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Add thermal growth, possibly non-uniform temp. profile if line is not running full, which could create bending stresses. "Unrestrained section" - still, there is friction with whatever supports do exist, or against the ground if laid directly on the surface.
Disregarding friction and bending from self-weight plus contents, wind loads, etc., primarily they arise from pressure acting on end caps. All pipes are a pressure containing system, if they have more internal pressure than external pressure.
End cap pressure, line pressure, is multiplied by cross-sectional area of flow to give an axial tension force in the pipe wall.
Now including bending, the bending compression in the top-most part of the pipe and tension in the bottom-most part of the pipe are algegraicaly added to the axial tension. Those axial forces are combined with the hoop stress, and shear stress and torsional stresses, if any, and radial stress (equal to internal pressure), to arrive at a maximum equivalent shear stress, which is checked against the allowable equivalent shear stress.
What would you be doing, if you knew that you could not fail?
In ASME B31.4 , For unrestrained section they are considering a longitudinal stress due to fluid pressure with value pd/4t . But since this is an unrestrained pipe how will that stress arise ? . The pipe will be free to expand in its longitudinal direction.ryt ??.So i cannot think of the possiblity of a longitudinal stress . Somebody please explain this scenario.
Ya. The pipe is being pulled, stretched, by the pressure acting on the end caps. End cap stress = pressure x X-sectional flow area / X-sectional area of steel .... also = pd/4t
What would you be doing, if you knew that you could not fail?
okie thanks..but now my question is in ASME B31.4 , for restrained section they are considering a value of 0.3 times hoop stress as longitudinal stress (which is the lateral component of circumferential stress) . But for unrestrained they are considering this 0.5 times hoop stress(pd/4t).
But ideally there is the effect of this lateral component of circumferential stress should be added to pd/4t in unrestrained section..ryt ?
When UN-restrained, the Axial tension stress component due to Poisson effect, 0.3 * S[sub]hoop[/sub], is NOT present, as the pipe when UN-restrained is free to contract. End cap load is there however, stress = PD/4/t, since that is not the result of trying to stop a strain; it is the result of an actual direct load on the pipe, pressure acting on the end caps. If Free Contraction is allowed, there will not be any associated stress due to the Poisson effect, as you have applied no load to resist the strains and it will freely contract.
When the pipe is REstrained, with anchors at each end, meaning anchors at the joint between the end caps and pipe, end cap loads are dumped into the anchors and not transmitted past the anchors into the pipe, provided it is a full, 100% longitudinal restraing. The longitudinal stress in a REstrained pipe segment is only due to radial pressure (and makes a tension load of 0.3 * S[sub]hoop[/sub] plus the thermal expansion stress.
What would you be doing, if you knew that you could not fail?
but where from that end cap load comes during normal operating condition ?
What my understanding is end caps are used for future tie-ins and to close the pipe during construction.
Pipe is a pressure containing system. That includes pressure on the circumference and on the ends. Pipelines have ends right. Think of a short manifold, with end caps, add a tee-off next to one end cap and a pipe attached to a compressor.
What's the pressure on the end cap. It equals, nearly, the compressor discharge pressure, say 1000 psig.
Put a reducer on the other end. Due to flow losses in the manifold, 1 psi, the pressure at the reducer is now 999 psig. From that reducer add a pipe 100 miles long. Do a free body analysis on the reducer and the 100 mile long straight unrestrained pipe. The unbalanced forces show that the opposite end cap force is carried into reducer and the 100 mile long pipe as an axial load of 999 psig x cross-sectional area of flow, - flow friction losses in the 100 miles, balanced by the remaining pressure at the end of the pipeline x the cross-sectional flow area.
If you restrain the 100 miles of pipe by filling the trench with soil, the soil restraint will extract the axial load due to the one end cap at the manifold from the 100 mile long pipe, due to soil friction on the pipe in around the first 300 meters of pipe length. Only flow loss differential pressures need be balanced by being taken up by the pipe wall and also distributed to the soil after that.
What would you be doing, if you knew that you could not fail?
much appreciated bro..
Hey here goes another question ,
In ASME B31.4 , they are taking lateral component of hoop stress as poisson's ratio times hoop stress.On the contrary, in ASME B31.8 ,they took 0.3 times hoop stress .
What is the reason behind this ?
Hey i have got a basic doubt.
In ASME B31.4 definition of unrestrained section goes likes this ,
It is free to displace laterally and to strain axially..
displacement happens means there is strain..ryt ?
so that means it is free to strain in all directions..ryt ?
