# Positive displacement pump

Come friends today we will know about positive displacement pump. A pump is a device that moves a fluid ( liquid or gas ), or sometimes slurry, by mechanical action , usually converting electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method used to move the fluid: direct lift , displacement , and gravity pumps.

Pumps are driven by some mechanism (usually reciprocating or rotary ), and consume energy to perform mechanical work to move the fluid . Pumps operate through a number of power sources, including manual operation, electricity , engine or wind power , and come in many sizes, from micro to large industrial pumps for use in medical applications.

Mechanical pumps serve in a wide range of applications such as pumping from water wells , in aquarium filtering , pond filtering and aeration , in water cooling and fuel injection for the car industry , oil pumps for the energy industry and operating for natural gas or cooling Towers and other components of heating, ventilation and air conditioning systems. In the medical industry , pumps are used for the development and manufacturing of biochemical processes in medicine, and as prosthetic replacements for body parts, particularly Artificial heart and prosthetic penis .

When a casing has only one rotating impeller , it is called a single-stage pump. When a casing contains two or more rotating impellers, it is called a double- or multi-stage pump.

In biology, many different types of chemical and biomechanical pumps have evolved ; Biomimicry is sometimes used in developing new types of mechanical pumps .

## Type

Mechanical pumps may be immersed in the fluid they are pumping or may be placed outside the fluid.

Pumps can be classified by their method of displacement into positive-displacement pumps , impulse pumps , velocity pumps , gravity pumps , steam pumps and valveless pumps . There are three basic types of pumps: positive-displacement, centrifugal and axial-flow pumps. In centrifugal pumps the direction of flow of the fluid changes ninety degrees as it flows over the impeller, whereas in axial flow pumps the direction of flow remains unchanged.

### Positive-displacement pump

A positive-displacement pump moves the fluid by forcing (displacing) the trapped volume into a fixed volume and into the discharge pipe.

Some positive-displacement pumps use an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the cavity expands on the suction side and the liquid exits the discharge when the cavity collapses. The volume is constant through each cycle of operation.

#### Positive-displacement pump behavior and safety

Positive-displacement pumps, unlike centrifugal ones, can theoretically produce the same flow at the same speed (rpm), no matter what the discharge pressure. Thus, positive-displacement pumps are continuous flow machines . However, a slight increase in internal leakage as pressure increases actually prevents a constant flow rate.

A positive-displacement pump must not operate against a shutoff valve on the discharge side of the pump, as it has no shutoff head like centrifugal pumps. A positive-displacement pump working against a closed discharge valve continues to produce flow and the pressure in the discharge line increases until the line bursts, the pump is severely damaged, or both.

Therefore a relief or safety valve on the discharge side of a positive-displacement pump is necessary. The relief valve can be internal or external. The pump manufacturer usually has the option of supplying an internal relief or safety valve. Internal valves are generally only used as a safety precaution. An external relief valve in the discharge line, with a return line to the suction line or supply tank provides increased protection of both human and equipment.

#### Positive-displacement type

A positive-displacement pump can be further classified according to the mechanism used to move the fluid:

• Rotary-type positive displacement: internal or external gear pump , screw pump , lobe pump , shuttle block, flexible vane or sliding vane, circumferential piston, flexible impeller, helical twisted inertial (such as Wendelkolben pump) or liquid-ring pump
• Reciprocating-type positive displacement: piston pump, plunger pump or diaphragm pump
• Linear-type positive displacement: rope pump and chain pump

#### Rotary positive-displacement pump

These pumps move the fluid using a rotating mechanism that creates a vacuum that captures and pulls the liquid. [3]

Advantages: Rotary pumps are very efficient [4] because they can handle highly viscous liquids with high flow rates as the viscosity increases.

Drawbacks: The nature of the pump requires very close clearance between the rotating pump and the outer edge, allowing it to spin at a slow, steady speed. If rotary pumps operate at high speeds, fluids cause corrosion, which eventually causes increased clearance that liquid can pass through, reducing efficiency.

