“Civil engineers just love pouring that grey stuff up against timber. You would think they had shares in a concrete supply company. Seriously, I continue to argue with civil engineers who just don’t get the fact that a fully welded pipe or restrained pipe does not need thrust blocks. There must be a problem in university teaching or text books, but try as I might civil engineers go on burying concrete blocks. I point to refineries where the pipes are above ground with no thrust blocks to no avail.”[i]- Strainer
Similar was my situation when I was trying to convince a highly placed civil engineer that a fully-welded pipe does not require a thrust block and you know what, he treated me as if I had lost my mind.
Consider this simple example: There is a pipe that starts from a point A and ends at point B; in between it takes two 90deg turns, one at point C and another at point D. For now, forget about the point E.
Fluid density: ρ, OD = d, Pressure inside pipe = P, Velocity = V
Condition: Pipe is flanged bolted with the equipments at point A and point B. Fluid flows in the pipe from A to B or B to A (doesn’t matter) at pressure P and Velocity V.
So What Now: Thrust will be generated at both the turns (points C & D) due to pressure force inside the pipe, i.e., Pressure * Area of Pipe and also because of fluid’s momentum, i.e., Mass flow * Velocity. The same needs to be calculated for both X and Y directions and accordingly the resultant force shall be computed[ii]. It must be noted that thrust force due to pressure inside the pipe is considerable large than that due to mass flow and velocity and hence the latter can be ignored for simplicity.
If you look at the figure carefully then you will observe that both the Y-components of resultant thrust force are equally[iii] opposite to each other and hence shall get cancelled. But yes, in this case the stretch of pipe between points C & D shall be under tension and it would be really amazing to see if someone takes this mild steel pipe apart because of this tensile force!
And how do you think the X-components are gonna behave. Does it seem like that they are forming a couple about point E and are really on the verge of twisting this pipe, is it so? Well, think again then. Because first they will have to break the bolts which have tied this pipe with the equipments at A & B and only then will they have the luxury to form a couple, meanwhile they will have to languish alone, and as they say not in this life guys.
So what’s happening then: Well, these stupid X-components don’t get the point and thus they also put the pipes between A & C and D & B under tension. I hope there is no need to tell as to what will happen of them. Let them have fun in trying to tear the pipe, hahaha…
Now do you seriously think that we require a thrust block in a fully welded and fully restrained pipe and if you do, then pray tell me how?
[i] Strainer (Mechanical), 16 Feb 11 17:51, http://www.eng-tips.com/viewthread.cfm?qid=292032
[ii] This is an online calculator which is available for free along with the theory behind this calculation. http://www.engineeringtoolbox.com/forces-pipe-bends-d_968.html
[iii] an assumption that pressure drop between points C and D is insignificant
