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01 November 2003

Unplugging a pressure situation

By Ron Szanyi, Mike Raterman, and Evren Eryurek

Refinery transmitters show diagnostic capabilities

We all know time is money, and when it comes to refineries, time means big money. That is why refiners want advanced diagnostic technologies that will help avoid unexpected process shutdowns.

One notorious cause of shutdowns is blockage in the impulse lines of pressure transmitters at fluid catalytic cracking (FCC) units in refinery applications. Though an experienced operator might have a feel for blockage of the impulse lines during normal operations, it is usually well after the fact. When the impulse lines plug up, the control system will not get an accurate pressure reading (the pressure sensor will be reading the trapped pressure between the sensor and the blockage in the impulse line).

Blockage of impulse lines can be very costly in a refinery. Depending on the capacity of the refinery, a process shutdown due to an impulse line blockage during operation of an FCC unit could cost as much as $1 million per day if the FCC operation completely shuts down. Further, it might take up to seven days to restart the FCC operation. The FCC unit of a refinery has a large impact on profits. Early detection of possible upsets can significantly enhance refinery profits.

With that potential economic impact in mind, the ExxonMobil Research and Engineering group decided to put advanced diagnostic technologies to the test and see where and how they might help avoid production outages in refineries.

The FCC unit ExxonMobil decided to test comes equipped with 18 levels of aeration taps on the regenerated catalyst standpipe. Several ring headers connect to a single flow controller, which controls the total flow to the group. The restriction orifice then sets aeration flow to each point on the standpipe within a grouping. There are three flow controllers for the 18 different levels of aeration, forming three groups.

The upper 17 aeration levels have pressure transmitters to aid in diagnosing flow instability problems and to help optimize the aeration distribution.

Line plugging has long been an issue for flow and level measurements in many process applications. Processes with dense materials, such as crude oil, or those in colder climates are susceptible to impulse line plugging.

In a typical process, length of impulse lines could vary from one foot to longer than ten feet. Although recent close-coupled designs should eliminate this problem, industry standards or the process conditions require impulse lines for flow and level measurements.

When the impulse lines of a pressure transmitter become blocked, operators and the control system can no longer rely on the measurement. Once impulse lines plug, the reliability of that measurement becomes very questionable, because only the trapped pressure level between the sensor and the point of blockage is measurable—not the actual process pressure.

There are three types of problems associated with the pressure transmitters and purge systems on an FCC unit and with the process itself:

1. Loss of a reliable signal due to a plugged pressure tap caused by the catalyst restricting the outlet
2. Plugged restriction orifice or filter resulting in diminished purge flow and possible loss in signal sensitivity
3. Circulation woes in the FCC unit caused by stick-slip flow condition

Although problems 1 and 2 may seem similar in nature, the first one involves the blockage of the pressure tap of the standpipe, not the plugging of the impulse lines of the pressure transmitter.

The third problem is a process problem, and is essentially a function of catalyst circulation rate, standpipe, and the fluidization properties. Under normal conditions, gas draws into the standpipe and travels downward between the catalyst particles (emulsion phase) as bubbles. These bubbles compress as they travel downward, forming smaller bubbles. In addition they will merge to form larger bubbles, which can subsequently break apart. This leads to pressure fluctuations or noise within the standpipe.

Sudden noises

Under certain conditions (low circulation, poor catalyst fluidization properties), the catalyst will over deaerate as the bubbles travel down the standpipe. The compression effect will then cause the bubbles to disappear. When this happens, the pressure buildup along the length of the standpipe is no longer smooth but becomes erratic. Under severe conditions the catalyst will bridge across the standpipe, momentarily stopping and then breaking loose again. This sudden stopping and starting of the catalyst flow is generally referred to as "stick-slip flow," which produces a very noticeable chugging noise with pressure fluctuations that become less random but more severe. If uncorrected this can result in severe damage to the standpipe system, particularly at expansion joints.

Normally, the "noise" from the standpipe should have no distinguishable pattern as a result of the random size and population of gas bubbles in the standpipe. When the catalyst bridges, the noise becomes more regular. The "noise" at the bridging condition shows up as large pressure fluctuations to the pressure transmitters that are generally in use today, and detecting this condition before it becomes a serious issue has been a costly challenge.

One goal of the field test was to determine if the statistical process monitoring diagnostic capabilities of the Foundation fieldbus-based pressure transmitters could detect noise anomalies in the standpipe early enough to allow the operators to prevent the bridging condition.

