ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 2 Part 10 pptx

Valves, Fittings, and Piping Details 461 (text continued from page 453) Insulation for personnel safety is required only when accidental con- tact of the hot surfaces could be made by personnel within normal work or walk areas. Isolation may be in the form of guards or barriers and, in special cases, warning signs. Hot surfaces associated with natural gas compressors and pumps han- dling volatile flammable fluids should be insulated since the equipment itself is a source of hydrocarbon liquids or gases. Generators, electric motors, and engine-driven equipment such as fire water pumps, wireline units, welding machines, hydraulic equipment, and the like do not them- selves cause the area to become classified from an electrical standpoint, However, they may be in a classified area due to other equipment and thus require insulation or barriers. Turbo-chargers, exhaust manifolds, compressor heads, expansion bottles and the like (including associated piping), which cannot be insulated without causing mechanical failure, are not normally insulated. In these cases, warning signs, barriers, gas detectors, or other methods for the protection of personnel and minimiz- ing exposure to hydrocarbon liquids and gases are acceptable. MISCELLANEOUS PIPING DESIGN DETAILS Target fees Where 90° turns in piping are required, standard long radius ells (ell centerline radius equals 1.5 times pipe nominal diameter) are usually used. In sandy service, the sand has a tendency to erode the metal on the outside of the bend. Target tees, such as shown in Figure 15-25, are often specified for such service. The sand builds up against the bull plug and provides a cushion of sand that is constantly being eroded and subject to deposition by the sand in the flow stream. Chokes The flow of fluid leaving a choke is in the form of a high-velocity jet. For this reason it is desirable to have a straight ran of pipe of at least ten pipe diameters downstream of any choke prior to a change in direction, so that the jet does not impinge on the side of the pipe. 462 Design of GAS-HANDLING Systems and Facilities Often on high-pressure wells two chokes are installed in the flow- line—one a positive choke and the other an adjustable choke. The adjustable choke is used to control the flow rate. If it were to cut out, the positive choke then acts to restrict the flow out of the well and keep the well from damaging itself. Where there are two chokes, it is good piping practice to separate the chokes by 10 pipe diameters to keep the jet of flow formed by the first choke from cutting out the second choke. In practice this separation is not often done because of the expense of sepa- rating two chokes by a spool of pipe rated for well shut-in pressure. It is much less expensive to bolt the flanges of the two chokes together. No data has been collected to prove whether the separation of chokes is justi- fied from maintenance and safety considerations. Whenever a choke is installed, it is good piping practice to install block valves within a reasonable distance upstream and downstream so that the choke bean or disc can be changed without having to bleed down a long length of pipeline. A vent valve for bleeding pressure off the seg- ment of the line containing the choke is also needed. This is particularly true in instances where a positive choke is installed at the wellhead and an adjustable choke is installed hundreds of feet away in a line heater. If block valves are not installed downstream of the positive choke and upstream of the adjustable choke, it would be necessary to bleed the entire flowline to atmosphere to perform maintenance on either choke, Flange Protectors The full faces of flanges never really touch due to the gaskets or rings that cause the seal. The space between the two flange faces is a very Figure 15-25. Target tee. Valves, Fittings, and Piping Details 463 good spot for corrosion to develop, as shown in Figure 15-26. Flange protectors made of closed-cell soft rubber are sometimes used to exclude liquids from penetrating this area. Stainless-steel bands and grease fit- tings are also used. Closed-cell flange protectors are much less expensive than stainless bands. However, if not installed properly they can actually accelerate cor- rosion if a path is created through the material to allow moisture to enter, Flange protectors should not be used in H 2 S service. They may trap small leaks of sour gas and keep them from being dispersed in the atmosphere. Figure 15-26. Flange protector types. 