Progressing Cavity Pump Guide and Design

27 January 2020

What are progressive cavity pumps?

Progressive cavity pumps, also known as PC pumps, progressing cavity pumps, eccentric screw
pump and mono pumps are a type of rotary positive displacement pump designed for the conveying
of liquids and sludges from 1cst to 1Million. They handle not only viscous fluids and solids but also
gassing or multiphase liquids containing gas slugs typical during crude oil extraction.

The volume of liquid pumped is proportional to speed providing a linear predictable pumping rate
across a range of pressures. This technology delivers one of the highest flow and pressures available
from a positive displacement pump being up to 600M³H and 48bar, with efficiency ranging from
55% to 75%. This technology is most suited for fluids more viscous than 5cst.

What is the design of a Progressing cavity pump and Working Principle?

The design consists of a motor at the drive end which is connected to a gearbox as pc pumps operate
at low rpm compared to centrifugal pumps. The output shaft from the gearbox connects to a rotor via
a universal pin joint which rotates a metallic rotor within a rubber stator. Stators contain cavities, and
the rotor pushes fluids through the cavities in a slow rotating fashion.

View an animation here 

A pumps pressure generating ability will depend on the number of cavities within the pump, with
high pressure designs often consisting of more than one stator and rotor. Each rotor will typically
produce 6 bar enabling pressures up to 48 bar to be achieved through its modular design.
The clearances between the rotor and stator are very small thus meaning any fluids which are
transferred must be lubricating, within a consistent temperature band as the stator can expand at
certain temperature bands leading to stator scoring or damage if the two contact.

This design of pump is better suited for viscous lubricating fluids, which can contain solids. Short
stator life can be experienced with abrasive slurries at which point a peristaltic pump can be a
preferred option.  Eccentric Screw Pumps viscosity handing is unrivalled, and they are usually
specified when there are no other suitable options.

Rotors are usually a range of metals from cast iron, and tool steel, to stainless steels and duplex’s
which at times are hardened to cater for aggressive and/or high-pressure applications. Hollow rotors
allow for higher operating speeds, long life spans, produce low vibration and near silent running.

Stator designs consist of two types - equal and non-equal walled. Equal walled stators ensure a lower
starting and running torque, lower pulsations and reduced power consumption, high volumetric
pumping efficiency, and lower replacement costs. Materials are usually types of rubber being NBR,
FKM but not PTFE meaning solvents cannot be handled.

Applications
 Applications for units can be found across a range of industries and include:

·         Wastewater - Thickened & Treated Sludge Transfer & Polymer dosing.

·         Oil & Gas – Cutting Transfer, Drilling Mud transfer and recovery, Separator Feed, Crude Oil
Transfer, MOL (Main Oil line Pump), Multiphase transfer and injection in remote areas.

·         Biogas – Digestate feed & Biomass transfer.

·         Paper and Pulp – Pulp Transfer, recycled paper transfer, Filler transfer, fixative & dye metering.
·         Edible Oil – Crusher feed, Oil Malaxing (mixing), oil extraction, oil separator feed, filter press

feed.

·         Wine  - Wine filtering, grape pressing, must & pomace transfer.

They can often be an ideal replacement for conveyors.

Advantages
There are many advantages to such pumps being:

Low pulsating flow - Due to its rotary motion at low rpm, flow pulsation is limited with low
amounts of acceleration head produced. Coupled with even wall design of stators and long pitch rotor
design reduce pulsations further.
 
Low shear - Ensures gentle handling of the most difficult to pump fluids such as resins, viscous
foods, oil and water emulsions without change in consistency to the liquid. They are often use in oily
water separators as the design ensures oil droplets remain intact and was rated by SPE (Society of
Petroleum Engineers) in Paper SPE18204 as the preferred pump to use for oil droplets which were
disturbed the least during handling and a comparison of lobe, vane and screw technology.

Reversible – Units are reversible with reduced output pressure as standard meaning hoses can be
emptied, or if blockages are encountered pump can be reversed to assist with clearing. It also enable
the pump to be versatile for situations such as tanker loading and offloading.

Wide fluid handling capabilities – Designs can handle viscous liquids, large solids, abrasive
materials, fibrous solids and gas slugs without issue making it one of the most versatile pumps
available. This design has Unparalleled Viscosity handling viscosities from 1cst to 1Million means
there are no comparable pumping technologies.

Self Priming – Due to the tight tolerances in its construction, it has high suction capabilities priming
up to 8M, with a corresponding low NPSH. Designs are available which can withstand gas slugs for
up to 30 minutes.
 
