The power part of the diesel fire pump used in a certain offshore oil emergency station adopts a John Deere 6135 series electronic fuel injection diesel engine. During multiple weeks of testing, it was found that the diesel engine exhaust pipe turned red, the exhaust pipe temperature exceeded the standard, the speed was unstable, and there was severe shaking. After inspecting the intake pipe, oil circuit, twin turbine mechanism, and intercooler of the diesel engine, it was finally found that the exhaust hose was broken, the cable was corroded, the turbine mechanism was damaged and worn, and the intercooler was corroded and leaked seawater. Through the maintenance of the faulty parts and the replacement of the intercooler, the above problems of the diesel fire pump were perfectly repaired and the equipment was restored to normal operation.
Basic structure of diesel fire pump
The emergency fire pump is the last firefighting facility in case of a fire on the platform, which can save the lives of everyone on the platform and protect the safety of all equipment at critical moments.
Therefore, emergency fire pumps are an indispensable part of platform equipment. Given the importance of emergency fire pumps, the engine part of the fire pump must undergo regular maintenance, inspection, and repair.
The John Deere diesel fire pump was designed and installed on the 17 meter deck of the offshore platform in 2012 (outdoor placement). After 8 years of operation, the diesel engine body suffered severe corrosion in multiple areas due to the high salinity and humidity environment at sea. Based on this, independent pry block protection was installed on the platform at the end of 2020, and remote control and other electronic control modifications were carried out.
The general operation principle of diesel fire pump is as follows:
After the diesel fire pump is started (by pressing the start button locally or remotely starting from the central control room or changing the outlet pressure of the fire pipe network), the 24Vdc electric motor is driven by the battery to drive the flywheel to rotate. The flywheel drives the crankshaft to move, and the crankshaft drives the piston to compress the air and diesel mixture. After the misty mixture is pressurized, it is ignited sequentially through six sets of spark plugs to start the diesel engine.
After being filtered, the fresh air will increase in temperature after turbocharging, and then be cooled to 25-60 ℃ by the intercooler to prevent the compressed air temperature in the cylinder from being too high and the air flow from being diluted, resulting in insufficient diesel combustion and decreased power.
The intercooler in this device is cooled by seawater. When the fire pump is running normally, seawater will be drawn from the seawater outlet pipeline to the intercooler, where it will exchange heat with compressed air. After heat exchange, the seawater will be discharged into the sea, and the cooled compressed gas will enter the cylinder for combustion, allowing the diesel engine to output power.
Troubleshooting, problem identification, and process handling
1. Exhaust circuit soft
In the shutdown state due to a broken connection, a significant fracture gap was found in the exhaust hose connected to the exhaust elbow and intercooler inlet pipeline on the right side of the diesel engine;
The fixed connecting rods on both sides of the hose are severely corroded and have detached from the fastening bolts.
After analysis, the exhaust hose is fixed by fixed connecting rods on both sides, which play a role in fixing and shock absorption. Due to the harsh marine environment, the connecting rods are corroded and broken. The exhaust hose lacks support and fixation. During multiple weekly tests, the high-temperature and high-pressure exhaust pressure vibration caused the hose to vibrate violently for a long time, ultimately resulting in mechanical fatigue and cracking and air leakage.
Diesel engine exhaust hose rupture
Solution:
Replace the exhaust hose with a new one and the stainless steel locking buckle;
Remake a new hose fixing rod and restore installation after anti-corrosion treatment.
2. Turbocharging mechanism malfunction, cable damage
According to the maintenance records, this diesel fire pump underwent maintenance in mid October 2019. Due to a blockage in the turbocharger mechanism, a new turbocharger was replaced.
During the testing period of 2019-2022, it underwent multiple repairs and currently shows the following performance:
After the engine starts, it runs normally for one minute. When the pump water pressure reaches 10MPA, the exhaust pipe turns red and overheated, the engine speed is unstable, and there is an oil truck phenomenon, emitting thick black smoke. The engine shakes significantly, and sparks are generated by the friction between the turbocharger blades and the volute.
Turbocharger temperature abnormal and burned red
Abnormal operation of diesel engine, including:
1. The transmission mechanism of the turbocharger is malfunctioning, and the turbocharger cannot be opened normally. After about two minutes of operation, the turbocharger housing burns red.
