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MEBS 6008 Sea Water Heat Rejection System Revision 2. No change to the following pages of the power point from previous version. Reasons for popularity of sea water cooling in Hong Kong is mainly due to :. Sea Water is easily available along the coastal regions
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MEBS 6008 Sea Water Heat Rejection System Revision 2
No change to the following pages of the power point from previous version
Reasons for popularity of sea water cooling in Hong Kong is mainly due to : • Sea Water is easily available along the coastal regions • The higher first equipment and installation cost are far outweighted by the lower operating cost in electricity • The shortage of space for installation of air cooled heat rejection system
Description of sea water system - 1 Layout and components of seawater plants are varied according to the specific site conditions such as location, capacity and reliability required, space limitations and economic considerations. Some of the seawater intakes are located near bottom of sea walls have settlement chambers to reduce entry of suspended solids in the circuits. Flat mesh screens mounted beside intakes to enable primary filtration so as to enhance screening area and thus reduce the frequency of cleaning. After primary filtration, seawater will be pumped through pipelines to plant rooms which are usually located at basement levels of buildings
Description of sea water system - 2 For buildings which are relatively far from sea water intakes, small pump houses are situated right beside sea walls that are usually located below ground level. Long suction pipelines running across road and pedestrians are common features for sea water systems in Hong Kong and booster pumps are often required. Before entering condensers in plant room, seawater will pass through a secondary filtration system which are in-line strainers. Some plants employ drum type manually cleaned strainers and some with automatic strainers.
Description of sea water system - 3 Direct Sea Water cooling using tube and shell type condensers are commonly used. Indirect sea water cooling using titanium plate type heat exchangers are also found. Seawater is usually used to cool and condense refrigerant directly and then discharged through pipeworks back to sea within plate heat exchanger. Fresh-water is cooled by seawater in plate heat exchangers and then passed to a second stage tube and shell type heat exchanger where refrigerant is condensed. Seawater is discharged after passing through plate heat exchangers while the fresh water re-circulates.
Description of sea water system - 4 After compression and condensation, refrigerant is passed to chillers where it absorbs heat from the chilled water. The low temperature chilled water is then circulated to AHU or FCU where cool air is required. Since high frequency of cleaning and maintenance are needed for some of the components in the circuits, employment of operators are preferred as technical skill is essential for smooth operation of the systems. Therefore, stand-by components such as screens, pumps, strainers and condensers are commonly adopted.
Material for Sea Water Equipment - 1 Condensers Shell and Tube Type This type of condenser is most commonly used in Hong Kong. Shell: The shell is usually made of cast iron or mild steel with or without epoxy coating. The more expensive shells are made of gunmetal, silicon bronze, aluminium bronze or nickel alloys. Tubes: The tubes are made of the following materials: 70/30 or 90/10 cupro-nickel (more resistant to impingement attack and polluted waters) Aluminium brass (preferred if relatively higher working temperatures are involved) Bronze Stainless Steel Titanium (very high corrosion resistance)
Biofouling Causes • Biofouling may be caused by: • Deposition of large quantities of microbes on surfaces in contact with the seawater so that formation and development of biofilms on the surfaces. • The attachment of larvae of invertebrates, mussels and the spores of marine algae settling and growing on submerged surface.
Biofouling Problems caused by Biofouling • Reduction in flow rate • Deterioration of heat transfer rate • Increase erosion and corrosion caused by cavitation around fouling build-up
Bio-fouling Preventive Measures – 1 Screening • Use of mechanical screening or filtration of the sea water to remove large size matured organisms • Seaweeds, debris and large-size matured organisms can be kept out of heat exchangers by traditional mesh screens followed by secondary screens. • However, such screening systems cannot prevent the passage of small particles. • Spawn, mussels and barnacles enter the cooling water intakes and attach to the walls of the water boxes. • The steady stream of relatively slow-moving water supplies food and oxygen for their development so these organisms grow to adult size and multiply.
Control of Bio-fouling Chemical Treatment • Application of chlorination and/ or biocides to kill small size organisms such as spawn, mussels and barnacles. • Chlorination by: • - Dosing of chlorine gas (0.1 to 2 ppm) • - Adding sodium hypochlorite solution • - Using electro-chlorinator to produce hypochlorite ions by electrolysis of sea water • Application of biocides to prevent growth of small size organisms
Bio-fouling Preventive Measures – 3 Flow Control & Thermal Soak • Maintaining a proper flow velocity to inhibit settlement of fouling organisms • Velocity greater than 1.5 m/s can inhibit settling of various fouling organisms • Application of thermal soak to kill fouling organisms. Recycling of water through condenser until the water temperature reach a level that organisms cannot tolerate.