Rotary positive-displacement pumps come in 5 main types:

• Gear Pump – A simple type of rotary pump where liquid is pushed around a pair of gears.
• Screw Pump – The shape of the internal part of this pump is usually two screws that turn against each other to pump the liquid
• rotary vane pump
• Hollow disc pumps (also known as eccentric disc pumps or hollow rotary disc pumps), similar to scroll compressors, have a cylindrical rotor in a circular housing. As the rotor spins and spins somewhat, it traps the fluid between the rotor and the casing, pulling the fluid through the pump. It is used for highly viscous liquids such as petroleum-derived products, and can also support high pressures of up to 290 psi. [6] [7] [8] [9] [10] [11] [12]
• Vibratory pumps or vibration pumps are similar to linear compressors with the same operating principle. They work by using a spring-loaded piston with an electromagnet connected to AC current through a diode. The spring-loaded piston is the only moving part, and is placed at the center of the electromagnet. During the positive cycle of AC current, the diode allows energy to pass through the electromagnet, generating a magnetic field that moves the piston backward, compresses the spring, and generates suction. During the negative cycle of the AC current, the diode blocks the flow of current in the electromagnet, decompressing the spring, moving the piston, and pumping fluid and pressure, like a reciprocating pump. Due to its low cost, it is widely used in inexpensive espresso machines. Although, Vibratory pumps cannot be operated for more than a minute, as they generate a large amount of heat. Linear compressors do not have this problem, as they can be cooled by the working fluid (which is often a refrigerant).[13] [14]
##### Reciprocating positive-displacement pump

Reciprocating pumps move fluid using one or more oscillating pistons, plungers, or membranes (diaphragms), while valves restrict fluid movement in the desired direction. For suction to occur, the pump must first pull the plunger outward to reduce the pressure in the chamber. Once the plunger pushes back, it will raise the pressure chamber and the inward pressure of the plunger will then open the discharge valve and release the fluid into the delivery pipe at high velocity.

Pumps in this category range from simplex with a single cylinder to quad (four) cylinders or more in some cases . Many reciprocating types of pumps are duplex (two) or triplex (three) cylinders. They can be either single-acting with suction during one direction of piston motion and discharge on the other, or double-acting with suction and discharge in both directions. can be. Pumps can be operated manually, by air or steam, or by a belt driven engine. This type of pump was used extensively in the 19th century—in the early days of steam propulsion—as a boiler feed water pump. Now reciprocating pumps typically pump highly viscous fluids such as concrete and heavy oils, and operate in specialized applications that demand low flow rates against high resistance. Rear hand pumps were widely used for pumping water from wells. Common bicycle pumps and foot pumps use reciprocation for inflation.

These positive-displacement pumps have an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pumps as the cavity on the suction side expands and the liquid exits the discharge when the cavity collapses. Each cycle of operation is given a volume constant and the volumetric efficiency of the pump can be achieved through regular maintenance and inspection of its valves. [15]

Typical reciprocating pumps are:

• Plunger Pump – A reciprocating plunger pushes fluid through one or two open valves, which are closed by suction on the way back.
• Diaphragm Pump – Similar to a plunger pump , where the plunger exerts pressure on hydraulic oil which is used to flex the diaphragm in the pumping cylinder. Diaphragm valves are used for pumping hazardous and toxic liquids.
• Piston Pump Displacement Pump – Usually simple device for manually pumping small amounts of liquid or gel. Common hand soap dispenser is one such pump.
• Radial Piston Pump – A form of hydraulic pump where the piston propels in a radial direction.
##### Various positive-displacement pumps

The positive-displacement principle is applied in these pumps:

• rotary lobe pump
• progressive cavity pump
• rotary gear pump
• piston pump
• diaphragm pump
• screw pump
• gear pump
• hydraulic pump
• rotary vane pump
• peristaltic pump
• rope pump
• flexible impeller pump
###### GEAR PUMP

It is the simplest form of rotary positive-displacement pumps. It consists of two forged gears that rotate in closely spaced casings. Teeth spaces trap fluid and force it around the outer circumference. Fluid does not flow back to the mesh part, as the teeth mesh closely in the center. Gear pumps see wide use in car engine oil pumps and various hydraulic power packs.

