Energy and Power Engineering, 2013, 5, 642-645
doi:10.4236/epe.2013.54B124 Published Online July 2013 (http://www.scirp.org/journal/epe)
The Influence of Overlap Degree Research on Nozzle
Governing Characteristic
Lin Ma1, Jian-qun Xu 1*, Xue-yu Dong2, Tao Yang1
1School of Energy and Environment, Southeast University, Nanjing, China
2Electrical Power Simulation and Control Engineering Center of Nanjing Institute of Technology, Nanjing, China
Received January, 2013
ABSTRACT
To solve the problems existing in the flow characteristics of steam turbine unit, the influence of valve overlap degree on
nozzle governing steam turbine had been studied. The combined flow characteristics of given valve overlap degree were
obtained for a 600 MW steam turbine unit by the method of theoretical calculation combined with simulation test, and
the influence of valve overlap degree on governing stage efficiency and steam chest pressure had been also analyzed.
This paper discussed the selection of rational overlap degree and introduced a new method of building model for gov-
erning stage efficiency of steam turbine in constant pressure operation condition, which provided theoretical guidance
for optimization research on nozzle governing steam turbine operation.
Keywords: Overlap Degree; Flow Characteristics; Valve Point; Governing Stage Efficiency; Operation Mode
1. Introduction
In China, many 600 MW and above units are widely us-
ing DEH control system, which provides the function of
valve management and the operation control for single
valve to sequence valve. Unfortunately, due to the effect
of some factors, site installation, for example, there are
differences between actual flow characteristics and ini-
tialize flow characteristics in DEH system. In this case, it
will lead to load disturbance and more security risks with
operation under design condition [1]. Furthermore, the
complexity is increased by the fact that a turbine is gen-
erally operated by two or four control valves which do
not necessarily work parallel over the complete opera-
tional range. Therefore, in order to ensure the operation
of single valve to sequence valve can be stably switched,
certain overlap degree is needed when control valves
open or close. However, big overlap degree will cause
large throttling losses and low thermal efficiency. Con-
versely, small overlap degree will cause poor linearity of
combined flow characteristics, which is disadvantageous
to electric power control. Thus, rational overlap degree
should be set before the function of sequence valve put
into operation, in order to ensure the security and reduce
the influence on important parameters (vibration, bearing
temperature, etc) of steam turbine during the switching
process.
The value of overlap degree not only directly affects
static characteristics of steam distribution mechanism,
but also affects governing stage efficiency. Therefore, the
selection of rational overlap degree to further improve
the economic efficiency is of great significance.
The aim of this paper is to discuss the selection of ra-
tional overlap degree, and analyze the influence of over-
lap degree on governing stage efficiency and steam chest
pressure.
2. Valve Overlap Degree
To the “sequence valve” operation mode with nozzle
governing units, the subsequent valve didn’t open until
the previous valve opened completely, its flow charac-
teristics can be illustrated by the following Figure 1, the
solid cam line. In this case, the static characteristics of
speed variation rate were also cam curve, it was not con-
form to the design requirements of control system. Thus,
it is necessary to find certain valve overlap degree so as
to compensate for the previous valve’s nonlinear charac-
teristics, the dotted line as shown in Figure 1. Generally,
valve overlap degree is expressed as
p
ζ .
3. The Selection of Overlap Degree
Valve flow characteristics must be necessary so as to
obtain rational overlap degree. Assuming that the indi-
vidual valve characteristic is known, the key problem is
how to determine the flow distribution among semi-open
valves[2]. From the point of mathematical view, building
*Corresponding author.

L. MA ET AL. 643
mathematical model of overlap degree, in fact, aims at
taking a nonlinear distribution upon steam mass flow
through control valves.
Nevertheless, due to the existing variable condition
calculation without considering the overlap degree, addi-
tionally, the actual flow characteristics are too hard to
compute. To comply with this task, this paper present a
method of theoretical calculation combined with simula-
tion test, look as the following Figure 2:
4. Application and Analysis
An application example is provided with a 600MW
steam turbine unit.
Figure 1. Flow characteristics of control valves.
Figure 2. Flow chart of research method.
a) In practice, according to the concept of overlap de-
gree, the thermodynamic calculating program with given
overlap degree for governing stage in variable operation
condition were compiled together with the simulation test
by means of APROS simulation platform for constant
pressure operation (16.7MPa)with four valves[3,4]. The
percentage, here, refers to the flow ratio that main steam
mass flow in variable condition and rated condition.
Valve overlap degree are, respectively, 0, 0.121, 0.278,
the corresponding valve combined flow characteristics
and pressure characteristics are separately indicated by
Figures 3(a) and (b).
In Figure 3 above:
1) The linearity of combined flow characteristics are
worst when valve overlap degree is zero, then, the linear-
ity turns better with the increase of overlap degree, as
shown in Figure 3(a).
(a)
(b)
Figure 3. (a) Valve combined flow characteristics; (b) Pres-
sure characte ristic s.

