Even expansion joints are impacted by operational changes

Consistent with other presentations, expansion joints also are affected by a change in operations. For gas-turbine outlet fabric joints, as an example, cycling can increase thermal stress and cause cracks, hotspots, and fabric failure.

Dekomte’s Jon Tarrant addressed fabric expansion-joint technology for HRSG inlets and outlets, penetration seals, air intakes, and gas-turbine exhaust applications. These joints, the speaker said, reduce gas leakage, air ingress, and a host of related short- and long-term concerns.

Two-shifting and cycling have perhaps the most destructive impact on expansion joints and require review of equipment in these new modes of operation. Results can include duct-temperature gradients, duct fatigue and stress caused by these gradients, irregular stresses caused by movement, and both acid and water dew-point condensation.

In one example presented, the inlet joint on a hot casing will be impacted by inside dimension, gas velocity and pressure, axial and lateral movement, thermal transients, stress, and cycles (normal starts). Short-term solutions could include weld repairs to frame and duct, regular replacement of fabric, and external insulation. Long-term solutions are more complex:

      • New steel part arrangement.

      • Improved duct interface.

      • New external insulation.

      • New fabric and bolster design.

      • New fixing and convector design.

Material options were also given, particularly for HRSG outlets. These included double-coated PTFE fabric, a single layer of EPDM/Viton, and multi-layer joints with a Viton gasket sealing the flange. Related component designs were then reviewed.

Penetration seals attracted strong attention, along with the pros and cons of both OEM and retrofit designs. The most common OEM offerings are metal bellows, packed gland seals, mechanical seals, and fabric seals. Retrofits include metal bellows, mechanical seals, and fabric.

Reinsulating to mitigate casing hot spots. This session ended with an interesting discussion on pumpable insulation, a viable fix when thermal surveys reveal casing hotspots. The cause normally is movement or decay of internal insulation materials. Pumpable insulation can be injected online and monitored through thermal imaging as it spreads. This is a low-risk option to replacement of generally inaccessible block insulation. The selected material, Isofrax®, has low thermal conductivity, good strength and vibration resistance, and can withstand temperatures up to 2300F. The material can be removed, and injection ports can be reused.

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