You know how to spend it
You know how to blow it up, blow it up, blow it up
It's getting so expensive
You know how to screw it up, screw it up, screw it up
Up, up, up, up,
Screw it up, up, up, up, up.

("Red Flag," by Gwen Stefani et al., 2016)

 

THE SINGLE-SCREW, TWO-SCREW, AND THREE-SCREW ROTARY-MOTION PD PUMP

This PTOA Segment #214 features the Screw Rotary-Motion PD Pump, a pump that is encroaching on what has traditionally been the territory of Centrifugal Pumps and Reciprocating-Action PD Pumps!

In the lower right quadrant of the nearby "Classification of Pumps Chart … with Examples'" the four types of Rotary-Motion PD Pumps are designated.

Screw Rotary-Motion PD Pumps appear as the third of the four classifications of Rotary-Motion PD Pumps shown.

Screw Rotary-Motion PD Pumps have been around so long that a design by Archimedes was used to vertically move water.  The Archimedes vertical water pump design is still used to "lift" wastewater and storm water and for land drainage.

The popularity of the Screw Rotary-Motion PD Pump is increasing because of technological advances which have made it possible for Screw Rotary-Motion PD Pumps to transport a large Capacity/Flowrate of extremely viscous, heavy crude oils produced in places like Canada and Mexico.

These unconventional crude sources require dilution with steam or other means just to get the oil to flow from the reservoir feeder lines to the main pipeline. The resulting multiphase fluid composed of heavy crude oil and crude oil/water emulsions can successfully be moved by the Screw Rotary Motion PD Pump in large capacities. A Centrifugal Pump could not move the viscous fluid and would also vapor lock.

Three-Screw Rotary-Motion PD Pumps are the pumps that are used to boost the PV Pressure of multiphase crude oil mixtures that are gathered in lines and transported to the main crude oil pipeline.

Two-Screw Rotary-Motion PD Pumps do the same duty for the main crude oil pipeline; they add the PV Pressure to the fluid, boosting the flow of viscous crude through the main oil pipeline from Point A to Point B.

Guess what?

Screw Rotary-Motion PD Pumps are also used in the hydraulic systems of submarines! Their low pulsation translates into a low-noise pump.

Low pulsation also means no Pulsation Dampeners (PTOA Segment #204) or pipeline support systems are needed. This hardware would be required for a Reciprocating-Action PD Pump. Reciprocating-Action PD Pumps were featured in PTOA Segments #209 and #210.

 

 

To recap, Screw Rotary-Motion PD Pumps are used to move a large Capacity/Flowrate  of fluids with:

• A wide range of viscosities.
• Fluids containing vapors.

 

Additionally,  Screw Rotary-Motion PD Pumps have:

• Low pulsation.
• Low noise generation.

 

 

THE FORM AND FUNCTION OF SCREW ROTARY-MOTION PD PUMPS

Screw Rotary-Motion PD Pumps have a long shape because the Power Rotor and Idler Rotor(s) are snuggly enclosed in a specially machined Housing/Casing.

The hardware of the Screw Rotary-Motion Pump shares some similarities with the Gear Rotary-Motion Pump (PTOA Segment #212):

• Suction (Inlet). There might be more than one Inlet or otherwise the flow that enters the Screw Rotary Motion PD Pump might split and flow to opposite ends after entering the pump.
• (Machined) Casing/Housing
• Power Rotor… this Rotor is driven by a motor or other prime mover and extends through the Casing.
• A helical screw-shaped Impeller is attached to the Power Rotor.
• Idler Rotor(s) with attached helical screw-shaped Impeller(s). The Impellers on the Idler Rotor(s) mesh with the Impeller on the Power Rotor. Idler Rotors do not extend through the Casing.
• Timing Gears.
• Discharge (Outlet) which will often be situated at a 90° angle from the Inlet(s).

 

As the Power Rotor on the Single-Screw, Two-Screw, or Three-Screw Rotary-Motion PD Pump rotates, the pumped fluid is drawn into the pump at the Inlet.

As the single Impeller or intermeshed Impellers continue to turn, each rotation moves the trapped fluid further axially down the length of the Rotor. Finally the fluid is displaced from the pump at the Discharge Outlet.

Flow through a Screw Rotary-Motion PD Pump is always in the direction of the Power Rotor.

Since the fluid does not rotate … but rather moves more or less in a straight line … the pumped fluid does not pulsate as it would in a Reciprocating-Action PD Pump.

Single-Screw, Two-Screw, and Three-Screw Rotary-Motion PD Pumps move fluid axially.

