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around the casing and out the discharge nozzle. A comparison of the resulting performance of these two different pump types is shown in Figure 9.

The conditions of service will usually determine the best pump for an application. For instance, for constant pressure at varying flow rates, a centrifugal pump would be a good choice. An example of this is a municipal water system where consistent pressure must be maintained over a wide range in usage levels. By contrast, for a constant flow in the presence of varying back pressures, a rotary pump would be better. An example of this is an oil pipeline, where system economics dictate constant flow rates, regardless of any system pressure variations from changes in viscosity or pipe diameter.

Other differences exist between centrifugal and rotary pumps as well. The performance curves, affinity laws, and terminology used to describe rotary pumps are all different. And since rotary pumps are primarily for viscous fluids, the applications and markets served by these two pumps are also different. One of the few direct comparisons that can be made between centrifugal and rotary pumps is with single- versus multi-stage pumps. Even here though, the analogy is not a perfect one, and certain rotary pumps, such as progressing cavity pumps, fit the description better than others. Other factors, such as vertical versus horizontal mounting, metal versus non-metallic materials, sealless and magnetically driven versus dual-containment mechanical seals and conventional drivers, are similar whether considering a centrifugal or rotary pump.

Rotary Pump Curves Centrifugal pump curves plot the flow on the X-axis with the discharge head on the Y-axis. However, rotary pumps develop the flow against a system backpressure, rather than developing head with a corresponding flow rate. Their performance curves therefore show the flow on the Y-axis with differential pressure along the X-axis, as shown in Figure 10.

The influence of differential pressure on the flow is greatest with lower viscosity. Since this represents the worst (least) case for flow, it is the point around which the flow rate is established. Conversely, the maximum viscosity represents the worst (most) case from a power standpoint and is therefore the point around which the driver is sized.

Other considerations, such as flat versus steep curves or matching a system curve to a pump curve, also cannot be applied to rotary pumps the way they can with a centrifugal

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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