GB/T 33463.1-2017Technology specification of seawater desalination for iron and steel industry - Part 1: Low temperature multiple effect distillation (English PDF)

4.1 Quality of seawater spray in MED system

The quality of seawater spray in MED system shall comply with the requirements of Table 1. When the heat-exchange tube is made of aluminum alloy, the ion content of aluminum, copper, nickel, manganese and iron shall be monitored. The total amount of these five substances in the seawater spray ought to be not more than 0.5 mg/L.

Table 1 gives, for each item, a recommended value and a permissible value. Suspended solids (SS): recommended not more than 20 mg/L, permissible not more than 50 mg/L. Diameter of suspended particle: permissible not more than 100 micrometres. Turbidity: recommended not more than 5 NTU, permissible less than 10 NTU. Water temperature: recommended 30 degrees C to 40 degrees C, permissible 2 degrees C to 40 degrees C. Petroleum: recommended not more than 0.50 mg/L, permissible less than 1 mg/L. Salinity: recommended 2.0 % to 4.0 %. Free chlorine: permissible 0.01 mg/L to 0.1 mg/L. Total iron: recommended not more than 0.05 mg/L, permissible not more than 0.1 mg/L.

4.2 Quality of seawater for heat exchange

The quality of seawater for heat exchange shall comply with the requirements of Table 2.

Table 2 lists thirteen items with their units, recommended values and permissible values. Suspended solids (SS): recommended not more than 20 mg/L, permissible not more than 30 mg/L. Diameter of suspended particle: recommended not more than 100 micrometres. Turbidity: permissible not more than 10 NTU. Methyl orange alkalinity (as CaCO3): permissible not more than 350 mg/L. Calcium ion: permissible not more than 1 000 mg/L. Magnesium ion: permissible not more than 3 200 mg/L. Total iron: permissible less than 1.0 mg/L. Chloride ion: permissible not more than 42 000 mg/L. Sulfate ion: permissible not more than 6 000 mg/L. Petroleum: permissible less than 5 mg/L. pH: permissible 6.8 to 8.8. Water temperature: permissible 4 degrees C to 40 degrees C. Total count of heterogeneous bacteria: recommended less than 1 000 CFU/mL, permissible less than 500 000 CFU/mL.

4.3 Steam conditions for LT-MED (low temperature multi effect distillation) seawater desalination

4.3.1 The heating steam shall be the steam that the iron and steel plant may provide economically and steadily, and low-parameter waste heat steam should be preferred.

4.3.2 Parameter requirements for heating steam in LT-MED: a) The extra low pressure steam in iron and steel plant shall have absolute pressure ranging from 0.02 MPa to 0.04 MPa and a temperature range of 60 degrees C to 76 degrees C. b) The low pressure steam from a steam pipe network or turbine extraction in iron and steel plant, with an absolute pressure of 0.3 MPa to 1.2 MPa, enters the thermal compressor through decompression, temperature reduction or other pressure regulating means.

4.3.3 The supply method of LT-MED seawater desalination steam source is determined according to the following principles: a) In order to avoid the shutdown of the desalination system, the steam supply system may use parallel dual header, annular dual header or another similar means. The minimum steam supply is to meet the minimum steam demand of the desalination system. The steam supply source should adopt multiple steam sources in parallel. In the case of limited number of steam sources, the iron and steel plant start-up boiler can be used as a commissioning and emergency steam supply source if conditions permit. b) The LT-MED system in iron and steel plants with backup water source may adopt the single-header steam supply system.

4.4 Desalted water quality requirements

4.4.1 The total dissolved solids of the LT-MED desalted water may confirm according to water requirements and should not exceeding 5 mg/L.

4.4.2 The temperature of the LT-MED desalted water shall be below 33 degrees C if it enters the circulating cooling water system of iron and steel plants; The temperature of the desalted water shall be below 40 degrees C if further desalination treatment is required.

4.4.3 The quality of the desalted water shall meet the corresponding water quality requirements according to different uses: a) The quality of desalted water shall comply with the requirements of GB/T 1576 when it is used for universal pressure boiler feed water. b) The quality of desalted water shall comply with the requirements of GB 50050 when it is used as the cooling water of water-cooled generator and makeup water for closed loop cooling systems in iron and steel plants. pH adjustment measures shall be considered when it is directly used as industrial water. c) The quality of the desalted water shall be further treated when it is used in high-requirement application. d) The quality of the desalted water shall comply with the requirements of GB 5749 when it is used as drinking water. e) The quality of the desalted water shall comply with the requirements of GB 17323 and GB 19298-2014 when it is used for bottled purified drinking water.

5.1 Sea water source and water intake system

5.1.1 It is required to understand the characteristics of sea water quality and its variation patterns in the water intake and drainage area, as well as the requirements of the surrounding marine environment.

