Gimbaled Pantograph Expansion Joints

The Best Design for Supporting the Center Spool on FCC Standpipe Expansion Joints

The standard UOP design for FCC stand pipe expansion joints includes a pantograph assembly on each side of the universal expansion joint to provide support for the center spool. Typically, the pantograph is pinned to the expansion joint at three points on trunnion type support, similar to the photograph below. This confi guration demands that the lateral offset is parallel to the plane of the pantographs. If the lateral offset is 90 degrees to the plane of the pantographs, a simple 3-pin pantograph will bind and become highly stressed.

Excessive stresses in the pantograph will result in buckling of the pantograph bars, bending of the supports, or unanticipated distortion of the expansion joint bellows. These conditions should be avoided if possible.

A typical standpipe specifi cation requires that the expansion joint be designed for .5 degrees of end rotation in any direction and .5” to .75” of lateral offset perpendicular to the plane of the pantographs. These movement are generally not real and it is fortunate that they do not occur in most cases.

Although, the use of simple 3-pin pantographs has been satisfactory for most of the standpipe expansion joints, there have been some noteworthy cases where the pantographs have buckled due to “excessive rnovements” perpendicular to the plane of the pantographs. The amount of “excessive movement” is always diffi cult to determine.

For a 48” diameter by 120” long standpipe expansion joint, a .75 inch movement perpendicular to the plane of the pantographs would result in .375” of interference between the pin on the trunnion and the center of the pantograph on a simply pinned system. The .375” of distortion could buckle the pantograph linkages or severely distort the bellows.

Senior Flexonics Pathway’s solution is to provide a gimbal on the center support for pantograph type expansion joints. The gimbal feature shown in the photograph above allows the standpipe expansion joint to move freely in any direction without binding. Based upon fi eld experience, use of the simply pinned pantograph is a calculated risk. For the small cost of the gimbaled pantograph and high cost of an unplanned outage due to an expansion joint failure, it is probably not worth taking. In certain eases where there are known movements perpendicular to the plane of the pantographs, there is really no other solution.

A users list of customers who have purchased the gimbaled pantograph style of support is available on request.