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?

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/globle 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?

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

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