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

486 Design of GAS-HANDLING Systems and Facilities low as possible, it may be necessary to unload the driven equipment dur- ing start-up. Figure 16-16 shows the performance characteristic of a split-shaft tur- bine where the only power output limitation is the maximum allowable temperature at the inlet of the turbine section. In actual practice a torque limit, increased exhaust temperature, loss of turbine efficiency, and/or a lubrication problem on the driven equipment usually preclude operating at very low power turbine speeds. The useful characteristic of the split-shaft engine is its ability to supply a more or less constant horsepower output over a wide range of power turbine speeds. The air compressor essentially sets a power level and the output shaft attains a speed to provide the required torque balance. Compressors, pumps, and various mechanical drive systems make very good applications for split-shaft designs. Effect of Air Contaminants The best overall efficiency of a turbine can be ensured by maintaining the efficiency of the air compressor section. Conversely, allowing the air compressor efficiency to deteriorate will deteriorate the overall thermal efficiency of the turbine. Air compressor efficiency can be drastically reduced 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 air compressor and reduce its compression efficiency. The effect will be decreased mass flow, reduced compressor discharge pressure, reduced horsepower, 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 appropriate filter system to use. Desert environments where a large amount of sand particles could be expected in the ambient air may use an automatic roll type of filter that allows new filter material to be rolled in front of the inlet without frequent shut-downs to change filters. Arctic or extremely cold locations may use pad type filters, snow hoods to prevent blockage, and exhaust recirculation to prevent icing. Filter assemblies for offshore marine environments may include weather louvers, demister pads, and barrier elements for salt and dirt removal. Screens may be used for insect removal 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 the air 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. from forming. In general, frequent detergent washing will ensure compressor cleanliness. Steam cleaning with an appropriate detergent is also very effective. Abrasive cleaning should be avoided and only be necessary as the result of improper frequency or technique of detergent washing. ENVIRONMENTAL CONSIDERATIONS Air Pollution Exhaust emissions will vary with the type and age of engine and the fuel used. Current environmental regulations must be consulted. It may be necessary to submit a permit to install the new equipment. Each coun- try, state, or county has variations of the maximum emissions. In general, liquid fueled engines tend to have increased emission lev- els of particulates and unburned hydrocarbons over those of gaseous fueled engines. Due to the large quantities of excess air, gas turbines tend to have lower emission levels of particulates and unburned hydrocarbons than reciprocating engines. Gas turbines do, however, tend to produce greater quantities of nitrogen oxides (NO X ). The formation of NO X depends on combustion temperature and residence times at high tempera- tures, both of which are higher in gas turbines than in engines. Engines, 488 Design of GAS-HANDLING Systems and Facilities on the other hand, tend to have greater concentrations of carbon monox- ide, CO, in their exhausts. Fuel quality will greatly affect emissions and can also have consider- able effect on engine life. Manufacturers' specifications will generally specify fuel quality for proper operation. In addition to carbon monoxide (CO) and unburned hydrocarbons (UHC), the most significant products of combustion are the oxides of nitrogen (NO X ). At high temperatures, free oxygen not consumed during combustion reacts with nitrogen to form NO and NO 2 (about 90% and 10% of total NO X , respectively). Improvements in engine and turbine design, along with the use of aux- 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. When a hydrocarbon fuel such as natural gas is burned in an engine or turbine, 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, in order to obtain maximum thermodynamic efficiency, the engine would be designed for a slightly greater than stoichiometric mixture. Because air and fuel are never perfectly mixed at the time of ignition, excess air must be present to burn all the fuel. The normal amount of excess air that achieves this efficiency is around 15-20%. Under these conditions, Fig- ure 16-17 shows that a relatively large amount of NO X will be formed. NO X emission controls in large engines and turbines are based on the same principles. However, special designs must be applied to accommo- date differences in the combustion process. Methods to control NO X include the following. NO X Reduction in Engines 1. Lean Burn As shown in Figure 16-17, at very high A/F ratios—greater than 30:1— the production of NO X can be very low. The problem with simply increasing A/F ratio is that, because the air/fuel mix is not uniform, 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 design solves 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, and ignites the fuel. 2. Catalytic Converter Catalytic converters are designed to oxidize the unburned UHCs and CO. The resulting combustion (oxidation) converts them into 490 Design of GAS-HANDLING Systems and Facilities water and CO 2 . A recent catalytic converter design called three-way converters also controls NO X using a reduction process. Three-way converters contain two catalytic "bricks," one for reduction and the other one for oxidation. The oxidation process with CO takes place as; 2CO + O 2 <-» 2CO 2 The oxidation of UHC is: 2O 2 + CH 4 <-» CO 2 + 2H 2 O And finally, the reduction of NO X with CO results in N 2 and CO 2 : NO 2 + CO <-» NO + CO 2 2NO + 2CO <-» N 2 + 2CO 2 Catalytic reactions occur when the temperature exceeds 500°~6QO°F (260°-316°C). Normal converter operating temperatures are 9QO°-1200°F (482°-649°C). Excessively rich A/F ratio causes converter operating tempera- tures to rise