LIQUID LEVEL SIGNAL CORRECTION SYSTEM AND METHOD

Abstract

A system and method for correcting a measured liquid level signal from a bypass chamber comprises calculating a density error correction to modify the liquid level signal and indicate a correct liquid level corresponding to the level of liquid inside of a vessel to which the bypass chamber is in fluid communication.

Description

BACKGROUND

The present disclosure relates in general to liquid level indicators, and more particularly, to a system and method for correcting density errors in bypass level indicators for steam boiler drums or vessels.

All of the subject matter discussed in the Background section is not necessarily prior art and should not be assumed to be prior art merely as a result of its discussion in the Background section. Along these lines, any recognition of problems in the prior art discussed in the Background section or associated with such subject matter should not be treated as prior art unless expressly stated to be prior art. Instead, the discussion of any subject matter in the Background section should be treated as part of the inventor's approach to the particular problem, which in and of itself may also be inventive.

Bypass level indicators are commonly installed outside of a vessel to provide a convenient visual indication of liquid levels inside of the vessel. The bypass level indicator may comprise a vertical chamber or tube that is in fluid communication with the vessel contents, wherein the level of the liquid inside of the bypass chamber corresponds to the level of liquid inside of the vessel, with markings along the chamber or tube to indicate that level. An example of such common level indicator is described in U.S. Pat. No. 5,988,701 by Wu, for example.

However, where the liquid contents of the vessel are subject to high temperatures such as to create steam in a steam boiler, the discrepancy in density between the liquid in the vessel versus the liquid in the bypass chamber may lead to inaccurate level indications on the bypass level indicator, leaving people to often inaccurately estimate what the liquid level may be in the vessel. Having proper, real-time measurements of levels are extremely important because too much liquid may lead to a vessel exploding, or otherwise damaging downstream components to which the vessel is connected.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

A system and method for correcting a measured liquid level signal from a bypass chamber comprises calculating a density error correction to modify the liquid level signal and indicate a correct liquid level corresponding to the level of liquid inside of a vessel to which the bypass chamber is in fluid communication.

In another aspect, a liquid level signal correction system comprises a vessel for containing liquid and saturated steam; a bypass chamber configured to be in fluid communication with the vessel, liquid and saturated steam; a level transmitter for measuring a level of the liquid in the bypass chamber and generating a bypass level signal; a first temperature sensor for measuring a temperature of the saturated steam and generating a steam temperature signal, and a second temperature sensor for measuring a temperature of the liquid and generating a liquid temperature signal; and a signal modifier configured to receive the bypass level signal, steam temperature signal and liquid temperature signal, and to calculate a density error correction based on the steam temperature signal and liquid temperature signal to modify the bypass level signal to indicate a correct liquid level corresponding to the level of the liquid inside the vessel.

In another aspect, a method for correcting a measured liquid level signal from a bypass chamber comprises measuring a level of liquid in a bypass chamber and generating a bypass level signal; measuring a temperature of saturated steam in a vessel and generating a steam temperature signal; measuring a temperature of a liquid in the vessel and generating a liquid temperature signal; sending the bypass level signal, steam temperature signal and liquid temperature signal to a signal modifier; and using the signal modifier to calculate a density error correction based on the received steam temperature signal and liquid temperature signal to modify the bypass level signal to indicate a correct liquid level corresponding to the level of the liquid inside the vessel.

Other aspects and advantages will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a simplified side view of a liquid level signal correction system according to the present disclosure.

FIG. 2 is a diagram showing an example measurement and calculation method for the signal correction system.

FIG. 3 is a flowchart showing an example process of using the signal correction system.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram showing a simplified side view of vessel 1, liquid 5, saturated steam 5S, liquid level 6, bypass level 7, density error 8, valves 9, analog level transmitter 10, bypass chamber 20, temperature sensors 30 including water temperature sensor W and steam temperature sensor S, and signal modifier 40. Vessel 1 may also be referred to as a steam boiler drum for purposes of the present disclosure.

