Off-design

If detailed manufacturer-specified performance curves are not available for a different size of the pump or operating condition, a best estimate of the off-design performance of pumps can be obtained through similarity relationship or the affinity laws. These are ... 

Off-design performance of an expander-compressor set can be derived from the curves typically provided by tlie equipment manufacturer. Actual... 

Modern eompressors ineorporate a parallel wall, vaneless diffuser that yields exeellent off-design performanee. On these maehines, the... 

USING EXPANDER-COMPRESSOR CURVES TO PREDICT OFF-DESIGN PEFORMANCE... 

While it is, of eourse, entirely possible to prediet expander-eompressor off-design performanee using modern eomputer software, tliere are a large number of variables to be eonsidered and it is not always easy to obtain a rapid overview. However, sereening smdies will benefit from graphieal metliods that allow tlie proeess designer to quiekly determine tlie expander and eompressor performanee over the enthe range of operation. 

Most ethylene plants operate continuously with the expander functioning at or near design point. However, by using inlet guide vanes, the expander can still provide optimum performance at off-design conditions. Also, the expansion process generates power, which is used by the compressor. The ethylene enters the expanders at approximately 26 bar (377 psia) and exits at approximately 6 bar (87 psia). The expanders generate over 2,000 hp for gas compression. 

CASE 14 IMPROVING FCC EXPANDER RELIABILITY UNDER OFF-DESIGN CONDITIONS ... 

Rotating machinery usually performs efficiently if it works under design point conditions. However, off-design conditions require a predictive model of the machine s performance. In a FCC power train system, mass flow deviation is quite common for adjusting production capacity to meet the requirements of petrochemical product markets. 

Evaluating off-design performance is strongly recommended for the purpose of improving the operating reliability of both power recovery sets and processing plants. To do this evaluation, a simulation analysis should be based on velocity diagrams, the law of similarity, and expander performance maps. 

It would be an understatement to say that the gas expander turbine forms a major part of the power train. Evaluating off-design performance for the expander installed in the Sinopec Jinan Oil Refinery is presented as an example of an appropriate evaluation method. It should be noted that calculations are based on actual field measurements and plant experience. 

The power train (Figure 8-10) was eommissioned in May 1989. Table 8-1 provides data on the maehine in question. Tables 8-2 and 8-3 show flue gas analysis from the regenerator to the gas expander turbine inlet and the relevant metallurgy, respeetively. There are many possible failure modes in gas expanders, whieh inelude erosion, eatalyst deposition, and exeessive meehanieal vibration. Obviously, these faetors may also eause power loss, and some power trains do indeed fall short of produeing the expeeted power. Nevertheless, in some eases operation at off-design expander system eonditions eould be the primary eause of performanee defieieneies. 

However, exeessive eatalyst flow may be experieneed during oeea-sional proeess upsets. Off-design eondition analysis ean antieipate and prepare for these negative eireumstanees. 

Off-Design Performance—This is an important eonsideration for waste heat reeovery boilers. Gas turbine performanee is affeeted by load, ambient eonditions, and gas turbine health (fouling, ete.). This ean affeet the exhaust gas temperature and the air flow rate. Adequate eonsiderations must be given to bow steam flows (low pressure and high pressure) and superheat temperatures vary with ehanges in the gas turbine operation. 

Water injection, or steam injection systems, are being used extensively to augment power. Corrosion problems in the compressor diffuser and combustor have not been found to be major problems. The increase in work and efficiency with a reduction in NO makes the process very attractive. Split-shaft cycles are attractive for use in variable-speed mechanical drives. The off-design characteristics of such an engine are high efficiency and high torque at low speeds. 

Flow coefficients and pressure coefficients can be used to determine various off-design characteristics. Reynolds number affects the flow calculations for skin friction and velocity distribution. 

Caleulating the performanee of a eentrifugal eompressor in both design and off-design eonditions requires a knowledge of various losses eneountered in a eentrifugal eompressor. 

The calculation of the performance of an axial-flow compressor at both design and off-design conditions requires the knowledge of the various types of losses encountered in an axial-flow compressor. 

Incidence loss. This loss is minimal at design conditions but will increase with off-design operation. These losses vary from about... 

The boundary layer along the blade surfaces must be well energized so that no separation of the flow occurs. Figure 8-16 shows a schematic of the flow in a radial-inflow impeller. Off-design work indicates that radial-inflow turbine efficiency is not affected by changes in flow and pressure ratio to the extent of an axial-flow turbine. 

The off-design characteristics of a turbine are as important to define as the design-point characteristics. Figure 9-29 shows the effect of the speed-to-... 

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