When the unit is on turning gear and 88QA running, it delivers 5.5 bars, but when the unit runs to full speed no load and the main pump running, it is observed that the pressure drops to 2.7 bars.

We have shut down the unit to investigate. The I/C team have checked 96QA-2 and it gave the desired reading. I opened the compartment where the VPR-2 -1 is and found that there is a test point for I/C but I was a little confused that it wasn't closed. At that point, I was not sure if it is supposed to be so.

Suggestion is now being made for us to regulate the VPR-2-1 when we run up the unit.

Again, I must say at this point that we also checked for leakages along the line but there was none.

Please, I implore this honourable house filled will vast array of knowledge to share if any have had this problem and how did they go about solving it.

Thanks in anticipation of your usual support.

 

humble3,

When did this problem start? After a maintenance outage? Suddenly?

We presume you are measuring the bearing header pressure downstream of VPR2-1 (using 96QA-2 AND the pressure gauge on the Accessory Gage Cabinet for comparison's sake).

The L.O. System P&ID (Piping Schematic) should tell you whether the "I/C" test point should be open or closed (an X on the tap means it should be plugged/closed; a solid valve means it should be closed; an outline of a valve means it should be open). Sometimes there is a manual valve shown in the closed position with an X on the outlet of the valve, meaning there should be a pipe plug or pipe cap on the outlet of the valve.

VPR2-1 should have a bypass with an orifice in it to ensure flow to the bearing in the event that VPR2-1 should close (for some strange reason). The Main (Accessory Gear-driven) L.O. Pump has a relief valve on it to regulate the discharge pressure from the positive displacement pump. There is no pressure regulator (or, rather, there is usually no pressure regulator) on the discharge of the Aux. (AC motor-driven) L.O. Pump, which is a centrifugal pump.

There is usually a gauge for the discharge pressure of each pump (upstream of VPR2-1) on the Accessory Gage Cabinet. What do EACH of those gauges read (on turning gear, and at rated speed)? The Main L.O. Pump discharge pressure (set by the relief valve, VR1-1, if I recall correctly) is listed in the Device Summary drawing. Sometimes the typical discharge pressure of the Aux. L.O. Pump is shown on the P&ID drawing.

There have been a couple of different types of pressure regulating valves used for VPR2-1; usually it is a diaphragm actuated differential pressure regulating valve that is set by adjusting a large "nut" beneath the diaphragm. There is a sensing line which may have a leak, or there may be air in the diaphragm--both of which can cause problems. And, the diaphragm has been known to rupture (though that's extremely unusual for this particular valve and is usually only the result of improper adjustment/connection of sensing line after a maintenance outage).

There is also a check valve on the discharge of each L.O. Pump, and sometimes these are swing check valves and they do occasionally stick. (Note if you open/examine the swing-check valves there is SUPPOSED to be a small orifice hole drilled in the face of the check element--see the P&ID).

Use the P&ID to logically check all of the possible problem elements.

And, please write back to let us know what you find!!!