Calculating Steam flow through and open ended vent pipe

[AlbertaMecchie]

Hello,

I am looking for some guidance wih respect to calculating steam flow through an open ended vent pipe.

The vent is from a superheater header at 2000 psig and 1000 F. The vent pipe is a 2.5" XXS 122 foot long pipe vented to atmosphere. At the upstream end of the pipe there is a 2.5" valve, likely a gate valve.

Also looking for some help determining the pressure in the pipe 10 feet upstream of open end.

Thanks,

 

[georgeverghese]

Depends on what percent of the full stroke of the valve it will be opened. Do you have a graph of the gate valve Cv vs % stem travel ?

Beyond a certain % of stem travel, the velocity of the steam through the vent pipe will be unsafe. A safer option, if this vent line is going to be a permanent fixture, is to install a safety restriction orifice sandwiched between the gate valve flange and its mating flange on the pipe that will restrict flow to a safe value so that the vent pipe doesnt shake itself to death. Obviously, you will also have to get a piping engineer to design some supports for this vent line.

You also need to tell us if you want to limit the throughput to some safe value where the noise is acceptable, or if you want the throughput to be much higher, but still below high vibration limits. If you go for this much higher throughput, then approved ear muffs will be required for several metres radius around the vent exit.

Also send us a diagram of the vent line arrangement with elbows and turns included, including details of this location 10ft from the exit.

 

[davefitz]

You can use the same method that is recommended for safety valve exhaust vent piping contained in B31.1 or section VIII. The flowrate is based on choked flow thru the minimum flow area which is likely a globe root valve, and that valve's Cv and Xt together with the choked flow equation contained in the ISA handbook for control valves gives the flowrate .

Once the flowrate is known, the pressure at the outlet of the vent can be determined based on Fanno choked flow at that pipe's open area at a mach number of M=1. The pressure at the inlet of the vent pipe ( outlet of the gate valve) is based on the pressure ratio Pi/Po obtained from the referenced code or from Fanno flow rules.

If in fact only a single gate valve was supplied instead of a combination globe + gate valve and its internal geomentry is more constrictive than a smooth bore pipe,then there will be another oblique shock wave across the valve .

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[goutam_freelance]

Refer link below:

https://www.eng-tips.com/viewthread.cfm?qid=476634

You can formulate an iterative approach by assuming different mass flow rates. You know valve throat area and upstream conditions. With the assumed flow you can calculate the Mach No downstream of valve which is inlet to pipe, With this inlet Mach No you can calculate the length required to achieve M=1 at outlet. If this L is less than pipe length then increase flow and do another iteration.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[LittleInch]

Jon,

You will not get anywhere until you decide how much flow is going down your vent pipe or what is going to be your limiting factor.

If the valve opens fully ( and a gate valve is not much good at anything else), then you will be hitting sonic flow at the end after not very much opening of the valve. This will make a VERY loud noise and possibly a huge amount of vibration.

Your vent pipe might not even survive unless it is dead straight and even then it's not a good move.

So please go away and do some thinking about this or give us the calculated / required steam flow in mass/sec and we might get somewhere.

Why you're interested in a pressure 10 ft from the end of a 122 ft pipe also needs some explaining.



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