DESC:FIELD
The present disclosure relates to an apparatus and a method for measuring dryness fraction of steam.
DEFINITIONS OF TERMS USED IN THE SPECIFICATION
As used in the present disclosure, the following term is generally intended to have the meaning as set forth below, except to the extent that the context in which they are used indicate otherwise.
The expression ‘dryness fraction’ used hereinafter in the specification refers to as the ratio of weight of dry steam Wds present in known quantity of wet steam to total weight of Wet steam (Wds+Wws). It is a unit less quantity and is denoted by x. Thus,

Critical pressure ratio formula for choking flow condition is given by:

where Po- Pressure at the orifice outlet;
Pi – Pressure at the orifice inlet; and
? – Adiabatic exponent or index of isentropic expansion or compression, or polytropic constant.
The above definitions are in addition to those expressed in the art.
BACKGROUND
The background information herein below relates to the present disclosure but is not necessarily prior art.
Steam dryness is important for process heating, as a poor dryness fraction can lead to a higher steam consumption and a lower system efficiency. Throttling calorimeter is a simple method for dryness fraction measurement but the currently used method for using the throttling calorimeter has several imitations and is unreliable.
In the throttling process, the pressure of steam is reduced by using a valve or an orifice and both temperature and pressure are measured at the outlet of the valve or the orifice. Throttling is a constant enthalpy process and causes superheating of steam. As a function of pressure and temperature enthalpy of superheated steam at the outlet is calculated and equated with the inlet enthalpy. Inlet enthalpy is the function of pressure and dryness fraction. The following equation is used for the calculation of the steam dryness:

