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ONCE UPON A TIME IN THE LAND OF ROTARY-MOTION PD PUMPS …

The Process Technology and Operator Academy Posted on by

 

Once upon a time I could control myself
Once upon a time I could lose myself
Once upon a time I could love myself
Once upon a time I could love you

("Once," by E. Vedder  and S. Gossard of Pearl Jam, 1991)

 

ONCE UPON A TIME IN THE LAND OF ROTARY-MOTION PD PUMPS

Once upon a time there was a family of pumps whose main goal in life was not to add the PV Pressure to the pumped fluid but rather to make that fluid move forcefully out of the pump.

A pump that adds Velocity but not the PV Pressure … and thus adds "Force" to a fluid …  falls into the category of Hydraulic Pumps.

Many truck-mounted Hydraulic Systems use Gear Pumps.

And some members of this Pump Family did not even move liquids … but rather moved gases and vapors … like air at Atmospheric Pressure!

A pump that is able to get air at Atmospheric Pressure to move falls into the category of Vacuum Pumps.

 

This Rotary Vane Pump is a Vacuum Pump!

 

 

Some models of Rotary-Motion PD Pumps are Hydraulic Pumps.  Other models of Rotary-Motion PD Pumps are Vacuum Pumps.

The versatile family of Rotary-Motion PD Pumps are featured in PTOA Segments #211-214.

 

DIFFERENCES BETWEEN ROTARY-MOTION PD PUMPS
AND RECIPROCATING-ACTION PD PUMPS

Brilliant PTOA Readers and Students … meaning those who are reading the PTOA Segments in the intended, sequential order … recently concluded the PTOA Focus Study featuring Reciprocating-Action PD Pumps.

As was stated in PTOA Segment #204, a graphic of a Rotary-Motion PD Pump might be mistaken for a Centrifugal Pump.

However, nobody will mistake a  Reciprocating-Action PD Pump for a Rotary-Motion PD Pump for the reasons listed below.

 

Obvious Differences Between Rotary-Motion PD Pumps
and Reciprocating-Action PD Pumps

This Reciprocating-Action Triplex Pump is much heavier, takes up much more "floor space," and costs much more than the Rotary-Motion External Gear Pump shown nearby.

Compared to Reciprocating-Action Positive Displacement Pumps, Rotary-Motion Positive Displacement Pumps are much more simple, compact, lightweight, and cost a bunch less to purchase.

Rotary-Motion PD Pumps do not have a Suction Valve and a Discharge Valve, but rather a Suction Port and a Discharge Port.

The Suction Port and Discharge Port do not open and close like the Suction Valve and Discharge Valve on a Reciprocating-Action Pump do. The Ports are always open and simply permit the pumped fluid continuous entry to and exit from the Rotary-Motion PD Pump.

Although exceptions exist, the Suction Port and Discharge Port are typically situated at the middle of the Rotary-Motion PD Pump's Casing. The action of the rotating hardware within the Rotary-Motion PD Pump moves the pumped fluid from the Suction Port, to the interior wall of the Casing, thence to the Discharge Port.

Not So Obvious Internal Differences of Rotary-Motion Pumps

The Suction Port … (or is it the Discharge Port?) … of this Rotary-Action PD Pump is easy to see at the midpoint of the pump's Casing.

The Cylinder of a Reciprocating-Action Positive Displacement Pump provides the captive liquid volume that is displaced by the Piston or Plunger. Ergo, the Cylinder has a important role to play in Reciprocating-Action PD Pumps. Nevertheless, the Cylinder itself is not a difficult piece of hardware to fabricate.

The internal surface of the Rotary-Motion PD Pump Casing is fabricated with the intention of maximizing the Volumetric Efficiency of the pump. Otherwise stated, the interior surface of the Casing is fabricated to ensure that the maximum fluid is sucked into the pump, forced to flow between the outer edges of the rotating hardware and the Casing, and thence flows out of the pump through the Discharge Port.

Brilliant PTOA Readers and Students learned all about the pumping terminology "Volumetric Efficiency" in PTOA Segment #209 and "Slip" which was featured in PTOA Segment #205.

THE PTOA DEPARTMENT OF SLIGHT CORRECTIONS NEEDS TO MAKE A SLIGHT CORRECTION:

The statement was made in PTOA Segment #205 that a PD Pump does not allow for "Slip" or "Slippage" yet the Centrifugal Pump design allows for Slip. That statement is factually accurate. Centrifugal Pumps are designed to allow for some Slip whereas PD Pumps are designed not to have any Slip.

However …

Any time the Volumetric Efficiency of a PD Pump is declining, Slip or Slippage is increasing. Otherwise stated, there is less fluid getting moved to the Discharge Port of the Rotary-Motion PD Pump.

