20- metallurgical differences among Carbon steel, Alloy steel and Austenitic (Stain less) steel tubes in Boilers.

SL No.	Carbon steel Tubes	Ferritic alloy steel tubes	Austenitic (Stain less) steel
1	Tubes are seamless & metal temperature is up to 454 deg C	Tubes are seamless, Metal temperature for T1 & T2 materials is 538 deg C and T5, T9, T11, T12, T22 and T91 is 649 deg C	Tubes are seamless Metal temperature is 816 deg C for TP 304, TP310, TP 316, TP 321 and TP 347 tubes.   And 427 deg C fr TP 304 L & TP 316 L materials
2	Are both hot finished and cold drawn tubes	Are both hot finished and cold drawn tubes	Are both hot finished and cold drawn tubes
3	Steel is manufactured from Open hearth or electrical process or any other oxygen process	Steel is manufactured from Open hearth or electrical process or any other oxygen process	Steel is manufactured from Open hearth or electrical process or any other oxygen process
4	Carbon content is more and in the steel of Grade A, B & C varies from 0.06 to 0.35% maximum	Carbon content is medium and in the steel of Grade T91 to T1 varies from 0.05 to 0.2% maximum	Carbon content is less and in all grades of steel. And it varies from 0.04 to 0.1% maximum
5	Manganese content in the steel of Grade A, B & C varies from 0.27 to 1.06% maximum	Manganese content is medium and in the steel of Grade T91 to T1 varies from 0.3 to 0.8% maximum	Manganese content is more and in all grades of steel. And it varies up to 2.0% maximum
6	Silicon content in the steel of Grade A, B & C varies from 0. to 0.25 % maximum	Silicon content is more that is 0.1 to 1%	Silicon content is up to 0.75%
7	Sulphur and Phosphorous content limit to 0.035%	Sulphur and Phosphorous content limit to 0.025%	Sulphur and Phosphorous content are 0.03 to 0.04%
8	No chromium content	Chromium content varies from 0.5 to 10%	Chromium content is more & varies from 16 to 24% in different grades of austenitic steel.
9	No Molybdenum content in carbon steels	Molybdenum is present in the range of 0.4 to 1.2%	Molybdenum is present in only steel grades of TP 310 S, TP 316, TP 316 H&L. And the range is 0.75 to 3%
10	No Vanadium content in carbon steels	Is present in only T 91 materials	No Vanadium content in Austenitic steels
11	No Nickel content in carbon steels	No content in all grades of Ferritic alloy steels except 12 X 1 M steels	Nickel is present in all grades of steel varying from 8 to 22%
12	Hot finished tubes are not heat treated	Hot finished tubes also heat treated	Hot finished tubes also heat treated
13	Cold drawn tubes are heat treated. Shall be given a sub critical annealing, full anneal or normalising heat treatment.	Cold drawn tubes are heat treated as per procedure.	Cold drawn tubes are heat treated as per procedure.
14	Permissible variation in out side diameter of the tubes after manufacturing is 0.1 to 0.4 mm over size for the tubes having out side diameter ranging from 25.4 mm to 228.6 mm	Permissible variation for all grades of tubes in outside diameter of the tubes after manufacturing is 0.1 to 0.4 mm over size for the tubes having outside diameter ranging from 25.4 mm to 228.6 mm	Permissible variation for all grades of tubes in outside diameter of the tubes after manufacturing is 0.1 to 0.4 mm over size for the tubes having outside diameter ranging from 25.4 mm to 228.6 mm
15	Permissible variation in outside diameter of the tubes after manufacturing is 0.1 to 1.14 mm under size for the tubes having outside diameter ranging from 25.4 mm to 228.6 mm	Permissible variation for all grades of tubes in outside diameter of the tubes after manufacturing is 0.1 to 1.6 mm under size for the tubes having outside diameter ranging from 25.4 mm to 228.6 mm	Permissible variation for all grades of tubes in outside diameter of the tubes after manufacturing is 0.1 to 1.6 mm under size for the tubes having outside diameter ranging from 25.4 mm to 228.6 mm
16	Permissible variation in thickness for seamless hot finished tubes for over size is 28% to 40% for tubes having Out side diameter up to 101.6 mm and Thickness from 4.6 mm to 2.4 mm.   Note: Variation is allowable for over thickness not under thickness. And Tubes having lesser thickness have higher tolerance on over side and tubes having higher thickness have lesser tolerance on higher side.  Read>>>Procedure for pre and post weld heat treatment For cold drawn tubes variation in thickness is allowable up to 20-22%	Permissible variation in thickness for seamless hot finished tubes for over size is 28% to 40% for tubes having Outside diameter up to 101.6 mm and Thickness from 4.6 mm to 2.4 mm.   Note: Variation is allowable for over thickness not under thickness. And Tubes having lesser thickness have higher tolerance on over side and tubes having higher thickness have lesser tolerance on higher side.   For cold drawn tubes variation in thickness is allowable up to 20-22%	Permissible variation in thickness for seamless hot finished tubes for over size is 28% to 40% for tubes having Outside diameter up to 101.6 mm and Thickness from 4.6 mm to 2.4 mm.   Note: Variation is allowable for over thickness not under thickness. And Tubes having lesser thickness have higher tolerance on over side and tubes having higher thickness have lesser tolerance on higher side.   For cold drawn tubes variation in thickness is allowable up to 20-22%
17	Tests carried out during manufacturing are Tensile test, Hardness test, flattening test, Expanding or flaring test and Hydraulic test.	Tests carried out during manufacturing are Tensile test, Hardness test, flattening test, Expanding or flaring test and Hydraulic test.	Tests carried out during manufacturing are Tensile test, Hardness test, flattening test, Expanding or flaring test and Hydraulic test.
18	Yield strength for Grade A, B & C of carbon steel tubes varies from 180 Mpa to 275 Mpa	Yield strength varies from 205 Mpa to 415 Mpa	Yield strength varies from 200 Mpa to 300 Mpa
19	Tensile strength for Grade A, B & C of carbon steel tubes varies from 325 Mpa to 485 Mpa	Tensile strength varies from 380 to 585 Mpa	Tensile strength varies from 200 Mpa to 300 Mpa
20	Rockwell Hardness B for Grade A, B & C of carbon steel tubes varies from 77 to 89 HRB	Rockwell Hardness B varies from 80 to 90 HRB	Rockwell Hardness B varies from 70 to 90 HRB