2. The unstable speed of the diesel engine results in unstable pressure and displacement of the fire pump.
3. During the operation of the diesel generator set, the exhaust pipe emits heavy black smoke.
After inspecting various systems of the diesel engine, the maintenance personnel preliminarily determined that there was a mechanical failure inside the turbocharger of the diesel engine.
The maintenance personnel found during inspection that:
The wear of the copper sleeve and shaft of the turbocharger causes an increase in longitudinal and axial clearance. Friction occurs between the turbocharger intake turbine blades and the volute shell, and the generated debris enters between the valve and the valve seat ring through the intake duct. The tight cover between the valve seat ring and the valve is worn, resulting in poor sealing. Some high-temperature and high-pressure gases will pass through the intake valve, intake pipe, and intercooler until they are flushed out of the air filter intake port, causing increased intake resistance, insufficient intake, incomplete combustion, and incomplete combustion of diesel fuel in the exhaust pipe. The exhaust pipe turns red, the power is insufficient, the speed decreases, and other phenomena occur.
Starting the load test of the diesel fire pump, it was found that there was severe air leakage in the exhaust manifold, resulting in a decrease in gas flow and heat conversion on the turbocharger turbine side, causing a low turbocharger speed and a decrease in the amount of air entering the cylinder, leading to high exhaust temperature and red exhaust pipe.
Dealing with exhaust pipe leakage issues:
1. Remove the muffler and bellows
2. Drain the cooling water from the unit
3. Remove the turbocharger and disassemble for inspection (excluding any damage to the turbocharger)
4. After removing the exhaust manifold and other corresponding components, it was found that the exhaust pipe gaskets of cylinders 5 and 6 were damaged, causing air leakage. Six new exhaust pipe gaskets were made of graphite material
5. Reinstall the exhaust manifold, reinstall the turbocharger, reinstall the muffler, repair and install it, and test the unit. First, bypass the actuator of the turbocharger, wrap the control circuit plug with electrical tape, and let the turbocharger power reach its maximum;
During the first test, the lever of the turbocharger actuator was lifted upwards in the wrong direction and not fully opened, resulting in the exhaust pipe still turning red;
The second test was conducted by pulling down the actuator rod in the correct direction. The turbocharger speed was normal, the exhaust pipe did not turn red, and the diesel engine speed was also normal. After running for more than 10 minutes, the machine stopped normally and the test was completed.
The reason for the damage to the exhaust pipe gasket may be that the diameter between the corrugated pipe of the exhaust pipe and the muffler has become smaller, and the diameter of the inlet end of the muffler is also very small, causing an increase in exhaust back pressure. Over time, the exhaust manifold gasket is damaged.
It is recommended to reprocess the exhaust pipe. The diameter of the exhaust pipe will increase with the number of bends and the length, and there is no possibility of it becoming smaller than before.
Wrap the plug of the actuator control circuit of the turbocharger with electrical tape and do not need to be reconnected.
Does not affect usage. (Originally, the platform personnel also fixed the actuator of the turbocharger by pulling down the rod, but did not remove the control line plug, which could easily damage the actuator. Therefore, the plug circuit is now removed).
After replacing the turbine mechanism and conducting multiple tests, it was started and operated for 5-10 minutes with the rated output load. The exhaust temperature remained stable at 400-500 ℃, and the turbocharger area no longer turned red. The problem was resolved.
3. Multiple fault codes are displayed locally
During the initial maintenance, it was discovered that multiple fault warning codes were displayed on the local monitor, including the communication warning SPN3822 for the 2nd stage exhaust circulation pump (Second ECR sensor cable) and the disconnection warning SPN641 for the turbine drive.
The fault alarm displayed on the local control box module is SPN3822 code, which is due to the previous inspection of the turbocharger jamming problem. As the turbocharger cannot automatically detect and execute the opening degree to prevent the turbine mechanism from misoperation, the connecting cable of the exhaust circulation pump was manually disconnected. Only by restoring the turbine mechanism later and connecting the cable, the relevant fault code can be eliminated.
The local control box module displays another fault alarm code SPN641, indicating a communication failure between the engine turbocharger actuator VGT valve and ECM. On site inspection found that there were two corroded and broken wires in the wiring harness between the ECM and the local control box, which happened to be the working power supply wire (J1-H3AI-H4) of the actuator VGT valve, causing communication failure between the ECM and VCT valve.
