Block Valve and Rupture Disc: how can I avoid overP?

[BNJR]

Hi, I have this one rupture disc (set at 2.5 kscg) in a line with 3.0 kscg Steam line for extinguishing internal fires in a Tank. The disc porpouse is to avoid steam contact with tank's atmosphere, so I have a block valve normally closed upstream the RD to be opened when fire is detected. Is there any means (apart from excess flow valves) for avoiding overpressure in the middle of the two devices? Should it be enough with a TSO valve?

Any guidance is more than welcome

 

[EGT01]

BNJR,

I can't say I have any good suggestion for your problem but here's some observations to think about if you haven't already.

Just to re-state what I think you said, the rupture disc is not intended for overpressure protection but for temporary isolation. The disc is purposely set below the operating pressure of the steam so that it will burst when you open the steam valve.

When you say "avoiding overpressure", I assume you mean avoiding pressure buildup between steam valve and rupture disc such that the disc bursts before it is intended, for example, if the block valve leaks through. In that case, the disc alone can not provide temporay isolation and requires additional means to achieve what is desired.

A block valve + rupture disc + excess flow valve arrangement sounds similar to a double block and bleed system. A double block valve arrangement would seem to be less of a maintenance problem since you would never have to worry about replacing a rupture disc but maybe it is not quite as quick as having to open a single block valve in response to a fire event. If the downstream block valve was a quarter turn ball valve, maybe that wouldn't be so bad. Just be sure your bleed valve or excess flow valve was vented to a safe location.

When you ask if a TSO valve is sufficient, are you referring to a tight shutoff valve? If so, and you deem TSO sufficient to prevent blowing the rupture disc, then it would seem the TSO valve alone could be used without the need for a rupture disc.

Is it possible to use some other form of isolation device rather than relying on one that is pressure sensitive? For example, could a fusible plug be used that would be sensitive to fire temperature but not steam temperature?

 

[MortenA]

Are you concerned with the valve leaking?

The easiest solution would be to make a small "hole" in the line. The steam loss when the line is in actual use would (propably) be small. The hole might however be a safety risk if the steam escaping could hit something/someone. This should be considered.

This is more or less the same advice as given above.

Best regards

Morten

 

[sailoday28]

What is a "TSO"?
Similar to the above, consider:
If the steam valve leaks, won't heat transfer between the steam valve and rupture disk result in condensation. A trap in the line could allow no loss of steam and no excessive pressure build up.

 

[hacksaw]

the space between the RD and valve is traditionally vented to a safe place.

 

[sailoday28]

sailoday28 (Mechanical) Sep 29, 2004
What is a "TSO"?
hacksaw (Mechanical) Sep 29, 2004
the space between the RD and valve is traditionally vented to a safe place

Then why is there a problem with leakage?

 

[BNJR]

First of all, thanks to everyone for replying... and sorry for the "TSO" thing, as EGT01 said, that means Tight Shut Off (we are used to call it that way and extended to the rest of the world )
The configuration for isolation is a must in the line, so I should better work on that (anyway i'll note other options down for anytime in the future)
Actually i have this 3/4" weep hole and a 3/4" drain (normally close, of course) in between the valve and the RD, but i don't know yet if the RD supplier considers those as a guarantee for avoiding the pressure buildup issue. Those are intented to eliminate condensates in the line but I believe that pressure build up caused by leakage would probably make the disc burst before heat transfer leads to condensation and then liquid removal.

Thanks again

P.S. i must confess that i didn't think that with a TSO valve i wouldn't have no leakage problem ...

 

[sailoday28]

BNJR (Industrial) writes:
Actually i have this 3/4" weep hole and a 3/4" drain (normally close, of course) in between the valve and the RD,
Is the weep flow, in effect the flow from the blocked valve?
If these flow are blocked, then in time the block valve downstream pressure will build up and tend to equalize with that of the upstream and burst the RD.

With a small vent line and a trap for condensate upstream of the RD----An energy balance can determine the pressure upstream of the RD