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: –
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: –
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: –
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: –