thermal engineering of nuclear power stations balance-of

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Thermal Engineering of Nuclear Power StationsBalance-of-Plant SystemsCharles F. Bowman, Seth N. Bowman

High-Pressure Turbines

Publication detailshttps://www.routledgehandbooks.com/doi/10.1201/9781003011606-3

Charles F. Bowman, Seth N. BowmanPublished online on: 21 Jul 2020

How to cite :- Charles F. Bowman, Seth N. Bowman. 21 Jul 2020, High-Pressure Turbines from:Thermal Engineering of Nuclear Power Stations, Balance-of-Plant Systems CRC PressAccessed on: 07 May 2022https://www.routledgehandbooks.com/doi/10.1201/9781003011606-3

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3 High-Pressure Turbines

3.1 HIGH-PRESSURE TURBINE CONTROL VALVES

HP turbine control valves limit the amount of main steam flow through the HP tur-bine and thus control the reactor power by throttling the main steam flow rate. HP turbine control valves are sized to pass the main steam flow that will result in 100% reactor power while not fully open to provide some controllability at full power oper-ation. However, in the case of a PWR, if the SG is fouled, the heat transfer between the primary and secondary sides of the HX is reduced, resulting in a reduced SG pressure. As shown in Figure 3.1, at some point as the fouling increases, the SG pressure is reduced to the point that the HP turbine control valves became wide open (VWO). Subsequent fouling means that 100% reactor power can no longer be main-tained. When that happens, the SGs are often replaced.

Historically, the different turbine vendors have offered different HP turbine control valve designs. General Electric, as well as most foreign turbine vendors, offered the full-arc design, whereas Westinghouse offered the partial-arc admis-sion type.

Figure 3.2 illustrates full-arc admission into the HP turbine. With full-arc admis-sion, the steam is admitted from all valves at once to first stage nozzles of the HP tur-bine, resulting in parallel steam flow through all of the first-stage nozzles at all loads. This design requires more throttling, which results in higher throttling loss if the turbine is operating at less than VWO. Figure 3.3 illustrates partial-arc admission into the HP turbine. With partial-arc, the valves open one at a time as load increases to admit steam from each valve to only a portion of first-stage nozzles of the HP tur-bine, resulting in full steam flow through some of the first-stage nozzles and less or none through others. This design requires lower throttling loss because some valves are wide open, but it increases cyclic blade loading as the blades alternately pass by nozzles that are admitting steam and those that are not admitting steam.

Although there is a definite pressure drop through the HP turbine control valve, main steam passing through the valve is an adiabatic process (i.e., no heat transfer occurs), so there is no change in the enthalpy. Figure 3.4 shows the expansion of the main steam through the HP turbine in an enthalpy vs. entropy diagram (known as a Mollier diagram). The slightly wet steam is first expanded through the HP turbine control valves(s). If the HP turbine were 100% efficient, the steam would expand in an reversible and adiabatic (or isentropic) manner and the entropy of the steam would be constant. However, only a portion of the energy is converted to work, whereas a portion of the energy excites the steam molecules as entropy is increased.

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-3 12 Thermal Engineering of Nuclear Power Stations

Inactive Arc

FIGURE 3.3 HP turbine with partial-arc admission. (After Steam Turbine Performance Seminar Lecture Notes by Kenneth C. Cotton.)

Steam Generator and Throttle Pressure

400

500

600

700

800

900

1000

1100

1200

0 20 40 60 80 100Unit Load (%)

Pre

ssu

re (

PS

IA)

DesignS.G.VWO

LoadLimitedThrottlepressure

Control valve pressure drop

Load reduction

FIGURE 3.1 PWR steam generator and throttle pressures.

FIGURE 3.2 HP turbine with full-arc admission. (After Steam Turbine Performance Seminar Lecture Notes by Kenneth C. Cotton.)

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-3 13High-Pressure Turbines

The efficiency of the HP turbine is defined as follows:

ηturbineactual

isentropic S

ww

h hh h

= =−( )−( )

1 2

1 2

(3.1)

where:η = efficiencyw = workh = enthalpy.

Knowing the steam flow rate and the HP turbine efficiency, one may calculate the work done by the HP turbine.

