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.
From a practical standpoint for in-line blending applications, the tighter the pump-performance band, the better the metering accuracy under varying process conditions. At the same time, the wider the performance/flow rate band, the more flexibility in handling formulations that require a wide range of possible ingredient input flows.
This article explores these new concepts that take pump performance to the next level. In addition to the pump just simply working, the correct application of these pump concepts allows refinement of the transfer process, permitting new, enhanced applications that were previously not possible or reliable. The in-line blending process described above is one example. Other examples include coating, spray drying, filling, filtering and heat-exchange processes that require controlled flow with tight pump performance bands.
Tight Versus Loose Pump Performance
The root issue with rotary PD pumps is that the flow performance on all pumps is to some degree affected by internal clearances that result in slip. The degree of slip changes with:
- Viscosity changes
- Differential pressure changes
- Clearance allowances for temperature change
- Wear (resulting in an increase in clearance)
Given these product/process variables, tight performance occurs when the pump maintains close to its theoretical displacement independently of changes to the above variables. The definition of a PD pump is a pump that transfers a set displacement per unit operation, such as revolution or stroke.
Tight versus loose pump performance is the extent to which, under a given range of conditions, the pump maintains high volumetric efficiency. High volumetric efficiency is the extent (ratio) in which the true displacement of the pump approximates its theoretical displacement for given process/product conditions. Pump slip is the difference between the theoretical displacement and the actual displacement. Therefore, the lower the pump slip in any condition, the tighter the pump's performance under conditions of changing viscosity, pressure, temperature or wear.
Classifying a pump as simply positive displacement without quantifying the tightness of its performance band can greatly affect the desired results in an application. The extreme example is one in which, regardless of the pump speed, the slip is 100 percent. That is, all fluid that is pumped forward then flows (slips) back through the pump's internal clearances to produce no net fluid transfer. While sounding dramatic, it is not uncommon that a pump reaches this point (total loss of flow) before it is taken out of service to be repaired or replaced.
To understand slip for traditional PD pumps, see Figure 5. It illustrates the possible loose-performance range (the yellow area) of a typical PD pump when operating in variable conditions (changes in viscosity, back pressure, temperature and wear). This graph shows how flow for a given pump speed (A) can vary from the theoretical (intersection BA) to an extreme (intersection EA), which indicates no flow. This condition occurs in pumps with worn pumping elements, for example.
PICTURE
Figure 5
Even in non-extreme cases, such as when needing a flow rate of (B), the pump would need to be accelerated from (A) to (F) to achieve the flow (B). This can prove to be an automation challenge and result in a reduction of reliability. If the automation system does not have a way to compensate for loss of flow and the pump remains at the same speed (A), the flow rate (D) would be inadequate. An actual curve for such a pump with 0.153 gallons/revolutions can be seen in Figure 6.
PICTURE
Figure 6
Most users specifying pumps realize this and attempt to control the extreme variabilities of viscosity, pressure, temperature and wear simultaneously. In many applications, this variation is sufficient to produce a challenging operational scenario. In some cases, advanced automation can help, such as using flowmeters with speed/pressure control loops. However, there are cases for which the possible variation cannot be compensated without recalibration or retuning the processes. These methods can prove costly or unfeasible, and could also increase system complexity (thus reducing reliability).
Figure 5 illustrates a tight performance band, which is shown as the green performance band range superimposed on the same graph. Even with large variations in pumping conditions in its published performance limits, the maximum variation in flow versus pump speed would be between (B) and (C) instead of (B) and (E), illustrated by the yellow loose-performance band. An actual curve band for such a pump can be seen in Figure 7. Both pumps (Figures 6 and 7) have a theoretical displacement of 0.15 gallons/revolution, but the curve in Figure 6 shows how loose the pump's performance is at 250 rpm, producing as much as 28 gpm of slip while attempting to pump 38 gpm. The pump shown in Figure 7 has only 4 gpm of slip under the same conditions.
PICTURE
Figure 7
Today's advanced pump manufacturers provide the tools that permit evaluating the possible slip for a given application. Curves are supplied that demonstrate how to down-rate the flow given changes in back pressure, viscosity or change of internal component clearance to handle certain temperature ranges. These tools are helpful for compensating for performance. At times, however, these performance changes cannot be adequately or reliably compensated and may not produce optimal control.
Next month, we will look at the effects of pump component wear and a narrow versus wide performance band.
This article builds on the principles presented in the Hydraulic Institute's "PD Pump Fundamentals, Design and Applications (Part One)" (Pumps & Systems, February 2009, available on www.pump-zone.com).
Pumps and Systems, April 2010
