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How to size steam blowdown line with choked flow?

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jimhokie

Mechanical
Feb 16, 2005
50
I need to size a steam blowdown line to vent a pressure vessel initially at 100 psig. The flow will be initiated by opening a butterfly valve in the line near the pressure vessel. The blowdown line will be on the order of 10 to 15 feet long, then dump into a 36" duct. The backpressure at this point will be on the order of 20 psig. I expect the flow to choke either across the open butterfly valve or where the blowdown line joins the larger 36" duct.

I need to determine what size the blowdown line needs to be (as small as is feasible due to space limitations) such that it can pass at least 350 lbm/sec saturated steam (~40% quality). All the literature I read on the subject deals with pretty nozzles or clean orifices. How do I go about estimating mass flow capability vs. size in a real world pipe with a butterfly valve, a few bends, and an exit into a larger duct? Also, any thoughts on whether the choke point would be at the valve or at the exit into the larger duct?

Any assistance offered will be greatly appreciated!
 
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For simple calculations I use a graph published by "Tube Turns" some years ago. I think Tube Turns was an American Co. Not sure if it still exists.

athomas236
 
Imagine that a 5/16thin in. oriffice will dump 350pph in one hour,
then you will need 60 x 5/16th in hole AREA to accomplish your job. buterfly or not.
 
Thanks for the responses, athomas and GenB. I am trying to locate the Tube Turns reference--I believe I have seen that before.

GenB, are you saying all I need to know is the open area through the butterfly valve and treat it as an orifice of the same area? Is this just a rough approximation of the flow capability, or can it be considered to be reasonably accurate? (Also, in your example, if a 5/16" orifice passes 350 lb/hr, wouldn't it take 3600 x 5/16" area to pass 350 lb/sec, not 60 x 5/16" area? I'm confused about your statement "...will dump 350pph in one hour".

Also, how do I go about determining if it will choke at the valve or where the blowdown line exits into the larger duct?

Thanks again for your assistance!
 
Will find graph and reference tomorrow (18 Feb), its all on the graph

athomas236
 
a butterfly valve is not suitable for throttling a choked flow- you should design the system so that the flow is choked at the outlet of the 15 ft long small bore pipe.

The printed form of the ASME steam tables has a chart in the end that lists the choked mass flow ( lb/hr/ft2) vs inlet pressure and vs enthalpy.
 
Be careful in calculating the line pressure drop with two phase flow.
Also make sure that your back pressure is not affected with two phase flow.
 
davefitz is right as usual

there are usually two points where choking can occur, the valve and the end of the pipe where it expands either into the vessel or the manifold

I usually arrange for the pipe from the drum to be vertical and of a diameter so the static head is greater than the friction loss. In this way the conditions at the inlet to the valve are single phase water. That makes the valve calculations easier.

I usualy make an iterative calculation on flow rate to see which one chokes first the valve or the expansion

athomas236

 
I find it interesting that there are no responses/comments to my post with regard to line flow losses with two phase one component flow.
 
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