As a chemical engineering intern over the 2011/2012 summer I was exposed to a number of different pumping mechanisms. As the process fluids I was dealing with were of high viscosity, the most common pumping system I came across was the gear pump. Hopefully the following (summary of my notes) helps anyone looking for information on gear pumps. I will follow this up with a post on piston pumps soon.
Class: Positive displacement > Rotary > Multiple rotor pump
A basic gear pump will have a rotary housing containing two or more gears (toothed wheels), helical gears or lobed cams. Tight tolerances are required between the casing & gears, bore & gears and between the gears, more on this later. A typical housing will have an inlet and outlet, for suction and discharge respectively. There are two main types: external gear pumps (Exterior-bearing type) which use two external gears (Figure 1, below) and internal gear pumps (Internal-bearing type) which use internal and external gears (Figure 2, below). The term positive displacement for gear pumps describes the fixed amount of fluid they move for each revolution.
Rotation of the two gears causes suction at the inlet and a subsequent discharge at the outlet (Figure 1 & 2, Below). The liquid is carried around the casing to the outlet by the teeth where they eventually mesh, causing the fluid to discharge via the outlet.
General viscosity range: 2 to 400,000 cSt (EPW, 2012)
An external gear pump uses two external gears (Figure 1, below) that displace non-lubricating fluids (gears are oil lubricated). The mechanism is usually driven by one of the toothed gears, which in turn drives the other. Three factors are involved in the regulation of flow: volume of cavity between the teeth, speed of gears, and the amount of fluid that slips back to the inlet (tolerance dependant) via the mechanism. There are three main types of external gears: spur, helical and herringbone. Helical and herringbone deliver more flow at higher pressure while also being quieter, but may require a greater inlet pressure than spur (EPW, 2012)
An internal gear pump uses internal and external gears (Figure 2, below). The gears themselves are lubricated by the fluid, which is of a lubricating nature. The internal design is seen as being reliable, easy to operate and maintain – due to only two moving parts being present. Only one drive gear is required for the mechanism to function but it is possible to use two. The pump will usually contain at least one bushing (EPW, 2012). The design can also be modified to include a crescent shaped portion that improves performance when pumping high viscosity fluids (Figure 2). Internal gear pumps have relatively low speed and inlet pressure requirements.
Gear pumps are capable of moving small suspended solids but due to the meshing of gears they can be damaged by pumping large solids. When solids are present in the fluid they may act as abrasives, causing damage to the gears and increasing downtime. In terms of construction materials, gear pumps can be made from a wide variety of materials, ranging from bronze, iron and stainless steel to cast iron, depending on the application and fluid properties (abrasive, corrosive etc.)
Marine, terminal unloading, chemical, petrochemical, food and general industrial
Transfer, lubrication, processing and hydraulic.
High viscosity fluids, fuel oils, lube oils, various chemicals, resins, paints, pulps, acids, bitumen etc.
Green, D. W., & Perry, R. H. (2008). Perry’s Chemical Engineer’s Handbook. McGraw-Hill.
G.L.Hayett. (2004). Engineering Handbook: Technical Information. New York: Industrial Press Inc.
EWP. (2012). Introduction to Pumps – Positive Displacement Pump. Retrieved 06 2012, from Explore the World of Piping: http://www.wermac.org/equipment/pumps_positive_displacement.html