That statement means the fluid moves linearly in the direction of the Power Rotor.

Even when the helical-shaped Impellers and their respective Rotors are operating at high rotational speeds, the fluid that is being pumped will not pulsate and quiet, continuous pump operation results.

 

THE THREE TYPES OF SCREW ROTARY-MOTION PD PUMPS

 Single-Screw Rotary-Motion PD Pumps

The Single-Screw Rotary-Motion PD Pump has a long, single helically-spiraled Rotor/Impeller that rotates eccentrically (meaning not in the center).  The Archimedes Screw Pump is an example of a Single-Screw Rotary Motion PD Pump.

Enclosed Single-Screw Rotary-Motion PD Pumps have a tight tolerance between the helical Screw and the Housing/Casing which minimizes leakage, even at very high speeds.

The Single-Screw Rotary-Motion PD Pump can operate at very high rotating speeds while moving liquids with low lubricating value. The low-lubricity liquid is simultaneously used to lubricate the pump internals. The crucial need for lubrication in Rotating Equipment was featured in PTOA's Tribology Focus Study, PTOA Segments #177 through #180.

 

Two-Screw  and Three-Screw Rotary-Motion PD Pumps

The Two-Screw Rotary-Motion PD Pump has two parallel Rotors that are intermeshed like the gears of a Rotary-Motion Gear Pump.

The Power Rotor is externally driven by a motor or other Prime Mover.

Timing Gears synchronize the rotation of the Idler Rotor.

The Power Rotor extends out of the Housing/Casing; however, the Idler Rotor does not. The meshed Impellers on the Power Rotor and Idler Rotor rotate while snuggly fitting into a  Casing/Housing.

Close scrutiny of the nearby graphic reveals that both the Power Rotor and Idler Rotor are made of half "left-handed helices" and the other half "right-handed helices."  This pump design balances the pump and greatly reduces Axial Thrust. Axial Thrust was featured in PTOA Segment # 182.

In a balanced pump, the flow that enters the Inlet is divided and sent to the far ends of the Impeller; the common Discharge Outlet is in the middle of the pump.

The center Rotor of a Three-Screw Rotary-Motion PD Pump is the Power Rotor. One Idler Rotor/impeller is above the Power Rotor/Impeller. A second Idler Rotor/Impeller is below the Power Rotor/Impeller.

The helices of the three Impellers are intermeshed and the Housing/Casing that surrounds them is machined to a tight tolerance.

Two-Screw and Three-Screw Rotary-Motion PD Pumps are widely used for low pulsation transportation of oils or liquids with naturally good lubricating characteristics at medium to large Capacity/Flowrate.

Two-Screw and Three-Screw Rotary-Motion PD Pumps can be used to generate medium to high PV Pressures, however in high PV Pressure service leakage will occur axially.

Bearings are required to reduce the wear created by the Impellers contacting the Housing/Casing. Sleeve bearings or bushings will be lubricated by the fluid that is being pumped. The form and function of Bearings in Rotating Equipment was covered in PTOA Segment # 182.

 

WHAT REMAINS TO BE EXPLORED ON THE "CLASSIFICATION OF PUMPS CHART …  WITH EXAMPLES"?

Brilliant PTOA Readers and Students … meaning those who are reading the PTOA Segments in the intended, sequential order … are very familiar with the nearby chart which illustrates the "Classification of Pumps … With Examples."

This chart shows that all the pumps in the world fall into one of two categories, Dynamic Pumps and Positive Displacement Pumps as was highlighted in PTOA Segment #204.

The Dynamic Pump Family Tree members are illustrated in the middle of the chart.

The Dynamic Pump Family has two main branches, Centrifugal Dynamic Pumps and Special Dynamic Pumps.

 

The Centrifugal Dynamic Pump sub group members include the:

• (Radial) Centrifugal Pumps which were featured in PTOA Segment #162 through PTOA Segment #176 and PTOA Segment #206.
• Mixed Flow Centrifugal Dynamic Pumps and Axial Flow Centrifugal Dynamic Pumps which were featured in PTOA Segment #206.

 

The Special Dynamic Pump sub group members include the:

• Ejectors and Eductors which were featured in PTOA Segment #207.
• Gas Lift Pumps and Electromagnetic Pumps which were featured in PTOA Segment #208.

 

 

 

The Positive Displacement (PD) Pump Family Tree members are illustrated on the right hand side of the chart.

The PD Pump Family has two main branches:

Reciprocating-Action PD Pumps are located in the upper right part of the chart.

