David Kent

Thrust Restraint

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 I am taking some time out from my campaign for world supremacy to have a rant about something that continues to annoy me. This is the subject of thrust restraint and how a relatively simple subject seems to be confusing to so many, especially structural engineers. I will start by giving an overly condescending explanation of where thrust comes from.

Myth: The force comes from the change in direction as the water goes around the bend. So it is momentum based and the direction of flow is important.

Fact: Almost all the thrust in almost all water pipelines we will be involved in comes from pressure, not momentum. Momentum is only significant if your pressure is very low and the velocity very high. So whether the water is flowing or not is irrelevant in most cases we will encounter. Usually the highest thrust load happens under test pressure when the water is stationary. Go to here and have a play. http://www.engineeringtoolbox.com/forces-pipe-bends-d_968.html If you enter in our typical pressures and velocities, then you see that momentum thrust is a very small percentage of pressure thrust.

Myth: the water is acting on the inside of the pipe, so I use the internal diameter in my calculations.

Fact: For RRJ pipes, always use the outside diameter. Sure the water around the bend is acting on the inside, but it is also pushing on the annulus (the area of the ring bounded by the OD and the ID) so the total effect can be calculated using the OD rather than the ID.

Myth: I have to put thrust blocks on a welded PE or steel pipe.

Fact: Any pipeline with restrained joints (welded, flanged, Victaulic, Tyton-lok, etc.) does not need thrust blocks as the thrust is carried in the wall of the pipe. Your hose at home has plenty of bends but it doesn’t need thrust blocks. The only time the hose starts to whip around is when there is an open nozzle on the end, and this is from nozzle thrust.

Myth: I have an unequal tee so I calculate the thrust based on the main diameter, not the branch diameter.

Fact: Always use the branch diameter. It is the hole in the wall of the main line that is resulting in the unbalanced force.

So where does the thrust come from? At a dead end or valve or tee it is easy to envisage the pressure pushing on the disc and easy to work out (T=PA). But in a bend, it is not so easy to see but it is still due to acting on an area. If you look at a bend in plan, the length of the outside curve is more than the length of the inside curve. So the area on one side of the bend is more than on the other. This is where the net thrust comes from on a bend (at least that’s how I envisage it – the formula used to calculate it doesn’t include bend radius so it is based on a single mitre bend as the worst case scenario).

There is one special case with pipe with restrained joints and that is at an unrestrained end. If your PE or steel welded pipe ends in a RRJ at a different pipe material, then this end will need restraint. And the restraint load is not just from pressure thrust. Two additional effects that will be acting are thermal contraction and Poisson’s contraction. Either you need to accommodate this contraction or restrain it. With PE pipe, this contraction can be hundreds of millimetres so the first RRJ will just pop out (it has happened). So you will need an anchor wall on the end of the main. Thermal contraction happens when the operating temperature is less than the installed temperature. Usually this is in the order of 10 to 15 degrees. Poisson’s contraction happens when the pipe is under hoop pressure. The pipe wants to get longer circumferentially so it tries to shorten longitudinally. Fortunately, we have spreadsheets to calculate the loads and design the thrust walls, so email me if you want them. I may even get around to putting them in the water Toolbox.

One last rant is about geotechnical engineers. Any time you ask them for a horizontal bearing pressure to use in your thrust block design, they put so many factors of safety on it you end up designing your block for a soil with the consistency of custard. And it gets even more confusing when they don’t even tell you if it is for working stress design (allowable bearing pressure) or limit state design (ultimate bearing pressure). So treat what they tell you with caution and get a second opinion from someone in the know.

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