Recycle valves

The essence of surge protection is determining when and how much to open or close the recycle valve. In the simple application of Figure 6-45, the antisurge controller (UIC) will position the recycle valve using a control algorithm based on AP, AP, and data on the location of the surge limit. 

The first objective of the antisurge control system is to protect the compressor. This can be accomplished for some disturbances by using the PI algorithm with a large value of bj. However, it is also necessary to maximize the region in which the compressor can operate with the recycle valve closed. This increases the efficiency of the compressor at lower throughputs. Steady-state operation with recycle is extremely inefficient. Therefore, from this perspective, small values of bj are highly desirable. 

Most gas lift, flash gas, and vapor recovery compressors require a recycle valve because of the unsteady and sometimes unpredictable nature of the flow rate. Indeed there may be periods of time when there is no flow at all to the compressor. 

At a constant speed, a constant volume of gas (at suction conditions of pressure and temperature) will be drawn into the cylinder. As the flow rate to the compressor decreases, the suction pressure decreases until the gas available expands to satisfy the actual volume required by the cylinder. When the suction pressure decreases, the ratio per stage increases and therefore the discharge temperature increases. In order to keep from having too high a discharge temperature, the recycle valve opens to help fill the compressor cylinder volume and maintain a minimum suction pressure. 

A speed controller does not ehminate the need for a recycle valve, flare valve, or suction throttling valve, but it will minimize their use. The recycle valve and suction throttling valve add arbitrary loads to the compressor and thus increase fuel usage. The flare valve leads to a direct waste of reservoir fluids and tlius loss of income. For this reason, engine speed control is rec-... 

A recycle valve is needed for surge control as well as for the conditions listed above for reciprocating compressors. At constant speed the head-capacity relationship will vary in accordance with the performance curve. For a constant compressor speed ... 

If the flow rate to the compressor decreases, the compressor approaches the surge point and a recycle valve is needed. 

The use of speed control and/or a recycle valve is covered in Chapter 10. Our discussion in this chapter will concentrate on cylinder inlet valve unloaders and changing cylinder clearance. 

Ensure the recycle valve is closed if water is to be measured at the local water outlet or the system discharge valve is closed if water is to be recycled into storage. 

A recycle valve was provided around the pump to enable discharge pressure regulation and flow-rate variation. This, however, combined with shearing across the individual separator level-control valves, was probably responsible for a large percentage of droplet breakup, casting some uncertainty on the effectiveness of the pump. 

A number of runs were carried out with the pump stopped but liquid flowing through the wide-open recycle valve w h an upstream pressure of 500 to 600 kPa [73 to 87 psi). This resulted in little change in performance over the pumping mode. This, combined with the Ap observation, suggests that most of the droplet breakup is taking place in the separator-level control valves. 

A technique called shave/recycle, mentioned earlier, allows separtion of a pair of close resolving peaks. To use shave/recycle, it is necessary to plumb the HPLC system so that the output from the detector can be returned to the HPLC pumps through small diameter tubing and switching valves (Fig. 11.2). Twenty-thousandths tubing is used to connect the detector output to valve 2, the waste recycle valve 0.02-in tubing connects from valve 2 to valve 1, the solvent select valve and, finally, a third valve 3, the collect valve, can be placed in the waste line from valve 2. 

At this point, switch to recycle by turning valve 1 and 2 at the same time switch valve 3 to waste. We want to send the contaminated portion between peaks back through the pump head back to the column head for further separation. We continue to recycle until the detector shows we are well down on the backside of peak B. (Remember, A is tailing into B.) Change the collection flask while we are recycling to collect flask 2. Shut recycle valves 1 and 2 and switch valve 3 to collect in flask 2. Stop collecting 2 when we reach the baseline on the recorder and switch valve 3 back to waste and change to a clean collection flask 3. 

Assembling a line of reactors Linear, divergent as well as convergent multistep syntheses are also feasible Issue solvent switching. If reaction times are not similar, switching and recycling valves are required... 

