You got me going in circles
(oh round and round I go)
You got me going in circles
(Oh round and round I go)
I'm spun out over you

("Going in Circles," written by J.Peters and A. Poree and made famous by The Friends of Distinction, 1969)

 

THE MANY SERVICES OF THE POPULAR GEAR ROTARY-MOTION PD PUMP

The Rotary-Motion family of Positive Displacement Pumps is located in the lower right quadrant of the familiar diagram which illustrates "The  "Classification of Pumps … with Examples."

The Rotary-Motion PD Pump Family includes:

• Vane Pumps
• (Rotary) Piston Pumps
• Screw Pumps
• Gear Pumps

 

This PTOA Segment #212 focusses on Gear Rotary-Motion PD Pumps. Vane Rotary-Motion PD Pumps and Screw Rotary-Motion PD Pumps will be featured in upcoming PTOA Segments.

Why are Gear Rotary-Motion PD Pumps so Popular?

Tis a fact that Gear Rotary-Motion PD Pumps cannot match the Capacity/Flowrate of a Centrifugal Pump nor the PV Pressure discharged from a Reciprocating-Action PD Pump. The reasons why Gear Rotary-Motion PD Pumps have a wide application of service is explained in painstaking detail below.

One reason is because the form and function of Gear Rotary-Motion PD Pumps makes them capable of achieving higher Capacity/Flowrates and Discharge Port PV Pressures than their relatives, the Lobe Rotary-Motion PD Pump and the Vane Rotary-Motion PD Pump.

 

EXTERNAL GEAR ROTARY-MOTION PD PUMPS

Brilliant PTOA Readers and Students …  meaning those who are reading the PTOA Segments in the intended, sequential order … recently learned in PTOA Segment #211 about the generation of hydraulic power in hydraulic power systems.

For their ability to convert mechanical power into hydraulic power External Gear Rotary-Motion PD Pumps are used in a variety of mobile and industrial hydraulic systems. The hydraulic force of the pumped fluid can be used to open and close valves, or lift, or rotate.

Consequently, External Gear Rotary-Motion PD Pumps will be found in hydraulic lifting systems wherein they service fluids which have ideal lubricity and viscosity characteristics.

External Gear Rotary-Motion Pumps are also used in heavy duty trucks as lubrication pumps for hydraulic power systems. They will be found in lawn care vehicles and log splitters.

Furthermore,

External Gear Rotary-Motion PD Pumps can be operated in either direction!

This feature means that the Power Gear (defined below) can be rotated in reverse, thus creating a hydraulic motor!

In a remote location without convenient electricity, having a hydraulic motor for power is handy!

The nearby photo shows an External Gear Rotary-Motion pump in a heavy duty tractor.

Tractors use their engines to drive the External Gear Rotary-Motion PD Pumps for hydraulic power service.

No way is the use of External Gear Rotary-Motion pumps limited to hydraulic power systems!

Because the rotational speed of the Power Gear is directly related to the Capacity/Flowrate of the pump, External-Gear Rotary-Motion Pumps are excellent blending and metering pumps.

Because of the tight tolerance between their gear teeth, the fluid that flows into the Suction Port of the External-Gear Rotary-Motion Pump is sucked in with a strong vacuum. Good suction means the External-Gear Rotary-Motion Pump can move harsh chemicals with less fear of leakage into the external environment.

In summary, the industrial applications of the External-Gear Rotary-Motion Pump include:

• Metering and mixing of chemical additives.
• Moving acids and caustics.
• Moving low Capacity/Flowrates of compatible fluids.

 

 

DESIGN OF EXTERNAL-GEAR ROTARY-MOTION PD PUMPS

The External Gear Rotary-Motion PD Pump is also popular because of its relatively simple construction compared to either Centrifugal Pumps or Reciprocating-Action PD Pumps.

External Gear Rotary-Motion PD Pumps are easy to identify because of the Figure 8 construction of their Casing.

One gear is situated in the upper half of the Figure 8 and an identical gear is located in the bottom half of the Figure 8.

A Suction Port is fabricated into one side of the pump and a Discharge Port is fabricated into the opposite side of the pump.

Note that both of these ports are situated at the center of the pump where the top circle and bottom circle of the Figure 8 shape would touch.

 

One of the gears is driven by a motor (or other type of Driver/Prime Mover).

 

This gear is called the "Power Drive" or "Gear Drive" because its rotary motion drives the "Driven Gear" … which is sometimes called the "Idler."

 

 

The nearby graphic labels the important features of an External Gear Rotary-Motion PD Pump:

• Suction Port
• Power Gear aka Drive Gear
• Driven Gear aka Idler Gear
• Casing (not labelled)
• Discharge Port

 

As the intermeshed gears rotate, fluid is trapped in the spaces between the gears' teeth and the internal surface of the pump's Casing.

