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5. PD Pumps Cannot Handle Non-Lubricating Liquids

Non-lubricating liquids range from thin to thick viscosity. Many thin liquids have poor lubricating properties, but so do some thick liquids. For example, number 6 fuel oil (sometimes used for heating or diesel fuel) can be as thick as 15,000-cps, but it is not very lubricating.

Manufacturers tend to limit the load from pumping element contact in PD pumps by limiting maximum pressure with non-lubricating liquids. Doing this minimizes pumping element wear on most non-lubricating liquid applications.

The critical area in PD pumps is pumping element support, which can be either a journal bearing operating in the liquid pumped or an external antifriction bearing. PD pump manufacturers have a wide array of journal bearing and shaft materials to handle low lubrication situations. The key is choosing the right materials for the particular liquid characteristics to get the best pump life.

External antifriction bearing support takes away the material problem, but in most designs this moves the support farther away from internal pump loads, resulting in higher bearing loads that may also limit maximum pressure on a particular design.

6. PD Pumps Do Not Run Fast

Browsing product information for a number of different pump designs and manufacturers reveals this is not true. Granted, PD pumps must run more slowly as viscosity increases and also when flow is above 200-gpm, but direct motor speed operation is not uncommon.

Many smaller displacement pumps run at direct motor speeds of 1800-rpm, with some even going to 3600-rpm. Motor speed PD pumps provide an economical solution to liquid transfer for any number of applications where liquid properties like viscosity, abrasiveness and shear sensitivity are not a problem.

7. PD Pumps Are Expensive to Own

The initial cost of most PD pumps is more than centrifugal pumps, but one must consider the total cost of ownership. Using a simplistic approach to cost of ownership, the main contributors are first cost, repair costs and energy to operate.

Assuming an average pump life of seven years, a couple of repairs during its life span and rather modest pressure requirement energy to operate the pump equals one half of the total cost of ownership. First cost is the lowest, with repair parts coming in second. Figure 1 shows the detail. PD pumps can be quite efficient. A small increase here can save considerable money over the life of the pump.

Figure 1

8. PD Pump Repairs Are Expensive

Repair parts costs vary between manufacturers. Since these costs differ, understanding them before the pump purchase is always a good idea. Misconception 7 above assumed an average pump life of seven years with two lower level repairs and a major overhaul (replacing all critical wear parts) during the life of the pump. Cost for the parts required for a major overhaul is only two-thirds the price of a new pump. PD pumps are usually easy to work on too, so labor costs are not high.

9. PD Pumps Have Pulsing Flow

This is true for reciprocating PD pumps, but not for most rotary PD pumps. This may seem counterintuitive since these pumps move liquid by delivering confined volumes of liquid from the suction to the discharge port. Although it would seem that confined volumes or buckets of liquid would result in flow pulsations, they actually don't.

In theory, gear, vane, some lobe pumps and all types of screw pumps deliver a continuum of liquid resulting in very little, if any, flow pulsations. There may be slight differences in slip within the pump, depending on rotational position of the pumping elements that result in minimal pressure pulsations, but theoretical output remains constant. Traditional lobe pumps with three lobed rotors do have theoretical flow pulsation, but again this is minimal.

10. PD Pumps Cannot Run Dry

PD pumps self prime, meaning they are capable of pulling a liquid from a level below the pump into the pump port. This means the pump must run without liquid for the time it takes to get liquid up the elevation. In other situations, PD pumps are asked to empty tanks, which many times results in periods of running with little, if any, liquid inside the pump.

Most PD pumps can run dry for short periods of time without damage. In many cases there is a small amount of liquid in the pump, which keeps the parts wetted to the point damage does not occur. Obviously, extended periods of run dry are not recommended and some designs are more tolerant than others. In any case, run dry situations are more common than anyone likes to admit and PD pumps can usually handle them.

Summary

Rotary PD pumps are (for the most part) simple devices, but more complex in application. Understanding the basic operating characteristics of PD pumps and system requirements is a great place to start in correctly applying a pump.

Recognizing these common misconceptions opens up a whole new application area for these pumps to improve performance, extend service life or operate more efficiently. Consider all the alternatives to a pumping problem before making a decision - you'll be surprised by how many choices you have.

John Petersen is vice president, technical customer service and CH Tan is regional director for the Asia Pacific Region of Viking Pump, Inc., a unit of IDEX Corporation, 406 State Street, P.O. Box 8, Cedar Falls, IA 50613-0008, 319-266-1741, Fax: 319-273-8157, http://www.vikingpump.com/.

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