P180 Rotating Enquipment Engineering Machinery Basics

8/6/2019 P180 Rotating Enquipment Engineering Machinery Basics

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Shell Exploration & Production

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P180 Development & Operation of

Production SystemsRotating Equipment Engineering –Machinery Basics

David Moore



Driven Machines and Drivers

DRIVERS

T R A N

S M I S S I O N

DRIVENUNIT

Please click on notes

Machinery, also known as Rotating Equipment or (less often) RunningEquipment, is vital to E&P production operations. Rotating Equipment may bedivided into two broad categories: 1. Driven machines. 2. Drivers.



DRIVERS

T R A N

S M I S S I O N

DRIVENUNIT

PUMPCOMPRESSORGENERATOR

Driven Unit

The driven machines used in E&P are generally pumps, compressors and generators. Pumps andcompressors are required in various parts of the production process to increase the pressure of oil,gas, water or some other process fluid. Generators are required to provide electrical power at remotelocations, or to back up mains supplies in an emergency.



Drivers

DRIVERS

T R A N

S M I S S I O N

DRIVENUNIT

GAS TURBINEELECTRIC MOTORDIESEL ENGINEGAS ENGINE

BASEPLATEENCLOSURELUBRICATIONCOOLING

VENTILATION AIR INTAKE ETC

Drivers, or prime movers, provide the energy input to the driven unit. They maybe categorised as either fired or unfired. Fired drivers are machines which burnfuel to produce energy. In E&P the main examples of this type of driver are gasturbines, gas engines and diesel engines. The main type of unfired driver in E&Pis the electric motor: this does not burn fuel, but converts electrical energy intomechanical power.



Transmission

DRIVERS

T R A N

S M I S S I O N

DRIVENUNIT

GEARBOXCLUTCHFLUID COUPLINGFLEXIBLE COUPLINGS

A transmission is required to connect the output of the driver to the input of thedriven machine. This transmission may be a simple drive shaft connecting onemachine to the other. However, the rotation speeds of the driver and drivenequipment are often not ideally matched and a speed changing device, agearbox, has to be placed between the two units..



Machinery Package

DRIVERS

T R A N

S M I S S I O N

DRIVENUNIT

GAS TURBINEELECTRIC MOTORDIESEL ENGINEGAS ENGINE

PUMPCOMPRESSORGENERATOR

BASEPLATEENCLOSURELUBRICATIONCOOLING

VENTILATION AIR INTAKE ETC

GEARBOXCLUTCHFLUID COUPLINGFLEXIBLE COUPLINGS

Most major equipment used in E&P applications is procured as Packages,consisting of driver, driven machine and transmission as well as supportsystems and auxiliaries. The Package is procured from a Lead Vendor who ismade responsible for the many interfaces involved in successfully specifying amachinery package. The principal variable in driver package supply is thequality of accessories, these are the legitimate concern of a user to specifyproper quality for his application. This means precise application of the ShellDEPs



Main Classes of Fluid Machine

• Turbomachines:– Centrifugal– Axial

• Displacement Machines:– Reciprocating– Rotary

The two major classifications of fluid machinery are: 1. Turbomachines, also known asRotodynamic machines or just Dynamic machines2. Displacement machines, also known as Positive Displacement machines. These two

classifications apply to machines handling any fluid, whether it is a gas or a liquid: the samebasic principles apply. However, the working principles of Turbomachines and Displacementmachines are radically different. Turbomachines work by putting kinetic energy into the fluid

and then converting that kinetic energy into pressure. Displacement machines work byenclosing fluid in a moving part of the machine and transporting both fluid and machine partfrom inlet to outlet. The difference may be likened to two ways of carrying water to put out afire: we could either fill buckets with water and then throw the water toward the fire (theturbomachine), or we could fill the bucket, carry it to the fire and empty it (displacementmachine).



