Skimmer lids blowing off

[Mayorb]

This one is for the pool techs out there.
I've been working pools for almost 30 years. This is a new one for me.
When the pump shuts off the handles on the skimmer baskets pop the lids off the skimmers.
Here's the caveat- there is ZERO air getting into the system.
The pump (Pentair VS) has 100% full prime. When I open the air relief on the filter (Pentair 420 cartridge) no air comes out at all.
It's a 6 year old pool/spa combo.
What may be causing the blowback to the skimmers?
This just started about 2 weeks ago, and it's happening every day.
My well-seasoned veteran pool tech buddies are scratching their heads also.
Something maybe weird going on downstream from the filter?
Otherwise filter PSI is normal, and water flow to the pool is strong.
Any ideas?

 

[JamesW]

You can put a check valve between the pump and the filter.

Is the filter plumbed correctly in and out?

Can you show a video of the pump, skimmer and pressure gauge when the problem happens?

What is the filter pressure?

What is the pump speed?

 

[JamesW]

Blowback like that is almost always due to air because air is compressible and water is not.

If it is air, you should be able to see it in the pump lid as it happens.

If the filter is plumbed backwards, the water will be going inside the cartridge and the cartridge will swell up and it might decompress when the pump is turned off.

 

[Mayorb]

Thanks guys!
It's a very standard setup, installed about 5-6 years ago. I have been servicing the pool since day 1.
Pentair VS pump, Pentair 420 cartridge, Pentair 400K heater, Pentair EasyTouch control system.
There's a possibility it has to do with the EasyTouch or the heater.
A few weeks ago, I found that I could not actuate the heater from the EasyTouch power center. But the H/O could fire it up from the app on his phone. It may be possible the the heater is not turning off.
That confuses me in two ways.
One, if heater is in "runaway" condition, why aren't the sensors in the safety loop shutting it off (boiling water inside the exchanger, which doesn't indicate a stuck gas valve as it still turns on and off).
The other is why isn't the pool getting way too hot.
Also how could overheated water (no banging) inside the heater exchanger blow the skimmer lids off.
May be a compound problem.
Tomorrow I'm going to take the heater offline and see what happens.
I'll update post.

 

[JamesW]

I don't see how the heater would cause the blowback.

Have you watched the skimmer and pump basket when the pump is shut off?

Does it decompress immediately when the pump shuts off?

Can you show the system?

 

[Mayorb]

Update.
Problem has been resolved, somehow.
H/O saw that his heater had been running at 103 degrees even in pool mode, which confused me because pool temp was not above normal. He put it back right so 103 was only in spa mode. The problem went away.
????!!!
I still can't figure it. The ONLY times I seen blowback to the skimmers was when a suction leak loaded the filter with air. In one case about 25 years ago I saw both skimmer baskets blown all the way out onto the deck.
Possibly a bypass stuck? The pressure on filter was at its normal 22 psi all along.
I'm not sure we found the root of the issue.

 

[JamesW]

I doubt that the heater had anything to do with the blowback problem.

The information provided is insufficient to make any sort of reasonable diagnosis.

 

[Dirk]

Do you own a Mogwai by any chance?

 

[Mayorb]

I doubt that the heater had anything to do with the blowback problem.

The information provided is insufficient to make any sort of reasonable diagnosis.

I can't see any connection either.
All the components are functioning at normal parameters. no indications of any other issue.
If the problem recurs I'll need to be on-site when the pump shuts off during its normal cycle. I could not replicate any back-pressure at all while running the pump manually, even at max rpm.

 

[Bill1974]

Are there any valves downstream of the heater that are close with automation? Only thing I can thing is that there is a residual heat build up in the heater and it causes a boiling when the pump shuts off (reduction in pressure and water will boil at a lower temperature). And somehow the pressure is released back through the pump and filter. Seems it should be easier to releave pressure to the returns unless it is a such a larger amount of water flashing to steam that it can't get out the returns fast enough.

Doesn't take much water to flash to steam to move a lot of water. If i recall it's an a lbs of steam is 1600 times larger than 1 lbs of water.

 

[Mayorb]

Customer will stand by pump at end of daily run time and report back any indication of air in pump.
To this point, there has been zero air showing under pump lid (I call it "magnifying glass effect").
Will update when I find out more.

 

[JamesW]

If I recall it's an a lbs of steam is 1600 times larger than 1 lbs of water.

 

[JamesW]

The salt cools down rapidly and as it converts from liquid to solid, it dumps a lot of heat in a short time causing some of the water to boil and convert into steam.

520 joules per gram x 50 grams = 26,000 Joules of energy are released as salt converts from liquid to solid.

26,000 joules / 2,260 jouls per gram for water = 11.5 grams of water converted to steam.

11.5 ml of water will convert into about 20 liters of steam.

I suspect that initially the salt has low surface area in contact with the water. However, when the water begins to boil, I suspect that it disperses the salt enough to greatly increase the surface area in contact with the water resulting in a big heat transfer from the salt to the water.

 

[JamesW]

Only thing I can thing is that there is a residual heat build up in the heater and it causes a boiling when the pump shuts off (reduction in pressure and water will boil at a lower temperature). And somehow the pressure is released back through the pump and filter.

Seems unlikely, but not impossible.

 

[Bill1974]

If the heater stays on for a few seconds after the pump turns off, it may be possible. Yeah a bit of a stretch. The boiler for my house would kettle when it first kicked on becuase the circulator was on it's way out and took a couple of minutes to get he water going. That is only a 120kBtu boiler. It's a 2 stage boiler, I disabled one of the stages and it stopped a 60kbtu (took me a while to figure out the circulator was bad). Not sure how much water and thermal mass a pool heat exchanger adds up to, but a 400kbtu burner can heat things up pretty quickly. If the heat exhanger hold a 1/2 gallon of water it could go from 85°F to 212°F in 6 seconds in a 400Kbtu 85% efficient heater. (https://www.omnicalculator.com/physics/water-heating)

All a bit of guess not knowing any of the equipment or layout.

 

[JamesW]

If the heat exchanger hold a 1/2 gallon of water it could go from 85°F to 212°F in 6 seconds in a 400Kbtu 85% efficient heater.

You also have to figure in the energy required to convert the water to vapor.



4,800 kJ = 4550 BTU.

At 93 btu/sec, it would take 49 seconds to heat 0.5 gallons from 77 F to 212 F and convert all of the water to steam.

1/2 gallon of water will convert into 107 cubic feet of steam.

 

[Bill1974]

Good catch.

With the pump running and if there is pressure in the system (from restrictions) the water could be slightly superheated As soon as the pump is turned off, the water could have small pockets that flash to steam as the pressure drops. Doesn't take much water to convert. I am guessing it could also condense back to water just as quickly.

Only takes about a tablespoon of water to convert to wet steam to displace 1 cu-ft of water (7.5 gallons).

 

[Mayorb]

Thanks everyone! Sorry for the late reply.
The heater is a Mastertemp 400K btu.
Water boiling in the exchanger would be the result of failure of 3 sensors in the safety loop. But I've been in the business long enough to know not to rule anything out.
The thermopile that tells heater what temp water is (max 105), and the high limits (135, and 150) all should have each killed the fire independently when it hit those temps.
"Should have" are the key words in that sentence.
I'll have H/O crank heat back up to see if we can replicate the blowback.
Will update soon.

 

[JamesW]

In my opinion, the heater hypothesis is very unlikely to be true.

 

[Bill1974]

Is there a height difference between the spa and pool? Is water draining from the spa to the pool (through the skimmer lines)? Lazy check valve