Because of the tight clearance between the upper and lower gears, the fluid is forced to move to the outer diameter of the gears (because that is easier than trying to flow through the intermeshed gears).

The fluid is thus carried to the Discharge Port, allowing more fluid to be sucked into the Suction Port.

The tight tolerance between the intermeshed teeth is directly related to how well the pumped fluid is prevented from leaking back to the suction side of the pump as well as the success of sucking more fluid into the Suction Port.

 

Rotary-Motion Gear Pumps Must Have Tight Tolerance Between Gears 

The Rotary-Motion Gear Pump presents a great case study for the need of tight tolerances between the two rotating gears and the internal surface of the pump Casing which was detailed in PTOA Segment #211.

When tight tolerances are lost with wear:

• Slippage is increased.
• Volumetric Efficiency is decreased.
• Discharge Port PV Pressure is decreased.
• Suction Port PV Pressure is decreased, which means less of the pumped fluid is sucked into the pump.

 

Furthermore:

The Volumetric Efficiency of Gear Pumps and all Rotary-Motion PD Pumps greatly decreases at low Capacity/Flowrates and rotational speed.

 

Therefore,

 

Rotary-Motion PD Pumps should always be operated at their recommended speed.

 

External-Gear Rotary-Motion Pump Leak Prevention and Vibration Control

Those same brilliant PTOA Readers and Students who read the PTOA Segments in the intended, sequential order can also predict that a means of leak prevention (perhaps a gasket) and some means of bearing support (perhaps bushings) must be part of the pump package.

More than likely the rotating hardware within the pump is lubricated with the fluid that is being pumped.

When that is not the case, a means of external lubrication must be included in the pump design.

 

THE THREE TYPES OF GEARS FOUND IN GEAR PUMPS

The gears in Gear Rotary-Motion PD Pumps come in three varieties.

Spur Gears are cheapest and cost less to maintain. However, Spur Gears are noisy and thus at high rotational speeds significantly contribute to decreasing the pump's Overall Mechanical Efficiency.

Helical Gear and Herringbone Gears make it possible for large External Gear Rotary-Motion PD Pumps to operate at high speeds.

A higher rotational speed means more Capacity/Flowrate through the pump  …

which means more Hydraulic Horsepower is transferred from the pump!

Helical Gears and Herringbone Gears also yield smoother fluid flow which translates into a smoother transfer of hydraulic power!

For example:

A Herringbone Gear Rotary-Motion PD Pump with a Name Plate Capacity/Flowrate of 200 gpm and PV Discharge Port Pressure of 160 psi can achieve 93% Volumetric Efficiency and 85% Overall Mechanical Efficiency.

What exactly is the relevance in the Real World of the lofty sounding statement above?

With 200 gpm Capacity/Flowrate and 160 psi Discharge Port PV Pressure, the Hydraulic Horsepower of the External Gear Rotary-Motion Pump (defined in PTOA Segment #211) is:

Hydraulic Horsepower (Hp) =
(200 * 160)/1714 = 18.7 Hp

However, the Real World Volumetric Efficiency is stated to be 93%. Ergo, the Real World Flowrate/Capacity is

(200 gpm * 0.93) =186 gpm.

The Real World Overall Mechanical Efficiency is stated to be 85%.

That statement means that the Real World Overall Mechanical Efficiency is:

18.7 Hp*0.85 = 15.9 Hp

Guess What?

The only Real World Discharge Port PV Pressure that can be multiplied by 186 and thence divided by 1714 to get 15.9 Hp is 146.5 PSI.

Otherwise stated:

Real World Hydraulic Horsepower = (186 gpm * 146.5 psi)/1714 = 15.9 Hp

Don't stress, Fred!

The intent of the above calculating exercise was to clarify the differences between the theoretical Hydraulic Hp of a Rotary-Motion PD Pump design and the Real World Hydraulic Horsepower that is typically observed once the pump is installed in a system.

Although the Process Technology curriculum insists that Process Operators need to be aware of the difference, Your Mentor has never met a Process Operator who has had to prove they know the difference between theoretical and Real World Hydraulic Horsepower!

Suffice it is to state that the Name Plate Capacity and Discharge PV Pressure of a pump are always somewhat more than what is observed in the Real World once the pump is installed in a Real World system! Like, Duh!

 

 

LOBE ROTARY-MOTION PD PUMPS

The Lobe Rotary-Motion PD Pump is one of the earliest Rotary Pump (and Blower) designs.

Before Centrifugal Pumps took their place, large Lobe Rotary-Motion PD Pumps achieved an 80-85% Overall Mechanical Efficiency in pump services requiring low Discharge Port PV Pressure and high Capacity/Flowrate service … like 35,000 gpm.

