Akber Pasha - Academia.edu (original) (raw)

Papers by Akber Pasha

The International Gas Turbine Congress : book of abstracts, Oct 1, 1987

Am. Soc. Mech. Eng., (Pap.); (United States), 1982

The design of heat recovery systems (HRS) has undergone considerable changes in the last 10 years... more The design of heat recovery systems (HRS) has undergone considerable changes in the last 10 years. Previously the emphasis had been to recover the energy from waste gases in the least expensive manner. However, due to the rising cost and shortage of energy, initial cost of the HRS is not the sole criteria. The emphasis has shifted to recover the maximum amount of energy and to optimize the entire system. This has led to various innovations and developments in the art of heat recovery system design. The paper describes the various option available including multi-pressure boilers, combinations of boilers and feedwater heaters, etc. Economic considerations for various arrangements are discussed which may eliminate some alternates in the preliminary design making the ultimate selection easier. Effects of offload performance is also discussed.

Power; (United States), Oct 1, 1982

In combined-cycle power generation schemes, multiple heat exchangers arranged for maximum heat tr... more In combined-cycle power generation schemes, multiple heat exchangers arranged for maximum heat transfer over the cycle temperature range are replacing the simple single-pressure, heat-recovery boiler. As a result, the heat-recovery systems (HRS) influence the selection of steam tu

A simple, low cost approach to uprating existing steam plants is presented. The proposed uprating... more A simple, low cost approach to uprating existing steam plants is presented. The proposed uprating eliminates or reduces much of the boiler and turbine work required by conventional uprating methods. Waste heat from a gas turbine's exhaust can be transferred into the steam cycle of an existing plant creating a combined-cycle unit with greater output than the combined capabilities of the two independent units and improved overall efficiency. When using or relocating an existing gas turbine the cost of the additional midrange capacity (steam plant uprate plus gas turbine) will be far less than the cost of new fossil capacity. The additional capacity is gained without creating a new emissions source. BENEFITS OF COMBINED CYCLE RETROFIT Existing gas turbines can be integrated into the steam plant's cycle to form an efficient combined cycle plant. The proposed retrofit has a number of benefits that should be of interest to utilities seeking to add new capacity or reduce production costs. The retrofit can result in up to 25% increase in Steam Turbine-Generator output without an increase in boiler main steam flow.

The design of a gas turbine exhaust heat recovery system (HRS) depends upon evaluating various pa... more The design of a gas turbine exhaust heat recovery system (HRS) depends upon evaluating various parameters. Basically for an unfired heat recovery system the heat contained in the gas turbine exhaust is fixed and output is determined based on the system's effectiveness. One of the design objectives is to maximize the output and thus maximize the effectiveness. However, increase in effectiveness will increase required heat transfer surface and thus the cost of the HRS. The increased cost (and benefits) must be evaluated to establish whether the higher effective system is economically justifiable. The evaluation criteria of a heat recovery system involves analysis of various design parameters. This paper presents the general design procedure, the effect of each parameter on the design and basic criteria used to develop the HRS design.

The design of a Heat Recovery Steam Generator behind a gas turbine depends upon various input par... more The design of a Heat Recovery Steam Generator behind a gas turbine depends upon various input parameters such as gas turbine exhaust flow, exhaust temperature, etc. Most of the input parameters are either measured with tolerances or calculated based on experimental correlations. The design of the heat recovery steam generator itself utilizes various correlations and empirical values. The errors or measurement tolerances in these variables affect the performance of the steam generator. This paper describes the various design parameters, the possible magnitude of errors in these parameters and the overall effect on the steam generator's performance. By utilizing the information given in this paper, it is possible to develop a performance envelope based on the possible error margins of the input variables. The steam generator performance can be deemed acceptable if it is within this envelope.

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The heat recovery steam generator (HRSG) is an integral part of the combined cycle power plant wh... more The heat recovery steam generator (HRSG) is an integral part of the combined cycle power plant which includes combustion turbine and steam turbine in addition to heat recovery steam generator. The start-up of the heat recovery steam generator, therefore, has an influence on the start-up of the total plant. The paper discusses various constraints, both external and internal, which affect the Steam Generator start-up and in turn influence the start-up of the total plant. Considerations in the design of the steam generator to accommodate the plant start-up requirements, along with the effect of the cyclic or base loaded operation are also discussed. The paper also presents a procedure which may be adopted in the conceptual design of the plant for an optimized system, a system which can accommodate the total plant start-up requirements without undue constraints on the availability of the full plant output.

