Fix the pumps

Wednesday, April 18, 2007

For the record

I need to post some corrections to my earlier post regarding sizing of the hydraulic pipes. While my conclusions remain correct, my methodology was incorrect in some respects.

First off, I cited the wrong standard. Instead of ASME B31.1, I should have cited ASME B31.3. That's because B31.1 is used for power piping (boilers, etc.), while B31.3 is used for process piping, which is what we have at the floodgates. However, the formula to figure out the required thickness of a pipe is still the same in each standard. It's the following:

T > t + c

The left side of the inequality represents the actual, physical diameter of the pipe as it's received in the field. The right side is the calculated minimum thickness, which - because it is a minimum - must be less than the real thickness of the pipe.

Pipe is made with a certain tolerance. In the case of standard carbon steel A106B pipe, like what we have here, the tolerance is +0,-12.5%. That means that while the nominal thickness might be "X," it could be "X" minus 12.5% of "X," or 0.875 times "X." For 3" schedule 80 pipe, the nominal thickness is 0.3". But when manufacturer's tolerance is included, the actual thickness could be 0.875 x 0.3, or 0.2625". This is the thickness we must beat on the right side of the inequality. If we don't, then the next highest thickness should be chosen.

On the right side of the inequality "t" can be found using the following B31.3 formula:

t = P*D/(2*(S*E+P*Y))

where each of those letters represents the following:
P = internal design gage pressure = 3000 psi
D = outside diamter of pipe as listed in tables of standards or specifications or as measured = 3.5"
S = stress value for material from B31.3 - Table A-1 = 20,000 psi for A106, Gr. B
E = quality factor from B31.3, table A-1A or A-1B = 1
Y = coefficient from B31.3, table 304.1.1, valid for t less than D/6 and for materials shown = 0.4

t = 0.2476"

Ah ha! It would appear the pipe is fine. 0.2476" is less than 0.2625". Case closed. Move along here, nothing to see.

Before I debunk that, let me point out another of my previous errors. Previously, I used 15,000 psi as the stress value for ASTM A106, Gr. B. That's the value from the material table in standard B31.1. However, in B31.3, the corresponding value is 20,000 psi. All other values in the equation above are exactly the same.

Now, what about the apparent victory for Corps/MWI engineering? Well, there's still the matter of "c" in the original equation. Little "c" is the sum of mechanical, corrosion, and erosional allowances. These are additional factors which take into account wear in the pipe and threadedness. We have seamless welded pipe, so I'm going to generously not include any mechanical allowance. However, we obviously have corrosion, and we also have erosion. Both are natural processes of fluids flowing through pipes. Normally one only has to worry about such things happening on the inside of the pipe. However, the corrosion in this case is unnaturally bad, since corrosion protection has not been applied to the pipe for over a year.

Picking corrosion/erosion allowances is part art and part science, and I'm not going to pretend to be an expert on it. I've seen numbers ranging from 1/32" (0.03125") all the way up to 1/8" (0.125"). From what I understand, 1/8" is pretty stringent, usually being used for salty environments such as an offshore platform. However, one can see that even applying the smallest of allowances pushes the calculated minimum thickness above 0.2625". So what I thought I'd do is just lay out the results for various allowances:

1/32": t + c = 0.2788"
1/16": t + c = 0.3101"
3/32": t + c = 0.3414"
1/8": t + c = 0.3726"

All of these are greater than 0.2625", and most are greater than 0.3". Thus, the pipe is not up to the applicable code, and a greater thickness should have been used.

Then, going one step further, I subtracted the various corrosion/erosion allowances from the actual pipe thickness (0.2625"), and calculated the actual pressure rating of the pipe:

1/32" allowance (wall thickness = 0.23125"): pressure rating w/ existing pipe: 2790 psi
1/16" allowance (wall thickness = 0.2"): pressure rating w/ existing pipe: 2395 psi
3/32" allowance (wall thickness = 0.16875"): pressure rating w/ existing pipe: 2006 psi
1/8"allowance (wall thickness = 0.1375"): pressure rating w/ existing pipe: 1622 psi

All of these are less than 3000 psi.

What these results say is that while the pipe is okay when you first load it with fluid, you can't actually flow any fluid through it at pressure for the anticipated life cycle (regular maintenance runs a few times a year, plus one or two storm events a year, for an indeterminate, but ever-increasing, number of years; the Corps now says to expect permanent pumping stations in 2012, and frankly it's not clear that the current installations won't be part of those stations). So the piping's kind of useless in its current application.

Keep in mind I've been pretty conservative in this analysis. The fact is that the hydraulic pumps on the drive units can put out more than 3000 psi. Also, the pipe is currently rusting every day at an unknown rate. Also, there are unaddressed flaws in the piping where welding rods, welding torches, and steel-toed boots have locally deformed it. At those points, where the wall thickness could be considerably less than 0.2625", all bets are off.

There's one other correction I need to make. I earlier said that the pipes could burst if put under pressure. That's not technically true. For a pipe to burst, it would have to have its yield strength exceeded. The yield for A106B is 35,000 psi, far higher than the minimum tensile strength of 20,000.

What could happen is the tensile strength is exceeded? It could put failure stress on welds and on areas of local deformation. It could also weaken the elbows in the pipes. In any case, what it boils down to is that there is no factor of safety built into the thickness of the pipes, and they can't be trusted. After all, would you trust a system to protect an entire city that can't even protect itself from rust?


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