FIELD OF INVENTION

Present invention provides a device to measure force of the valve spindle of a steam control valve assembly. More particularly, the present invention relates to measurement of force on Steam Control Valve Assembly to estimate the mechanical strength of the components of the valve. The invention specifically relates to measurement of force, exerted by steam on valve spindle while opening and closing of the valve during actual operation.
BACKGROUND OF INVENTION
Technically, a valve is a device which will control flow of fluid or vapour in any system. In case of steam, the major applications are to reduce the pressure of the inlet steam for process application. However, in addition to controlling pressure, a steam control valve will also control temperature.
Boilers will typically work at high pressures, as low pressure operation will result in carryover of water. High pressure steam has lower specific volume, which will allow pipes to carry less weight. In effect, distribution of steam becomes easier and cheaper due to the cost reduction in piping and insulation. Industrial process applications use steam at a lower pressure. The reason for this is that the steam at a low pressure has higher latent heat, which increases energy efficiency to a large extent. Steam pressure and temperature are related, and hence temperature is automatically controlled

through the control of steam pressure. The reduction of steam pressure is also related to the required plant safety. Steam pressure can be controlled by using Steam Control Valve.
In the present invention a steam control valve assembly is a device which regulates the steam flow into the turbine for controlling the speed or load requirements of the driven machine which can be a generator, a compressor or a pump. It can be used to raise or lower to speed or load requirements of the driven machine based on the requirement and also regulate the same to maintain it constant. Position of the steam control valve assembly in a steam turbine power plant shown in the fig.1.
In the present invention, one component of the control valve assembly is converted to load cell by use of strain gauges to develop a device to measure the force, exerted by steam on valve spindle while opening and closing of the valve during actual operation.
OBJECTS OF THE INVENTION
Therefore it is an object of the invention to propose a device to measure force of the valve spindle of a steam control valve assembly which is capable of measuring force for validation of design and mechanical strength of various components of the valve assembly.
Another object of the invention is to propose a device to measure force of the valve spindle of a steam control valve assembly where four strain gauges are installed

on a groove of the connecting spindle in full bridge configuration to achieve higher sensitivity and temperature compensation during measurement at higher temperature of 250 dig. C and 30 Kilo Newton (KN) of load.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 shows a typical steam turbine with steam control valve assembly.
Fig.2 shows a typical steam control valve assembly.
Fig.3 shows the drawing of the original connecting spindle.
Fig.4 shows the drawing of the modified connecting spindle converted to load
cell.
Fig.5 shows Strain Gauge Configuration in Full Bridge.
Fig.6 shows Schematic diagram of strain gauge location and connection.
Fig.7 shows 2-Element 90˚(T)rosette strain gauges installed on the connecting
spindle.
Fig.8. shows the Wheatstone Bridge for strain gauge.

Fig.9 Shows the instrumented connecting spindle (converted to load cell) with
strain gauges installed and sealed.
Fig.10 shows Calibration set up for load calibration of the Connecting spindle in
Universal Testing Machine (UTM).
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
It has been felt that the forces on the control valve spindle of a newly designed steam control valve assembly (V) is required to be measured for validation of design and mechanical strength of various components of the valve assembly. Measurement of force is planned in a test bed while opening and closing of the valve during its operation.
Connecting spindle as shown in fig.3 & fig.4, which exist in the load transfer path is chosen for conversion into load cell. Temperature during the operation of the valve is expected to be in the order of 150°C to 250°C. Strain is estimated in the connecting spindle for designed/expected load on the valve spindle. Connecting spindle is modified and converted to load cell using strain gauges suitable for temperature upto 250°C. Strain gauge was installed and connected in full bridge configuration in order to increase the sensitivity of the load cell as well for temperature compensation at higher temperature. Strain gauges on the connecting spindle has been suitably protected and calibrated using Universal Testing Machine (UTM). The device i.e. connecting spindle load cell thus invented is capable of measuring force upto 30 Kilo Newton (KN) with

measurement accuracy of +/- 0.1 Kilo Newton (KN) and resolution of 0.01 Kilo Newton (KN). Connecting spindle Load cell can be easily retrofitted to the Valve assembly without modification in the mating components in the assembly. Accordingly in order to increase the sensitivity of the load cell and also for protection of the strain gauges to be installed on the connecting spindle a groove has been designed and made in the connecting spindle as shown in the fig.5. A load cell usually consists of four strain gauges in a Wheatstone bridge configuration. Load cells of one strain gauge (Quarter Bridge) or two strain gauges (half bridge) are also available. In the current invention, as depicted in Fig. 5, full bridge configuration is used. The advantage of full bridge configuration is higher sensitivity and temperature compensation during measurement at high temperature as explained later. Full bridge configuration involves installing two strain gauges in the axial direction and the other two strain gauges installed in transverse direction. The Strain gauges used are two 2-Element 90˚ ‘T’ rosette type with a gauge factor of 2.05±0.5% and placed 180˚ opposite to each other on the modified connecting spindle.
The actual representation of Strain gauging of 2-Element 90˚ (T) rosette strain gauges is shown in Fig. 6 and Fig.7, wherein the bridge is given power supply across two terminals and output is measured across another pair of terminals. As seen in the Fig. 8., a DC supply voltage, VS is supplied (top to bottom) across the bridge, which contains four resistors (two parallel legs of two resistors each in series).The output voltage is measured across the legs in the middle of the bridge.
The output voltage VO is calculated from Ohm’s law using

Arrangement in the full bridge configuration eliminate the strain increase due to temperature and hence enable the load cell for measurement at high temperature without any error due to change in temperature in the strain gauges.
Calibration of the load cell was done using a standard load cell with known sensitivity and the instrumented connecting spindle sequenced in an Universal Testing Machine (UTM) machine to obtain strain vs load plot.

A standard load cell is a four arm Wheatstone bridge, which is pre-calibrated and the relationship between load and strain (Fig.10.) is already known. This relationship of the standard load cell is used to calibrate the connecting spindle load cell.
Fig. 10 shows the calibration setup for obtaining the load versus strain measurements on a Universal Testing Machine (UTM) for connecting spindle load cell.
Fig.11 gives the plot between strain and load for the connecting spindle load cell.
EXAMPLES / PREFERRED EMBODIMENTS:
Device can be used for measurement and monitoring of force on the valve spindle of a steam control valve assembly (V) up to temperature of 250DegC and 30 Kilo Newton (KN) of load during actual operation of the steam turbine. Concept can be extended to control valve of similar but of different ratings.

WE CLAIM

1. A Device to measure force on the valve spindle of a steam control valve assembly (V)
upto temperature of 250 deg. C and 30 Kilo Newton (KN) of load, the said device comprising;
a modified connecting spindle converted to a load cell with strain gauges characterized in that said strain gauges are installed and connected in full bridge configuration for increasing the sensitivity of the load cell as well as for temperature compensation at higher temperature; wherein the connected spindle load cell device is capable of measuring force upto 30 Kilo Newton (KN) with measurement accuracy of +/- 0.1 Kilo Newton (KN) and resolution of 0.01 Kilo Newton (KN).
2. The device as claimed in claim 1, wherein a groove is made on the connecting spindle
for installing strain gauges.
3. The device as claimed in claim 1, wherein full bridge configuration involves installing
two strain gauges in the axial direction and other two strain gauges installed in transverse direction.