Spillback

Verify the position of the wet gas compressor spillback. Determine if the compressor turbine needs water washing. Trend the level of inert gases in the dry gas. 

Spillback bypasses the compressor discharge gas, back to the compressor suction. This is a relatively energy-inefficient way to increase the evaporator temperature. Spillback is discussed in Chap. 28, Centrifugal Compressors and Surge. ... 

But, suppose we must maintain a constant pressure in the wet-gas drum. The pressure in this drum may be controlling the pressure in an upstream distillation column. To hold a constant pressure in the drum, we will have to resort to spillback suction pressure control, illustrated in Fig. 28.5. 

Great. But what about Dr. Mayer What about Eq. (28.2) It is true that the compression ratio (P2/P1) has been held constant. But as the molecular weight has increased, the number of moles N has also increased. Why Because, the spillback valve has opened. And as N increases, so does the amperage drawn by the motor. 

Why, then, in ordinary process plant practice, do we see an increase in the amps on a motor driving a centrifugal compressor as the gas becomes heavier Does it take more work to compress a mole of propane [44 MW (molecular weight)] than it does to compress a mole of methane (16 MW) Certainly not. It s just that compressing a heavier gas forces the spillback to open, to prevent the pressure from falling in the wet-gas drum. This extra gas recirculating through the compressor is the factor that increases the amp load on the motor driver. 

If the molecular weight of the gas increases, the gas density will increase. The AP will increase. The pressure in the wet-gas drum will drop. The new suction throttle PC valve will start to close. This will restore the pressure in the wet-gas drum, without increasing the flow of gas through the spillback valve. But what happens to Px How does closing the suction throttle valve affect the actual compressor suction pressure ... 

Of course, we may also control the wet-gas drum pressure very nicely by the spillback PC valve, shown in Fig. 28.5. But this mode of control causes N, the number of moles, in Eq. (28.2), to rapidly increase. And the motor amps will increase just as fast as the number of moles. 

If the molecular weight rises by 20 percent and we control the drum pressure by spillback control, the motor amps will increase by about 20 percent. 

Both the suction throttle pressure control (PC) valve and the spillback [flow-control (FC)] valve are in a nice operating position. All we wish to do is save electricity. 

But, as we reduce N, we are moving toward the surge point in Fig. 28.3. If we try to save too much electricity, by forcing the FC to close too much, then the centrifugal compressor may be forced into surge. That is why we call the FC spillback valve the antisurge valve. 

There are three basic ways to control throughput in a centifugal compressor spillback (bypassing), suction throttling, and variable speed. Figure 7.2 shows each of these structures and illustrates how they work in terms of their compressor curves. 

Figure 7.2 Compressor controls.

Compressor surge is prevented by using a low-flow controller that opens the valve in the spillback line from compressor discharge to compressor suction. 

This simple and direct method (Fig. 17.4a) is usually the best choice when the column has a vapor product (77). The controller directly manipulates vapor inventory and, therefore, coliunn pressure. The equivalent method commonly used to control vacuum columns (Fig. 17.46) has the pressure controller varying the quantity of spillback to the ejector suction. The spillback control method can also be applied to pressure columns where the vapor product is compressed. 

Figure 17.4 Pressure control by vapor rate variations, (a) Product rate variation, superatmospheric columns (6) vapor spillback variation, vacuum columns (c) product rate variation, with internal condenser, (d) product rate variation, poorly piped system. (Part d from Tfnusual Operating Histories of Gas Processing and Olefins Plant Columns, H. Z. Kister and T. C. Hower, Jr., Plant/C rations Progress, vol. 6, no. 3, p. 163 (July 1987). Reproduced by permission of the American Institute of Chemical Engineers.)...

In both leaf and candle filters, the cake deposits onto vertical surfaces. It should not be shocked or disturbed by sudden changes in flow rate. It is more important with polishing filters than with bed filters to maintain steady brine feed rates. The same technique of controlled flow with spillback to a feed tank is often used. The instrument diagrams of Figs. 11.6 and 11.7 illustrate this approach. 

The intention of defining key indicators is to describe the process and energy performance with a small number of operating parameters. A key indicator can be simply an operation parameter. Some examples of key indicators are reaction temperature, distillation temperature and pressure, column overhead (ovhd) reflux ratio, column overflash, spillback of a pump, heat exchanger U value, and so on. The parameter identified as a key indicator is important due to its significant effect on process and energy performance. 

The pressure in vacuum distillation columns with is similarly controlled by manipulating the spillback around the ejectors, as shown in Figure 12.51. The installation of the pressure transmitter needs special attention. It is important that the impulse line is self-draining back to source otherwise a liquid head can build up and cause a false pressure measurement. The liquid may also boil and cause a noisy measurement. 

Throttling either the inlet or discharge from an ejector, or throttling the motive fluid, is not generally successful because of the impact they have on ejector performance. However it is common for the ejector spillback to be closed and the pressure controller on manual. This is not a reflection on the performance of the scheme. It can be economically very attractive to operate the column at the lowest possible pressure, even if this means the pressure fluctuating somewhat. We will cover later in the chapter techniques for compensating for such fluctuations so that product composition is not affected. And we will also return later to pressure optimisation. 

Van Knoop, V, Zuylen, H., Hoogendoorn, S. (2008). The need of spillback modelling when assessing consequences of blockings in a road network. European Journal of Transport and Infrastructure Research 8 287-300. 

Spillback A spillback allows fluid to recycle from the discharge back to the suction of a machine. It s one way to stop a centrifugal compressor from surging. 

But we are working with an ordinary AC (alternating-current) motor—which is a fixed-speed device. There are then three methods available to control the temperature in the evaporator spillback, discharge throttling, and suction throttling. 

Open the spillback around the discharge check valve to warm the pump case. For pumps with the suction on top of the pump s case, open the connection allowing the hot spill-back flow to recirculate to the bottom of the pump. The suction valve needs to be partly open. Wait until the pump case is uniformly warm. Starting a pump that is cold or unevenly... 

Meaning, suction throttling just a bit causes the spillback valve to throttle back a lot. Or, N, the number of moles of gas compressed, will drop a lot, but P /P, the compression ratio, will increase just a little. 

To summarize, suction throttling moves us closer to surge, but saves energy by forcing the spillback valve to close. 

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