Control system and method for detecting plugging in differential pressure cells

• US 20040068392A1
• Filed: 10/07/2002
• Published: 04/08/2004
• Est. Priority Date: 10/07/2002
• Status: Active Grant

• Current Assignee: Honeywell International Inc.
• Original Assignee: Honeywell International Inc.
• Inventors: Mylaraswamy, Dinkar
• Granted Patent: US 6,904,386 B2
• Time in Patent Office: Days
• US Class Current: 702/183
• CPC Class Codes: G05B 23/0227 Qualitative history assessment, whereby the type of data acted upon, e.g. waveforms, images or patterns, is not relevant, e.g. rule based assessment; if-then decisions
• CPC Class Codes: G05B 9/02 electric

Abstract

A method and control system that monitors the high frequency or commonly referred to as the “noise” component of a measurement signal to detect plugging conditions in fluid flow systems monitored by DP-cell based sensors. This high frequency component has contributions from the process factors like disturbances, user actions and random effects like turbulence. A test statistic θ(t) has been developed that monitors the proportion of variance introduced by the first two factors and the third factor. By monitoring this proportion, it is possible to detect a frozen sensor that is characterized by a dramatic reduction in the variance due to process factors over a sufficiently long detection window. The method has successfully detected a frozen level sensor using historical data from a large petrochemical plant. Unlike spectral analysis, this method works with measurements sampled at frequencies commonly achievable in a process environment. This increases the practical utility of the method. In addition, relatively simple calculations make it computationally attractive.

Claims

1. A method for detecting a plugging condition in lines that provide a signal in a system, said method comprising:

(a) developing a variance factor; and

(b) signaling an occurrence of said plugging condition in response to changes in said variance factor.

2. The method of claim 1, further comprising:

(c) processing said signal to detect a zero mean velocity condition, and wherein step (b) signals said occurrence of said plugging condition when said variance factor and said zero mean velocity condition are developed simultaneously.

3. The method of claim 1, wherein said lines are impulse lines of a DP-cell, wherein said signal is proportional to pressure and velocity of a fluid, and wherein said variance factor measures the relative importance of variance imposed by turbulence and process dynamics at a current time t, with respect to a baseline value and a predetermined threshold.

4. The method of claim 3, wherein step (a) uses a current value of said sensor signal, and wherein said baseline value is based on a value of said sensor signal that is earlier in time than said current value.

5. The method of claim 3, wherein said baseline value is dynamically updated.

6. The method of claim 4, wherein said current value is based on a window of N samples of said sensor signal, wherein said window of N samples advances at least one sample at a time to provide a plurality of windows and wherein step (a) is repeated for each of said plurality of windows.

7. The method of claim 6, wherein said baseline value is dynamically updated based on a value of said baseline developed prior to said window of said current value.

8. The method of claim 7, wherein said baseline value is updated during each of said windows over a detection window that spans a plurality of said windows of N samples.

9. The method of claim 8, wherein said baseline value is updated using an exponentially weighted moving average of said plurality of sample windows.

10. The method of claim 6, wherein N is selected to minimize the effect of slow process dynamics.

11. The method of claim 10, wherein said developing step (a) compares said baseline value and said threshold to a ratio of high frequency components of said signal due to turbulence effects and to process effects so as to substantially eliminate effects due to working conditions of said system.

13. The control system of claim 11, wherein said operations further comprise:

(c) processing said signal to detect a zero mean velocity condition, and wherein operation (b) signals said occurrence of said plugging condition when said variance factor and said zero mean velocity condition are developed simultaneously.

12. A control system for detecting a plugging condition in lines provide a sensor signal in a process system, said control system comprising:

a controller including a processor, a memory and a program that causes said processor to perform a plurality of operations comprising;

(a) developing a variance factor; and

(b) signaling an occurrence of said plugging condition in response to said variance factor.

14. The control system of claim 12, wherein said lines are impulse lines of a DP-cell, wherein said signal is proportional to pressure and velocity of a fluid, and wherein said variance factor measures the relative importance of variance imposed by turbulence and process dynamics at a current time t, with respect to a baseline value and a predetermined threshold.

15. The control system of claim 14, wherein said baseline value is dynamically updated.