Bio-fouling Preventive Measures – 4 Mechanical Cleaning - 1 • Brushes • Traditional manual cleaning method of pushing long handled brushes through condenser tubes • High pressure water gun • New method includes the use of high-pressure water gun to force nylon brushes down the tubes to scrape scale and force debris out the outlet end. • Nylon brushes are shuttled through the individual tubes by reversing the cooling water flow in the system.
Bio-fouling Preventive Measures – 4 Mechanical Cleaning - 2 • Sponge Balls Method • Automatic online mechanical cleaning of condenser tubes by frequently wiping of the water side surfaces of the condenser tubes while the unit is in operation. • Sponge balls are re-circulated constantly from the inlet to outlet water boxes. • Since these sponge balls are slightly larger in diameter than the tubes, they are compressed and clean the surface as they travel the length of the tube. • The balls are caught in a screening device installed directly in the pipeline. • A collection unit would reroute them back to the condenser inlet ball injection nozzles.
Corrosion Control Causes of corrosion in sea water system • Corrosion arises in the waterside heat exchangers using seawater as the cooling medium as a result of: - • Impingement Corrosion - fairly high velocity of flow. Impingement attack is usually 1ocalized, e.g. at inlet ends or partial obstruction, • Sand erosion, • Pitting - the effects of polluted water, • Galvanic corrosion - between tube plate and tubes, • Crevice attack underneath deposits and corrosion induced by bio-fouling countermeasures, • Failures by stress corrosion or corrosion fatigue cracking.
Seawater corrosion in the Hong Kong harbour • The fresh water discharged from the Pearl River greatly influences the hydrology of the western side of Hong Kong. The salinity of the water will vary with the seasons and fresh water flow over a wide area and the salinity will vary daily with the tides in a narrow region. • The city discharges into the harbour minimally treated wastes which include sewage effluents and industrial effluents. Parts of the harbour are land-locked that cause these effluents not efficiently dispersed. • The in-takes of many seawater handling systems are positioned close to the sea-bed or are affected by reclamation. Thus suspended solids and debris drawn into the in-takes can cause erosion.
Corrosion Approaches to solve • Design and Construction Features • Employing different preventive measures • Selection of appropriate tube materials • Correct operation and maintenance of plant
Corrosion Control Design and Construction Features • Proper selection of seawater intake location • Provision of efficient screening and filtration system • Proper design of condenser to eliminate local turbulence • Maintaining adequate flow can reduce the possibility of inlet impingement, attack due to deposition and foreign body partial blockage • The fatigue failure is supporting plate normally avoided by suitable design of spacing • Stress corrosion cracking by reducing residual stresses at the rolled-in portion of tube through special heat treatment.
Corrosion Control Employing different preventive measures • Nylon inserts for protection of the inlet tube ends. • Addition of ferrous ions by dosing ferrous sulphate or iron corrosion products. This promote the formation of protective firm on the condenser tubes and reduce the harmful effect of any residual chlorine ( as low as 0.2 ppm may be very harmful). • Provision of cathodic protection to the tube inlets, the tube plates and water boxes using sacrificial zinc or iron. • High molecular weight inhibitor for pre-treating tubes for protecting tube surface. • Application of artificial protective films on the tube surface.
Corrosion Control Selection of appropriate tube materials • Condenser tube material : • Aluminium brass • 90/10 Cu/Ni • 70/30 Cu/Ni • Stainless steel condenser tubes (suffers crevice and stress corrosion attacks) • Titanium tubes or plates (high cost)
Corrosion Control Correct operation and maintenance of plant • Cleaning and Maintenance • Proper periodic cleaning and maintenance is important • To avoid substantial accumulation of deposits and stagnant water condition • To follow the correct procedure • To be carried out during period of down time. • Tube cleaning • Manually • By flow driven brushes • Re-circulating balls system.
