Sliding pressure operation on a constant pressure unit

[wayuu1981]

Hello,

I have been reading about sliding (variable) pressure operation and constant pressure operation for load regulation on steam turbines. The way I see it, sliding pressure operation (or hybrid pressure, about 0-90% sliding and 90-100% constant) has some advantages over pure constant pressure for partial load operation. I think constant pressure is a tougher design condition than sliding, and so a Unit designed for sliding pressure must not be operated at constant pressure. But I wonder whether a Unit designed for a constant pressure operation could be operating safely at sliding pressure (or hybrid pressure), what do you think?

By the way, as a reference, my concern is related to a 165MW PC and a 300 MW ACFB coal fired subcritical units, natural circulation, condensing turbine with reheating section and two Main Stop Valves (MSV) and four Control Valves (CV) on HP turbine, and 2 RSV and 4 ICV on IP turbine.

Javier Guevara E.
Projects, Mechanical Engineer

 

[macmet]

I suggest you post this in the B&PV or Mechanical Engineering forum as it gets more traffic.

I work for HRSG company and we have some boilers on constant pressure and others on sliding pressure. We seem to be able to accommodate both methods fairly easily. Unfortunately I do not have a lot of experience working with those types of boilers.

 

[davefitz]

This topic was covered pretty completely in the 1980's- you may want to visit a tech library that has old copies of "the proceedings of the american power conference".

The drum boiler you described might have issues with VP operation- if it was designed for CP ops, then the superheater metal selection would need to be upgraded , the spray water capacity increased, the superheater supports upgraded,and the control logic modified in order to operate at VP.

The issue with drum type boilers is that all the heating surfaces upstream of the drum water separators ( ie the waterwall and economizer) contribute to generating steam , and the supereheaters of course superheat. The raio of those aras was based upon the enthalpy needed to generate steam at the orignal full load operating pressure , typically 2700 psig . If you operate at a lower pressure, such as 1500psig, th optimum area ratio changes. Also, during fast VP load changes, significant overfiring must occur since extra heat is needed to increase the drum pressure ( due to thermal ineratia of waterwalls and the the contained water inventory). The end resutl is that the superheaters get toasty warm during a VP load increase, and you are also likely to run out of spray water capacity and overheat the HP main steam line.

All that for a 1/2% improvment in heat rate. Go figure.

"Nobody expects the Spanish Inquisition! "