Pump not Pumping

[Pool Pumper]

This pool is an inground cement pool with 28,000 gallons of water. The pump & pool cleaner (Hayward) operate properly at start-up. After running about 45 minutes the pool cleaner stops moving. I have serviced the pump (open face impeller, 2" bronze pump; impeller to volute clearance reduced from 0.090" to 0.015" increasing efficiency) replaced the sand in the filter (Sta-Rite HRPB24 w/3.1 sq. ft. of filter area 47-62GPM; 250# of #20 Silica Sand) and I changed the water. Upon initial start-up (filter backwashed, baskets clean...) the suction guage registers 9-11" of vacuum and the filter pressure gauge indicates 11 PSI. When the system starts to decline the vacuum shows 5-6" of vacuum and the filter pressure gauge is now at 23-26 PSI. There is no air in the suction basket and the filter bleed valve spews water only when cracked. I shut the pump off, wait one minute and re-start and the system returns to normal start-up readings and runs fine. However, within a few minutes it is back to the state of decline and the sweeper is motionless. I have also used a Kreepy Krauly with the same results. I have serviced the plunger valve o-rings used in the backwash cycle on the chance that a bypass could have been causing recirculation without a positive result. Any ideas would be appreciated.

[mas985]

Sounds like your filter is just clogging up with debris. Are you trying to clean up an algae bloom? Also, is the filter a DE filter? You may need to clean the grids.

[Pool Pumper]

Thanks for the reply. The sand filter has constantly been back-washed since the replacement sand was installed. There is no evidence of an algae bloom. Very little debris is present and I continually clean and inspect the inline basket on the suction hose as well as the inlet basket at the pump suction.

[mas985]

It could also have to do with the sand replacement. Sometimes a lateral can break during the process.

But the only reason the filter pressure will increase like that is if something is getting clogged in the filter or there is an issue with the filter so I would focus on the filter.

[Pool Pumper]

I checked the laterals and they are pristine. The sand filter operates in a quasi non-mechanical state during normal filtration cycles. Does anyone know of an occurrence involving a sand filter that is properly sized, containing new sand that meets manufacturers spec and is in "good repair" concerning the internal manifolds/laterals that has initiated or was the source of a hydraulic mishap? I do not wish to start replacing components in an effort to remedy the problem without the benefit of proper troubleshooting to identify the source of the troublesome part or element.