Steam turbine heat rate upgrades: Big bang for fewer bucks

There are various upgrades available to reduce steam turbine heat rate. Some come at a high cost but also deliver much savings. But some require little investment but provide substantial benefits. At a presentation at Power Gen International, Muhammad Saqib Riaz, Manager Steam Turbine Engineering at Mitsubishi Hitachi Power Systems explained the upgrades that can help users make choices that provide the most benefits at least cost. Below is a summary of his presentation.

Blade path improvements are produced by F3D blades, updated radial seals as well as control stage optimization.

Improvements in last stage blades include using integral shrouded blades, LP end blade sizing. In some cases, increasing the annulus area of the last stage blades may improve efficiency and reduce exhaust losses.

Seal leakage is determined by the type of seal, the inlet pressure, and the differential pressure across the seal. High pressure, high differential pressure, large radial clearance implies large leakage, typically large heat rate.

Rubs can increase dummy leakage over time. Updating and upgrading the dummy seals can reduce leakages, improving heat rate and performance.

Seal configuration can affect how the heat rate reacts to changes in leakage flow. If an equilibrium or balance pipe is included, changes to either the HP or the IP dummy leakage will have a stronger effect on the heat rate and power, depending on the direction of flow. If the HP dummy leakage is greater than IP dummy leakage and excess HP dummy leakage flows to the HP exhaust, then HP dummy leakage has a stronger impact on heat rate and power. If the HP dummy leakage ins more th IP dummy leakage and the remainder IP dummy leakage flows from the HP exhaust, then IP dummy leakage has a stronger impact on heat rate and power.

Active clearance control on seal is typically used in HP dummy upgrade. Benefits include increased clearance at start-up, preventing rubs. Clearance is decreased during normal operation to minimize leakage.

Leaf seal is typically used in HP dummy upgrade. Benefits include increased clearance at start-up, preventing rubs. Clearance decreased during normal operation to minimize leakage.

HP inlet bushing leakage wastes some of the highest energy steam in the turbine, and has the highest pressure. Changing from bell seal to stack ring provides better leakage protection.

Feedwater heater operation. During normal operation, cutting the high pressure heater can improve power output, but at the cost of heat rate. Heater performance also has an impact on heat rate. Accurate feedwater level measurements can improve heater performance. The further downstream the heater is, the more impact its performance has. The following numbers are for standard configurations of 6-7 heaters.

Feed water low load optimization. For partial load, the extraction temperature tends to be cooler. Feedwater heaters are not able to get the optimal performance. An optional extraction line further upstream of the existing extraction line can improve the heater performance at low load, at the cost of power produced.

Operating valves away from valve points can introduce throttling losses.  Less pressure drop means more available pressure for the turbine.

Attemperator spray flow cools steam that has been overheated by the boiler. Minimizing the attemperator flow will reduce heat rate. Maintaining proper boiler temperature will reduce the need for attemperator spray.

Reducing the condenser pressure is key to improving heat rate. A low condenser pressure will increase the pressure drop across the last stage blade, increasing power output and overall heat rate. Heat rate improvements of up to 1.4% are possible with condenser vacuum boost.

Greater than 0.2% heat rate improvement is obtained by blade path improvement: LP > HP > IP, Condenser pressure boost, feedwater heater optimization, feedwater low flow optimization, valve point optimization and reheater spray optimization.

Heat rate improvements of 0.05% to 0.2% are possible through HP inlet bushing, last stage blade optimization, HP dummy seal 0.2% (ACC seal application) and bearings.

Other methods have only a minor impact on heat rate.

Condenser Pressure (if no modifications are needed), valve point optimization, feedwater low flow optimization, reheater spray optimization, feedwater heater optimization, boiler spray optimization are relatively low cost upgrades. HP dummy seal (ACC), HP/IP/LP dummy leakage reduction, and LP leaf sale come at a moderate cost of more than $500,000. Blade path improvement, last stage blade optimization, HP inlet bushing and others come at high cost (more than $1m).