Side stories aside, the force and stiffness of the expansion joint are both functions of the pressure.

If the pressure is low, the load from the joint inflating is also low, and the joint stiffness is low, thus the expansion joint absorbs the load, thereby dissipating the load as seen by nozzle and piping.

Conversely, if the pressure is high, the load is high and the joint stiffness is high.

If the existing joint ran smoothly for 10 years, it's probably because the pipe never saw the pressure you're probably being asked to design for.

Only the manufacturer can give you these numbers, unless you buy one and test it yourself, but the manufacturer will generally give you these values for a specific design condition.

However, I'll note that for "low pressure" applications, it's common practice just to ignore loads being distributed through and by the expansion joint.

But "low pressure" is generally accepted as "no more pressure than that provided by the hydrostatic load from an atmospheric tank."