When a Backup Isn’t a Backup

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The International Association of Engineering Insurers found that the highest frequency of steam turbine failures worldwide is due to loss of oil pressure. Most of these failures are caused by an unreliable backup system to maintain oil pressure to the bearings should the primary AC-driven lube oil pumps fail. These AC motors are powered by either the turbine’s output or the grid, and will fail if the turbine or generator trips, or if there is an external outage.

Modern turbines have backup powered DC oil pumps mounted on the oil tank which are triggered by a pressure switch in the event of a loss in oil pressure. It is very important to conduct tests with the AC and DC oil pumps during scheduled maintenance inspections to ensure that the DC pump engages as required. Such tests can be referred to as cascade pump pressure inspections. These tests will confirm the pressures when the DC oil pump will engage after the AC oil pump is actually turned off. Backup batteries should also be verified. These tests should be performed on a regular basis when the unit is down and mandatory tests should be performed before the unit is placed in operation after an overhaul.

Older turbines can use steam-driven pumps as backup. On these designs, a pressure regulator will sense the drop in bearing oil pressure and turn on the steam supply to the blade wheel of the pump. While these pumps are usually very reliable, they still must be manually tested on a regular basis and after an overhaul. Care must be taken to not overspeed the pump or it will cause internal component damage and may even completely destroy the pump.

Some older turbines use gravity lube oil tanks.  These tanks are mounted above the unit on stands and are controlled by a check valve type of arrangement.  There are no pumps involved; gravity provides the bearings with sufficient lubrication in an emergency situation. While less complicated than DC or steam powered backups, their operation must still be routinely checked.

Bottom line, a backup is not a backup unless it is reliable. And it can only be reliable if it is tested.