Supercritical Steam Blows

[hacksaw]

What are the particulars of supercritical steam blows used for line flushing?
Only involved indirectly at the time, and after the fact, but was puzzled at the fluid velocoities claimed.

 

[davefitz]

It has been a while since I have witnessed this, but modern supercritical steam generators do not have any valves installed in the "boiler" ( aka steam generator) HP section. The objective of steam blowing is to generate more momentum in the flowing steam than is created during full load operation, to strip away any internal deposits from main steam line, cold reheat pipe, and hot reheat pipe. This can be accomplished by using a lower pressure steam than normally used at full load while minimizing the amount of fuel combusted during the steam blows. I suspect one starts by calculating the full load momentums for each of the 3 pipes, then calculating the required lower steam pressure needed in each pipe under the assumption the steam generator is being fired at 10-15% MCR capacity. A temporary pressure reducing valve would need to be installed in the inlet of the HP main steam line, and the HP bypass valve be controlled so that the ratio of inlet P to outlet P is at least 2:1.

An alternate solution is to use air blows, as currently accomplished on combined cycle power plants. To minimize the size of the air compressors , a large air pressure vessel can be used between the air compressors and the lines to be cleaned. There would need to be a fast acting stop valve between the pressure vessel and the line to be cleaned. Large air pressure vessels can be built in the field using "ribbon wound " technology, but it probably is more economical to ship a 10'-12' dia pressure vessel and re-use it at later jobs.

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

 

[hacksaw]

Fully aware of the need for steam blows, and saw what happened when a plant the piping was clean over the objections of the turbine supplier. The had to replace the turbine, and consented to a steam blow for the restart.

The supercritical blow techs, claimed that supersonic flows were achived in the piping, not at the discharge. Claiming it was supported by CFD calculations, inspite of my skepticism. Apparently the entire boiler contents were dumped over a 20 minute period.

Was aware that supercritical flows can actually heat up as the pressure drops, but their claim still surprised me, thus the post.