464 Design of GAS-HANDLING Systems and Facilities Vessel Drains If vessel drain valves are used often, there is a tendency for these valves to cut out. As the valve is opened and shut, there is an instanta- neous flow of a solid slurry across the valve that creates an erosive action. Figure 15-27 shows a tandem valve arrangement to minimize this potential problem. To drain the vessel, the throttling valve is shut and one or more drain valves are opened. These valves open with no flow going through them. Then the throttling valve is opened. To stop draining, the throttling valve is closed, flow goes to zero, and the drain valves are shut. The throttling valve will eventually cut out, but it can be easily repaired without having to drain the vessel Vessel drain systems can be very dangerous and deserve careful atten- tion. There is a tendency to connect high-pressure vessels with low-pres- sure vessels through the drain system. If a drain is inadvertently left open, pressure can communicate through the drain system from the high- pressure vessel to the low-pressure vessel. If this is the case, the low pressure vessel relief valve must be sized for this potential gas blowby condition. The liquid drained from a vessel may flash a considerable quantity of natural gas when it flows into an atmospheric drain header. The gas will find a way out of the piping system and will seek the closest exit to atmosphere that it can find. Thus, a sump collecting vessel drains must be vented to a safe location. Figure 15*27. Drain valves for a separator. Valves, Fittings, and Piping Details 465 Open Drains Open, gravity drains should not be combined with pressure vessel drain systems. The gas flashing from vessel liquids may exit an open drain system at any point and create a hazard. On open drain piping leaving buildings, a liquid seal should be installed as further protection to assure that gases flashing from liquids from other locations in the drain system will not exit the system in the building. The elevation of gravity drain systems must be carefully checked to assure that liquids will flow to the collection point without exiting the piping at an intermediate low point. Piping Vent and Drain Valves At high points in piping, vent valves are required to remove air for hydrotesting and for purging the system. At low points, drain valves are required to drain liquids out of the system to perform maintenance. Nor- mally, vent and drain valves are H-in. or %-in. ball valves. Control Stations Whenever it is necessary to control the process level, pressure, temper- ature, etc., a control station is installed. A control station may be as sim- ple as a single control valve or it may contain several control valves, block valves, bypass valves, check valves, and drain or vent valves. Where there is a control valve, block valves are often provided so the control valve can be maintained without having to drain or bleed the pressure from the vessel. Typically, the safety-systems analysis would also call for a check valve at this point to prevent backflow. Drain or vent valves are often installed to drain liquid or bleed pressure out of the sys- tem so that the control valve can be maintained. In smaller installations drain and vent valves may not be provided and the line is depressured by backing off slightly on flange bolts (always leaving the bolts engaged until all pressure is released) or slowly unscrewing a coupling. This is not a good practice although it is often used for small-diameter, low-pressure installations. Bypass valves are sometimes installed to allow the control valve to be repaired without shutting in production. On large, important streams the bypass could be another control valve station. Manual bypass valves are 466 Design of GAS-HANDLING Systems and Facilities more common. The bypass valve could be a globe valve if it is anticipat- ed that flow will be throttled through the valve manually during the bypass operation, or it could be an on/off valve if the flow is to be cycled. Because globe valves do not provide positive shutoff, often globe-bypass valves have a ball or other on/off vaive piped in series with the globe valve. The piping around any facility, other than the straight pipe connecting the equipment, is made up primarily of a series of control stations. Flow from one vessel goes through a control station and into a piece of pipe that goes to another vessel. In addition to considering the use of block valves, check valves, etc., all control stations should be designed so that the control valve can be removed and any bypass valve is located above or on a level with the main control valve. If the bypass is below the con- trol vaive, it provides a dead space for water accumulation and corrosion. CHAPTER 16 Prime Movers * Both reciprocating engines and turbines are used as prime movers in production facilities to directly drive pumps, compressors, generators, cranes, etc. Reciprocating engines for oil field applications range in horsepower from 100 to 3,500, while gas turbines range from 1,500 to in excess of 75,000. Prime movers are typically fueled by natural