Method and apparatus to reduce thermal stress when starting combined cycle power systems

Abstract

Apparatus and method to wet start the HRSG and combined cycle in the fastest time with minimum thermal stress. This wet start provides the earliest possible cooling steam during acceleration, reducing stress in the superheater, reheater and steam turbine. The gas turbine is started and loaded to full power in the fastest possible time without holds. A once-through HRSG filled with saturated boilerwater including the superheater generates dry steam during acceleration. Start apparatus positions the dryout zone in each superheater tube, controls surge swell, ensures uniform pressure rise, and controls the steam temperature. The superheater generates temperature controlled steam to cool the tubes while heating headers. Superheater and reheater tubes and headers start at saturation and increase to operating with minimum differential temperatures. The superheater evaporating boilerwater supplies constant low temperature dry steam to start the steam turbine without the use of attemporators.

Description

BACKGROUND OF THE INVENTION[0001]The invention describes methods and apparatus for rapidly starting combined cycle heat recovery steam generators (HRSGs) and steam turbines while minimizing damaging effects of the thermal stress. Reducing component differential temperatures will decrease or eliminate most of the cyclical-life reducing problems recorded in conventional HRSGs and steam turbines. Thousands of Combined Cycle (CC) power plants have been installed and are operating throughout the world. In the first forty years, most were installed as base load and designed to be started and slowly warmed up a few dozen times per year. Operating experience with these CCs proved that start-stop cycles led to the greatest damage requiring major repairs, replacement and reduced availability for many plants in only a few hundred start cycles. Problems recorded for HRSGs include: superheater and reheater header cracking, weld failures, tube buckling, and drum cracks. This long recorded history...

AI Technical Summary

Benefits of technology

This patent describes a method to start a gas turbine quickly while minimizing thermal stresses on the components. The method involves using a once-through flow HRSG with a horizontal drain manifold and a circuit drain system. The apparatus and start method control the filling of the high pressure superheater with saturated boilerwater to a specific level important to the start up method. This reduces the gas temperature entering the reheater and protects the components from over temperature and high differential temperature thermal stress. The method also generates dry low controlled temperature steam suitable to start the steam turbine as the gas turbine is loaded to full power as fast as possible. The apparatus and method achieve the goals of starting up a power plant quickly without the need for interstage attemperators.

Problems solved by technology

Operating experience with these CCs proved that start-stop cycles led to the greatest damage requiring major repairs, replacement and reduced availability for many plants in only a few hundred start cycles.

Problems recorded for HRSGs include: superheater and reheater header cracking, weld failures, tube buckling, and drum cracks.

This long recorded history has caused reduced availability and expensive repair costs.

Steam turbine components are also at risk, requiring special methods to minimize thermal stress and maintain adequate clearances in starting, warm-up and loading.

High differential temperatures between rotor components and thick case, or across thick walls, cause cracks and rub damage.

This requirement can be damaging to thick wall HRSG components and steam turbines have greatly increased the challenge to solve the drum and header problems For the fast start requirements, after ignition, the exhaust gas rapidly increases in temperature to about 1,100° F. in a few minutes as the gas turbine is rapidly accelerated and fully loaded as fast as possible without a hold.

A major cause of high differential temperatures in conventional HRSGs is that superheaters and reheaters are dry during a start.

As a consequence, differential temperatures cause high header and tube stresses and tube buckling loads as steam flow and tube-to-tube cooling may not be uniform.

When starting the gas turbine and rapidly accelerating to full power the steam produced is too hot to start the steam turbine.

Because steam flow is delayed for many minutes the tubes go to about gas temperature limiting the effectiveness of attemperators until high velocity steam is generated to atomize and evaporate the injected cold water.

Attemperator over-spray of cold feedwater water, and or poor water atomization, has also been identified as another major cause of damaging thermal stress quenching of piping, tubes and headers.

Moreover, the attemperator's hot metal components are exposed to quench cracking and malfunctions in normal operation.

Higher steam temperature and pressure specified to improve efficiency of CCs further increases start problems.

Nevertheless most HRSG designs still retain the 19th century natural circulation evaporators that require a thick wall and its related problems.

The temperature differentials across the thick wall of drums in fast starts typically result in unacceptable cyclical fatigue life reduction for conventional steam drum designs.

Both of these HRSG configurations are specially designed to meet fast start specifications, but they result in extra complications and expense.

A limitation to these modifications is that both have a fixed surface area for superheating.

This limits the HRSG operating range for starting, part load and the ambient air temperature performance optimization.

Additionally, the fixed area superheater requires attemperators at part load reducing efficiency with some gas turbines.

Another limitation of the Benson and reduced diameter drum HRSG start solutions is they both incorporate the relatively rigid thermal harp heat exchanger in their modules.

Although this does not eliminate tube-to-tube differential expansion or tube-to-header differential temperature stress the single row header reduces much of the calculated stresses.

However the design fixes don't solve the basic problems: high differential wall temperatures of drums, headers-to-tube differential thermal stress and the means to control the steam turbine start temperatures without attemperators.

However, vertical tube once-through circuits are not drainable.

Conventional gravity draining dozens of bottom headers each with three drains, plus drums may result in reliability problems to completely drain and dry.

This expensive alloy is competitive in small HRSGs for many reasons including: dry operation, dry starting, self-draining, corrosion protection and simple water treatment.

For utility CCs as disclosed herein, the expensive alloy is cost prohibitive.

However, the start apparatus for level control of boilerwater in the superheater is not described nor is a detailed method to start the steam turbine described.

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