Turbomachinery Working Principle

• A high speed rotating component - the ROTOR or IMPELLER - adds kineticenergy to the fluid

• The fluid then flows into a static component - the STATOR or DIFFUSER in which the kinetic energy is converted to pressure energy

– diffuser: a flow duct whose crosssectional area increases in thedownstream direction

– as flow area increases, fluid velocity decreases

– as fluid velocity decreases, pressureincreases

Bernouilli’s equation:p + ½ ρ V 2 = constant p = pressureρ = density

V = velocity

Turbomachines work by putting kinetic energy into the fluid and then convertingthat kinetic energy into pressure. The kinetic energy is put into the fluid by arotor, usually known as the impeller. The kinetic energy is converted topressure by reducing velocity. This is done by increasing the flow area afterimpeller discharge and hence reducing the velocity so that velocity is convertedinto pressure in accordance with Bernouilli’s equation. Flow through aturbomachine is continuous. There is a direct flow path between the suction anddischarge of the machine: if it stops rotating the fluid will flow back through themachine.



Turbomachine Rotors

Centrifugal Impeller Axial Impeller

The rotor or impeller of a turbomachine may be either centrifugal or axial. Inthe centrifugal impeller, the fluid at inlet moves parallel to the axis of rotation.The inlet of a centrifugal impeller is known as the eye of the impeller. Fluid isthen accelerated and is discharged from the outer diameter of the rotor withhigh radial and tangential (“whirl”) velocity. In the axial impeller, the fluid is notaccelerated radially. The general motion of the fluid is parallel to the axis of rotation and the acceleration is tangential, that is whirl velocity increases.Turbomachine rotors may be a mixture of these two types in which the outletfluid flow has radial, tangential and axial velocity: these rotors are known asmixed flow impellers. Axial turbomachines are used for low pressure, highvolume applications. Centrifugal machines are used for low flow, high pressureapplications.



SCROLLDIFFUSER

RADIALDIFFUSER

AXIAL (“BOWL”)DIFFUSER

Turbomachine stators: diffusers

As stated earlier, the stator of a turbomachine is a flow duct whose crosssectional area increases in the direction of flow. This increase in area willdecrease the velocity of the fluid and increase its pressure. The stator actuallyhas three functions - “the three Cs” – 1. C ollect: all the fluid discharged by therotor. 2. C onvert: decrease the fluid velocity and increase its pressure, in otherwords, convert Kinetic energy to Pressure. 3. C onvey: the fluid to where it isrequired next - either the outlet connection of the machine, or the nextcomponent in the machine. In principle, a stator can be any arbitrary threedimensional shape as long as it does these three things, and the shape chosendepends on the overall design of the machine. There are a few generic types of stator or diffuser. The simplest type is the scroll diffuser, or volute, which isessentially a length of pipe bent around the outlet of the impeller. Fluid entersthe volute through a slot on its inside diameter. The diffuser has a flow areavery much greater than that of the impeller outlet and the fluid velocity isconverted to pressure. the scroll is arranged in a sort of helix around theimpeller, and the outer end also serves as a connection to the dischargepipeline. The radial diffuser consists of a solid ring in which tapered tangentialslots have been cut. These slots widen outward from the impeller dischargehence causing the velocity decrease and pressure increase required. The radialdiffuser is most frequently combined with a volute type stator.

The previous types of stator have been in the same plane as the impeller.However the bowl diffuser is arranged axially. Here the fluid discharges radiallyand is turned into a direction parallel with the axis of rotation, while the flowarea increases. Discharge from this stator can be taken into the eye of anotherimpeller to form a multistage machine.



Turbomachine Performance Curve

Flow

P r e s s u r e

IncreasingSpeed, Diameter,Density

All turbomachines have the same generic pressure - flow characteristic. For agiven design of machine, the following relationships with impeller speed anddiameter hold:● Flow is proportional to speed and diameter. ● Pressure is proportional to fluiddensity, speed squared and diameter squared. ● Power absorbed is proportionalto fluid density, speed cubed and diameter cubed. (Strictly, these relationships

hold for geometrically similar rotors.) There is a practical limit to the pressurethat can be generated by a simple turbomachine: for higher outputs it iscommon to build a machine with a series of impellers in the same casing.



Displacement Machines: Reciprocating

CONNECTING ROD

In a displacement machine, discrete volumes of fluid are moved from suction todischarge and there is no direct connection at any time between the suction anddischarge pipes. In the reciprocating displacement machine, a rotating crankconverts the rotary motion of the drive motor into reciprocating motion throughthe connecting rod. The connecting rod is attached to the piston rod at thecrosshead bearing which ensures that the end of the piston moves linearly. Thepiston moves inside the cylinder and physically moves the fluid to and fro.When the piston moves so as to increase the enclosed volume, the suctionvalve opens and fluid enters the cylinder while the discharge valve remainsshut. When the piston moves so as to decrease the enclosed volume, thesuction valve is pushed shut while the discharge valve opens.When both sides of the piston are used as shown, the machine is said to bedouble acting. When only one side of the piston and cylinder are used, themachine is single acting. In the reciprocating pump or compressor, the suctionand discharge flow are intermittent.