###### SCREW PUMP

A screw pump is a more complex type of rotary pump that uses two or three screws with opposing threads – for example, one screw rotates clockwise and the other counterclockwise. The screws are mounted on parallel shafts that have gears that are forged so the shafts are turned together and everything stays in place. Screws turn the shaft and drive fluid through the pump. As with other forms of rotary pumps, the clearance between the moving parts and the pump casing is minimal.

###### PROGRESSIVE CAVITY PUMP

Widely used for pumping hard materials such as sewage sludge contaminated with large particles, this pump has a helical rotor, about ten times longer than its width. It can be visualized as a central core of diameter x , typically, a curved spiral wound around half the thickness x , although in fact it is produced in a single casting. This shaft fits inside a heavy-duty rubber sleeve, also usually x of wall thickness . As the shaft rotates, the rotor slowly moves the fluid up the rubber sleeve. Such pumps can develop very high pressure in a small volume.

###### ROOT-TYPE PUMP

Named after the Rootes brothers who invented it, this lobe pump displaces trapped liquid between two long helical rotors, each fitted at 90° perpendicular to the other, rotating inside a triangular shaped ceiling line configuration , both at the point of suction and at the point of discharge. This design produces a constant flow of equal volume and without vortices. It can operate at low beat rates, and provides the gentle performance required for certain applications.

Applications include:

• High capacity industrial air compressors.
• Root superchargers on internal combustion engines.
• A brand of civil defense sirens, Federal Signal Corporation’s Thunderbolt.
###### PERISTALTIC PUMP

peristaltic pump is a type of positive-displacement pump. It consists of fluid within a flexible tube that is mounted inside a spherical pump casing (although linear peristaltic pumps have been built in). Several rollers , shoes or wipers attached to the rotor compress the flexible tube. As the rotor turns, part of the tube under compression (or under occludes ), forcing fluid through the tube. Additionally, when the tube opens to its natural state after the cam has passed, it draws ( restores ) fluid into the pump . This process is called peristalsis and is used in many biological systems such as the gastrointestinal tract.

###### PLUNGER PUMP

Plunger pumps are reciprocating positive-displacement pumps .

These consist of a cylinder with a reciprocating plunger. Suction and discharge valves are mounted in the cylinder head. In the suction stroke, the plunger is retracted and the suction valves open allowing fluid to be suctioned into the cylinder. In the forward stroke, the plunger pushes the liquid out of the discharge valve. Efficiency and Common Problems: In plunger pumps with only one cylinder, the fluid flow varies between maximum flow when the plunger moves from the middle position, and zero flow when the plunger is in the final position. When fluid is accelerated in a piping system a lot of energy is wasted. Vibration and water hammer can be a serious problem. In general, problems are compensated by two or more cylinders not working in phase with each other.

###### TRIPLE-STYLE PLUNGER PUMP

Triplex plunger pumps use three plungers, which reduces the vibration of single reciprocating plunger pumps. The pump ripple , or ripple graph of the pump transducer, can be further smoothed by adding a pulsation damper at the pump outlet . The dynamic bonding of high pressure fluid and plunger usually requires high quality plunger seals. Plunger pumps with a large number of plungers have the advantage of increased flow, or smooth flow, without a pulsation damper. A drawback is the increase in moving parts and crankshaft load.

