Issues associated with two-shift operation of combined cycle generating plant

Abstract

Diverse experiences associated with two-shifting of conventional generating units are largely relevant to the ‘balance of plant’ in combined cycle generating units (CCGTs). Although two-shifting has adversely impacted upon reliability and availability, and incurred substantial additional maintenance costs on many conventional units, this has not been the case for all installations. Reference 1, for example, reports that four large coal-fired units, which have been extensively two-shifted for more than 15 years, have experienced no major and just a few minor problems attributable to many shutdown-startup cycles. These units are capable of more than 200,000 hours running with more than 6,000 starts; only a small additional maintenance cost is attributable to two-shifting.Startups and shutdowns impose more onerous conditions on critical parts of many components of most fossil-fired generating plant than experienced when operating continuously at maximum rating (the steady-state condition at which unit and component design verification analysis is generally performed). The generic root cause of many of the major and the miscellany of lesser problems attributable to two-shifting has been a general omission by equipment designers, architect-engineers and purchasers to determine, during the design phase, (i) realistic transient conditions imposed on various components during unit startups and shutdowns. and (ii) the consequential life expenditure from cyclic thermal-mechanical cyclic loadings and other mechanisms of material degradation caused by mechanical load cycles.It is feasible to design CCGTs capable of very extensive two-shifting without thermally induced failures in the balance of plant [2–4]. However, CCGTs do have additional difficulties to understand and manage by appropriate design and operation. For example, CCGTs:have to consider the additional operational and cost implications associated with two-shifting the gas turbine (GT);have more complex cycles, which are additional complications to operation during shutdowns and startups;impose more severe conditions on the superheater and economizer of the HRSG during both the shutdown and startup process than is experienced in conventional boilers.Therefore CCGTs require even more design attention than conventional units to enable them to twoshift without adversely impacting reliability and availability and without incurring excessive additional maintenance costs for the balance of plant.For either a CCGT or conventional coal-fired generating plant, one of the requirements for troublefree two-shift operation is that all parts subjected to significant heating and/or cooling during CCGT startups and shutdowns require good thermal flexibility. Thermal flexibility in the critical parts of the plant is achieved by a combination of:appropriate conceptual and detailed design of the key equipment; i.e., the GT, steam turbine (ST), and HRSGappropriate design of key auxiliary systems which influence the conditions imposed on HRSG and ST;unit operating procedures for CCGT shutdowns and startups that minimize thermal stresses in all critical parts of GT, ST and HRSG.The importance of having good thermal flexibility to minimize distress from thermally-induced low cycle fatigue (LCF) caused by unit shutdowns and startups has been well-publicised. There have been many failures of tubes at attachments to superheater and economizer headers most of which are attributable to transient high stresses developed during startups (in some cases in less than 200 startups). Despite this, CCGTs continue to be installed with design weaknesses that would not be difficult nor costly to correct on a new installation, and operating procedures unnecessarily harmful to the HRSG continue to be widely used.Good thermal flexibility is only one of the crucially important objectives for CCGTs intended for twoshifting. This paper discusses several other technical challenges to be overcome for a CCGT to provide high startup reliability without adverse impact on CCGT reliability and availability. It also addresses the minimum additional maintenance cost for the ‘balance of plant’ that is attributable to two-shifting.