Process control of Larnaca 51,000 m³/d seawater RO plant

A special process control was required to cope with partial production during peak hours in order to take profit of lower electricity tariffs. The process control philosophy was based on separation of the system to independent subsequences with the purpose to achieve fully automatic start-up and normal operation of plant at major flexibility. The plant control was divided on main independent sequences as follows: a) intake system; b) pretreatment plant; c) booster pumps, first stage high-pressure pumps and RO trains; d) second stage high-pressure pumps and RO train; e) post-treatment plant; f) product delivery system; g) auxiliaries. During start-up, the operator selects the number of first stage RO trains to be operated. The intake and pretreatment plant initiate according to the logic of operation. Once the desired filtrated water quality is reached, the boosters are operated and the high-pressure pumps are started one after the other. High-pressure pump start-up procedure is based on partial load of the motor. When the system reaches normal operation, the start-up procedure is stopped and all independent sequences continue running in parallel. At peak hours, three high-pressure pumps are stopped for 3 h. The main sequences reach new working conditions automatically: a) the intake system includes the self-cleaning screens operation and the intake pumps. Independent of the number of RO trains in operation, the number of working pumps is adjusted according to the inlet flow rate required to the pretreatment plant. A control valve controls the inlet flow to the plant, in order to maintain constant level in the filtrated seawater tank; b) in the pretreatment plant an outlet control valve controls the seawater level in each one of the filters. The filter is backwashed automatically according to the pressure losses on it; c) booster pumps, high-pressure pumps and RO trains: each RO train is directly connected to one high-pressure pump. The number of booster pumps working is directly related to the number of high-pressure pumps in operation; d) second stage highpressure pumps and RO train: the feed to the second RO train is controlled by the number of operating pumps; e) in the post-treatment plant the flow rate to the filters is adjusted by a control valve. The filters are refilled and backwashed periodically; f) in the product delivery system, the number of product pumps working controls the water level in the product tank, and g) when a high-pressure pump stops, the auxiliaries operate by activating the flushing procedure.