gas or diesel. Dual fuel turbine units exist that can run on natural gas and can automatically switch to diesel. So-called "dual fuel" reciprocating engines run on a mixture of diesel and natural gas. When natural gas is not available, they can automatically switch to 100% diesel. Most prime movers associated with producing facilities are typically natural gas fueled due to the ready availability of fuel. Diesel fueled machines are typically used to provide stand-by power or power for intermittent or emergency users such as cranes, stand-by generators, firewater pumps, etc. Due to the extremely wide variety of engines and turbines available, this discussion is limited to those normally used in production facilities. The purpose of this chapter is to provide facility engineers with an under- standing of basic engine operating principles and practices as necessary for selection and application. The reader is referred to any of the many texts available on engine and turbine design for more in-depth discussion of design details. ^Reviewed for the 1999 edition by Santiago Pacheco of Paragon Engineering Services, Inc. 467 468 Design of GAS-HANDLING Systems and Facilities RECIPROCATING ENGINES Reciprocating engines are available in two basic types—two-stroke or four-stroke cycle. Regardless of the engine type, the following four func- tions must be performed in the power cylinder of a reciprocating engine: 1. Intake—Air and fuel are admitted to the cylinder, 2. Compression—The fuel and air mixture is compressed and ignited. 3. Power—Combustion of the fuel results in the release of energy. This energy release results in increase in temperature and pressure in the cylinder. The expansion of this mixture against the piston converts a portion of the energy released to mechanical energy. 4. Exhaust—The combustion products are voided from the cylinder and the cycle is complete. In this manner the chemical energy of the fuel is released. Some of the energy is lost in heating the cylinder and exhaust gases. The remainder is converted to mechanical energy as the expanding gases move the piston on the power stroke. Some of the mechanical energy is used to overcome internal friction or to sustain the process by providing air for combustion, circulating cooling water to remove heat from the cylinder, and circulat- ing lube oil to minimize friction. The remainder of the energy is available to provide external work. The amount of external work that can be devel- oped by the engine is termed its "brake horsepower" or bhp. The amount of work required to sustain the engine is termed its "friction horsepower" or fhp. The work developed by the power cylinders is termed the "indi- cated horsepower" or ihp. The indicated horsepower is the sum of both the friction horsepower and the brake horsepower. ihp = bhp + fhp Four-Stroke Cycle Engine The four-stroke cycle engine requires four engine strokes or 720 degrees of crankshaft rotation to complete the basic functions of intake, compression., power, and exhaust. All flow into and away from the cylin- ders is controlled by valves directly operated by a camshaft that is driven at 1 A engine speed. Figures 16-1 and 16-2 illustrate a cross section and an idealized P-V diagram for a four-cycle spark-ignited engine, respectively. Prime Movers 469 Figure 16-1. Cross section of 4-cycle, spark-ignited engine. Intake Stroke (Point 1 to Point 2) With the intake valve open, the piston movement to the right creates a low pressure region in the cylinder, which causes air and fuel to flow through the intake valve to fill the cylinder. Compression Stroke (Point 2 to TDC) The intake valve is now closed as the piston moves from the bottom dead center (BDC) to top dead center (TDC), compressing the fuel/air mixture. At Point 3, just prior to TDC, a spark ignites the fuel/air mixture and the resulting combustion causes the pressure and temperature to begin a very rapid rise within the cylinder. Power Stroke (TDC to Point 4) Burning continues as the piston reverses at TDC and pressure rises through the first portion of the "power" or "expansion" stroke. It is the increase in pressure due to burning the fuel that forces the piston to the right to produce useful mechanical power. The piston moves to the right until BDC is reached. 470 Design of GAS-HANDLING Systems and Facilities VOLUME % of Piston Displacement Figure 16-2. Idealized P-V diagram for a 4-cycie, spark-ignited engine. Exhaust Stroke (Point 4 to Point 1) With the exhaust valve open, the upward stroke from BDC to TDC creates a positive pressure within the cylinder, which forces combustion products from the cylinder on the "exhaust" stroke. Two-Stroke Cycle Engine Two-stroke cycle engines require two engine strokes or 360 degrees of crankshaft rotation to complete the basic functions of intake, compres- sion, power, and exhaust. Figures 16-3 and 16-4 illustrate a cross section and a P~V diagram for a two-cycle engine, respectively. In this common type of engine, the piston in its traverse covers and uncovers passages or [...]