A - RESERVOIR B - PUMP C 0 1 jim FILTER D INJECTION VALVE E - COLUMN F - DETECTOR G - VOLURATOR H RECYCLE VALVE I - WASTE RESERVOIR J COMPUTER K CONSOLE L - PRINTER... 

Adjust recycle valve to maintain 140 psig to 150 psig. 

Open discharge and close recycle valve keeping pump pressure 140 psi to 150 psi. 

FIGURE 6-7. Recycle Chromatography, (a) Schematic. (b) Example of a recycle valve. 

Turn the recycle valve knob to the collect position. 

After the recorder pen starts forming the next peak, wait 15 sec and turn the recycle valve knob to the recycle mode. 

However, many differences arise among the schemes when looking at how production rate is set and how liquid level and pressure in the reactor are controlled. For example, production rate is set in various strategies via fresh feed Fo0 flow, condenser cooling water flow, separator liquid flow, stripper base flow, or fresh feed FoC flow. Reactor level is controlled by fresh feeds FoD and F,lK, separator temperature setpoint, compressor recycle valve, or fresh feed FoC flow. Reactor pressure is controlled by reactor cooling water flow, purge flow, or FcA feed flow. In one strategy reactor pressure is uncontrolled and allowed to float. 

Step 2. This process has 12 degrees of freedom. One of these is agitation rate, which we simply hold constant. This leaves 11 degrees of freedom four fresh feeds F,A. F0d. FoE. and FcC, purge valve gas recycle valve separator base valve stripper base valve steam valve reactor cooling water valve and condenser cooling water valve. 

Step 8. Control of the individual unit operations has been established and all of the control valves have been assigned except for the gas recycle valve. 

Fig. 5,10. Approach to the matrix-independent removal of fluorescent compounds. (A) Controlled FMASE RV recycling valve. (B) Flow-injection manifold IV injection valve. (Reproduced with permission of the American Chemical Society.)...

ENVIRONMENTAL ASPECTS OF RECYCLING VALVE-REGULATED LEAD-ACID BATTERIES... 

It is not always possible to obtain complete resolution of components following a single preparative HPLC run. An alternative to further method development, or to the use of a longer HPLC column, is to recycle that portion of the eluent that contains the incompletely resolved components through the column a second time. Preparative LC instruments equipped with a recycling valve can provide a speedy solution to difficult separations, at the same time offering additional substantial savings in solvent consumption in certain cases. 

The rate of heat removed from the vessel is varied by manipulating the rate of solvent evaporated. The pressure and, thus, the evaporation rate are manipulated by varying the recycle rate of the stream exhausted from the steam ejector. A control loop that links the recycle valve position directly to the operating temperature would not permit compensation for short cycle variations in the steam supply pressure, permitting rapid swings in pressure and surface-solution temperature that could result in spontaneous nucleation. Therefore, a cascade configuration that uses the temperature measurement in a master loop and a pressure measurement in a slave loop is employed. 

MF - eluent container, DG - degasser, F - filters, P - pnniping system, I -sample injector, C - columrr(s), R - recycling valve, D - detector(s), V - volmneter, FC - fiaction collector... 

The recycling valve R is in its construction similar to injecting valve, however it has only four ports. It allows returning selected fraction of column effluent via the pump back to the column C. Recycling procedure enables increasing selectivity of separation but its application is rather limited because the front fraction easily catches the rearmost one. 

In a plant producing liquid chlorine, the compressed gas goes next to the liquefaction system. Rather than impose a pressure drop between the processes, the gas is allowed to flow freely into liquefaction. A valve on the uncondensed gas venting from the liquefaction unit (Section 9.1.7.2) controls the pressure on both systems. When chlorine is sent to another process without liquefaction, it would be possible to withdraw it on downstream pressure control and let the compressor outlet pressure fluctuate. This approach leads to variability in the differential pressure across the compressor recycle valve. Fluctuations in this flow can cause fluctuations in the compressor suction pressure and therefore in the cellroom chlorine header. It is better to control the compressor outlet pressure itself, even at the cost of another pressure control loop at the destination. Section 11.3.2.6 describes instrumentation hardware and the problems of transferring chlorine to more than one destination. 

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