Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2000-06-05
2002-11-19
Hilten, John S. (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S049000, C702S050000, C702S029000
Reexamination Certificate
active
06484108
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of leak detection in process systems and, more particularly, for leak detection performance in boilers such as black liquor recovery boilers of any other area where the detection of leak created mass imbalances using online measurements is of interest.
BACKGROUND OF THE INVENTION
Early detection of recovery boiler leaks continues to be an important objective of power and recovery operations because of the serious consequences of a water leak into the recovery boiler furnace. The leak detection techniques currently in use can generally be classified into four categories: (1) operator observations; (2) acoustic systems; (3) chemical mass balance systems; and (4) water mass balance systems. Each method has its own inherent strengths and weaknesses. The need for multiple methods of detection as a means to overcome individual weaknesses and ensure reliable detection also has been documented.
The application of the present invention is directed to providing boiler operators with tradeoffs among sensitivity, false alarms and offline periods of leak detection systems that use water or chemical mass balance methods around a recovery boiler. For a water mass balance (WMB), flow meters around the waterside of the boiler are used to calculate the balance of water entering and leaving the boiler. The chemical mass balance (CMB) technique relies on a combination of flow measurements and chemical concentration measurements to calculate the mass balance of a specific stable and non-volatile species (such as phosphate or molybdate) around the waterside of the boiler. In either case, if a statistically significant loss is calculated a water leak is suspected and an alarm is triggered to alert the operator.
Typically, there is interest in detecting leaks of 1,000 to 10,000 lb/hr or 0.1% to 1% of a typical 500,000 lb/hr total flow. This presents a challenge when one considers the magnitude and type of noise or variation that exists in a calculated water or chemical mass balance signal. For a water mass balance system, noise arises from the inherent variability of steam and water flows, the flow meters measuring them, and the drum level control circuit. An indication of the noise associated with a calculated water mass balance is shown in Table 1. The calculated standard deviation of a water mass balance is shown for five study recovery boilers at times when their loads were relatively stable.
Three observations can be made from Table 1: First, the magnitude of the noise presents a distinct challenge in meeting the stated leak detection goal (less than 2% of steam load). Second, the magnitude of the noise varies among boilers. The differences are primarily due to the differing degrees of sophistication and care taken in tuning the drum level control circuit. The mass balance noise is primarily related to the variation in time response (lag) between an altered steam flow and the responding change in feedwater pumping rate. Third, the noise is variable for a given boiler over a daily and even weekly basis. Any water mass balance method requires some way to manage this flow-related noise.
For chemical mass balance, the situation is improved as the number of measurements and their noise levels are lower than water mass balance. One of two related approaches have been used. In the first, the concentration of a tracing or treatment chemical (entering at fixed concentration) and exiting the boiler are determined while holding the ratio of feedwater to blowdown flow fixed. In the second, the pumping rate of a chemical of known concentration is measured while the blowdown chemical concentration and flowrate are measured.
In the first case, the measurements are chemical concentrations entering and exiting the boiler. In the second, they are product chemical concentration (fixed), pumping rate of that chemical, blowdown flow and blowdown chemical concentration. Noise levels for the individual measurements of the second method have been determined and are shown in the Table 2.
In addition to the random noise discussed above, steam loads in recovery boilers often vary due to liquor heating value variation, control of liquor supply, operation of other boilers in the system, and other process influences.
FIG. 1
shows the duration vs. % load drops in five recovery boilers taken over ¾ year to 1 year time periods. The area within ±20% on the y-axis is assumed to be normal boiler load variations and were not plotted. As can be seen from the plots and tables, significant load changes are a regular occurrence with recovery boilers. Also, these load changes vary in duration by quite a wide range of times. Three of the five boilers studied only decrease their steam load from “normal” steaming rates; two boilers both increase and decrease load. Steam load changes affect water mass balance leak detection systems in one of two ways: (1) Load swings alter the steam to water ratio in the boiler and thus the total mass. With a lower steam to water ratio expected at lower load, the boiler water mass increases. As the load is decreased, the mass increases which may lead to a false alarm; (2) Flow meter calibration errors vary with steam load. Demonstration of the combined effect is shown in
FIG. 2
where a load drop from 500 klb/hr to 350 klb/hr leads to an apparent 15 klb/hr “leak” in a raw water mass balance.
Load changes also affect chemical mass balance systems. As the load decreases, the amount of water present in the boiler increases which dilutes the tracer or treatment chemical potentially leading to a false alarm. When the load increases back to normal, the mass of water decreases making the tracer concentration increase. The characteristic of this type of change is a sharp change in chemical concentration as the load is changed.
As can be seen from these curves plotted in
FIG. 3
, there is a strongly likelihood that such load drops can lead to false alarms. Given the number and duration of these load changes, mass balance systems not correcting for these will spend significant time in a false alarm state. Using the data from the five boilers shown in
FIG. 1
, estimates were made as shown in Table 3.
Based on the data from these five boilers, a mass balance not correcting for load changes could expect false alarms due solely to load changes on average every seven to fourteen days with times in alarm condition between 2% and 9%. Mass balance systems which shut down when load changes occur would be offline at these times. Alternatively, if a system were designed to avoid false alarms, but was not designed to provide load swing correction or disabling, detection limits would be relaxed to the point where the system would not be a useful detection tool.
There are other system changes that can affect mass balance measurements. One with a potentially large impact are boiler startups especially those where the boiler has been down for more than a day.
Mass balances (chemical and water) are unstable during startups. The flows will be outside normal operation and the boiler water will change as cold water is converted to a mixture of steam and water with increased steam load. To better understand this phenomena, an extensive analysis of ten boiler startups was completed for one boiler system.
FIG. 4
shows steam flow and a smoothed raw water mass balance for a typical boiler startup.
The overall mass balance does not stabilize for fifteen to twenty hours. A similar situation is observed for chemical mass balance systems. An effective mass balance-based leak detection system must be able to avoid the false alarms associated with mass balance instabilities.
There are other situations where the mass balance (especially water mass balance) is briefly upset. Some of these include over-pressurization venting, momentary drum level upsets, and manual blowdown. Additionally, some boiler processes have periodic oscillations such as drum level variation (fast) or flow meter drift (slow). An effective system must deal with these without generating un
Burgmayer Paul R.
Gunther John C.
Hong Ke
Caesar Rivise Bernstein Cohen & Pokotilow Ltd.
GE Betz, Inc.
Hilten John S.
Lau Tung S
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