... internal-combustion engines in the manner in which the expanded gases are employed The principle of operation is to direct a stream of hot gases against the blading of a turbine rotor As shown in Figures 16-6 and 16-7, the gas turbine consists of 478 Design of GAS-HANDLING Systems and Facilities Figure 16-6 Schematic of single-shaft gas turbine Figure 16-7 Cutaway view of typical turbine Prime Movers 4J9 three basic... turbine that provide the 484 Design of GAS-HANDLING Systems and Facilities Figure 16-13 Air compressor discharge pressure as a function of its speed Figure 16-14 This two-shaft turbine illustrates how power turbine wheels may be placed on separate shafts Prime Movers 485 output work and the speed of the wheels of the power turbine driving the air compressor are independent of each other The output shaft... the "gas producer") at Point 1 under normal atmospheric pressure and temperature, PI and Tl It is then isentropically compressed to Point 2 where the pressure and temperature are now P2 and T2 From Point 2 the air flows into the combustion chamber where fuel is injected and burned at constant pressure, raising the temperature to T3 and expanding the volume to V3 From the combustion chamber the heated... the hot exhaust gases, and 10% is lost to radiation and the lube oil system Simple cycle industrial gas turbines burn more fuel than comparable reciprocating machines There are, however, several methods available to 480 Design of GAS-HANDUNG Systems and Facilities Figure 16-8 This Brayton cycle describes the basic operation of a gas turbine Figure 16-9 The regeneration cycle of a gas turbine uses recovered... periods of negative pressure to induce air nor of high positive pressure to completely expel exhaust gases While a twocycle engine has the ability to produce power with each down motion of the piston, it is at the expense of some external means of compressing enough air to fill the cylinder and to expel the combustion products from the previous cycle ("scavenge" the cylinder) as well 4 72 Design of GAS-HANDLING. .. 4 Reduced maintenance 5 Generally simplified maintenance procedures 6 Reduced overall installation cost due to size and weight Disadvantages 1 Scavenging system required to allow self-starting 2 Prone to detonation at high ambient temperatures 474 Design of GAS-HANDLING Systems and Facilities 3 Lower exhaust temperature reduces available waste heat, 4 Power cylinders require frequent balancing 5 Very... section of a two-cycle engine, only a trained eye can identify the engine porting that distinguishes it from a four-cycle engine (Courtesy of Electro-Motive Division, General Motors Corp., and Stewart and Stevenson Services, Inc.) ports in the lower cylinder wall that control the inflow of air and the outflow of exhaust gases This type of engine is called a piston-ported engine Inasmuch as both intake and. .. output of the steam turbine The energy used to drive the steam turbine contributes to the overall thermal efficiency as the steam is generated without the expense of any additional fuel consumption A combined cycle system can increase overall thermal efficiency to the 40% range, Figure 16 -10 This schematic illustrates how waste heat can be recovered and used to heat process fluids 4 82 Design of GAS-HANDLING. .. Design of GAS-HANDLING Systems and Facilities Figure 16*11 The combined cycle of waste heat recovery can be used to generate steam, Effect of Ambient Conditions Available horsepower from a gas turbine is a function of air compressor pressure ratio, combustor temperature, air compressor and turbine efficiencies, ambient temperature, and barometric pressure High ambient temperatures and/ or low barometric... example, an 1100 -hp gas turbine (Solar Saturn) handles approximately 12 lbm/sec or almost 22 tons of air per hour A comparable reciprocating engine will use only about 1A that amount Piston engines use almost all the air for combustion (a small amount may be used for cylinder scavenging), while turbines use only about 25 % of the air flow for combustion The remainder, or about 75%, is used for cooling and to . reached. 470 Design of GAS-HANDLING Systems and Facilities VOLUME % of Piston Displacement Figure 16 -2. Idealized P-V diagram for a 4-cycie, spark-ignited engine. Exhaust Stroke . well. 4 72 Design of GAS-HANDLING Systems and Facilities Figure 16-4. Idealized P-V diagram for a 2- cycle engine. The Compression Stroke As the piston begins its leftward stroke . wellhead and an adjustable choke is installed hundreds of feet away in a line heater. If block valves are not installed downstream of the positive choke and upstream of the adjustable