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Buckling of Pig Sealing Discs

When the combination of disc oversize, thickness and the spacer diameter dimensions are right – or wrong, depending on the application – then a sealing disc can buckle. A typical “accidental” or undesired buckle is shown below: –

Figure 1 – Buckle on sealing disc showing loss of seal
Figure 1 – Buckle on sealing disc showing loss of seal

The view shows how the seal is lost as there is now a flow path past the buckle. This can happen in lines with internal diameter (ID) variations or with a badly designed seal.

Buckling can also have a positive effect, especially in dual diameter pigging. In order to allow a large sealing disc to fold into a small diameter line (for example a 16-inch sealing disc in a 10-inch pipeline), then forcing the seals to buckle is one way to reduce the stresses in the disc and to minimise the friction in the small bore pipe: –

Figure 2 – View on 16-inch seal buckled or folded into the 10-inch pipeline creating a regular rose type pattern. The forced buckling is performed using buckle inducers moulded onto the disc as indicated in the right hand image.
Figure 2 – View on 16-inch seal buckled or folded into the 10-inch pipeline creating a regular rose type pattern. The forced buckling is performed using buckle inducers moulded onto the disc as indicated in the right hand image.

Discs can also be designed not to buckle. This is useful for certain pig types, especially dual diameter and bi-directional pigging. It is also valuable as one seal can be used to work in several diameters in the line: –

Figure 3 – Pull through test on non-buckling 16-inch seal in 12-inch pipe (probably an extreme example of this technique)
Figure 3 – Pull through test on non-buckling 16-inch seal in 12-inch pipe (probably an extreme example of this technique)

A simple test can be performed to show that the seal will not buckle and will recover from an induced buckle from contact with an obstacle in the pipe for instance: –

The question is then when do seals buckle and when do they not. This information can be used to design dual and multi-diameter pigs by judicious selection of buckling with buckle inducers and non-buckling seals. To answer this, work has been performed to allow a selection guide based on the ratio of hoop to radial stress. The program is shown below as an example: –

Figure 4 – Buckling analysis for 12-inch x 10-inch line with requirement to provide a force buckle in the 10-inch line but no buckling in the 12-inch line. The green area shows the best design - 105 mm flange with and 330 x 17 mm seal. Practical fitment to the pig means that the flange diameter must be large. The red area is where both seals would buckle or not buckle. The grey area is just that – a grey area where it is not fully known in the seal will buckle or not.
Figure 4 – Buckling analysis for 12-inch x 10-inch line with requirement to provide a force buckle in the 10-inch line but no buckling in the 12-inch line. The green area shows the best design – 105 mm flange with and 330 x 17 mm seal. Practical fitment to the pig means that the flange diameter must be large. The red area is where both seals would buckle or not buckle. The grey area is just that – a grey area where it is not fully known in the seal will buckle or not.

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