Tandem Steam Turbine forCogeneration Power Plant(Optimization for Min. LoadOperation)
POWER SYSTEMS HEADQUARTERS
INDUSTRIAL ENERGY POWER SYSTEMS DEPARTMENT

In power-generation facilities with steam turbines for cogeneration power plants, there has been a heightened needto operate the facilities at minimum load during nighttime
hours and on holidays, when the electric power demand
within factories for in-house power generation is low.
Mitsubishi Heavy Industries, Ltd. (MHI) has long
been manufacturing tandem steam turbines in the 5,000kW class using an engage/disengage clutch. MHI has
recently designed and delivered a facility with a capacity exceeding 20,000 kW. The facility has operated
smoothly since delivery. We introduce it here.

on whether consumers use power during the days, nights,
and weekends and holidays. For customers who own
steam turbines for cogeneration power plants, operation
with the minimum required equipment at night and on
holidays reduces the plant operation costs. In the caseFig. 1of general extraction condensing turbines (Fig.
1), operation can be continued at reduced loads by cooling theinside condensing zone of the turbine casing. To maintain this condition, a minimum amount of steam (about
15 to 20% of the rated amount) must continue to flow.
The cost to maintain this flow of steam goes to waste.
In the tandem steam turbine equipped with the engage/disengage clutch (Fig. 1), on the other hand, thecondensing turbine can be completely stopped while still
maintaining the operation of the back pressure turbine.
This eliminates the cost demerit.

1. Product specifications
Table 1 shows the product specifications for the tandem steam turbine power-generation facility recently
manufactured by MHI.

2. Product features
The electric power rates contracted with power companies now vary greatly from hour to hour, depending
Table 1 Product specifications for tandem steam turbine
power generation facility
Item

Specifications

Back pressure turbine
Inlet steam pressure
Inlet steam temperature

(MPa)

2.45

o360

( C)

Exhaust pressure

(MPa)

0.36

Rotational speed(min-1)

6 910

Turbine output (at generation terminal) (kW)10 300To steamprocess To condenser
Extraction condensing turbine

Condensing turbine
Inlet steam pressure(MPa)0.35

Exhaust pressure(kPa)(min-1)

6 017

Turbine output (at generation terminal) (kW)10 700

Rotational speed-92.1

Back pressure
turbineTo steamprocess

Generator
Rotational speed
Generator rated output

Condensing
turbineTo condenser(min-1)

1 800

Tandem steam turbine(kW)

21 000Fig. 1 Extraction condensing turbine andtandem steam turbine

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)1

About 18m

About 5m
Condensing turbine

Engage/disengage clutch

Condenser

Generator

Reduction gear

About 3.5m

Reduction gear

About 4m

Back pressure turbine

Fig. 2 Outline drawing of 21 000 kW tandem steam turbine

The adoption of the tandem type also brings in new
cost factors, however, such as more turbine casings,
more reduction gears, and the need for an engage/disengage clutch. Fortunately this does not lead to a large
demerit, as the turbine casing and rotor structures aresimplified compared with the extraction condensing
turbine.
The most important new feature in the recently de2 is the ability to easily
livered product, shown in Fig. 2,start and stop a condensing turbine with a capacity exceeding 10,000 kW (rated flow = About 100 t/h) everyday.

3. Development in the future
The electric power rates of domestic power companies
are expected to be ever-more segmented in the future. The
difference in the electric power demand between day andnight will continue to increase even overseas (and the electric power rates will vary during the day and during thenight). Thus, our future developments will be applicableeven to turbines with larger capacities. This will allow
us to meet the expectations of the users of cogeneration
power plants, including those in foreign countries.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 4 (Dec. 2007)2