Power, 2003

Designing and operating HRSGs for cycling duty requires close cooperation between the manufacture... more Designing and operating HRSGs for cycling duty requires close cooperation between the manufacturer and the user. Once HRSGs are in service, a monitoring system should provide a thorough picture of the mechanisms affecting the life and integrity of the boiler.

The combined cycles plants installed in mid nineties to early 2000s were mainly designed to opera... more The combined cycles plants installed in mid nineties to early 2000s were mainly designed to operate at base load with planned starts. However the realities of the gas deregulation made it imperative to operate them with almost daily cycling. This operational mode change was implemented with much caution and apprehension as to the life expectancy of the HRSGs. The concern became more pronounced when it was found that simple addition of fatigue in the life cycle analysis to the existing creep effect is not enough. The alloy material used for superheaters and reheaters now need to consider the added effect of creep and fatigue interaction, which essentially results in further lowering of the life expectancy. The paper described various damaging mechanisms encountered by the HRSG during fabrication and operation and the impact of cycling on these. However in addition to the traditional Life Cycle Analysis (LCA) , the paper also introduces the concept of a Cycling Operational Analysis (C...

The relative ease of manufacturing and erecting a combined cycle power plant (CCPP) and the consi... more The relative ease of manufacturing and erecting a combined cycle power plant (CCPP) and the considerable shortening of time required to achieve commercial operation has made the CCPP the plant of choice for utilities as well as independent power producers. This popularity has led to many innovations for increasing the power output and the efficiency. Of the three main components of the CCPP—gas turbine (GT), heat recovery steam generator (HRSG) and steam turbine (ST)—the GT has gone through numerous innovations and improvements. The ST traditionally has the capacity to accept higher amounts of steam at higher pressures, so very little improvements to the ST were needed. However, the HRSG, which is sandwiched between the GT and ST, has to undergo many changes due to the impact of higher amounts of gas at higher temperatures from the GT and the requirement of higher-pressure steam at a higher temperature to boost the ST efficiency. HRSGs for the new generation of CCPPs have to be designed with the proper material to accept the higher temperatures and pressures. The new plants are also required to be on line faster and are subjected to cycling operations. These additional constraints require design innovations using advanced analysis techniques. New Combined Cycle Plants The combined cycle units using advanced class of gas turbines from various manufacturers have certain common aspects. These can be listed as: • Higher efficiency at about 60 percent or beyond. However, 60 percent seems to be the limit for conventional Brayton and Rankine cycles. Generally, the size of GT makes it improbable to have higher than 40 percent efficiency for the GT by itself. The steam turbine efficiency adds about another 20 percent. • Higher gas turbine exhaust mass flows and temperatures. Higher GT firing temperatures result in increasing the GT efficiency, however, these also mandate that the exhaust temperature to the HRSG be high so the GT can have an economical size. • Higher GT exhaust temperature can allow for higher steam temperatures. Higher steam temperatures for superheated and reheated steam can deliver better steam turbine efficiency. These requirements pose design challenges for the mechanical integrity of the HRSG. Critical among these are: • Material to withstand higher metal temperatures due to higher gas and steam temperatures • Mechanical stability for higher gas velocity and turbulent nature of the flow • Larger and thicker steam drums and steam piping due to higher steam flows and pressures

Today's merchant plants are required to come on line or shut down at short notice which puts ... more Today's merchant plants are required to come on line or shut down at short notice which puts a strain on the HRSG. This article describes various mechanisms which affect the integrity of the boilers and discusses monitoring requirements to keep the boiler from developing forced shutdowns.

Fast-starting combined-cycle plants are designed for a certain operating life based on a customer... more Fast-starting combined-cycle plants are designed for a certain operating life based on a customer-specified set of operating scenarios . During that design phase, periodic inspection and maintenance procedures to benchmark equipment actual wear and tear should be developed, but seldom are. Without an accurate assessment of remaining equipment life for components subjected to fast and frequent start-ups and Shutdowns, the real operation and maintenance cost is only a guess.