Bypass chamber 20 is attached to vessel 1 to provide a suitable platform for mounting the analog level transmitter 10, and to provide a manometer-based communication with the vessel 1, which may comprise a pressure vessel or steam boiler drum. Bypass chamber 20 may comprise a fabricated piping structure consisting of a vertically mounted chamber tapped to the vessel 1 or steam boiler drum with the purpose of maintaining a liquid level 7 within the bypass chamber 20 reflective of liquid level 6 within the vessel 1 or drum. The bypass chamber 20 may be isolable from vessel 1, such as via valves 9, and may be connected by welding, flanges or unions, and used as a device to mount various instruments as needed.

The analog level transmitter 10 is mounted to bypass chamber 20 as shown in FIG. 1, and may comprise a commercially available level transmitter that utilizes time domain reflectometry, differential pressure, displacement, float/buoyancy, capacitance, pressure or other means of providing liquid level measurement of bypass level 7 in chamber 20. The analog level transmitter 10 may comprise a probe, transducer or sensor, displacer and/or a float to determine the bypass level 7.

In a closed circuit, saturated steam application, the temperature of the steam 5S in all locations (whether in vessel 1 or bypass chamber 20) is equal to the temperature of the liquid 5 in the vessel 1, but not the liquid in the bypass chamber 20, where steam has condensed and cooled into liquid water. Thus, where liquid 5 is subject to high temperature or pressure such as to create saturated steam 5S, the density of liquid 5 in vessel 1 will be different than the density of liquid in the bypass chamber 20, thus causing a density error 8 between the indicated bypass level 7 and actual liquid level 6. Typically when the temperature of a liquid increases, the volume of the liquid also increase which lowers the density of that liquid.

Thus, to correct for the density error 8 and indicate an accurate liquid level 6 using analog level transmitter 10, the system and method of the present disclosure are provided. This includes temperature sensors 30, including water temperature sensor W and saturated steam temperature sensor S, which transmit data to signal modifier 40 to dynamically correct the analog signal of the level transmitter 10 to compensate for the calculated error in liquid density, as will be described in further detail below. Temperature sensors 30 may comprise thermocouple or resistance temperature detectors, or other suitable temperature measurement device capable of converting a temperature measurement into an electrical signal. Temperature sensors 30 may be mounted to the bypass chamber 20 or vessel 1 with direct insertion or thermowells to measure temperature of either the liquid 5, such as water, or saturated steam 5S. Temperature sensor 30 may also measure temperature of the vapor side of the bypass chamber in communication with steam 5S from vessel 1.

The signal modifier 40 may comprise a density correction processor or circuit that receives the signal input from the two or more temperature sensors 30 and dynamically modifies the analog level transmitter 10 signal in real-time based on a calculation of the density difference between liquid 5 and saturated steam 5S. The conditioned signal may be used to provide a corrected level indication or used as an input for feedwater control. The signal may comprise a 4-20 mA signal or other suitable means. The processor looks up the density of the saturated steam and water, and runs the calculation, then corrects the 4-20 mA signal. The circuit can provide a means of indicating a real-time liquid level with a discrete visual display in one or several locations. Alternatively, the steam temperature may be calculated by using pressure data and ASME Saturated Steam tables.

The density correction circuit of signal modifier 40 may also comprise a processor, correction circuit, and local and/or remote displays, all fed by a standard power supply. This may optionally be used to provide the current required to operate the level transmitter 10 or temperature sensors 30, and to control and power a remote or local liquid level 6 display or be used as input data for vessel 1 control.

FIG. 2 shows an example method of calculating and indicating the actual liquid level 6, represented by H_d, as well as calculating the density error 8, here represented as Hs, wherein D_d is the density of liquid 5 (such as water) in vessel 1, H_g is the bypass level 7 as measured by level transmitter 10, D_g is the density of the liquid in bypass chamber 20, and Ds is the density of saturated steam 5S from vessel 1, for example (or in the vapor side of bypass chamber 20). The required inputs and calculations shown in FIG. 2 may be received and calculated by the density correction circuit of signal modifier 40. ASME saturated steam tables may be referenced by a processor of the programmed signal modifier 40 to convert the temperatures of liquid water 5 and saturated steam 5S (measured by water temperature sensor W and steam temperature sensor S) into the required input densities D_d and Ds respectively. More than one temperature sensor 30, e.g., thermocouple, may be used to measure an average temperature of water in the bypass chamber 20 to improve accuracy of the calculation, each generating a signal provided to the signal modifier 40. In such case the temperature sensors 30 may be distributed along different locations of the bypass chamber, and the average temperature may be calculated by the signal modifier 40 density correction circuit by inputting the average temperature value into calculation of the density of liquid 5, or D_d.

