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Questions Related to Pipe Under Stress:- Part-1

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1. What are primary loads? Mention some of primary loads?
These are typically steady or sustained types of loads. These loads have their origin in some force acting on the pipe causing tension, compression, torsion etc leading to normal and shear stress. Primary loads are not self limiting. Some of the primary loads are as:-
A. Internal fluid pressure
B. External pressure.
C. Gravitational forces acting on the pipe such as weight of the pipe & fluid.
D. Forces due to relief or blow down.
E. Pressure waves generated due to water hammer effects.
2. What do you mean by self limiting?
It means that the stresses continue to exist as long as long the load persists and deformation does not stop because the system has deformed into a no – stress condition but because strain hardening has come into play.
3. What are secondary loads? Mention some of the secondary loads?
Secondary loads are caused by displacement of some kind. Some of the secondary loads are as:-
A. Force on piping due to tank settlement.
B. Vessel nozzle moving up due to expansion of vessel.
C. Pipe expansion or contraction.
D. Vibration due to rotational equipment.

4.What is the most used choice of co-ordinate system for defining the stresses?
In a pipe subjected to internal pressure or any other load, the most used choice of co-ordinate systemis as :
A. Axial or longitudinal direction.
B. Circumferential or hoope’s direction.
C. Radial direction.
The stresses in the pipe wall are expressed as axial(SL), hoope’s (SH) and radial (SR). These stresses which stretch or compress a grain / crystal are called normal stress because they are normal to the surface of the crystal.
5. What do you mean by hoop stresses and how do you calculate it?
Stresses which are generated circumferentially due to the action of Internal pressure of pipe are called hoop stress.
It is calculated by, hoop stress (Sh) = Pdo/ 2t
Where
P = Force acting from inside.
do= OD of pipe.
t = pipe thickness.
6. How does Hoop Stress affect the system?
As per membrane theory for pressure design of cylinders, as long as hoop stress is less than yield stress of Moc, the design is safe. Hoop stress induced by thermal pressure is twice the axial stress (SL). This is widely used for pressure thickness calculation for pressure vessel

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