Industrial Pumps Blog

The vacuum pump was invented over 350 years ago by German scientist, Otto von Guericke.  The vacuum pump’s first application was in scientific experiments which required the removal of gas molecules to create a vacuum or a partial vacuum. The humble vacuum pump has modern applications, but there are now several variants which use the fundamental operating principal.

Broadly, there are three types of vacuum pump:

• Positive displacement pumps;
• Entrapment pumps; and
• Momentum transfer pumps.

Positive displacement pumps continuously create a vacuum by manipulation of a diaphragm or rotary action. Positive displacement pumps are by far the most common variant of vacuum pumps in operation today. The primary application of positive displacement pumps are in fluid transfers because they actively move the fluid (hence positive displacement and because a fluid cannot be “pulled”).

The entrapment pump works by using temperature.  Cold temperatures are used to create condensation from gases and convert them into a solid state. Molecules of gas are trapped in a confined space (hence entrapment) which is known as a “cold trap”.  Here the gas molecules are turned into condensation or a liquefied state, while ionized gas is used to power an ion pump.

Momentum transfer pumps, also known as molecular pumps, work by means of a high speed jet of fluid, or alternatively, a very high speed rotor.  In either case, molecules are “knocked out” of the pump chamber and the gas molecules are transferred to the exhaust stage of the pump. There are two main types of momentum transfer pump – diffusion pumps and turbo-molecular pumps.  The diffusion model uses a jet of dense fluid such as mercury to create molecular displacement, while the turbo-molecular pump uses the high-speed rotor method. Typically, both these types of pumps work at very low pressures, i.e. 1kPa, and in both cases the pumps will fail if the gases are vented directly into the atmosphere or exhaust environment of similar pressure, then the pumps will fail, so it is necessary that they vent into low-pressure vacuum which will require the use of another mechanical pump.

Positive displacement pumps operate by trapping fluid and physically moving it through the pump to the discharge outlet. There are two principal types of positive displacement pumps; rotary and reciprocating.  In this post we will deal with reciprocating types of positive displacement pumping systems.

Examples of reciprocating pumps include pistons, hand water pumps and windmills. Though they are still in use today for certain applications, they have largely been displaced by more efficient pumping mechanisms.  However, in the 19th Century and during the height of the Industrial Revolution, reciprocating pumps played a huge role in the economic development of both Europe and the United States.

Reciprocating pumps operate by the use of a plunger, piston or some form of diaphragm.  The operation of the plunger or piston acts to create a vacuum which acts on the fluid (i.e. suction), and this moves the fluid in the direction of the vacuum. The reciprocating pump may be a “simplex” or single cylinder pump or multiple cylinders may be used.  Reciprocating pumps tend to be produced in duplex and triplex forms, though there are some examples of quad cylinder mechanisms.

In addition to the number of cylinders, reciprocating pumps are further classified by how they “fire”, i.e. the pump can be a single action suction creating and discharge mechanism or it may have a double action so that suction operates in both directions (as does discharge).

Reciprocating pumps can be powered by a variety of energy forms including water, air, steam and belt-driven motors.  Clearly, these pumps are forerunners of modern pumping systems, but they are relatively inefficient and suitable for pumping relatively small amounts of fluids, typically water.  Modern applications of reciprocating pumps include managing highly viscous fluids, such as concrete.  Reciprocating pumps tend to be chosen where there is a need for a low flow rate to be delivered against high resistance.

Positive displacement pumps come in two main flavors: rotary and reciprocating.  Both have similar operating function; however there are distinct differences and applications for each type of industrial pump.

Rotary displacement pumps use rotation to create a vacuum which creates suction to draw fluid through the pump. The vacuum created displaces the fluid through the pump into the discharge pipe, hence the name positive displacement.  Rotary pumps are extremely efficient in operation as air is removed from the pump lines by the rotary action and this removes any need for air bleeding from the lines which is a manual operation.

There are some drawbacks to rotary pumps.  The differential between the moving, rotating parts and the pump enclosure are extremely close, which means the pump speed is limited to avoid erosion and excessive wear. High speed operation of rotary pumps generally leads to enlargement of the differential and a loss in pumping efficiency.