SEA WATER TREATMENT BY ELECTROCHLORINATOR - 1 • Electro-chlorinator designed for production & injection of on-site sodium hypochlorite solution. • Such system shall take water under pressure upstream of the main sea water pump (usually in pump chamber) and after passing through the electro-chlorinator, inject the solution back into the pump inlet side. • The operation of the electro-chlorinator shall be interlocked to work or stop as the seawater pumps. • Duplicate electro-chlorinator plant shall be required to ensure continued treatment when one unit is 'off-line' for maintenance.
SEA WATER TREATMENT BY ELECTROCHLORINATOR - 2 • The electrodes shall be made of high corrosion resistant material, in particular, anodes shall be made of uranium substrate coated with protective oxide. • Integral safety facilities in the electrolytic cells such as water flow switch and cell voltage imbalance detector shall be provided to prevent build-up of hydrogen gas by-product. • Vent and other necessary facilities shall be provided to dilute and disperse the hydrogen gas from the degas tank (note to FSD’s requirements). • An adequate electrical supply point shall be made available to the equipment plant room.
SEA WATER TREATMENT BY BIOCIDES General Approach • The biocides used shall inhibit the growth of marine organisms throughout the system and inhibit the formation of scale and slime by acting as an efficient dispersant. • This shall be achieved by the appropriate biocide treatment being carried out at a regular interval such that the whole system shall contain a 6-ppm concentration of biocide solution for approximate 1 hour during treatment is sufficient to kill all organisms. • Continuous dosages shall not be applied at lower ppm levels lest they allow the organisms to build up immunity.
SEA WATER TREATMENT BY BIOCIDES Properties of Biocides The biocide for sea water condenser cooling systems shall have the following characteristics: - • Strong biocidal properties • Dispersing effect on deposits • Filming properties providing a protective film to internal of pipe work • pH value of 6.8 • Non-flammable • Easily application by chemical metering pump with or without dilution • Being sufficiently bio-degradable to avoid pollution (impact life of marine organism) and acceptable to the Environmental Protection Department
SEA WATER TREATMENT BY BIOCIDES System Sizing and Design - 1 • The advice of approved water treatment specialist shall be sought on the dosages required, together with calculation, for a specified systems water volume. • In some cases where fixed sea water pumped quantities apply, metering pumps can be set for a particular input rate to achieve the appropriate dosage and duration. In such cases it is usually possible to pump biocide direct from its supply container.
SEA WATER TREATMENT BY BIOCIDES System Sizing and Design - 2 • Where variable pumping flows exist, the metering pump/pumps concerned shall have the facility to automatically vary the amounts of chemicals pumped in to correspond in proportion with the actual sea water flow. • This can be achieved by metering pumps which respond to the reading of a flow rate sensor inserted into the main sea water pipe. • Variable pumping rates shall be required where a number of pumps are 'stage' controlled or where individual pumps are being 'speed' controlled.
SEA WATER TREATMENT BY BIOCIDES System Sizing and Design - 3 • Sets of sea water pumps can be supplied with fixed biocide input from one metering pump per main sea water pump with inlet before the pump and as near as possible to the primary inlet sea water gate valve. • Such metering pumps would be controlled from the individual pump circuit such that they only operate when their respective pump is operated.
SEA WATER TREATMENT BY BIOCIDES System Sizing and Design - 4 • Sets of pumps can also be supplied with a single source variable duty metering pump controlled from a flow metering device placed after the main header outlet. • The chemical injection shall be distributed to all pump inlets via solenoid valves with each controlled to open when their respective main sea water pump is switched on.
Problems encountered in Sea Water Heat Rejection System • Rapid blockage of primary screens and strainers by floating or semi-floating objects (especially plastics bags) and various marine foulants. • Other frequently found problem are corrosion of screens and filtration elements of strainers along welded lines. • Corrosion of seawater valves, erosion corrosion of pump impellers and inlet attack of condenser tubes. • Guidance for effective usage of preventive measures is imperative for sea water cooled systems in Hong Kong.
Sea Water System and Equipment Maintenance -1 • Frequent cleaning and maintenance of screens, strainers, and condensers are necessary for sea water cooled systems in Hong Kong, especially during the peak operation period of summer. • Frequent cleaning and maintenance are needed as a consequence of rubbish and heavily polluted harbor waters by untreated sewage, the peaks of marine fouling growth, and the directions of wind and sea water current during different seasons. • Maintenance works and frequency depend on the types of preventive measures and equipment used, sea water quality and etc. • Furthermore, different components of the plant may also require different frequencies of cleaning in resume efficient operation.
