Pressure reducing and desuperheating steam velocity and straight length pipe requirements
Pressure reducing and desuperheating steam velocity and straight length pipe requirements
(OP)
I currently have a project I'm working on which is an existing power plant designed to asme section 1 that has some letdown valves that have desuperheating capabilities. The pressure on the high side is 1500 psig and the low side is 425 psig. Boiler feed water is introduced within the valve to reduce the temperature of the steam to 700 deg F. The power plant is existing and I'm assuming the design velocity upstream and downstream of the valve was 200 ft/sec. I'm working on trying to increase the capacity through the valves which would require increasing the line size to maintain the 200 ft/sec velocity. My question is what is the velocity range you should keep the flow downstream of the let down valve for the boiler feed water to properly reduce the temperature? Also, is this in a standard or a guideline somewhere? One more thing; I'm also concerned about the straight length of pipe required as well as whether a reducer impedes the de-superheating.





RE: Pressure reducing and desuperheating steam velocity and straight length pipe requirements
RE: Pressure reducing and desuperheating steam velocity and straight length pipe requirements
Maybe i didn't explain this well in my original post, but what I'm trying to do is increase the mass flow rate through the valve. The system is existing and since my mass flow is increased now my velocity is going to increase downstream, and I'm concerned that the velocity will be too fast for the water droplets to evaporate properly to cool the steam. That is why i asked if there is a velocity range you need to keep the steam at for the boiler feed water to perform the de-superheating, and what straight length of pipe will be required for the de-superheating. Is it based on the velocity (i.e. .1 sec * velocity (ft/sec)). If the velocity is indeed too fast then i will need to replace the existing piping with larger pipe to keep the velocity below the max level.
One more thing to mention. These are turbine bypass valves. This is medium pressure supply steam to the plant.
RE: Pressure reducing and desuperheating steam velocity and straight length pipe requirements
RE: Pressure reducing and desuperheating steam velocity and straight length pipe requirements
RE: Pressure reducing and desuperheating steam velocity and straight length pipe requirements
In my opinion, I would go with a seasoned DS vendor and take his recommendation for the installation.
There are trade-offs on design: pressure drop, price, erosion resistance, turn-down etc. etc.
https://www.s-k.com/pdf/6_desuperheater_bulletins_...
http://www.graham-mfg.com/usr/Product%20Manuals/IO...
http://www.spiraxsarco.com/Resources/Pages/Steam-E...
You also may want to consider a change to a higher alloy piping material downstream of the DS..... This is an area prone to erosion and cracking.
There is a lot to consider here ...
MJCronin
Sr. Process Engineer
RE: Pressure reducing and desuperheating steam velocity and straight length pipe requirements
RE: Pressure reducing and desuperheating steam velocity and straight length pipe requirements
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Pressure reducing and desuperheating steam velocity and straight length pipe requirements
Thank you MJCronin for the reference links. I will review the information. The existing piping downstream of the valve is currently P91 and if we were to increase the line size we would replace the material in kind.
Thank you also Quark for your responses.
RE: Pressure reducing and desuperheating steam velocity and straight length pipe requirements
RE: Pressure reducing and desuperheating steam velocity and straight length pipe requirements