Slide valve pressure balance

Higher catalyst circulation usually requires opening the regenerated and spent catalyst slide (or plug) valves. Higher circulation increases the pressure drop in the riser and in the reactor cyclones, lowering the differential pressure across the slide valves. This causes the valves to open further, until the unit finds a new balance. 

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. 

The pressure balance should be examined to determine the normal pressure readings in the reactor, regenerator, air system, flue gas system, and main fractionator and overhead system. These need to be followed on a time basis and plotted against variables such as feed rate, wet gas rate, and dry gas rate to see if and where problems may occur. Adjustments may be possible if the spent or regenerated catalyst slide valve delta P is at a minimum to provide more operating room. 

To achieve a pressure balance, Reh did not require a standpipe of considerable height. Providing a slide valve for his operating temperature, 1,100°C, might be difficult, and he elected a device with no moving parts, the J-leg, for balancing the pressure drop across his fast bed. 

Pressure Balance - Operability. Overall operation, as in catalytic cracking, depends on use of standpipes to generate the pressure differentials necessary to cause shale and balls to flow into the process vessels. Excess pressures are taken out by slide valve control which also dampens the transfer of pressure surges between vessels. 

Since FCC catalyst is kept above minimum fluidization conditions everywhere in this catalyst circulation loop, the fluidized catalyst is free to flow from one place to another. Thus the catalyst circulation is driven by the overall pressure balance of the unit, and the circulation rate is regulated by the two slide valves, i.e., the stripper slide valve and the regenerator slide valve. A minimum pressure drop across each slide valve is set in the control system to guard against flow reversal, which is a very serious safety issue. For instance, a reverse flow of hydrocarbon vapor from the reactor to the oxygen-rich regenerator can lead to a sudden increase in combustion reaction and regenerator temperature. In the extreme case, a catastrophic explosion could occur. The overall pressure balance of the unit determines the pressure drops available for the slide valve control and hence the maximum catalyst circulation rate. 

The next step is to estimate transition pressure losses, whieh inelude riser liftpot, riser termination, cyclone, spent catalyst distributor, and other flow transitions. By subtracting these transition pressure losses from the hydraulic pressure, the remainder would be the pressure drop available for slide valve control. While other parameters in pressure balance seldom change much at all, it is not uncommon to have the standpipe apparent density lower than the typical... 

In most cases, such an error on the conservative side in predicting pressure drop is perfectly acceptable since, if anything, the plant will experience less pressure drop through the cyclone installation and this seldom creates an operational problem. However, if there is some delicate pressure balance across a slide valve, for example, which relies on an accurate knowledge of the cyclone s pressure drop, then one should try to acquire the most accurate estimate possible. 

In addition, as the pressnre balance is obvionsly affected when the reactor pressure is increased to compensate for increased vapor line pressnre drop, catalyst circulation and resnlting catalyst/oil ratio may be rednced due to low slide (plug) valve differential pressnre. 

A number of features common to most sliding-spool valves are shown in Figure 9.10. The small ports at either end of the valve housing provide a path for fluid that leaks past the spool to flow to the reservoir. This prevents pressure from building up against the ends of the pistons, which would hinder the movement of the spool. When these valves become worn, they may lose balance because of greater leakage on one side of the spool than on... 

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