Steam Conditioning Valve

[Bran8]

Background:
We designed a steam letdown station, which has since been installed and in operation. The system consists of a pressure reducing valve and a de-superheater right after the valve (all in one unit). Inlet conditions are 450 psig, 750 F steam in an 8" pipe to the conditioning valve and is brought down to 12 psig, 275 F with a 28" outlet pipe.
After the conditioning valve there is 30' of straight length of pipe (followed the manufacturers minimum straight length requirements) then there is a long radius elbow and other ~20' of pipe where it ties into the main distribution pipe. Right before the tie in where we have the pressure and temperature transmitters that controls the conditioning valve (again followed the manufacturers minimum pipe length requirements to the temperature sensor to ensure water droplets are not hitting the thermowell and giving false temp reading). My apologies if I am giving too much detail.

Issue:
Using a infrared temp gun we measured the pipe wall temp 5' before the elbow, at the elbow and 5' after the elbow. The temp at the elbow is 35 F hotter than then the temps before and after the elbow. Is there an explanation to why the elbow temperature is hotter? Also is there any research papers or anything to support the explanation?

I've search the internet and this forum with no good explanation to this phenomenon. If the elbow temp was lower then the rest of the pipe, it would be the de-superheating spray water hitting the elbow pipe wall but the opposite is happening.

Thank you in advance!

 

[willard3]

The film coefficient/velocity profile could be less where you are measuring.

 

[EdStainless]

In the straight sections there is a boundary layer with no flow, the steam near the walls will be cooler. In the elbow the flow is turbulent and well mixed, allowing the hotter steam (core flow) to impinge on the wall.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Would imagine all this piping is lagged, so how did you get this IR gun to give you a credible reading? Some exposed section of piping? And given this is a rough IR gun measurement, did you calibrate / compare the readout from this gun against the control temp transmitter further down ?

 

[davefitz]

Is the elbow thermocouple at the extrados (outer part of el) or intrados?If it is at the intrados, the +35F offset seems plausible , implying the water droplets have not completlely evaporated and or separating from the steam due to the elbow centrifugal forces.

There have been several recent papers in the last 10 yrs that use latest lab laser testing techniques that compare the spray nozzle design, droplet size ,and liquid subcooling with the required distance for complete droplet evaporation.

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

 

[BronYrAur]

Just guessing here. Is the steam fully-desuperheated before hitting the elbow. If not, then the temperature profile within the pipe may not be uniform. As previously mentioned, the elbow turbulence could cause hotter steam to reach the surface.

Are the elbow and pipe the same material?