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Apr
28

Identifying Equipment Performance Problems

Plants often consist of many production units and trains of equipment. The production units may produce raw materials as intermediates in a series of processes that lead to predominant product. Often intermediates or byproducts are also sold. Trains of equipment feed into a common tank that may feed other trains of equipment. Recycle streams further complicate the process. The production usually culminates as a feed to an area for final processing, such as purification, drying, and packing. Equipment diagnostics and the automation of corrective actions are essential for these plants. The equipment diagnostic of widest applicability involves the measurement of heat transfer.

The measurement of heat transfer can be done by a flow measurement and inlet and outlet temperature measurements of a principle stream to compute the net gain or loss in energy of the stream. For vessels, the stream is a utility such as cooling water or steam. For heat exchangers without reactions, measurements of a process stream can be used. For synchronization in the heat transfer calculation, the inlet temperature should be passed through a deadtime block with the deadtime equal to the residence time before the difference between the inlet and outlet temperatures is computed. The residence time is simply the volume between the inlet and outlet temperatures divided by the flow. The heat transfer rate can be used as an inferential measurement of the rate of condensation crystallization, evaporation, drying, and reaction for monitoring unit operation performance. The integral of the rate is representative of the total amount of material condensed, crystallized, dried, and reacted. Probably, the largest opportunities occur in the monitoring of crystallizations and reactions. These rates may be limited by the heat transfer coefficients particularly if the process surfaces can be coated or fouled.

The computation of the overall heat transfer coefficient is complicated by the driving force for heat transfer being the log mean temperature difference between the two sides of the heat transfer surface and the variable heat transfer surface area covered in a vessel. While theoretically 4 temperature measurements (inlet and outlet of the fluid on each side of the heat transfer surface) is needed along with the heat transfer area covered, an inferential measurement can simply use the minimum difference between the outlet temperatures of the process and utility stream. This minimum difference is known as the “approach temperature.” For a relatively constant process load and utility supply temperature an increase in the “approach temperature” or an increase in the utility flow is indicative of a decrease in heat transfer coefficient.

Fouling can generally be measured by differential pressure measurements across a portion of a unit operation. An increase in differential pressure at a given flow is indicative of an increase in fouling of coils, filters, tubes, and trays. Fouling of heat transfer surfaces from chemicals in the process or biological growth in cooling tower water that decreases the heat transfer coefficient can show up as simply an increase in pressure drop on the fouled side. A more exact calculation would use the sizing equation for the flow coefficient (Cv) of a control valve. For liquid flow, the equivalent flow coefficient is proportional to the volumetric flow multiplied by the square root of the ratio of fluid specific gravity to pressure drop. For vapor flow, the computation gets more complex. The rate of decrease in the flow coefficient is a measure of the fouling rate.