Monoblock isolation joint or isolation gasket plus sleeved bolts?

[ScottyUK]

Electrical engineer straying a little outside his field of expertise so bear with me.

I've received a TQ regarding the type of isolation joints we would prefer to install on the pipework on a liquified gas storage facility. I'm familiar with the old-fashioned type of joint using an isolation gasket and sleeved bolts and have used this in an operational environment on cooling water service in a power plant environment without any difficulty other than inadvertent connections across the joint by instrument impulse lines, valve actuator cable armour, and the like. The pro's and cons of each type of joint in hydrocarbon service is new to me. The process fluid is propane and pressure is up to approx. 40 BarG. Process pipework sizes are in the 200mm - 600mm range, possibly including some small-bore lines too.

Can anyone offer any insight or references which would be the more suitable isolation method in this instance, and why?

 

[cloa]

What is the pipework made of? Alloy steel or stainless steel- stainless steel would be more of a worry.

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[cloa]

What is actually service- liquified propane or chilled or room temperature? Significant downtime expected either before operation or during operation?

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[LittleInch]

Monolithic isolation joints are the isolation joint of chocie as they :
1) Eliminate a flanged joint which is notmally the pipeline side of the ESD / isolation valve
2) Have far higher relaibility in terms of voltage breakdown and lack of galvanic corrosion onthe bolts
3) No crevice corrosion
4) Virtually as storng as the main pipe in torsion, and bending
5) Valid for all presures and temperatures

If the process fluid can transmit electrical current you need to be careful to internally coat the pipeline side to avoid internal corrosion, but that would also affect a flanged joint

The real key is item 1 for most operators as a leak int he flange on the pipeline side of your islation valve is very bad news

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

 

[ScottyUK]

cloa -

I'm not certain but my understanding is that vessels are likely to be alloy steel and pipework will be stainless. Fluid is liquified propane and will normally be at or slightly above ambient temperature although from memory the design window is fairly wide from approx +60°C to -40°C. The vessels and pipework in question form part of a mounded storage facility which is in FEED at present.

LittleInch,

Thanks for the list, that seems to weigh fairly heavily in favour of the monoblock. I assume cost is significantly higher for the monoblock, but are there any technical disadvantages? I agree that minimising the probability of leaks is our number one objective, closely followed by minimising corrosion problems.


Should have stated before that it's an impressed current system and likely to be a fairly powerful one based on the initial estimates of required current from the piping guys.

 

[LittleInch]

They are not as cheap as flanges, but last for 25yrs + so in the long run save you money. Many times problems on cp systems get traced to faulty isolating flanges, usually because some operator replaced the insulating sleeves with normal bolts....

I hope you got the current requirements from the corrosion engineer, not the piping man... High current on a new system is not good, but doesn't mean high voltage all the time.

My motto: Learn something new every day

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

 

[ScottyUK]

Current estimate from the piping / vessels guys. I'm sceptical of their estimate - at least 10x - 20x higher than I expected for a coated vessel - but it's early days and I expect the corrosion engineers to get a more realistic figure.

Longevity will be a big plus from my perspective.