What is a Pump?

What is a pump? This is a seemingly elementary question. A person might say it’s something that pulls water out of a basement or pushes water through pipes. Or, it's a radiator coolant circulation device that sometimes fails at the most inopportune moment. Or it’s part of a swimming pool filtration system. The following will offer a more complete answer to the question “What is a pump?”

In the context of moving or pressurizing a pure liquid or, for that matter, any mixture or paste, most people would quickly come up with at least one or maybe a few examples of what a pump is.

In the following paragraphs, essential concepts will be presented along with fundamental working principles and functional comparisons, operating regimes, typical applications, and pros and cons for the main liquid pump categories will be presented.

The key to any technology is understanding its terminology. Let us begin with the definition of a pump: a device that causes the movement of a liquid or a gas into or out of something. It is also a device that develops pressure, causes flow, or both.
 
Figure 1. Basic pump system diagram

The majority of pumps fall into two main type classifications: rotodynamic – not to be confused with the term ‘rotordynamic’ – and positive displacement. There are few pump types that fall outside of these main categories and those can be parked under a residual classification called other.

Rotodynamic Pumps

Rotodynamic pumps apply rotational motion and torque to a fluid, transferring mechanical to hydraulic energy. The resultant pump internal fluid velocity is dynamically converted to pressure – hence ‘roto’ + ‘dynamic’.

With rotodynamic pumps one encounters the term head. Rotodynamic pumps work by means of kinetic action – velocity – which converts to pressure in addition to flow. Pressure is density dependent. For a given rotational speed and internal velocities – its kinetic motion – a rotodynamic pump will achieve different design pressures depending upon the density of liquid being pumped. The term head normalizes pressure to energy per unit weight, expressed as a height of liquid. This conveniently reduces the number of catalogue performance curves that manufacturers might otherwise need to make available for various liquids and temperatures. It also simplifies comparisons for those who specify pumps and use their performance curves.

It must be noted that prior to the term 'rotodynamic' entering contemporary literature there was long-established use of the term 'centrifugal pump' which covered the largest share of the classification category, particularly when the context was understood to apply to the entire group with bladed impellers in common: ‘centrifugal pump,’ ‘mixed-flow pump,’ and ‘axial flow pump.'

A rotodynamic pump’s passageways do not “wall off” portions of the pumped product. Pump internals are often curved in three-dimensional shapes leaving no see-through visibility from inlet to the exit in a single stage. However, liquid will readily flow through the pump with minimal resistance or the slightest elevation differential.

Another aspect of rotodynamic pumps is that its pressure versus flow characteristic is intimately tied to the flow resistance of the system it is connected to. The operation of a control valve demonstrates this. With the valve wide open, flow is maximized and backpressure at the valve is minimized. With the valve throttled, the system flow is reduced and backpressure on the upstream side of the valve is higher.

The following are the categories of rotodynamic pumps:
o      Centrifugal
o      Mixed-flow and axial flow
o      Disc (vaneless)
o      Pitot tube
o      Regenerative turbine

A basic diagram of a centrifugal pump is shown in Figure 2. Centrifugal, mixed-flow, and occasionally axial flow pumps may come in multiple stage configurations where the flow travels through the entry impeller, through a diffusing stator and on to subsequent pump stages. This achieves an additive higher total head and the number of stages can be up to a dozen or more depending upon the pump configuration.

Figure 2. Left, diagram of a single-volute centrifugal pump, right, photo of a centrifugal pump

In many applications rotodynamic pumps, especially the centrifugal, mixed-flow, and axial flow types, across a broad range of sizes and specific models, exhibit good-to-excellent efficiency when well matched to their respective systems.

Centrifugal pump efficiency and total head are substantially reduced with high viscosity liquids. There are no rules of thumb when it comes to estimating corrections to a baseline water performance to obtain its viscous performance. The Hydraulic Institute’s standard ANSI/HI 9.6.7 Effects of Liquid Viscosity on Rotodynamic Pump Performance is the best publication available covering this topic.

