External hydrostatic test pressure for "hot tap"

[gibsi1]

Upon review of our internal procedure covering "hot tap" connections and associated testing, I've found a lot of discrepancy on acceptable calculation methods to determine appropriate external hydro test pressure and the required wall thickness to support it.

We often perform "hot tap style" connections onto lines that are not in service to prevent loading the entire system with water during a hydro test. There is no hot tap machine: we are simply welding the reinforced attachment onto the main line and pressure testing against the exterior wall before removal of the coupon. This is pretty common across the industry. In these scenarios, the API recommendation to never exceed 10% over internal operating pressure of the main line is not accurate. The same API recommendation really really isn't reasonable for true hot tap connections made on lines that operate at low pressure with the potential for high pressure excursions (i.e. vent lines), either.

I've used various calculations at different times to confirm typical test pressures (150% design) would not damage the pipe. Many folks used the unstayed flat plate calc as a very conservative check. I've also use external pressure calculations for convex heads. Does anyone have a clean calculation to determine the maximum test pressure that can be applied to the exterior wall of an existing piping using a branch (o-let) connection. All necessary data would be know (i.e. wall thickness, material, ID of branch). I would assume the calculation would be made without considering the reinforcement as a safety factor.

 

[MTPipeliner]

I've never used the following formula for your exact use, but it may be useful for you.

P=16,250,000*(T/D)^3

P=Collapsing pressure in psi
T=Thickness of pipe in inches
D=Diameter of pipe in inches

**** There is a safety factor of 4 built into this equation

It is a formula to predict the collapse pressure of a thin wall pipe in a vacuum. I've used it as a quick check of the likelyhood of collapsing a pipe that is under a water body, like a HDD under a river.

You may want to look into offshore pipelines for some more information.

I suspect that the reinforcing sleeve will actually add quite a bit of strength, but I think you're correct in taking the conservative approach in neglecting it.

I would be hesitant about pushing the limits too much because of the unknowns that may affect the strength of the pipe such as out of roundness.

I've done quite a few hot taps, but never hydro-tested them prior to drilling them out. Not saying it's not a good idea, just never done it.

 

[BigInch]

I don't think you can use a formula that assumes equal hydrostatic pressure all around the pipe as being topographically similar to localized pressure on a flat or curved wall. I think a flat plate analogy would be a lot closer, but still too far away from reality if the tapped pipe had a small enough diameter in relation to the branch such that any arching action would arise. For a small diameter tap on a large diameter, flat circular plate with edge stiffness might be pretty close... maybe. Somehow underneath it all, I don't see much realistic value in these kind of calculations. It doesn't seem like you could hardly fail a bit of pipe wall that had just been reinforced by adding all that edge stiffness around it anyway, unless failure is defined as a bit of permanent bending. But you're going to cut it out anyway and I wouldn't think the pipe past the tap would be affected either, so is it really important?

If it was something I really had to know, I'd build an exact replica test section and put the killer hydro on it.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[gibsi1]

I agree that you can't use the same calculation as when an entire pipe is exposed to the same external pressure.

The value in all of this is preventing catastrophic failure that could lead to a release or expose water into a system where it shouldn't be. Some of these hot taps are done on critical services that can severely harm people and/or the environment. We've hot tapped as large as 18" and 20" connections... that's a lot of exposed area. Large diameter connections into thin wall stainless steel or corroded carbon steel is where things become questionable.

Now, performing a hot tap style connection onto a line that isn't currently in service doesn't really have much risk other than damaging the existing pipe. Small diameter connections aren't much of a concern, either.

Let's change the question a bit:

When using the flat plate calc as a safety check before pressure testing a branch connection against the exterior wall of a pipe, which UG-34 connection (and associated C factor) would best represent the scenario?

 

[BigInch]

4" on 20 yes, but in large diameter to large diameter, there's that big curve in the tapped pipe and it cannot be considered flat. I think if you insist on calculating these things, to have some realistic answers you need to do/commission some FE work. Remember what Columbus said, "I have seen the shadow of the Earth on the Moon and I know its round".

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[gibsi1]

I realize that it can't be considered flat and agree that accuracy is not very high. However, elevated safety factor is desired in these scenarios and the procedures are a worst-case guidline. Working from the extremely conservative flat plat calculation is a good thing. If it passes the flat plate calc, then you know it's good. If it doesn't, you're forced to either delve deeper into more complex (accurate) calcs or discuss whether or not this is the avenue you want to go down. It takes 6-8 signatures from various levels of management and technical supervision to get a hot tap approved. They really scrutinize these things. They're often the result of emergency work (e.g. stopple a damaged line to run temporary bypass while keeping the unit running) when there may not be time for detailed FE unless absolutely necessary.

 

[BigInch]

What kind of emergency requires 6 to 8 signatures?

You could easily do a number of FE to cover a wide range of typical cases and have them signed off in advance and ready to go. Peace of mind and no run arounds at the last minute making a whole lot of mistakes.

If you're happy with Flatworld, I'm happy, but it was you that said its too conservative. I was just agreeing.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[MTPipeliner]

I guess that I read into the OP that it was a full encirclement sleeve type hot tap fitting, which now that I re-read it I see that it wasn't specified. I agree that for a branch type fitting that the formula I posted would not work.

 

[gibsi1]

Lots of emergencies require multiple signatures. You cannot sign off in advance. We can certainly have example calcs ready for use with each package, and for that FE would be great. I think that is the best path forward to re-write this entire procedure. It will take a bit of work to model enough cases to form a baseline that all involved parties would be happy with. These connections greatly vary in material of construction, type of connection, wall thickness, branch diameter, base diameter, process condition, etc.

I was simply soliciting ideas from anyone who has run into this before and might to confirm use of flat plate calcs with a particular "C value" or suggest something a bit better.

 

[BigInch]

I think you can standardize quite a lot of them.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain