ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 2 Part 11 ppsx

In actual practice a torquelimit, increased exhaust temperature, loss of turbine efficiency, and/or alubrication problem on the driven equipment usually preclude operating atvery low pow

low as possible, it may be necessary to unload the driven equipment ing start-up.

dur-Figure 16-16 shows the performance characteristic of a split-shaft bine where the only power output limitation is the maximum allowabletemperature at the inlet of the turbine section In actual practice a torquelimit, increased exhaust temperature, loss of turbine efficiency, and/or alubrication problem on the driven equipment usually preclude operating atvery low power turbine speeds The useful characteristic of the split-shaftengine is its ability to supply a more or less constant horsepower outputover a wide range of power turbine speeds The air compressor essentiallysets a power level and the output shaft attains a speed to provide therequired torque balance Compressors, pumps, and various mechanicaldrive systems make very good applications for split-shaft designs

tur-Effect of Air Contaminants

The best overall efficiency of a turbine can be ensured by maintainingthe efficiency of the air compressor section Conversely, allowing the aircompressor efficiency to deteriorate will deteriorate the overall thermalefficiency of the turbine Air compressor efficiency can be drasticallyreduced in a very short time when dirt, salt water mist, or similar air con-

Figure 16-16 Performance characteristics of a multi-shaft turbine.

Prime Movers 487

taminants enter the inlet air Contaminants will accumulate in the aircompressor and reduce its compression efficiency The effect will bedecreased mass flow, reduced compressor discharge pressure, reducedhorsepower, and higher-than-normal engine temperatures

Effective inlet air filtration is required to ensure satisfactory operation

of the engine The location of the unit determines the most appropriatefilter system to use Desert environments where a large amount of sandparticles could be expected in the ambient air may use an automatic rolltype of filter that allows new filter material to be rolled in front of theinlet without frequent shut-downs to change filters Arctic or extremelycold locations may use pad type filters, snow hoods to prevent blockage,and exhaust recirculation to prevent icing Filter assemblies for offshoremarine environments may include weather louvers, demister pads, andbarrier elements for salt and dirt removal Screens may be used for insectremoval prior to filtration in areas with bug problems

Cleaning the air compressor can be accomplished by injecting water,steam, detergent, and/or abrasive material (such as walnut hulls) into theair inlet Engine life and performance will be improved if cleaning is done

on a periodic basis so as to keep any hard deposits of oil, dirt, etc fromforming In general, frequent detergent washing will ensure compressorcleanliness Steam cleaning with an appropriate detergent is also veryeffective Abrasive cleaning should be avoided and only be necessary asthe result of improper frequency or technique of detergent washing

lev-to have lower emission levels of particulates and unburned hydrocarbonsthan reciprocating engines Gas turbines do, however, tend to producegreater quantities of nitrogen oxides (NOX) The formation of NOXdepends on combustion temperature and residence times at high tempera-tures, both of which are higher in gas turbines than in engines Engines,

on the other hand, tend to have greater concentrations of carbon ide, CO, in their exhausts.

monox-Fuel quality will greatly affect emissions and can also have able effect on engine life Manufacturers' specifications will generallyspecify fuel quality for proper operation

consider-In addition to carbon monoxide (CO) and unburned hydrocarbons(UHC), the most significant products of combustion are the oxides ofnitrogen (NOX) At high temperatures, free oxygen not consumed duringcombustion reacts with nitrogen to form NO and NO2 (about 90% and10% of total NOX, respectively)

Improvements in engine and turbine design, along with the use of iliary equipment such as catalytic converters, selective catalytic reduc-tion (SCR) units and the use of steam and water injection into turbines,combine to reduce overall emission levels

aux-When a hydrocarbon fuel such as natural gas is burned in an engine orturbine, the concentration of pollutants is dependent on the air to fuel(A/F) ratio as shown in Figure 16-17 If pollution was not a concern, inorder to obtain maximum thermodynamic efficiency, the engine would

be designed for a slightly greater than stoichiometric mixture Becauseair and fuel are never perfectly mixed at the time of ignition, excess airmust be present to burn all the fuel The normal amount of excess air thatachieves this efficiency is around 15-20% Under these conditions, Fig-ure 16-17 shows that a relatively large amount of NOX will be formed

NOX emission controls in large engines and turbines are based on thesame principles However, special designs must be applied to accommo-date differences in the combustion process Methods to control NOX

include the following

NOX Reduction in Engines

1 Lean Burn

As shown in Figure 16-17, at very high A/F ratios—greater than30:1— the production of NOX can be very low The problem withsimply increasing A/F ratio is that, because the air/fuel mix is notuniform, increasing A/F ratio in the cylinder increases the probabili-

ty that the mixture at the point of the spark plug may be too lean,thus leading to a misfire

Installing a pre-combustion chamber (PCC) in the engine designsolves this problem In this design, a normal A/F ratio fuel is intro-duced into a PCC at the time of ignition This creates ignition torch-