Rotary-Motion PD Pumps are located in the bottom right quadrant of the chart.

The Reciprocating-Action PD Pump sub group members include Piston, Plunger and Diaphragm Reciprocating-Action PD Pumps (which were featured in PTOA Segments #209 and #210).

The form and function of common Rotary-Motion PD Pumps and their role generating hydraulic power and force were featured in PTOA Segment #211.  The Rotary-Motion PD Pump sub group members include:

 

• Gear Rotary-Motion PD Pumps which were featured in PTOA Segment #212.
• Vane Rotary-Motion PD Pumps which were featured in PTOA Segment #213.
• Screw Rotary-Motion PD Pumps which are featured in this PTOA Segment #214.

 

OMG, FRED! That means the only pump that has yet to be featured is the Piston Rotary-Motion PD Pump!

 

 

 

VERY BRIEF ATTENTION GIVEN TO PISTON ROTARY-MOTION PD PUMPS

Like many other sub group members in the Rotary-Motion PD Pump Family, Piston Rotary-Motion PD Pumps are used as hydraulic pumps. The use of Rotary-Motion PD Pumps to generate hydraulic power and force was featured in PTOA Segment #211.

Piston Rotary-Motion PD Pumps are too complicated to cover in the PTOA because they combine reciprocating hardware  … for example, the plungers and pistons/cylinders which are typically associated with a Reciprocating-Action PD Pumps … as well as some type of rotating surface or member to actuate the displacing hardware.

Examples of the Piston Rotary-Motion PD Pump include the Radial Plunger (shown in the nearby gif), the  Axial Piston, and the Swash Plate Rotary-Motion PD Pumps.

Besides their use delivering hydraulic force and power, these complicated machines can be adapted for variable Capacity/Flowrate. Otherwise stated, there is no need for a Variable Speed Driver to be able to change the Capacity/Flowrate of these pumps.

In the Radial Plunger Rotary-Motion PD Pump shown in the nearby gif, the Capacity/Flowrate can be changed by varying the eccentricity between the plunger-carrying body and the ring that drives the plungers.

Hey!

Big Internet High Five to all PTOA Readers and Students because they just completed learning about the family members in the Pump Family Tree. There is just one more thing to cover about pumps before moving on to Compressors:

When is a pump not a pump?

 

TAKE HOME MESSAGES: The popularity of the Screw Rotary-Motion PD Pump is increasing because technological advances have made it possible for this type of pump to transport the extremely viscous, multiphase crudes being produced in Canada and Mexico. Centrifugal Pumps could not handle the wide viscosity range of these non-conventional crudes nor their vapor content.

The pumped fluid is sucked into the Suction Inlet as the Power Rotor on the Single-Screw, Two-Screw, or Three-Screw Rotary-Motion PD Pump rotates, As the single Impeller or intermeshed Impellers continue to turn, each rotation moves the trapped fluid further axially down the length of the Rotor. Finally the fluid is displaced from the pump at the Discharge Outlet.

Screw Rotary-Motion PD Pumps are low-noise, low-pulsation pumps. They are used in the hydraulic oil systems of submarines. Reciprocating-Action PD Pumps would not be used in submarines because they are noisy, require Pulsation Dampeners as well as stabilizing piping.

Screw Rotary-Motion PD Pumps have structural similarities to Gear Rotary-Motion Pumps.  Like Gear Rotary-Motion Pumps, Screw Rotary-Motion Pumps have Rotors with Impellers that tightly intermesh. The Rotors/Impellers fit snuggly within the internal surface of the Housing/Casing.

Like Gear Rotary-Motion PD Pumps, the Two-Screw and Three-Screw Rotary-Motion PD Pumps have a Power Rotor and Idler Rotor. The Three-Screw Rotary-Motion PD Pump has one Idler Rotor above the Power Rotor and one below it. Timing Gears are used to synchronize the rotation of the Rotors.

A balanced Screw Rotary-Motion PD Pump has half of its Impellers with left-handed helices and the other half with right-handed helices. These blades on these Impellers mesh with the mating Impeller blades on nearby Rotors.

Screw Rotary-Motion PD Pumps move fluid axially in the direction of the Power Rotor.

A brief introduction to Piston Rotary-Motion PD Pumps was featured in this PTOA Segment.

PTOA Readers and Students completed learning about the family members in the Pump Family Tree and their relationship to each other.

 

©2020 PTOA Segment 0214

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

You need to login or register to bookmark/favorite this content.