Some Chemical Applications Require the Range of a Positive Displacement Pump

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Chemical manufacturing includes complex processes. In fact, chemical manufacturing processes are so intricate that, typically, several unit operations exist within an overall process. These may include cracking, distillation and evaporation, gas absorption, and scrubbing and solvent extraction. Within these operations, transferring is the process of transporting fluid from one point to another—stands out because it is important to the whole manufacturing process. Fluid transfer is a jack-of-all-trades with responsibilities along the whole chain. Some examples are moving raw materials into storage tanks, raw materials into blending or mixing tanks, final formulations into holding tanks and finished products into intermediate bulk containers for delivery or two-gallon jugs for store shelves. Because of transferring’s importance, facility operators should identify the best pumping technology for the job—one that is versatile, reliable and efficient. For many years, centrifugal pumps were the go-to technology. However, positive displacement (PD) pumps specifically sliding vane and eccentric disc pumps—can
be the right pump technology for many chemical transfer operations.

In a basic explanation, the volume of fluid sent from Source Tank A will increase in Destination Tank B (see Figure 1). As this operation occurs, the only variable in the hydraulic system is the static head, which will change as the level in Tank A decreases and the level in Tank B increases.

In many cases, when the tanks are large enough, the static head variation is assumed to be insignificant, and a centrifugal pump is sized for a specific performance point. In reality, a centrifugal pump operates in a range in the curve of its hydraulic performance. The size of this range is specific to each application and should be evaluated. In Figure 1, the performance of an equivalent PD pump is the yellow line (QM), which represents what a PD pump must do to deliver the same volume in the same time as the centrifugal pump that is operating in a specific range. Also, PD pumps, particularly those with self-adjusting volumetric efficiency capabilities, such as eccentric disc or sliding vane pumps, will consistently deliver the same flow rate across all pressure variations, regardless of the pumping system’s static head. As the discharge pressure changes, PD pumps provide a consistent flow rate. A centrifugal pump’s operating range becomes more critical when the fluid must be transferred from one source tank to several points or tanks within the plant. In this case, the operating range will be wider, and the delivery parameters will be different from tank to tank. Chemical manufacturers have traditionally chosen centrifugal pumps for transfer applications for the following reasons:

  • They are commonly the first choice for moving water like fluids. PD pumps are usually considered when the fluid is viscous.
  • They are a well-known technology and familiar to most operators.
  • They are believed to have a lower initial cost than PD pumps. However, this is not necessarily the case.

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