By pass system for steam turbine

[engy74]

I have to study a reducing and cooling station for a steam turbine by-pass system.
The reducing plant is controlled in pressure and temperature; it uses water, which injected at a certain temperature via spray nozzles with fixed area, to reduce the temperature of steam.
I want to understand the distance at which to put the PT 100 to measure the temperature on the reduced steam: but, for this purpose, I need to know the time necessary to evaporate the water flow completely, from the initial situation of water to the final situation of steam.
Does anybody know if exist a way to calculate this value?
Thank You very much

 

[TBP]

You're looking at installing a de-superheater. This technology has existed for a very long time, so there's no need to re-invent the wheel. Talk to some de-superheater manufacturers. They should have this information, along with lots more.

 

[davefitz]

The value used in the past was a steam transit time of at least 0.10 sec to evaporate all water droplets, assuming the final superheat after spray was at least +10 F.

There are other correction sometimes used. If the spray water is cold condensate at 100 F , rather than heated water from the BFP at 300F, then additional transit time is needed. The measuring thermocouple should be at least 0.15 sec transit time downstream of the spray nozzle.

 

[arup213]

hi,
u r talking of a desuperheater, for that u need to contact the vendor for it, they would suggest the distance, as because after the desuperheater , there is a 2 phase flow for which there is a high chance of erosion of the PT if u put it near to the DS. my guess is about a dist. of 4-6m dowstream.

regards,

arup

 

[davefitz]

Actually, the distance recommended by Fisher Contec , and other vendors, is 0.1 sec min transit time to first downstream elbow ( which uses 1% Cr to limit erosion-corrosion) and 0.15 sec mintransit time to the monitoring thermocouple. Alternately, if one wishes to spray to below saturated conditions, a downstream thermocouple is not used and then one computes an on-line real time heat and mass balance . In that case, you monitor the spray water flowrate, and the steam flow is calculated using the chike flow equation based on monitored bypass valve position (z), upstream Pi and upstream Ti:

steam flow= C*z*Pi/SWRT (Ti+460)(assumes linear valve characteristic)
inlet steam Hs= f( Pi, Ti)
spray Hw = constant