Reviewing a traditional block and bleed system it consist of two isolation valves and a bleed valve within the pipeline configuration. Removing these three valves and retorfitting it with a single double block and bleed valve within the steam system seems to be appropriate, especially if fitting such valve in a confined space. However, I have to prove that a single dbb is more energy efficient than the traditional dbb arrangement. Any suggestions?
I guess you are speaking of a single valve with two disks and a bleeder on the body between the disks. I have seen these proposed but I don't believe it fully satisfies the need for positive 100% isolation from energy source. One valve means only one component needs to fail to allow the energy source to pass through. And personally I would not enter a vessel if that was your single isolation point against a hazard.
The energy loss should be negligible. Half of negligible is still negligible.
Given the years of safety history behind the adoption of the double block and bleed safety isolation, I suggest that you abandon your present course of enquiry.
I saw plant down for several days due to an issue that would not have arisen had the original double block system been left in place. Energy savings would not replace the three days lost revenue in 100 years. The cost of parts not installed was, well, negligible.
And on the fourth day, there was a new face in place of the man responsible for the mess.
Bill
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"Why not the best?"
Jimmy Carter
I don't think the advantage of double block and bleed valves lies in energy efficiency, and telling you to make your case based on that measure is a bit like trying to choose to drink Bunnahabhain instead of Teachers on the basis of which is the less ozone depleting.
The DB&Bs I've seen are two interlocked ball valves with an intervening bleed valve all in a single body small enough to sit in place of a traditional single isolating valve. The isolating valves are sufficiently separate to give you effective two valve isolation.
By retrofitting them in HP air systems, we're now able to achieve both elements of a two-valve isolation scheme at a single location (which is great, because it restricts the amount of the system which has to be taken out of service, and the amount of pipework which gets depressurised and will inevitably leak when subsequently repressurised).
You minimise the risk of having the kind of experience WAROSS had by fitting DB&B's at all the locations previously fitted with single isolation valves. This buys you flexibility at the expense of significant capital outlay. For us, with a complex ringmain system with numerous branches, distributed consumers, storage and supplies and a requirement for damage tolerance and reconfigurability, it really makes sense. I can see simpler systems where you really don't need that flexibility and where the up-front cost of all that extra valvery is never going to be justified.
