DOI 10.1007/s10749-015-0540-3 Power Technology and Engineering

Vol. 48, No. 5, January, 2015

CALCULATING THE EQUIVALENT SERVICE LIFETIME OF POWER GENERATING UNITS IN THERMAL POWER PLANTS1,2 R. Z. Aminov,3 A. F. Shkret,3 and M. V. Garievskii3 Translated from Élektricheskie Stantsii, No. 8, August 2014, pp. 16 – 18. To evaluate the effect of variable loads on the operating efficiency of generating units at thermal power plants we propose using an indicator of the effective run time of the turbine. Formulas are given and an example of calculating the equivalent service lifetime of a power generating unit for different loads is provided. Keywords: power generation unit; steam turbine; operating modes; service lifetime; equivalent operating time

of type i, h; ni is the number of startups of type i; Y is the total number of operating modes with different loads; bj is a time coefficient for the equivalent run time for operation with load j; tj is the run time with load j (e.g., with stepped variation of the load beginning at the base level and ending at the engineering minimum). The coefficients ai and bi, which characterize the influence of various operating conditions on the lifetime of a steam turbine, are estimated using an indicator of the total damage dS (corresponding to breakdown of the turbine) that is determined by summing over the damage factors [2, 3]

Quantitative evaluation of the reduction in the economic efficiency and reliability of the power generating units in thermal power plants operating in variable modes is of great significance for predicting and setting up the structure of power generation in electric power systems, establishing the need to introduce peak and semipeak power sources, and choosing the most favorable combination of working equipment, optimum repair cycle duration, and establishing valid methods for support of base loading of nuclear power plants. For evaluating the effect of variable loads on the reliability and operating efficiency as applied to the operation of steam-turbine power generating units we use an approach that involves calculating the equivalent service (run) lifetime of a unit in different operating modes. Compared to its base operating mode, running the generating units of thermal power plants in variable modes leads to accumulated damage in the equipment. The standards document GOST 52527–2006 of the Russian Federation [1] is based on the use of the concept of equivalent service life (run time) for determining the periods of engineering maintenance and repair, as well as for predicting the service lifetime. The following expression is proposed for calculating the equivalent service lifetime Teq [1, 2]: N

Y

i =1

j =1

Teq = å a i n i + å b j t j ,

2

3

Ni

i =1

[N ]i

dS = å

Y

t op j

j =1

t*j

+å

,

(2)

where Ni is the number of load cycles of type i over the specified service time; [N ]i is the permissible number of load cycles of type i based on the design curves for low-cycle is the steady state run time over the prescribed fatigue; t op j service lifetime for nominal load j; t*j is the time to failure for nominal load j determined using the endurance limit equation. The time coefficient for the equivalent service lifetime for each type of startup can be defined by [2]

(1)

where N is the total number of startups of the various types; ai is a time coefficient for the equivalent service for startup 1

N

ai =

In order of discussion (From Editorial Board). This work was supported by the Russian Foundation for Basic Research, grant No. 11-08-00052a, “Development of methods for system studies with a search for effective ways of ensuring base load of newly introduced nuclear power plants.” Saratov Scientific Center, Russian Academy of Sciences, Saratov, Russia; e-mail: [email protected]

Tlt

1 , d S [N ]i

(3)

where Tlt is the lifetime of the most important part of the turbine, the rotor, as set by engineering conditions during manufacture, h. 391 1570-145X/15/4805-0391 © 2015 Springer Science + Business Media New York

392

R. Z. Aminov et al.

The time coefficient for the equivalent service lifetime for each operating mode is defined by bj =

Tlt 1 . d S t*j

(4)

It should be noted that the lifetime of a generating unit is customarily equated to the lifetime of the turbine and its lifetime, in turn, is equated to the lifetime of the high-temperature rotor. It is assumed that replacement of this most important and costly component of the turbine makes it unprofitable and inappropriate to extend the operating lifetime of the other parts and components of the unit. We note that the nominal (as set by engineering conditions during manufacture) lifetime of the high-temperature components of the turbine is equated to the lifetime of the stock, which assumes production of similar design types in terms of construction, materials, and operating conditions for thermal power engineering equipment such that accident-free operation is ensured.4 For an example of calculating the equivalent lifetime we use the following data. According to the standards document GOST 24278–89 [5], condensation turbines must be designed for a total number of startups over their operating lifetimes of at least: 1000 from an uncooled state (after stoppage for 24 – 55 h), 2000 from a hot state (after stoppage for 5 – 8 h), and 100 from the cold state. The commercial (nominal) lifetime of 50 – 300 MW turbines for steam parameters of 13 – 24 MPa is 220,000 h [6], i.e., Tlt = 220 × 103 h. The designated service lifetime of a power generating unit (for its important components) corresponds to an accident-free running time which is calculated based on the nominal endurance limit sen. The standards for estimating endurance require that the endurance limit óen for a given service lifetime should be a factor of 1.5 times the permissible stress óps, i.e., óps = óen/1.5 [7]. The time to failure will exceed the designated service lifetime by a factor of 1.5, i.e., for operation at base load t*base = 330 × 103 h while for operation with daily unloading t*unl is estimated to be 280 × 103 h (based on operating data). We take the endurance reserve with respect to the number of cycles to be 10 (i.e., [N ]i = 10Ni ) [3] and the run time in base mode to be 17 h/day (0.71 of the total operating 4