Video clip: Reciprocating machine.

This clip shows how the rotary motion of an engine or motor is converted toreciprocating motion by the “slider crank mechanism”. As the piston movestoward the end of the cylinder it pushes fluid out, the discharge valve opensand the suction valve shuts. As the piston moves away from the cylinder endthe suction valve opens, fluid is pulled into the cylinder, and the discharge valveshuts.



Displacement Machines: Rotary

THREE LOBE PUMP

As with reciprocating machines, discrete volumes of fluid are moved fromsuction to discharge. However, rotary machines do not use valves. Instead, thedesign of the rotors and casing are such that direct communication betweensuction and discharge ports is prevented. Suction and discharge flow arecontinuous. There are many designs of rotary machine but all work on thisprinciple.



Video clip: Rotary machine

In this video clip of a rotary machine flow enters at the top and is discharged atthe bottom. You can see that volumes of fluid are enclosed between the lobes of the rotors and the casing, then displaced from suction to discharge. By meshingtogether, the rotors prevent backflow from the discharge. NOTE: The variationin rotation speed is for illustration purposes only! These machines normallyoperate at a steady speed.



sp acemen ac ne: er ormanceCurve

FLOW

P R E S S U R E

Unlike turbomachines, displacement machines are essentially constant flowdevices and are insensitive to fluid density. The pressure:flow characteristic issteep, however it is not vertical due to: 1. internal leakage. 2. compressibility(in the case of gas compressors).



System Resistance Curve

Machine

Reservoir

Pressure = P r

Process resistance curve

Flow

P r e s s u r e r e q u

i r e

d

P r

Real machines are integrated into systems and the question arises: how domachine and system inter-relate? We need to consider the characteristics of thesystem into which the machine delivers its flow. The system resistance curveshows the pressure required at the inlet to a system in order to sustain a givenflow rate. The curve is the sum of two parts: 1. Static resistance at zero flow(eg a fixed height in a pumping installation or a large reservoir of pressure in agas lift application). This static resistance has to be overcome regardless of whether the machine is operating at extremely low flow, or maximum flow. 2.Frictional resistance, which is proportional to the square of the fluid flow rate.Frictional resistance increases with: ● pipe length ● number of bends ● changesin flow area ● number of valves ● number of branches



Machine and System: Operating Point

0

5

10

15

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0 100 200 300 400 500 600

FLOW

P R E S S U R E

SYSTEMRESISTANCE

CURVE

TURBOMACHINEOPERATING POINT

DISPLACEMENTMACHINE OPERATING

POINT

TURBOMACHINEPERFORMANCE

CURVE

DISPLACEMENTMACHINE

PERFORMANCELINE

The operating point is found by plotting, on the same diagram, the machineperformance curve and the system resistance curve. The operating point iswhere the two curves intersect - this is where the machine produces enoughoutlet pressure to balance the losses in the system at the given flow rate. Thediagram shows the operating point for both a turbomachine in a given system,and a displacement machine in the same system – the same rule applies toboth types.



Turbomachines

0

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10

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0 100 200 300 400 500 600

FLOW

P R E S S U R E

NOMINAL SPEED

INCREASED SPEED

REDUCED SPEED

CONTROL RANGE

SYSTEMRESISTANCE

CURVE

It was stated earlier that the pressure output of a turbomachine increases withincreasing speed. Therefore, varying the speed of the machine provides amethod of controlling or regulating flow in the system and the machine. Therule that the operating point is defined by the intersection between the machinecurve and the system resisitance curve still applies as shown above. Hencethere will be a variation of system pressure as well as flow.



Machines

0

5

10

15

20

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0 100 200 300 400 500 600

FLOW

P R E S S U R E

CONTROL RANGE

SYSTEMRESISTANCE

CURVE

Varying the speed of a displacement machine will vary the output flow of themachine: in fact the flow is directly proportional to speed.As with the turbomachine, the operating point is determined by the intersectionof machine performance curve and system resistance characteristic, hence avariation in speed will cause variation of both flow and pressure.



