Tank Wall Penetrations

[mflam]

What do you think?
We are going to install a low-voltage mixer (48 VDC) in a 2 MG water standpipe, 60 ft diameter 110 feet high.
I can either loop the power cable all the way up to the roof of the tank and down, or I can drill through the 1 3/8 inch thick plate and intall a water tight fitting. Then the cable can go through the fitting.
Obviously the fitting is going to be subject to the hydraulic pressure at the bottom of the tank ( 40 psi +/-) and the possibility of a leak. I'm inclined to be cautious and run the wire to the roof and down.

 

[bimr]

There really is not much risk of a leak. However, it will be simpler to replace the cable when it wears out if you enter through the top.

 

[hydrae]

All pumps require maintenance (or replacement), if you go through the wall you will need to drain the tank to work on the pump.
compare the cost of at least one maintenance cycle (alternative storage or pressure control, draining, cleaning, disinfecting, fill, sample etc).

Most of the water tight fittings in the water industry are expecting equal pressure on both sides, such as on submersible pumps, the water or oil inside the motor is pressure balanced through a filter or diaphragm. You may need to look at other industries (submarine or Nuke plant) to find a reliable water tight fitting that can take the pressure difference.

If you design the over the top method correctly, you could work on the pump with water in the standpipe.

I am with you, and would go over the top.

Hydrae

 

[bimr]

Water tight fittings are typically constructed of epoxy pottings with electrical connections on each side of the fitting rather than through the fitting. The fittings can usually be repaired without draining the tank (diver is used).

 

[JedClampett]

I'm not an electrical nor environmental engineer. But I would lean toward penetrating the wall. Here's some factors to keep in mind:
Power losses; That's a lot of cable to get up and down the tank wall. And you're going to pay for that power loss every day.
Physical Damage; The cable on the tank wall is subject to wind, snow and other exterior factors. If it runs in the tank and ice forms on the surface, that's another way to damage it.
UV Exposure; Depending on where you are, this can deteriorate conduit and cable.
Lightning Protection; I've heard of lightning strikes knocking out whole SCADA systems. Your cable might be a path for lightning damage.

 

[stanier]

Unless you are going to use divers to inspect/clean the tank you will have to get in there some day. You can maintain/change the pump when the tank is emptied for inspection/clean out.

Many authorities use divers to vacuum the sludge out these days.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[mflam]

Thanks, everyone for your input.
I'm going to go ahead with the bulkhead fitting. The risk to the cable is the deciding factor. If we get a little ice in the tank and the water level changes, it will rip the cable. We'll be draining the tank to install the mixers so now is the time to drill through the tank wall and do it permanently.