Rotary-Motion PD Pumps have a Suction Port and Discharge Port that are always open. There must be tight tolerance between the displacing hardware and the internal pump casing to insure the liquid is moved from the Suction to Discharge and not allowed to "slip." Over time wear increases on the rotating hardware, "Slip" increases and "Volumetric Efficiency" decreases.

The Volumetric Efficiency of a Rotary-Motion PD Pump is dependent upon the tight clearance … aka tolerance … between whatever rotating hardware is doing the displacement and the internal surface of the pump's Casing.

The greater the spacing between the rotating hardware and the internal surface, the more Slippage occurs … meaning the  pumped-up liquid leaks around the displacing members and remains in the pump's body.

The amount of Slippage significantly impacts the Volumetric Efficiency of the Rotary-Motion PD Pump as well as decreases the Overall Efficiency of the pump by wasting some of the utility expense used to power the pump's Driver.

Spoiler alert! A Rotary-Motion Gear PD Pump fitted with a Herringbone Gear can achieve a 93% Volumetric Efficiency which is generally considered the high end of Rotary-Motion PD Pumps.

 

Hydraulic Pumps Must Pump Oils with Sufficient Lubricity and Viscosity

Who amongst the brilliant PTOA Readers and Students who have already read the PTOA Tribology Focus Study is surprised to learn that the tight clearances required to achieve Volumetric Efficiency create friction which eventually causes the rotating members to wear down?

A comparison of worn and new Internal Gear Rotary-PD Pumps.

Unfortunately, no successful method of packing the moving surfaces of Rotary-Motion PD Pumps to compensate for this wear has been developed.

 

For this reason Rotary-Motion PD Pumps are only suitable for pumping hydraulic fluids or similar oils that have a sufficient lubricity tendency so that the moving parts are lubricated while the hydraulic fluid and/or oil is being pumped!

The ideal fluid to pump with a Rotary-Motion PD Pump would be an oil with good lubricity characteristics AND sufficient Viscosity to prevent excessive leakage out of the pump into the external environment (Viscosity … the tendency to resist flowing …  was featured in PTOA Segment #162).

The more viscous a fluid is the greater its tendency to NOT flow!

 

SIZING UP ROTARY-MOTION PD PUMPS

The diameter of the Rotary-Motion Discharge Port determines the nominal size of the pump.

Rotary-Motion PD Pump Sizing Based Upon Discharge Port Capacity/Flowrate

The Capacity/Flowrate that exits a Rotary-Motion PD Pump ranges from the fraction of a gallon-per-minute for the typical Rotary-Motion PD refrigerator pump to the 5000+ gpm pumps used in marine service.

This Rotary-Motion Gear Pump moves 9.4 gpm with a Discharge Port PV Pressure of 200 psi. By Capacity/Volume this is a "small Rotary Pump." By PSI Pressure this pump is a "Moderate PV Pressure Pump."

The classification of Rotary-Motion PD Pump size based upon Capacity/Flowrate is as follows:

  • A "Small" Rotary-Motion PD Pump moves up to 50 gpm.
  • A "Moderate" Rotary-Motion PD Pump moves 50-500 gpm.
  • A "Large" Rotary-Motion PD Pump moves 500+ gpm.

 

Rotary-Motion PD Pump Sizing Based Upon Discharge Port PV Pressure in PSI

The classification of Rotary-Motion PD Pump size based upon Discharge Port PV Pressure in PSI is as follows:

  • A "Low PV Pressure" Rotary-Motion pump is up to 100 psi.
  • A "Moderate PV Pressure" Rotary-Motion pump is in the range 100-500 psi.
  • A "High PV Pressure" Rotary-Motion pump is in the range 500+ psi.

 

Although the PV Pressure at the Discharge Port of some Rotary-Motion PD Pumps can achieve 1000 psi, the typical PV Pressure of the Rotary-Motion PD Pump is in the range of 25-500 psi.

Rotary-Motion PD Pumps are best suited for low-to-medium PV Pressure applications because in this range they have a more even discharge flow compared to a Reciprocating-Action PD Pump. A more even discharge flow translates into a steady velocity of flow exiting a Hydraulic Pump and less maintenance for all kinds of Rotary-Motion PD Pumps.

 

HYDRAULIC POWER SYSTEMS

Very soon PTOA Readers and Students will read about certain Rotary-Motion PD Pumps in Hydraulic Pump service.  Ergo, right now is the ideal time to learn what the phrase "Hydraulic Power Transmission" means.

Hydraulic Power: The Third Way to Transmit Power

In this Centrifugal Pump, the green motor on the right provides the electrical power that rotates the Impeller that is enclosed in the blue pump. The electrical power is turned into mechanical power.

