by Wallace Wittkoff

Technical professionals understand a variety of fluid-transfer performance concepts. The principles have much to do with evaluating if an individual pump will succeed in accomplishing its fluid-transfer duties with a reasonable degree of dependability. This includes evaluation of inlet/discharge conditions, flow, speed and power requirements, as well as durability.

This article explores a segment of the positive displacement (PD) pump arena where precise flow control is needed from a rotary PD pump (Figure 1). Despite the PD style of operation for these pumps, their use in precise metering applications has to be approached with caution because of the potential for excessive slip, which induces errors. Historically, instead of rotary pumps, reciprocating pumps have been favored in these types of applications. However, some processes cannot accept reciprocating pumps because of their inherent pulsation, cost, automation complexity or other parameters. The NPSHR and stuffing needs of reciprocating pumps are also a challenge.

The Challenge

Figure 1 outlines the concepts to evaluate the suitability of rotary PD pumps for applications needing precise flow control with variable process conditions, while factoring in pump wear.

Figure 1

The application of advanced fluid-transfer concepts on a macro (or process) scale will enable entire processes to become efficient in addition to aiding the efficiencies of any specific pump. Users can find ways to produce a product at the least cost considering factors such as plant-wide labor, floor space, capital investment, cleaning infrastructure and total process energy usage (Figure 2)

Figure 2

For instance, users could replace batch-blending processes with continuous in-line blending processes. New pumps with good metering/predictable flow performance are enabling this process method switch.

In its simplest form, a batch process (Figure 3) first involves sending ingredients in the correct amounts to a processing tank. Subsequently, and possibly in a distinct step, the products are mixed within the tank to produce the desired blended product. In contrast, with an in-line continuous-blend process (Figure 4), the ingredients are fed proportionally correct amounts and instantly combined as they are transferred within a common manifold. This manifold may also contain shearing devices to make sure the ingredients are properly mixed.

Figure 3

Figure 4

A full analysis of the benefits and drawbacks of truly continuous over batch processes are not possible in the scope of this article. In summary, however, continuous-batch processes can yield:

  • Large reductions in floor space (no multistage blend tanks needed)
  • Possible quicker product-formulation changes to match needs
  • Reduced cleaning surfaces (eliminating multistage tanks)
  • Capability of high degree of automation (recipe control)
  • Reduced product losses and waste treatment

Several drawbacks in the use of continuous-blend processes have been caused by limitations in the pumping technology employed. Past and existing systems can be effective, but cannot accommodate wide changes in process parameters like flow rates (affecting proportion limits) and viscosity (ingredient flexibility). Additional issues with existing continuous in-line blending processes include stability as a result of start-up/shutdown conditions, equipment aging and process upsets.

New pump technologies, as well as correct selection of existing technologies, are now enabling the wider use of continuous-blending processes that require more flexibility and stability.

Details on Pump Performance

A pump's "performance band" is the family of duty points (pump speed versus delivered flow rate) resulting from pump slip for a range of possible process conditions, including viscosity, back pressure, temperature and even pump wear during its lifetime. The pump performance band can be described as either tight or loose, which indicates how much the flow can change (think of slack) for a fixed pump speed. The performance band can also be described as wide or narrow to indicate the possible range of speeds the pump can run while producing flow.