The Freshwater Generators is an important auxiliary equipment necessary for ships to sail in the ocean. The Alfa Laval JP-26 watermaker is compact and easy to operate, and is often available on ocean-going vessels. With the aging of water-making machine equipment, abnormal faults occur frequently, which has a certain impact on the operation of ships.
1. Alfa Laval JWP-26 principle and important components
1.1Principle of freshwater generators
The Alfa LavalJP-26 watermaker is a vacuum boiling seawater desalination unit, and its water-making system is shown in Figure 1. The device is composed of seawater pump, plate condenser, plate evaporator, mist eliminator, jet pump, feed water pressure regulator, condensate pump, salinity meter, control panel, etc., and is heated by the main engine jacket water. Seawater Path: From the seawater pump outlet of the watermaker into the condenser. Then it is divided into two ways from the condenser outlet, one way enters the evaporator through the feed water pressure regulating valve for evaporation seawater, all the way enters the jet pump to provide working water, and the jet pump outlet is discharged to the outboard. Desalination process: the main engine jacket water enters the evaporator plate, heats the seawater, the seawater evaporates at a relatively low temperature under a higher vacuum, the water vapor enters the condenser plate through the mold remover for cooling, and the cooled fresh water is collected in the lower space of the plate and discharged to the fresh water tank by the condensate pump.
1.2 Important parts
1.2.1 Seawater pumps and jet pumps
The seawater pump is a single-stage centrifugal pump, which has the following functions: to provide cooling seawater for the condenser, to provide working water for the integrated pump for salt discharge and vacuuming, and to provide seawater feed water for the evaporator. The jet pump is designed as an integrated pump for salt discharge and vacuuming, with 2 nozzles: one nozzle is made of stainless steel for vacuum salt discharge; The other nozzle is made of red copper, which has the function of vacuuming and maintaining vacuum. The working condition of the jet pump directly affects the vacuum degree of the device.
1.2.2 Check valve
There are 3 check valves designed on the watermaker system.
1) Check valve of vacuum pipeline,
Its function is one-way flow, and the non-condensable gas and part of the uncondensed steam in the condensed space are pumped away by the jet pump to maintain the vacuum of the space. The original spare part is equipped with a visual ball that prevents seawater from backing up into the condenser when the back pressure at the outlet of the jet pump is too high. The vacuum line is hot during normal operation and cold when the water generator is first turned on. During the start-up process of the water generator, when fresh water is generated and the condensate pump is not started, the condensate water level gradually rises to the suction port of the pipeline, and the condensate is pumped away by the jet pump, and the vacuum pipeline is cold at this time. The deterioration of the plexiglass material of the check valve will cause leakage and affect the vacuum level.
2) Condensate pump outlet check valve,
There is a ball and an outlet pipeline in the body of the check valve, which is used to isolate the water generator body and the outlet pipeline of the condensate pump at the beginning of the water generator start-up, so as to maintain the vacuum degree of the device.
3) Dosing pipeline check valve,
Its function is to prevent the water-making device from backfilling the dosing barrel after the shutdown.
1.2.3 Evaporator and condenser
The evaporator consists of the innermost ES plate, the middle EV and ED plates, and the outermost EE plate, one for ES and EE, and 49 for EV and ED, for a total of 100 plates, and the installation sequence is ES-EV-ED-.-EV-ED-EE. Each plate is composed of a substrate and a sealing strip, and there are two specifications according to the thickness of the plate, namely 0.5 mm and 0.4 mm (the thickness of the titanium alloy material of the substrate), and the sealing strip (rubber material) is adhered to the groove around the plate, and the overall thickness of the new plate is about 7 mm (substrate plus sealing strip). The total length of the plates after installation and compression is L=nx3.5 mm, n is the number of plates, and the total length of the plates of the new machine is 350 mm.
The condenser is composed of the innermost KS plate, the middle KV, KD plate, the outermost KE plate, one KS and one KE plate, 49 pieces each of the model KV and KD, a total of 100 pieces, and the installation sequence is KS-KV-KD -….- KV-KD-KE. The thickness of the plates and the overall length after compression installation are the same as those of the evaporator, which is 350 mm.
