Process flow diagram

[image] As shown in Process Flow Diagram, seawater desalination process includes 30 steps described by simple verbs, each declaring some objective to implement. The above-mentioned steps shall be typical and with repeatable implementation and good operation records in industry. This raw PFD stripped on any qualitative data may only serve as a proof of moving in the right direction; a promise that the customer requirements regarding the product quality can be met. To make it more meaningful, Crenger overloads this PFD with the data on design and off-design conditions, modularity, functionality, reliability and project risks.

The S1001 project is similar to the Nemmeli one (India, 2016), but still some differences exist.

  1. The selected SWRO pumps tandem does not match variation in the seawater salinity of 30000-40400 ppm. To achieve this, the high pressure pump shall be equipped with VFD. That makes the discharge isolation and throttling valves redundant.
  2. Two-day long flares of NTU and TSS up to 80 and 200 accordingly affect the quality of the selected pretreatment. It is partially mitigated by oversizing the filtration system installed before ultrafiltration and increasing the chemical dosing (by at least 100%). The plant production shall be decreased in such days by 30 - 50%.
  3. The surge tanks for filtered feed are sized according to the 5 minutes of operation instead of 15minutes.
  4. The intake station pumps number does not match the tender provisions.
  5. The standby capacities selection is not optimized due to a lack of data on the product consumption hourly rates.

67 generic P&ID symbols are described here.

Intake station

[image] This P&ID shows the implementation of the PFD first 4 steps: filtration, pumping, chlorination and cleaning.
It includes 2 rotating band screens operating all the time, each being designed for the 100% flow rate. Each screen may be isolated with the stop logs to perform necessary inspection and maintenance of submerged parts. The rotating band screen is equipped with the cleaning system and the debris disposal one. The first cleans the screens by high-velocity jets produced in specially designed nozzles.
The anticipated pumping turn-down ratio and spinning reserve capacity are provided with 6 pumps connected in parallel, each pump being driven by the variable speed drive (VSD). There is no standby pump; its capacity is equally divided between operating pumps.
As seen this P&ID includes the backwash and sludge collecting tank with the vertical pump. This type is the proven solution for situations when this tank accept unusually big quantities of water from bursting piping or tanks overflow. For commissioning purposes the intake station discharge piping is connected to the backwash tank.
Ancillaries include the pigging system, the chlorination system, the compressed air one and the sacrificial-anode protection of the screens against corrosion (not shown on P&ID).
The chlorination system is simple and rugged in design as it is used only on rare occasions. The shock-chlorination dosing rates are monitored at the station discharge manifold, after the lamella settler and before SWRO membranes. The compressed air is fed to the intake head to build the bubble screen that should scare away the jelly fish.
The selected configuration of the intake sump features minimum volume and amount of concrete works.


[image] The selected standard filtration process and implementation match the following ultrafiltration (targeting SDI of 3.5 for SWRO feed) at the incoming seawater NTU below 30.
First seawater goes through the static mixers (for fast mixing) and then slow mixing chambers before entering the lamella settler - a rack of inclined plates, which cause flocks of suspended solids material to precipitate from water that flows across the plates. The flocks sludge - settle at the bottom and are collected and fed by a slow-rotating scraper to the progressive-cavity pump through the sludge outlet located at the clarifier center. Small part of settled sludge is used for seeding; it is extracted just above the scraper and re-circulated to the flocculation chambers.
From the lamella settler the seawater stream enters DAF system including the air saturator and the recirculating pumps. DAF uses minute bubbles generated from dissolved air to attach and float flocculated particles (flocs) to the top of the clarification basin for removal by a mechanical skimming device or by flooding (hydraulic removal). About 6-8 percent of the effluent is pumped and recycled through an air saturator tank to the front of the DAF tank. The proposed DAF clarification rate is set to 60 m3/h/m2. Such an increase is possible due to higher air saturation rate, proprietary design of air nozzles and optimized hydraulics of the DAF tank.
Flocculation objectives for DAF and sedimentation are different because of the reverse principles. Sedimentation requires larger flocs for gravity settling while DAF needs smaller flocs of 10-30 µm to obtain high rise velocities to minimize tank size. As bigger flocs grow from smaller ones, logically, in contrast to the project tender recommendations, the DAF process should be initiated in the slow-mixing flocculation chamber. This approach is implemented in the S750 project.