Car washes often use these triple-style plunger pumps (perhaps without pulsation dampers). In 1968, William Brugman reduced the size of the triplex pump and increased the lifespan so that car washes could use equipment with smaller footprints. Durable high pressure seals, low pressure seals and oil seals, rigid crankshafts, rigid connecting rods, thick ceramic plungers and heavy duty ball and roller bearings improve reliability in triplex pumps. Triplex pumps are now available in a myriad of markets around the world.

Triplex pumps with shorter lifetimes are common to the home user. A person who uses a home pressure washer 10 hours a year may be satisfied with a pump that lasts 100 hours between rebuilds. At the other end of the quality spectrum industrial-grade or continuous duty triplex pumps can run for up to 2,080 hours a year. [16]

The oil and gas drilling industry extensively uses a semi-trailer-transported triple pump, called a mud pump, which pumps drilling mud, which cools the drill bit and returns the cutting to the surface. goes. [17] Drillers use triple or quintuplex pumps to pump water and solvents into the deep shale in an extraction process called fracking . [18]

###### COMPRESSED-AIR-POWERED DOUBLE-DIAPHRAGM PUMP

A modern application of positive-displacement pumps is the compressed-air-operated double-diaphragm pump. Run on compressed air, these pumps are intrinsically safe by design, although all manufacturers offer ATEX certified models to comply with industry regulation. These pumps are relatively inexpensive and can perform a variety of duties, from pumping water from dams to safe storage to pumping hydrochloric acid (depending on how the pump is constructed – elastomers/body construction). These double-diaphragm pumps can handle viscous liquids and abrasive materials with the ideal pumping process for transporting shear-sensitive media. [19]

###### ROPE PUMP

Designed in China as chain pumps over 1000 years ago, these pumps can be made from very simple materials: a rope, a wheel and a PVC pipe are enough to make a simple rope pump. The efficiency of rope pumps has been studied by grassroots organizations and the techniques to build and operate them are continuously improved. [20]

### Impulse pump

Impulse pumps use the pressure created by gas (usually air). In some impulse pumps, the gas trapped in the liquid (usually water) is released and stored somewhere in the pump, creating a pressure that can push part of the liquid upward.

Conventional impulse pumps include:

• Hydraulic Ram Pump – The kinetic energy of a low-head water supply is temporarily stored in an air-bubble hydraulic accumulator, then used to propel the water up to the high head.
• Pulsar pumps – run on kinetic energy, from natural resources.
• Airlift pumps – run on air pumped into the pipe, which pushes the water upward as the bubbles move up

Instead of a gas accumulation and release cycle, pressurization can be created by burning hydrocarbons. Such combustion powered pumps directly transmit the impulse from a combustion event through the actuation membrane to the pump fluid. To allow this direct transmission, the pump must be made almost entirely of an elastomer (such as silicone rubber). Therefore, combustion causes the membrane to expand and thus push the fluid out of the surrounding pumping chamber. The first combustion-powered soft pump was developed by ETH Zurich. [21]

#### Hydraulic ram pump

A hydraulic ram is a water pump driven by hydroelectricity. [22]

It takes water at relatively low pressure and high flow rate and produces water at high hydraulic-head and low flow rate. The device uses the water hammer effect to develop pressure that elevates a portion of the input water which moves the pump above the point where the water started.

Hydraulic rams are sometimes used in remote areas, where there is a source of low-top hydroelectricity, and there is a need to pump water to a location higher in elevation than the source. In this situation, the ram is often useful, as it does not require a source of power other than the kinetic energy of the flowing water.

### Velocity pump

A rotodynamic pump (or dynamic pump) is a type of velocity pump in which kinetic energy is added to a fluid by increasing the flow velocity. This increase in energy is converted into a gain in potential energy (pressure) when the velocity first decreases or when the flow exits the pump into the discharge pipe. This conversion of kinetic energy into pressure is explained by the first law of thermodynamics , or more specifically by Bernoulli’s principle .