L. MA ET AL.
644
2) The pressure characteristics are precisely shown by
Figure 3 (b): the steam chest pressure is mostly propor-
tional to the change of total steam mass flow. The pres-
sure after governing stage at valve point fluctuates
enormously.
b) Combining the ideas of the previous sections, the
variable condition calculating program for nozzle gov-
erning 600 MW steam turbine in constant pressure (16.7
MPa) was compiled [5-7]. According to the thermody-
namic calculation, the change law of relative internal
efficiency for governing stage is accurately explained by
the following Figure 4.
In Figure 4 above:
1) Sequence valve operation mode in constant pressure
condition: when the overlap degree is zero, the corre-
sponding governing stage efficiency is the highest, but
getting lower with the increase of overlap degree.
2) Sequence valve operation mode in constant pressure
condition: the tendency of efficiency curve is parabolic
shape in valve point condition, and getting lower with the
decrease of the valve point.
Relative internal efficiency at valve point ri , can be
expressed as the function of main steam mass flow G,
main steam pressure
η
0
p
, overlap degree
p
ζ :
(, ,)p
ri
f
Gp   ζ
0 (1)
Assuming
p
ζ , 0
p
are given:
()
ri
f
G

 (2)
Thus, parabolic equation can be obtained by fitting the
efficiency curve of valve point:
2
12
()
ri 3
f
GeGeGe   (3)
Since the coefficients are linked to
p
ζ , 0
p
and gov-
erning stage structure, they can be determined by the
design of units.
Figure 4. The change law of relative internal efficiency.
3) Sequence valve operation mode in constant pressure
condition: the efficiency of semi-open valves condition
among valve points are approximately tend to sine curve,
which was caused by throttling losses.
Thus, can be determined by G,
ri
η0
p
,
p
ζ and flow
area A:
(,, ,) p
ri
f
GA p   ζ
0 (4)
Since flow area A is a single-valued function of valve
opening degree D. When
p
ζ, 0
p
are given, can be
expressed as: ri
η
(, )
ri
f
GD


(5)
According to the research above, a simplified model,
which is uniquely linked to the operation mode, G and D,
can be inverted:
()sin( )
ri
f
GaDbc


 (6)
Where a, b and c are all constant.
Consequently, ri

can be calculated quickly by the
simplified model, especially when design parameters are
known.
The subsequent table displays a computational com-
parison between the simplified model and thermody-
namic calculation (overlap degree is zero, for example),
as shown in Table 1.
Table 1 shows the accuracy of simplified model, for
the maximum error is 0.472%.The new mathematical
model has been successfully implemented on fast calcu-
lation of efficiency for governing stage in constant pres-
sure operation of sequence valve.
5. Consequence
1) Unreasonable overlap degree will affect static char-
acteristics of steam distribution mechanism, may cause
system operation unstable. Rational overlap degree can
improve governing stage efficiency. Therefore, the selec-
tion of rational overlap degree should consider better
linearity and higher efficiency as criterion.
Table 1. Efficiency comparison.
Value operationLoadEfficiency of simplified
model /%
Efficiency of
thermodynamic
calculation /%
Absolute
error/%
0.585149.014 48.789 0.225
0.637250.640 51.091 -0.441
0.692253.953 53.511 0.442
1 fully
open 1 throttle
0.733156.161 55.689 0.472
0.851261.612 61.312 0.300
0.911363.088 62.788 0.300
2 fully
open 1 throttle0.968864.808 65.121 -0.313
1.029267.886 67.543 0.343
1.069468.036 67.832 0.204
3 fully
open 1 throttle1.112368.229 68.610 -0.381

L. MA ET AL.
645
2) Simplified model for governing stage efficiency has
high feasibility and accuracy. The research’s thought and
method provided governing stage operation with theo-
retical guidance for economic evaluation standard.
6. Conclusions
1) This paper adopted the method of theoretical calcu-
lation combined with simulation test to study overlap
degree that was different from other previous research
work, which is convenient for optimization research on
nozzle governing steam turbine.
2) This paper introduced valve point operation condi-
tion to take a ‘multiple’ comparison with higher accuracy,
which can more accurately reflect the influence of over-
lap degree.
3) This paper determined the pressure characteristics
and analyzed the influence of overlap degree on steam
chest pressure. The pressure characteristics were helpful
to reflect the importance of rational overlap degree.
4) This paper put forward a new simplified model
based on nozzle governing 600 MW steam turbine unit,
provided theoretical reference for different capacity units.
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