The Progressive Cavity Pump and It's Geometry

A progressive cavity pump is a type of positive displacement pump and is also known as a progressing cavity pump, eccentric screw pump or cavity pump. PCP systems use a rotor and stator system that, once turned, causes the cavities to progress upward and move fluid from the intake to the discharge end of the pump.

Rotors are made of Hardened Steel or Stainless Steel and are covered with a Chrome Plating to give resistance to corrosive and abrasive materials. Some liquids affect the Chrome Plating and in those applications a Non-Plated Rotor should be used. The Rotor forms a single helix and rotates eccentrically in the Stator.

Stators are metal tubes with internally molded cavities of Synthetic or Natural Rubber. The Stator has a double helix cavity double the total volume of the rotor.

The Rotor seals tightly against the flexible rubber stator as it rotates, forming tightly sealed cavities which move toward the discharge port, carrying the liquid. Liquid acts as the lubricant between the pumping elements. When combined, as the rotor turns, cavities or pockets nearly half of the total volume are formed in the stator which push the product from the suction toward the discharge end of the pump.

Geometry of Rotor

• Crest to Crest
• Circular cross-section
• Machined in a helical shape similar to a corkscrew

Geometry of Stator

• Oval shaped cavity cross-section.
• Similar geometry as Rotor but uses a double helix.
• Available in several elastomers, metal, or urethane construction materials

Cavities

• As Rotor orbits (turns eccentrically) inside the Stator.
• The motion creates cavities and progresses them from suction to discharge.

Application Variables
Application Influences "The Major Three"

• Viscosity
• Temperature
• Abrasion

Abrasion Effects

• Abrasive fluids = Wear
  *Wear is proportional to speed; minimize speed to minimize wear.
  *De-rate pressure per stage to limit slip amount ... 6 Bar for no abrasion; 1.4 Bar for heavy abrasion.
  *Specify oversize Rotor to increase interference fit = longer life.
  *Use abrasion resistant Stator material or softer durometer elastomers
  *Double chrome rotor for additional Rotor base metal protection.

Abrasive Characteristic	Fluids	Press / Stage
None	Water, Polymer, Oil	6 Bar
Light	Milk or Lime	4.5 Bar
Medium	Sludge, Clay or Gypsum Slurries Chocolate, Drilling Mud	3 Bar
Heavy	Emery Dust, Lapping Compounds, Grout, Sand, Granulated Sugar	1.4 Bar

Temperature Effects

• Stator Elastomers swell from 70 to 130^o (physical Rotor dimensions require adjustment above this 130^o temperature) and Elastomers shrink with Lower temperature (Below 50^o).
• Metal parts such as the rotor and drive train tend to expand and contract at a negligible rate than elastomer counterparts.
• Since Stator is bonded to a metal tube, the rubber can only swell inward on the rotor, or shrink away from the rotor.
• This changes the compressive fit between the rotor and stator. Again, to keep a standard fit, the Rotor requires under sizing above 130^o, and over sizing below 50^o.
• Under extreme heat or cold, elastomer Stators may not be appropriate.
• Metal Rotor and Stator combinations can be used for extreme temperature applications because they swell or shrink at the similar rates.

Viscosity Effects

• The more viscous a fluid, the slower the pump will have to run in order to permit the fluid to flow into the cavity.
• Even at reduced speeds, the pump may not develop 100% volumetric efficiency and this must be accounte
  
  1 CPS = Above 1800 RPM
  100 CPS = 700 RPM
  1000 CPS = 150 RPM
  10,000 CPS = 30 RPM

Material	Viscosity (centipoise)
Water e 70deg F	1-5
Blood or Kerosene	10
Anti-freeze or Ethylene Glycol	15
Motor Oil SAE 10 or Corn oil	50-100
Motor Oil SAE 30 or Maple Syrup	150-200
Motor Oil SAE 40 or Castor Oil	250-500
Motor Oil SAE 60 or Glycerin	1-2 thousand
Karo Corn Syrup or Honey	2-3 thousand
Blackstrap Molasses	5-10 thousand
Hershey Chocolate Syrup	10-25 thousand
Hershey Chocolate Syrup	50-70 thousand
Tomato Paste or Peanut butter	150-250 thousand
Crisco Shortening	1-2 million
Caulking compound	5-10 million
Window Putty (glazing compound)	100 million

How does a SYNO-PCP Pump work?

The standard PC Pump consists of a Rotor (metal) which rotates within an elastomeric Stator

• The Rotor has a circular cross-section and is machined in a single helix like a corkscrew.
  
• The Stator cavity is molded as a double helix with an oval cross-section. The helix geometry is similar to the Rotor to create an interference fit.
  
• As the Rotor turns inside the Stator it orbits on an eccentric (at an offset around the center axis), this motion creates cavities that progresses from suction to discharge; moving product and building pressure
  

Advantages Of Progressive Cavity Pumps

• The SYNO design creates a low sheer, metered and pulse-less flow.
• The PC Pump is able to effectively handle "water-like" to super viscous fluids including levels of air or gases.
• It can gently pump large particulates and handle abrasive solids.
• Provides excellent suction capabilities and does not air lock.
• The SYNO design creates a low sheer, metered and pulse - less flow

The flexible geometry of the SYNO Pump allows:

• Multiple drive end choices (power) and multiple stages (pressure).
• Syno Pump allows ump precise control of the interference fit.

For more details

Download PDF