Brilliant PTOA Readers and Students already know that Rotary Equipment … Pumps and Compressors … must be powered by a Driver to be able to perform the work of adding the PV Pressure to a liquid, gas/vapor, or mixed fluid.

The Electrical Power supplied by a Motor (PTOA Segment #187) is typically transferred into Mechanical Power which makes it possible to rotate hardware in Rotating Equipment (e.g. the Impellers of a Centrifugal Pump).

Alternatively, Mechanical Power could be powered by:

 

There is one more way to transmit power in the industrial world:

Hydraulic Power is the third method available for power transmission.

Force must be applied to the Small Diameter Piston (on the right). The Hydraulic Fluid/Oil is thence displaced via the Connecting Line into the Large Cylinder (on the left). The displaced liquid flows into the Large Diameter Cylinder and forces the Large Piston upward. Whatever is connected to the Large Piston will likewise be lifted. Hydraulic Fluid flows back to the Hydraulic Fluid Reservoir when the Blue Valve is open.

Process Operators in a large processing plant will be surrounded by dump trucks, graders, cranes, back hoes and bulldozers … the equipment used to maintain and improve the facility. A very large processing plant that repairs its own vehicles will have a hydraulic lift system in the company garage and a hydraulic press would be found in the machine/fabrication shop.

The brake and power steering systems of an automobile are also Hydraulically Powered. Likewise the brakes, landing gear, and wing flaps on an airplane are Hydraulically Powered.

YouTube and Chill!

Landing an aircraft without any hydraulic controls was heroically attempted when United Airlines Flight 232 lost all hydraulic fluid, hence lost all hydraulic control. Life was lost. But the landing is a famous aviation success story because dozens survived.

This 14 minute You Tube gets interesting around minute 4. Do not access any of the irritating pop up messages … especially the ones that may claim something is wrong with your lap top or device.

 

 

Hydraulic Power Transmission is for "Straight-Line" Motion 

The common denominator between the heavy equipment, the hydraulic lift in the vehicle shop, the hydraulic press in the machine/fab shop, and the hydraulic controls in an airplane is the need for straight-line, push-or-pull motion … either vertically up/down or horizontally left/right.

Straight-Line motion … horizontal left/right motion … or vertical up/down motion … is the main reason to use Hydraulic Power transmission.

Hydraulic Power is extremely efficient and easy to control … as long the hydraulic fluid remains in the system!

The components of the basic Hydraulic Power Transmission System were first  introduced in PTOA Segment #147. The popular,"Real World" example of a Hydraulic Power Transmission System is the hydraulic lift found in every commercial vehicle repair shop.

As shown in the nearby graphic of a hydraulic lift, the Small-Diameter Piston and Cylinder is connected to a Large Diameter Piston and Cylinder via a Connecting Line.

The volume of the Cylinders and the Connecting Line are filled with hydraulic fluid/oil.

When no force is applied to either Piston, both Pistons are at the same height in their respective Cylinders.

When a force is applied to the Small Diameter Piston (on the left):

  • Hydraulic fluid is displaced and flows through the Connecting Line into the Large Diameter Cylinder (on the right).
  • The increased volume of liquid in the Large Diameter Cylinder pushes the Large Diameter Piston upward with a force!
  • If the Large Diameter Piston were connected to something other than the platform supporting a car, whatever the Large Diameter Piston was connected to would likewise be move straight upward.

 

The architecture of a Small Diameter Cylinder and Piston connected to a Larger Diameter Cylinder and Piston makes it possible to transmit a much greater force to the Large Diameter Piston from the downward movement of the Small Diameter Piston.

The larger the diameter of the Large Diameter Piston, the more force will push the Large Diameter Piston upward.

Because: Pressure = (Force/Area,) Force = (Pressure * Area)

Why? All brilliant PTOA Readers and Students who read PTOA Segment #146 know that Pressure = Force/Area.

Rearranged to apply to the transfer of Hydraulic Power:

Upward Force on the Large Piston  =

(Pressure applied to the  Cylinder * Cylinder's Surface Area).

And a Large Diameter Piston has much more Surface Area than a Small Diameter Piston!

The transmission of Hydraulic Power would not work without the much smaller diameter Connecting Line.

 

It is the Connecting Line which transmits the Hydraulic Power from one Cylinder to the other.

 

A large amount of Hydraulic Power can be transmitted through small flexible hoses and tubes.

 

Modern Hydraulic Cylinders Transmit Hydraulic Power

The modern Hydraulic Cylinder shown nearby looks like a miniature  Double-Acting Reciprocating-Action PD Pump featured in PTOA Segment #210.

A single Piston is forced to expand and contract when hydraulic fluid/oil is forced into or out of either side of the the Cylinder.