Dynamic pumps can be further sub-divided according to the means of obtaining speed advantage. [23]

This type of pumps has a number of features:

1. sustainable energy
2. Conversion of excess energy to increase in kinetic energy (increased velocity)
3. Conversion of increased velocity (kinetic energy) into increase in pressure head Conversion

A practical difference between dynamic and positive-displacement pumps is how they operate under closed valve conditions. Positive-displacement pumps physically displace the fluid, so closing a positive-displacement pump’s downstream valve creates a constant pressure build-up that can lead to mechanical failure of the pipeline or pump. Dynamic pumps differ in that they can be operated safely under closed valve conditions (for short periods of time).

Such pumps are also called centrifugal pumps. The fluid enters along the axis or center, is accelerated by the impeller and exits at right angles to the shaft (radial); An example is the centrifugal fan, commonly used to implement vacuum cleaners. Another type of radial-flow pump is a vortex pump. In them the fluid moves in a tangential direction around the working wheel. The conversion from the mechanical energy of the motor to the potential energy of the flow occurs through a number of revolutions, which are excited by the impeller in the working channel of the pump. Generally, a radial-flow pump operates at a higher pressure and lower flow rate than an axial- or mixed-flow pump.

#### Axial flow pump

These are also called all fluid pumps. The fluid is pushed outwards or inwards to make the fluid move axially. They operate at much lower pressures and higher flow rates than radial-flow (centrifugal) pumps. Axial-flow pumps cannot be driven up to speed without special precautions. If at a lower flow rate, the total head increase and higher torque associated with this pipe would mean that the starting torque would have to become a function of acceleration for the entire mass of liquid in the pipe system. If there is a large amount of fluid in the system, accelerate the pump slowly. [24]

Mixed-flow pumps act as a compromise between radial- and axial-flow pumps. The fluid experiences both radial acceleration and lifts and exits the impeller somewhere between 0 and 90 degrees from the axial direction. As a result mixed-flow pumps operate at higher pressures than axial-flow pumps, providing higher discharge than radial-flow pumps. The exit angle of the flow determines the pressure head-discharge characteristic with respect to radial and mixed-flow.

It uses jets of steam, most often, to create low pressure. It absorbs the low pressure fluid and pushes it into the high pressure region.

### Gravity pump

Gravity pumps include a siphon and a Heron fountain . The hydraulic ram is also sometimes called a gravity pump; The water in gravity pump is lifted by gravitational force and so called gravity pump

### Steam pump

Steam pumps have long been mainly of historical interest. These include any type of pump driven by a steam engine and also pistonless pumps such as the Thomas Savery or Pulsometer steam pumps.

Recently there has been a resurgence of interest in low-power solar steam pumps for use in small-hold irrigation in developing countries. Small steam engines have previously not been viable due to increasing inefficiencies as steam engines reduce in size. However the use of modern engineering materials along with alternative engine configurations means that this type of system is now a cost-effective opportunity.

### Valveless pump

Valveless pumping aids in fluid transport in a variety of biomedical and engineering systems. In a valveless pumping system, no valves (or physical barriers) are present to regulate the direction of flow. The fluid pumping efficiency of valveless systems, however, is not necessarily less than that of valved ones. In fact, many fluid-dynamic systems in nature and engineering rely more or less on valveless pumping to transport the working fluids in it. For example, blood circulation in the cardiovascular system is maintained to some extent even when the heart valves fail. Meanwhile, the embryonic vertebrate heart begins to pump blood long before the development of clear chambers and valves. In microfluidics, impedance pumps without valves are fabricated, and is expected to be particularly suitable for handling sensitive biofluids. Ink jet printers operating on the piezoelectric transducer principle also use valveless pumping. Due to the low flow impedance in that direction, the pump chamber is emptied by means of a printing jet and refilled by capillary action.