Advanced diagnostics block of the fieldbus pressure transmitter.

Follow the pattern

Plugged impulse line detection technology is pattern recognition with built-in intelligence that is aware of the environmental conditions of the pressure and differential pressure transmitters widely used in the process industries. Basically, the pattern recognition algorithm embedded in the pressure transmitters receives the sensor updates. The faster the response time, the more information it can capture about the process noise. This becomes important, especially for differential pressure applications, to differentiate a single-leg plugged condition from one that has both legs plugged.

In general, the measurement signal contains fluctuations superimposed on the average value of the pressure or differential pressure of the process, called process noise or signature. These fluctuations come about because of the flow, and are a function of the geometric and physical properties of the system. The time domain signatures of these fluctuations do not change as long as the overall system behavior stays the same. In addition, small changes in the average value of the flow variables do not affect these signatures. This offers an advantage in identifying and isolating line plugging, which is part of the underlying pattern recognition technology developed to solve the problem of line plugging.

When the lines between the process and the sensor start to clog through fouling and buildup on the inner surfaces of the impulse tubing, or loose particles in the main flow get trapped in the impulse lines, the time and frequency domain signatures of the fluctuations start to change from their normal states. The clogging decreases or increases the effect of damping on the pressure noise of the main flow signal. As the impulse lines clog, the noise levels of the measurement change.

Operational details of plugged impulse line detection technology come in two distinct sections once the system is properly configured, which consists of simply selecting a few parameters.

Learning phase: The algorithm first observes its environment, including the level of process noise and temperature conditions. At the end of this phase, the algorithm establishes the basic signature for the pressure transmitter used in that process. It establishes various parameters that represent the behavior of the process and keeps them in its memory so it can use them during the monitoring phase. The learning phase also has a verification phase to establish the repeatability of the process behavior.

Monitoring phase: This is where the algorithm periodically monitors the process and looks for changes in the process signature. Once it detects and verifies a change in the process conditions, the pressure transmitter sets its alert bit to inform the operator, because the plugging could cause a major process upset.

Traditionally, fault detection has been part of the control system, where analysis occurs by using the data collected by process historians. There are various reasons for this implementation choice; most important is that the field devices could not handle the tasks required of fault-detection methodologies. This is mainly due to the limited firmware capability of the older technologies. However, with the help of the advanced silicon technology and with the digital fieldbus technologies, today's smart transmitters are capable of providing more information regarding the process and its conditions in addition to traditional process variable information.

One can group process anomalies into five categories, and these are common for all sensor types and processes: pressure, temperature, flow, level, and others. Using advanced pattern recognition and statistical analysis methods, fieldbus transmitters and smart valves can now detect drift, bias, noise, spike, and stuck behaviors of each process.

Trials begin

The following were the focus of the field trials: Loss of a reliable signal due to a plugged pressure tap caused by the catalyst restricting the outlet; plugged restriction orifice or filter resulting in diminished purge flow and possible loss in the signal sensitivity; and circulation problems of the FCC unit caused by stick-slip flow condition.

The testing on the unit was broken into two days. On day one the plugged tap and loss of purge scenarios underwent testing, and on day two officials tested a circulation problem. Prior to starting the test officials calibrated each instrument to establish new baseline values for the diagnostics analysis, and plugged line diagnostics and statistical process monitoring (SPM) features of the transmitters started to learn the process and establish baseline patterns. The pressure transmitter successfully detected every test scenario.

Purge detection

Engineers tested the loss of purge flow condition by closing the purge source valve. They expected that either the built-in impulse line plugging detection feature or the statistical data collected at the fieldbus transmitter via SPM would provide sufficient data. The test results showed both diagnostic features indicated the loss of flow.

The transmitter's SPM diagnostic technology continuously samples the process signal from the sensor at high frequencies and performs additional calculations on it. The transmitter calculates the mean value of the signal and how that changes with time. It also calculates the standard deviation in the noise from the process signal. The standard deviation calculation should allow us to detect a change in the "white noise" characteristic long before transition into "stick-slip flow." This will allow operations to take corrective actions before problems develop.

The data gathered from the advanced diagnostics block suggests ExxonMobil could detect catalyst problems as much as thirty minutes in advance in some cases. This could be very useful for avoiding possible process upsets when the diagnostic data integrates into the operation. P

Behind the byline

Ron Szanyi and Mike Raterman are with ExxonMobil Research and Engineering, and Evren Eryurek is with Emerson Process Management.