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Friday, 12 October 2018

GEAR PUMPS

          The most common pumping system used in the process of chemicals having high viscocity is the Gear pump. The gear pump was invented around 1600 by Johannes Kepler.

Design:
          A basic gear pump consists of a rotary housing or casing or stator containing two or more intermeshing gears (Helical and Herringbore gears) or lobed cams. Tight tolerances are required between the casing & gears and between gears (The fluid being pumped will lubricate this small clearence and help prevent friction and therefore wear of the rotors and casing). Atypical housing will have an inlet and outlet, for suction and discahrge respectively. There are two main types of gear pumps: 1) External gear pumps (Exterior bearing type) which use two external gears (Figure 1) and 2) Internal gear pumps (Internal bearing type) which use internal and external gears (Figure 2). The term positive displacement for gear pumps describes, they compel or force a fixed amount of fluid they move for each revolution.

Working:
Ø  In this type of pump, only one of the rotor is driven. The intermeshing gears rotate the other rotor. As the rotors rotate, the liquid or gas, (this type of mechanism can also be used in compressor), enters from the suction line and fills the space between the teeth of the gears and becomes trapped forming small 'Slugs' of fluid between teeth.
Ø  The slugs are then carried round by the rotation of the teeth to the discharge side of the pump.
Ø At this point, the gears mesh together and, as they do so, the fluid is displaced from each cavity by the inermeshing teeth.
Ø  Since the fluid cannot pass the points of near contact of the intermeshed teeth nor between the teeth and casing, it can only pass into the discharge line via the putlet.
Ø  As the rotation continues, the teeth at the suction end are opened up again and the same amount of the fluid will fill the spaces and he process repeated. The liquid at the discharge end is constantly being displaced.

General viscosity range : 2 to 4,00,000 cst (EPW, 2012)
Pump formulas:
· Flow rate in US gal/min = Pump Capacity X rpm
· Power in hp = US gal/min X (lbf/in³)/1714

Types:

1) External Gear Pump:
Image Credits : www.oleoflex.it
          In external gear pump two identical external gears (Figure 1) that displace non-lubricating fluids (gears are oil lubricated). The mecahnism is usually driven by one of the toothed gears, the intermeshing which inturn dives the other (Driven or Idle gear). The both gears are supported by bush bearing or ball bearing it is based on the application of load. The drive gear driven by motor directly through coupling. Drive shaft leak is controlled by gland rope or by mechanical seal. There are three factors in the regulation of flow; Volume of cavity between the teeth, gears speed, and the amount of fluid that slips back to the inlet (tolerance dependent) via the mechanism. There are three main types of external gears: Spur, Helical and Herringbone gears. Helical and Herringbone gears deliver more flow at higher pressure while also being quieter, but many require a great inlet pressure than spur gear.

2) Internal Gear Pump:
          An internal gear pump uses internal and external gears (Figure 2). The gears themselves are lubricated by the fluid, which is of a lubricating nature. The internal design is reliable, easy to operate and maintain- due to the presence of only two moving parts. 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. The design can also be modefied to include a cresent shaped portion that improves performance when pumping hugh viscosity fluids. Internal gear pumps have relatively low speed and inlet pressure requirements. 

Considerations:
          Gear pumps are generally operated at high speed and thus give a fairly pulse-free discharge flow and pressure. Where these pumps are operated at slower speeds, as in pumping viscous liquids, the output tends to pulse due to the meshing of the teeth.
          Any air gap drawn into the pump with the liquid, will be carries through with the liquid and will not cause cavitation. This action of the pump means that it is a 'self priming' pump. The discharge pressure may however, fluctuate.
The output from this type of pump is directly proportional to the speed of operation. If the speed is doubled, the output will be doubled and the pressure will have very little effect. (Due to higher pressures, due to the fine clearences between the teeth and between the casing and the rotors, a small leakage back to the suction side will occur resulting in a very small drop in actual flow rate. The higher the discharge pressure, the more likely that internal leakage will occur).
          Rotory pumps are widely used for viscous liquids and are self-lubricating by the fluid being pumped.
          This means that the external source of lubrication can't be used as it would contaminate the fluid being pumped. Gear pumps are capable of moving small suspended solids but due to the meshing of gears they can be damaged by pumping large solids. However, if a rotory pump is used for dirty liquids or slurries, solid particles may act as abrasives, can get between the small clearences and casue wear of the teeth and casing. This will result in loss of efficiency and expensive reapir or replacement of the pump.
          In terms of construction materials, gear pumps can be made form a wide variety of materials, ranging from bronze, iron and stailess steel to cast iron, depending on the application and fluid properties.

OTHER TYPES OF ROTARY PUMPS
The following types of pumps have same charecteristics to the gear pump, they are 
a) Lobe pump
b) Sliding vane pump
c) screw pump

Advantages of Rotary Pumps
· They can deliver liquid to high pressures.
· Self - priming.
· Relatively smooth output, (especially at high speed).
· Positive Acting.
· They can pump viscous liquids.
Disadvantages of Rotary Pumps
· Compared to centrifugal pumps these are more expensive.
· Should not be used for fluids containing suspended solids.
· Wear is more if not pumping viscous material.
· Must NEVER be used with the discharge closed.

Areas:
Marine, Chemical, Petrochemical, Food and General industries.
Applications:
Transfer, lubrication, processing and hydraulic.
Fluids/Materials:
High viscosity fluids, fuel oils, lube oils, various chemicals, resins, paints, pulp, acids, etc.,

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