Double Wall Component Testing

Without going into too much detail, I’d much rather see 500,000 cycles on a test bench, than a supercomputer running an engineering simulation based on algorithms. The value of validated simulations is fully appreciated at AF Pipes, and we regularly use simulations to optimise and scale designs (..and create beautiful pictures for presentations). Testing is absolutely critical in order to gain trust and familiarity with the components, and we would therefore like to share some details about a few specific test performed on the inner pipe flex support. There are also different tests performed on the other double wall products available from AF Pipes, but they all serve the same function, which is to produce a set of installation specifications that is trusted by the designer, and can be used to reliably predict the life-time of the entire system.

 

Ventilation Flow Testing

The ventilated double wall pipes, or soon to be know as AF Flex double pipe, consist of two ends open to atmosphere. At one end a ventilation fan is mounted to pull the air all the way through the annular space between the inner & outer pipe, and the double wall components that are installed in the double wall system. In order to select a suitable fan the designer typically needs to know the required flow rate, and pressure loss of the system at that flow rate. The flow rate can be calculated relatively simply from the volume of the annular space (including dead space volume from components in the piping system), and the required number of exchanges per hour, which, if not already specified, can be determined in order to achieve a high dilution factor. The pressure loss of the system can be calculated from the sum of all pressure losses within the system, which will consist of the pressure loss from the piping surfaces & bends, pressure loss from supports, and pressure loss from any other components in the double wall system. The pressure loss from the piping can be estimated by calculation, but the pressures losses from supports & components must come from empirical testing (or validated simulations), from which a pressure loss co-efficient can be deduced. For the inner pipe flex support, the test is fairly simple; install some (in this case 6) supports in a straight piece of double wall pipe with known annular cross section, block off the inner pipe, install a fan at one end, install a dP measurement just before the fan to measure vacuum against atmospheric pressure, and measure the flow speed. There’s an example here:  https://youtu.be/oMLOc_xxyCc

 

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Do this a few times at different flow speeds and you have a well specified pressure loss versus flow rate (the flow rate can be calculated with the flow speed, and the annular cross section), which can be used to specify a pressure loss coefficient.

 

Durability Testing

The inner pipe support is a flexible support, and the flexibility is designed and specified with the support, in conjunction with the carefully selected material. This is described in more detail in the previous blog post. As the design, and the material, go hand in hand, then it is essential that the support is produced with the correct material to ensure premature failure does not occur. In order to test for durability, a small mock-up of a double pipe bend is made, together with a reciprocating cylinder that can run with a specified deflection, continuously, for a long period of time. The test can be seen here:  https://youtu.be/xWb1Yun6X7I

Durability

The cyclical loading simulates, within a few days, a lifetime of thermal or pressure expansions of the piping, and provides the specification for the longevity of the inner pipe flex support. As an example of a representative number of cycles, if the supports are installed in a vessel which is designed for 25 years of service, and the gas supply is operated 2 times daily (start & stop), that’s 4 cycles per day. After 25 years then that’s 36,500 cycles, for these tests, there are typically around 300,000 cycles prior to failure. It should also be noted that incorrect installation of the flex support, will reduce the liftetime by about half, and it is actually therefore that AF pipes typically specifies these supports for 150,000 cycles at max deflection.

In addition to the two tests mentioned above there are many additional tests performed on the inner pipe flex supports, including corrosion, wear, stiffnes, and vibration testing.

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