Pump

FIGURE 5 Typical automatic bypass system based on pump differential head

The bypass pressure reducing orifice handles the high velocity and erosive forces of the bypass flow. However, the handling of cavitation and flashing by this fixed restriction is limited. The orifice is sized to operate properly at one bypass flow condition. At low flows, the orifice does not provide the correct pressure reduction as designed.

The check valve is placed in the main line, as shown in Figure 4, to prevent reverse flow through the pump. The pump and motor bearings can be damaged if rotated in reverse. Pressure drop through the check valve must be considered in pump sizing in addition to other piping and valve pressure losses.

A typical automatic bypass system based on pump differential head is shown in Figure 5. This system is identical to the flow controlled bypass system except that pump differential head is measured instead of flow. This system can only be used with continuously rising pump head characteristic curves. The system will have to be adjusted for each individual pump characteristic and later adjusted if this characteristic changes over the life of the pump.

Depending on the bypass valve and control instrumentation selected, these systems can provide either "Modulating" or "On-Off" bypass control. Valves, instrumentation, and controls must be individually sized for the service conditions. All components must be integrated together to provide bypass flow control that meets the pump and process operational design criteria.

The Automatic Recirculation Control (ARC®) Valve This valve provides bypass flow control, pressure reduction, and reverse flow pump protection all within a single unit. This single valve combines the functions of the check valve, pressure reducing orifice, pipe tee, control instrumentation flow meter, and bypass control valve that are all required in the instrumented control loop bypass system. Valves can be designed to provide either "Modulated" or "On-Off" bypass control. The operation of all ARC valves is fundamentally the same. The flow sensing element (check valve disc) responds to the process flow demand and opens or closes the self-contained bypass control valve. Differences in ARC valve design center around bypass valve actuation method, repairability, serviceability, and adjustability. Figure 6 illustrates a typical ARC valve bypass system.

Pilotless Trim Design The simplest of ARC valve designs operate the bypass flow control valve directly by the flow sensing element. The construction of a typical ARC for low pressure service is shown in Figure 7. The basic operation of this valve is shown in Figure 8. At zero main flow ("A"), the bypass provides full recirculation flow. As main flow increases ("B"), the bypass modulates proportionally closed. At full main flow ("C"), the bypass is shut. The pressure reduction from the pump discharge to the bypass is accomplished by the characterized orifices in the bypass element that is attached to the disc. However, the differential pressure from inlet to bypass imposes static and dynamic forces on the disc assembly. These unbalanced forces interfere with the normal disc motion and limit the maximum differential pressure for pilotless trim. There is an upper differential pressure limit at which this design will not operate satisfactorily and pilot operated bypass control must be used.

Pilot Operated Trim Pilot operated bypass control valves utilize a combined hydraulic-mechanical force to control the bypass flow. Only a relatively small mechanical force is necessary to hydraulically generate the large forces required to operate a bypass flow control valve in a high-pressure system. Figure 9 illustrates a pilot-operated bypass that utilizes a lever connected to the check valve disc to actuate the pilot. Figure 10 illustrates a pilot operated multistage bypass operated by an in-line pilot valve. Pilot-operated valves require more components and seals than unbalanced valves and are therefore more costly.

BACK-PRESSURE REGULATOR „

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