A “Solid” Solution to Pump Selection


Written by:
Sergio Avila, Almatec

 

Tests show that solid-body plastic pumps are the best choice for oscillating applications.

The demands that oscillating-displacement pumps place on their materials of construction are extreme due to the constant alternating stress to which they are subjected. In contrast to rotating machines— such as centrifugal pumps, where the load builds gradually when the machine is started and then remains constant—the components of an oscillating machine (diaphragm or piston pumps) are subjected with each single discharge stroke to continuous alternating stress between the maximum delivery pressure and the maximum vacuum in suction operation.

In the ongoing contest to determine the best pump materials of construction, the general opinion about metal versus plastic can be reduced to the following relationships:

  • Metal is “tough but restricted.”
  • Plastic is “weak but intelligent.”
  • Plastic Advantages

However, when you get right down to it, the big advantage of using plastic as a pump material lies in their exceptional resistance to all types of chemicals. On the other hand, metals—such as stainless steels to the more exotic alloys—only cover a narrow chemical-resistance band. These metals are also expensive and can be difficult to machine. In the simplified resistance list of selected materials shown in Figure 1, the weaknesses of stainless steel with respect to acids, alkalis and salts can be seen quite easily. Aluminum does not need to be discussed at all.

Figure 1. Resistance of materials

In an era in which pump manufacturing has been dominated by steel, we have developed a feeling for knowing how much a millimeter-thick steel sheet, or an M8 bolt, is capable of withstanding. In the case of plastics, this feeling is virtually non-existent. When choosing plastic over steel, it is of profound importance to know that the mechanical strength value of thermoplastic is just one-tenth the strength value of steel.

Several other properties of plastics must be kept in mind—such as deformation under load, narrow temperature application limits, high thermal expansion and cold flow. The design of a component must be suitable for plastic and cannot simply be derived from metal. This applies, in particular, to the housing parts of oscillating machines.

Material Choices

Creating the best plastic pump relies on selecting the best materials of construction for the required processes. The most common plastic pump materials are polyethylene (PE) and polypropylene (PP), which generally possess good chemical-resistance properties. Both are polyolefins with almost identical chemical and temperature behaviors.

These materials differ considerably in several other properties, however. Tests that were conducted based on the sand-slurry method confirm that PE has an abrasion resistance that is seven times higher than PP and 1.6 times higher than steel (Figure 2). For numerous applications, this high abrasion resistance plays an essential role—for example, for pickling baths in electroplating, printing inks, the transport of milk of lime for wet desulfurization, ceramic material and glazes in the ceramic industry.

Figure 2. Sand-slurry method wear factors

A further difference between PE and PP lies in the fact that PP is capable of being injection-molded, whereas PE is normally not. PE can also be rendered conductive (antistatic) by the addition of conductive pigments. This is not possible with injection-molded PP.

Polytetraflouroethylene (PTFE) and the related perfluoroalkoxy (PFA) have almost universal chemical resistance, albeit with low strength values. PFA is capable of being injection-molded with considerable expenditure (Hastelloy molds) but not in a conductive version. PTFE, also intermixed with conductive pigments, must be pressed and sintered and can subsequently only be mechanically machined.

Higher resistance requirements can also be met with the use of polyvinylidenefluoride (PVDF), although it does not have the chemical-resistance properties of PTFE. PVDF is capable of being injection-molded, but it cannot be rendered conductive.

To a small extent, polyacetal (POM) is also normally used in injection-molded form, but it is only of secondary importance due to its limited chemical resistance.

Body Type Options

After the proper plastic is selected, the next crucial question is which type production process should be used: injection molding or solid-body. Despite the fact that an injection mold is expensive to produce, the individual part is low-cost. Injection-molded parts should be thin-walled and have the same wall thickness throughout. This is not a problem for the unstressed housing of a kitchen appliance, but it is for the pump chamber of a diaphragm pump subjected to alternating stress.

Efforts are being made to exploit the maximum processible wall thickness and to stabilize the design by means of diverse ribbing. Nonetheless, external metal parts for stiffening purposes can hardly be dispensed with. Plastic pumps are now used, particularly due to their resistance, for the transport of aggressive media because aggressive atmospheres constantly prevail, attacking and corroding any metal parts.

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