Prime Movers 489

Figure 16-17 Emission trends vs A/F ratio for a typical engine/turbine.

es that enter the main cylinder, which has the lean A/F ratio, andignites the fuel

2 Catalytic Converter

Catalytic converters are designed to oxidize the unburned UHCsand CO The resulting combustion (oxidation) converts them into

water and CO2 A recent catalytic converter design called three-wayconverters also controls NOX using a reduction process Three-wayconverters contain two catalytic "bricks," one for reduction and theother one for oxidation The oxidation process with CO takes place as;2CO + O2 <-» 2CO2

The oxidation of UHC is:

2O2 + CH4 <-» CO2 + 2H2O

And finally, the reduction of NOX with CO results in N2 and CO2:

NO2 + CO <-» NO + CO2

2NO + 2CO <-» N2 + 2CO2

Catalytic reactions occur when the temperature exceeds 500°~6QO°F(260°-316°C) Normal converter operating temperatures are9QO°-1200°F (482°-649°C)

Excessively rich A/F ratio causes converter operating tures to rise dramatically, thus causing converter meltdown On theother hand, if the A/F ratio is too lean, the excess O2 will react withthe CO, and the reduction of nitrogen with CO will not take place.Thus, catalytic converters cannot be used where there is excess air

tempera-3 Selective Catalytic Reduction (SCR)

Selective catalytic reduction is based on selective reactions of acontinuous gaseous flow of ammonia or similar reducing agentswith the exhaust stream in the presence of a catalyst The reactionthat occurs is as follows:

4NH3 + 6NO <-» 5N2 + 6H2O

SCR units require handling, storage, and continuous injection ofthe reducing agent The temperature level is critical because theSCR operates in a narrow temperature range between 550°~750°F(260°-399°C), and thus an exchanger is necessary to cool theexhaust stream This leads to a complicated and costly process sys-tem that must be added to the engine exhaust

Prime Movers 491

NO X Reduction in Turbines

1, Inject Steam or Water

This system is called wet NOX control Water or steam is injectedinto the primary combustion zone This method has been used effec-tively in the past Current installations are using this system whenthe water or steam is readily available or if they are already part ofthe process Maintenance costs are higher when compared with drycontrol, because this method requires high quality water If highquality water is not used, the corrosion associated with dissolvedminerals in the water may prematurely damage the turbine

2 Lean Premised Combustion

When air and fuel are mixed and burned in standard turbine bustion systems, incomplete mixture occurs Areas of rich A/F ratiosexist, which cause local high temperatures called "hot spots." Nor-mal turbine combustion temperatures can reach 2800°F (1538°C).Because NOX formation rate is an exponential function of tempera-ture, decreasing the combustor temperature can substantially reduce

com-NOX production One method of reducing hot spots is to premix alean A/F ratio prior to combustion Lower temperature levels areachieved by using stages (multiple sets of air and fuel injectors) and

by adding special instrumentation to control the appropriate tion of air and fuel Excess air is used to further reduce the overallflame temperature Using this approach, most gas turbine manufac-turers are able to guarantee about 25 ppmv

propor-3 Selective Catalytic Reduction (SCR)

SCR is described above It is important to note that SCRs require

a lot of space, are relatively expensive, and use toxic metals fore, they may not be practical and may be too costly to install andoperate compared with other methods

Based on the results of the catalytic combustor, there is an effortunder way to develop this concept into a practical, field-proventechnology.

Noise Pollution

Increased public awareness of noise as an environmental pollutant and

as a hazard to the hearing of personnel requires that attention be given tothis problem during the design phase When a prime mover installation isplanned, enough silencing should be installed to ensure that the noiselevel will be acceptable to the community and meets all governmentalrequirements The requirements will vary substantially depending uponsuch factors as location, population density, operating personnel in thearea, etc

17

Electrical Systems*

This chapter introduces some concepts concerning electrical systemdesign and installation that are particularly important from the standpoint

of safety and/or operational considerations for production facilities Thereader is referred to texts in electrical engineering and to the variouscodes and standards listed at the end of this chapter for a more detaileddescription concerning the design of electrical circuits, sizing conductors,and circuit breakers, etc This chapter is meant merely as an overview ofthis complex subject so that the project facilities engineer will be able tocommunicate more effectively with electrical design engineers and ven-dors who are responsible for the detail design of the electrical system

SOURCES OF POWER

The required power for production facilities is either generated on site

by engine- or turbine-driven generator units or purchased from a localutility company For onshore facilities the power is generally purchasedfrom a utility However, if the facility is at a remote location where there

*Reviewed for the 1999 edition by Dinesh P Patel of Paragon Engineering Services, Inc.

493

is no existing utility power distribution, an on-site generating unit may beconsidered A standby generator may be required if utility power is notsufficiently reliable The standby generator may be sized to handle eitherthe total facility load or only essential loads during periods of utilitypower failure.

