Hello Akber and Ross,

Before I get to your questions, Akber, I want to underline Ross's excellent observations. You must be aware of the forces due to flow of fluid and design to accommodate these.

There are several forces involved here and perhaps it would be useful to consider them individually:

Internal pressure is equal in all directions. For the most part the forces impinge upon the inside of the pipe and they cause compressive forces on the inside diameter and tensile forces on the outside diameter (both of theses stresses are circumferential and they are considered by B31 Codes in the process of determining the pipe wall thickness (P * D / 2 * t)) - the continuous wall of the pipe allow these equal and opposite forces to counteract each other.

It should be noted that some types of expansion joints (e.g., bellows) also have an area that is greater than the internal bore area. A bellows has convolutions that include an internal "annular space". Normally all of the convolutions will have opposite and equal areas to counteract the longitudinal pressure - EXCEPT the two that are on opposite ends of the bellows. This feature of these types of expansion joints results in an additional longitudinal force that must be added to the "bore thrust" when the system is designed.

The longitudinal forces eventually impinge upon something that is aligned with (perpendicular to) the pipe inside bore - changes in direction come to mind. When these internal areas of impingement (e.g., inside pipe wall at bends) are connected together by a continuous pipe wall, the load path is continuous, allowing the opposite and equal forces to counteract each other - the stresses in the pipe wall caused by this are longitudinal stresses and they are addressed in the B31 Codes as part of the "additive longitudinal stresses due to weight (bending) and internal pressure (P * D / 4 * t)) " (i.e., "sustained longitudinal stresses").

Yet another force results from flowing fluids. The flow of fluids moving through the pipe must be "turned" by the pipe wall (change in the direction of flow) when a bend is encountered. This impingement of flowing fluid (energy) causes a force to be applied to the pipe that also must be counteracted. Obviously, the magnitude of this force is a function of the mass that is having its direction changed by the bend. If the force is great, it MAY require that an "anchor block" or similar restraint be used to transfer the fluid flow force from the pipe to surrounding soil or adjacent structure.

Further, some perturbations in flow will cause discontinuous or impact type applications of forces to the pipe ID at bends (and other changes in direction). As an example, if liquids are allowed to accumulate in a piping system that normally carries a gas or vapor, the "puddle" of liquid MAY be "picked-up" by the flowing medium and "thrown" downstream as a “slug” (this is called a “slug flow event”) until the "puddle" IMPACTS upon a pipe wall (bend). This is a very destructive event. AND, if that were not enough, the pipe designer must be aware of "steam hammer" and "water hammer" which occurs as a result of acoustic waves being propagated through the process medium in alternating compression and rarefication waves. These also result in impact loadings. “Hammer” events may be caused by product phase changes, “bubble collapse”, fast opening (or closing) valves or other such unpleasantness. These types of events are analyzed as a function of time and they have been well discussed in previous threads on this board.

Now, having said all of that lets look at you questions:

1)Does this mean that it is not required to use a thrust block at the elbow(high flow rate)?

NO.
Keep in mind what Ross is saying above. The flow of product through the pipe will cause a force to be applied that is a function of the mass of the flowing fluid and its velocity. This is a different force that that caused by internal pressure.

2)Is longitudinal pressure thrust in a continuous pipe covered by (PD/4t)

YES, in so far as the pipe stresses are concerned. It is part of the B31 Code "sustained stresses". The circumferential stresses caused by internal pressure are considered in the wall thickness calculation.

3) If there is change in momentum (like a Wye with different flows in and out)does the same criteria apply?

Yes, the same design issues are presented but remember that the "bore thrust" due to internal pressure is a function of the inside diameter and if the "Y" changes the diameter the designer must address this change in force - keeping the pressure-caused forces "balanced" is very important. This is VERY well explained and illustrated (with good diagrammatic figures) in the Standards of the Expansion Joint Manufacturers (EJMA).

The forces applied to changes in direction due to flow of product through the pipe must be countered by including thrust blocks, struts, restraints or by other design considerations.

I hope some of this will be of value to you. Good luck with your projects.

Best regards, John.