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Re: Orifice Plate Steam Flow Meter Question

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Posted by Bob D. on November 18, 2005 at 10:57:43:

In Reply to: Orifice Plate Steam Flow Meter Question posted by Ben on November 18, 2005 at 07:43:18:

I'm not aware of a specific "Code" (B31.1/31.9, API, etc.) for this, but there is "accepted practice". Is this an existing installation that has gone through several/many warm-up cycles before? If yes, I wouldn't worry about it excessively other than to ensure that the Operators know to warm the lines up slowly and thoroughly before establishing "normal" flows. I would like to see an offset and drip leg above the orifice (just beyond the required straight pipe distance) purely to ensure that the whole downstream pipe system doesn't have to drain back through the orifice, just the minimum straight pipe above the orifice. I would have a concern about installing a trap leg just above the orifice, within the minimum straight length, for a couple of reasons: (a)I might disturb the pressure gradient that the orifice is establishing. That would make my meter readings even less accurate across the board. Maybe a weld-o-let with a nicely finished inner surface wouldn't make that much difference, it all depends on how much accuracy I really have to have. (b) The lower edge (invert) of the trap leg would have to be right at the plate surface to make much difference. That's just not physically possible with the "upper" orifice flange. So, assuming this installation uses flange taps, I might consider adding a tee on the upper tap take-off, before the condensing pot, and running that to a trap. This allows the wet leg from the condensing pot to the transmitter to remain flooded & cool, and allows draining of the "dry leg" between the flange tap and the pot under "flooded" conditions, and it's out of the steam flow path. When the trap cycles, as it normally will, I would see a flow "spike" from "low leg" level/pressure fluctuation, but at least I would know what that small anomoly is from. Just thinking out loud . . . . - Bob D.

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