This connector belongs to a specialized connector and cannot be disassembled for maintenance, so it can only be handled simply on site. The two VCT valve power lines of J-H31-H4 are crimped onto a circular terminal block, then inserted into a round pin and tied tightly with zip ties to prevent poor contact. After processing, communication between ECM and VCT is normal and SPN641 code no longer appears.
4. Corrosion, rupture, and water leakage of the intercooler
After removing the exhaust soft connection, using a flashlight to shine inside the intercooler, it was found that there were continuous water droplets on the cooling fins of the intercooler. When touched by hand, there were granular water droplets with a strong fishy seawater smell, which was determined to be corrosion and perforation of the intercooler heat exchange tube.
The intercooler uses accompanying seawater to cool the fresh air after turbocharging. Seawater is highly corrosive, causing water leakage in the intercooler's heat exchange pipes. Due to the high position of the intercooler, at the highest point of the engine, and the fact that seawater only reaches the intercooler after starting the engine. When the engine is running, seawater passes through the intercooler and enters the intake manifold before entering the cylinder. Poor combustion in the cylinder causes carbon deposits and water vapor, which further corrode the valves and seat rings. The wear inside the cylinder intensifies, and the piston and piston ring wear.
After determining the corrosion perforation of the intercooler, considering that although seawater is corrosive, the amount of seepage is not too large, and the diesel fire pump is a critical emergency equipment and is in online backup, it is not allowed to dismantle and repair the intercooler for a long time. It is recommended that the oilfield site:
1. Reduce the frequency of equipment testing and switch from weekly testing to bi monthly testing;
2. Urgently purchase intercooler and replace it in a timely manner.
3, Summary of Investigation and Backup Suggestions:
After a detailed inspection by various maintenance forces in the oil field, it was discovered and resolved that the diesel fire pump had insufficient power and turbine heating caused by corrosion, abnormal high temperatures, and other reasons. Eventually, the diesel fire pump was restored to normal standby by replacing spare parts and changing operating procedures.
Due to the fact that the equipment is imported from the United States, the spare parts ordering cycle is relatively long, and it takes some time to recover from equipment failures. In addition, combined with the special emergency function of the diesel fire pump, the recommended operating procedures are as follows:
1) Do not start the diesel engine unless it is an emergency to prevent further damage to the engine;
2. Replace the intercooler;
Replace the turbocharger and exhaust pipe assembly;
4) Perform medium or major repairs, replace internal components such as valves, seat rings, piston rings, etc
This fire pump belongs to emergency equipment and is the most important emergency fire-fighting equipment in offshore oil fields. If there is a short-term abnormal state during the normal production period of the oil field, there is a certain safety risk for the oil field, as follows:
Risk 1
During large-scale fire sprinklers, it may cause low pressure in the fire pipe network and insufficient fire water supply.
Control measures:
1) Replace hot work with cold work and minimize the number of hot work as much as possible.
2) It is prohibited to carry out hot work simultaneously in multiple hazardous areas. Suspend open flame operations in hazardous areas.
3) The control measures for thermal work carried out in hazardous areas should be upgraded.
4) Strengthen thermal monitoring during hot work operations, equip fire extinguishing equipment, and improve the emergency response capabilities of fire observers.
5) Encrypt and inspect explosion-proof electrical equipment and electrical devices to eliminate fire hazards.
6) Check the status of international shore connections and hoses on the docking deck to ensure reliability and usability.
Risk 2
In the event of maintenance or platform power failure of the electric fire pump, the electric fire pump cannot start normally, and the diesel fire pump cannot be used, resulting in the failure of the fire protection system.
Control measures:
1) Encrypt and test the electric fire pump, and extend the maintenance period of the electric fire pump shutdown appropriately.
2) Spare one electric fire pump and make sufficient preparations before replacing the pump to shorten the maintenance time of the pump replacement operation.
3) Add portable, wheeled dry powder and foam fire extinguishers on site to enhance the ability to extinguish initial fires.
4) Check the international shore connection and hose status of the docking deck to ensure reliability and usability.
5) The power department encrypted and tested the emergency generator, attempting to supply power to the fire pump in the event of a power loss on the platform.
6) Promote the interconnection and intercommunication of fire protection pipelines between this platform and adjacent platforms.