The ends of the Cylinder are capped so the hydraulic fluid cannot leak out.

The Rod that sticks out of one side of the Cylinder moves outward when the Piston is contracting and inward when the Piston is expanding.

This Piston Rod could be attached to the boom arm of an excavator of the platen of a press forcing the excavator arm or the plate press to move up and down or left and right as required.

This Nissan auto uses an electro-hydraulic system for power steering. The Hydraulic Cylinder delivers a horizontal Force.

Or several Hydraulic Cylinders might be attached to make the platform of a Flight Simulator gyrate!

By the way … here's a quick You Tube that illustrates how NOT to unload a Hydraulic Cylinder:

 

HYDRAULIC ROTARY-MOTION PD PUMPS

Several models of Rotary-Motion PD Pumps are used in Hydraulic Power Systems.

The most popular Rotary-Motion Hydraulic Pump is the Internal Gear Pump.

The Peristaltic Rotary-PD Pump has a flexible hose between its Suction Port and Discharge Port.

Other Rotary-Motion PD Pumps used in Hydraulic Power transmission include:

  • Vane Pumps
  • Screw Pumps
  • Piston Pumps (like the Triplex PD Pump and Rotary-Motion Radial Piston PD pumps)
  • Peristaltic Pumps.

 

The goal of any Hydraulic Pump is not to add the PV Pressure to the fluid being pumped but rather to deliver a constant, forceful velocity of flowing liquid from the pump's Discharge Port.

A Hydraulic Pump provides forceful flow into a downstream system just like forceful flow enters the Connecting Line after a force has been applied to the Small Cylinder in this hydraulic auto lift.

The fluid discharged from the Rotary-Motion Hydraulic PD Pump has a forceful velocity and is able to enter the pressurized system that is connected to the downstream side of the pump.

The role that the Rotary-Motion Hydraulic PD Pump plays in a Hydraulic System is similar to the spot in the hydraulic auto lift where the displaced hydraulic fluid exits the Small Cylinder and enters the Connecting Line.

The amount of Hydraulic Horsepower transmitted via a Hydraulic Pump is easy to calculate if the Flowrate/Capacity and Discharge Port PV Pressure are known:

Hydraulic HP = (GPM * PSI) / 1714

For example, the Rotary-Motion PD Gear Pump featured above has a Capacity/Flowrate of 9.4 gpm and a Discharge Port PV Pressure of 200 psi.

The Hydraulic Horsepower the pump is capable of is:

Hydraulic HP = (9.4 * 200)/1714 = 10.96 Hp

No pump is 100% mechanically efficient. In the Real World, this pump might be 93% efficient, which means in the Real World the pump delivers 10.2 Hp.

The next PTOA Segment features Rotary-Motion Gear Pumps.

TAKE HOME MESSAGES:  Unlike Reciprocating-Action PD Pumps, Rotary-Motion PD Pumps take up less floor space and are less expensive. A Rotary-Motion PD Pump has a Suction Port and Discharge Port that are always open and which are typically situation in the middle of the pumps casing.

The Rotary-Motion PD Pump family can service Capacity/Flowrates ranging from fractions of a gallon per minute to 500+ gpm. The most successfully operating Rotary-Motion PD Pumps have Discharge Port PV Pressures in the range of 25-500 psi. The nominal size of the Rotary-Motion PD Pump is determined by the diameter of the Discharge Port.

The interior surface of a Rotary-Motion PD Pump is fabricated to help limit "slip" and a loss of "Volumetric Efficiency" between the rotating hardware and the interior surface of the pump's Casing. No packing has been invented to eliminate the wear that still happens between the rotating hardware thus Rotary-Motion PD Pumps are limited to pumping hydraulic oils and fluids with ideal lubricity and viscosity characteristics.

The third way to transmit power is by Hydraulic Power Transmission.  Hydraulic Power Transmission delivers a straight line, forceful velocity of fluid. The typical example of the transmission of Hydraulic Power is the automobile lift with two Piston-Cylinders attached via a Connecting Line. The Connecting Line transmits the Hydraulic Power. A large amount of Hydraulic Power can be transmitted through flexible hoses and tubes.

Several types of Rotary-Motion PD Pumps are Hydraulic Pumps. Their purpose is to deliver constant, forceful flow into a pressurized system. Hydraulic Pumps play the analogous role of the liquid delivered from the Small Cylinder of an automobile lift into the much smaller diameter Connecting Line. In the Real World the Connecting Line is the Discharge Line connected to the pump's Discharge Port.

Hydraulic Horsepower of Hydraulic Pumps can be calculated if the Capacity/Flowrate and Discharge Port PV Pressure are known:

Hydraulic HP = (GPM * PSI) / 1714

 

©2020 PTOA Segment 0211

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

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