As Figure 3.5 shows, turbines served by partial-arc control valves are at their point of best efficiency when the valves are either fully open or fully closed, whereas turbines that are served by full-arc control valves are relatively less efficient at par-tial loads because all of the valves are throttled but may be more efficient when all of the valves are fully open.

Figure 3.6 shows a sectional view of the top half of a typical Westinghouse HP nuclear turbine, which is a two-flow machine. The design philosophy adopted for the Westinghouse HP turbines with their partial-arc control valve design is carried forward into the design of HP turbine’s first-stage blades. Steam is first admitted into two donut-shaped bowls (one in each direction of steam flow) that distribute the steam around the turbine casing (the stationary part of the turbine). The steam is then distributed through nozzles to the first stage of blading, which is attached to

Ent

halp

y

1.4 1.5 1.6 1.7

1100

1200

1000

1300

Entropy

1.0

2.5

10

502001000 500oF

400oF

95%

90%

2

2s

1

FIGURE 3.4 HP turbine expansion line on the Mollier diagram.

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the turbine rotor (the moving part of the turbine). In the Westinghouse design, the first-stage blades are said to be of the impulse design.

Figure 3.7 illustrates the impulse blade design with the blades viewed as if they were rolled out on a table. In this design, the entire pressure drop takes place in the stationary nozzles as the steam pressure is converted into kinetic energy, which is transferred to the moving blades (buckets), depleting the steam velocity as the steam impacts the turbine blades as they pass the nozzles, like a kick in the seat. In this design, the maximum work is achieved when the steam velocity is twice the veloc-ity of the moving blades. The pressure of the steam beyond the first-stage blades

Bowl

First stage

FIGURE 3.6 Westinghouse HP nuclear turbine.

Full Arc

η HP

85

65

70

75

80

90

0.4 0.6 0.8 1.0

Flow Ratio

Partial Arc

FIGURE 3.5 HP turbine efficiency. (After Steam Turbine Performance Seminar Lecture Notes by Kenneth C. Cotton.)

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is known as the first-stage pressure (a very important parameter in nuclear turbine cycle analysis beyond the first stage blades are designed as reaction blades and the steam flow [and thus the power] through the remainder of the turbine cycle is roughly proportional to the first stage pressure).

Figure 3.8 illustrates the concept of the turbine blading beyond the first stage being thought of as simply a series of orifices. The flow-passing capability of the HP turbine is governed by the following relationship:

m k Pv

first stage

first stage= (3.2)

where:k = turbine stage flow coefficientP = pressureV = specific volume.

Over a wide range of operating conditions,

ν first stagefirst stageP

≈ 1 (3.3)

Stea

m I

n

Nozzle Vanes Moving Buckets

Steam Pressure

Steam Velocity

FIGURE 3.7 Impulse blade design. (After Steam Turbine Performance Seminar Lecture Notes by Kenneth C. Cotton.)

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Therefore,

≈m k Pfirst stage (3.4)

Because unit output is proportional to through flow, the first-stage pressure is an important indicator of unit load and should be trended.

The design philosophy adopted by the rest of the world for HP turbines is to use the full-arc control valve design. Steam is first admitted into the bowls, which dis-tribute the steam around the turbine casing. The steam is then distributed through nozzles to the first stage of blading as with the Westinghouse design except that the first stage is designed as reaction blading similar to the rest of the stages of blading, as shown in Figure 3.9. In this design, the bowl pressure is analogous to the first-stage pressure in the Westinghouse design.

With the reaction blade design, the pressure drop takes place in both the station-ary nozzle and the moving blade. Lower fluid velocities are used, resulting in greater efficiency; however, because there is pressure drop across the moving blade, leakage around the tip of the blade is of concern.

In turbine acceptance tests, the measured first-stage pressure is typically higher than the design value shown on the vendor’s heat balance (HB) diagram. The rea-son is that the turbines when delivered typically exhibit tighter clearances between the rotating and stationary blades (i.e., design margin). Other possible reasons why the first-stage pressure might be high would be due to erosion in the first-stage noz-zles (impulse blade design only), partially plugged turbine nozzles downstream of the first stage, and speeding (i.e., the reactor power is higher than advertised). Possible reasons for low first-stage pressure include erosion of turbine nozzles and blading in the turbine, degraded turbine seals, poor cycle isolation, high moisture carryover, and lower than advertised reactor power (possibly due to feedwater nozzle fouling).