where ho is the steam enthalpy at the outlet of the valve or the orifice and is calculated as a function of the outlet pressure and the temperature. hf and hfg represents the liquid enthalpy and latent heat of the steam at the inlet of the valve. x is the dryness fraction of the steam.
In the prior art, the difference of kinetic energy is neglected for small velocities. This is the limitation in the prior art. The changes in the kinetic energy cannot be neglected for steam with low dryness fraction. In such a case, a significant pressure drop is required to generate super-heated steam at the outlet of the valve.
In most applications, ratio of the outlet pressure to the inlet pressure is less than the required critical ratio, causing sonic speed at the orifice and flow choking. In such scenarios, change in kinetic energy is significant and cannot be neglected. If the temperature is measured near the orifice, velocity is near the sonic velocity and can cause significant error. If the temperature is measured at a distance from the orifice, the steam expands due to the Venturi effect of the orifice, and the velocity decreases below sonic level. This decrease in velocity causes an error in the measurement of the dryness fraction.
There is, therefore, felt a need for an apparatus and a method for measuring dryness fraction of steam that mitigates the drawbacks mentioned hereinabove.
OBJECTS
Some of the objects of the present disclosure aimed to ameliorate one or more problems of the prior art or to at least provide a useful alternative are described herein below:
An object of the present disclosure is to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
Another object of the present disclosure is to provide an apparatus for measuring dryness fraction of steam.
Yet another object of the present disclosure is to provide a method for accurate and reliable measurement of dryness fraction of steam.
Other advantages of the present disclosure will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present disclosure.
SUMMARY
The present disclosure envisages an apparatus for measuring dryness fraction of steam. An inlet pipe is provided for drawing steam from a source at a constant mass flow rate. A preliminary outlet pipe is connected with the inlet pipe through an inlet pipe flange and an outlet pipe flange. An orifice of a predetermined diameter is placed between the inlet pipe flange and the outlet pipe flange. A diverging outlet pipe is connected with the preliminary outlet pipe. The diverging outlet pipe has a diverging cross-section to reduce change in kinetic energy. A main outlet pipe is connected with the diverging outlet pipe. The diameter of the main outlet pipe is higher than the diameter the preliminary outlet pipe. At least one pressure transmitter and at least one temperature sensor is configured to sense pressure and temperature respectively at a pre-determined location in the main outlet pipe and generate a sensed pressure signal and a sensed temperature signal respectively. A second pressure transmitter and a second temperature sensor is placed near the outlet of the main outlet pipe. A computing unit is configured to compute dryness fraction of the steam from the source based on the sensed pressure signal and the sensed temperature signal.
In an embodiment, the orifice is of a small diameter is selected to reduce the velocity at the inlet to reduce the difference in kinetic energy.
In another embodiment, the orifice is selected of an optimum diameter to increase the kinetic energy component in the preliminary outlet pipe.
In still another embodiment, if the diameter of the orifice is selected to provide a pressure ratio across the orifice than the critical pressure ratio.
Advantageously, the area and diameter of the orifice is calculated by using the sonic velocity, density and allowed mass flow rate at the outlet of the orifice.
In an embodiment, a first pressure transmitter and a first temperature sensor is configured to sense pressure and temperature respectively at a first location in the main outlet pipe near the inlet of the main outlet pipe and generate a first sensed pressure signal and a first sensed temperature signal respectively. A second pressure transmitter and a second temperature sensor is configured to sense pressure and temperature respectively at a second location in the main outlet pipe near the outlet of the main outlet pipe and generate a second sensed pressure signal and a second sensed temperature signal respectively. The computing unit is configured to calculate dryness fraction value corresponding to both the first location and the second location based on the sensed pressure signals and sensed temperature signals.
In another embodiment, the computing unit includes a repository and an arithmetic unit. The repository is provided for storing a steam table and thermodynamic equations and the arithmetic unit is communicatively coupled with the pressure transmitter(s), the temperature sensor(s) and the repository.
A method for obtaining a measure of the dryness fraction of steam, the method comprising:
• drawing steam from a source at a constant mass flow rate via an inlet pipe;
• throttling the steam through the orifice;
• measuring the pressure (Pi) at the inlet of apparatus;
• calculating the pressure (Po) at the outlet of orifice by using critical pressure ratio formula for choking flow condition;
• calculating the density (?) corresponding to pressure (Po) at the outlet of the orifice;
• calculating the sonic velocity (?s) corresponding to orifice outlet pressure (PO);
• calculating the orifice area (Ao) and mass flow rate (m) for choking condition;
• calculating the density (?i) corresponding to inlet pressure (Pi);
• calculating the velocity(Vi) in the inlet pipe,
• measuring the outlet pressure (Po) in an outlet pipe;
• calculating the density (?o) corresponding to outlet pressure (Po);
• calculating the velocity (Vo) at outlet condition;
• measuring the outlet temperature (To) in the outlet pipe;
• calculating the enthalpy (ho) corresponding to measured pressure (Po) and temperature (To) by superheated steam properties formula;
• calculating the inlet enthalpy (hi) by using the first law of thermodynamics for steady state throttling process; and
• calculating the dryness fraction(x) of steam.
In an embodiment, the temperature is measured at a predetermined distance from the orifice such that steam coming out of the orifice gets sufficient time for expansion.
In another embodiment, a first pressure and a first temperature are measured at a first predetermined distance from the orifice and a second pressure ad as second temperature are measured at a second predetermined distance from the orifice that is larger than the a first determined distance.
In yet another embodiment, the choking condition is mathematically calculated and inlet and outlet kinetic energy is calculated for the choking flow and the dryness is corrected mathematically.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
An apparatus and a method for measuring dryness fraction of the steam, of the present disclosure will now be described with the help of accompanying drawing, in which:
Figure 1 illustrates a schematic view of the apparatus, in accordance with the present disclosure.
LIST OF REFERENCE NUMERALS
100 – Apparatus
102 – Inlet pipe
104 – Inlet pipe flange
106 – Orifice
108 – Outlet pipe flange
110 – Outlet pipe
112 – Diverging outlet pipe
114 – Main outlet pipe
116 – First pressure sensor
118 – First temperature sensor
120 – Second pressure sensor
122 – Second temperature sensor
DETAILED DESCRIPTION
Embodiments, of the present disclosure, will now be described with reference to the accompanying drawing.
Embodiments are provided so as to thoroughly and fully convey the scope of the present disclosure to the person skilled in the art. Numerous details are set forth, relating to specific components, and methods, to provide a complete understanding of embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.
The terminology used, in the present disclosure, is only for the purpose of explaining a particular embodiment and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly suggests otherwise. The terms "comprises," "comprising," “including,” and “having,” are open ended transitional phrases and therefore specify the presence of stated features, elements, modules, units and/or components, but do not forbid the presence or addition of one or more other features, elements, components, and/or groups thereof.
The terms first, second, third, etc., should not be construed to limit the scope of the present disclosure as the aforementioned terms may be only used to distinguish one element, component, region, layer or section from another component, region, layer or section. Terms such as first, second, third etc., when used herein do not imply a specific sequence or order unless clearly suggested by the present disclosure.
In the throttling process, the pressure of steam is reduced by using a valve or an orifice and both temperature and pressure are measured at the outlet of the valve or the orifice. Throttling is a constant enthalpy process and causes superheating of steam. As a function of pressure and temperature enthalpy of superheated steam at the outlet is calculated and equated with the inlet enthalpy. Inlet enthalpy is the function of pressure and dryness fraction. The following equation is used for the calculation of the steam dryness:

Where ho is the steam enthalpy at the outlet of the valve or the orifice and is calculated as a function of the outlet pressure and the temperature. hf and hfg represents the liquid enthalpy and latent heat of the steam at the inlet of the valve. x is the dryness fraction of the steam.
Therefore, methods known in the prior art for measurement of dryness fraction of steam neglect the difference of kinetic energy in the energy balance equation:


This is correct for a throttling process with small pressure drop. However, change in kinetic energy cannot be neglected for a steam with low dryness fraction, where the significant pressure drop is required to generate superheated steam at the outlet of valve.
The present disclosure provides an apparatus and a method for measuring dryness fraction of steam. The method results in more accurate and reliable measurement of dryness fraction of steam, wherein the effect of kinetic energy is also considered for the measurement.
In one aspect, the present disclosure provides an apparatus 100 for obtaining the measure of dryness fraction of steam, the steam having liquid and vapor components. The schematic view of the apparatus 100 is illustrated in Figure 1. The apparatus 100 comprises an inlet pipe 102, a preliminary outlet pipe 110, a diverging outlet pipe 112, a main outlet pipe 114 and an orifice 106. The inlet pipe 102 is configured for drawing steam from a source at a constant mass flow rate. The preliminary outlet pipe 110 is connected with the inlet pipe 102 through an inlet pipe flange 104 and an outlet pipe flange 108. The orifice 106 of a predetermined diameter is placed between the inlet pipe flange 104 and the outlet pipe flange 108.
The diameter of the orifice 106 is selected such that the difference in kinetic energy of the steam in the inlet pipe 102 and the preliminary outlet pipe 110 is reduced. The diverging outlet pipe 112 is connected with the preliminary outlet pipe 110. The area of the outlet is increased by using a diverging channel to reduce change in kinetic energy. The main outlet pipe 114 is connected with the diverging outlet pipe 112. The diameter of the main outlet pipe 114 is higher than the diameter the preliminary outlet pipe 110.
A first pressure transmitter 116 and a first temperature sensor 118 is placed at the inlet of the main outlet pipe 114. A second pressure transmitter 120 and a second temperature sensor 122 is placed near the outlet of the main outlet pipe 114.
The orifice 106 with a small diameter is selected to reduce the velocity at the inlet to reduce the difference in kinetic energy.
The cross-section of the outlet pipe is increased to reduce change in kinetic energy. The area of the main outlet pipe 114 is increased to reduce the change in kinetic energy. The main outlet pipe is connected by using the diverging outlet pipe 112 to reduce the difference in kinetic energy.
The orifice 106 is selected of an optimum diameter. A relatively higher diameter of the orifice 106 will increase the kinetic energy component in the outlet pipe. A relatively small diameter of the orifice 106 will result in the steam taking more time for expansion. The optimum diameter of the orifice 106 will provide a pressure ratio Po /Pi just higher than the critical pressure ratio.
In another aspect, the present disclosure provides a method for obtaining a measure of the dryness fraction of steam. In an embodiment the method comprises the following the steps:
i. drawing steam from a source at a constant mass flow rate via the inlet pipe 102;
ii. throttling the steam through the orifice 106;
iii. measuring the pressure (Pi) at the inlet of apparatus 100;
iv. calculating the pressure (PO) at the outlet of orifice 106 by using critical pressure ratio formula for choking flow condition,

Po- Pressure at the orifice outlet
Pi – Inlet pressure
? - Adiabatic exponent;
v. calculate the density (?) corresponding to pressure (Po) at the outlet of orifice 106;
vi. calculating the sonic velocity (?s) corresponding to orifice outlet pressure(Po),

vs- Sonic velocity
? – Steam density;
vii. calculating the orifice area (Ao)and mass flow rate (m ?) for choking condition,

m ? – Steam mass flow rate
?i– Density at the inlet pressure
Ao – Orifice area;
viii. calculating the density (?i) corresponding to inlet pressure (Pi);
ix. calculating the velocity(Vi) in inlet pipe,