Liquid characteristics, inlet conditions, discharge pressure, temperature, the size and hydraulic design of the pump, the sealing system, the driver, materials of construction, test requirements, industry and buyer specifications, number of units, manufacturer experience and capabilities, the installation environment, manufacturing and delivery schedules, and possibly additional parameters are considered in the selection process. 

After the performance losses are estimated, and if found to be substantial, a positive displacement pump may be the better option for liquid viscosities greater than that of water.

Rotodynamic pumps are often smaller in size and cost relative to positive displacement pumps. The compact footprint of a vertically suspended pump type can be a compelling advantage.

Rotodynamic pumps tend to fare better and in many cases are the only practical alternative for pumping abrasives and suspended solids.

Figure 3. The pros and cons of rotodynamic pumps

Entrained gases in excess of three to five percent of the total pumped volume can cause a loss of suction prime for the typical centrifugal pump. Special pump inlet and product wetted bearing designs are possible for increasing gas handling capacity. Such conditions necessarily require coordination with the manufacturer.

Where there exists a choice between selecting a positive displacement pump versus a rotodynamic pump, a decision to go with the latter might be based on smoothness of operation. While each type produces pressure pulsations, the rotodynamic types tend to have lower levels and usually do not need pulsation dampeners.

Positive Displacement Pumps

A positive displacement (PD) pump, as its nomenclature suggests, establishes an enclosing boundary around the pumped medium. Through rotational or reciprocating action, a captured liquid volume is mechanically acted upon directly moving the fluid from the pump inlet to the outlet.

The rate of flow is determined by the enclosing displacement volume and is essentially linearly proportional to the pumping speed. The pumping pressure is determined by the external system resistance or its built-in or external pressure relief valve.
Here is a “mostly complete” list of PD pump types:
o    Sliding vane
o    Gear
o    Lobe type rotary
o    Circumferential piston
o    Screw type
o    Progressive cavity 
o    Axial piston
o    Flexible member peristaltic
o    Reciprocating piston
o    Plunger
o    Reciprocating plunger
o    Diaphragm

“Mostly complete” because within each of these listed PD pump types, there can exist several sub-types and hybrids which would extend this list considerably.

Figure 4. Basic diagram of a gear pump

 
Figure 5. Timed gear twin rotor helical screw PD pump

PD pumps generally exhibit good efficiency, especially for low flow, high head and high viscosity liquid applications. Because the rate of thru-flow for a PD pump varies directly with its driven speed or stroke, often the best overall efficiency, especially for a single-pump operation, is accomplished by matching the pump output to the system process demand using a variable speed drive (VSD).

There are many different PD pump designs and a number of these have carved out industry niches. The reciprocating plunger pump, Figure 5 and its recognizable pumpjack is a notable example and in some regions such as the Texas Permian Basin or Kern County California, they are a ubiquitous sight.

The physical equipment size of a PD pump tends to be larger than that of a rotodynamic pump when comparative performance, at least in terms of flow and head, can be made. Larger size generally translates into higher equipment and greater footprint related costs. But where a PD pump offers better performance characteristics, those will almost always govern selection and cost considerations.

The constant flow characteristic of a PD pump independent of system pressure can be an advantage in situations where the system resistance is either very low, or it varies all over the map, but the process demand is for a constant or a metered flow that can be readily adjusted by PD pump speed (or stroke) control.

There are a wide range of PD types available for pumping essentially anything that will flow – viscous liquids, pastes, and even sludge. Most, but not all, PD pumps are built to handle relatively clean liquids and mixtures that do not contain abrasive solids.

In certain applications, the ability of PD pump to create vacuum and self-prime is cost-saving or vital attribute. For example, the ability to ingest slugs of gas without losing suction prime is an essential function of multi-phase screw pumps used in oil production. 

