WHY CENTRIFUGAL PUMPS NEED ARC VALVES
A centrifugal pump relies on the fluid it pumps to carry away heat generated by rotating members and bearings. As long as the pump operates at high flow rates, the fluid volume is usually sufficient to prevent overheating; but at lower flow rates, cooling can be inadequate and the risk of premature pump failure due to bearing over-heating and seizure or excessive ther-mal expansion of impeller blades can arise.

To assure safe pump operation at all flow rates, particularly at less than 40 percent of rated capacity, requires recirculation (or bypass) systems. A recirculation systems function is simple: to maintain flow at the rates required for effective pump protection, discharging excess flow (that is, flow not required by the process) into the same reservoir or sump from which it was originally pumped.

Recirculation systems can also be used to maintain stable flow rates especially for the high-volume centrifugal pumps used in utility plants and for small, high-speed centrifugal pumps that rely on speed and varying impeller diameter for pressure and capacity control. Adequate recirculation can help maintain both stability and accuracy.


HOW ARC VALVES WORK

The disc is shown in the closed position in Figure 3. In this position there is no process flow and the bypass is full open. This protects the pump against planned or accidental “dead heading” which can result from a closed down stream pump isolation valve or process control valve.

As the disc lifts (Figure 4) in response to an increase in flow to the process, the bypass element which is integral to the disc, closes the bypass flow orifices reducing recirculation flow. Recirculation flow is controlled with disc position. This modulation feature assures that the total of process flow and recirculation flow exceed the minimum flow through the pump as specified by the pump man-ufacturer.

When the disc is set at full lift position, as in Figure 5, the bypass is closed. As process flow decreases, the reverse action occurs and the recirculation flow again increases. Flow enters the bypass element at the bot-tom of the disc assembly and is controlled by characterized orifices inside the disc stem. Flow continues through an annulus in the bypass bushing and is directed to the outlet of the valve.

The valve provides for single phase flow in the bypass eliminating the possibility of flashing or cavitation. This is accomplished by the valve design, and if necessary, an integral second stage pressure letdown device or external back pressure regulator.




BACK PRESSURE REGULATORS
In high pressure pumping applications the system often does not provide adequate pressure in the bypass line to prevent cavitation or flashing. Either of these conditions is undesirable in that it can cause damage to both valves and the pipe system or cause a reduction in flow below the minimum desired, jeopardizing the pump protection system. All PRV’s will experience a velocity induced recovery effect which will limit the amount of pressure drop a valve can take and cause a reduction in flow capacity.

The requirement of backpressure is generic to all pressure reducing applications. Pressure reduction even by multiple stage cascading can minimize the requirement, however no valve design will redefine a fluid’s physical properties.

This becomes especially important in modulating systems. A fixed orifice will not provide the proper backpressure at all flow levels. As the flow in the bypass line is reduced, the orifice becomes less effective. Proper system design should be used to optimize valve pressure reduction and consider all fluid dynamic effects downstream of any pressure reducing device.

When adequate backpressure is not available downstream of a pressure reducing valve, vapor bubbles will form in the zone just downstream of the valve last stage control surface. This zone is defined as the “Vena Contracta and represents the point of highest fluid velocity and lowest pressure. The potential for 1) damage to downstream piping components and 2) flow reduction exists from this point. When line pressure remains below the fluid vapor pressure, any existing bubbles will remain and expand as piping friction further reduces line pressure. This can be defined as ‘FLASHING CONDITION” and is characterized by a polished appearance on affected surfaces. When the line pressure drops below the fluid vapor pressure and then recovers, any entrapped vapor bubbles will collapse (implode). This is defined as a “CAVITATION CONDITION” and is characterized by a cinder like appearance on affected surfaces.... The resolution of either condition is best addressed by eliminating vapor formation. This can be assured by the provision of adequate backpressure. The “Backpressure Factor” is key to reliable system operation and must not be ignored in piping design considerations.

As such we feel it is the obligation of a responsible Automatic Recirculation Control Valve manufacturer to analyze the system needs and supply a Back Pressure Regulator (BPR) when it is warranted by the laws of fluid dynamics. For on/off systems this could be a simple orifice, but for modulating conditions, it must be a device like the BPR noted herein.