?i– Density at the inlet pressure
Ai – Area of the inlet pipe
Vi – Velocity in the inlet pipe
x. measuring the outlet pressure (Po);
xi. calculating the density (?o) corresponding to outlet pressure (Po);
xii. calculating the velocity (vO) at outlet condition;
xiii. measuring the outlet temperature (To) at a point on the main outlet pipe 114;
xiv. calculating the enthalpy (ho) corresponding to measured pressure (Po) and temperature (To) by superheated steam properties formula;
xv. calculating the inlet enthalpy (hi) by using the first law of thermodynamics for steady state throttling process; and
xvi. calculating the dryness fraction (x) of steam by using following equations:
,
where,

wherein hf and hfg are the specific enthalpy of saturated water and latent heat of the steam at the inlet of the valve.
In one embodiment of the method, the source of steam is an industrial boiler.
In another embodiment, the area and diameter of the orifice 106 is calculated by using the sonic velocity, density and allowed mass flow rate at the outlet of the orifice 106.
In the present disclosure, choking condition is mathematically represented and inlet and outlet kinetic energy is calculated for the choking flow and the dryness is corrected mathematically.
The temperature is measured at a predetermined distance from the orifice 106, so the steam coming out of the orifice 106 can get sufficient time for expansion.
In yet another embodiment of the method, the pressure and the temperature are measured at two predetermined points on the main outlet pipe 114 one near the orifice 106 and another at a distance from the orifice 106. Calculation is done at both places to measure the expansion in the outlet pipe and confirm accurate and reliable value of dryness fraction of the steam.
In use, a steam sample is taken from the small diameter pipe 102. This pipe is connected with the outlet pipe 110 through the inlet pipe flange 104 and the outlet pipe flange 108. The orifice 106 of suitable diameter is placed between these two flanges. The orifice 106 is critically selected to have significantly low value of kinetic energy. The steam coming out of the orifice 106 enters the preliminary outlet pipe 110, which has the same diameter as the inlet pipe 102. Since, the orifice 106 has a reduced diameter it acts as a venture. The steam coming out of the orifice 106 will expand along a significant length of the preliminary outlet pipe 110. After the preliminary outlet pipe 110, the steam enters the diverging outlet pipe 112. The diverging outlet pipe 112 joins the preliminary outlet pipe 110 to the main outlet pipe 114. The main outlet pipe 114 has a larger diameter than the preliminary outlet pipe 110 to reduce the effect of change in velocity. A first pressure transmitter 116 and a first temperature sensor 118 are placed at the inlet of the main outlet pipe 114. Similarly, a second pressure transmitter 120 and a second temperature sensor 122 are provided near the outlet of the main outlet pipe 114. The pressure near the outlet of the main outlet pipe 114 is approximately equal to atmospheric pressure. However, depending on the inlet condition, the steam can partially expand, fully expand, over expand. This can change the pressure at the outlet of the main outlet pipe 114. Therefore, the length of the main outlet pipe is so selected to achieve full expansion of steam within the pipe itself.
If the dryness fraction of the steam is higher than its limiting value, steam at the outlet of calorimeter will be in a superheated condition. Steam enthalpy is calculated by using the superheated properties equation. The first pressure transmitter 116 and the first temperature sensor 118 are used to verify the outlet pressure and temperature measured by second pressure transmitter 120 and second temperature sensor 122 by using the energy balance equation.
The foregoing description of the embodiments has been provided for purposes of illustration and not intended to limit the scope of the present disclosure. Individual components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.
TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of an apparatus and the method for measuring dryness fraction of steam that:
• is cost effective;
• is efficient;
• provides accurate and reliable measurement of dryness fraction of stream; and
• provides accurate reliable measurement even when the steam velocity at orifice is near the sonic velocity.
The foregoing description of the specific embodiments so fully reveals the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, step, or group of elements, steps, but not the exclusion of any other element, step, or group of elements, or steps.
While considerable emphasis has been placed herein on the components and component parts of the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other changes in the preferred embodiment as well as other embodiments of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