dramatically, thus causing converter meltdown. On the other hand, if the A/F ratio is too lean, the excess O 2 will react with the CO, and the reduction of nitrogen with CO will not take place. Thus, catalytic converters cannot be used where there is excess air. 3. Selective Catalytic Reduction (SCR) Selective catalytic reduction is based on selective reactions of a continuous gaseous flow of ammonia or similar reducing agents with the exhaust stream in the presence of a catalyst. The reaction that occurs is as follows: 4NH 3 + 6NO <-» 5N 2 + 6H 2 O SCR units require handling, storage, and continuous injection of the reducing agent. The temperature level is critical because the SCR operates in a narrow temperature range between 550°~750°F (260°-399°C), and thus an exchanger is necessary to cool the exhaust 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 NO X control. Water or steam is injected into the primary combustion zone. This method has been used effec- tively in the past. Current installations are using this system when the water or steam is readily available or if they are already part of the process. Maintenance costs are higher when compared with dry control, because this method requires high quality water. If high quality water is not used, the corrosion associated with dissolved minerals in the water may prematurely damage the turbine. 2. Lean Premised Combustion When air and fuel are mixed and burned in standard turbine com- bustion systems, incomplete mixture occurs. Areas of rich A/F ratios exist, which cause local high temperatures called "hot spots." Nor- mal turbine combustion temperatures can reach 2800°F (1538°C). Because NO X formation rate is an exponential function of tempera- ture, decreasing the combustor temperature can substantially reduce NO X production. One method of reducing hot spots is to premix a lean A/F ratio prior to combustion. Lower temperature levels are achieved by using stages (multiple sets of air and fuel injectors) and by adding special instrumentation to control the appropriate propor- tion of air and fuel. Excess air is used to further reduce the overall flame temperature. Using this approach, most gas turbine manufac- turers are able to guarantee about 25 ppmv. 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. There- fore, they may not be practical and may be too costly to install and operate compared with other methods. 4, Catalytic Combustor A recent innovation includes use of a catalytic combustor. Several tests with large turbines indicate that this alternative can reduce NO X emission to less than 5 ppmv. The catalyst inside the combustion chamber actually causes a portion of the fuel to burn without the pres- ence of a flame. This significantly reduces combustion temperature. 492 Design of GAS-HANDLING Systems and Facilities Based on the results of the catalytic combustor, there is an effort under way to develop this concept into a practical, field-proven technology. 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 to this problem during the design phase. When a prime mover installation is planned, enough silencing should be installed to ensure that the noise level will be acceptable to the community and meets all governmental requirements. The requirements will vary substantially depending upon such factors as location, population density, operating personnel in the area, etc. CHAPTER 17 Electrical Systems* This chapter introduces some concepts concerning electrical system design and installation that are particularly important from the standpoint of safety and/or operational considerations for production facilities. The reader is referred to texts in electrical engineering and to the various codes and standards listed at the end of this chapter for a more detailed description concerning the design of electrical circuits, sizing conductors, and circuit breakers, etc. This chapter is meant merely as an overview of this complex subject so that the project facilities engineer will be able to communicate 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 local utility company. For onshore facilities the power is generally purchased from 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. 493 494 Design of GAS-HANDLING Systems and Facilities is no existing utility power distribution, an on-site generating unit may be considered. A standby generator may be required if utility power is not sufficiently reliable. The standby generator may be sized to handle either the total facility load or only essential loads during periods of utility power failure. In the case of an offshore facility, electrical power is generally gener- ated on site by engine- or turbine-driven generator sets using natural gas or diesel as fuel. Most installations are designed to handle the total elec- trical load even if one generator is out of service. To minimize the size of standby equipment, some facilities have a system to automatically shed non-essential loads if one generator is out of service. Some offshore facilities are furnished power from onshore via high-voltage cables. The cables are generally laid on the ocean floor and are buried in shallow water. In some cases a single cable (usually three-phase) is used for such applications to minimize initial project cost. However, if a fault develops in this single cable, the facility could be shut in for extended periods of time. To avoid extended shut-ins, either a spare or alternate cable can be installed 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 gener- ating plants, overhead transmission lines, power substations which reduce 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). The power from the distribution line voltage is converted to facility distribution voltage using a "step-down" transformer, providing power to facility switchgear and motor control centers. The facility distribution voltage selection depends upon the length of the distribution system, the size, 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 transformers deliver these voltages from the facility distribution system. Electrical Systems 495 The electrical distribution system design and equipment selection must consider requirements of the utility company for protection and metering. Available short circuit currents from