("My Type," by Saint Motel, 2013)

 

QUICKIE REVIEW  … WHY IS A PUMP NEEDED IN THE FIRST PLACE?

By now every PTOA Reader and Student can reply in unison when asked what the purpose of a pump is:

"To add the PV Pressure to a flowing liquid!"

 

And just why would it be necessary to add the PV Pressure to a flowing liquid?

"Because flowing liquids need to be moved from Point A to Point B … and sometimes liquids need to be circulated around the same loop or path, like cooling water in a cooling water loop and lubrication oil supplying lubricant to hardware."

One last review question in the form of a question:

Why does the PV Pressure in a flowing liquid diminish after the liquid is discharged from the Pump?

PTOA Segment #165 explained how the Friction generated between a flowing liquid and the interior pipe wall will diminish the ΔP needed for a liquid to flow until … Poof! … the ΔP is gone and the fluid will not flow!

Great! Time to move on!

This PTOA Segment #204 explains the fundamental differences between Dynamic Pumps and Positive Displacement Pumps and the process service that would favor selection of one type of pump over another.

 

THE TWO MAIN TYPES OF PUMPS

The novice Process Tech Operator in a large industrial processing plant will observe pumps that can be distinguished by their horizontal or vertical positioning, their axial or horizontal bolted casing, whether or not they are single stage or multi stage, or "single acting" or "double acting," etc.

Although there may seem to be many types of pumps, all of them fall into just two classifications:

• Dynamic Pumps and
• Positive Displacement Pumps

 

 

DYNAMIC PUMPS

There are Three Dynamic Pump Subsets:

• Centrifugal Pumps, the most popular industrial pump which were thoroughly featured in PTOA Segments #163 through PTOA Segment #176.
• Axial Pumps (a specific type of Propeller Type Pump that DOES NOT rely on Centrifugal Action to build up the PV Pressure).
• Specialty Dynamic Pumps are so "special" that they will be featured in an upcoming PTOA Segment.

 

Centrifugal Pumps are the most popular of industrial pumps because they are a cheaper initial investment  and have a wide range of applications.

Centrifugal Pumps require little maintenance and when they do they are typically easy to repair.

 

 

When to Select a Dynamic Pump

Dynamic Pumps are the go-to pump whenever these two criteria exist:

• A large volume of clean, low viscosity liquid must be moved ... but
• A high Discharge Pressure is not required.

 

Alternatively stated:

Dynamic Pumps have a large Pump Capacity but achieve modest to mid range Discharge Pressures.

Oops! There's always a caveat:

As the nearby graphic illustrates, when two Centrifugal Pumps are piped "in series" where the Discharge Pressure from Pump 1 becomes the Suction Pressure for Pump #2, then a significantly greater Overall Total Dynamic Head (labelled Head ΔP in the below graphic) can be achieved. Add more Centrifugal Pumps in series and even greater overall TDH will be realized!

 

Centrifugal Pump and Axial Pump Service

Centrifugal Pumps would be a good selection for emergency firewater, pumping water out of the ground, and just reliably transferring and circulating low viscosity, clean liquids.

Brilliant PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already learned in PTOA Segment #163 that a bank of Centrifugal Pumps circulate large amounts of cooling water throughout a processing plant.

NOTE:  In circulating loops, the Discharge PV Pressure will be the highest PV Pressure in the Loop. So nobody should be surprised that the Suction PV Pressure will be the lowest PV Pressure in the Loop.

 

Axial Pumps  are in-line flow pumps with the specific duty of moving a lot of liquid fast. They are easy to identify because they become part of a piping system that has an "elbow" or 90° change in flow direction.  This specific architecture makes it possible to connect the Axial Pump to an Motor driver.

 

The Operating Principle of Dynamic Pumps

Like most text book definitions, the text book definition of a Dynamic Pump is not very enlightening: "a pump that converts the energy of a liquid in motion into the PV Pressure."

To be more descriptive:

• 

  Axial Pump on the left, Centrifugal Pump on the right. Both convert liquid velocity to the PV Pressure.

  Centrifugal Flow Action (explained in PTOA Segment  #173) sucks the flowing liquid into the eye of an Impeller and then flings the rotated liquid outward from the Impeller, increasing the liquid's Velocity. Then the intricately manufactured Volute and Diffusers convert the Velocity into the PV Pressure (explained in PTOA Segment #175).

• Axial Flow Action also converts the liquid Velocity into the PV Pressure. However the liquid is not flung outward from the eye of the Impeller but rather appears to flow "parallel to the axis of the Shaft." Axial Flow Action will be better explained in the upcoming PTOA Segments featuring Axial Flow Compressors so do not stress about understanding the theory now.

 

Both Centrifugal Pumps and Axial Flow Pumps build up the PV Pressure in a flowing liquid via the PV Pressure ↔ Fluid Velocity Swap which was described in detail in PTOA Segment #159.

