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Rerating (additional design) in existing lines 2

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Stouthamer

Chemical
Joined
Jul 12, 2007
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4
Location
CH
How to rerate an existing 150# line when hydraulic testing is not possible. The design is 10 Bar and water testing @15Bar 30 yrs ago. Due to introducing a slow closing EBV 70 sec. the surge is still 18 Bar. Acc code B31.3 this shall be the new design pressure and therefore water tested again but is practical not possible. Is there a other possibility?
 
30 year old line? What was the design life at construction? Most of the time it is around 20-25 years to keep the wall thickness from being too great (corrosion rate/year * expected life = corrosion allowance). I would bet that as you dig into this you'll find that you are obligated by code to evaluate the line (with something like smart pigging) before establishing a new MAWP, and that you'll also have to do a new static test.

Every time I've ever been asked to look at 20, 30, or 40 year old lines the problems looked like a can of worms that had been run through a blender.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist
 
This overaged line has been monitored by the GMISS-team and the wallthickness is still within the pipe spec rating.
The normal oparating is 6Bar but when the emergency block valve closes at high level high high on the vessel(Probably never happens anyway) we have to take in acount a calculated surge pressure of 18BAR. I'm wondering if an assessment approach can give us the credit to operate this line.
Sounds this familair?
 
It sounds very familiar. The normal answer is "let sleeping dogs lie" because if the line is functioning properly and you have ongoing operational data that says that the 30 year old line still meets mill specs everywhere (which would have difficulty passing the red face test), and the 18 barg surge is theoretical, I would look really hard at how many safety factors were folded into the theory. If you find (as is usually the case) that the surge calcs are ultra-uber conservative (instead of just uber-conservative) then you can get that monkey off of your back and leave the line alone.

If you just must re-rate the line then you have to go through the whole re-rate process, which today's best management practices (BMP) would say that you have to smart pig and hydro it.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist
 
Zdas04, as usual, offers excellent advice here. I confess I don't know what GMISS is and mr google didn't seem to know either, but I'll assume it is some sort of integrity system.

Don't forget B 31.3 offers you up to 33% over the design pressure for less than 10 hours per year. (302.2.4 f)1)a) ) so your comment that "this shall be the new design pressure " is not necessarily true depending on the duration and likelihood of the event.

I don't know any other means of re-rating pipework like this other than a hydro test, but even that depends on what "spare" you have in your wall thickness to up your design pressure, but as noted above, it doesn't have to be to the full 18 bar, just 3/4 of it if this event is very rare.

Basically you need to look at how to alleviate the surge pressure before anything else and make sure it is properly analysed on a transient model, but it seems quite high to me. Surge tanks, relief valves, accumulators etc can all help to get this down to less than the 13.3 bar allowed.

A few more details might help - size of line, wall thickness, flow velocity, temperature, fluid, length, that sort of thing.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way
 
Thank you for the value answers!!
GMISS is a team that outlines requirements to assure that equipment used to process, store or handle chemicals are designed, constructed, installed and maintained to minimize the risk of release of chemicals.
This team did an assesment and this unloading line from ship upto holding tank is still within the pipe spec rating.
I'll will challenge this surge calculations to see how conservative this was done. Probable the closing time can increase as well wich helps to decrease this surge pressure.
Thanks again!
 
Loading lines are notorious for having high velocities and hence high surge pressures, but this should be your first option to reduce this to a level your pipe can accept.

I assume you've checked and the original pipe wall thickness can actually still be within the design of 31.3 for the new design pressure??

70 second closure is very slow - I doubt any slower will make much of a difference to be honest, but you should run this in a transient analysis program. Turing the pump off faster is usually a lot better.

My motto: Learn something new every day

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