FIG. 3 is a flowchart showing an example method or process of outputting a corrected liquid level 6 signal using signal modifier 40 and the calculation methods of FIG. 2 as described above, and utilizing the system described in the present disclosure.

For example, a method for correcting a measured liquid level signal 7 from a bypass chamber 20 may comprise measuring a level of liquid in the bypass chamber 20 and generating a bypass level 7 signal; measuring a temperature of saturated steam 5S in a vessel 1 and generating a steam temperature signal; measuring a temperature of a liquid 5 in the vessel 1 and generating a liquid temperature signal; sending the bypass level 7 signal, steam temperature signal and liquid temperature signal to a signal modifier 40; and using the signal modifier 40 to calculate a density error 8 correction based on the received steam temperature signal and liquid temperature signal to modify the bypass level 7 signal to indicate a correct liquid level 6 corresponding to the level of the liquid inside the vessel 1. The signal modifier 40 may calculate the density error 8 correction by first calculating a density of the liquid 5 and a density of the saturated steam 5S based on the liquid temperature signal and the saturated steam temperature signal. Optionally, the signal modifier 40 may output the correct liquid level 6 for use in feedwater control, and/or for visual display. In another aspect, such as to improve accuracy of the density error correction, multiple temperature measurements of the liquid 5 in the vessel 1 may be taken to generate multiple measured liquid temperature signals, which can be sent to the signal modifier 40 for calculating an average of the liquid temperature signals for use in calculating the density error 8 correction.

While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

1. A liquid level signal correction system, comprising: a vessel for containing liquid and saturated steam;a bypass chamber configured to be in fluid communication with the vessel, liquid and saturated steam;a level transmitter for measuring a level of the liquid in the bypass chamber and generating a bypass level signal;a first temperature sensor for measuring a temperature of the saturated steam and generating a steam temperature signal, and a second temperature sensor for measuring a temperature of the liquid and generating a liquid temperature signal;a signal modifier configured to receive the bypass level signal, steam temperature signal and liquid temperature signal, and to calculate a density error correction based on the steam temperature signal and liquid temperature signal to modify the bypass level signal to indicate a correct liquid level corresponding to the level of the liquid inside the vessel.

2. The system of claim 1, wherein the signal modifier calculates the density error correction by calculating a density of the liquid and a density of the saturated steam based on the liquid temperature signal and the saturated steam temperature signal.

3. The system of claim 1, wherein the signal modifier is further configured to output the correct liquid level for feedwater control.

4. The system of claim 1, wherein the signal modifier is further configured to output the correct liquid level for display.

5. The system of claim 1, further comprising a plurality of the second temperature sensors distributed along different locations in the bypass chamber for generating multiple measured liquid temperature signals, and wherein the signal modifier calculates an average of the liquid temperature signals for use in calculating the density error correction.

6. A method for correcting a measured liquid level signal from a bypass chamber comprises; measuring a level of liquid in a bypass chamber and generating a bypass level signal;measuring a temperature of saturated steam in a vessel and generating a steam temperature signal;measuring a temperature of a liquid in the vessel and generating a liquid temperature signal;sending the bypass level signal, steam temperature signal and liquid temperature signal to a signal modifier; andusing the signal modifier to calculate a density error correction based on the received steam temperature signal and liquid temperature signal to modify the bypass level signal to indicate a correct liquid level corresponding to the level of the liquid inside the vessel.

7. The method of claim 6, further comprising using the signal modifier to calculate the density error correction by first calculating a density of the liquid and a density of the saturated steam based on the liquid temperature signal and the saturated steam temperature signal.

8. The method of claim 6, further comprising outputting the correct liquid level from the signal modifier and using it for feedwater control.

9. The method of claim 6, further comprising outputting the correct liquid level from the signal modifier for visual display.

10. The method of claim 6, further comprising taking multiple temperature measurements of the liquid in the vessel to generate multiple measured liquid temperature signals, sending the multiple measured liquid temperature signals to the signal modifier, and using the signal modifier to calculate an average of the liquid temperature signals for use in calculating the density error correction.