I know you know all the pros and cons
They help you get to everything you want

("Crime Pays", by Hall and Oates, 1982)

 

THE PROS AND CONS OF DYNAMIC AND POSITIVE DISLACEMENT PUMPS

Pump Capacity

The nearby graphic of a pared down Centrifugal Pump Characteristic Curve (first explored in PTOA Segment #166 through PTOA Segment #168) reminds PTOA Readers and Students that the flowrate through a Centrifugal Pump (aka Capacity) varies with the pump's Discharge PV Pressure. The Discharge PV Pressure is shown as Total Dynamic Head or TDH, labelled as "Head in Feet" on the Y axis.

The range of the Centrifugal Pump's Capacity in gallons per minute is shown as a blotchy black curve.  Each Centrifugal Pump has an optimally efficient Capacity, yet can be operated above or below that Capacity with an understood sacrifice in operational efficiency.

In the same nearby graphic the sole PD Pump Capacity is shown as a vertical red line that cannot be turned up or turned down. However, the Discharge PV Pressure (represented as TDH and shown as "Head in Feet" on the Y axis of the diagram) is significantly higher than any TDH the Centrifugal Pump can attain at any Capacity.

The logical conclusions to draw from the graph are:

Compared to a Centrifugal Pump, a Positive Displacement Pump can achieve a much higher Discharge Pressure while pumping a specific flowrate of liquids or fluids.

and:

Centrifugal Pumps cannot achieve the high Discharge Pressures that PD Pumps can, however, they do have a range of liquid flowrate throughput (aka Capacity).

 

Liquid Viscosity

The nearby graphic shows the relationship between the Flow Rate through the Pump (aka Capacity) on the Y axis as the Viscosity of the pumped-up liquid increases (X axis).

The Flow Rate through a Centrifugal Pump (aka Capacity) is shown as a dotted line. The schematic clearly shows that the Flow Rate (aka Capacity) declines rapidly as the Viscosity of the pumped-up liquid increases.

In summary,

A Centrifugal Pump cannot efficiently pump high viscosity liquids.

The Flow Rate through the PD Pump is shown as a solid line. The graph shows that the Flow Rate (aka Capacity) of the PD Pump slightly improves as the liquid Viscosity increases and then evens out at the Flow Rate that the PD Pump is designed to pump.  Thus the Capacity of the PD Pump is not impacted by increasing liquid viscosity.

In summary,

PD Pumps can pump high viscosity liquids/fluids at a single flowrate.

 

Centrifugal Pumps have "Slip" and PD Pumps Do Not

To understand the concept of "Slip" presume Fred the Process Operator started up a Centrifugal Pump but forget to open the Discharge Valve.

The pumped-up liquid is supposed to be able to flow from the pump's Volute through a short length of Discharge Pipe and thence through the open Discharge Valve as shown in the nearby schematic.

When the Discharge Valve is closed, this normal flow to the process that needs the pumped-up liquid is stopped.

Since there is no way out, the flow begins to form an eddy ... circular flow ...  back toward the Volute wherein the flow is greeted with a slightly higher PV Pressure flow and circles back toward the closed Discharge Valve.

This circulating eddy flow continues between the Volute, Discharge Pipe and the closed Discharge Valve.

Meanwhile the liquid within the body of the pump continues to circulate around and around because all Centrifugal Pumps ... even when working as designed with an open Discharge Valve .. have a design feature known as Slip.

Slip is the difference between how much liquid flow a Centrifugal Pump can feasibly transfer out of the pump compared to how much liquid flow the Centrifugal Pump actually does transfer out of the pump.

Otherwise stated:

A percentage of liquid that enters the Eye of the Centrifugal Pump's Impeller will "slip by" the Cutwater/Tongue of a Single Stage Centrifugal Pump (explained in PTOA Segment #175) and/or "slip by" the internal clearances of the pump's casing and diffusers and therefore not be discharged from the pump.

Some Centrifugal Pumps are designed to handle as much as 100% Slip!

Operating a Centrifugal Pump with the Discharge Valve closed is not efficient because the utilities to power the Driver are still being paid for, yet no pumped-up liquid can flow from the pump.