High Accuracy – Due to flow being directly proportional to pump speed, and due to its cavity
design, it enables flows to be very predictable enabling it to be used in metering and dosing
applications

High Flows – Flows of up to 600M³H can be achieved due to rotary positive displacement design
which is second only to large screw pumps

High Pressure - Producing pressures up to 48 bar makes this one of the highest pressure producing
PD Pumps.

Model Variants:

Eccentric screw pumps are available in a multitude of designs dependent on the application. Variants
include:

Hopper Pump – A pump is fitted with a hopper of various designs, designed for viscous liquids,
materials containing high amounts of dry matter, large solids requiring breaking up and materials
which plasticise

Food – Models of hygienic construction, with materials specified in accordance with 3A, or EHEDG
for ease of cleaning. Units can be trolley mounted for relocation between processes.

Macerator – Inline grinder to macerate solids before entering the pump.

Vertical Immersion – Designed to be immersed in the fluid such as in tanks eliminating NPSH
issues, as units can be accepting of an NPSH as little as 0.5M (canned design) making them ideal for
open or closed drain applications. 

Multiphase Design - Baseplate mounted unit for multiphase boosting, with accessories allowing
pump to handle viscous oil, gas slugs, sand and water, with automatic remote operation. 

Wobble – Wobble pumps are like a small pc pump but are of single joint design meaning the internal
rotor does not rotate in 360° motion but wobbles.
Units can be further configured through the use of accessories, and in construction by being close
coupled in design, having a separate bearing housing for demanding applications, and modular

bearing design for the longest bearing design life.

Types of Hopper Design:

The inlet of such pumps can be fitted with a hopper for the conveying of viscous fluids. Options
include:

Bridge Breaker – For the breaking up of large solids within dehydrated sludge. Motorised paddles
rotate within the hopper ensuring particles are broken into sizes which can be accommodated by the
pump preventing blockages

Auger Feed – For high dry solids content / dry sludge.  A rotating screw which can run dry rotates
within the hopper, feeding the pump ensuring difficult to pump and non-flowing liquids do not clog.

Combined Bridge breaker and auger feed – As above combined

into one design for the most viscous and solid laden fluids

Motorised wheel – Feeding of liquids with high dry solid content and materials which plasticize into
the main pump. When materials such as liquid mortar, resins, mud, blocks of fat, or butter are
pumped they can plasticise meaning they change shape rather than break up. To ensure they are fed
into the rotor and stator, a motorised wheel ensures materials are broken up when other technologies
may mean materials clog.

Liquid injection port – Typically used for the biogas sector, this unit has a separate injection
port for accepting liquid manure which is combined with materials in the inlet containing high dry
solids content (such as digestate, straw, corn, grass, rye, vegetable and food waste ) ensuring
pumpability.

Customisable Hopper – Shape is designed to match pipework being either circular or rectangular.

Pump curve
PC Pump curves are different to a centrifugal curve as it is linear demonstrating the units ability to
handle liquids of varying viscosities with little impact on pump performance, with the bottom axis
being speed rather than flow as flow is proportional to speed. Unit speed is much lower than
centrifugal, operating from as little as 50rpm

Pc pump vs gear pump vs screw pump

PC Pump Gear Screw
Handle Solids. Non-Clog Not designed for particles. Typically designed for clean
design Accelerated teeth wear with fluids
even small particles
Self-Prime up to 8M Self-Prime up to 8M Self-Prime up to 7.5M
Handle up to 1M Cst Up to 55,000cst Up to 15000cst
Up to 48bar Up to15bar Up to 120bar
Mechanically Sealed or Magnetic coupling or Magnetic coupling or
cartridge seal Mechanically Sealed Mechanically Sealed
Not suitable for solvents All metal parts means All metal parts means
solvents can be transferred, solvents can be pumped.
although some designs may
have bearings within liquids
and should be avoided
 

Accessories
Pumps are available with a variety of accessories to ensure suitability for your process:

Dry Run Sensor - Prevents the dry running of stators through the use of a PT100 temperature probe
sensing when temperature of stator is over a set temperature and stopping the motor.

Product Sensor - Automates the pump to switch on when product is sensed in the hopper or pump
inlet.

CIP Connection – For ease of cleaning in between batches or changes in fluids.

Heating Jackets - Both hopper and stator can be fitted with heating jackets ensuring solidification of
material does not take place.

Pressure switch – to control pump based on inlet pressure

Pressure relief valve – May be fitted on pump or externally on pipework to prevent the build up of
excess pressure which can damage pump or system.

There are a range of symbols that can be used to symbolise Progressing Cavity Pumps in Process and
Instrumentation Drawings -