1.2.4 Other parts
Other parts include mist eliminator, feed water pressure regulator and throttle orifice plate, condensate pump, salinity meter, control panel, condensate outlet pipeline, and other accessories (auxiliary equipment) of the body, these parts are relatively simple in structure and have a low failure rate, so they will not be repeated here.
2. Failure instances
2.1.1 Fault phenomenon
A container ship sails in the ocean with a stern draft of 14.3 m, a water generator, a vacuum degree of 60%, a pressure of 0.32 MPa into the jet pump, and a back pressure of 0.11 MPa in the jet pump, which cannot make water.
2.1.2 Troubleshooting process
Dismantle and inspect the seawater pump of the watermaker, and find that the impeller, the upper and lower leakage rings and the pump cover are seriously pitted, replace the impeller, the old parts of the lower leakage ring in slightly better condition, install the retest machine, the vacuum degree is 80%, and the injection pump is 0.38 MPa, which still cannot make water.
It is suspected that the condenser is not smooth and the injection pump is ineffective. Dismantling and inspecting the evaporator and condenser, it was found that the plates were not very dirty; The jet pump was dismantled and inspected, and the stainless steel nozzle was intact, and it was found that the hole diameter of the red copper nozzle was significantly larger, and the inner wall of the diffuser pipe was uneven. After reloading, the vacuum degree is increased to 85%, and the vacuum degree is reduced to 70% after opening and heating, and no water can be made. At this time, it was found that the liquid level of the observation mirror was full, and the adjustment of the feed water pressure regulator had no effect, and the dismantling and inspection of the feed water pressure regulator found that the throttle orifice plate was corroded, and the aperture was equivalent to the pipe diameter and the throttling effect had been completely lost. Remake the throttle plate with a pore size of 10 mm, adjust the liquid level to 1/3 height after reloading, the vacuum degree is increased to 90%, and water can be produced, but the salinity (200 mg/kg) cannot be reduced, and the vacuum degree fluctuates in the range of 85%~90%. Dismantling the pipeline for pumping non-condensable gas, it was found that the jet pump was incoherent in suction, and there was a phenomenon of seawater backflow. After turning off a group of central coolers, the inlet pressure of the seawater pump is increased to 0.26 MPa, the outlet is 0.55 MPa, and the jet pump is 0.41 MPa, the vacuum degree is stable at 90%, and the salinity fluctuates in the range of 100~200 mg/kq.
2.1.3 Causes and treatment of failures
Comprehensive analysis, the cause of the failure of the water generator is the poor efficiency of the jet pump. After the spare parts are supplied to the ship, the red copper nozzle and diffuser pipe of the jet pump are replaced. When the draft of the same stern is 14 m, the vacuum degree of the watermaker reaches 100%, the salinity is maintained below 10 mg/kg after the water production is stable, and the water production is 24 t/d (the water production can reach 30 t/d after adjusting the heating capacity), and the watermaker returns to normal.
2.2 The failure of the check valve of the outlet pipeline of the condensate pump leads to the inability to produce water
2.2.1 Fault phenomenon
For a container ship, after the water generator of a certain voyage is started normally, the condensate pump is turned on, but there is no pressure at the outlet, and other parameters are normal.
2.2.2 Troubleshooting process
Touching the vacuum pipeline with your hand and finding that the pipeline is cold, it means that the water production is normal, the condensate pump does not work, and the condensate water level rises to the suction port of the vacuum pipeline and is pumped away by the jet pump. Stop the machine to dismantle and inspect the condensate pump, check that the impeller and shaft seal are normal, and the failure to restart the machine is still the same. Finally, the outlet pipeline of the condensate pump was dismantled and inspected, and the outlet check valve was faulty, and the stop blocking the ball in the check valve was missing.
