There are many special problem areas to address on expansion joints for Gas Turbine exhaust applications. They are unique because of rapid temperature rise at turbine start up, which causes thermal shock and may consequently crack the expansion joint frame.
Differential temperature distribution in transient stages between liners and frames may cause binding, warping and cracking. Temperature differentials between externally and internally insulated duct where the expansion joint becomes a transition element need to be addressed. Attachment and geometry of internal insulation is important to the free expansion of all components to avoid binding, erosion and extensive stress. Poor methods of expansion joint fabric mounting to metal flanges may cause leaks, heat migration and premature failure of the fabric in the attachment area.
Bachmann® GTJ-HC (Hot to Cold) frame is selected to describe and illustrate solutions to the problem areas described above. The cross section shown below is an example of a transition joint between an internally insulated duct off a turbine exhaust and a insulated duct or Bachmann® diverter.
Paragraph numbers below refer to the balloon numbers on the illustration.
1. Externally insulated gas turbine neck or duct exhaust: Both the turbine exhaust flange and the expansion joint inlet flange may be a bolted or welded connection, and must be designed of like materials with the same coefficient of thermal expansion to withstand the high temperature (thermal growth) common of gas turbine exhaust. (1250 F or 675 C)
2. The inlet flange of the expansion joint is also externally insulated for equal thermal expansion. The total inlet frame is allowed to heat to prevent differential thermal growth which causes severe warping and cracking.
3. The inlet frame/liner connection forms a channel with at least one wall of the channel sloped with respect to the direction of the flow. This reduces stress of the hot frame due to radial expansion on round joints and comer stress due to longitudinal expansion on rectangular joints.
4. The flexible acoustic/insulation material (ceramic) is wrapped in stainless steel wire mesh and further wrapped in high temperature silica based cloth to prevent erosion due to possible migration turbulent flow. This material serves as an acoustic barrier and insulation for the flexible element. Installation geometry is also important to prevent erosion due to movement of liner against the flexible elements.
5. The flexible insulation/acoustic barrier is pinned and clipped to each inlet and outlet liner. This will allow full return of the flexible material after compression and help prevent hot spots on the flexible element (Fabric Belt).
6. The internally insulated duct: The outlet flange of the expansion joint is internally insulated to mate with the customer duct or the Bachmann® diverter inlet. Both carbon steel mating flanges are below the temperatures, which require expensive alloy steels and expand at a much lower rate.
7. The outlet liner is subject to the high gas temperature and is normally made from 409 stainless steel to help keep costs low. The liner is sloped on the outlet end to help prevent erosion to the adjoining internally insulated equipment. Liners are fabricated in short sections with a gap to allow for thermal expansion. The gaps are covered with a liner clip to prevent erosion of the insulation materials. Each liner comer piece of a rectangular joint is mounted independently and overlaps the straight sides to allow for thermal growth into the comers.
8. The liners are attached to the cold side frame using annealed stainless steel liner pins or tabs as shown below. The hot floating liner is independent of the cold outlet frame and is allowed to thermally expand without effecting the cold side frame.
9. The flexible element of the expansion joint is designed to withstand the full gas temperature without the insulation/acoustic material in the joint cavity. The individual layers of the fabric belt are selected for a high quality product, which will provide longevity of the total expansion joint. Bachmann® experience and thermal analysis showed that it is important to give special attention to the clamping area of the hot inlet interface between the fabric and hot frame. Extra insulating material is used in the gas turbine fabric belts to prevent heat migration through the fasteners and clamping to the outer cover material. Special geometry folds of the outer cover prevent overheating of the outer cover and provide long service life.
10. The channel and angle clamping method is designed to dissipate heat due to migration through the metal parts. A gas side non-porous barrier is used to prevent gas leakage.