The question begs asking, “Everyone knows centrifugal pumps are best suited for handling thin liquids. Why even bother whether PD pumps will work for such applications?”
Before we address this, let’s first understand what thin liquids are. Thin liquids are liquids having low viscosity. Viscosity is internal resistance of the liquid to flow, meaning thin liquid can flow easily. Water, gasoline, diesel, Liquefied Petroleum Gases (LPG), and liquor- most common liquids that we encounter everyday- are all thin liquids. Other examples of low viscosity liquids are solvents, refrigerants (Freons such as R134a, Ammonia), hot oils for food industry, Liquefied gases, crude oil, naptha, additives, glycerine, ethanol,bio-diesel, surfactants, monomers etc.
With this knowledge, now let’s explore what difficulties one can possibly encounter handling thin liquids with a PD pump. For example, how can a gear pump be used on a thin non-lubricating liquid when one gear drives the other?
First challenge is that most thin liquids are non-lubricating. So if the pumping elements are running with close tolerances, the parts will wear and tear faster as there is no liquid film between the internal parts. The wear and tear will increase proportionately with increase in differential pressure for the pump.
Secondly, if the clearances between the rotating and stationery parts are high, the slip (reverse flow of liquid from discharge to suction ports due to pressure differential) can be significant, reducing overall volumetric efficiency of the pump.
Lastly, certain materials such as stainless steel will gall if two SS elements are running against each other and the liquid is non-lubricating. The pump will seize in such a case.
Nevertheless, the fact remains that most PD pumps- including gear pumps- can be used on thin
liquids.The challenges listed about can be dealt with by selecting proper pump materials that can handle low lubricity and viscosity. For instance, when using SS 316 Rotor, one can satisfactorily use SS 770 idler or non-metallic material such as Ryton in a gear pump.
The critical area in PD pump is pumping element support which can be either a journal bearing operating in the liquid pump or external antifriction bearing. PD pump manufacturers have a wide array of journal bearing and shaft materials to handle low lubrication situations.
So PD pumps can handle thin liquids with proper material selection, bearing support to maintain close tolerances. In fact, Jens Nielsen invented Viking ® Internal Gear Pump to drain water out from his limestone quarries!
Still the question remains, “Why PD and not centrifugal for such applications?”
Let’s first investigate difference between operating principles of a centrifugal pump and a PD pump. Centrifugal pump imparts high velocity to the liquid as it travels through the impeller vanes and at volute, discharges liquids at high pressure by converting kinetic energy into pressure energy. On the contrary, a PD pump traps certain amount of liquid between the pumping elements and stationery casing at the inlet, traverses it through the internal pockets, and pushes out at the outlet as the fluid cavities are collapsed. In short, a centrifugal pump develops pressure and flow is result; whereas a PD pump develops flow and pressure is result.
Herein lies the answer. There are certain circumstances that demand a PD pump:
Suction lift or Low NPSH applications: For example, any close vessel transfer where system pressure is low or drawing liquid from vacuum
High pressure applications: Multistage centrifugal pumps are required for such applications having poor mechanical efficiency
Constant flow irrespective of varying pressure: For pipeline sampling application where it is critical to have positive feed for the analyzer
Low flow applications: Centrifugal pumps are typically constructed for high flow applications, PD
pumps can handle very small flow rates
Metering applications: For example, to make PU foam, low and precise amount of water has to be added along with Polyol and TDI/ MDI
Energy Usage: With the cost of energy increasing every year, it makes sense to choose the most energy efficient pumps available. Over most of their performance range, whatever the viscosity, Viking® did extensive testing and concluded that its iron internal gear pumps and stainless steel vane pumps consume less energy than ANSI centrifugal pumps, and frequently allow use of smaller motors
To summarize, conventional wisdoms says that a PD pump must be used over centrifugal pump for high viscosity liquids (over 150 cps). Though this is a sound criteria, but an oversimplified one. End users will benefit by calculating life cycle costs for both centrifugal and PD pumps, regardless of
viscosity, before making any new pump selection or replacement.
Have you handled low viscosity liquids with PD pumps? Share your thoughts with us!
References:
1) Ten Misconceptions on Rotary PD pumps: By John Petersen, and CH Tan
Source: Pump &Systems Magazine
Link: http://www.pump-zone.com/topics/pumps/rotary-pumps/ten-misconceptions-rotary-pd-pumps
2) When to use a PD pump vs. centrifugal pump
Link: http://www.pumpschool.com/intro/pd%20vs%20centrif.pdf
3) Viking® Pump Applications
Link: http://vikingpump.com/en/products/custom_solutions/index.html
4) Understanding Pumps, Motors, and Their Controls
Link: http://www.pumped101.com/