Horizontal sand filter crack\burst !

[Canadiannascar]

Hello, I've been reading\learning a lot from this site, the internet is so awesome ! lol
Anyways, I wasn't too sure where to put this thread, but here goes...

I work at a city run facility, with 2 pools the one in question is a 25m 132,000 gallon fresh water pool. We have 2 horizontal sand filters with usually around 11lbs pressure on the tanks, after night shift staff left the lower tank burst with about a 3ft crack\gash horizontally along the mid section. It is a fiber glass shell about 60in diameter and maybe 8ft long. The pump room was flooded with several inches of water, and the pool water level was done about 1ft. We valved off the affected filter and by the end of that night we were able to get the pool clear and ready to re open.
I know a back wash done and normally completed with no issues 2hrs. before staff left.
I was just curious if this is a fatigue issue or if improper valving during backwash could have caused this ? I can't see that low of a pressure causing this damage, & I would think if it was valved improperly during backwash, it would have burst during the backwash cycle.

Input is appreciated, and thanks in advance, here are some pictures...

 

[PoolGuyNJ]

It could easily have been either or both. I don't think there will be any way to prove either though there may be additional indications inside the tanks that are visible once the sand is removed.

How old is the tank?

What size is the pump for that tank?

Scott

 

[Canadiannascar]

It's only 10yrs old, motor is 15amp, not sure what size, I'm at home now.

Thank for the reply.

 

[duraleigh]

Normally, pool pumps are only capable of producing around 35 psi even if the water is shut completely off. Most all residential filters are rated @ 50psi so a burst filter should never be an issue.

Unless your pump is capable of more than 35 psi, the filter seems at fault.

 

[Canadiannascar]

I'm hoping that is the case, I can not see the pressure building after the back wash,I know the valving was correct after the back wash.
This kind of catastrophic failure would have been noticed if it happened during backwash procedure.
The manufacture is coming to check it out, so I assume they will diagnose the cause.

It is a commercial pool, 600v motor..

Thanks again !

 

[JamesW]

If a valve is closed while the pump is running, then a phenomenon known as Water Hammer can happen. Water hammer can be very destructive to plumbing and equipment. Under the right conditions, closing a valve quickly while the pump is running can create pressure well in excess of 100 psi.

If you can provide more information, such as pump horsepower, flow rate, pipe size and length etc. that will help.

For example, water moving at 6 feet per second through 3-inch schedule 80 will generate close to 130 psi if the water is suddenly stopped by a valve.

http://www.corr-tech.com/EG-all.pdf (See pages 22-23)

 

[PoolGuyNJ]

@15 amps, a 600V motor is likely to be about 5 or 6 HP. That's a lot of water movement and momentum. Is the single pump used on both tanks?

The pump should be off when changing valve settings.

Scott

 

[Canadiannascar]

Yes the pumps were off during valve movement and the one pump does both tanks.
I will get more info tomorrow on horse power, etc.

 

[JamesW]

The center of the crack seems to line up with a bolt that is securing the cross members of the frame. It looks like there might be some sort of damage due to the bolt contacting the side of the tank. Under pressure, the tank will swell and it might have repeatedly pressed against the bolt eventually causing a stress crack, which finally broke through the shell.

Another possibility is that the bolt was pushed into the tank by something hitting the cross member.

 

[Canadiannascar]

That bolt & cross member is a few inches from the side of the tank, I don't think that caused it.

Here is some mechanical info :

132,000 gals
turn over time is 3.5hrs.
flow rate is 632 gpm
pump, 575v 14.5a 15 (11) HP

6" intake (exhaust throttled 40 degrees) & exhaust on pump, to a 3" inlet\outlet on the filters.

Price on new tank is almost 10k !!

 

[JamesW]

Perhaps it's a reflection, but it looks like there's an indentation and scratches on the white outer layer of the tank in front of the bolt. Can you look closely and feel the side to see if there is any sort of impact evidence?

 

[Ohm_Boy]

Just to clarify:

if improper valving during backwash could have caused this

Yes, it could. Especially with waterhammering.

Yes the pumps were off during valve movement

I think you mean to say "Yes, they TOLD ME that the pumps were off during valve movement". Just sayin'. I've worked a lot of backshifts in my time.


Don't rule out that the integrity could have been compromised some time before the tank actually let go, anywhere from days to minutes before. If there are indentations in the tank side from the bolt on the crossbraces, it's likely there from the tank side hitting the bolt when the tank ruptured.

Regardless, I'm pretty sure that the next step is to graciously off the manufacturer an opportunity to weigh in. They might offer some assistance.

 

[Canadiannascar]

JamesW
Yes there is a very small indentation and scratch, but it is not where the crack is, and looks to be slightly higher then the bolt.

Ohm_Boy
I understand the water hammering, but all 4 valves would have to be closed exiting the tanks. I don't think that is the case, the water would at least have a route to the drain or back into the system circulation. Also, I think only an idiot would actuate the valves while the pump was running, I'd assume there would be a lot of resistance during this motion.

I find it hard to believe it ruptured under proper valving during normal operation, under only 12lbs. pressure. Even after the air was bled off both tanks.

The manufacture has been contacted, the rep of our installers looked at it also.
I guess I will have to wait & see after the new tank is installed (4 to 6 weeks !) to see the inside for any clues.

Again thank you everyone for your input, much appreciated.

 

[Ohm_Boy]

Sounds good. Please let us know what is learned.

 

[Canadiannascar]

For sure I will update this thread when I get more info..

 

[JamesW]

I understand the water hammering, but all 4 valves would have to be closed exiting the tanks.

Only one valve would need to be moved for water hammer to happen. Also, it's more common than you think. Someone gets distracted or tired and forgets to shut off the pump before turning the valve.

 

[Canadiannascar]

Yea no kidding on distractions, being tired, & stuff, sometimes we are human.. lol

For water hammering to be the cause, I think all outlet valves on BOTH tanks would need to be closed. The pic below, has the tank isolated, the valves on the right are the outlet valves, both tanks would have to have all 4 outlet valves (on the right, top & bottom tanks) closed. So, theoretically the water (pump) would push all the water into the bottom tank only, having no where to go, it stressed the tank, then after back wash, during normal operation (valving) the tank ruptured.. Hard for me to believe, more believable if it happened during back wash, which it did not ..

Heard that the tank has a 10 year warranty with a 15yr life expectancy, the tank is just over 10yrs old.

Thanks James

 

[JamesW]

Each filter has four valves. During filter or backwash, two valves are open and two valves are closed. If one of the outlet valves is closed, then there will be a water hammer in one filter and all flow will be forced into the other filter.

Have you opened the hatch to get a look from the inside?

 

[Canadiannascar]

I hear what your saying, & totally know I'm being a PITA but, I know at least the top tank outlet valves were positioned correct, water would take the path of least resistance, so can't understand why it would water hammer. It still had an outlet from the top tank...

I was thinking about opening the hatch, but water is still leaking from the crack. I'm assuming the inlet valves that are now closed, are leaking a wee bit, so I'm reluctant to remove the hatch.

 

[Ohm_Boy]

It doesn't hammer from having nowhere to go, per se. It hammers from the moving column slamming into a solid obstruction.