Process flow diagram

[image] Shown above PFD (flow of processes diagram) is in fact the block diagram showing the methods and sequence needed to deliver the product with guaranteed quality and quantity. Each constituent process is concisely defined by a simple verb. In fact, each node represents the P&ID piece.

67 generic P&ID symbols are described here.

Intake station

[image] Two seawater pipelines each designed for 100% of nominal seawater flow connect the intake heads with the pumping station. The latter includes self-cleaning travelling screens and vertical pumps housed in a light ventilated building for their climate protection and noise isolation. These systems are equipped with automatic active cathodic protection system. The number of pumps in operation will vary according to the plant production rate. It generally depends upon daily and seasonal demand and electrical tariffs. All of the pumps are equipped with variable frequency converters. The selected intake sump configuration footprint is the smallest compared to the conventional designs. Ancillaries include the pigging system, the chlorination system and the compressed air one. Pigging is a batch process which is started every time the water level in the sump approaches the minimum submergence for the main pumps.


[image] Pretreatment system includes multimedia gravity filters containing gravel, quartz sand and anthracite. The filtration velocity is limited to 10 m/hour when 2 filters in backwash modes. The backwash water after MMF is treated in the sludge treatment system before being pumped to the brine outfall tank. The clear water from the pretreatment system is pumped with the low pressure booster pumps to the SWRO units. 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 SBS into seawater if needed. The filtration quality is periodically checked through water sampling to SDI- monitoring system.


[image] The train design is selected. Comparing to the pressure center configuration, the previous has much higher flexibility in operation, less expensive in the high-pressure piping (by about 40%), more efficient in operation with high fouling rates, and has higher safety standards. Before entering the trains seawater goes through horizontal double-split micron filter. Comparing to the conventional vertical single-flow design they have the following advantages.

  1. The smallest footprint and compact design
  2. The same axial direction for inlet and outlet
  3. The easy access to the cartridge filters headers for replacement
  4. No ladders and pedestals
  5. In cartridge headers replacement, piping is not dismantled at all
  6. The shortest time for the cartridge headers replacement

Selected implementation of the seawater reverse osmosis desalination is built round DWEER - energy recovery device brand produced by Flowserve company. As shown on P&ID SWRO membrane array is fed with 2 streams of seawater. First stream is pressurized in the high pressure booster pump and the high pressure pump connected in train. The second stream is pressurized in DWEER through the energy recuperation from the brine reject. Due to the brine pressure being below the one at the SWRO membranes inlet, and inevitable energy losses in DWEER, the second stream is additionally pressurized in the DWEER booster pump before being fed to SWRO membranes. The high pressure booster pump serves 2 purposes; it accommodates the pressure variation in the SWRO process and, secondly, it boosts the pressure before the high pressure pump to avoid cavitation incipience. The permeate streams extracted from the membranes front end and the rear one differ in quality generally defined by remained TDS and Br content. By varying the ratio between the permeate streams the front-end quality may be tailored to that of the final product delivered to the client. High pressure pump forced oil lubrication system serves both the pump and the motor. For higher reliability of this system, one oil pump is coupled to the shaft of the high pressure pump. At power supply interruption this pump continues pumping oil to the bearings till the complete stoppage of the pump set. To cool the oil, the lubrication system is plugged into a cooling system common for all water-cooled motors.

SWRO membrane array

[image] This P&ID shows SWRO membrane arrays arrangement and manifold fittings. As seen every membrane location is described by row, column, and ordinal number inside the pressure vessel. This membrane address is extensively used by the membrane tracking software.

SWRO dosing

[image] This P&ID shows the antiscalant and SBS daily storage and dosing systems. Such an implementation is safe and simple in maintenance. Batch recharging of the dosing systems is quick and fully automatic. The typical storage system includes an open tank with a spill berm, the group of transfer pumps with 100% reserve capacity, and the strainer installed at the pumps common suction line. The storage system is common for all SWRO units. The dosing system has 50% redundant capacity and a means to check the metering pump calibration (measuring bucket and/or mass-meter).


[image] This P&ID shows Brackish Water Reverse Osmosis (BWRO) unit needed to raise the quality of the rear end permeate produced by SWRO unit, the permeate suck-back (surge) tank, and the water cooling system of the main pumps. BWRO unit consists of three groups, each including the feed pumps and the RO membranes arrays. The permeate streams collected from the membrane arrays are transferred to posttreatment stage.

BWRO membrane array

[image] This P&ID shows BWRO membrane arrays arrangements for the second pass and for the combination of the third and fourth passes. The ratio between the passes is 7 : 2.6 : 1.


[image] This P&ID describes the remineralization of front-end permeate in the limestone reactors to make it more stable and less corrosive. After remineralization it is mixed with the permeate from the second, third, and fourth passes. As the permeate water used for the reactor backwashing has high content of solids, it is treated by the sludge treatment system. Re-mineralized water is stored in the product tank before being pumped to water mains.


[image] The CIP system is an auxiliary system used to prolong the membrane life before it being replaced for a new one. Before being fed to SWRO membranes the CIP solution is passed through the micron cartridge filter. The CIP interconnecting piping is designed for maximum allowable velocity to cut down the piping volumes to be flushed after each cleaning.

Sludge treatment

[image] Shown on this P&ID system treats 2 effluents to make them suitable for final disposal: the water after the multimedia filters backwashing, and the permeate stream after the limestone reactor backwashing. All the streams are collected in the waste water pool, from which the water is pumped at the constant flow rate to the flocculation chambers before being admitted to the lamella thickener equipped with rotating scraper. After the lamella settler the clean stream is returned to the pretreatment system, while the liquid sludge is pumped to the centrifuge for final dewatering and disposal.

Plant layout

[image] The plant layout provides minimum 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. Neither SWRO membrane vessel ends nor BWRO ones overlook the maintenance areas which are mostly frequently visited.

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