Bypass Control

There are several general heuristic guidelines for heat exchanger bypass systems. However, this very much remains an open research area since these guidelines are not always adequate to deal with all of 

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The fix for the erratic reflux drum pressure problem was to provide for separate pressure control of the fractionator column and the reflux drum. A new pressure control valve was installed upstream of the condenser and the old condenser outlet control valve was removed. A hot gas bypass, designed for 20% vapor flow, was installed around the pressure control valve and condenser. A control valve was installed in the hot gas bypass line. The column pressure was then maintained by throttling the new control valve upstream of the condenser. The reflux drum pressure w as controlled by the hot gas bypass control valve and the psv saver working in split range. The new system is shown in the figure below. 

Usually, major pumps are spared, even if the philosophy of shutting down an entire train is adopted. Other spares or bypasses are often left out for such a philosophy, but standard bypasses (control valves, major block valves, etc.) are normally included otherwise. The utility area will have normal sparing (for example, three 3-capacity electric generators), even if train shutdown philosophy dictates the process area. 

Total flow vacuum pump Exhaust control with bypass controller valve... 

GRBVB/GRBVA GC refrigerant bypass control valves B/A fail open... 

In general, flooded condenser pressure control is the preferred method to control a tower s pressure. This is so because it is simpler and cheaper than hot-vapor bypass pressure control. Also, the potential problem of a leaking hot-vapor bypass control valve cannot occur. Many thousands of hot-vapor bypass designs have eventually been converted—at no cost—to flooded condenser pressure control. 

Rotary pumps deliver a nearly constant flow at a given speed, regardless of the pressure. Bypass control is the usual method, with speed control in larger sizes. Reciprocating pumps also may be controlled on bypass if a pulsation damper is provided in the circuit to smooth out pressure fluctuations Figure 3.21(c) shows this mode. 

Bypassing-Controlled Trayed or packed columns operate with countercurrent flow and can achieve many equilibrium stages in series by good distribution of gas and liquid, and careful control of details. Other devices such as sprays are vulnerable to bypassing and are limited to one equilibrium stage. 

In the PCU during rapid transients the rate at which inventory must be changed according to the load schedule may not be physically achievable. The helium fill and bleed system has limited capacity. In this case turbine bypass control is used to more quickly vary the power output of the shaft. In this scheme the power output of the turbine is changed by bypassing high pressure compressor outlet coolant to the exit of the turbine. The pressure drop across the turbine is reduced so power is reduced while at the same time the frictional losses through the rest of the PCU circuit increase. The result is a rapid reduction in shaft power. However, PCU efficiency is reduced under turbine bypass control and so control is typically transitioned back to inventory control over time. 

A rapid 10% reduction in generator load with bypass control was simulated. The generator load is decreased from an initial steady state of 100% power down to 90% power over one second. The pre-cooler and intercooler water flow rates are also decreased. 

In the conventional control loop, the measurement lag is only part of the total time lag of the control loop. For example, an air heater might have a total lag of 15 minutes. Of this lag, 14 minutes is contributed by the process lag, 50 seconds by the bulb lag, and 10 seconds by the control valve lag. Bypass control is often applied to circumvent the dynamic characteristics of heat exchangers, thus improving their controllability. Bypass control can be achieved by the use of either one three-way valve or two two-way valves. 

Figure 5.8 Bypass control of process-to-process heat exchangers, (at Controlling and bypassing hot stream (b) controlling cold stream and bypassing hot stream

Figure 5.2S Control of packed adiabatic plug-flow reactor with FEHE and bypass control.

BYPASS CONTROL-TO SE FURNISHED BY KELLOGG FLANGE STUDS TO SE FURNISHED BY PUMP MFR... 

If the exchange is between two process streams whose flows are fixed, bypass control will have to be used, as shown in Figure 5.24b. 

Figure 5.24. (a) Temperature control of one fluid stream, (b) Bypass control, (c) Air cooler with bypass control,... 

FLUSHING OIL PIPING BY BYPASS CONTROL-TO BE FURNISHED BY KELLOGG FOUNDATION 80LT8-T0 BE FURNISHED BY KELLOGG. FLANGE STUDS—TO BE FURNISHED BY PUMP MFR. 

It is permissible to switch the flame supervision out of the safety circuitry for a furnace zone when the zone temperature is at or above 1400°F (760°C), per NFPA 86 (sect. 5-9.1). Burners without flame supervision shall be interlocked to prevent their operation unless the furnace is at or above 1400°F (760°C). A MOOT (760°C) bypass controller must be used for this purpose per NFPA 86 (sect. 5-17). The bypass controller and temperature-sensing element must be independent from any other controller or element. Failure of the element must cause the bypass controller to sense a temperature below MOOT (760°C) and therefore shut off the combustion system. Visual indication must be provided to indicate that the bypass controller action is in effect. 

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