[image] [image] Clear water stream from the DAF is directed to standard ultrafiltration unit through the self-cleaning filters of 150 - 250 micron protecting the UF fibers. The ultrafiltration quality is periodically checked through water sampling to SDI- monitoring system. The system auxiliaries include the backwash system and CEB one. Additionally the UF modules are connected to the CIP system of the plant.
After UF feed water goes through the micron filters (to shave off occasional flares in SDI values) and is pumped with common booster pumps to the SWRO trains.
Comparing to the conventional vertical single-flow design, the double-split micron filter have a number of advantages: smallest footprint, compact design, the same axial direction for inlet and outlet, easy access to the cartridge filters headers for replacement, neither ladders nor pedestals, the fastest cartridge headers replacement without piping dismantling.

Chemical dosing

[image] Shown are the antiscalant and SMBS dosing systems with the 100% metering pump redundancy and a means to check the metering pump calibration (measuring bucket and/or mass-meter). The storage systems are optional. Each 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. Reagents are dosed to the common suction of the common booster pumps installed before the micron filters group.


SWRO unit

[image] As shown in 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 enters the ERI energy recovery device (PX300 or similar) where its pressure is increased 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 ERI, the second stream is additionally pressurized in the ERI 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 high pressure feed pump is driven by the water-cooled AC motor with soft starter and equipped with the forced oil lubrication system. Water-cooled motors have low noise emission and better efficiencies comparing to the air-cooled motors.
The oil lubrication system serves both the pump and the motor. To make it reliable, 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 common cooling system.
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.

Table 1 Sample of SWRO train performance prediction for pumps selection
Nu Category Value
1 Seawater pressure, kPa 6800
2 Seawater temperature, oC 25
3 Seawater salinity, kg/kg 0.04
4 Seawater flowrate, kg/s 641
5 Permeate pressure, kPa 160
6 Number of RO modules in vessel 8
7 Membranes vessels number 288
8 Fouling factor 0.73
9 Head/rear permeate flowrate ratio 0.3
10 Membrane manufacturer TORAY
11 Recovery 0.4601
12 Head permeate salinity kg/kg 0.00016
13 Rear permeate salinity kg/kg 0.00042
14 Permeate maximum flux, kg/sq.m*h 22.03 (first membrane)
15 Permeate flowrate, kg/s 294
16 Brine pressure kPa 6706
17 Brine flowrate kg/s 346
18 Brine salinity kg/kg 0.0738
19 Membranes stack cost, $USA 952000

SWRO membrane array

[image] This P&ID shows SWRO membrane arrays arrangement (12 rows and 24 columns) 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.

Clean-in-place system (CIP)

[image] CIP system is used for membrane cleaning off scale or other organic/non-organic matter deposition and for flushing after the unit emergency shutdown. Cleaning is a batch process that may be sensitive to the solution temperature. So CIP scope additionally includes a water heater and cooler, the pH-control, and the neutralization tank.


[image] RO permeate is aggressive and would pose a high risk of corrosion in the water distribution system. Remineralization and pH adjustment of the permeate make it potable and non-aggressive, with low potential to form scale.
Remineralization is typically achieved by dosing carbon dioxide and calcium hydroxide, in the form of lime water, to the untreated RO permeate to increase bicarbonate alkalinity according to the following equation.

2CO2 + Ca(OH)2 -> Ca(HCO3)2

This process is the simplest as it requires only chemical storage and simple dosing system. Alternative process uses more sophisticated limestone filters, batch control and requires substantially more instrumentation. It should be noted that considerable reduction in chemical usage is possible through use of a corrosion inhibitor.

Plant layout

[image] The plant layout provides minimum footprint (specific area is 4.8 m3/h/m2) 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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