Evaluating thermal shock under Attemperator (Desuperheater) conditions 1

[shmultz]

Hello,

My company measures and analyzes Acoustic Emission in power stations.

We were planning an experiment in which we reproduce thermal shocks which occur in Cold Reheat lines in the area of the Attemperator (Desuperheater) unit.

Thus, we would be able to differentiate more accurately of signals which are generated as a result of water spray hitting the pipe from actual flaws emanating from the thermal shocks the piping suffers off.

We were thinking to heat a small piece of steel pipe with an Acoustic Emission sensor attached to it to a temp. of about 710F with a blowtorch and than spraying it using cold water from a household sprayer until it reaches 640F (imitating true temperature differentials in a real Attemperator)

this is of course just the first stage of the experiment. later on, we are planning to stress the steel pipe in order to also reproduce the pressure in the pipe.

My questions for you, if you will, first is if this course of experiment is close to what happen in reality, and second is how to effectively generate such conditions in a lab, which would be as much as possible close to those in the field.

Much thanks to any help granted,

Samuel,
Margan Physical Diagnostics

 

[davefitz]

Well , if you wanted to be as close as possible to the real thing, just visit your local large coal fired boiler and ask the operators to remove the insulation around the CRH attemporator , attach the sensors, and let' er rip. In general it takes about 0.15 seconds to evaporate most the spray water, so you could calculate howlong a section of pipe to expose. It is also informationve to take an infrared thermograph photo/ movie of that section of piping.

The values you gave (710 to 640 F) are correct for that attemporator- the more interesting case is the spray at the bypass valves that spray from 1050 F down to 350 F- the thermographs are very surprising. Don't even think of calculating the thermal stresses implied by the severe gradients. .

Some other obscure effects are occurring. If the initial pipe wall temperature is more than 50 F hotter than the saturation temperature , then the wall temp is above the "liedenfrost " temperature, which means that the water droplets that hit the wall will bounce off the wall without evaporating, just like spitting in a hot frying pan. Unusual acoustic effects occur due to that effect. Also, for pipes of wall thickness greater than 1" thk, normally one supplies a thermal liner inside the pipe to reduce thermal stress and water erosion. The contraction, and vibration, of that liner provides additional acoustic effects.

 

[shmultz]

Dear Davefitz,

Thanks for the quick reply and help, it is very much appreciated.
could you please also give an estimation of the speed of water sprayed and angle of spraying or do you know where I can get access to such info?

Thanks very much for any assistance,
Samuel.

Best Regards,
Samuel Rosenberg
Margan Physical Diagnostics

 

[davefitz]

The important speed is the speed of the steam that accepts and carries the droplets- not the initial water speed. It variesa at each site. It should not exceed 150 m/sec at attemporators, but one never knows what is being provided by the designer until you make the calculation yourself.