Which is better: a fabric penetration seal, or a metal one?

An easier question might be: Who’s the best quarterback in professional football? At least you’ll have some stats for comparison purposes. When it comes to penetration seals for heat-recovery steam generators, most decisions seem guided by personal preference.

   The editors asked several users and a couple of boiler OEM reps at the HRSG User’s Group exposition what they preferred. All offered opinions but no compelling reasons for their choices. Next stop: Mike Green of KE-Burgmann USA Inc, perhaps the largest supplier of penetration seals in the world and one of few companies to offer both types. His is a familiar face at user-group meetings.

   Not an easy question for Green either. Fabric and metal do the intended job, he said, which is to prevent leakage of hot gas where pipes intersect the casing, thereby minimizing energy loss and emissions, reducing noise, and protecting maintenance personnel.

   Both types of seals are designed to accept axial, lateral, and angular movements, he continued. Such flexibility minimizes stresses on the pipes to which they are attached. Large lateral movements favor fabric, particularly in confined spaces where vertical height is at a premium. Reason: The amount of lateral movement required directly impacts the length of a metal joint.

   Fabric seals also are capable of greater axial movement than metal, and their unique clamping design prevents over-extension failure.

   Metal seals are more compact than fabric ones, so they often are specified when several pipes are in close proximity or pipes are close to structural members. However, if two or more pipes are very close to each other, an alternative is to wrap all within one fabric joint.   

   All metal seals are gas-tight when installed, Green said; fabric seals can be gas-tight but require a special layer to achieve zero leakage. All fabric seals are not created equal, he stressed. Regarding pipe size and temperature, both metal and fabric seals can be made to accommodate the largest steam lines as well as small-diameter drain lines, and handle temperatures up to 1400F. Generally speaking, fabric is less expensive at high temperatures but metal should last longer.

   Green also told the editors that fabric seals run cooler in most applications. They are internally insulated and have an outer cover temperature ranging between 200F and 350F, depending on ventilation and proximity to other heat sources. Metal seals are more likely to survive a tube leak.

   Many plants have a mixture of fabric and metal penetration seals, particularly when equipment was sourced from different OEMs. Keep in mind that seals might be selected by the gas turbine manufacturer when it is supplying a turnkey combined cycle; other times it may be the HRSG supplier, EPC contractor, or the end user.

   Wear and tear, especially the kind associated with daily starts and fast ramping, shortens the lives of both types of seals. Replacement of fabric is significantly less expensive than metal. The clamshell shown in the photo to the left facilitates change-out of metal penetration seals, but installation labor is expensive. Top skills are required to make the quality convolution weld required.

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