Otherwise stated:

Operating a Centrifugal Pump with a closed Discharge Valve is not at all efficient (like, DUH!), but will not damage the pump.

BIG CAVEAT: The Friction created between the eddying liquid and interior piping surface will cause all liquids to heat up and expand. Therefore, a liquid that is near its boiling point can vaporize, expand, and damage the pump. For this reason some Centrifugal Pumps will be designed with a PV Pressure Relief Valve piped into the Discharge Line.

Now presume Fred the Process Operator starts up the PD Pump and forgets to open the Discharge Valve.

Kapow! Game Over!

Fred has some explaining to do because he apparently did not read this PTOA Segment #205.

PD Pumps have no Slip because their design success is based upon moving an exact amount of liquid (and maybe mixed regime fluid) with every stroke or rotation of the displacer hardware.

Like the Centrifugal Pump, when the Discharge Valve is closed but the PD Pump is operating, the liquid/fluid has no exit.

However, with every stroke or rotation of the displacer hardware, more and more PV Pressure builds up and very quickly and the PV Pressure is radiated outward in all directions

More than likely the increasing PV Pressure will be released at flange connections. However the Discharge Piping, the PD Pump, and even the Driver ... or Process Operator ... could be damaged!

For this reason PV Pressure Relief Valves protect every PD Pump ...  because humans are humans and at somebody somewhere is going to forget to open the Discharge Valve during a PD Pump Startup (but that person WILL NOT BE a PTOA Reader or Student).

Time to You Tube and Chill, again!

The below Viking Pump Internal Relief Valve You Tube illustrates how Viking's Internal Relief Valve protects a Rotary PD Pump.

Or access the Viking You Tube by clicking on this link: Viking Pump Internal Relief Valve .

The Viking Internal Relief Valve You Tube is a  "sanitary design" for a PV Pressure Relief Valve. The design is considered "sanitary" because the dead headed liquid/fluid is returned to the pump's suction and not discharged freely into the surrounding environment.

A "sanitary PV Pressure Relief Valve" design is required for all pumps that pump-up noxious or corrosive fluids. This type of PV Pressure Relief Valve design is just a good idea. Who wants to clean up the mess of liquid/fluid being spewed into the surroundings?

 

 

PD Pumps are Self-Priming, Centrifugal Pumps are Not Self Priming

Centrifugal Action requires that every Centrifugal Pump must be Primed ... aka filled with liquid … prior to starting the pump. "Priming the Pump" is required to insure that the Centrifugal Pump will not Cavitate (explained in PTOA Segment #171 and remedied in PTOA Segment #172).

Conversely ..

All Positive Displacement Pumps are Self-Priming, meaning the air that the pump has inside of it after a maintenance interval does not need to be displaced with liquid/fluid prior to PD Pump Startup.

The PD Pump will operate just fine with the air and liquid/fluid mixture. Eventually the liquid/fluid will displace all the gas that was left in the PD Pump prior to Startup.

 

 

Centrifugal Pumps Have Minimum Flow Lines, PD Pumps Do Not

The most brilliant PTOA Readers and Students have already slogged through understanding Centrifugal Pump Characteristic Curves and Performance Curves (PTOA Segments #166 through PTOA Segment #168). Thus they already know that every Centrifugal Pump has a specific Capacity that is its Best Efficiency operating Point (BEP).

The nearby graphic indicates the optimal Capacity-BEP point on the Characteristic Curve of a Centrifugal Pump

Brilliant PTOA Readers and Students also know that zero Capacity (no flow through the pump) occurs at maximum TDH. Otherwise stated …

Closing the Discharge Valve is characterized as moving leftward on the curve.

Precisely because the engineering and design of all pumping systems works well on paper but never matches the Real World situation, the Discharge Valve of a Centrifugal Pump will be closed off somewhat during normal operation.

As the graphic shows, moving more and more leftward on the curve by closing off the Discharge Valve more and more can cause the formation of eddys mentioned above, reduce the operating life of Impellers, Bearings, and Seals, and even cause the pump to Cavitate.