## Pump repair

Checking pump repair records and average time between failures (MTBF) is very important for responsible and conscientious pump users. With that fact in mind, the preface to the 2006 pump user manual points to the statistics for “pump failure”. For convenience, these failure statistics are often translated into MTBF (in this case, the life established before failure). [25]

In early 2005, Gordon Buck, John Crane Inc.’s chief engineer for field operations in Baton Rouge, Louisiana, examined repair records of several refinery and chemical plants to obtain meaningful reliability data for centrifugal pumps. A total of 15 operational plants with around 15,000 pumps were covered in the survey. The smallest of these had about 100 pumps in the plant; Many plants had over 2000. All facilities were located in the United States. In addition, one is considered “new”, others are considered “upgraded” and still others are considered “installed”. Many of these plants—but not all—were alliance arrangements with John Crane. In some cases, the Coalition contracts with John Crane Inc. on-site to coordinate various aspects of the program. Technician or engineer involved.

However, not all plants are refineries, and different results occur elsewhere. In chemical plants, pumps have historically been “throwaway” items because chemical attacks limit life. Things have improved in recent years, but the somewhat limited space available in “old” DIN and ASME-standardized stuffing boxes places limits on the type of seal that fits. Until the pump user upgrades the seal chamber, the pump only accommodates more compact and simpler versions. Without this upgrade, lifetimes in chemical installations are typically around 50 to 60 percent of refinery values.

Unscheduled maintenance is often one of the most significant costs of ownership, and failure of mechanical seals and bearings are among the leading causes. Keep in mind the potential value of choosing pumps that cost more initially but last longer between repairs. An improved pump’s MTBF may last one to four years longer than its non-advanced counterpart. Consider that the published average value of surviving pump failures ranges from US$2600 to US$12,000. This does not include the lost opportunity cost. One pump fires every 1000 failures. Having fewer pump failures means fewer catastrophic pump fires.

As mentioned, a typical pump failure, based on actual year 2002 reports, costs an average of US$5,000. This includes the cost of materials, parts, labor and overhead. Increasing the pump’s MTBF from 12 to 18 months would result in a savings of US$1,667 per year – which may exceed the cost of upgrading the reliability of the centrifugal pump. [25] [26] [27]

## Application

To know about the positive displacement pump, now we will know about the application

Pumps are used throughout the society for various purposes. Early applications included the use of a windmill or watermill to pump water. Today, pumps are used for irrigation, water supply, gasoline supply, air conditioning systems, refrigeration (commonly called a compressor), chemical agitation, sewage movement, flood control, marine services, and more.

Because of the wide variety of applications, pumps come in a plethora of shapes and sizes: from very large to very small, from handling gas to handling liquid, from high pressure to low pressure, and from high volume to low volume.

### Priming a pump

Typically, a liquid pump simply cannot draw air. The feed line of the pump and the internal body surrounding the pumping mechanism must first be filled with the liquid that requires pumping: an operator must introduce liquid into the system in order to begin pumping. priming itcalled a pump. Prime loss is usually caused by air ingress into the pump. The clearance and displacement ratio in pumps for liquids, whether thinner or more viscous, usually cannot displace air because of its compressibility. This is the case with most velocity (rotodynamic) pumps – eg, centrifugal pumps. For such pumps the position of the pump should always be below the suction point, if not then the pump must be filled with liquid manually or a secondary pump must be used until all air from the suction line and pump casing is not removed.

Positive-displacement pumps, however, have a sufficiently tight sealing between the moving parts and the casing or housing of the pump, which can be described as self-priming . Such pumps may also serve as priming pumps , so called when they are used to meet the need for other pumps in exchange for action taken by a human operator.

### Pump as public water supply

One type of pump that was once common around the world was a handheld water pump, or ‘pitcher pump’. It was usually installed on community water wells in the days before piped water supply.

