Back Pressure Valve in Oil Distribution
Lube oil console are used in turbo machinery packages for e.g. steam turbine, gas turbine, compressor packages etc. for supplying clean and cool lubricating oil to the seals & bearings.
The pumps operate in a primary/auxiliary setup, where the primary pump drives the system pressure to desired condition with the auxiliary pump providing a backup in the event of primary pump downtime or maintenance. The auxiliary pump turns on when the system pressure drops below the optimal condition, or it can be started manually by the user in order to turn off the primary pump case of scheduled maintenance.
Why we need Back Pressure valves in lube oil system:
As we have discussed earlier the primary and auxiliary pump can switch back and forth between them. At times while the primary pump is running, the auxiliary pump is started, causing both pumps to act together for some time before primary pump is switched off and maintenance work performed on it. For that time span where both pumps act together there is a sudden high flow in the system and there is a need to regulate this pressure so that it does not damage the system. As you can see in fig:1 there is safety valves to protect the pump and vent to the reservoir. But it’s wrong to think that these safety valves will protect the system by popping up when there is over pressure. It’s so because ideally these safety valves are so designed that it should not lift during standard operation. Here safety valves act as last line of defence and for large swings in flow that results from pump switching, commonly a direct operated back pressure regulator tied into the pump header is used which senses pressure changes and adjust flow accordingly. Obviously, the location from where it senses the pressure and regulates the pressure is also important.
The proper operation of turbo machinery bearings relies on clean lubricating oil appropriate for the application and efficient transfer of heat out of the bearings. Because the lube oil filter and heat exchanger see a considerable pressure drop, it is common for the back pressure regulator to use an external line which senses pressure downstream of the heat exchanger and filter and keep constant, controlled pressure supplied to the downstream bearing and seals.
How a Back Pressure valves works:
Relief or back pressure valves respond to changes in upstream pressure or the pressure which it senses from the external control line. First the desired pressure is set by adjusting screw present at top of the valve body. Pressure changes register under the diaphragm (see Figure 1) through a registration hole in the valve body or through an external control line. When this pressure increases above the spring setting, the pressure underneath the diaphragm overcomes the spring compression. This causes the valve plug to move away from the orifice. The flow path through the valve is open and excess pressure is vented. When upstream pressure drops below set point, the valve closes.