For this reason a designed "Minimum Flow" is constantly circulated from the Centrifugal Pump Discharge to the Centrifugal Pump's Suction Tank.

The design considerations for Minimum Flow are way beyond the scope of the PTOA to explain nor are they necessary for any Process Operator to understand.

The nearby schematic shows a design schematic for continuous Minimum Flow for a Centrifugal Pump.

A portion of discharged flow from the pump is circulated through a restriction orifice (labeled RO ... do not stress about that now) back to the tank from which this Centrifugal Pump draws suction.

Basically all PTOA Readers and Students just need to be able to recognize the Minimum Flow Line of a Centrifugal Pump and understand its purpose.

 

FUNDAMENTAL SIMILARITIES OF EVERY KIND OF PUMP

The above content reviewed significant differences between Centrifugal Pumps and PD Pumps.

The below content covers fundamental hardware that is needed by ALL PUMPS  … and all Rotating Equipment.

The below content has been thoroughly covered in the PTOA Segments that are cited and will not be rigorously featured  and explained again.

Friction will cause Wear between all moving metal parts thus Lubrication will be needed to keep rotating parts working (Tribology was defined and explored in PTOA Segments #177 through PTOA segment #180).

The Pump will create high PV Pressure as it is supposed to do and thus a means to keep the pumped-up liquid from leaking into the pump surroundings ... for example, a Stuffing Box … must be part of the Pump design. (PTOA Segment #181).

Bearings are needed to control Axial Movement and Radial Movement of all Rotating Equipment (PTOA Segments #182 and PTOA Segment #183).

A method of Sealing contaminants from entering the pumping system must be part of the pumping design (PTOA Segment #184).

The pump must be expertly Aligned and "Coupled" with the Prime Mover/Driver (PTOA Segment #185) because the Prime Mover/Driver supplies the rotational mechanical movement (aka Torque and Horsepower) to the Shaft of the Rotated Equipment (PTOA Segment #186).

The selection of a Driver/Prime Mover is specific to all Rotating Equipment, not just Pumps but Compressors and Electricity Generators, too.

The Driver/Prime Mover might be:

A Motor (PTOA Segments #187 through PTOA Segment #190)

An Engine (PTOA Segment #191 and PTOA Segment #192)

A Steam Turbine (PTOA Segment #193 and PTOA Segment #194)

A Gas Turbine (PTOA Segment #195 through PTOA Segment #202)

 

TAKE HOME MESSAGES: Centrifugal Pumps have a range of Capacity but cannot achieve the Discharge PV Pressures that PD Pumps can achieve.

PD Pumps have only one Capacity but can achieve much higher Discharge Pressures than Centrifugal Pumps can.

PD Pumps can add the PV Pressure to viscous liquids, but Centrifugal Pumps cannot.

Centrifugal Pumps are designed with Slip but PD Pumps are not. Slip is the amount of liquid that the Centrifugal Pump is designed to move from the Eye of the Impeller to the Volute compared to the amount of liquid that the Centrifugal Pump actually does move. Some Centrifugal Pumps are designed with 100% Slip.

All PD Pumps must have a PV Pressure Relief Valve piped in to protect the pump from over pressuring. Centrifugal Pumps that add the PV Pressure to liquids near their boiling point also need PV Pressure Relief.

Sanitary PV Pressure Relief systems circulate the relieved liquid/fluid to the Suction Side of the Pump or maybe even the Suction Tank of the Pump.

Thank you to Viking Pump for allowing use of The Viking Internal Relief Valve You Tube.

Centrifugal Pumps must be "Primed" meaning 'liquid filled" before Startup. PD Pumps are self-priming and do not need to be liquid filled.

Centrifugal Pumps have "Minimum Flow Lines" which return a portion of the discharged liquid to the Suction Side of the Pump or the Suction Tank. PD Pumps do not need "Minimum Flow Lines."

All Rotating Equipment … Pumps and Compressors and Electricity Generators … have a variety of similar hardware dedicated to combatting Friction and Wear, Axial and Radial Movement, Leakage and Sealing, Alignment and Coupling, and decision making regarding the appropriate Driver.

©2020 PTOA Segment 0205
PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT PUMPS

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