Hello All,
Previously we had 25MW Condensing Steam Turbine. When it was Stopped or Tripped Barring was starting at @250 rpm spped and STG rotor was rotating at 250 rpm speed (STG supplied by Siemens). We have replaced same Turbine with Extraction Condensing type with same make and capacity. In this Turbine barring starts at Zero speed and rotor is shifted by 15 deg Angle with interval of time (self sequencing logic).
In a Frame6 Gas turbine Rotor is rotated 47 deg Angle after every 3 minutes.

My question is on what basis it is defined that rotor sould be rotated either at some speed (say 200 rpm) Or The Rotor should be
Shifted (change its position) with some Time interval. What is the Criteria?

Thank You

 

There are many gas turbines which use a "turning gear" sometimes called "barring" to keep the rotor from bowing when shut down. Most F-class GE machines use a turning gear instead of a ratchet for cooldown operation. Many Frame 7Es and 9Es use "slowroll" through the torque converter; while newer Frame 7EAs use a ratchet mechanism.

GE-design heavy duty gas turbines probably use ratchets for several reasons. One, it allows some ability to rotate the shaft very slowly and control where it stops more easily during maintenance or troubleshooting.

Second, the amount of rotation on a ratchet stroke is supposed to correspond to the number of through-bolts holding the axial compressor together (which is the longest and heaviest part of a gas turbine's shaft and the part that's most susceptible to bowing after shutdown; remember, the turbine is only two or three stages and is much shorter and lighter than the axial compressor). I'm told the idea is to keep the weight "on the bolts" when the turbine shaft stops turning between ratchet strokes as opposed to between the bolts, but I've never really understood exactly how this could be true. The ratchet has no knowledge of where the rotor bolts are when it starts or stops a ratchet stroke, so I've always been kind of suspect of that explanation.

Third, it's a compact design and can be operated with a small DC motor in the event of loss of AC power. Also, with DC motor-driven pumps operating periodically when there is no AC, the bearings are protected and the rotor can still be safely cooled while waiting for AC to be restored so the unit can be re-started (if it's not a black start unit).

There are probably more reasons, but those are the ones I've heard used the most.

 

Hello Mr.CSA
Previously I was thinking that as Steam turbine shaft is solid ( having more cross sectional area) it is required turning gear and GT shaft being hollow ( having less cross sectional area ratcheting is okay for cool down. But now a days Steam turbines are also have ratcheting mecahnism, That is why a question raised in my mind, on what basis the shaft speed is decided for cooling

Thank you

 

The only disadvantage with ratchet is long cooldown time. The time between machine shutdown and the machine handing over to maintenace is very high.

People are not happy with even turning gear speed and doing forced cooling to save time. some kind of rotating(few degree) mechanism is required for even spare rotor lying in store to avoid bow due to self wight.