Evaluation of the Fluid Forcein the Main Feed Water Control
Valve for APWRs

1. Introduction

KOJI TACHIBANA*1TOSHIKAZU MAEDA*1

HIDEYUKI MORITA*2

HIDEYUKI SAKATA*2

TAKAHARU HIROE*2

KOICHIRO OKETANI*3

plishes water flow control by vertically moving the plug (connected to the actuator via the stem). Because a pressure
distribution is formed around the plug by fluid flow, a load
(fluid force) is imposed on the plug. If the amount of fluid
force which vertically acts on the plug is significant compared with that of the force delivered by the actuator, thevalve's ability to control fluid flow can be adversely affected.

In pressurized water reactor (PWR) power plants, themain feed water control valve plays an important role incontrolling the water level in the steam generator by reguFig
lating the feed water (flowing to the steam generator) (FigFig.. 1).
Mitsubishi Heavy Industries, Ltd. (MHI) is now in theprocess of designing an advanced pressurized water reactor(APWR), which puts out about 40% more power than a conventional PWR of comparable size. Since the increase inreactor output also causes feed water flow to grow by roughly
40%, it follows that the fluid force acting on the plug of themain feed water control valve rises considerably. Fluid force
acting on the valve plug in a vertical direction affects thevalve operation. Therefore, it is necessary to ascertain thatthe main feed water control valve, even when exposed tosuch increment in fluid force, will be capable of controllingthe steam generator water level stably in response to thecommand signal. In the absence of any widely known experiences that have enabled a quantitative assessment asto how the feed water valve's controlling performance isinfluenced by the fluid force to which it is subjected, wehave carried out a flow test to determine the amount offorce that works on the plug of the main feed water control
valve and run a computer simulation considering the fluid
force to evaluate the valve's controlling capability.

3. Flow test
3.1 Test setup
A flow test was conducted in order to grasp the amountof fluid force that vertically acts on the plug. Used as a test
specimen was a 1/3 scale model, which was exactly analogous to a production model in terms of the internal flow
pass contour. Fluid force measurements were taken bymeans of a strain gage. An external view of the test setup
is given in Fig. 3
3.
3.2 Test conditions
The flow test was performed at room temperature. Flowvelocity was made equal to that found in actual installations so that the stress and strain imposed by fluid force on
the plug would become identical with those found on-stream,
except for the influence of fluid density.
SpringActuator

Diaphragm

2. Construction of the main feed water control valve

StemControl
airFig. 2 is a conceptual illustration of the main feed water
control valve. This is an air-operated valve which accom-Glandpacking
Plug

Cage

Pressurizer
Steam

Fluid flow
Fluid force

Main feed water
control valve

Fig. 2 Main feedwater control
valve concept

Turbine Generator
Steamgenerator

Fuel
ReactorPrimarycoolant

Test piece

Condenser

Feed
water

Primary coolant
pump

Reactor containment vessel

Condensate pump

Circulatingwater pump

Fig. 3 External viewof test setup

Fig. 1 Schematic diagram of PWR power plant*1 Kobe Shipyard & Machinery Works*2 Takasago Research & Development Center, Technical Headquarters*3 Nuclear Energy Systems Headquarters

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 43 No. 1 (Jan. 2006)

1-1.0
-1.5
-2.00

20

40
6080
Valve lift (%)

Fig. 4 Fluid force coefficient
Value obtained by making fluidforce, which has been obtained
from a flow test, dimensionless
100 by dynamic pressure and representative area

On the other hand, there were no specific efforts to ensure inlet pressure matched that observed on-line, because
it had no impact on the flow situation inside the valve. TheReynolds number during the flow test was lower comparedwith that observed in actual units, but the flow situation
within the valve was considered almost the same as that
noted in real installations, because it was well within theturbulence range.
3.3 Test Result
Fluid force can be calculated from the strain in the stem,
which has been determined through the flow test, using
equation (1): Ffluid = Sstem x σ = Sstem x EεWhere:
Ffluid: fluid force
Sstem: cross-sectional stem area
σ : stress developed in the stem
E: modulus of longitudinal elasticity
ε : strain
Since fluid force is in proportion to dynamic pressure andthe area on which it works, it can be made dimensionless bydynamic pressure and its working area into equation (2):

Steam generator
water level (%)-0.5

Valve lift (%)

Fluid force which is madedimensionless

0.0: when fluid force is factored in: when fluid force is not factored in60
50
400100

200

300

400

500

100

20030040050067
6155
0Time (s)Fig. 5 Examples of behaviors of the steam generator
water level and main feed water control valve lift,
found at a given change in operating conditions

control valve itself into that representing the entire plant.
Note that the model of the main feed water control valve
was formulated to contain an equation of motion based on
the plug being the material point. This means that the fluid
force in actual installations, which had been computed fromthe fluid force coefficient obtained from the test, is factored
in as an external force.
4.2 Simulation results
The simulation revealed that the main feed water control valve would be able to control the steam generator waterlevel reliably under varying transient conditions encounteredduring operation. As an example, FigFig.. 5 shows the behaviors of the steam generator water level and the main feedwater control valve lift, noted at given change in operating
conditions. For reference, the figure also illustrates the behaviors they would exhibit if fluid force were not factored
in. It has been affirmed that there is some difference inthe behavior of the main feed water control valve, depending on whether or not fluid force is involved, but the steam
generator water level remains barely affected.

Where:

ζ : fluid force which is made dimensionless(hereinafter referred to as fluid force coefficient)ρ : fluid density
v: representative velocity
A: representative area
In a range where the Reynolds number is adequately
high, the flow situation within the valve stays roughly identical, irrespective of the Reynolds number. Thus, the fluidforce coefficient remains virtually constant, regardless ofsuch conditions as fluid velocity or scale. Fig. 4 shows therelationship between the fluid force coefficient and the valve
lift that were determined by the test. A negative fluid force
coefficient indicates that fluid force is acting in a downward direction. Representative velocity was taken as mean
fluid velocity across an orifice and representative area, thearea of the orifice's opening.

5. Conclusion
In PWR or APWR, the main feed water control valve
plays a key role in regulating the steam generator waterlevel. Hence, we assessed its performance taking fluid force
into account and determined that it can achieve stable control of the generator water level.

Koji Tachibana

Toshikazu Maeda

Hideyuki MoritaHideyuki Sakata

Takaharu Hiroe

Koichiro Oketani

4. Assessment of controlling performance
4.1 Simulation model
In order to verify the controlling ability of the main feedwater control valve, a computer simulation was conductedby incorporating a model embodying the main feed water

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 43 No. 1 (Jan. 2006)

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