Troubleshooting using

One of the usual responsibilities of an engineer is problem solving, e.g., process troubleshooting. Using an established technique of problem analysis, you can become a better problem solver by knowing which problem to attack first. The Pareto chart is one of the most powerful tools in problem solving, yet is often overlooked in U.S, industry. 

The second major troubleshooting use of simulation during startup was on the natural gas feedstock control system. After the trip of one of the methanol plants, severe transients in the feedstock system caused the second methanol plant to trip due to overpressure lifting PSV s and tripping on low steam to carbon ratio. This led to a complex trip and a downtime of days. The problem was found to be in the natural gas letdown controls (see figure 5). Simple as this control loop appears, it caused a number of trips during startip. 

Simple Diagnostics and Troubleshooting Using the RTD for Ideal Reactors... 

Ensure that sufficient space exists around the electrical equipment or circuit in order to maintain or operate it safely. Personnel must remove personal metal jewelry. De-energize electrical equipment before testing or repairing in accordance with the OSHA Lockout Tag-out Standard 29 CFR 1910.147. If de-energizing electrical equipment or circuits increases the potential of an electrical hazard or requires troubleshooting, use appropriate tools and PPE as mandated by the situation. 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

A silver stain is used when proteins exist in a very small quantity or when analysis of as many bands as possible created by separation techniques is desired. One positive apphcation of silver stain is its sensitivity. A drawback of the silver stain, however, is that it is more complex and often requires more troubleshooting to obtain the desired results. 

The three vertices are the operating plant, the plant data, and the plant model. The plant produces a product. The data and their uncertainties provide the histoiy of plant operation. The model along with values of the model parameters can be used for troubleshooting, fault detection, design, and/or plant control. 

The vertices are connected with hues indicating information flow. Measurements from the plant flow to plant data, where raw measurements are converted to typical engineering units. The plant data information flows via reconciliation, rec tification, and interpretation to the plant model. The results of the model (i.e., troubleshooting, model building, or parameter estimation) are then used to improve plant operation through remedial action, control, and design. 

Extended Plant-Performance Triangle The historical representation of plant-performance analysis in Fig. 30-1 misses one of the principal a ects identification. Identification establishes troubleshooting hypotheses and measurements that will support the level of confidence required in the resultant model (i.e., which measurements will be most beneficial). Unfortunately, the relative impact of the measurements on the desired end use of the analysis is frequently overlooked. The most important technical step in the analysis procedures is to identify which measurements should be made. This is one of the roles of the plant-performance engineer. Figure 30-3 includes identification in the plant-performance triangle. 

Data Acquisition As part or the understanding, the measurements that can be taken must be understood. A useful procedure to prepare for this is to develop a tag sheet for the process (Lieberman, N.P., Troubleshooting Refinery Processes, PennWell Books, Tulsa, 1981, 360 pp). An example of a simplified sheet is given in Fig. 30-5. 

Focus For the purposes of this discussion, a model is a mathematical representation of the unit. The purpose of the model is to tie operating specifications and unit input to the products. A model can be used for troubleshooting, fault detection, control, and design. Development and refinement of the unit model is one of the principal results of analysis of plant performance. There are two broad model classifications. 

Measurement Selection The identification of which measurements to make is an often overlooked aspect of plant-performance analysis. The end use of the data interpretation must be understood (i.e., the purpose for which the data, the parameters, or the resultant model will be used). For example, building a mathematical model of the process to explore other regions of operation is an end use. Another is to use the data to troubleshoot an operating problem. The level of data accuracy, the amount of data, and the sophistication of the interpretation depends upon the accuracy with which the result of the analysis needs to oe known. Daily measurements to a great extent and special plant measurements to a lesser extent are rarelv planned with the end use in mind. The result is typically too little data of too low accuracy or an inordinate amount with the resultant misuse in resources. 

The above assumes that the measurement statistics are known. This is rarely the case. Typically a normal distribution is assumed for the plant and the measurements. Since these distributions are used in the analysis of the data, an incorrect assumption will lead to further bias in the resultant troubleshooting, model, and parameter estimation conclusions. 

Overview Interpretation is the process for using the raw or adjusted unit measurements to troubleshoot, estimate parameters, detect faults, or develop a plant model. The interpretation of plant performance is defined as a discreet step but is often done simultaneously with the identification of hypotheses and suitable measurements and the treatment of those measurements. It is isolated here as a separate process for convenience of discussion. 

Too few people eonsider the equipment with the seal seleetion. In most eases, poor equipment will give poor seal performanee, regardless of the seal or arrangement ehosen. Also, beware that different pumps with the same shaft diameter and TDH may present different sealing problems. (Note These same eonsiderations may be used for troubleshooting.)... 

Lieberman gives two rules of thumb for troubleshooting fractionators that could also be used as checks on a design. First, the pressure drops across a section of trays must not exceed 22% of the space between the tray decks, to avoid incipient flood. Mathematical , hold... 

For column analysis and troubleshooting it is important to have pressure drop measured with a DP cell. The differential pressure can also be used to control column traffic. A good way to do this would be to let the differential pressure control the heating medium to the reboiler. The largest application for differential pressure control is with packed columns where it is desirable to run at 80 to 100% of flood for best efficiency. 

Temperature sensing points at various points along the tower shell are often a useful troubleshooting tool. Many... 

The simple checks need to be made first. These include column liquid level, temperature profile, pressure profile and stream flow rates. In addition, a Delta-P. survey using test gauges should be made in the field and if not by the troubleshooter then under his direct supervision. Just... 

Table 1 presents a ehecklist for troubleshooting distillation eolumns. Hopefully it can be used as a starting point for a specific unit. The order will depend on which process variables are most important. Again, the simple checks, which can be made quickly, should be done first. 

A colloid fog will scatter a beam of light. This is called the Tyndall Effect and can be used as a troubleshooting tool. 

Glycol plant troubleshooting information is presented from the article Dehydration Using TEG by Manning and Thompson. 

Maintenance and production records, along with the used lean and rich glycol analyses, can be very helpful to the troubleshooter. A history of filter element, carbon, tower packing, and firetube changeouts can sometimes be very revealing. The frequency of pump repairs and chemical cleaning jobs is also beneficial. With this type of knowledge, the troubleshooter can quickly eliminate and prevent costly problems. 

Corley, J. E., Troubleshooting Turbomachinery Problems Using a Statistical Analysis of Failure Data, Proceedings of the 19th Turbomachinery Symposium, Texas A M University, College Station, TX, 1990, pp, 149-158. 

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