Minimum flow bypass

A jockey pump maintains pressure on the firewater system at all times. The set pressure will typically be in the range 8—lObarg. Generally, the jockey pump will mm itself on and off depending on the header pressure. If the pump mns continuously its discharge should recirculate through a minimum flow bypass. 

When a blocked discharge could be a problem, or when large turndown ratios are anticipated, the installation of a minimum flow bypass should be considered. This type of bypass opens when the flow of liquid leaving the pump falls below a minimum value. The bypass returns discharge liquid to the suction of the pump. Since the liquid is heated as it goes through the pump, it may be necessary to put a cooler on the recycle stream, or to have the recycle stream flow to a suction tank or vessel that contains a large inventory of liquid. 

At high pressure experiments the reactor should be installed in a pressure cell. All check valves before it, and the filter with the flow controller after it, can be kept in the vented operating room. As a minimum, the bypass valve and the flow controller must be accessible to the operator. This can be done by extended valve stems that reach through the protecting wall. Both the operating room and the pressure cell should be well ventilated and equipped by CO alarm instruments. 

Step 2. System calculations over the entire range of possible operating conditions are required. The range must cover from the air blower minimum flow point to the expander bypass point for all reasonable variations in the applicable parameters of COj/CO ratio, fresh feed rate, flue gas temperature, ambient air temperature, and so forth. 

Step 3. The final selection of a specific air blower and turboexpander must be made after fully considerating the results of Steps 1 and 2. The normal operating point must be located on the system operating map (similar to Figure 4-66) so that reasonable latitude is available in operating variables between normal operating point and air blower minimum flow, and between operating point and expander bypass point. 

A minimum flow controller ( antisurge ) sets the valve in the compressor bypass line to prevent the flow rate through the compressor from dropping below some minimum flow rate. 

Minimum-flow loop left open at normal rates, or bypass around control valve is open. 

Close minimum-flow loop or control valve bypass valve. 

The controller output, OP, to be manipulated by the controller, as a percentage of its full range. This variable is usually either a stream flow rate or the rate of heat transfer of an energy stream. Grenerally, its minimum value is specified as zero and its maximum is taken as twice its nominal value. Note that occasionally the nominal value is not positioned midway between the minimum and maximum values (e.g., when the nominal flow rate of a bypass stream lies near its maximum or minimum flow rate). 

Depending on the bypass flow rate and the minimum flow required, the control valve could be a small rubber-lined butterfly valve or an all polytetrafluoroethylene (PTFE)-lined globe valve. The sizing of this valve can be difficult, because there is very little pressure drop available. Butterfly valves should never be required to control at an open angle of less than 20°. 

If upon a reduction in load, the speed of a compressor is reduced, Vd — Pt mill also decrease, and with it, the minimum flow requirement for surge protection. Thus with speed as the manipulated variable, the bypass valve usually does not need to be opened. Together with the reasons mentioned under centrifugal pumps, this makes manipulation of speed economically attractive. 

This boiler is designed to maintain a minimum flow inside the furnace water wall tubes to prevent tube overheating during all operating conditions. This flow must be established before startup of the boiler. A bypass system, integral with the boiler, turbine, condensate, and feedwater system, is provided. 

The manufaeturer of this system has developed a eontrol strategy by whieh the bypass valves are eontrolled in the event of an expander trip. This enables the valves to preeisely assume the position in whieh, in eonjunetion with the downstream orifiee ehambers, they preeisely emulate the flow loss eaused by the expander prior to the trip event. A properly adjusted and ealibrated eontrol output jump funetion is applied so that the pressure fluetuations that oeeur at a given duty point are kept to a minimum. 

Figure 3.23. Control of centrifugal compressors with turbine or motor drives, (a) Pressure control with turbine or motor drives, (b) Flow control with turbine or motor drives. SC is a servomechanism that adjusts the guide vanes in the suction of the compressor, (c) Surge and pressure control with either turbine or motor drive. The bypass valve opens only when the flow reaches the minimum calculated for surge protection.

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