Flexibility in Tortion
subjected to bending and tortion. In case of a beam subjected to moments as .shown in figure, the angle θ (The change in slope) is given by;
The circumferential stress in pipe wall due to moment M can be many times the value calculated as (My/I) as per ordinary bending theory for structural members. The factor by which the circumferential stresses exceed the longitudinal stresses in bend is called the “Stress intensification factor” often denoted as S.l.F.
Effect of Pressure on Stress Intensificaition Factor and Flexibility Factor
T - Nominal wall thickness of the fittings for elbows and miter bends, inches
r2 - Mean radius of matching pipe, inches
R1 - Bend radius, inches
P - Gauge pressure in psi
EC - Cold modulus of elasticity, psi
Stresses in a Piping System
- Safety is the main consideration
- The above alone will not govern the final specification for any piping installation.
- Code is not a designs hand book.
- It does not do away with the need of designer or competent engineering judgment.
· basic design principles
· supplemented by specific requirements to assure uniform
application of principles and guide selection of piping materials code prohibits designs and practices known as unsafe and contains warnings where caution, but not prohibition is warented.
Code Section Includes
ASME B 31 Code for Pressure Piping
(1998) - Electric power generation station.
· Central district heating systems.
(1999) - petroleum refineries
· cryogenic plants
(1998) hydro car-bons and other liquids
(1996) - industrial
· public buildings
(R1998) - aqueous slurries
- fluid temp.
- ambient temp
- heating or, cooling medium.
Design Minimum Temperature
The mill tolerance 12.5%
Min. thickness = T – mill tol. > t + C
P - Internal design pressure gage.
D - Outside diameter
S - Stress value for material. From table A-1
Seamless pipe E = 1.0
ERW E = 0.85
Furness butt welded E = 0.6
Electric fusion welded E = 0.95
100% radio graphed E = 1.0
Y - Coefficient from table 304.1.1 t < D/6