Main Steam

1 2 3 4 5

Stage Numbers

ThrottlePressure

BowlPressure

FirstStagePressure

ControlValves

FIGURE 3.8 Steam moving through a turbine with an impulse first stage. (After Steam Turbine Performance Seminar Lecture Notes by Kenneth C. Cotton.)

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As shown in Figure 3.10, each stage of a turbine is designed for a specific volu-metric flow rate.

As Figure 3.11 shows, when the turbine is operating at some volumetric flow rate other than the design value, the steam flow mismatch results in reduced stage efficiency. Figure 3.12 shows a representative cross section of a turbine with alternat-ing stationary and rotating blades. In an HP turbine, a shroud encircles the rotating blades. The stationary casing has sealing strips at the interfaces between the station-ary and rotating parts to reduce the leakage around the tips of the stationary and rotating blades.

V2

V1V1

V2

Stationary blade row

Rotating blade row

FIGURE 3.10 Each stage of a turbine is designed for a specific volumetric flow rate. (After Steam Turbine Performance Seminar Lecture Notes by Kenneth C. Cotton.)

Stea

m I

n

Nozzle Vanes Moving Buckets

Steam Pressure

Steam Velocity

FIGURE 3.9 Reaction blade design. (After Steam Turbine Performance Seminar Lecture Notes by Kenneth C. Cotton.)

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Conventional blade designs are straight with uniform steam flow across the blade. More advanced computer modeling has led designers to develop advanced flow pat-tern blade designs that concentrate more on the steam flow in the center of the blade and away from the blade tips. The result is less tip end leakage and more efficient blade stages.

Rotatingblade

Stationaryblade

Steam

Turbine rotor

Turbine casing

Shroud

Sealingstrips

Sealing strips

FIGURE 3.12 Cross section of a turbine. (After Schofield, P., Efficient Maintenance of Large Steam Turbines, paper for Pacific Coast Electric Association 1982 Engineering and Operating Conference, San Francisco, CA, 18821.)

0.8

1.0

0 1.0 2.0

0.6

0.4

0.2

v/vdesign

Eff

icie

ncy

FIGURE 3.11 Turbine stage efficiency as a function of volumetric flow rate.

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Figure 3.13 shows the advanced flow pattern blade design. Turbine blade deposit buildup and blade erosion, as shown in Figures 3.14 and 3.15, are potential sources of reduced turbine blade stage efficiency. These problems are not nearly as preva-lent in nuclear turbines as in coal-fired units. However, copper buildup was an issue before most of the nuclear plants made a concerted effort to eliminate copper in their condensate and feedwater systems, and erosion is an issue only where high moisture carryover is a problem.

FIGURE 3.14 Deposit buildup on turbine blades. (From Schofield, P., Efficient Maintenance of Large Steam Turbines, paper for Pacific Coast Electric Association 1982 Engineering and Operating Conference, San Francisco, CA, 1882.)

FIGURE 3.13 Conventional and advanced flow pattern turbine blade design.

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High moisture carryover from the reactor (BWR) or SG (PWR) can reduce the HP Turbine efficiency. As Figure 3.16 shows, each percent of moisture in the main steam reduces the HP turbine efficiency by about 1%.

10%0.85

0.90

0.95

1.00

8% 6% 4% 2% 0%

Moisture

Eff

icie

ncy

Cor

rect

ion

FIGURE 3.16 HP turbine efficiency correction for main steam moisture.

FIGURE 3.15 Turbine blade erosion. (From Sumner, W.J. et al., Reducing Solid Particle Erosion Damage in Large Steam Turbines, paper for American Power Conference, IEEE, 19852.)

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REFERENCES

1. Schofield, P., Efficient Maintenance of Large Steam Turbines, paper for Pacific Coast Electric Association 1982 Engineering and Operating Conference, San Francisco, CA, 1882.

2. Sumner, W. J., et al., Reducing Solid Particle Erosion Damage in Large Steam Turbines, paper for American Power Conference, IEEE, 1985.

thermal engineering of nuclear power stations balance-of

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