Having a battery of parallel operating pumps with variable speed drive system (VSDS) saves a great deal of energy especially for big pumps with large throughputs. Back in 80s and even 90s, however, relying on variable speed drive pumps with high target reliability and 100% availability was a big task mainly due to the complexities of the associated power electronics and the control issues. In the present era, sophisticated control equipment and philosophy allows taking the full advantage of VSDS pumping system whereby the pump generates a dynamic head only as much as plant requires. As a result the operator ends up in saving substantial operating expenditure in terms of energy bills and simultaneously reduces the environmental impact due to reduction in CO2 emission associated with the power generation. The larger the plants, the larger the consequent savings thus demanding due design attention at early stages.

The proceeding paper discusses the salient design and operational aspects of a large cooling sea water project in Ras Laffan, one of the largest industrial sites in the world, situated 80 km North of Doha in the state of Qatar. This project is unique in terms of its seawater throughput of nearly 1 Million m3/hr by variable speed drive pumps, through a set of very large diameter GRP pipelines to various end users spread throughout the industrial city, with varying seawater demand. These series of 27 pumps each around 7.5 MW with a future provision of additional 6 pumps are located in three separate pump houses. The project also boasts one of the largest electro chlorination plants in the world. The paper describes in sufficient detail the energy saving aspects of a large VSDS driven cooling water system and the associated reliable control with no allowable downtime that will jeopardize the water supply to any end user at any point of time. The design target reliability and availability are 99.7% and 100% respectively. The potential opportunities of pump head savings are identified and the associated benefits in power and CO2 emission are evaluated by comparing the various operational scenarios. The paper also describes the specialties and application of the control systems.

In the proceeding work it is found that on the current project alone, more than 400 GWhr of energy per annum can be saved by the application of VSDS system translating into a reduction in emission of nearly 170,000 tones of CO2 on annual basis. An efficient and robust control strategy has resulted in less operating costs in terms of energy bills and retarding the pollution of the environment.

With the use of cooling water plants around the world inevitably on the rise as more oil & gas facilities of mega size are being built, it is concluded that all viable power saving options should be explored at the early concept stages. Sufficient emphasis should be placed on energy optimization and the resultant environmental benefits.

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