Energy savings and better performances through variable speed drive application in desalination plant brine blowdown pump service

Abstract

Abstract Brine blowdown represents an essential service for thermal desalination (both MSF and MED) and its operation is linked to the overall process reliability and availability. Even if the brine blowdown pump represents a relatively minor utility in MSF plants, this utility is often the object of a service of very delicate nature. Different options are available in the market to design the blowdown pumping system, reducing EPC and operating costs and increasing reliability. In this paper, the use of variable speed drive has been analysed for a centrifugal pump brine blowdown service in a desalination plant application. Today, the price of variable speed drive up to 250 kW is lower than 10 years ago, allowing the application of these devices in industrial applications with no price increase for the customer. Due to the particular nature of the service (level regulation inside evaporator), the combination of low head and large capacity imposes the selection of high Nq hydraulics with consequently a flat performance curve. Under these conditions, regulation by throttling is awkward because a small variation in the system losses results in a large variation of pumped quantity. Moreover, high Nq hydraulics show a relatively narrower operating range that increases the regulation problem. Therefore, the centrifugal pump selection is difficult because the machine works badly when too far away from the design point and, thus, the use of an inverter associated to the pump’s electric motor could allow to regulate the pump speed according to the pump requested performances. This solution improves performance because it allows the machine to work not too far away from the best efficiency point thus reducing power absorption. In addition, the pump works at BEP with less vibration and requires less maintenance over time. Briefly, the main characteristics of the most used pump in this kind of service have been analysed and a simple type of control logic has been proposed. Furthermore, the present paper aims at overviewing the different project options, comparing costs and taking corrective actions on the factors causing operational troubles.