Isolating a flange on an outlet line from a water storage tank

[Faisale]

Hi,

Would just like information please on whether or not it is a ASME/ISO standard requirement to have some kind of isolation for a flange (for ability to take it offline for maintenance) or just an industry practice.

I currently am in discussion with the site engineer that the firewater tank's outlet line (4"-flange-2"-pump) should have some kind of isolation mechanism for the flange in between the 2 lines(i.e. a valve to be able to stop the flow from the water tank before the flange), since with its current assembly, the whole water tank needs to be stripped if any leakage happens or recertification is required for the flange. His argument is that it is a low pressure activity and leakages rarely happen (17m high water tank)

Would like your thoughts on the above and thanks in advance.

 

[LittleInch]

Up to the buyer / owner.

There is no "standard requirement", just good practice. Is this on the line to the pump inlet? and there's no valve?

please draw a sketch or schematic or a drawing so we can be sure what you mean.

For the sake of a low pressure 4" valve you will waste more money talking about it than it costs to supply it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Faisale]

Hi and thank you for your reply.

I'm sorry for the low quality picture, but this is basically the idea, the outlet's set up currently has no way of the isolating the flange if one is to repair/change it.

Therefore, once the water tank is full, the only way to do any kind of maintenance on the flange is to completely strip the tank and disconnect it.

Hope this clears the idea.

 

[LittleInch]

Flanges are highly reliable and if it doesn't leak at the start, is very unlikely to leak or be "recertified". If it leaks tighten it up.

But essentially this falls into the maybe not great but nothing really wrong with it category. No prescriptive requirements AFAIK.

If there was a valve there there would also be a flange which if it leaked on the upstream side is exactly the same. At some point you either need a welded connection (rare) or still have a flange connection that if it leaked would need the tank drained to get access, or some sort of complicated plug device.

But you have a 17m high tank for a 2" outlet? Are there other outlets on this tank?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Faisale]

There are other outlets yes,

I appreciate your answer thank you very much.

 

[LittleInch]

The key issue normally is how long is your 2" pipe before you get to an isolating valve.

The risk of leak or damage to the pipe is far higher than a flange issue, but if you get a leak then you can't do anything until the water runs out.

If this is the only fire water tank then you might need to shut the entire plant when you take it down or arrange alternative fire water supply.

And all for a 2" valve?? I really don't understand the site engineers issue here.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

I agree with LittleInch ...

An old general rule of thumb for process plant design is to locate an isolation valve close to any storage tank outlet, The valve is supposed to allow work to be done on the downstream part of the system and protect against a site flood.

Tank flanges and tank piping systems do not suddenly spring leaks or normally require maintenance.

The problem, of course, with these rarely used tank isolation valves is that they sometimes do not work when needed ....

I am aware of maintenance programs that periodically (yearly?) cycle these isolation valves to keep them alive.

We will now get "the rest of the story" from the original poster ....

MJCronin
Sr. Process Engineer