Pressure drop erratic

If the unit pressure balance indicates that either the pressure gain in the standpipes is inadequate or the delta P across the slide valves is erratic, standpipe aeration and instrumentation should be examined. Redesigning the aeration systems or replacing the standpipes can gain valuable pressure drop. Proper instrumentation can include independent aeration flow to each tap, flow indicators/controllers on each, and differential pressure indicators between the taps. 

Dissolved air is not readily drawn out of solution. It becomes a problem when temperatures rise rapidly or pressures drop. Petroleum oils contain as much as 12% dissolved air. When a system starts up or when it overheats, this air changes from a dissolved phase into small bubbles. If the bubbles are very small in diameter, they remain suspended in the liquid phase of the oil, particularly in high viscosity oils. This can cause air entrainment, which is characterized as a small amount of air in the form of extremely small bubbles dispersed throughout the bulk of the oil. Air entrainment is treated differently than foam and is typically a separate problem. Some of the potential effects of air entrainment include pump cavitation, spongy and erratic operation of hydraulics, loss of precision control, vibrations, oil oxidation, component wear due to reduced lubricant viscosity, equipment shutdown when low oil pressure switches trip, microdieseling... 

Where there are large pressure drops across the valve, double-seated valves are used to balance the pressures on the plug and ensure easy operation. To avoid erratic control action due to valve sticking the controller output may be applied to a valve positioner, which is a servomechanism (position controller) designed to fix the valve position as required by the controller. 

Ultimately, purely mechanical difficulties arise. High pressure drop may lead directly to flooding. With a large pressure difference in the space between trays, the level of liquid leaving a tray at relatively low pressure and entering one of high pressure must necessarily assume an elevated position in the downspouts, as shown in Figure 4.6. As the pressure difference increases due to an increased rate of flow of the gas, the level in the downspout will rise further to permit the liquid to enter the lower tray. Ultimately, the liquid level in the downcomer may reach that on the tray above and the liquid will fill the entire space between the trays. The tower is then flooded, the tray efficiency falls to a very low value, the flow of gas is erratic, and liquid may be forced out of the gas exit pipe at the top of the tower. 

When a process-flow control valve is opened past a point, it may start to vibrate. The flow through the control valve becomes erratically low, and the pressure drop through the valve becomes erratically high. The vibrating sound coming from the valve indicates that the seat inside the control valve is loose. To prove this, leave the valve on automatic FRC and open up the control valve bypass a few turns. If the vibration stops, you can be sure the control-valve seat is loose. 

FIGURE 8-11. Example of pressure drop increase as a result of cell flooding (IS) (note the erratic cell voltage pattern as a result of flooding). 

The changes in the anode composition and morphology are also responsible for the erratic response of the current density vs. potential difference tendency and the decrease in the electrocatalytic activity. Thermo-corrosion and thermal stress failure, stress corrosion cracking during gas evolution, and overcritical local pressure conditions can cause extensive pitting and large ohmic voltage drops. 

Operators who have problems with loss of reboiler capacity often attribute these problems to condensate backup. This is usually true. To drop the level of water out of channel head, either the steam trap or the condensate drum is bypassed by putting the condensate to the sewer. Sometimes the float of the trap is sticking, but mostly the difficulty is an erratically high pressure in the condensate collection... 

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