Instrumentos de Pesaje de Funcionamiento Automático, (IPFA)

The state-of-the-art methodology in axial steam turbine thermal design takes into account many factors, including optimal stage-to-stage loading (enthalpy drop). Other design factors include:

• Airfoil shapes used in nozzles, diaphragms, rotating blades/buckets (three-dimensional designs being increasingly used)

• End-wall shape (that is, the inner and outer boundaries of the flow passage

• Blade/bucket shroud configuration

• LP turbine stage wetness removal

• Blade/bucket tie-wire losses

• Interstage sealing

The dry internal efficiency of modern steam turbine sections ranges from the high 80s percentage range to the low 90s percentage range. Wetness losses in both the HP (nuclear) and LP (nuclear and fossil) sections of units reduce this efficiency somewhat, depending on the wetness level.

Individual stage efficiency, particularly in the superheated early stages of the LP turbine, can exceed 90%. However, the overall multistage expansion is always less than the individual stage efficiencies.

Turbine-Generator Condition Assessment – In Service

As part of the overall condition assessment, thermal performance degradation is used as an indirect indicator for identifying problems that may be developing within the HP, IP, or LP turbine steam paths that in some manner inhibit or disrupt the flow so that noticeable losses are produced. These losses are reflected as a higher turbine cycle heat rate or loss of power output.

To facilitate the interpretation of performance-related test data, Tables 1-4 and 1-5 reflect common changes in the condition of different components of the turbine steam path in terms of their consequence on measurable performance cycle parameters. Table 1-6 provides general guidance in terms of the impact on heat load due to various forms of steam leakage into the condenser.

To make this assessment, the most current data should be gathered and recorded using Data Sheet #4. Data assembled on 4 (a) should be trended at the same load point to further assess performance degradation over time. Often, a key in the interpretation of overall performance data is to isolate the cause within the respective section of the overall steam path.

Identifying the source of a suspected flow restriction can be approached in a systematic manner:

• Steam extractions, reheat conditions, and moisture separators are logical cycle points that can be used to identify turbine sections or plant systems that may be deteriorating.

• The first HP (control) stage and last LP stage have a significant influence on the overall unit performance. Although most turbines consist of a large number of individual stages, only the first and last stages tend to be significantly influenced by changes in the flow rate. First stage performance is primarily more sensitive to variations in load. The last stage is more sensitive to variations in both load and backpressure.

• Throttle flow factor is usually an indication of increased nozzle erosion. Trending this value will give an indication of the rate of nozzle degradation that may be occurring for the specific unit. Evaluation of other performance parameters may show other types of deterioration, such as nozzle area closure or significant deposit formation on stationary and rotating blades.

• Excessive erosion on the leading edge of a last stage blade may be an indication of high feedwater levels in the neck heaters or of problems in the moisture removal system in the LP section. If significant erosion is seen, such as 1/8" (3.18 mm) or greater on the leading edge of a last stage blade since the last inspection, attention to feedwater heater controls should be considered along with a detailed inspection of the LP section moisture removal system at the next scheduled overhaul. If numerous tube leaks have occurred in these heaters, immediate actions should be taken to eddy current inspect and plug suspect tubes. A long-term fix may be to re-tube these feedwater heaters.

• Operating at low loads and high backpressures can result in excessive moisture droplet erosion in the last stage blades or the development of fatigue cracks due to a vibratory condition referred to as flutter. This can occur as a result of increased cycling duty. In the current utility financial operating environment, many units designed in the 1960s and 1970s for base load operation are now being run to match peak consumption swings, sometimes on a daily basis. When performing a condition assessment, changes to the condenser system or operating pattern of the unit should be noted, and the condition of the LSB monitored for

Turbine-Generator Condition Assessment – In Service

Performance-related problems such as blade erosion or nozzle deterioration are generally not catastrophic, unless a severe condition is allowed to persist. The incentive to address the thermal performance issues is therefore typically based on the loss of efficiency produced by the

deterioration of the steam path. However, the location and extent of work that may be required to restore a component should be seriously evaluated so that damage that would require only small repairs is not allowed to escalate into a major scope of activities that compound the risk of an extended outage due to unforeseen circumstances.

For example, erosion in the HP nozzles can normally be tracked by periodic visual inspection with corrective actions planned for the next overhaul. Monitoring performance parameters such as those identified in Tables 1-4 and 1-5 can assist in identifying consequences of such erosion.

However, if erosion cuts or erosion are allowed to become deep, the time required for repair will be extended based the amount of weld repair required to correct the problem. The location of the repair may also increase the level of risk assigned to a reliability issue. For example, extensive erosion to the HP nozzle plates can then result in significant erosion to the HP first stage buckets.

Unintended removal of material from the buckets can make them more susceptible to resonant vibration, particularly at certain valve points. It is this type of secondary consequence that should always be considered when assessing the potential need and urgency to address an original problem.

A partial list of research published by EPRI related to turbine-generator performance is as follows (organized by year of publication):

Assessment of Supercritical Power Plant Performance, EPRI, Palo Alto CA: 1986. CS-4969.

