Calculating Flow Throuh a Control Valve
Calculating Flow Throuh a Control Valve
(OP)
Hello to all, I am recient college graduate (University of North Dakota 2009). I am working in the nitrogen fertilizer business and one of the tasks given to me is to perform a 120 # steam balance over our major users. I am attempting to figure out how to calculate the flow through a pressure control vale that regulates the 120 # head pressure. The valve vents directly to the atmosphere. I am confused when I read about "critical pressure drop" does this mean the discharge at the point of exit of the valve is not actually at atmospheric pressure? Also, I cant find anything on how to calculate this "critical pressure drop" just that its about 42 % of the inlet pressure. So, what I am looking to do is to calculate the mass flow rate of 120 # steam that is being vented based on valve position and I cant find any equation to aid in this calculation. This may sound like a lot but I could really use some guidence on this. Thank you so much to those who respond.





RE: Calculating Flow Throuh a Control Valve
ht
This is a good book, should provide answers and it is free too.
If the outlet is atmospheric, the vena contracta will be negative. Intuitively I want to ask "Really?". If so, why isn't the air sucked into the valve from the outlet?
Life.
RE: Calculating Flow Throuh a Control Valve
Regards
Tarlan
RE: Calculating Flow Throuh a Control Valve
Not exactly. Critical pressure drop is the downstream pressure at which the flow will not increase additionally if the downstream pressure is further reduced.
Re-Phrasing: Flowrate is proportional to SQRT Delta-P until you hit Dp crit. Then the downstream pressure can be anything lower and the flowrate is not affected by additional change.
Imagine a standing shockwave in the valve. The molecules downtream of the shock cannot communicate back up thru the shock for their buddies to "c'mon down-there's plenty of room to expand".
Opening the valve further still gives you more flow.
RE: Calculating Flow Throuh a Control Valve
RE: Calculating Flow Throuh a Control Valve
Other than Emerson, there are also texts by the valve vendors , such as Fisher, that use their proprietary coeficients, but to be non-discrimantory, I use the ISA method.
For compressible choked flow, such as dry steam above 15 psig inlet pressure, the max flow will be a function of the valve Cv ( at current % open) and the valve throat's "Xt". The Xt is a representation of the degree of oblique shock waves formed at pressure ratios less than critical, and will vary according to valve internal geometry.
Some valves , such as CCI Self drag valve, have no acoustic choking, so Xt=1.0, but a typical globe valve has an Xt= 0.85, and a streamlined ball valve may have a Xt=0.15 . So, using the ISA equations, the critical presure ratio is not simply a function of ratio of heat capacities but is also a function of internal geometry.
RE: Calculating Flow Throuh a Control Valve