The service lifetime of equipment can essentially be extended to infinity if the equipment is subjected to timely, high-quality technical inspection and its components are repaired or replaced in timely fashion upon completion of their operating lifetime. For example, three units of the Konakovskaya GRES power plant, which have run for almost four decades, have been modernized with replacement of worn out equipment, including the turbine rotor. Besides increasing reliability, this has raised the power of the individual units and improved the technical and economic performance. Here it is assumed that the service lifetime of the renovated equipment will be at least 40 years (http://nedvizhimost-konakovo.ru/konakovo_gres.html).

time) with unloading of up to 50% of the nominal power for 7 h/day (0.29 of the total operating time). For these initial conditions, Eq. (2) yields the following total susceptibility to damage: dS = +

100 1000 2000 + + + 1000 10,000 20,000

0.71´ 220´ 103 0.29´ 220´ 103 + 330´ 103 280´ 103

According to Eq. (3), the coefficients of equivalent run time for the three types of turbine startup over the designated run time [5] are a1 — 220 h for a single startup from the cold state; a2 — 22 h for a single startup from an uncooled state; and a3 — 11 h for a single startup from the hot state. The coefficients for the equivalent service lifetime for the base load and unloading modes to the technical minimum according to Eq. (4) are bbase — 0.667 h for one hour of operation, and bunl — 0.786 h for one hour of operation. For these initial data and the resulting coefficients for each of the of startup and loading modes for the turbine, Eq. (1) yields an equivalent service lifetime for the turbine of Teq = 220 × 100 + 22 × 1000 + 11 × 2000 + 0.71 × 0.667 × × 220 × 103 + 0.29 × 0.786 × 220 × 103 = 220 × 103 h. As an example we have calculated the equivalent service lifetimes of a 300 MW power generating unit with obligatory 100 startups from the cold state for the following operating modes in the course of the nominal service life of 200,000 h [5]: 1 — operating in base mode for 7000 h/year; 2 — daily unloading for 7 h to 50% of nominal load; 3 — 7 h stoppage for the night per day; 4 — stoppage for Saturday and Sunday and unloading for 7 h on weekdays; 5 — stoppage on Saturday, Sunday, and at night on weekdays; 6 — limited startups: 1000 from an uncooled state, 2000 from the hot state, and the rest of the time the unit is run with base load; and, 7 — limited startups: 1000 from an uncooled state, 2000 from the hot state, and the rest of the time the unit is unloaded to 50% of nominal load on Saturday, Sunday, and at night on weekdays. Table 1 lists the calculated equivalent service lifetime of a turbine with an obligatory 100 startups from the cold state, a nominal lifetime of 220,000 h, and a design time to failure at base load of 300,000 h. Table 1 shows that the equivalent service lifetime exceeds the nominal lifetime of the turbine only in the variant

Calculating the Equivalent Service Lifetime of Power Generating Units in Thermal Power Plants

393

TABLE 1. Calculated Equivalent Service Lifetimes for Different Loading of a 300 MW Unit Number of startups from states hot

uncooled

hot

Average annual use of installed power, h/year

Equivalent service lifetime

Teq/Tlt

100 100 100 100 100 100 100

— — — 1302 1302 1000 1000

— — 9114 — 6510 2000 2000

7000 5979 4958 4270 3542 5028 4285

166,739 174,271 224,771 159,409 195,492 169,906 175,382

0.758 0.792 1.021 0.724 0.888 0.772 0.797

Version 1 2 3 4 5 6 7

with daily stoppages of the power generating unit during the drop in load at night. In the other operating modes the equivalent service lifetime of the unit is shorter than the nominal lifetime. The ratio Teq/Tlt of the equivalent service lifetime of the power generating unit to its nominal service lifetime is of greatest interest for estimating the effect of variable loading on the economy and reliability of equipment. This property, along with other factors, can be used for setting up differential pricing, creating a structure for the power generated by the units of power systems, choosing the most favorable composition of the working equipment, and establishing efficient support of the base load of nuclear power plants. CONCLUSION 1. It is proposed that an indicator of the equivalent service lifetime be used for estimating the effect of variable operating modes for the power generating units of thermal power plants on their operating efficiency. 2. Equivalent service lifetime coefficients have been evaluated for different types of startup, stepwise unloading of power generating units, and operation at base load. 3. The equivalent service lifetime of a 300 MW power generating unit has been estimated for various possible operating modes during the nominal lifetime of the turbine.

REFERENCES 1. State Standard GOST R 52527–2006 (ISO 3977-9:1999), Gasturbines. Reliability, Availability, Maintainability, and Safety [in Russian], Standartinform, Moscow (2006). 2. Yu. A. Radin and T. S. Kontorovich, “Use of the principle of equivalent service lifetime for evaluating equipment reliability in steam-gas units,” Élektr. Stantsii, No. 1 (2012). 3. A. G. Kostyuk, Dynamics and Durability of Turbine Machinery [in Russian], Izd. dom MÉI, Moscow (2007). 4. V. F. Rezinskikh, “Once more about the service lifetime of power generating equipment,” Nadezhn. Bezopasn. Énerget., No. 4 (2009). 5. State Standard GOST 242–89. Fixed Steam Turbine Systems for Electric Generator Drives in Thermal Power Plants. General Technical Specifications [in Russian], Izd. Standartov, Moscow (1989). 6. Regulating Document RD 10-577-03. Standard Instructions for Monitoring of Metal and Extending the Service Lifetime of Boiler, Turbine, and Piping Components in Thermal Power Plants [in Russian], Izd. Gosgortekhnadzor Rossii, Moscow (2003). 7. T. G. Berezina, N. V. Bugai, and I. I. Trunin, Diagnostics and Prediction of Metal Durability in Thermal Power Plants [in Russian], Tekhnika, Kiev (1991).