26 Sep 01 9:21
A few more thoughts.
The design of pulverized coal fuel piping systems is an art as well as a science. Typically, these systems are highly articulated as most power boilers are "hung" from the top of the structure and the vertical (down) movement of the burners as the boiler goes from cold to hot is significant. Special in-line pipe couplings (Dresser or Grey-lok) are included in the design to allow for the movement a (and to essentially release a lot of the torsion in the pipe by axial rotation).
You will notice that these systems are explicitly excluded from the scope of the B31.1 Power Piping Code (see B31.1, paragraph 100.1.3(K)) and reference is made to NFPA Standard 8503.
These systems are complicated as the various pulverizer mills have to supply any of several burners under various operating modes - this leads to a "maze" of piping which all has to be supported. "Box gang" supports are commonly used to support several "runs" of pipe at one point - some of these include spring hangers. Most of these systems employ large "constant force" spring hangers located at the "four corners" of the boiler above the burner elevations.
The pulverized coal is very abrasive, especially at changes in direction. Over the last 20 (or so) years, there has been a trend to replacing the worn pipe elbows with steel, ceramic lined (internally) elbows which are much heavier that the original (or even the most resent replacement)components. Unfortunately, sometimes these new components are installed without a proper structural analysis and the result is that the original pipe supports (and the structures that they are attached to) are over stressed. Usually, the constant force hangers are "bottomed-out" and the special joints located in-line are forced to try to accommodate rotations beyond their design capacity. The loadings placed on the burners and the mill nozzles under these circumstances are very high.
There is a need to perform comprehensive structural analyses on pulverized coal fuel piping systems whenever changes and or updates are made.