You really need to provide more information when you post a question like this for a proper response; we presume you are talking about a GE-design heavy duty gas turbine in the discussion below given the lack of information provided in the original post.
But, any L.O. system will likely have a cooler and some kind of temperature controller, so the concepts are applicable.
L.O. Header temperature is usually controlled by a temperature regulating valve. If we're talking about a GE-design heavy duty gas turbine, that temperature regulator is usually an adequate but somewhat dated design made by Robertshaw, which usues a sensing bulb connected to a bellows by a capillary tube. The concept is exactly the same as many temperature switches; as the temperature changes, the fluid in the sensing bulb and capillary expands or contracts, increasing or decreasing the pressure on a diaphragm. In the case of the Robertshaw actuator, the action is "multiplied" through the use of a bellows.
There are usually two issues with the Robertshaw actuators. First, they are usually slow to respond. Second, most people don't bother to read the manufacturer's manual about how to adjust the setting and think the manual crank on the side of the actuator is used to change the actuator's setpoint, which is not true.
So, if the unit is started/stopped frequently (once per day, or so), it's quite common for the L.O. Header temperature to not be spot on during the start-up and to take a while to reach stable operating temperature (presuming the actuator is working properly, and is adjusted properly).
The capillary tube is also usually not very well protected or secured, and quite frequently gets damaged during maintenance outages. This means that the actuator has to be manually adjusted at all times, since there is no feedback if the fluid has leaked out. (The fluid is usually a flourescent green, and doesn't smell very good, and evaporates fairly quickly, and leaves a sticky residue.) So, you should inspect the entire length of the capilllary from the actuator bellows to the sensing bulb. There should be no tight right-angle bends or kinks in the capillary, and it should not be broken or punctured.
The good news is that Robertshaw sell a replacement bellows/capillary tube/sensing bulb (they are one integral unit) unit.
The second thing is the adjustment of the actuator. The manual hand-wheel is only for manual "over-ride" of the automatic mechanism. The automatic mechanism is adjusted by turning a triangular-shaped piece of brass which changes the tension of the spring against which the bellows pushes. The triangular-shaped piece has three round openings, into which a the point of a long-bladed Phillips screwdriver can be inserted to use as a tool for rotating the brass piece one third of a revolution at a time.
Adjusting the setting of this valve takes patience--and the setting of the manual hand-wheel <b>MUST</b> set to the AUTO setting. Make small adjustments (one-third of a revolution) when the unit is operating at stable load and wait about 10 or 15 minutes to see the effect of the change before making another change, if necessary.
RTFM. Read The Fine Manual for more information.
As for low lube oil header pressure, it sounds like the L.O. Header temperature is too high, making the oil "thin" and it's possible the L.O. Pressure Regulator isn't working correctly? But, you haven't provided us much information about when the pressure is low (when the temperature is high? or when it's low? or ?).
Please try to remember: We aren't there on your site and we don't know the exact configuration of your system. While most GE-design heavy duty gas turbines are very, very similar, they are not identical. And, if you're talking about something other than a GE-design heavy duty gas turbine, we need to know that, too. If you want a quick, concise answer. If a stranger asked you why his room was hot, how would you respond? Open the window; open the door; turn on the fan; turn on the air conditioner; turn down the heater; close the shade on the window. You would likely ask him some information to provide a meaningful response. No difference in these Internet forums, either. The quality of your response is going to be directly related to the quality of the information provided in the post.
