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2 July, 2012 Posted by Christopher Jenner Posted in Process and Instrumentation, Pumps
7 comments

Gear Pumps

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

Design:

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.

Actions:

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)

Type:
External

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)

External Gear Pump - Exterior-bearing type

Figure 1: External Gear Pump – Exterior-bearing type. Arrows indicate flow direction and movement of gears.

Internal

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.

Internal Gear Pump - Interior-bearing type

Figure 2: Internal Gear Pump – Interior-bearing type. Arrows indicate direction of flow and movement of gears.

Considerations:

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.)

Areas:

Marine, terminal unloading, chemical, petrochemical, food and general industrial

Applications:

Transfer, lubrication, processing and hydraulic.

Fluids/Materials:

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

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7 comments

  1. zoynul abedin says:

    helpful for begineers

  2. Mark says:

    Thanks for the information. I am trying to design a liquid transfer system and would love your thoughts on basic design, such as: 1) most economical pump to use, 2) most reliable pump to use, 3) best type of pump to use.

    The system transfers liquid chemical from a non-pressurized tank to a pressurized (max 200 psi) tank.

    Thoughts?

  3. jyoti says:

    What should we consider technically if we have to run our external gear pump for soft water with inject dye liquid. This liquid have Specific gravity 1.0 , I want to reduce noise level in this application.
    Our design gear pump generate noise level between 90dB to 100dB.

    If you say that we should use centrifugal pump for my this application but let me know to you that I need Bi-directional flow in my application of the given liquid.
    Your prompt reply is highly appreciable.

  4. hello sir,
    great this is your post.i am very impressed your post and i value your post highly.these post are very useful about of all person so i hope next time your article are the better and I can fetch some information related to our new informative piece of content on helical gear pumps . I might add this, and yes a link to this post.I am waiting you next post.”

  5. H Harvey says:

    What is the difference between 1 gear and 2 gear driven pumps? Pros and cons would help as well. The pump I’m looking at buying is for pumping herbicides.

    Thank you,

  6. Arshi says:

    My experimental setup is a container with 1kPa pressure and containes some salt solution. I need to pump solution from bottom of container to the top where it get sprayed to bottom again. For such a low pressures which gear pump is useful?

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