Previous valves had often stuffing boxes housing 7 and more packing rings. Today’s experience with modern packing material limits the recommended amount of rings between 4 and 6 depending on the sealed pressure. The rest of the available space in a stuffing box is filled with distance rings made for example from 2 pressure resistant filled carbon. These rings are usually split in 2 halves for ease of installation.
As well other temperature and pressure stabile materials can be used. Physical sense behind this measure is to distribute the available compression force from the gland bolts more equally.
A reduction from 7 to 5 rings can result for an internally rough packing with a low K-Value = distribution factor of applied axial force into radial sealing force of a packing in less than half gland pressure. This reduces the risk of extrusion of packing material between gland and stem and still delivers the same compression at the bottom product next packing ring.