2.2.3 Causes and treatment of failures
Comprehensive analysis, the cause of the failure of the water generator is the failure of the check valve of the outlet pipeline of the condensate pump, and the inner ball is stuck in the outlet elbow, resulting in the pump not being able to discharge water and has been in the state of “stuffy” pump (that is, the pump outlet valve is not opened during the pump operation), and the check valve is troubleshooting after repair.
2.3 The lack of the valve core of the outlet valve of the seawater pump leads to the inability to make water
2.3.1 Fault phenomenon
In a container ship, when the watermaker is turned on on a certain voyage, the outlet pressure of the seawater pump and the inlet pressure of the jet pump are basically normal, but the vacuum degree of the watermaker is only about 20%.
2.3.2 Troubleshooting process
Dismantling and inspecting the jet pump, it was found that the main body in front of the stainless steel nozzle was seriously corroded, and a hole with a diameter of about 2cm appeared.
Temporarily use iron and cement to repair the rotten parts, and the watermaker will return to normal. After a period of time, the water generator had problems again, and the vacuum was low and could not make water, this time the outlet pressure of the seawater pump was normal, but the inlet pressure of the jet pump was low.
Check whether the condenser is dirty and blocked, and dismantle and clean the condenser to find that the condenser is not dirty. A re-examination of the jet pump also found no abnormalities. However, when closing the outlet valve of the seawater pump, it was found that the non-resistance valve stem could be closed to the end. Immediately dismantled and inspected the outlet valve of the seawater pump, found that the valve was missing, dismantled the seawater pipe section by section, and found the broken valve core blocked in the middle of the pipeline, and after taking it out, the fault was solved.
2.3.3 Causes and treatment of failures
According to the comprehensive analysis, the cause of the failure of the water generator was caused by the decline of nozzle performance for the first time; The second time is that the fragments of the valve core of the outlet valve of the seawater pump are blocked somewhere in the pipeline, which causes the pressure and flow rate of the incoming jet pump to decrease, so that the vacuum pumping capacity of the jet pump is greatly reduced.
2.4 Marine organisms block the pipeline to the sea, resulting in the inability to produce water
2.4.1 Fault phenomenon
A container ship, which has been running on domestic trade coastal routes for a long time, is rarely used as a watermaker. When the watermaker is turned on on a certain voyage, the outlet pressure of the seawater pump and the inlet pressure of the jet pump are basically normal, but the vacuum degree is only about 35%, and the water cannot be produced.
2.4.2 Troubleshooting process
The back pressure at the outlet was 0.18 MPa, and the draft was about 11 m at that time, and the normal back pressure should be about 0.09 MPa, which was suspected to be blocked by the pipeline to the sea. In the state of keeping the seawater pump of the watermaker open, the pipeline after knocking on the oscillating sea valve found that the back pressure dropped significantly, and after continuous knocking for about 1h, the back pressure dropped to 0.1 MPa and no longer decreased, the vacuum degree had been restored to 95%, and the watermaker returned to normal.
2.4.3 Causes and treatment of failures
According to a comprehensive analysis, the reason for the failure of the watermaker is that the watermaker of the domestic trade ship has less use time, which leads to the accumulation and growth of marine organisms in the pipeline behind the sea valve, blocking the sea pipeline, causing the back pressure of the sea pipeline to become high, thereby affecting the efficacy of the jet pump. Troubleshooting after cleaning the pipeline after the sea valve.
3. Management and maintenance
3.1 Key points of equipment maintenance
3.1.1 Seawater pump maintenance
The key points of maintenance of seawater pump are as follows,
1) Pay attention to the working conditions of the seawater pump in daily work, if the pressure at the outlet of the pump drops too much, check the working conditions of the impeller, pump cover and leakage ring, whether the pressure is affected by excessive cavitation.
2) Pay attention to the inlet filter of the seawater pump, the pore size of the factory filter is large, and the fine filter screen needs to be installed by yourself.
