Design Procedure for Boiler Joints

Design procedure for Boiler Joints In a steam boiler the two types of joints are longitudinal joint and circumferential joint. Longitudinal joint is used to join the ends of a plate to form a cylindrical shell and is generally made by butt joint with two cover straps. Circumferential joint is used to get the required length of the shell and to close its ends. Lap joint is widely used for this purpose. Design steps for Longitudinal Joint (Butt joint) 1. Select a suitable butt joint. The selection depends upon the diameter of the shell and the desired efficiency of the joint.(Table 5.2, page 68 in DDB) 2. Determine the thickness of the boiler shell plate + 1 to 2 mm as corrosion allowance (eqn 5.1) Where pi = steam pressure in N/mm2 D = Internal diameter of the pressure vessel in mm σt = allowable tensile strength of the shell plate and η = efficiency of the joint 3. Select the standard value of thickness of steel plates for boiler from table 5.3c page 68A 4. Calculate the diameter of the rivet hole by the Unwin’s formula √ The diameter of rivet can be obtained from the table 5.3b page 68A.The diameter of rivet hole shall not be more than 1.5 mm larger than the diameter of rivet. 5. Calculate the pitch of the riveted joint by equating the tearing strength of the joint and shear strength. (Eqn 5.11 and 5.13) According to IBR the maximum shorter pitch where k1 is a constant (table 5.4a, page 68A). 6. Calculate the transverse pitch (pt) for the given condition. Use appropriate equation from eqn 5.31a to 5.33b given in page 66. 7. Calculate the thickness of the cover plate. Use appropriate equation from eqn 5.4 to 5.7 given in the pages 62-63. 8. Calculate the margin from

9. Calculate the diagonal pitch the joint is a zig-zag one

or 2.25 to 2.5d

Design steps for Circumferential Joint (Lap joint) 1. The diameter of rivet hole and rivet should be same as that of used in longitudinal joint. 2. Calculate the total number of rivet required for the joint from equating the shear strength of rivet to the shearing load Where N is the total number of rivet required 3. Assume double riveted lap joint calculate the number of rivet per row (n) = N/2 OR assume suitable value for efficiency (60-70 %) 4. Calculate the pitch (

)

5. Calculate the efficiency of the joint

6. Calculate the margin from 7. Select the transverse pitch in a similar way as in the design of the longitudinal joint Design Procedure for Lozenge Joint A Lozenge joint is a special type of double cover plate butt joint which is mainly used in structural joints namely bridges, girders etc

1. Calculate the strength of the joint in different modes a) Shear strength of one rivet in double shear = b) Crushing strength of one rivet = c) Tearing strength of the plate at outer row = (

)

2. Calculate minimum number of rivets required, N= 3. Draw the layout of the joint 4. Calculate the strength of the joint in different modes of failure

a) Shearing strength of all rivets = b) Crushing strength of all the rivets = c) Tearing of plate along different sections, starting from the outer row section A-A (contains only one rivet) ,B-B , C-C and so on Tearing of plate along section A-A = (

)

Tearing of plate along B-B + shearing of one rivet at section A-A Tearing of plate along C-C + shearing of rivets at section A-A and B-B

5. Calculate the efficiency of the joint =

6 Calculate the margin from 7 Calculate the pitch along different sections like B-B , C-C etc 8 Calculate the transverse pitch = 2.5 d