5.1.2 The project shall obtain sufficient data regarding tides, water temperature and water quality analysis in recent years. a) Tide data include the relevant data under the highest and lowest tide levels in the local sea area over the years (including salinity, suspended solids, permanganate index, and etc.), as well as the highest and lowest tide levels and time of occurrence in each month in recent years. b) No less than 4 copies of complete water quality analysis data of the seawater source in each season shall be obtained prior to design, as well as the annual sea temperature monitoring data of the water intake. For the seawater desalination project in the northern region, special attention shall be paid to the seawater temperature in winter. Sea water quality analysis shall include the sand content and particle size distribution (see Annex A).

5.1.3 The water intake of the LT-MED seawater desalination system shall be selected reasonably according to the specific requirements of the desalination process, the water supply type of the unit, and the differences of water temperatures in cold and hot seasons, as well as the water intake requirements of short term and long term engineering plan. The sea water of relatively low temperature shall be used as the cooling water for the final effect to reduce the heat exchange area of the final effect; the seawater spray may use the heated seawater after the final effect or the relatively high temperature seawater cooling system blowdown of the iron and steel plant.

5.1.4 The water intake of the LT-MED seawater desalination shall be designed according to the following principles: a) When seawater is directly taken as the source water, the water intake facilities for seawater should be taken into consideration in combine with the water intake facilities of the cooling water system or makeup water system of the iron and steel plant. Bank side, tidal water flume and pipe culverts are generally used for water intaking. Water intake structures shall comply with relevant regulations. b) When the source water is the seawater from circulating cooling water system in the steel plant, the intake may be drawn directly from the supply or return pipe of seawater.

5.1.5 The design of the seawater intake system shall fully consider the impact of sediment, ice, wind, waves, marine organisms, red tides and other marine hydrology on the water intake facilities. The seawater intake channel is required to install screens to intercept the marine organisms and to inhibit its growth by fungicides dosing.

5.1.6 The conveyance of source water of the seawater desalination system shall be considered together with the seawater intake system of the iron and steel plant based on the amount of water taken and the conveyance distance. The number of conveyance pipes shall be no less than 2, and when one is under maintenance, the remaining pipes shall be able to bear 70 % of the maximum design capacity.

5.2 Seawater pretreatment system

5.2.1 For a LT-MED seawater desalination process, whether pretreatment is required shall be based on the water quality requirement of the equipment manufacturer and the seawater quality of the local sea area.

5.2.2 If needed, the clarification process shall use clarifier or sedimentation tank with coagulation and flocculation function or horizontal flow sedimentation tank etc., and it shall be designed based on source water quality, hydraulic capacity, water quality requirements of subsequent unit, as well as local conditions.

5.3 LT-MED seawater desalination system

5.3.1 There are many types of steam in iron and steel plants, and the LT-MED process should select a process with thermal vapor recompression (LT-MED-TVC).

5.3.2 The GOR (Gained Output Ratio) of LT-MED shall be in the following range: a) LT-MED: 3 to 7; b) LT-MED-TVC: 6 to 14.

5.3.3 The TTB of LT-MED system shall be reasonably determined based on the solubility of calcium sulfate. The maximum operating temperature of LT-MED system shall be lower than 66 degrees C.

5.3.4 The capacity variation of the LT-MED seawater desalination should be designed as 50 % to 110 %. The design capacity adjustment range shall be determined based on the system requirements, equipment conditions and external conditions.

5.3.5 The yield of condensate water of heating steam and vacuum steam shall be deducted when the yield of desalted water of LT-MED is calculated.

5.3.6 The LT-MED unit shall be designed according to the following principles: a) The LT-MED unit shall give priority to the use of low-pressure waste steam from the iron and steel plant. b) If the pressure of heating steam is low and cannot meet the steam pressure requirements of the vacuum equipment, an additional vacuum equipment steam supply system may be installed. c) If the LT-MED unit contains aluminum parts, metal ions of copper, nickel, mercury and etc. shall be removed before seawater spray entering the evaporator. d) If the vacuum system adopts a steam ejector, an ejection starter and a vacuum ejector is required to be installed. The steam at the outlet of the ejection starter can be directly discharged. The vacuum ejector shall be equipped with a heat exchanger to condense the outlet steam and preheat the seawater. The steam condensate water of the vacuum ejector shall be recovered. The capacity of the steam extraction system should reach the starting conditions of the evaporator within 40 min to 60 min, and the vacuum ejector should be designed to be with 2 to 3 levels for normal operation. If the inlet steam pressure of the vacuum system is unstable, a pressure stabilizer is required to be installed. e) If the seawater desalination device is used as the steam turbine condenser, the vacuum generator is required to consider the vacuum requirements of the steam turbine same time. f) Before the seawater spray enters the device, a self-cleaning filter shall be installed. The filtration accuracy of the filter is determined based on the requirements of the device and the quality of the source water. g) Exhaust device is required to be installed at the highest point of the water supply pipeline.

5.3.7 The LT-MED system shall be equipped with acid cleaning system with reserved connection port for acid wash. The acid cleaning system may adopt fixed equipment or mobile equipment for temporary use.

