Many positive displacement pumps deliver the same flow rate regardless of the static head.
by Edison Brito, Pump Solutions Group

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.

The Challenge

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.

Figure 1. Typical transferring process

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.

In reality, PD pumps can quantifiably counteract these advantages. PD pumps are appropriate for fluids with high viscosity, but they can easily move other fluids, from liquefied gases and water-like liquids (sliding vane pumps) to medium and very viscous fluids (eccentric disc and sliding vane pumps). PD technologies have operated successfully in the chemical manufacturing industry for more than a century, and their initial costs can be similar when all the equipment, accessories and controllers are evaluated. In many cases, the total cost of ownership is lower over a PD pump’s operational lifespan.

centrifugal pumps work best when they operate at their best efficiency point (BEP). Unfortunately, the BEP is rarely realized for an extended period during fluid transfer operations, resulting in flow rates that can fluctuate constantly. Many facility operators are willing to accept fluctuations in flow rate. However, consistent off-BEP operation can lead to potential problems in the equipment’s operation, the production process and how the chemical is formulated. Note that the system, not the pump, dictates the operating conditions in which the pump must work.

During the chemical process, the amount of fluid sent must adhere to specific guidelines and quantities that are sometimes only known by the chemical manufacturer. In these instances, a centrifugal pump will not provide constant flow unless it is controlled with proportional-integral- derivative loops, flow meters, recirculation lines and variable speed drives. These components complicate the pumping system and introduce electric and electronic components that may be required to operate in hazardous areas and require special ratings.