3) The suction port of the seawater pump comes from the seawater main pipe or the main seawater pump outlet header, and it is recommended that the suction port should be designed as the main seawater pump outlet header, which has the advantage of increasing the inlet and outlet pressure of the seawater pump of the watermaker, and can reduce the impeller cavitation while maintaining the positive pressure of the suction port. This method has no effect on the central cooling system. There have been examples of retrofitting of old ships, and new ships have changed the suction port of the seawater pump to the main seawater pump outlet. Since most of the outboard pipelines of the jet pump are at the bottom of the engine room, the back pressure is high (taking a certain round of measurement as an example: the back pressure of the 11 m draft jet pump is 0.06 MPa, and the back pressure of the jet pump can reach up to 0.12 MPa when the draft is 14 m at full load normally. The design of the main seawater pump outlet main has one advantage: the pressure of the seawater main can be increased by shutting down a group of central coolers to maintain the normal cooling needs of the engine room equipment, so as to increase the inlet and outlet pressure of the seawater pump.
3.1.2 Jet pump maintenance
The maintenance points of the jet pump are as follows.
1) The stainless steel nozzle is not easy to break, the red copper nozzle needs to be replaced according to the situation, the nozzle becomes larger, the efficiency of the jet pump becomes worse, the maximum wear of the diffuser pipe is 20%, and it needs to be replaced if it exceeds the standard.
2) There are two major factors that affect the working conditions (suction capacity) of the jet pump when the design of the jet pump is finalized: one is the working water pressure of the jet pump, the higher the working water pressure of the jet pump, the better the effect of the jet pump; The second is the outlet back pressure of the jet pump, the lower the outlet back pressure of the jet pump, the better the efficiency of the jet pump. If you want to improve the vacuum of the water generator, you can start from these two aspects.
3) There are three influencing factors of the inlet pressure of the jet pump: the efficacy of the seawater pump, the dirty blockage of the condenser, and the feed water rate. The poor efficacy of the seawater pump directly affects the inlet pressure of the jet pump; The working water at the inlet of the jet pump comes from the outlet of the condenser, and the dirty blockage of the condenser will cause the inlet pressure of the jet pump to be low; If the feed water rate is too large, the inlet pressure of the inlet of the injection pump will be lower and the flow rate will be smaller.
4) The biggest influencing factor of the outlet back pressure of the jet pump is the ship draft. The draft becomes larger, the back pressure becomes higher, and the jet pump becomes less effective. Most of the seagoing pipes of the water-making machines designed by ships are on the bottom floor of the engine room, and this design can rarely meet the requirements of back pressure less than 0.06 MPa when the ship is fully loaded.
3.1.3 Maintenance of feed water pressure regulator and throttle orifice
The maintenance points of the feed water regulator and the orifice plate control are as follows.
1) The normal feed water rate is 3~4, the feed water rate is too small, the evaporator is easy to scale, and the feed water rate is too large, which affects the working water pressure of the jet pump inlet and thus affects the vacuum degree of the device.
2) The manual requires that the height of the observation mirror is less than 20 mm, and the total visual height of the observation glass is about 70 mm, that is, the water level is required to be below 1/3 of the observation glass
3) The elasticity of the spring of the feed water regulator will be weakened, which will cause the pressure regulation failure, due to the limited length of the factory adjustment bolt, when the spring elasticity becomes weak, it needs to be replaced with an extended lead screw to regulate the pressure
4) The throttle orifice plate is generally self-made, and the middle hole of the copper plate (the normal optional aperture range is 6~14 mm) is installed behind the pressure regulator and cooperates with the pressure regulator to control the feed water rate. After long-term use, the copper plate will become larger due to corrosion, and it is difficult to achieve the adjustment effect only through the pressure regulator, so the throttle orifice plate needs to be replaced.
3.1.4 Maintenance of evaporator and condenser plates
The key points of maintenance of evaporator and condenser plates are as follows,
1) Pay attention to the difference between evaporator and condenser plates. The EV plate and KV plate substrate of the evaporator and condenser are exactly the same, the sealing strip arrangement is different, the upper opening is the same as the upper opening, the evaporator plate opening is connected to the seawater channel for evaporation, the cooler plate opening is the inlet of steam entering the condenser, and the lower left sealing strip of the condenser plate is left with a gap to connect the condensate outlet channel. The ED sheet is similar to the KD sheet in that the lower left corner of the ED sheet is notched with a sealing strip and the plate has a small hole that connects to the seawater feed channel for evaporation.
