Extend Expansion Joint Life With 2-Ply Testable Bellows

                                                                                                                                                                    A publication of Oakridge Bellows

Single ply limitations

Metal bellows expansion joints should provide 10 to 20 years of good service before needing replacement. Unfortunately, the bellows do not give any early warnings before developing leaks and they are very difficult, if not impossible, to weld repair.

Field inspections are limited to just checking for obvious signs of external damage. Corrosion attack and fatigue cracking are rarely detected prior to a leak.

Two ply details

The expansion joint inspection problem is solved with a redundant ply testable bellows. A testable ply bellows has two plies, each designed to handle the operating conditions independent of the other. If one ply fails (almost always the inner) then the other provides the pressure containment.

Redundant ply testable bellows enable field maintenance to pressure test between the plies and inspect for leaks. This usually occurs during shutdown inspections but can
also be performed on-line if conditions are safe and practical.

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Testing and equipment

A low pressure gauge should be used as the test pressure should be limited to less than 15 psig. Bellows are typically designed for internal pressure but this type of test creates external pressure on the inner ply. Excessive pressure between the plies during a leak test can damage the inner ply, but those pressures are usually well above 15psig.

The test equipment includes a gauge, pipe tee, and a shut-off valve. A favorite tool of maintenance personnel is also a tire pump for regulating the low pressure between the plies during a test. Care should be taken not to introduce moisture between the plies. Trapped moisture can flash into steam upon start-up. Many users leave the tap-plug off in order to allow free venting should that occur.

Cost savings

The extra ply will add between 15% to 30% in the expansion joint cost - a real bargain when considering added life and predictability.

The end-user can truly appreciate the extension of equipment life, but it is the maintenance department who will really see the value in equipment failure prevention.

Buying smart

OK, so you paid the extra money and bought an expansion joint with a 2-ply testable bellows. You were wise, very wise – but keep that wisdom going by following some recommendations not normally found (or buried deep) in the other expansion joint manufacturers long and boring literature. This tribal knowledge is now passed on to you.

The first start-up

The first start-up after installation is an important break-in step for the bellows. Although rare, there are some real horror stories about moisture from the hydro-test getting trapped between the plies, flashing into steam and imploding sections of the inner ply. These do not usually result in an immediate pressure boundary leaks, but have the uncanny habit of showing up just after warranty.

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Imploded inner ply discovered 2 years after start-up in 2003

To prevent this problem the tap should be open (with gauges or pop-up removed) so that any trapped moisture can vent out during the first start-up.

I recommend that the expansion joints be shipped WITHOUT gauges or pop-up’s installed – just an open vent with a one-way valve installed or a cap that has a small hole drilled in it. That way nobody has to remember to do anything to it during the fog of a turn-around. The gauges or pop-up’s should be sealed in a separate box labeled ‘Do not open until 7 days AFTER start-up!’.

Testing your testable

If the failure indicator, gauge or pop-up shows a failure, you should do a pressure test between plies just to make sure you don’t needlessly replace an expansion joint (while I’m thinking about it – nobody holds a patent on pop-up indicators, regardless of what their literature says).

Be careful how much pressure you put between the plies. The inner ply will see external pressure during the test and could buckle with pressures as low as 15 psig so use 5-10 psig. The guys over at RTI (Refinery Technology Inc), an independent engineering consulting group, use bicycle pumps to control a low volume of pressure during the test.

Often during a pressure hold-test between plies there will be a drop in pressure, giving a false indication of failure; what’s really happening is minor relaxations in the convolution side walls cause significant volume changes between the plies. That volume is very small to begin with. To minimize this effect I recommend using a small air tank (Wal-Mart, $19.99) between the shut-off valve and the bellows. This provides a volume reservoir unaffected by these small pressure changes.

The bottom Line – staying smart

An expansion joint with a 2-ply testable bellows is a great investment and now you know how to look after them – that makes you one wise guy.

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