Steam Turbine lube oil pressure

[Emkel]

Hello newbie here,

I'm just curious about something.

You know Turbine have some interlock specs.

This is our turbine lube oil press interlocks
Aux lube oil pump outlet press : 17.5 Bar
Lube oil press normal : 0.8 - 1.2 Bar
AC lube oil pump auto start : 0.69 Bar
DC lube oil pump auto start : 0.59 Bar
Turbin Trip : 0.4 Bar


I have a question, what things that makes changes in lube oil pressure? Sometimes it can be too low or too high.

And, you know from the interlocks spec if the pressure too low, AC pump will be turn on automatically. what is the relation of low pressure with AC pump? Because as i know, AC pump is used if the turbine speed is too low and replace main oil pump function.

 

[byrdj]

Oil pressure is determined by the equilibrum between discharge and restriction.
the condition of the pump can effect its discharge, ie, worn pump, lower discharge
the restriction is provided by the devices using the oil.
one use is the bearings, worn bearings can use more oil and thus lower the pressure
filters/strainers in the supply to the bearings can be wrong size of get plugged and thus increase the pressure

other devices that use oil could be valves. check valves not blocking. Broken pipes and leaks are also possible

besides the balance of discharge and restrictions, viscosity changes greatly effect pressure. the common change is as oil gets hotter, oil pressure goes down

 

[romke]

As mentioned: luboil pressure is the result of the balance of discharge and restrictions. luboil pressure also is rather simple to measure, and thus specifications for lube oil pressure in particular situations is more common then specifications of a certain amount of flow.

In a turbine system you will generally have two different lubricating systems, where each system may have one or more pumps. One system is used to lift the turbine shaft hydrostatically in its bearings before the turbine starts to rotate when started. That lubsystem thus is designed to prevent wear at the start. When the turbine runs the other systems supply a large flow of oil to the bearings to ensure that there is sufficient flow to the bearings to make sure that hydrodynamic lubrication can be achieved. That large amount of oil need not be supplied at high pressure, it is the flow that counts. In the bearing itself pressure buildup will take place due to the fact that the turbine shaft will run somewhat off centre, creating a "lubrication wedge" with sufficient pressure (several hundreds of bars) to prevent the turbine shaft to come into contact with the bearings. When the turbine is shut down the hydrostatic lubsystem will also prevent contact between shaft and bearings to prevent wear as long as the turbine still turns before coming to a complete standstill.

If you look at the socalled "Stribeck curve" you can see why this type of solution is chosen. In the right part of that curve there is hydrodynamic lubrication and essentially only fluid friction. In the left part there is very high friction (boundary lubrication) initially that changes to mixed lubrication when the rpm of a shaft in the bearing is increased. Incorporating a hydrostatic lubesystem will prevent wear and contact between shaft and bearing when operating within that part of the curve - a situation that occurs when starting up or stopping the turbine.