Both Centrifugal Pumps and Axial Flow Pumps have continuous liquid flow moving from the Pump Suction to the Pump Discharge, whilst the PV Pressure increases in the liquid. Albeit the Discharge Pressure from a Dynamic Pump can be impacted by system variations, the Discharge Pressure into the Discharge Header is still somewhat continuous. The same observation cannot be stated for Positive Displacement Pumps described below.

 

POSTIVE DISPLACEMENT PUMPS

The Operating Principle of Positive Displacement Pumps aka "PD Pumps"

What is meant by "Displacement?"

The dude in the nearby gif filled his bathtub with candy. Then he jumped in.

The amount of candy that spills out onto the floor has been displaced by the dude's body.

That's because of this fact: No two things can occupy the same space at the same time! Got it? That's the operating principle of all PD Pumps!

But what does "POSITIVE" Displacement mean? Don't worry about it! Just a distinction that is important to anal retentive engineers.

 

There are Two Positive Displacement Pump Subsets:

• Reciprocating-Action (PD) Pumps use a Piston or a Plunger or a Diaphragm to displace a liquid with back-and-forth or up-and-down repeated motion from Pump Suction to Pump Discharge.
• Rotary-Motion (PD) Pumps displace a fluid with rotating, spinning hardware which moves the fluid from Pump Suction to Pump Discharge. Look out! Because of their spinning action, graphics of Rotary (PD) Pumps are easy to confuse with with Centrifugal Pumps.

 

Reciprocating-Action and Rotating-Motion Displacers

No matter what the design, the following happens in every Positive Displacement pump:

• Liquid flows through a Suction Valve into a rigid chamber that has a specific volume.
• The Suction Valve closes and the liquid cannot escape from the rigid chamber.
• Then some kind of displacement hardware presses the liquid into a much smaller volume and/or moves the liquid along to the Discharge Valve.
• Eventually the now-higher-PV Pressure liquid triggers a PV Pressure sensor to open the Discharge Valve, hence the high PV Pressure liquid exits the PD Pump.

 

Guess what? Brilliant PTOA Readers and Students … those who are reading the PTOA Segments in the intended, sequential order ...  have already learned the principles of hydraulics which explain how and why the PV Pressure increases in the PD Pump. The subject will be reviewed in an upcoming PTOA Segment so do not stress about that now.

Just be aware that the Positive Displacement Pump DOES NOT rely on the PV Pressure ↔ Liquid Velocity Swap to build up the PV Pressure in the pumped-up liquid!

 

When to Select a Positive Displacement Pump:

• When a high PV Discharge Pressure is needed to move a liquid into a pipe or area that is already at a high PV Pressure. All PTOA Readers and Students who have read PTOA Segment #158 know a liquid can only flow from an area of higher PV Pressure to an area of lower PV Pressure.
• When a  specific amount of chemical needs to be injected into a process line. PD Pumps are particularly well suited to metering and measuring liquid injections because the chamber from which the pumped up liquid is displaced has a set, unchangeable volume. PTOA Readers and Students learned about chemical injections way back in PTOA Segment #75.
• When the liquid that is being pumped up has a high viscosity, or has particulate in it. Some PD Pumps can even handle fluids … liquids mixed with vapors and steam. Centrifugal pumps are for lower viscosity and relatively clean liquids.

 

Reciprocating-Action (PD) Pumps

In a Reciprocating-Action Pump, the displacer hardware is a Piston or a Plunger being moved in back and forth motion via a Connecting Rod and Crankshaft which convert the rotary motion of the Driver into back and forth motion. Diaphragms are used as the displacer hardware when the process liquid being pumped up is particularly corrosive or abrasive.

The constantly intermittent release of high PV Pressure liquid from the Reciprocating-Action (PD) Pump Discharge Valve is hard on the Discharge Header piping and also not desired by the downstream users of the pumped up liquid.

For these reasons Pulsation Dampener Hardware will be installed on the Suction Side and Discharge Side of the Reciprocating-Action PD Pump. Pulsation Dampener Hardware provides a volume of liquid from which more constant flow of liquid can be drawn into and discharged from the pump.

The PTOA DOES NOT advocate the use of any brand of a Reciprocating-Action PD Pump over another.

That being understood and stated, PTOA Readers and Students could "Google" the phrase "TRIPLEX PUMP" to attain a better understanding of this type of Reciprocating-Action Positive Displacement Pump which is capable of moving a wide range of corrosive, viscous, and particulate-laden 'heavy' liquids.

Large triplex pumps are used for drilling, hydraulic fracturing, cementing, and descaling operations.Triplex pumps can also be used for water jetting applications like surface preparation, hydro demolition or hydrostatic testing. Industrial uses of triplex pumps include  oil line pumping, mine dewatering, chemical and petroleum products transfer, and well servicing.

The process of removing water from recovered natural gas (aka, "dehydration) will be featured in a future PTOA Separation Processes Focus Study Area. Your Mentor is familiar with triplex pumps in dehydration service which draw little Suction PV Pressure from a partially-filled glycol tank and then add sufficient Discharge PV Pressure such that the discharged glycol easily flows to an elevated entry portal on a dehydration Contactor tower which operates at 450 psi.

