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Posted by Bob D. on April 23, 2008 at 16:04:32:
In Reply to: non return valve posted by tommy on April 07, 2008 at 08:44:15:
Hi Tommy - Just a shot in the dark, but I have a few thoughts on this. Theoretically, you're right on the money. The combination S&NR should stay shut, with the seating force equal to the surface area times the pressure differential across the "flapper". Some guys have always left the stem "open" when the boiler is idled or the plant at cold iron, never bothering to shut it tightly. But I think it is more likely we should be talking about habits, and good practice. (1) Not all plants use combination valves. Some use a non-return and a separate stop (gate). The gate probably can't be opened until the pressures equalize (unless you use a big "cheater", and risk scoring the seats or breaking the stem), so how many trips to the boiler tops do you want to make? One to open the non-return stem and then another to open the stop? I don't think so. (2) Also, in the past few years a lot of really smart people have been researching water hammer events where condensate and pockets of trapped steam interact poorly and "bad things happen". For example, say a small steam bubble gets exposed to some left over condensate. The cooler condensate sucks the heat out of the steam, and the steam bubble collapses. The void left by the steam bubble gets filled by the condensate, and at such a speed that the momentum and energy of the "water hammer" can burst valve bodies and pipelines. I don't pretend to understand it all, but browsing the web sure reveals some scary reports. I think I'd just wait until both sides of the valves were pretty much at the same temperature, then slowly open 'em up. (3) There's also some thoughts on the initial swinging of the "flapper" causing load instability between the boilers, but I haven't though much on this.
Anyway - 85% system pressure on the boiler side before opening the Comb. S&NR sounds like a good idea to me, if I don't have any other over-powering reason. Then again, I could be wrong.