Thrust Block design

[EntryLevelEIT]

I know....Civil form has topics on thrust blocks, they still don't answer my question. Actually, this is more of a statics question.

I'm reading that to design a thurst block both static pressure and flow velocity must be account for.

Flow velocity I get, but I can't envision how static pressure inside a pipe would transmit any force to an object outside the pipe. The static pressure is the same at any point in a pipe (assuming same elevation). What's the difference if this pressue is along the straight portion of the pipe run or an elbow. Doesn't the pipe have the same chance of rupturing at any point along the pipe run? So, why all of a sudden does the force from static pressure factor into play at bends as opposed to anywhere else outside the pipe? I though the pipe wall thickness accounted for static pressure.

Thanks for any help!

 

[dhengr]

You said... “I'm reading that to design a thurst block both static pressure and flow velocity must be account for.” Don’t be so thursty? or thirsty? the next time.

Otherwise, reread that to mean dynamic forces due to the flow and flow direction changes, etc., which you claim to understand. And, static forces such as expansion/ contraction, length changes over long pipe lengths or at shorter lengths btwn. two direction changes, friction btwn. pipe and soil, and how the thrust blocks interact or restrain these actions. What happens beyond flow induced forces if a pipe runs horiz. and then at some point turns up or down hill at 45° for a long run? I think these are the kinds of static forces they are talking about. Obviously, you must design the pipe for internal forces and stresses from pressure and the like, and these might end up being added to some of what is induced by the thrust blocks and reaction hardware above.

 

[EntryLevelEIT]

That makes sense.

Just to take it a bit further...
This is for a 30" horizontal pipe which makes a 90 degree bend to exit a pumping station, with 70 psi.

According to 2 different sources (DIPRA and

http://www.engineeringtoolbox.com/forces-pipe-bends-d_968.html)

the hydrostatic pressure at the bend = 1.5 * P * A. = 1.5 * 70 psi * 707 in^2 = 74,235 lbs. I find it hard to believe that 74 kips transmitts through the pipe and would need to be restrained by a thrust block, and that's without even taking velocity into account.

I agree with expansion/contraction and other conditions which may add a static force to a pipe per your explaination. But, the above number of 74,235 is what I was getting at in my original post.

 

[cvg]

valves, fittings and pipes can blow apart if they are not restrained at the joints, valves or at changes in direction. This is generally a problem with un-restrained bell and spigot pipe which is installed underground.

Think about a champagne bottle with a cork. let it get warm and shake it a bit, then release the restraint which is the wire. The static pressure will blow the cork out. There is no dynamic force involved.

 

[JStephen]

EEEIT, some joints restrain the pipe longitudinally (flanges, welded joints, Victaulic couplings, solvent-weld joints), some don't (slip-on joints, mechanical joints). The type of pipe used for long runs of large water mains is quite frequently a push-together style that does not restrain the pipe, thus the need for thrust blocks.

The dynamic effects will quite often be neglible in comparison to the pressure forces.

 

[rb1957]

how much force does a pressure create ?
F = p*A
i think the 1.5 factor is a safety factor.

alternatively, 'cause the pipe is bent 90deg, the same force is reacted in both directions, so the total force on the bend is 1.414*(p*A).

 

[EntryLevelEIT]

@ cvg: Ok, I strictly pictured a velocity force before. I'll continue going with the loads I calculated then and factor in velocity.

@ JStephen: The bid drawing shows 2 flange supports at the elbow in each direction. I'm not sure of the restraint capability of them. I'm looking into that also. I attached a jpg of this.

@rb: Yes, 1.5 is just the safety factor. 1.5 * P * A is actually called out for a dead end, I though this would most nearly resemble the 90 degree bend in a conservative manner. Sorry, where is the 1.414 coming from?

Thanks guys.

 

[dhengr]

I’d suggest you get a couple good text books or handbooks on the subject and study them if you really want to become proficient at this type of design. Not just using the quick-fix, fast trip to the internet. I don’t know what your 1.5 is, some safety factor, MS or what; but otherwise you’ve calc’ed. the force on a 30" dia. capped pipe at 70psi, and that does exist. The bend is one cap in that length of pipe and the pump is the other. And the pipe might take this force as part of its internal stress condition, or the pump base and a trust block at the 90° bend might take some or all of this force to relieve the pipe of that stress. But in the first case the tensile stress is uniform around the pipe circumference, 74.2kips/(? d) = 790 lbs./in., fairly clean and straight forward, if the pipe can take that stress, but the pipe will grow in length because of this; in the second case the pump base/housing and the trust block at the 90° bend prevent this pipe expansion and take the load, but they also tend to concentrate these forces and stresses on the pipe plate ; therein lies the complete pipe system design problem. Velocity and mass flow forces are a whole additional loading consideration on the system, and they do exist. And, you must sum all of these to be sure you are not over stressing the pipe.

 

[cvg]

For a pump station with 30 inch discharge line, is there a chance for transient surges (water hammer) such as when the power fails, the pump stops and the check valve shuts? If so, your pressure could be way more than 70 psi. Is a FS of 1.5 high enough?

 

[EntryLevelEIT]

Our existing pumping stations have definitely had surges due to power failures and various reasons. In that case I'm not sure how much the 70 psi will increase or if the 1.5 factor of safety is enough. I need to contact the mechanical squad to gather more information. I appreciate the heads up, this needs to be designed for worst case. It's always just a matter of time before a pump trips.

 

[csd72]

Just to explain the whole pipe pressure thing in a little more detail.

When you look at a pressure diagram of a pipe there is the pressure on the walls that is resisted by the pipe ring pressure and cancels itself out. There is also the pressure along the pipe which is only resisted by the pressure of the other liquid or gas along the pipe.

As others have said this pressure along the pipe causes a force component that acts along the axis of the pipe and at bends this causes an out of plane force that needs to be resisted.

This is why even gas mains need thrust blocks even though the flow momentum is negligible.

As others have said, thermal forces also should be considered.

A useful tool is to apply method of sections to this cutting the pipe at the end of the curve on each side and looking at the resultant force components on the whole bend.