In the case of an offshore facility, electrical power is generally ated on site by engine- or turbine-driven generator sets using natural gas

gener-or diesel as fuel Most installations are designed to handle the total trical load even if one generator is out of service To minimize the size ofstandby equipment, some facilities have a system to automatically shednon-essential loads if one generator is out of service Some offshorefacilities are furnished power from onshore via high-voltage cables Thecables are generally laid on the ocean floor and are buried in shallowwater In some cases a single cable (usually three-phase) is used for suchapplications to minimize initial project cost However, if a fault develops

elec-in this selec-ingle cable, the facility could be shut elec-in for extended periods oftime To avoid extended shut-ins, either a spare or alternate cable can beinstalled or standby generators can be installed on the offshore platform.The choice of whether to purchase or generate electricity and decisions

on generator or cable configuration and sparing are often not obvious An

economic study evaluating capital and operating costs and system

relia-bility of several alternatives may be required

Utility Power

Utility companies have a power system network including large ating plants, overhead transmission lines, power substations whichreduce transmission line voltages to distribution line voltages, and over-head/underground distribution lines which carry power to the end users(such as a production facility)

gener-The power from the distribution line voltage is converted to facilitydistribution voltage using a "step-down" transformer, providing power tofacility switchgear and motor control centers The facility distributionvoltage selection depends upon the length of the distribution system, thesize, and location of the electrical loads to be served Most oil field elec-trical distribution systems in the United States are 4,160 or 2,300 volts.Typically, 480 volts is used for motor and other three phase loads; 240/

120 volts usually is used for lighting and other single phase loads, and

120 volts usually is used for control circuits Step-down transformersdeliver these voltages from the facility distribution system

Electrical Systems 495

The electrical distribution system design and equipment selection mustconsider requirements of the utility company for protection and metering.Available short circuit currents from the utility distribution network tothe primary of the facility's main transformer must be considered inselecting circuit protection devices for the facility distribution system,

Electrical Generating Stations

Where electricity is generated in the facility, generator sizing shouldconsider not only connected electrical loads, but also starting loads andanticipated and non-anticipated expansions In most installations this isdone by developing an electrical load list itemizing the various loads aseither continuous, standby or intermittent service Examples of continu-ous loads are electric lighting, process pumps and compressors required

to handle the design flow conditions, and either quarters heating or airconditioning, whichever is larger Intermittent loads would include quar-ters kitchen equipment, washdown pumps, cranes, air compressors andsimilar devices which are not in use at the same time The total demand

is normally taken as 100% of the continuous loads, 40 to 60% of theintermittent loads and an allowance for future demand Standby loads donot add to generator demand as they are activated only when anotherload is out of service

Generators must be sized to handle the starting current associated withstarting the largest motor On large facilities with many small motors,starting current usually can be neglected unless all the motors are expect-

ed to start simultaneously However, if the total load is dominated by eral large motors, the starting load must be considered

sev-In calculating generator loads it must be remembered that each motorwill only draw the load demanded by the process It is this load and notthe nameplate rating of the motor that should be used in the load list Forexample, even though a pump is driven by a 100 hp motor, if the processconditions only demand 75 hp, the total load that will be demanded fromthe generator is 75 hp

Generators are normally provided with static voltage regulators ble of maintaining 1% voltage regulation from no load to full load Whilerandom ("mush") wound stators are acceptable for smaller units, formedcoils are normally preferred for generators of approximately 150 kW orlarger Vacuum-pressure-impregnated (VPI) windings are recommendedfor all units operating in high-humidity environments

capa-Smaller generators, typical of those frequently used at productionfacilities, often cannot provide enough current to operate the instanta-neous trip of magnetic circuit breakers used as main circuit breakersunder certain conditions Manufacturers' data should be obtained forunits under consideration and, if necessary, a short-circuit boost option or

a permanent magnet rotor (PMG) option should be considered Theseoptions will assist the voltage regulator in delivering full exciter voltageand current during periods of severe generator overload and short circuitconditions This helps assure that the generators are capable of deliveringenough current to trip the main circuit breaker

When generators are specified, it should be realized that both ical and electrical requirements differ between units which will be usedfor standby service and units which will be operated continuously Typi-cally, standby units have less copper in their windings than continuousduty units, causing standby units to reach higher temperatures if operatedcontinuously, and thus reducing life Standby units, as classified by mostmanufacturers, are not to be confused with units which are alternatedweekly (or on some other regular basis), but which are operated continu-ously when they are "on line." This operating mode should be considered

Generators typically are Y-connected to provide three phases and aneutral for a 3-phase, 4-wire system The neutral can be grounded orungrounded, but a grounded neutral is usually preferred

Transformers can be delta-connected on both the primary side and thesecondary side for a 3-phase, 3-wire system or delta on the primary sideand Y on secondary side for a 3-phase, 4-wire system Transformers canalso be Y connected on both primary and secondary sides, but such is not

Apparent power is the total power of a circuit and is measured in VA

or kVA (1,000 VA) It is obtained by multiplying voltage and current

Figure 17-1 Three-phase connections.