Heat-Rate Improvement Guidelines for Existing Fossil Plants, EPRI, Palo Alto CA: 1986.

CS-4554

Fossil Unit Performance: 1965-1984, EPRI, Palo Alto CA: 1987. CS-5627.

Solid Particle Erosion Technology Assessment, EPRI, Palo Alto CA: 1994. TR-103552.

Thermal Performance Engineering Handbook, Volume 2: Advanced Concepts in Thermal Performance, EPRI, Palo Alto CA: 1998. TR-107422-V2.

Turbine Steam Path Damage: Volumes 1 and 2, EPRI, Palo Alto CA: 1998. TR-108943.

Turbine-Generator Condition Assessment – In Service

Table 1-4

Effect of Component Condition Changes on Fossil Cycle Performance Parameters (at Valve Wide Open Operation)

Condition Throttle

Low #/hr PT P1st PHRH PLP H. P.

Efficiency I. P.

Efficiency

Increase TT ↓ N. C. - ↓ ↓ ↓ -

Increase THRH - N. C. - ↑ ↑ - -

Increase A1st

(SPE) in HP

↑ N. C. ↑ ↑ ↑ ↓ -

Increase AHRH

(SPE) in IP - N. C. - ↓ - - ↓

Decrease A1st

(Deposits and Peening) in HP

↓ N. C. ↓ ↓ ↓ ↓ -

Decrease AHRH

(Deposits and Peening) in IP

- N. C. - ↑ - - ↓

Decrease A2nd

(Deposits) in HP ↓ N. C. ↑ ↓ ↓ ↓ -

Increase A2nd

(Rubs) in HP ↑ N. C. ↓ ↑ ↑ ↓ -

Decrease ALP

(Deposits and Damage) in LP

- N. C. - - ↑ - -

Note that every change in turbine condition results in a different three-key pressure pattern.

Turbine-Generator Condition Assessment – In Service

Table 1-5

Guidance for Interpreting Turbine Cycle Steam Flow and Unit Load Changes Type Problem Timing Throttle Flow Section

Efficiency

Electrical Load

SPE Gradual Increase Decrease (-HPη

decrease greatest at light load)

Increase or essentially constant

Deposits Gradual Decrease (may

increase after

outages Decrease Decrease Decrease

Peening (weld

bead) Abrupt following

boiler repairs Decrease Decrease Decrease

Mechanical Failure Abrupt anytime, usually during operation

Usually decrease

Decrease Decrease

Water Induction Abrupt anytime

during operation Slight increase Decrease Decrease

Vibration Abrupt, usually

most severe at first startup

Slight increase Decrease Decrease

Steam Whirl Abrupt at first startup

Slight increase Decrease Decrease

Internal Leakage (Balance Hole Plug)

Abrupt following

overhaul Increase HP turbine –

decrease Decrease

Internal Leakage

(Inner Shell) Gradual Slight increase Decrease Slight increase Broken Valve Stem Abrupt Decrease Decrease Decrease

Turbine-Generator Condition Assessment – In Service

Table 1-6

Effect of Leakage to the Condenser on Heat Rate and Load

Effect of 1% Leakage to the Condenser on Fossil Reheat Origin of 1% Leakage Flow On Heat Rate On Load

Throttle 0.83% 0.94%

HP Turbine Exhaust 0.53% 0.69%

Ahead of Intercept Valves 0.69% 0.56%

Crossover 0.44% 0.44%

Rules of Thumb

1% ηHP = 0.16% heat rate or 0.3% kilowatt 1% ηIP = 0.12% heat rate or 0.12% kilowatt 1% η_LP = 0.5% heat rate or 0.5% kilowatt 1% flow increase = 0.94% increase in kilowatts

1°F (0.56°C) temperature increase = 0.08% decrease in kWs and 0.024% better heat rate (VWO)

1 Btu TEL (total exhaust loss) = 0.1% poorer heat rate 10°F (5.6°C) decrease in TT increases ηHP 0.11%.

A 5% increase in stage pressure flow relationship is cause for alarm.

1% ∆P increase in steam path = 0.1% poorer heat rate

A 1% change in P1st due to a change downstream indicates a 1.5% change in flow for a 1.25 pressure ratio control stage (Curtis stage, not single Rateau stages).

A 1% change in P1st due to a change upstream of the 2^nd stage indicates a 1% change in flow . Bench mark η HP 2% is better than heat balance.

A 10% nozzle area increase due to SPE results in a 6½% loss in stage efficiency for the control stage and 3–4% for the other stages.

A 10% decrease in control stage nozzle area decreases the flow passing capacity by 3%.

% ∆P SV and CV 4% (VWO)

% ∆P IV 2%

% ∆P crossover 3%

A 1% increase in HP and IP turbine stage pressures due to a restriction downstream of the stage results in a 0.6% increase in pressure upstream for an impulse type stage and 0.7% for a 50% reaction stage.

Turbine-Generator Condition Assessment – In Service