Flow Regulation by System Control Valve

0

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10

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0 100 200 300 400 500 600

FLOW

P R E S S U R E

CONTROL RANGE

DISPLACEMENTMACHINE

PERFORMANCELINE

TURBOMACHINEPERFORMANCE

CURVE

Since the operating point is determined both by the machine and systemcharacteristics, flow may also be regulated by varying the system resistance.This is achieved by installing a control valve somewhere in the system to give avariable resistance to flow. Notice that, with a displacement machine, which hasan essentially constant flow for a given speed, varying the system resistancehas little or no effect on flow. Flow control valves are therefore not used toregulate displacement machines.



Flow Regulation by Recycle

NET FLOW TOSYSTEM

BYPASS FLOW

MACHINE FLOW

FLOW CONTROL VALVE

The flow delivered into a system by a machine may also be regulated by recycle(also known as bypass or spillback). In this case, the flow through the machineitself may remain constant. The amount of flow delivered into the system iscontrolled by a flow control valve which varies the amount of fluid which isrecycled to the suction of the machine. Net system flow = Machine flow -bypass flow. In principle, recycle control may be used to vary flow frommaximum to zero. This method of regulation is inefficient in energy termsbecause the full flow is pressurised and then part of it is depressurised again. If a large bypass flow is continuously recycled through a high-energy machine, itwill heat up. To limit the temperature achieved, either the flow should berecycled to a large capacity suction vessel or cooling should be providedsomewhere in the system. In some cases, where the fluid being handled is safe,non-toxic and cheap, the bypass may be an open cycle. For instance, inseawater pumping, the bypass may be routed to an overboard dump andreturned to sea. In air compression, surplus air may be discharged back to theatmosphere.



Machines in Parallel and Series

Machines in Series

Flow

P r e s s u r e

SYSTEMRESISTANCE

CURVE

ONE PUMP

TWO PUMPS

Machines in Parallel

Flow

P r e s s u r e

SYSTEM

RESISTANCECURVE

ONE PUMP

TWO PUMPS

When more than one machine is installed so that each has a common suctionand a common discharge they are said to be in parallel. When the discharge of one machine is connected to the suction of another, they are said to be inseries. Let us consider the case of two identical pumps installed in parallel andin series (go to next slide).



Machines in Series

Flow

P r e s s u r e

SYSTEMRESISTANCE

CURVE

ONE PUMP

TWO PUMPS


Flow

P r e s s u r e

SYSTEM

RESISTANCECURVE

ONE PUMP

TWO PUMPS


For the two pumps in Parallel, at a given pressure the flow from the two pumpscombined is twice the flow from a single pump at the same pressure. We cantherefore construct the performance curve of two pumps working together byadding the flows of the individual pumps at a series of different pressures. Theoperating point of the pair of pumps is where this combined performance curvecrosses the system resistance curve. The operating point of each individualpump will be at the same pressure as the intersection point but half the flow.Note that, although we have twice the number of pumps, neither the pressuregenerated against the system, nor the flow are doubled. In principle the sameprocedure can be used to generate a combined performance curve for twopumps of different performance characteristics. In practice it is unusual tooperate dissimilar machines in parallel. More than two machines may beinstalled in parallel: it is not unusual to find up to seven or eight pumps, orthree or four compressors in parallel.



Machines in Series

Machines in Series

Flow

P r e s s u r e

SYSTEMRESISTANCE

CURVE

ONE PUMP

TWO PUMPS


Flow

P r e s s u r e

SYSTEMRESISTANCECURVE

ONE PUMP

TWO PUMPS

For the two pumps in Series, the pressure at a given flow is the sum of thepressures of the individual pumps. Again, a combined performance curve can beconstructed at a series of different flows and the operating point is where thiscurve intersects the system resistance curve. Once again, although we havetwice the number of pumps, neither the pressure generated against the system,nor the flow through the system have doubled. It is normal for dissimilarmachines to be installed in series and the procedure described above can beused to construct the combined performance curve. In pumping installations, asmall booster pump is frequently used to provide higher suction pressure to alarger main pump. In compressor applications, a series of compressors may beused to achieve the required pressure and these compressors may be of different types, for example: centrifugal and reciprocating.



End of Machinery Basics.

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P180 Rotating Enquipment Engineering Machinery Basics