Figure 6. The pros and cons of PD pumps

 
Figure 7. Left, diagram below ground reciprocating plunger mechanism. Right, pumpjack in Kern County California.(Left image Source https://www.slb.com/resource-library/oilfield-review/defining-series/defining-rod-pumps)

Other Pump Types

There are a few pump types that do not fall into the main rotodynamic or positive displacement categories. They are often grouped into a separate and inspired classification called “Other.”

Magnetohydrodynamic (MHD) pumps use electromagnetic forces acting through a conductive fluid to propel the fluid directly. No moving mechanical parts contact the fluid. It is used for liquid metals, molten salts, seawater, or certain chemical processes.

MHD was popularized as the “caterpillar drive” in Tom Clancey’s 1984 novel The Hunt for Red October and later in the 1990 movie. The concept for naval submarines is a technical reality. However, large-scale quiet MHD propulsion technology has not yet been deployed. Developmental research remains on-going.

Jet pumps employ a motive fluid, either compressed gas or pressurized liquid, using a nozzle to form a high-velocity jet stream. This creates a low-pressure zone and by way of the Venturi effect the pumped fluid or liquid-solids mixture is entrained and mixes with the motive fluid through momentum transfer. The jet pump itself, independent of the liquid or gas pressurizing source, has no moving parts. These devices are widely used for well pumping, mixing, or handling hazardous fluids and require minimal suction pressure. The motive fluid can come from a separate pump, steam, or compressed air.

Air-lift (or gas-lift) pumps use compressed air or gas injected into a liquid column via a nozzle or diffuser at the bottom. The resulting aerated mixture rises due to buoyancy: the bubbly mixture lifts the liquid. These are simple devices and apart from the source of compressed air or gas they have no mechanical moving parts. They are used in wells, in wastewater processing, and for dredging.

Closing Remarks: The Range of Pumps

There is a fantastic variety of pump applications found in manufacturing, water treatment and distribution, wastewater, storm drainage, pipeline transport, pulp and paper, iron, steel and metals industries, swimming pools and spas, amusement rides and water parks, plastics, pharma, medicine, electric power generation, dishwashers and pressure washers, military, waterjet propulsion, chip fabs, computers and data centers, chemical and hydrocarbon industries, agricultural machinery and irrigation, car washes, residential circulators, sump pumps, food processing, HVAC, building services, fuel storage and distribution, fountains, aviation, rocket engines, mining, dredging, fire protection, marine services, construction equipment, street cleaners and dust control, dewatering, commercial vehicles, automotive, and others. The global economic scale of the pump industry is approximately USD 60B annually.

Descriptions of these services and the many types of pumps used are readily found in literature. Less commonly described is the collective coverage and range of these devices. Micro-scale pumps are available for flow rates on the order of less than one trillionth of a liter or per minute. At the other extreme are ultra-high pressure PD pumps capable of over 2K bar (30K psi) and large axial flow pumps capable of delivering approximately 500K m3/h (2.2M gpm). The chart, Figure 8, shows the enormous range of commercial pump performance covering approximately nine orders of magnitude in flow, six orders of magnitude in head and roughly eight orders of magnitude in power. This chart at its low end does not include the very smallest micro pumps, nor at the high end the monumental scale aqueduct lift pumps and reversible pump-turbines. These units may have developed heads of over 1K m (3K feet) with multi-staging and range and from tens of megawatts (MW) up to hundreds of MW in input power requiring a substantial portion of the output of an electric power generation plant.

Figure 8. Overall range of commercial pump performance covering all types

For an independent evaluation of the pumping equipment for your system, contact an experienced consulting engineer who can help with your specific application.

Tags: what is a pump, definition of a pump, pump classifications, rotodynamic pump classification, rotodynamic pump types, positive displacement (PD) pump classification, PD pump types, range of pump performance, rotodynamic pump pros and cons, positive displacement pump pros and cons, magnetohydrodynamic (MHD), The Hunt for Red October