In some parts of the British Isles, it was often called a parish pump . Although such community pumps are no longer common, people still use the expression parish pump to describe a place or forum where matters of local interest are discussed. [31]

Since water is drawn directly from the soil by pitcher pumps, the risk of contamination is high. If such water is not filtered and purified, its consumption can lead to gastrointestinal or other water-borne diseases. An infamous case is the 1854 Broad Street cholera outbreak. It was not known at the time how cholera spread, but physician Jon Snow suspected the water was contaminated and had the public pump handle removed; After that the outbreak subsided.

Modern hand-operated community pumps are considered the most sustainable low-cost option for safe water supply in resource-poor settings, in rural areas of developing countries. A handpump opens up access to deeper groundwater that is not often polluted and also improves well safety by protecting the water source from contaminated buckets. Pumps such as the Afridev Pump are designed to be cheap to manufacture and install and easy to maintain with simple parts. However, the lack of spare parts for these types of pumps in some regions of Africa has reduced their usefulness for these regions.

### Sealing Multiphase Pumping Applications

The increase in oil drilling activity has led to an increase in multiphase pumping applications, also known as tri-phase. Furthermore, the economics of multiphase production is attractive to upstream operations because it leads to simpler, smaller in-field installations, lower equipment costs, and better production rates. In short, the multiphase pump can accommodate all fluid stream properties with one piece of equipment, which has a small footprint. Often, two smaller multiphase pumps are installed in series rather than just one giant pump.

For midstream and upstream operations, multiphase pumps can be located onshore or offshore and connected to single or multiple wellheads. Basically, multiphase pumps are used to move untreated effluent stream generated from oil wells to downstream processes or collecting facilities. This means that the pump can handle a stream (well stream) of 100 percent gas to 100 percent liquid and every conceivable combination in between. Flow streams may also contain abrasives such as sand and dirt. Multiphase pumps are designed to operate under varying or fluctuating process conditions. Multiphase pumping also helps eliminate greenhouse gas emissions as operators attempt to reduce gas glare and flush out the tanks where possible. [32]

#### Types and features of multiphase pumps

##### Helico-axial (centrifugal)

A rotodynamic pump with a single shaft that requires two mechanical seals, this pump uses an open-type axial impeller. This is often called a Poseidon pump , and can be described as a cross between an axial compressor and a centrifugal pump.

##### Twin-screw (positive-displacement)

A twin-screw pump is composed of two inter-meshing screws that move the pumped fluid. Twin screw pumps are often used when pumping conditions have high gas volume fractions and fluctuating inlet conditions. Four mechanical seals are required to seal the two shafts.

##### Progressive cavity (positive-displacement)

When the pumping application is not suited to a centrifugal pump, a progressive cavity pump is used instead. [33] Progressive cavity pumps are single-screw types typically used in shallow wells or on the surface. This pump is primarily used on surface applications where the pumped fluid may contain significant amounts of solids such as sand and dirt. The volumetric efficiency and mechanical efficiency of a progressive cavity pump increase as does the viscosity of the liquid. [33]

##### Electric Submarine (Centrifugal)

These pumps are basically multistage centrifugal pumps and are widely used in oil well applications as a method for artificial lift. These pumps are usually specified when the fluid being pumped is primarily a liquid.

Buffer Tank In case of slug flow a buffer tank is often installed above the pump suction nozzle. The buffer tank breaks up the energy of the liquid slug, smoothing out any fluctuations in the oncoming flow and acting as a sand trap.

As the name indicates, multiphase pumps and their mechanical seals can withstand major changes in service conditions such as changing process fluid composition, temperature variations, high and low operating pressures and exposure to abrasive/erosive media . The challenge is selecting the appropriate mechanical seal arrangement and support system to ensure maximum seal life and its overall effectiveness.

## Specifications

Pumps are typically rated by horsepower, volumetric flow rate, outlet pressure in meters (or feet) of the head, inlet suction in feet (or meters) of the head. The head can be simplified because the pump can raise or lower a column of water under atmospheric pressure.