5.3.8 The LT-MED system seawater desalination shall be equipped with a vacuum protection system in case steam is lost.

5.3.9 The LT-MED system seawater desalination shall be equipped with overpressure steam protection device and overpressure steam discharge device.

5.3.10 Desalted water pipe and condensate pipe shall be equipped with off-spec discharge pipe, and the drain shall be recycled.

5.3.11 The filter on the sea water pipeline shall be designed based on the maximum seawater flowrate. The filter of seawater spray and heat-exchange sea water supply should meet the respective water supply requirement if they have separate supply system.

5.3.12 If the seawater desalination system uses steam turbine exhaust to produce water, a reliable isolation device shall be installed on the steam connecting pipeline to facilitate the operational mode switching of the evaporator.

5.3.13 If the seawater desalination system uses steam turbine exhaust to produce water, the inter lock program among the main steam valve of the steam turbine, seawater spray flowrate and the vacuum of the seawater desalination system shall be set.

5.4 Desalted water storage, treatment and water quality adjustment

5.4.1 Desalted water storage. The desalted water storage shall comply with the requirements as follows: a) The total effective volume of the desalted water storage tank (container) shall comply with the requirements of the iron and steel plant and external users on the quantity, supply method and its use. If the desalted water is used as the source of the desalted water for iron and steel plant, the effective volume of the storage tank (container) shall comply with the water demand of the desalted water when one LT-MED unit is under maintenance. If the desalted water is used as the source of the fresh water for iron and steel plant and the external users, the effective volume of the storage tank (container) shall comply with the water demand of the fresh water and the external users when one LT-MED unit is under maintenance. b) The retention time of water in the storage tank (container) should not exceed 3 d. c) The number of storage tank (container) shall be no less than 2 and should be placed near the desalination system. d) The highest level and lowest level of the storage tank (container) shall be determined and controlled according to the design and operating requirements, and set up clearly visible water level gauge, water level meter and level alarms. e) The vents, inspection and repair manholes of the water storage tank (container) shall have safety and health preventive measures.

5.4.2 Desalted water treatment and water quality adjustment shall comply with the requirements as follows: a) The desalted water shall undergo advanced treatment and water quality adjustment based on its treatment process and uses, so as to meet the requirements of relevant national regulations. b) If the desalted water is used for industrial purposes, the pH should be adjusted to 6.0-9.0 by adding ammonia or NaOH solution.

5.5 Wastewater treatment

Waste water generated by the desalination system shall comply with the requirement of national environment protection.

5.6 Treatment of brine

5.6.1 Brine should be comprehensively used.

5.6.2 The discharge of brine shall meet the environmental protection requirements of the discharging sea area.

5.7 Dosing system

5.7.1 Protective facilities such as emergency pool and safety eyewash equipment must be installed near the dosing system.

5.7.2 Each solution tank shall be equipped with level gauge and isolation valve, as well as level alarms; liquid discharge at the bottom shall be considered in order to completely empty the residual.

5.7.3 Unit dosing system or common header dosing system shall be adopted with metering pump equipped. Filters shall be installed at the inlet of the pump, and pressure stabilizers and safety valves at the outlet of the pump.

5.7.4 The bactericide of the seawater influent to the desalination system shall be sodium hypochlorite solution prepared by electrolysis of seawater, and the source seawater of the electrolysis for chlorine production may be seawater or the brine of the seawater desalination system.

5.7.5 The following principles shall be conformed to when the sodium hypochlorite is prepared by the electrolyzation of seawater: a) If the source water is seawater, the seawater shall be filtered before entering the electrolytic cell. b) Sodium hypochlorite storage tanks shall adopt reliable hydrogen exhaust measures. c) Acid cleaning unit for the electrolytic cell shall be equipped to facilitate descaling according to the structural requirements of the electrolytic cell.

6.1 Anti-corrosion and materials selection

6.1.1 The internal surfaces of all equipment, pipes, valves and structures that come into contact with corrosive media or have an impact on the quality of the water outlet shall be lined with a suitable anti-corrosion layer or use corrosion-resistant materials. The outer surface of equipment, pipelines, valves and structures affected by the corrosive environment shall be lined with a suitable anti-corrosion layer. The anti-corrosion requirements of seawater pretreatment equipment may refer to Table B.1 in Annex B, and the anti-corrosion requirements of pumps, valves and pipelines may refer to Table B.2.

6.1.2 The materials of the LT-MED seawater desalination system shall be resistant to seawater corrosion, and consider the operating temperature, the pH value of the seawater, the content of oxygen and carbon dioxide and the pollution of the seawater (sulfide, ammonium, etc.). The heat exchange tube may be made of stainless steel, Cu alloy, Al alloy or Ti alloy based on different corrosion requirements; container may be made of stainless steel, carbon steel coated anti-corrosion layer or carbon steel with cathodic protection. The top three rows of the heat exchange tubes in the evaporator should adopt titanium tubes. The corrosion requirement of the main equipment of LT-MED may refer to Table B.3.

6.1.3 The chemical storage tank and solution tank should be made of rubber lined carbon steel, glass fiber reinforced plastic or polyethylene.