I have problem with GE Frame 6, cables melt at the junction box and at the field instruments in the turbine compartment. Did anyone has come across this problem...

Did anyone has similar GE Frame 6 cables schedule.

 

farosham,

How old is the Frame 6 at your site? If it's a newer unit, possibly under warranty, have you contacted the packager about the problem?

There is usually a high turbine compartment temperature switch to alert conscious operators and technicians of a high turbine compartment temperature. Is there any Process Alarm indicating a high turbine compartment temperature? If there is no such alarm but there is a temperature switch (usually device number 26BT-n, where 'n' is a single digit number), has anyone checked the operation/function of the temperature switch to be sure it's working properly?

Also, there are usually vents with dampers, some of the gravity-operated (opened when the vent fans are in operation; closed by gravity when the vent fans are not in operation). Are these dampers working properly?

Also, there is usually some kind of damper vent that is mechanically latched open, and when CO2 is discharge after a fire is detected, the CO2 pressure releases the latch and gravity closes the damper to keep CO2 inside the compartment. Are these latched dampers properly latched during turbine operation?

The turbine compartment doors must remain closed when the turbine is running (and when on cooldown (ratchet)) for the turbine compartment vent fans to keep the compartment temperature at an acceptable level. Are the doors kept closed when they should be kept closed?

The turbine compartment is supposed to be forcefully ventilated to both remove any combustible gases and to cool the compartment. Sometimes it's positively pressurized by one or more turbine compartment vent fans. Sometimes it's under a negative pressure (slight vacuum) by one or more turbine compartment vent fans.

Some of the centrifugal turbine compartment vent fans supplied with GE-design heavy duty gas turbines have reverse tangential fan blades, meaning that they must be rotated in the opposite direction of the way one would expect the centrifugal fan to rotate. When operated in the wrong direction, the air flow through the turbine compartment is insufficient to maintain turbine compartment temperature at an acceptable level. Many times after a maintenance outage when fans are removed and then reinstalled, the direction of rotation gets improperly determined.

(For these fans, they usually draw higher than rated current when rotated in the opposite direction even though they aren't moving as much air <i>through the turbine compartment</i>. This usually results in actuation of the motor starter's thermal overload relay, which should cause another Process Alarm to alert a conscious operator or technician of a problem.)

There are not typically many devices/instruments mounted in the turbine compartment just because of this potential problem. For the devices that are mounted in the turbine compartment, wire and cable with an insulation with a high temperature rating (usually in excess of 100 deg C) is used for these devices. Fire detectors mounted in the turbine- and load compartments sometimes use mineral-insulated wires to protect them from failure in the event of a fire to allow detection of a fire which might melt normal insulation.

This kind of problem has occurred when site has replaced the as-supplied wire and cable with wire and cable with insulation that has a low temperature rating. If the unit has been in service for several years, this could be the case. Many times, site personnel find it "necessary" to replace wire and cable that is damaged during mechanical maintenance outages. And, they use wire and cable with an unsuitable temperature rating.

Also, sometimes maintenance personnel find they want to "extend" the length of wires and cables to devices in the turbine compartment to make maintenance outages easier, but they use wire and cable with the wrong insulation rating. And, worse, the allow the conduit and wire and cable to lay on or come into contact with the turbine casing, combustion wrapper, and combustion cans, which can even damage the originally supplied wire and cable!

I've seen originally supplied wire and cable damaged during maintenance outages and also because of improper re-installation after maintenance outages, and when that wire and cable is replaced with insulation with a lower temperature rating the problems get even worse.

But, to answer your question: The wire and cable supplied with most GE-design heavy duty gas turbines is rated to prevent heat damage under normal operating conditions--meaning that the turbine compartment ventilation fans are in service and the turbine compartment doors are kept closed and any gravity-operated dampers are working properly and any latched fire dampers on ventilation components are also properly latched during normal operation. The wires and cables in the turbine compartment should be routed and supported so that they don't come into contact with any hot surfaces (combustion cans; combustion wrapper; turbine casing; etc.). And if any wire and cable is replaced, then the new wire and cable should have an equal or higher insulation temperature rating than the wire and cable being replaced.

Check the turbine compartment temperature switches to be sure they are functioning properly. Check all of the doors and ventilation dampers to be sure they are operating properly. Check the direction of rotation of the turbine compartment ventilation fan(s) to be sure they are rotating in the proper direction (this can be verified by use of a clamp-on ammeter when the fan motor is running). Check to make sure that all wire and cable is properly supported and not in contact with hot surfaces.

Lastly, if any wire and cable has been replaced, check to make sure that the insulation rating is sufficient for the application.