Questions on Lift Oil and Effect of Orifice Pressure


1.We have 7 FA turbine, the lift oil pressure is 3500 psig, and the lockable flow control valve is adjusted so that the shaft is lifted by 5 mils approximately.I wonder what would happen to the pressure of lift oil if we increase the lifting distance and put is for example 7 mils instead of 5. Because in one hand we are opening the valve more which means less resistance but on the other hand the oil will be subjected to more load while lifting the shaft more distance.

The next question is not for a specific system, but to help general troubleshooting. What effect does the orifice have on the system pressure and flow rate if the only change was done on the system is the orifice? (upstream and downstream)

1. What would be gained by increasing the lift? The purpose of lift oil is to facilitate (make it easier) for the turning gear to rotate the shaft, and to prevent bearing damage at low speed when there's little oil wedge build-up. The bearings on most GE-design F/FA-class heavy duty gas turbines are tilting pad bearings, which require lift oil to protect the smaller, multiple bearing surfaces during low-speed operation. Once the shaft is "up to speed "--as determined by the OEM--lift oil is no longer required.

Without lift oil (sometimes called "jacking oil") the power required to turn the shaft at low speed would be much higher and the risk of bearing damage at low-speed operation would increase. Lift oil is an inexpensive way to achieve risk-free and "low power" low speed operation. If the OEM says 5 mils is good, what is to be gained by going to 7 mils?

2. This is just musing and speculation. The OEM has engineers who are knowledgeable and experienced in sizing orifices to achieve desired flow-rates and pressures to achieve safe and risk-free operation. This question is like asking, "What would happen if the turbine buckets were changed from impulse to reaction?" They can't be changed from one to the other--they are what they are.

We are (I am) happy to answer practical questions about systems and operation, but speculation and musing about changing design parameters for the sake of "discussion" is not productive. We are happy you have found; use the resource wisely and judiciously.
First, I am the happy one here since I have have someone (like you) answering my questions and others' question.

Second, I apologize if this was inappropriate. All I wanted is increasing my troubleshooting ability. I wanted to know how a change affects the system. Hopefully I will gain this troubleshooting ability by experience and by studying hard.

Best Regards.