Small wonder that Lobe Rotary-Motion PD Pumps found a niche in irrigation projects!

Close scrutiny of the Lobe Rotary-Motion PD Pump design results in classifying these pumps as a variation of the External Gear Rotary-Motion PD Pump.

Note the Figure 8 Casing shape with Suction Port and Discharge Port across from each other.

However, neither of the Lobes are "Idlers."

Unlike the External Gear Rotary-Motion PD Pump, each of the two Lobes in a Lobe Rotary-Motion PD Pump is separately driven by Pilot Gears which are external to the pump casing.

Contact "happens" between the Lobes and the internal surface of the pump Casing. At high Discharge Port PV Pressures, the Volumetric Efficiency of the Lobe Rotary-Motion PD Pump is greatly reduced.

For this reason, the service niche for the Lobe Rotary-Motion PD Pump remains limited to low Discharge Port PV Pressures and medium-to-large Capacity/Flow Rates. The nearby photo is a Lobe Rotary-Motion Pump in a water treatment (sewage) facility.

Any PTOA Reader or Student who is baffled by the descriptors "small" or "large" regarding Rotary-Motion Pumps should review Rotary-Motion Pump sizing which was featured in PTOA Segment #211.

 

 

INTERNAL GEAR ROTARY-MOTION PD PUMPS

Internal Gear Rotary-Motion PD Pumps have a wide range of service because their hardware can be altered to operate over a wide range of Capacity/Flowrates and Discharge Port PV Pressures.

In general, Internal Gear Rotary-Motion PD Pumps typically operate at lower rotational speeds than External Gear Rotary-Motion Pumps.

The Internal Gear Rotary-Motion PD Pump is easy to distinguish from an External Gear Rotary-Motion PD  Pump because the Casing is round, not shaped like a Figure 8.

Furthermore …

The Suction Port and Discharge Port are not round but rather oval in shape. The oval shape of the Ports helps to smooth out the flow of the discharged fluid.

Additionally ..

The ports are 90°apart, not 180°apart.

More structural differences are inside the Internal Gear Rotary-Motion Pump.

Unlike the two gears in an External Gear Rotary-Motion PD Pump, the  large-diameter Internal Gear and small-diameter External Gear enclosed within the Casing of an Internal Gear Rotary-Motion PD Pump are not identical gears nor are they adjacent gears.

As the nearby graphic shows, the important parts of an Internal Gear Rotary-Motion PD Pump are:

• A large-diameter Internal Gear whose teeth protrude radially outward from the Casing.
• A small-diameter External Gear which is mounted off-center.
• Depending upon the pump's service, either the large-diameter Internal Gear or the small-diameter External Gear could be the Power Gear. Obviously that means either of the gears might be the Idler.

The eccentric design of the Internal Gear Rotary-Motion PD Pump results in the meshing teeth engaging at one point only, which forms a tight seal.

The tight seal of the Internal Gear Rotary-Motion PD Pump is this pump's claim to fame!

The offset of the gear teeth is an important design parameter of Internal Gear Rotary-Motion PD Pumps. By how many teeth the gears are offset will determine if the pump's service will be for high Discharge Port PV Pressure and high speeds or limited to the small and medium ranges of Capacity/Flowrate and Discharge Port PV Pressure.

A Crescent keeps the two Gears apart. For this reason it is sometimes called "a spacer."

Depending upon pump design, the  orientation of the Crescent can be moved, allowing the pump to be operated in either direction.

Otherwise the Internal Gear Rotary-Motion PD Pump operates similarly to the External Gear Rotary-Motion PD Pump.

The Power Gear (whichever gear it is) rotates around the Driven Gear (aka Idler). As the nearby graphic shows:

Filling Step: On the Suction Port side of the Pump, the large-diameter External Gear and small-diameter Internal Gear come out of mesh revealing cavities that disappear when the gears are meshed together.

Transfer Step: The liquid that is trapped in the cavities is displaced when the gear teeth mesh together again. The meshed teeth displace the fluid, transferring the fluid from the Suction Port, around the Casing, thence toward the Discharge Port.

Discharge/Delivery Step: On the Discharge Port side of the pump, the Internal Gear and External Gear teeth mesh and the reduced volume forces the pumped fluid out of the pump at the Discharge Port PV Pressure.

The tight seal that results from the eccentric design of the Internal Gear Rotary-Motion PD Pump gives this pump superior suction capabilities. The vacuum that is created at the Suction Port makes the Internal Gear Rotary-Motion Pump ideal for transporting:

• Corrosive, caustic, and noxious liquids that must not leak into the surrounding environment. A list of corrosive liquids includes sulphuric acid, sodium hypochlorite, ferric chloride and sodium hydroxide.
• Shear-sensitive liquids like paints.
• Hot fluids (up to 400°C) which describes fluids like tar and molten sulphur.

