Sand filters are filtering ok, but pressure never rises?

[jamyers]

This is at a 144,000gallon church-camp pool with 2 pumps and filters that run 24/7.

Last summer we changed out the 10-year-old sand for new sand from a reputable local pool supply, and checked the laterals inside - everything looked/felt good.

But ever since then the filters seem to be working just fine except that the pressure never rises when they get dirty. They will get to the point that they stop filtering and the pool gets cloudy, but a good backwash and they'll clean the water again. After trial and error, I've been backwashing them once a week, which seems to work well.

I've verified that the gauges are working properly, I've taken the tops off the filters and looked/felt for channelling, the sand inside isn't clumpy or hard, it's easy to slosh it around inside. The free space is correct for the filters.

Any idea why the pressure doesn't rise, and/or should I be concerned about it?

 

[Bama Rambler]

Welcome to TFP.

Do both gauges read the same?
Was any plumbing changed when the sand was changed out?
Were the valves rebuilt when the sand was changed?

 

[UnderWaterVanya]

But ever since then the filters seem to be working just fine except that the pressure never rises when they get dirty. They will get to the point that they stop filtering and the pool gets cloudy, but a good backwash and they'll clean the water again. After trial and error, I've been backwashing them once a week, which seems to work well.

Does the flow drop when they "stop filtering"?
Can you post a full set of test values:

FC
CC
pH
TA
CH

How do you chlorinate?

I've verified that the gauges are working properly

How?

 

[jamyers]

Thanks for the welcome! Here are answers to all of the above questions:

* No, one gauge reads 17 psi and the other reads 14psi - same model filters, but one pump/motor is newer model than the other (old Challenger w/ bronze open-face-impeller pump vs. Challenger 2 w/ plastic closed-impeller pump). Both pump/filter systems are separate from each other except for a common waste line out of the building and a water-slide feed line that taps off of both "back-to-the-pool" lines in the pumphouse (about 10 feed downstream of the filters).

* No plumbing was has been changed in quite awhile apart from a leak repair in a water-well line.

*Valves weren't rebuilt when the sand was changed, but they both were rebuilt at the beginning of this season (May).

* No, the flow doesn't drop. Both systems just keep on flowing and reading the same psi.

* I just now pulled these test values (shocked it Sunday evening with 10% liquid CL from a 55-gal drum). Since changing the sand last year, I'm sure all the test values have been high and low at some point...
FC = 15
CC = 0 (no change from FC)
pH = 7.2
TA = 140 (which historically is low for this pool, the feed water coming out of the well is around 400, anytime I can get/keep the pool below 180-200 it's an accomplishment).
CH = I have no idea (I've ordered a better test kit that'll show CH).
CYA = 45

* To chlorinate, in years past we used a mix of the 3" CL tablets (each pump has it's own injector) and granulated CL to supplement / shock, at the beginning of last season (before changing the sand) I switched to tablets and liquid CL - because it seemed that at the higher TA levels the granulated CL was making the pool cloudy white and taking forever to completely dissolve. This year it's been all tablets and liquid.

* I confirmed the gauges by first swapping them with each other, then by plumbing a test gauge into the air purge fitting of each filter and confirming readings. I confirmed higher-then-normal pressure readings by partially closing off the valve on the waste line while backwashing and rinsing.

 

[UnderWaterVanya]

I feel like we are playing CarTalk's "Stump the Chump" game...

Have you considered adding DE to your sand filters?

EDIT - Channelling already covered. Removed from my post.

 

[jamyers]

I feel like we are playing CarTalk's "Stump the Chump" game...

Have you considered adding DE to your sand filters?

EDIT - Channelling already covered. Removed from my post.

Hahaha, I love Car Talk!

Never thought about adding DE, although Sunday I (for the first time) put Aluminum Sulfate in the pool as a floculant and then vacuumed to waste, and today the gauges show about a 4psi increase. I'm betting the alum will backwash out, maybe I could add some after every backwash?
Hmm... (wanders off to read up on DE and search "alum as a pool filter aid")