the utility distribution network to the primary of the facility's main transformer must be considered in selecting circuit protection devices for the facility distribution system, Electrical Generating Stations Where electricity is generated in the facility, generator sizing should consider not only connected electrical loads, but also starting loads and anticipated and non-anticipated expansions. In most installations this is done by developing an electrical load list itemizing the various loads as either 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 air conditioning, whichever is larger. Intermittent loads would include quar- ters kitchen equipment, washdown pumps, cranes, air compressors and similar 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 the intermittent loads and an allowance for future demand. Standby loads do not add to generator demand as they are activated only when another load is out of service. Generators must be sized to handle the starting current associated with starting 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 sev- eral large motors, the starting load must be considered. In calculating generator loads it must be remembered that each motor will only draw the load demanded by the process. It is this load and not the nameplate rating of the motor that should be used in the load list. For example, even though a pump is driven by a 100 hp motor, if the process conditions only demand 75 hp, the total load that will be demanded from the generator is 75 hp. Generators are normally provided with static voltage regulators capa- ble of maintaining 1% voltage regulation from no load to full load. While random ("mush") wound stators are acceptable for smaller units, formed coils are normally preferred for generators of approximately 150 kW or larger. Vacuum-pressure-impregnated (VPI) windings are recommended for all units operating in high-humidity environments. [...]... Electrical Systems Figure 17-3 Hazardous area classifications used in U.S and Canada, in accordance with Article 500, NEC Code—1984 (Courtesy of R Stahl, Inc.] 501 5 02 Design of GAS-HANDLING Systems and Facilities Electrical Systems 503 recommends that mixtures containing less than 25 % hydrogen sulfide be considered Group D; above this level they should be considered Group C Designating the "group" of a...496 Design of GAS-HANDLING Systems and Facilities Smaller generators, typical of those frequently used at production facilities, often cannot provide enough current to operate the instantaneous trip of magnetic circuit breakers used as main circuit breakers under certain conditions Manufacturers' data should be obtained for units under consideration and, if necessary, a short-circuit... properties similar to those of propane may exist Group IIA most closely matches the U.S and Canada Group D Group IIB is applicable to above-ground installations where hazards may exist due to gases or vapors with flammability properties similar to those of ethylene, and most closely matches the U.S and Canada Group C Group IIC is applic- 504 Design of GAS-HANDLING Systems and Facilities able to above-ground... (natural or artificial) that is sufficient to prevent the accumulation of significant quantities of vapor-air mixtures in concentrations above 25 % of their lower flammable (explosive) limit (LFL) 510 Design of GAS-HANDLING Systems and Facilities Figure 17-13 Compressor or pump in an adequately ventilated, enclosed area (top), and compressor or pump in an inadequately ventilated, enclosed area (bottom)... to those of hydrogen and acetylene Group IIC includes both U.S and Canada Groups A and B To promote uniformity in area classifications for oil and gas drilling and producing facilities, the American Petroleum Institute developed RP 500, "Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1, and Division 2. " Figures... Y-connected secondary Power 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 498 Design of GAS-HANDLING Systems and Facilities where E = line-to-line voltage, volts I = line current, amperes (amps) "Active power" is the portion of the apparent power that is consumed by the load to produce... occur in normal operations is designated Zone 1 An area in which an explosive gas-air mixture is less likely to occur, and if it does occur will exist only for a short time, is designated Zone 2 Zone 0 and Zone 1 correspond to Division 1 in the U.S and Canada System Zone 2 is equivalent to Division 2 Europeans characterize substances by Groups designated as I, IIA, IIB, and IIC Group I is applicable... first, one must designate the type of hazard or "class" that may be present—gas, dust, or fiber; second, one must designate the specific "group" of the hazardous substance; third, one must determine the probability that the hazardous substance will be present Figure 17-3 is a diagram showing the various classes and groups that are used in the U.S and Canada In oil and gas production facility design almost... Systems and Facilities Short Circuit Currents To avoid damage to equipment and harm to personnel, electrical components of the facility power system must be selected to withstand available short circuit currents and to isolate facility circuits quickly When a short circuit occurs, load impedance no longer limits current Only the power source capability and internal impedance limit the amount of short... encbsure (Reprinted with permission from API RP 500.) Electrical Systems 507 Figure 17-9 Process equipment vent in a nonencbsed, adequately ventilated area (top), ami instrument or control device vent in a nonenctosea, adequately ventilated area (bottom) (Reprinted with permission from API RP 500.) 508 Design of GAS-HANDLING Systems and Facilities Figure 17-10 Relief valve in a nonenclosed, adequately . CH 4 <-» CO 2 + 2H 2 O And finally, the reduction of NO X with CO results in N 2 and CO 2 : NO 2 + CO <-» NO + CO 2 2NO + 2CO <-» N 2 + 2CO 2 Catalytic reactions. "bricks," one for reduction and the other one for oxidation. The oxidation process with CO takes place as; 2CO + O 2 <-» 2CO 2 The oxidation of UHC is: 2O 2 + CH 4 . into 490 Design of GAS-HANDLING Systems and Facilities water and CO 2 . A recent catalytic converter design called three-way converters also controls NO X using a reduction process.