Gas expanders

When energy costs are high, expanders are used more than ever. A quickie rough estimate of actual expander available energy is 

AH = Actual available energy, Btu/lb Cp = Heat capacity (constant pressure), Btu/lb °F T = inlet temperature, °R Pi, P = Inlet, outlet pressures, psia K = Cp/C, 

For large expanders. Equation 1 may be conservative. A full rating using vendor data is required for accurate results. Equation 1 can be used to see if a more accurate rating is worthwhile. 

For comparison, the outlet, temperature for gas at critical flow across an orifice is given by 

The proposed expander may cool the working fluid below the dew point. Be sure to check for this. 

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The previous equation is only valid as long as there is no compositional change of the gas between the subsurface and the surface. The value of E is typically in the order of 200, in other words the gas expands by a factor of around 200 from subsurface to surface conditions. The actual value of course depends upon both the gas composition and the reservoir temperature and pressure. Standard conditions of temperature and pressure are commonly defined as 60°F (298K) and one atmosphere (14.7 psia or 101.3 kPa), but may vary from location to location, and between gas sales contracts. 

The elastomer process is very similar to the Dennis process. It involves a number of steps in which a gas, formerly carbon dioxide and now fluorocarbon, is mixed with a plastisol under pressure. When released to atmospheric pressure, the gas expands the vinyl compounds into a low density, open-ceUed foam which is then fused with heat. 

W. C. Meyers and L. M. Stettenbenz, "The Power Recovery Gas Expander," paper 64-PET, presented at Petroleum Mechanical Engineering Conference of AS ME, Sept. 1964. 

Hydroearbon gas expanders range in the order of 100 hp to 8,000 and more hp. The majority of these maehines are usually designed for power reeovery duty, with a proeess eompressor direetly driven by the expander. The gas is usually expanded from an inlet pressure in the 100 bar to 50 bar range, down to outlet pressures in the 50 to 15 bar range. This results in an expansion ratio of 2 1 to 4 1, a very suitable expansion for a single-stage expander. Typieal effieieneies range from 84% to 86%. 

The elevated pressure in Proeess 2, shown sehematieally in Figure 4-6, again ineludes a tail gas expander as part of the eombustion air eompressor train (Figure 4-7). A typieal eontrol and instrument diagram for Proeess 2 is given in Figure 4-8. The nomenelature deseribed in Proeess 1 also pertains here. 

Tail gas expanders are thus an integral part of modern nitrie aeid plants. However, these turboexpanders are also part of a eombined turbomaehinery train eomprised of a prime mover and two or more eompressor easings. 

Tail gas expander without or with adjustable inlet guide vanes (11) and guide vane setting motor (12), labyrinth shaft seals with air sealing... 

Although this hook deals almost exclusively with modern turho-expanders, the tail gas expander in nitric acid plants enjoys a somewhat special relationship with the compressors that are almost always associated with this turhotrain. 

Seleetion of a tail gas expander depends on the temperature and pressure on the inlet eonditions of the nitrous gas. The pressure equals that of the eompressor outlet minus the pressure losses in the eyele. Depending on proeess and plant size, these losses amount to 0.3-2.0 bar. The inlet temperature may vary widely from plant to plant. Figure 4-19 shows an expander in the 10,000 kW power output eategory. 

Figure 4-19. Tail gas expander for 7.4 bar, 700°C maximum inlet and power output at 10,000 kW.

Figure 4-20 shows typical duty ranges of hot gas expanders offered by one major manufacturer. 

Most of the large tail gas expanders are axial type. Multistage uncooled expanders, illustrated in Figures 4-21 through 4-23, are... 

Figure 4-21. Tail-gas expander for a 945-t/day two-pressure nitric acid plant P = 11,200 kW, n = 5,850 rpm. (Source GHH-Borsig.)...

Adjusting the stator blades leads to full are admission at all operating eonditions, and blade exeitations due to partial admission are thereby avoided. On well-designed hot gas expanders the adjusting meehanism is generally loeated between the blade earrier and outer... 

The smaller framed hot gas expanders are often equipped with two control valves, each controlling a segment of 30% and 20%, respectively, of the total inlet. It is possible to vary the inlet cross-sectional area of these segments, within limits, by later replacing individual nozzles with blind fillers or vice-versa. Other machines may feature full arc admission without control valves. 

Just as compressors are an indispensable element of the turbotrain of which the hot gas expander is a part, so is the principal driver. Therefore, the following briefly highlights the more important aspects of these drivers. 

Whenever a hot-gas expander is employed, the thrust bearing of the compressor train is located in the expander. This coupling arrangement minimizes the coupling overhung mass, thus reducing the risk of undue... 

Depending on plant size, the hot gas expander is supplied either with (Figure 4-40) or without variable inlet guide vanes. Adjustment features are not required in small plants where throughput at eonstant or full-load eapaeity is antieipated. 

Power is generated by the pressurized gas expanding through an 11,000 rpm single-stage, radial-inflow turbine expander, which drives a synchronous generator. Exhaust gas from the expander is liquified by air-cooled condensers and is pumped back to the heat exchangers to repeat the cycle. 

Figure 4-54. Cross-section of an Elliott single-stage hot gas expander.

The problem of eeonomie justifieation for the required investment is a matter that depends on the individual refinery situation. However, it is almost eertain that more refiners will be installing sueh equipment in the future. The ability of the equipment to operate satisfaetorily has been amply demonstrated and this will inerease the refiner s desire to eonsider and use flue gas expanders for FCC installations (Figure 4-55). For typieal sizes, refer to Figure 4-56. 

Figure 4-55. Elliott hot gas expander installed in an FCC power recovery string rated at 42,000 hp (31,330 kW).

Figure 4-56. Typical size chart for hot gas expanders in FCC units. (Source Elliott Company.)...

A satisfactory smdy of the application of the flue gas expander to a particular fluid catalytic cracking unit must include the following steps ... 

How long does it take to repau a hot gas expander, and what are tlie essential activities that are involved in effecting such repairs These valid questions are best answered by highlighting a specific example. 

Upon completion of tlie inspection process the results were reviewed, and the following requirements were defined for each major component of the hot gas expander and its respective subassemblies ... 

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