The application of pelton type impulse turbines for energy recovery on sea water reverse osmosis systems

Abstract

This paper discusses the background of impulse turbine technology, its applicability to sea water R.O. energy recovery systems, and the economic benefit of energy recovery on product water price. The growth of the Reverse Osmosis System in the production of fresh water and its emergence as a serious contender for large-scale sea water desalting schemes, combined with the increased cost of energy, have focused the need on energy recovery as an integral part of a competitive R.O. system. The Pelton Type impulse turbine with its simple, single-wheel construction and lack of close running clearances makes it highly suitable for the application. Pelton type impulse turbines have been used for many years in high-head hydro electric generating plants and thus have been developed to achieve very high hydraulic efficiency. An impulse turbine can replace the normal system relief valve and convert the Kinetic energy in the waste concentrate jet to rotating mechanical energy which can be used to drive an electric generator or can be coupled to the feed pump motor to reduce the normal electrical input. Up to 85% of the previously wasted energy can be recovered resulting in significant savings in product cost. The paper points out that impulse turbine technology is well-established and readily acceptable to energy recovery systems for sea water reverse osmosis. The Pelton type impulse turbine has a very broad or “flat” efficiency characteristic curve. It can be applied at flows ranging from 50% to 150% of its Best Efficiency Point (B.E.P.) performance with only a small reduction in efficiency. Additional nozzles can be added to increase its operating range of flow. The turbine wheel size can be adjusted and supplied in increments within a standard turbine frame size which allows the matching of system pressure and maximum efficiency. These characteristics enable the standardized impulse turbine to cover a very wide range of flows and heads with a minimum number of standard sizes. A typical standard range is shown with two basic frame sizes, with either one or two nozzles which are piped to the reject concentrate system, and six wheel sizes covering a range of flows from 75 GPM to 3000 GPM with inlet pressures from 700 to 1050 P.S.I.G. A proposed standardized range and optimum number of turbines required for systems with product outputs ranging from thirty thousand (30,000) to three million gallons per day are also given. A description of the machine, a standardized approach to application, the method of start-up and operation, the economic comparisons of operating costs and alternate systems are given in this paper. This topical paper concludes that as the world requirement for fresh water grows, combined with the demand for conservation of oil and the increase in energy cost, the attraction toward impulse turbine energy recovery systems will similarily grow over a wider range of economic conditions and plant sizes.