Has GE stopped giving AHOP in MS5001 machines? If so, what is the back up of MHOP? How GCV/SRV and IGV calibrated in gas Turbine stopped condition.

 

What is AHOP and MHOP?

 

> What is AHOP and MHOP?
AHOP = auxiliary hydraulic oil pump (usually electric motor driven)
MHOP = main hydraulic oil pump (usually accessory gear driven)

 

Auxiliary Hydraulic Oil Pump has always been an option on GE machines. If you don't have one, you need to do all calibrations at cranking speed (or rig up a temporary supply from the ratchet pump).

 

VJ,

The Auxiliary Hydraulic (Oil) Pump (AHOP) has been an option on GE-design Frame 5 heavy duty gas turbines for decades. Without an AHOP there is no back-up to the MHOP (Main Hydraulic (Oil) Pump).

When there is no auxiliary hydraulic pump, generally the machine has to be CRANKed during LVDT calibration. Some sites have been very creative and have some special pieces of tubing to use to port the output of the Hydraulic Ratchet Pump to the Hydraulic System for calibrating LVDT feedback. But, many Hydraulic Ratchet Pumps do NOT use filtered oil--they just take their suction from the Lube Oil reservoir, unfiltered. This means that it's very easy to contaminate the teeny, tiny passages of the servo-valves with dirty oil. (The last-chance filters at the servo-valve manifolds don't really do much more than catch large dirt particles, and quite frequently are never changed and because they don't have dp switches across them there's no alarm generated when the filters get dirty and they end up rupturing and spilling a lot of the dirt inside them into the servo-valves....)

It's really not a good idea to say one calibrates the SRV/GCV or IGVs--it has false implications. All one is ever doing is calibrating LVDT feedback--<b>not</b> the servo-valve or anything other than the LVDT feedback. LVDTs are used on aircraft, specifically because they are highly accurate and their output doesn't drift like many other position feedback devices. It should almost never be necessary to calibrate or re-calibrate LVDT feedback--and it should never be done without first recording the as-found condition to determine if re-calibration is even necessary. LVDTs are just like temperature switches and pressure transmitters and other kinds of feedback devices. One periodically checks to ensure they are working correctly and when doing so one records the as-found condition, and only if any adjustment (or replacement, if the device is found to be not working) is required will the calibration be adjusted.

Continuing to talk about "calibrating SRV/GCV and IGV" just perpetuates the myth that servo-valves and LVDTs are mysterious, magical and mythical devices and that AutoCalibrate can solve mysterious and unexplained nuisance operation and trips--which is simply false.

 

GE has informed that they have removed AHOP in Frame V machines since 2001. Is that so? What is your suggestion - whether MHOP is reliable to take care any situation?

 

VJ,

I believe (though I may be wrong) that an Auxiliary Hydraulic Pump is always an option which can be purchased. At additional cost. It was probably made an option to reduce the base cost of the machine, and the purchasing department didn't realize that.

As for the reliability of the Main Hydraulic Pump--it's essentially exactly the same pump as the Aux. Hyd. Pump. And, they are both pretty reliable. In my three decades-plus of experience I have never encountered a failed hydraulic pump--failed couplings (on both the Main- and Aux. pumps), failed Aux. Hyd. Pump motors, but not a failed hydraulic pump.

Is your unit critical to infrastructure? Critical for providing energy in the event of loss of grid power?

Early GE-design heavy duty Frame 3 and Frame 5 gas turbines did not have AC motor-driven Aux. Hyd. Pumps. To calibrate LVDT feedback, it was necessary to CRANK the units. The hydraulic pump has an internal pressure compensator which tries to maximize pressure and flow at low speed (both the Main and Aux. pumps--again, they are the same pump). So, while hydraulic pressure from a gear-driven Main Hyd. Pump may be a little less than rated at CRANKing speed, there is still sufficient flow for LVDT calibration--and again, one is not calibrating the valve or the IGVs or the servo-valve, one is only calibrating LVDT feedback and to do that it's only necessary to move the device to make the LVDT feedback change. Rate of change isn't really critical to this (unless the device isn't moving!), and it's only necessary to get two calibration data points so all that's really required is sufficient pressure and flow to move the device to make the LVDT feedback change.

Again, the criticality of the unit for the application will determine if you really need to have an Aux. Hyd. Pump. They are great to have, but they aren't required for every application. Most Main Hyd. Pumps are fairly easy to get to to change, and they're not very large or very heavy. Spare couplings and a spare pump are really excellent things to have if one doesn't have an Aux. Hyd. Pump.

Also, proper operation and adjustment of the hydraulic system pressure will make the pump last longer. MANY sites make the mistake of using the pressure relief valve to set and control the hydraulic system pressure. The relief valve is there to protect the system in the event the internal pump compensator fails and causes the pump output pressure to be too high (it is a positive displacement pump). If one uses the relief valve to set and control the hydraulic system pressure then there is excessive flow through the pump (being relieved and dumped to the reservoir) which will increase the torque required by the pump and can accelerate the wear of the pump internals.

The pump internal pressure compensator is adjustable, and to set/check the operation of the relief valve (the setting of which is above normal hydraulic system rated pressure) is to use the internal pump pressure compensator to increase the pump discharge pressure until the relief valve relieves, adjust the relief valve setting if necessary, then run the internal pump pressure compensator down to achieve the rated system hydraulic pressure. By doing this, you keep the flow through the pump to a minimum, reducing wear and the torque required to achieve system pressure while maintaining minimal flow through the pump. Most of the hydraulic coupling failures I have seen were caused by improper use of the hydraulic system relief valve to set and control hydraulic system pressure.

Hope this helps!