,CLAIMS:WE CLAIM:
1. An apparatus (100) for measuring dryness fraction of steam, said apparatus (100) comprising:
• an inlet pipe (102) for drawing steam from a source at a constant mass flow rate;
• a preliminary outlet pipe (110) connected with said inlet pipe (102) through an inlet pipe flange (104) and an outlet pipe flange (108);
• an orifice (106) of predetermined diameter, placed between said inlet pipe flange (104) and said outlet pipe flange (108);
• a diverging outlet pipe (112) connected with said preliminary outlet pipe (110), wherein diverging outlet pipe (112) having a diverging cross-section to reduce change in kinetic energy;
• a main outlet pipe (114) connected with said diverging outlet pipe (112), wherein diameter of the main outlet pipe (114) is higher than the diameter the preliminary outlet pipe (110);
• at least one pressure transmitter (116) and at least one temperature sensor (118) configured to sense pressure and temperature respectively at a pre-determined location in said main outlet pipe (114) and generate a sensed pressure signal and a sensed temperature signal respectively; and
• a second pressure transmitter (120) and a second temperature sensor (122) placed near the outlet of said main outlet pipe (114); and
• a computing unit configured to compute dryness fraction of the steam from the source based on said sensed pressure signal and said sensed temperature signal.
2. The apparatus (100) as claimed in claim 1, wherein said orifice (106) is of a small diameter is selected to reduce the velocity at the inlet to reduce the difference in kinetic energy.
3. The apparatus (100) as claimed in claim 1, wherein said orifice (106) is selected of an optimum diameter to increase the kinetic energy component in said preliminary outlet pipe (110).
4. The apparatus (100) as claimed in claim 1, wherein if the diameter of said orifice (106) is selected to provide a pressure ratio across said orifice (106) than the critical pressure ratio.
5. The apparatus (100) as claimed in claim 4, wherein the area and diameter of the orifice (106) is calculated by using the sonic velocity, density and allowed mass flow rate at the outlet of the orifice (106).
6. The apparatus (100) as claimed in claim 1, which includes:
a first pressure transmitter (116) and a first temperature sensor (118) configured to sense pressure and temperature respectively at a first location in said main outlet pipe (114) near the inlet of said main outlet pipe (114) and generate a first sensed pressure signal and a first sensed temperature signal respectively,
said second pressure transmitter (120) and said second temperature sensor (122) configured to sense pressure and temperature respectively at a second location in said main outlet pipe (114) near the outlet of said main outlet pipe (114) and generate a second sensed pressure signal and a second sensed temperature signal respectively, and
said computing unit is configured to calculate dryness fraction value corresponding to both said first location and said second location based on the sensed pressure signals and sensed temperature signals.
7. The apparatus (100) as claimed in claim 1, wherein said computing unit includes:
• a repository for storing a steam table and thermodynamic equations; and
• an arithmetic unit communicatively coupled with said pressure transmitter(s), said temperature sensor(s) and said repository.
8. A method for obtaining a measure of the dryness fraction of steam, said method comprising:
• drawing steam from a source at a constant mass flow rate via an inlet pipe (102);
• throttling said steam through said orifice (106);
• measuring the pressure (Pi) at the inlet of apparatus (100);
• calculating the pressure (Po) at the outlet of orifice (106) by using critical pressure ratio formula for choking flow condition;
• calculating the density (?) corresponding to pressure (Po) at the outlet of said orifice (106);
• calculating the sonic velocity (?s) corresponding to orifice outlet pressure(PO);
• calculating the orifice area (Ao) and mass flow rate (m) for choking condition;
• calculating the density (?i)corresponding to inlet pressure(Pi);
• calculating the velocity(Vi) in the inlet pipe;
• measuring the outlet pressure (Po) in an outlet pipe;
• calculating the density (?o) corresponding to outlet pressure (Po);
• calculating the velocity (Vo) at outlet condition;
• measuring the outlet temperature (To) in said outlet pipe (114);
• calculating the enthalpy (ho) corresponding to measured pressure (Po) and temperature (To) by superheated steam properties formula;
• calculating the inlet enthalpy (hi) by using the first law of thermodynamics for steady state throttling process; and
• calculating the dryness fraction(x) of steam.
9. The method as claimed in claim 7, wherein temperature is measured at a predetermined distance from said orifice (106) such that steam coming out of said orifice (106) gets sufficient time for expansion.
10. The method as claimed in claim 7, wherein a first pressure and a first temperature are measured at a first predetermined distance from said orifice (106) and a second pressure ad as second temperature are measured at a second predetermined distance from said orifice (106) that islarger than said a first determined distance.
11. The method as claimed in claim 7, wherein the choking condition is mathematically calculated and inlet and outlet kinetic energy is calculated for the choking flow and the dryness is corrected mathematically.