External hydrostatic test pressure for "hot tap"
External hydrostatic test pressure for "hot tap"
(OP)
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.
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.





RE: External hydrostatic test pressure for "hot tap"
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.
RE: External hydrostatic test pressure for "hot tap"
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
RE: External hydrostatic test pressure for "hot tap"
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?
RE: External hydrostatic test pressure for "hot tap"
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
RE: External hydrostatic test pressure for "hot tap"
RE: External hydrostatic test pressure for "hot tap"
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
RE: External hydrostatic test pressure for "hot tap"
RE: External hydrostatic test pressure for "hot tap"
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.
RE: External hydrostatic test pressure for "hot tap"
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