Auxiliary Hydraulic Oil Pump Hard-Wired Logic


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We have 2 GE frame#9E 115 MW base load capacity gas turbines at our site. Both have been commissioned around 1999. HMI of Mark 5 had undergone up gradation from DOS based to Windows based, without any change whatsoever in control loops & systems. Recently, during a FSNL operation [initially the unit was in barring with auxiliary oil pump [AOP] in service], it was observed for one of the gas turbines that the auxiliary hydraulic oil pump [AHOP] failed to come into service. As a result of which the unit was kept in crank forced condition [Auto/Cable Remote/Start command was initially given from HMI for FSNL]. On checking the AHOP module it was found that QL [lube oil pressure adequate] switch had not energized during barring gear operation. As a result of which AHOP was not taking start command [both from Mark 5 as well as from local module start]. However, on checking Mark 5 rungs, it was found that L63QAL lube oil pressure switch adequate had got energized, & L4HQZ AHOP start command was going correctly from Mark 5. Finally, the switch was shorted from local & AHOP took start. On checking the other gas turbine also, the same hard wired logic was found. Since the AHOP takes suction from lube oil discharge header, the logic makes sense. But since this is already given from Mark 5, is a separate hard-wired logic also required? I have seen this fail safe hard-wired logic for emergency oil pump starting, but that is for saving the turbine bearings. Is it safe to remove this hard-wired logic or is it recommended by GE as additional safety for protection AHOP against dry run? Is it really required for AHOP module since the main hydraulic oil pump [shaft driven from accessory box] has no such extra protection? Is this logic incorporated in any other GE frame 9E or any other frame units?

There was a period of time, right around the commissioning time-frame you cited, where GE was transitioning from the hard-wired low oil pressure contact in the Aux. Hyd. Oil Pump motor starter control circuit to wiring the contact to the Mark V and then using software for the permissive to run AND the oil pressure permissive to run. That would mean there were two (2) contacts from the pressure switch--one to the Aux. Hyd. Pump motor starter control circuit, and one to the Mark V. Is that correct? Or, are there two pressure switches, one wired to the motor starter control circuit and the other wired to the Mark V?

Or, some units had a relay output which was driven by the low oil pressure switch input which was wired in series with the L4HQZ output to the MCC starter circuit (when the hardwired contact to the MCC starter was "moved" to the Mark V).

Part of the reasoning was that 63QL sometimes had two contacts--one for the Aux. Hyd. Pump motor starter control circuit, and one to the Mark* for a low lube oil system pressure alarm. I believe, the thinking was to eliminate some of the interconnecting wiring, AND to replace a switch which needed two separate sets of contacts (one for the AC motor starter control circuit) and one for the Mark V contact input (125 VDC) with a switch that only required one set of contacts (a small cost reduction--very small).

My presumption is that the commissioning person was not aware of the change-over, and should have deleted one or the other.

As you cited, it's important to have the Emer. L.O. Pump oil pressure permissive hardwired to the pump motor's motor starter control circuit to protect the turbine bearings.

If you remove either, there's a risk of the Aux. Hyd. Pump motor not starting on low pressure, and that presents a risk of tripping if the Main Hyd. Pump fails.

I would think deleting either would be acceptable, and the simpler one would be the best choice. You would have to add a jumper to the MCC starter circuit to "replace" the pressure switch contact, and you would have to modify sequencing to remove the pressure switch permissive from the L4HQZ output.