DESALINATION

67 generic P&ID symbols are described here.

Intake station

[image] This P&ID shows the seawater intake implementation. It includes 2 rotating band screens, each being designed for the 100% flow rate. Both screens are pull-out design and may be isolated with the stop logs. The pull-out design is at least as twice expensive as the conventional one, but provides mush higher reliability especially for seismic areas. The last factor is probably taken into account in selecting the screens' cleaning system; it includes the stand-alone vertical turbine pumps – more expensive solution comparing to the pumps fed by the seawater from the intake main pumps. The requested pumping turn-down ratio and spare (standby and rotating) capacity are provided with a number of pumps connected in parallel, each pump being driven by the variable speed drive (VSD). In addition the intake station is equipped with the brine dilution pumps. Such a solution provides the stable quality of dilution regardless of the brine output rate as compared to the passive dilution – by evenly distributed brine jets along the brine outfall piping. Ancillaries include the pigging system, the chlorination system and the compressed air one.

Pretreatment

[image] [image] Pretreatment includes 2 identical stages of multi-media filtration in the horizontal filters designed for pressure as high as 10 Barg. Before the 1st stage sulphuric acid, polymer and flocculent are admixed to seawater. Before the 2nd stage flocculant is added again. After the filtration there is an option to add the sulphuric acid and SMBS.
This type of filter requires periodical backwashing as the build-up of the filtrated material gradually plugs it. The selected backwashing system does not cause any dips in the plant production during its operation. The said system contains an open tank with the three low-head pump, and the air scouring system. The tank is continuously filled with the brine rejected from the reverse osmosis process or the filtrated water. After backwashing the filter goes through maturing phase during which the filtrated water quality returns to the normal one. This water flow is diverted and fed again to the filters. The filtration quality is periodically checked through water sampling to SDI- monitoring system. Free chlorine, turbidity and pH are monitored as well.

 

SWRO unit

[image] The SWRO unit is of conventional design built round ERI energy recovery device. As shown on P&ID after the feed water goes through the micron filter, antiscalant and SMBS are admixed to it. After the static mixer the sample is taken to the SDI analyzer. To decrease the risk of SWRO membrane fouling via scaling formation, the antiscalant is constantly added to seawater streams. During the intake chlorination, the free chlorine control is engaged, injecting SMBS into seawater if needed. Then the feed is split into 2 streams; one goes to ERD, where the stream pressure raised, and the second – to the high pressure pump (HPP). After ERD the feed pressure is further increased in the ERD booster pump. The said streams are entered into the membrane vessel array from the opposite sides.
The pressure of the feed after the micron filter should be higher than the NPSHR value for HPP. The latter is equipped with VSD – variable speed drive to accommodate for the variations in the seawater temperature and salinity and the train load. The HPP motor is selected water-cooled to decrease the noise level. This high-fidelity system is shown in the bottom-left corner of P&ID. As well the CIP close-looped line connections are shown on P&ID. The product connection additionally contains the non-return valve – protection against the inadvertent CIP in-leakage from the return piping. Rupture discs are installed on the product line and the suction line of the HPP to protect against the pressure surges during transient operation – startups and shutdowns. Another feature is the pressure-equalizing line connecting the product line to the feed one.

SWRO chemical dosing

[image] This P&ID shows the antiscalant and SMBS daily storage and dosing systems. Each SWRO train contains its own dosing system with the 100% redundancy. The bottom-left corner SMBS dosing system serves the second-stage filters. The storage system is common for all SWRO units.

Posttreatment

[image] This P&ID describes the re-mineralization of desalted water by adding soda ash to make it more stable and less corrosive. The process is controlled by a set of criteria such as alkalinity, hardness, pH and LSI. To make the process effective, the CO2 gas is injected into the influent. The mineral-enriched water stream is then discharged to the buffer tank, its volume being selected based on the residence-time criterion. This product water pH and LSI are constantly monitored.

Plant layout

[image] The plant layout provides minimum footprint and the length of interconnecting piping, clearly defines the project areas, and meets work safety and O&M requirements. All chemical storage tanks have direct access for trucks. Importantly, the SWRO membrane vessel ends do not overlook the maintenance areas which are mostly frequently visited.

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