Control floating pressure

Figure 17.7 Miscellaneous pressure control methods, (a) Recirculation temperature control (b) Column overhead control (c) inerts pressure control (d) drained condenser bypass (e) pressure-reboil control (/) floating pressure control, combined with coolant flow manipulation.

To prevent this rapid drop, Shinskey developed a floating-pressure control system, sketched in Fig. 8.S. A conventional PI pressure controller is used. The output of the pressure controller goes to the cooling water valve, which is AC so that it will fail open. The pressure controller output is also scot to another controller, the valve-position controller (VPC). This controller looks at the signal to the valve, compares it with the VPC setpoint signal, and sends out a signal which is the setpoint of the pressure controller. Since the valve is AC, the setpoint of the VPC is about 5 percent of scale so as to keep the cooling water valve almost wide open. 

Note that in the floating-pressure application, there was only one manipulated variable (cooling-water flow) and one primary controlled variable (valve position). In the reactor temperature-control application, there are two manipulated variables and two controlled variables (temperature and refrigerant valve position). 

We want to analyze the floating pressure valve position control system proposed by Shinskcy. Let the transfer function between cooling water flow and column pressure be... 

Regardless of the method elected, the principal concept of tower pressure control is the same. We control the pressure in the reflux drum by manipulating the temperature in the reflux drum. The tower pressure then floats on the reflux drum pressure. To lower the tower pressure, we must first cool the reflux drum. This cuts the vapor pressure of the liquid in the reflux drum. 

The choice of a filler depends on the number of bottle sizes being considered for use on it. For example, if 3 1 PET bottles are to be filled, a 126 mm pitch between filling valves is required. If only small bottles up to 500 ml are to be filled, then a 70 mm pitch will suffice. This has a direct effect on the size of the filler and its footprint on the factory floor. Unless the filler bowl level is kept within tightly controlled limits, pressure head variations will affect the rate of flow into containers. Systems such as that shown in Figure 7.15 need to be employed. Wherever possible there should be minimal contact between any instrumentation and the product. Conventional float valves should be avoided and simple capacitance probes, which are easily cleanable but small and very effective, should be used. It is not uncommon in older fillers to only have one float valve. This often gives rise to filler bowl flooding, which may lead to inconsistent fill level and poor counter-pressurisation of the container pre-filling. 

Most distillation columns are operated under constant pressure, because at constant pressure temperature measurement is an indirect indication of composition. When the column pressure is allowed to float, the composition must be measured by analyzers or by pressure-compensated thermometers. The primary advantage of floating pressure control is that one can operate at minimum pressure, and this reduces the required heat input needed at the reboiler. Other advantages of operating at lower temperatures include increased reboiler capacity and reduced reboiler fouling. 

Optimization and vacuum control strategies (a) minimizing (floating) pressure by maximizing coolant valve opening, (b) floating pressure control of partial condenser with vapor distillate, (c) floating pressure control when the distillate is both vapor and liquid. 

Floating pressure control. While tight pressure control is mandatory in distillation columns, it may be desirable to slowly manipulate the set point of the pressure controller. This is the essence of Shinskey s "floating pressure control (362). The method minimizes column pressure without violating constraints such as maximum condenser or column capacity, or the ability to send column products to downstream units. 

The objective of the preheat control system is to supply the column with a feed of consistent specific enthalpy (enthalpy per unit mass). With a single-phase feed, this translates into a constant feed temperature with a partially vaporized feed, this translates into a constant fractional vaporization. Maximizing feed temperature (if desired) is usually performed manually, by an advanced control system, or by a valve position controller similar to that used in floating pressure control (Sec. 17.2.4). 

However, the steam flow set point signal coming from the TC is also sent as the process variable signal to a valve-position controller (VPC) whose set point signal is the minimum steam flow rate. Valve-position control is a type of optimizing control suggested over three decades ago by Shinskey who used it to achieve floating pressure control in distillation columns. Many other practical applications have been described. ... 

FIGURE 4.75 Rectangular flotation unit with recycle pressurization (1) flash mixing and flocculation, (2) float discharge line, (3) float beach, (4) skimmer, (5) adjustable effluent weir, (6) effluent line, (7) recycled effluent, (8) recycle pump, (9) air-saturation tank, (10) air controls, (11) pressure-reduction valve, (12) settled solids removal and bottom solids discharge line, (13) influent and recycle blending and distribution system, and (14) influent. (Envirex.)... 

Level control is cascaded to flow control. With floating pressure columns and with the trend toward small control-valve pressure drops for energy conservation, this is virtually mandatory to counteract the effect of control valve up- and downstream pressure variations. The flow measurement must be linear if an orifice flow meter is used, it must be followed by a sqiaare-root extractor. 

Pressure pellets sink when placed in water, whereas under the proper conditions, floating pellets can be produced through the extmsion process. That is accomphshed when the feed mixture contains high levels of starch that expands and traps air as the cooked pellets leave the barrel of the extmder. This gives the pellets a density of less than 1.0. Eloating pellets are desirable for species that come to the surface to feed since the aquaculturist can visually determine that the fish are actively feeding and can control daily feeding rates based on observed consumption. 

The reactor pressure is not directly controlled instead, it floats on the main column overhead receiver, A pressure controller on the overhead receiver controls the wet gas compressor and indirectly controls the reactor pressure. The regenerator pressure is often controlled directly by regulating the flue gas slide or butterfly valve. In some cases, the flue gas slide or butterfly valve is used to control the differential pressure between the regenerator and reactor. 

Since the low-pressure float needs a solenoid valve for tight closure, this valve can be used as an on-off control in conjunction with a pre-set orifice and controlled by a float switch (Figure 8.2). 

On a single-evaporator flooded system, a float valve can be fitted which will pass any drained liquid from the condenser direct to the evaporator. The action is the same as that of a steam trap. The float chamber is at condenser pressure and the control is termed a high-pressure float (Figure 8.3). 

More formal inspections also verily the operation of safety and other controls such as low water and fuel cutoffs, level controls, fusible plugs, pressure gauges, water glasses, gage cocks, stop valves, safety/relief valves, and BD valves and lines. Also, FW pumps, flue and damper arrangements, combustion safeguards, name plate specifications, set pressures, boiler connections, floats, mercury switches, bellows, and other components may be inspected. 

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