There are three main types of positive displacement rotary pumps.  The simplest are gear pumps which use two gear mechanisms aligned in parallel with enmeshed teeth.  The turning of the gears creates a flow of fluid between the teeth and outer body of the casing, with the fluid being discharged. Gears with a large number of small teeth generate a regular flow while larger and fewer gear teeth generate a pulsing flow which tends to flow in gushes.

Screw pumps are more complex and are comprised of two screws with counter threads which turn in opposition to one another. The screws are placed upon shafts with gears on them which are enmeshed with each other to generate a uniform turning movement of the two threads. The turning motion generates a fluid flow through the pump, but again the differential spacing between the pump casing and rotating threads must be minimalized to ensure efficient operation.

Finally, moving vane pumps comprise of a rotor mounted inside a cylindrical casing.  The turning rotor causes fluid flow between the pump housing and rotor which creates fluid flow through the pump.

Globally, oil production trends have mitigated towards increasing pipeline diameters and greater oil volume throughput from both onshore and offshore oil fields to downstream production facilities.  In part this trend is fuelled by a need to reduce the carbon footprint of the producer but also to reduce the cost of production using economies of scale, reduced maintenance and replacement cycles and increasing efficiency and reliability.

In addition, much of the global transportation infrastructure is in the middle of a huge reorganization and refit, so many of the previously accepted standards are being revised in the light of modern and forecasted developments.

The largest three-screw crude oil pipeline pump is capable of being fitted into 24 inch diameter pipes and can pump 85,000 barrels of oil per day under 2,000 pounds per square inch.  The oil industry needs the capacity of these new generation pumps to meet the stringent constraints being imposed upon them by the environmental lobbyists and government agencies involved, but at the same time by shareholders who have experienced a severe battering in the economic situation prevailing at present.

Centrifugal pumps no longer can be relied upon to deliver the reduced carbon footprint or the financial benefits.  It is not simply these two key factors which have stimulated the development of positive displacement pump technology; new generation pumps are capable of being retrofitted to old-style infrastructure which provides even greater scope for cost savings and carbon reduction.

Crude oil pipeline designs are increasingly calling for positive vane displacement pumps because of the cost savings and the 30 year track record of this pump type in some of the most extreme and harshest environments encountered in the world.  Screw pump operation also delivers high volume/high operating efficiency with increased maintenance and replacement cycles such that where fitted, the operator can experience up to 30% reduction in overall costs associated with the installation.

Noise and vibration form a key side effect of almost any industrial application or process you can think of.  They are also negative factors for a number of reasons – they are pollutants, they may harm human operators and persons in the vicinity, they may adversely affect performance and indeed, excessive noise and vibration is symptomatic of energy and operational inefficiency involved in the process.

There are also instances where for operational reasons, noise and vibration must be minimized.  The French Navy has recently placed an order for ultra-silent industrial pumps to be fitted to its new class of “Suffren” submarines.  When a submarine is engaged on duty or operations, it is essential that the vessel can operate under the strictest standards of silence to evade detection by the enemy.

In such instances, there is no alternative but to deal with the noise and vibration issue.

Industrial pumps and fluid handling systems contribute a large portion of the noise and vibration associated with any industrial or military process (obviously this applies only in normal circumstances).  Traditional centrifugal pump technology utilizes techniques which are themselves, very noisy and contribute to vibration production.  By definition, greater part movement and friction creation is implicit in the operation of centrifugal pumps which makes them inappropriate for designs intended to meet high environmental and operating efficiency levels.

To this end, it is noticeable that positive displacement technology is increasingly being used by commercial and military operators seeking to deliver noise and vibration free side effects of their activities.  Positive displacement pumps and valves are also much more efficient in terms of consumption of energy and enjoy greatly reduced operating costs due to extended maintenance and replacement cycles.  The new French submarines for instance, are utilizing positive displacement pumps, rather than old-style centrifugal technology which cannot deliver the same operational standards of silence and reliability as the newer technology.