2) Pay attention to the installation requirements of evaporator and condenser plates. If the evaporator and condenser want to reduce the plates, they need to be reduced in pairs (V and D plates are paired), and the total installed length after the reduction is reduced by nx3.5 mm.
The fastening of the plate is realized by two guide rods, 4 fixing bolts and 1 pressing plate. When installing or dismantling and inspecting, the 4 bolts should be evenly pressed or loosened, and when the bolts are tightened or loosened, each bolt should be tightened or loosened at most 2 times to achieve the purpose of uniformity to prevent plate deformation). In the maintenance process, if it is found that the pressure test is still leaking after tightening to the standard length, it is necessary not to blindly continue to tighten, and it is necessary to mark the leakage point, check the cause of leakage, replace the deformed plate or remove the deformed plate. With the increase of service life, the rubber plate sealing strip will deteriorate, affecting the elasticity and sealing needs to be replaced. When the sealing strip has good elasticity, it seals reliably and can protect the plate substrate during the tightening process. If the sealant is aged and loses its elasticity, over-tightening the bolt will cause irreversible permanent deformation of the plate, resulting in the scrapping of the plate.
3.2 Key points of operation management
3.2.1 Meet water quality requirements
The average salt content of seawater is 3 500 mg/kg, and the salt content of freshwater is generally less than 1 000 mg/kg. The water quality requirements of each equipment in the engine room: the water quality of the cooling water of the main and auxiliary engines is required to be less than 100 mg/kg; The separator requires the salinity of the water to be less than 100 mg/kg; The boiler requires the water salinity to be less than 10 mg/kg. Therefore, attention should be paid to controlling the salt content of fresh water produced by the water generator to be less than 10 mg/kg.
3.2.2 Establish a parameter table of working conditions of the water generator
Establish a parameter table of working conditions of the water generator, indicate the parameter range and design value during normal operation, and record the parameters daily, including: vacuum, evaporation temperature, seawater inlet and injection pump pressure, condensate pump outlet pressure, jet pump back pressure, cylinder jacket water inlet and outlet temperature, seawater inlet and outlet condenser temperature, condensate salinity, seawater pump current, seawater pump inlet and outlet pressure, seawater main pressure, seawater temperature, stern draft, condensate flow. In this way, the operating conditions of the watermaker can be analyzed through the change of parameters, which is helpful to accurately judge and troubleshoot.
3.2.3 Dosing management and reduce evaporator fouling
The function of the water generator is to eliminate foam, improve the quality of water production, and inhibit the scaling of the evaporator. For example, the normal dosage of the Urator product VAPTREAT is 30 mL/t, and the dosage can be adjusted according to the amount of water produced, and the original scale can be removed when it is twice the normal dosage.
When the watermaker is stopped, first turn off the jacket water heating device and continue to run the seawater pump for a period of time, so that the body and the evaporator are completely cooled and then stop the seawater pump, put twice the usual amount of medicine in the dosing barrel and fill it with fresh water, do not open the vacuum destruction valve, and slowly suck the medicine in the dosing barrel into the watermaker through the vacuum degree of the device, so as to achieve the purpose of full water maintenance on the seawater side of the evaporator plate. During normal operation, the vacuum degree of the watermaker is 90%~95%, and the corresponding evaporation temperature is 50~40 °C.
3.2.4 Keep the condenser seawater inlet channel unblocked
The aperture of the seawater inlet channel of the condenser plate is small, and the garbage with large diameter in the seawater pipeline is easy to accumulate at the inlet of the pore, which causes the flow rate of cooled seawater to decrease and affects the effect of the watermaker. Pay attention to the operating parameters during the daily inspection, and when the pressure difference between the inlet and outlet of the condenser becomes larger, the seawater inlet pipeline of the condenser can be dismantled and inspected to remove the garbage attached to the plate channel.
