Pipe pressure change due to seismic acceleration

[Ktracid]

I have a question that may lead to interesting discussions.
Can seismic acceleration lead to transient pressure fluctuations in a pressurized pipe? Any hint or insight could be useful.
Thanks.

 

[BigInch]

A liquid under acceleration from gravity, change in velocity, or application of any unequal forces, from whatever cause, whether it is inside a tank, a pipe, or a jar, sphere, road tanker truck, etc. will exert pressure forces on the container = Mass of liquid * Acceleration / Area of contact

A tanker truck going around a curve, or stopping, produces accelerations that act upon the fluid, which may change the level of the fluid inside the tank, thereby also changing the pressure against the walls of the container.

These internal fluid pressures generated from small seismic accelerations would normally be small in relation to the typically large internal pressure stresses of a typical pipeline, hence such effects from seismic loads are typically ignored. Seismic loads generally are assumed to cause axial, lateral and vertical relative displacements between points of a pipeline as the seismic waves move along or across a pipeline. These are the stresses that are usually being analyzed in the seismic design of pipelines.

Independent events are seldomly independent.

 

[Ktracid]

Thanks BigInch; I fully agree with you on the tanker truck example. Let's forget about sloshing and the fact that the liquid level at the tank could be tilted. How about if we are dealing with a pressurized pipe? I think it still applies; what do you think?

Let me explain further. We have a large pipe that runs horizontally around 250 meters straight before a 270 degree bend. The seismic acceleration is as high as 0.6g. The operating pressure of the line is ~550 kPa and the line class maximum allowable pressure is 1000 kPa. The pressure increase due to seismic axial acceleration is ~1500 kPa, which could lead to instantaneous pressure of ~2000 kPag at the bend for this pipe.

Should we considering changing the line class to be prepared for the instantaneous pressure increase in this pipe?

 

[LittleInch]

How have you calacualted this? I don't htink there is any increase in pressure from a liquid filled pipe being shaken. There are some forces developed due to momentum changes but I can't see this being either anything significant or actually calcualable. Pipelines in seismic events break either because they are brittle or made from push fit joints or the gorund underneath them moves away. I think you're finding a problem which doesn't exist.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Ktracid]

LittleInch, I also believe there should not be any problem. In my opinion, the pressure increase due to seismic loads should be negligible but I just do not know how to justify this statement.

I calculated the additional pressure due to seismic acceleration using the rho X acceleration X height equation. In this case, it will be 1000 x 6 x 250 = 1500 kPa. Does it make sense? This pressure could be instantaneous (maybe a small fraction of a second) but I do not know if from a stress point of view, the pipe will withstand it or not. Do you have anything to add to it?

 

[rconner]

If you were to re-phrase your question slightly to e.g., "Is there evidence of very high pressure in some earthquake events?" I believe the answer is quite assuredly, "YES" (see e.g. two pictures of incidents in slide 35 at

http://www.pnws-awwa.org/uploads/PD...on Session 5 Tim Collins_Don Ballantyne.pdf).

While perhaps "seismic acceleration" is involved, I'm not aware so far of research defining that as sole causation.

 

[BigInch]

As I said above, that link also clearly indicates that failures of pipelines are not associated with any internal pressure increase due to seismic accelerations, but rather to relative displacements of the pipe. Displacements and resulting stresses from axial load, bending and shear are not contributing to pressure hoop tension stress.

Independent events are seldomly independent.

 

[77JQX]

I'd like to note that the ~1500 kPa pressure increase calculated above is valid if the earthquake instantaneously accelerated the entire mass of water in the 250m of pipe leading up to the elbow. That is, if the pipe + water behaved as a rigid unit. That would be the maximum pressure that could possibly be generated at a closed valve. This pressure would be mitigated be elastic deformation of the pipe and fluid as is found in classical water hammer analysis. I think the proper analysis is to use the Joukowsky equation and the expected change in velocity to figure out the expected surge in a closed pipeline.

If the pipeline is flowing freely, then I speculate the acceleration of the elbow won't appreciably accelerate the water within, and the earthquake wouldn't contribute to a significant surge.

 

[BigInch]

Sorry that analogy is not valid. First there is no such thing (by definition) as instantaneous acceleration of a body or mass of fluid. Earthquake accelerations are measured in 1/10ths of gravitational acceleration and are not constant in one direction only, so you would never obtain the velocities you would need to generate 1500 kPa from the "Joukowski equation". If what you said were true, you could easily generate accelerations a number of times stronger than gravity, which according to your analogy, must generate internal pipe pressures of 250 psig or more in a 1 ft long piece of thin walled PVC pipe just by juggling it around your head... where it would explode in your face.

Sorry, not buying any of that.

Independent events are seldomly independent.

 

[77JQX]

BigInch - we're in agreement. I think.

 

[BigInch]

Then we're not in agreement with ktracid.

Independent events are seldomly independent.

 

[Ktracid]

I am in agreement too. I think.

 

[btrueblood]

Yes, there is pressure rise due to acceleration of a fluid. Usually pipe engineers only consider accelerations due to gravity, but rocket scientists have to consider accelerations other than pure g.

Can a seismic event accelerate a 250m pipe at 0.6 g? Yes, Northridge and other events had that much ground acceleration, and the typical wavelength for those waves was on the order of 5 km, so it's possible.

Could the pipeline rupture due to such events? Well, you'd have to exceed the burst (or collapse pressure) of the pipe (which if you do your homework correctly, ought to be 2x to 3x of your design pressure, no?), and do so before ground motion caused the pipe to seperate anyway, so it ought to be pretty rare. But it does happen, again see rconner's link, specifically the two slides he pointed to (slide 35 shows a clear rupture, slide 36 shows a vacuum collapse).

There was a large wooden water pipeline, approx. 30 miles long and 400' or so of elevation drop, near my house where I grew up (seismically active area). Spaced every half-mile or so were surge towers, designed to relieve such pressure events. They did work, as we never heard of this line rupturing, and it survived at least two or three Magnitude 5 or better events.