Sand Filter Pressure Problems; Sand Bad in 3 MONTHS of USE?

[Sumo1]

Our church pool has four TR140 sand filters. The sand hasn't been changed in at least 15 years and does a great job. However, we replace the spider gaskets in the multiports each year. They're about $15. The multiport is a Triton model SM 20-3. We've had problems with people not pushing the handle ALL the way down before moving it. This seems to wear out the spider gasket faster. Hope this helps.
Hal
P.S. Someone told me always backwash three times. Don't know if that's just for a DE filter or if it's just hogwash but it's served me well so far.