From an initial design point of view, engineers often use a quantity called specific speed to identify the most appropriate pump type for a particular combination of flow rate and head.

## Pumping power

The force imparted in the fluid increases the energy of the fluid per unit volume. Thus the power relationship is between the mechanical energy of the pump mechanism and the conversion of fluid elements within the pump. In general, this is governed by a series of simultaneous differential equations, known as the Navier–Stokes equations. However, a more simple equation relating only to the different energies in a fluid, known as Bernoulli’s equation, can be used. Hence the power required by the pump, P:

P = \frac{\delta p Q}{\eta}


where p is the change in total pressure between the inlet and outlet (in Pa), and Q is the volume flow-rate of the fluid given in m 3 /s. Total pressure can have gravitational, static pressure and kinetic energy components; That is, the energy is distributed between changes in the gravitational potential energy of the fluid (moving up or down the hill), changes in velocity, or changes in static pressure. is the pump efficiency, and can be given by manufacturer information, such as the pump curve, [36] and is usually derived from either fluid dynamics simulations (i.e. Navier–Stokes solutions for particular pump geometries). is ), or by trial. The efficiency of the pump depends on the configuration and operating conditions of the pump (such as rotational speed, fluid density and viscosity, etc.).

\Delta P = {(v_2 ^ 2 - v_1 ^ 2) \over 2} + \Delta z g + {\Delta p _ {\mathrm {static}} \over \rho}


For a typical “pumping” configuration, the work is imparted on the fluid, and is thus positive. For the fluid imparting the work on the pump (i.e. a turbine), the work is negative. Power required to drive the pump is determined by dividing the output power by the pump efficiency. Furthermore, this definition encompasses pumps with no moving parts, such as a siphon.

## Efficiency

Pump efficiency is defined as the ratio of the power imparted on the fluid by the pump in relation to the power supplied to drive the pump. Its value is not fixed for a given pump, efficiency is a function of the discharge and therefore also operating head. For centrifugal pumps, the efficiency tends to increase with flow rate up to a point midway through the operating range (peak efficiency or Best Efficiency Point (BEP) ) and then declines as flow rates rise further. Pump performance data such as this is usually supplied by the manufacturer before pump selection. Pump efficiencies tend to decline over time due to wear (e.g. increasing clearances as impellers reduce in size).

When a system includes a centrifugal pump, an important design issue is matching the head loss-flow characteristic with the pump so that it operates at or close to the point of its maximum efficiency.

Pump efficiency is an important aspect and pumps should be regularly tested. Thermodynamic pump testing is one method.

## Minimum Flow Protection

Most large pumps have a minimum flow requirement below which the pump may be damaged by overheating, impeller wear, vibration, seal failure, drive shaft damage or poor performance.[37] A minimum flow protection system ensures that the pump is not operated below the minimum flow rate. The system protects the pump even if it is shut-in or dead-headed, that is, if the discharge line is completely closed.[38]

The simplest minimum flow system is a pipe running from the pump discharge line back to the suction line. This line is fitted with an orifice plate sized to allow the pump minimum flow to pass.[39] The arrangement ensures that the minimum flow is maintained, although it is wasteful as it recycles fluid even when the flow through the pump exceeds the minimum flow.

A more sophisticated, but more costly, system comprises a flow measuring device in the pump discharge which provides a signal into a flow controller which actuates a flow control valve (FCV) in the recycle line. If the measured flow exceeds the minimum flow then the FCV is closed. If the measured flow falls below the minimum flow the FCV opens to maintain the minimum flowrate.[37]

As the fluids are recycled the kinetic energy of the pump increases the temperature of the fluid. For many pumps this added heat energy is dissipated through the pipework. However, for large industrial pumps, such as oil pipeline pumps, a recycle cooler is provided in the recycle line to cool the fluids to the normal suction temperature.[40] Alternatively the recycled fluids may be returned to upstream of the export cooler in an oil refinery, oil terminal, or offshore installation.