Your Mentor is also familiar with triplex pumps used to inject methanol into oil production lines that range from 400 psi to 2000 psi.

 

Rotary-Motion (PD) Pumps

In Rotary-Motion (PD) Pumps, the displacement hardware spins because it is attached to a Shaft that is supplied rotational mechanical power from a Driver.

Rotary-Motion (PD) Pumps cannot achieve the high PV Pressures that Reciprocating-Action (PD) Pumps can achieve. Nor can they match the Capacity of Centrifugal Pumps.

The Rotary-Motion (PD) Pump application niche is where there is a need for low to moderate Discharge PV Pressures delivered at a constant rate without the intermittent ramming that occurs with Reciprocating-Action (PD) Pumps. Otherwise stated, Rotary-Motion PD Pumps do not require Pulsation Dampening Hardware.

Furthermore Rotary-Motion(PD) Pumps can handle liquids that would gum up a Centrifugal Pump.

And PTOA Readers and Students will learn that certain types of Rotary (PD) Pumps are Vacuum Pumps! The important use of Vacuum Pressures in process industries was featured in PTOA Segment #151.

The displacement hardware gives the Rotary (PD) Pump its name.

 

Lobe (Rotary-Motion PD) Pumps

A Lobe Pump uses one or more Lobes as the displacement hardware.

Lobe Pumps are particularly useful moving fluids in environments that must be sanitary, like in food and beverage processing plants as well as pharmaceutical and biotechnology processing. So who is surprised to learn that Lobe Pumps are easy to clean and sterilize in place?

Lobe Pumps are also found in chemical and pulp and paper industries.

 

 

Sliding Vane and Flexible Vane (Rotary-Motion PD) Pumps

Vane Pumps excel at moving low viscosity liquids like  ammonia, alcohol, gasoline, refrigerants, and Liquid Petroleum Gas (LPG … propane put under pressure until it is forced into the liquid state).

The automotive industry is especially fond of Vane Pumps which can be part of the braking, power steering, automatic transmission and supercharging systems. Quite versatile, Vane Pumps are also found in espresso and soft drink dispensing machines and some air conditioning units.

Because Vane Pumps are a type of vacuum pump, these pumps are also found in industrial and laboratory settings where oil and water … even small amounts of gas/vapor … need to be moved.

 

 

Internal Gear and External Gear (Rotary-Motion PD) Pumps

Internal Gear and External Gear Pumps use Gears as the displacement hardware.

Internal Gear Pumps are particularly well suited to metering blending liquids because the output from the pump is directly proportional to how fast the gears are rotating and the rate of rotation is easy to adjust. Depending upon the materials of construction, Internal Gear Pumps can be designed to handle a wide range of viscosities and even move corrosive liquids... like sulphuric acid! Internal Gear Pumps have superior suction capabilities, which is desirable when handling corrosive liquids.

External Gear Rotary Pumps can achieve higher Discharge PV Pressures than Vane Pumps or Lobe Pumps can. So much so that External Gear Pumps are used in automotive hydraulic systems and lifting machinery. External Gear Pumps are used to pump oil and high viscosity liquids.

 

The (Rotary-Motion PD) Two Screw and Three Screw  Pump

The (Rotary-Motion PD) Screw Pump uses two or three Screws as displacement hardware (some models have up to 5 Screws). The current literature on Screw Pumps advocates for expanding their use to replace Centrifugal Pumps and Reciprocating Pumps on a wider scale.

Unlike Reciprocating-Action PD Pumps,(Rotary-Motion PD) Screw Pumps have an even Discharge Pressure delivery, eliminating the need for Pulsation Dampeners or other complicated pipeline support systems. (Rotary-Motion PD) Screw Pumps can also match the pumping Capacity of Reciprocating-Action PD Pumps.

And unlike the Centrifugal Pump, the (Rotary-Motion PD) Screw Pump can handle a large Capacity of liquids with a wide range of viscosities ... even multi-phase fluid mixtures that would cause a Centrifugal Pump to vapor lock (refer to PTOA Segment #177).

For example, the multi-Screw Pump excels in pumping high viscosity fluids like asphalt and multi phase emulsions that result when heavy crude shale is mixed with steam or additives so that the mixture can flow from reservoir to pipeline.

The popularity of the Two Screw Pump  and Three Screw Pumps may well increase as the Oil and Gas Exploration and Production Industry (E&P Industry) gains confidence in the technology

Screw Pumps are currently used in a variety of manufacturing, mining, and hydraulic systems.

 

Reciprocating-Action Pumps use Pistons, Plungers, or Diaphragms to displace liquid. Reciprocating-Action PD Pumps require Pulsation Dampening Hardware.

Like Reciprocating-Action PD Pumps, Rotary-Motion PD Pumps are identified by their Displacement Hardware. This PTOA Segment introduced the following Rotary-Action Pumps and their service application:

Lobe Pumps
Vane Pumps (Sliding Vane and Flexible Vane)
Internal and External Gear Pump
Screw Pumps

©2020 PTOA Segment 0204
PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT PUMPS

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