Rerating (additional design) in existing lines
Rerating (additional design) in existing lines
(OP)
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?





RE: Rerating (additional design) in existing lines
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
RE: Rerating (additional design) in existing lines
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?
RE: Rerating (additional design) in existing lines
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
RE: Rerating (additional design) in existing lines
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
RE: Rerating (additional design) in existing lines
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!
RE: Rerating (additional design) in existing lines
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