ADL - Engineering

CRYOGENIC GRADE STAINLESS STEEL

CRYOGENIC GRADE STAINLESS STEEL VS COPPER COLUMN FOR OXYGEN PLANT

STAINLESS STEEL	COPPER
Material Cryogenic grade stainless steel of grade ASTM A312 TP 304L is used for manufacturing the Air-separation-column of the Oxygen nitrogen plant, liquid Oxygen Nitrogen tanks , vessels by all American and European manufacturers like Praxair, Air Liquide ,Linde ,Etc due to the following reasons as below - ALL LIQUID OXYGEN COLUMNS TANKS VESSELS MUST BE MADE IN STAINLESS STEEL	COPPER material was used in 1930 for manufacture of columns when stainless steel was not available. Air separation column internally is not made of cryogenic grade stainless steel but made of copper and ordinary brass &mild steel. the trays / columns are not made in stainless steel as technology for stainless steel trays is not available in Asia. copper trays / columns and all pipes are also copper will result in corrosion as copper + oxygen=copper oxide. NOT COPPER
Low Temperature –very suitable SS 304L grade stainless steel is suitable for low temperatures upto –minus 220 . C even can withstand high pressures easily. Example: All liquid oxygen tanks are designed easily upto 18 or 30 bar	Low Temperature Copper is suitable for low temperatures upto –minus -190. C at low pressures only Example: NO liquid oxygen tanks are ever made in copper
High strength Cryogenic grade Stainless steel has got high tensile strength of 500-6070 MPa .	Copper has low tensile strength of upto 220 Mpa only
Corrosion resistance Is corrosion proof and does not react with oxygen	Reacts easily with oxygen to give greenish color & Is not corrosion proof.
Stainless steel condenser ASME QUALIFY High quality seamless tubes argon welded to tube plates makes the condenser 100% leakproof. Design pressure 5 bar Test pressure 25 bar	COPPER AND BRASS CONDENSER DON’T QUALIFY ASME Copper tubes soldered to the brass plate is not leakproof. Test pressure 6 to 8 bar
Column –stainless steel 304L Made in stainless steel can be made in tolerance of +.2 mm diameter to fit trays Very accurately	COPPER column is made by hand tools or hammering by old process and cannot get close tolerance due to which the column will always have some ovalty.
High tensile strength Stainless steel has high tensile strength due to which it can withstand high temperature & pressure variations/shocks and will not crack or explode	Low tensile strength Due to low tensile strength it is prone to cracks and leaks , resulting in explosion .
Welding ability Stainless steel can easily be argon welded with 100% radiography/ultra chronic testing making it leakproof .	Non Welding ability –has pores Copper and brass cannot be argon welded and so welded by hand gas torch and gas brazing is not leakproof
Formability Fabrication of SS trays are very accurate because it is made by Die&press work with fine tolerance .	Old type design with made with ordinary copper trays
Maximum Design and test pressures The upper and lower column in the air separation column made in SS can withstand 5 times of working pressure - Upper column - 25 Bar withstanding pressure Lower column – 25-30 Bar withstanding Condenser 25-30 Bar pressure Advantages: Very high resistance to pressure Will not explode or burst. Four times capacity to stand high pressure. Rupture disk built in the condenser even if the safety fails.	Low Design and test pressures The upper and lower column in the air separation column made in coppercan withstand only 1.3 to 1.4 times of working pressure - Upper column - 1 Bar withstanding pressure Lower column -6 to 7 Bar withstanding pressure EXPLODE AT 10 BAR Caution : in case the operator by mistake opens the expansion valve more and high pressure air enters and safety is stuck ,which can easily happen the lower column as has happened in many cases . the column and condenser will EXPLODE!!
COOLING TIME OF AIR SEPARATION PLANTS Stainless steel has no affect on the cooling time because it has good capacity to retain the liquid Oxygen for much longer time Even after stoppage of plant upto 12 hours Liquid Oxygen is retained in the stainless steel condenser/ column and the plant will come into production very quickly in 30 Min. approx only . Stainless steel easily cools as when a oxygen plants starts cooling the start time is 6 to 7 hours .so the stainless steel easily cools down to minus temperatures Stainless steel does not easily become warm	Copper also cools quickly but warms also very quickly so is harmful during restart after stoppage. Example: Liquid oxygen evaporates and cannot stay for long in a copper condenser or column After stoppage for 12 hours the plant will take 5-6 hours to come back in production, Hence wasting unnecessary cooling hours and power.
Heat transfer in exchangers The SS column is very successful because there is no heat transfer required in the exchanger jackets column because all the heat exchange takes place in the tubes of the exchangers i.e made of copper alloy .	Copper jackets in exchangers with brass headers are always leaking
Repairs – not required Since there are no cracks or leaks in a stainless steel vessel and pipelines and is 100% leakproof. STAILESS STEEL COLUMN DOES NOT LEAK Stailess steel welding /repairs are easily carried out if at all needed after 20 to 30 years by a large variety of weldable stainless steel electrodes .	Regular repairs required at Site Repairs – frequently required in copper columns and pipelines So people think that copper is easily repairable. Why there should be a leak first of all.Copper column are leaking regularly.
High quality engineering standards All parts of the air separation column including trays,condensor, upper & lower column, vessels qualify and are manufactured as per ASME/CE standards and can pass every test .100% radiography and ultrasonic test. Example. All liquid tanks made in 304 l SS are under high vacuum Pass helium leak test – ultimate test	Low engineering standards as cannot qualify any radiography. All parts of the air separation column including trays,condensor, upper & lower column are made in copper & Brass hence cannot be passed by and European(CE) or American (ASME). Cannot pass more that 15 % radiography and will 100% fail helium leak test

Advantages of using stainless steel over copper

STAINLESS STEEL	COPPER
It is TIG Argon welded	Cannot be argon welded, and is made by gas brazing which is old method and leads to cracks easily
Ss welding can easilypass radiography	Copper will pass 0% radiography test due to porosity in the brazing
Stainless steel tubes used in condensor and are TIG Argon welded to the ss tube plate and are 100% leakprood	Copper tubes are soldered to the brass tube plate and leaks easily.
SS welded vessels,exghangers/equipements confirm to ASME/DIN/CE standards	Copper vessels will not pass any engineering standard
During power failure , SS remains cold as it does not become hot easily like LOX tank under insulation which can retain liquid for many weeks and months	Liquid Oxygen inside copper vessel/copper column/copper condenser/copper tube will become hot in few minutes and evaporate after power tripping. Will take many many hours to come back into cooling, hence wasting lot of power .
All American/German oxygen plants are made in Stainless steel and not copper	Copper plants were banned in the early 1970's and discontinued by all European, American countries
SS equipment, machinery and vessels including ss columns, condeonsors have very long life of 30 to 40 years as it is non corrosive and strong material .	Copper is a weak metal and reacts with the Oxygen to form copper oxide and forms green deposit .