Common ways to break your main equipment and how to avoid them
If News 2/2017 Property. The lifeblood of any rotating machinery is the lubricating system. Steam and gas turbines in the power industry normally uses journal bearings that consist of a shaft that freely rotates, in a supporting metal sleeve, without rolling elements.
For the turbine shaft to move freely, it rides upon several lubricant-filled bearings into which the lubricating oil is pumped under high pressure. The oil lubricates the bearings through hydrodynamic lubrication. The bearings and the shaft are separated by a pressurized film of oil to prevent any metal-to-metal contact.
The purpose of the oil system is to:
- Lubricate journal and thrust bearings and the oil pumps
- Provide efficient cooling for the bearings
- Provide the control medium in the hydraulic governing system
- Supply the sealing medium for the hydrogen cooled generators. Image 2. Oil system (LIES, Lubrication Engineers)
Improvements and actions
During emergencies in the power industry, when the electricity supply is lost, the turbines are tripped and coasted down with emergency systems. These systems include a DC lube oil pump, possibly a DC seal oil pump and a battery bank as an electricity source. A major part of turbine related claims come from a malfunctioning DC system causing extensive damage to the turbine.
Far too many power plants simply rely on the assumption that they have an auxiliary pump, emergency generator and battery bank to protect the turbine. The design or operation of these items is taken for granted. There might be, however, disparity during the design phase of the systems between the process designer, the turbine OEM and the oil system component providers. For example, the provider of the DC motor might not understand that there is need for a run-to-failure type of equipment.
A couple of recent cases of lube oil system breakdowns, involving both modern and old turbine installations, have been the result of insufficient battery bank capacity. Although the emergency system, in these cases, worked perfectly, the capacity for operating the emergency lube oil system was insufficient and resulted in turbine failures, despite the emergency system, otherwise, working perfectly.
The actual capacity of the battery banks must be oversized, based on the calculated need in a worst case scenario, and the capacity should be tested regularly. Some operators prefer to procure batteries from different production batches in order to avoid possible manufacturing defects.
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A hazard and operability study (HAZOP) or similar process is recommended to ensure that the process and related equipment is suitable for emergency purposes. The HAZOP process is quite labour-intensive, but when risk areas are known, this process can be done in a more efficient manner.
Incorporating machinery breakdown data is a good starting point for an easier process. Please ask your If specialist for support with this process. When designing processes change, estimate the need and extent of HAZOP carefully. Some starting points for the HAZOP process are:
- Up-to-date and annual inspection of PI diagrams and system descriptions
- Connection of the emergency generator to the battery charger
- DC Motor protections should only send an alarm to the operators and trip functions should be disabled
- DC switchgear and switchboard protection settings in place
- Cabling solutions for DC motors including feed routing and fire retardant coating
- DC circuit breaker curve
- Fire protection for the DC system
- DC switchgear or board fire
- Oil leakages resulting in fire
- Battery bank capacity and testing. These should be included in all relevant operational and safety instructions.