 

Compared to External-Gear Rotary-Motion Pump, The Internal Gear Rotary-Motion Pump can operate across a wider range of temperatures and wider range of fluid viscosities (1 cP to over 1,000,000 cP). This wide range of viscosities includes water, solvents, fuel oil, asphalt, chocolate, and adhesives!

Internal Gear Rotary-Motion PD pumps are better than External Gear Rotary-Motion PD pumps with respect to adding velocity to fluids that contain suspended solids.

Like the other Gear Pumps, the Capacity/Flowrate of the Internal Gear Rotary-Motion Pump is directly proportional to the rotational speed of the Power Gear. Thus Internal Gear Rotary-Motion PD Pumps are commonly used for metering and blending service. For example the blending of polymers, hydrocarbon fuels, or chemical additives.

The next PTOA Segment focusses on Vane Rotary-Motion PD Pumps and Screw Rotary-Motion PD Pumps.

 

TAKE HOME MESSAGES: Gear Rotary-Motion PD Pumps are popular because of their wide range of service and simplicity compared to Centrifugal and Reciprocating-Action PD Pumps.

External Gear Rotary-Motion PD Pumps are found in vehicle lubrication systems and hydraulic systems that open and close valves, or push, or rotate, or lift.  Some External Gear Rotary-Motion PD Pumps are operated as hydraulic motors …  for example in heavy tractors. 

The Figure 8 structure of the External Gear Rotary-Motion PD Pump encloses identical gears on the top and bottom halves of the Figure 8.  The important hardware in an External Gear Rotary-Motion PD Pump includes:

• Suction Port
• Power Gear aka Drive Gear
• Driven Gear aka Idler Gear
• Casing
• Discharge Port located 180 degrees from the Suction Port

 

A tight tolerance between the meshing gear teeth and Casing is required to optimize Volumetric Efficiency, Hydraulic Horsepower, and Overall Mechanical Efficiency of all Gear Rotary-Motion PD Pumps.  All Rotary-Motion PD Pumps must also be operated at their recommended speed to optimize operational efficiency.

The three types of gears found in Gear Rotary-Motion PD Pumps are Spur Gear, Helical Gear, and Herringbone Gear. 

This PTOA Segment includes a demonstration calculation which explains the difference between the design and Real World observed Hydraulic Horsepower, Capacity/Flowrate, and Overall Mechanical Efficiency of a Herringbone External Gear Rotary-Motion PD Pump.

Lobe Rotary-Motion PD Pumps are considered a variation of External Gear Rotary-Motion Pumps. The main difference is that the two Lobes are powered separately by pilot gears (meaning there is not a Drive Gear and an Idler Gear).  The industrial service niche of the Lobe Rotary-Motion PD Pump is for low Discharge Port PV Pressure and high Capacity/Flowrate service. 

Internal Gear Rotary-Motion PD Pumps have an eccentric design which results in the teeth of the large-diameter External Gear and small-diameter Internal Gear meshing at one point, forming a tight seal for displacement as well as a strong vacuum at the Suction Port.  

The Internal Gear Rotary-Motion PD Pump includes the following hardware:

• Oval Suction Port
• Large-diameter External Gear protruding radially from the Casing
• Small-diameter Internal Gear held in place eccentrically by the
• Crescent or "Spacer"
• Casing
• Oval Discharge Port situated 90 degrees from the Suction Port

 

Either the large-Diameter External Gear or the small-diameter Internal Gear may be the Power Gear leaving the other to be the Driven … aka Idler Gear.

Internal-Gear Rotary-Motion PD Pumps can tolerate a much wider range of fluid temperatures and fluid viscosities than External Gear Rotary-Motion Pumps can as well as service fluids that contain suspended solids.

Both the External Gear Rotary-Motion PD Pump and the Internal Gear Rotary-Motion PD Pump have strong Suction. The strong vacuum at the Suction Port means the External Gear Rotary-Motion PD Pump is ideal to transfer acids and caustics and the Internal Gear Rotary-Motion PD Pump is ideal to transfer hot corrosive, caustic and noxious fluids in an environmentally sound manner.

Because the rotational speed of the Power Gear is directly related to the Capacity/Flowrate of the pump, industrial uses of  both the External Gear Rotary-Motion PD Pump and Internal Gear Rotary-Motion PD Pump include metering and blending.  However, Internal Gear Rotary-Motion PD Pumps typically operate at lower rotational speeds than External Gear Rotary-Motion Pumps. 

External Gear Rotary-Motion PD Pumps meter and blend chemical additives. Internal Gear Rotary-Motion PD Pumps meter and blend chemical additives, polymers, and hydrocarbon fuels. 

©2020 PTOA Segment 0212

PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT PUMPS

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