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What is the Function of Boiler start up vent Control valve??

 

The Start-Up Vent also called Start-Up Vent Line or Start-Up Vent Valve in a boiler plays a critical role during boiler start-up, shutdown, and low-load operations.

Start-up vent valve is generally manual, motorized or pneumatic operated. Now a days all Boilers have start up vent with motorized valve followed by pneumatic control valve.

Start-up vent line is always tapped from main steam out let line before or after safety valves and extended to suitable height. Start-up vent lines are equipped with silencers to reduce sound level of super heater steam upon venting. Steam carrying capacity of the line is 30-35% of Boiler MCR (Maximum continuous rating) or boiler capacity.

Generally, there is no flow meter for this line.

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Functions of start-up vent control valve.

Main function: Start up vent is used to provide minimum flow to Boiler:

As like high pressure pumps, Boilers should have minimum steam flow from Boiler during start-ups or low load operation. And this is the main function of the start-up vent line.

This minimum steam flow from start-up vent is for ensuring sufficient cooling steam is flowing through super heater coils. If NO or less steam flows through the SH coils will lead to starvation of the coils during boiler start-ups, shut downs and no-load operations.

There fore operator should ensure the opening of start up vent valve during start ups & sudden steam cut off due to process disturbance or grid failure etc.

Other functions of start-up vent valve/line are;

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1-Start up vent valve is used to open for increasing the load on the boilers during start up. This ensure the increase in steam temperature as the load on the Boiler increases.

2-Start up vent control valve allows initial steam generated during boiler start-up to be vented safely

3-Start up vent valve is used to vent out the moisture and non-condensed gases present in the super-heated steam to ensure the right quality steam.

4-Start up vent control valve is used to control the Boiler pressure during start-ups and shutdowns.

5- Start up vent control valve is used to reduce the steam pressure during Turbine trip, process steam cut off or Turbine load throw off.

6- Start up vent control valve helps in gradual increase of steam load on boiler. This ensures uniform heating & no stress development in pressure parts.

7-It prevents pressure surges and water hammering in downstream pipe lines.

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8- Start up vent control valve acts as safety valve if kept in Auto mode there by protects Boiler from over pressure.

9- Start up vent control valve in Auto logic operation helps in controlling main steam pressure there by avoiding Turbine tripping on High steam pressure.

10-Even start up vent valve is used to control the drum level during start up of safety valves floating.

When drum level rises suddenly, it can be controlled by sudden closing of start-up vent valve.

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In order to rise the drum level fast, start up vent valve is used to keep open.

Start up vent typically remains open during start ups and later can be closed gradually.

When do the Boiler start up control valve is used??

Used during;

1. Cold start up, Warm start up and Hot start up
2. During planned and emergency shutdown
3. During steam test (safety valve testing)
4. During Turbine interlocks testing
5. Start up vent is used during low load or partial load operation
6. During Home load operation
7. During grid failure
8. During cut off of process steam

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