frame had provision for adjusting the tension to release paper and foil at required rate. The frame was connected to a gear box through a drive to rotate at a slow speed. Separately from this frame a shaft was mounted on another drive rotating at a slow speed. This shaft can be tilted at various angles up or down. The inner vessel was mounted on this shaft.
By varying the inclination and the speed of the shaft one can vary the pitch at which foil and paper can be wound on the inner vessel of the cryo container. By varying the tension we can wind the paper and foil tightly or otherwise. This machine was a marvelous piece of development.
We needed one more tool.This tool should allow the assembled cryo container to be evacuated and seal the evacuated vessel. He designed a gadget which met these conditions.
The cryo containers required heating to accelerate degassing and remove moisture in the space between inner vessel and the outer vessel. A system was developed to heat and evacuate a number of cryo containers simultaneously. Heating temperature was controlled by a number of thermocouples. During evacuation vacuum was monitored continuously.
With all these systems in place we manufactured and sold hundreds of cryo containers to various customers.
This development gave us confidence to enter a new field of transporting liquid nitrogen and liquid oxygen over long distances without much loss.
One roll of aluminum foil and one roll of paper was mounted on a frame 180 degrees apart. This frame had provision for adjusting the tension to release paper and foil at required rate. The frame was connected to a gear box through a drive to rotate at a slow speed. Separately from this frame a shaft was mounted on another drive rotating at a slow speed. This shaft can be tilted at various angles up or down. The inner vessel was mounted on this shaft.
By varying the inclination and the speed of the shaft one can vary the pitch at which foil and paper can be wound on the inner vessel of the cryo container. By varying the tension we can wind the paper and foil tightly or otherwise. This machine was a marvelous piece of development.
We needed one more tool.This tool should allow the assembled cryo container to be evacuated and seal the evacuated vessel. He designed a gadget which met these conditions.
The cryo containers required heating to accelerate degassing and remove moisture in the space between inner vessel and the outer vessel. A system was developed to heat and evacuate a number of cryo containers simultaneously. Heating temperature was controlled by a number of thermocouples. During evacuation vacuum was monitored continuously.
With all these systems in place we manufactured and sold hundreds of cryo containers to various customers.
This development gave us confidence to enter a new field of transporting liquid nitrogen and liquid oxygen over long distances without much loss.
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