headsUP: 7FA

Leaking blank flanges on liquid fuel lines a safety hazard

The scene: Simple-cycle 7FA startup on a quiet Sunday afternoon. About 30 minutes after the button is pushed on the gas-only unit, compartment thermocouples read 300F; 15 minutes later, 350F. Operator checked the engine and saw a blowtorch-like flame shooting out from a combustor end-cover. The unit was shut down immediately. Inspection revealed the flame had burned through the blank flange covering a superfluous liquid fuel port. 

 

  

 

 

 

 

 

 

 

The cause: Investigators found that the seal ring under the blank flange permitted air in-leakage, thereby reducing the pressure drop across the fuel nozzle and allowing the flame to attach itself to the nozzle. An attached flame causes incidents such as this.

   This was another of those “first-time” incidents that others had experienced. The speaker identified five more plants that had experienced the same issue after making a few phone calls. He believes this to be an emerging fleet problem. Here’s what the user suggested:

* Check as soon as possible the fit-up of end caps on your units. The triangular blank flanges can be tricky to install without cocking. Proper torque-up sequence and torque settings should be reviewed beforehand.

* When removing the blank flange, use a flat-face lifting tool; a pointed tool, such as a scribe, is almost sure to make an indentation that could allow air to leak by.

* Check the origin of the seal rings. The OEM relies on two suppliers for this part and the part numbers are the same, but not equal. One of the seal rings is jacketed with a material that deteriorates above about 500F; the temperature in this region is roughly 600F.

* Finally, check the size of the cap screws. Don’t be surprised if you find a mixture of 1.25 and 1.5-in. fasteners. The first is too short to engage the HeliCoil locking feature and should not be used.    

Posted in Best Practices |

One response to “headsUP: 7FA”

  1. […] details, including pictures and recommendations are included in their article, here.  If you operate any 7FA’s – on any fuel – I recommend you take a look at the […]

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