Pressure services flooding

Water can cause severe operational problems in services which are not meant to handle it. One refiner (296) stated that 99 percent of his fractionator upsets are due to water other refiners (296) agreed with this evaluation. Most problems occur when the column separates water-insoluble materials such as hydrocarbons. The main adverse effects of water in such services are pressure surges, flooding, cycling, corrosion, hydrates, and off-spec products. Typical sources of water in this t5q>e of services are... 

A column flooded prematurely after being switched to a new service. Flooding was caused by an excessive pressure drop in a kettle reboiler circuit backing up liquid to above the rdboiler return nozzla Elevating vigKn inlet above liquid level solved the problem. Ladr of level indication made diagnosing the problem difficult. 

For general service the average centrifugal pump should lift about 15 feet of water on its suction side. However, since each process situation is different, it is not sufficient to assume that a particular pump wll perform the needed suction lift. Actually, certain styles or models of a manufacturer s pumps are often specially adapted to high lift conditions. On the other hand it is unnecessary to select a high lift pump when pressure head or flooded suction conditions prevail. Proper evaluation of suction lift conditions cannot be over emphasized. 

Unattended operations must be planned with automatic safety switches that prevent serious damage (fire, flooding, explosion) in case of accidental equipment failure or interruption of utility services such as electricity, water, or gas supplies. Of special concern are the constant flow of cooling water and the operation of high-temperature baths. In the case of water flow, a device should be installed in the water line to (1) automatically regulate the water pressure (so as to avoid surges that might disconnect or rupture a water hose), and (2) automatically turn off electrical connections and water-supply valves in case of a total loss of water supply. In the case of hot thermostat baths or ovens, a sensor/control device should be installed that automatically turns off the electrical power to all heaters if the temperature exceeds some preset upper limit. 

Figure 13.11 shows the principal characteristics of Sulzer CY packing for water distillation service [M6]. The optimum throughput is said to be at 75 percent of flooding, at which the F factor is 1.7. At this load, the gas-phase pressure drop is about 4 Torr/m, the Uquid holdup is about 6 percent of the packed volume, and the observed height of a transfer unit (htu) has been found to be between 6.5 and 12 cm. The observed variations in htu are attributed to variations in the wetting of the packing, which is impaired by traces of oil and other hydrophobic impurities in the water. 

Problems of establishing a liquid seal in the column downcomers are common in low-liquid-rate services during startup. In such services, the downcomer liquid seal may also occasionally be lost during operation. Symptoms of these problems are excessive entrainment and poor separation. As distinct from flooding, column pressure drop may remain low. These problems, and cures that can be implemented at the design stage, are described in Sec. 6.18. 

Manometers are often suitable for temporary differential pressure measurements at low pressures with nonhazardous systems. Manometers are portable, easily installed, and have a wide range of operation. Caution is required to ensure that the manometer used is suitable for the service. A comprehensive set of application guidelines for manometers in flood testing is available elsewhere (2). 

Flooding was being initiated from the bottom of the tower. I knew this because 79 in. of water-pressure drop represented a AP per tray of about 7 in. of water. Therefore, each of the trays below the LCO product draw tray was flooding. Put another way, the apparent fluid specific gravity on a flooded tray deck in hydrocarbon service is typically 0.3. For trays spaced 24 in. apart, the observed pressure drop corresponding to a fully flooded tray is then ... 

Air-cooled condensers seem to work as well as shell-and-tube water coolers in flooded condenser pressure control service. Some air coolers are sloped toward the outlet to reduce the ratio of the delta condenser surface area to delta height change of condensate backup. I don t think this is necessary, but it may help in fine-tuning pressure control with air-cooled flooded condensers. 

In designing controls for vacuum columns, the engineer should keep in mind that these columns have a narrow range of operation. The range in column pressure drop between flooding and tray instability for a perforated tray column in vacuum service may be no more than 10-15 percent. 

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