Hot Gas Expanders

Power recovery trains recover energy from the flue gas. The FCC starts to resemble a large jet engine air is compressed into a combustion zone and expanded across a turbine. Power recovery increases the efficiency of the unit but adds one more mechanical device to an already long list. Since they are too big to bypass, power trains need to be as reliable as the rest of the unit. 

The main concerns in the design and operation of a power recovery system are catalyst fines and temperature. Catalyst fines will lead to serious blade wear, deposits, power loss, and rotor vibration. Deposit occurs most frequently where flue gas velocities are at maximum levels, such as blade outer diameter. 

Causes of Blade Wear, Power Loss, and Rotor Vibration 

Increase in catalyst loading to the regenerator cyclones and third-stage separator 

Sodium and vanadium in the catalyst, resulting in the formation of low-melting eutectic, which makes the catalyst very sticky even at high temperatures 

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

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. 

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

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.)...

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 ... 

Determining Susceptibility of NijSj Formation Using a Hot Gas Expander Analysis... 

Until the late 1990s, Waspaloy was still the best alloy available for the majority of hot gas turboexpanders used in industry and, until recently, it continued to offer the many special characteristic needed for hot gas expander applications. However, a new development followed in 2000 when the Ebara Corporation (Japan) released data on a nickel-base superalloy. 

Figure 4-121. Hot gas expander wheel and shaft constitute the rotor. Note the splined engagement.

Proeeedings of Hot Gas Expander Conferenee, Elliott Company, Jeannette, Pennsylvania, 1992. 

A general speeifieation for modern power reeovery (hot gas) expanders in FCC serviee is given below. This speeifieation may be used as either a starting point for a plant-speeifie write-up, or as a generalized doeument that faeilitates bid evaluation efforts. Remember that operational referenees are a shared vendor/eontraetor/plant operator (owner) responsibility. 

Figure 5-13. The Elliott TH expander is a single-stage hot gas expander with an axial inlet and vertical exhaust. The TH-line expanders range in horsepower from 4,000 to 50,000 and are of overhung rotor construction. The expander consists of four major components the inlet casing and its supports, the exhaust casing, the bearing housing and support system, and the rotor.

Figure 5-14. Axiai entry hot gas expander, with principai parts identified.

Field testing of hot gas expanders is normally intended to establish the unit meets quoted performanee or as a set point for future eompari-sons, whieh eould be used to antieipate or plan maintenanee work. 

Sinee there is no standard eode whieh governs the testing of hot gas expanders, related or otherwise applieable eode praetiees should be used whenever possible. Thus, the relevant portions of the ASME PTC-6 and ASME PTC-IO-ABCD should be eonsidered. In partieular, attention should be given to the paragraphs relating to aeeuraey of instruments, speeifieally those in seetions 0.01 and 0.02 of PTC-6 on measurement uneertainty, whieh are quoted as follows ... 

The environment of a hot gas expander is partieularly harsh due to eatalyst fines and, therefore, some eompromise on instrumentation and test aeeuraey may be neeessary. The following speeifieation is intended to determine the baste requirements that should be eonsidered in any test. Test aeeuraey ean be determined onee the aetual plant eonfigura-tion is determined. 

Radial fit bolts are a special feature used in all types of turbomachinery. They are of increasing importance to users that are both reliability-minded and concerned about life-cycle costs. The radial fit coupling bolt was originally developed for coupling steam turbine shafts by Pilgrim International in Oldham, UK. It is equally useful to coupling other equipment, including, of course, hot gas expander shafts. 

Load sharing or selective load shedding is of interest to many users of hot gas expanders. A particularly successful European FCC application is illustrated in Figure 6-43. The addition of an expander-generator set to the FCC unit at a major refinery presented a challenge because a trip of the expander could upset the process. The company that is the subject of this application case study, GHH Borsig, solved this problem with the installation of a computerized control system and through computer simulation of trips. 

Compressors driven by steam mrbines, gas turbines, hot gas expanders, or variable speed electric motors usually employ variable rotation speed for capacity control. Generally, the surge limit slope varies witli rotation... 

A hot gas expander is typieaiiy deseribed by a map of shaft power versus mass flowrate (Figure 7-3). Notiee that there are four parameters ehanging in this partieuiar map w, J, Pj, and Tj. Figure 7-3 is most useful when the family of eurves (whieh are for a eonstant rotational speed) reduees to a single eurve in a two-dimensional spaee. Usually, expander eharaeteristies are a very weak funetion of angular speed. However, in eases where the variations due to rotational speed are important, a third dimension is required. This dimension should be equivalent speed, Ng. 

In a hot gas expander, the major problems associated with catalyst fines are centered on erosion of components and particulate plugging. Either problem can cause machine vibration and sometimes power train emergency shutdown. Because many failures have resulted from these factors, machinery manufacturers recommend that the maximum permissible solids concentration upstream of an expander not exceed 200 ppm. It is further stipulated that 97% of the particles be smaller than 10 p. Allowing concentrations of 160 ppm with 95% of the particle less than 10 p is considered reasonable. 

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