Cable ducts flooded

[tomato]

My Pulsar dome and adjacent warm room shed have been operational for just over 12 months now, recently one of my USB extension leads went down, and I was surprised to find the cable duct was full of water when I came to pull through a new cable. The whole run is encased in concrete and all joints were sealed with RTV when installed. I assumed the dome to plinth seal was breached and standing water was running under the false floor and down the duct under the pier.
 

However, when I removed the steel pier inside the dome, the floor under the pier was soaking wet but further out all of the floor was dry, so that killed my theory. Then I discovered about 500 ml of water inside the pier! It looks like the big steel tube  is acting as a condenser, and is getting replenished with moist air with each warm/cool cycle. The water could drain away into the cable duct, thereby creating a driving force for the cycle to continue. I sucked about 7 litres of water from the cable duct, not clever as there is a 240 v cable in there...

So I have now dried it all out and put foam plugs in the top of the duct, one in the hole at base of the pier, and a big one in the top of the pier. That will hopefully prevent a recurrence, but I will keep an eye on it.

 

[JamesF]

That's a bit of a shocker, and not something that would necessarily occur to me as a likely problem.  Hopefully your plugs will do the job.

James

[TerryMcK]

Interesting. I wonder if the condensation could be routed away in a different direction with another hole drilled in the base of the pier and a hose attached. Maybe a small pump could be run from time to time.

I wonder if other members have come across this before.

[tomato]

42 minutes ago, TerryMcK said:

Interesting. I wonder if the condensation could be routed away in a different direction with another hole drilled in the base of the pier and a hose attached. Maybe a small pump could be run from time to time.

I wonder if other members have come across this before.

Yes, I thought about drainage holes in the side of the pier, but managing the water would be a problem, given that it is in the centre of the dome. Hopefully, by effectively sealing the air inside the pier I can prevent it from forming in the first place.

[Hallingskies]

Might be worth sticking some desiccant in there as well, just to dry out the trapped air.

[Budgie1]

What about filling the void inside the pier tube to cut down on the amount of air in there to cause the condensation?

I take it the cables come up through the hole on the centre of the pier base? If you put those into some drainage pipe about the diameter of hole in the base and then fill the rest of the space with expanding foam (for a permanent solution) or packing of some sort if you want to remove it in the future. 

[Synchronicity]

If you have used external grade cables then sitting in standing water won't hurt them.  If you haven't you might need to consider it - especially for the mains power.

What is the length of the run in concrete (underground I'm assuming)? It's possible you have a crack somewhere which would let water seep in over time

All the best

Michael

[tomato]

Thanks for the comments and suggestions. The mains cable isn’t external grade, as I thought it would be protected in a nice dry duct!🙄

I guess it’s possible the plastic drainpipe ducting has cracked, but if it is encased entirely in concrete, how is the water getting in?

I like the idea of filling the pier with foam, that will be my next fix if the foam plugs don’t work.

[michael8554]

On 27/11/2020 at 21:02, tomato said:

The whole run is encased in concrete and all joints were sealed with RTV when installed.

Push-fit fittings ?

Or the other type, which are solvent welded, not sealed with RTV.............. :-<

1 hour ago, tomato said:

I guess it’s possible the plastic drainpipe ducting has cracked, but if it is encased entirely in concrete, how is the water getting in?

When the water in the concrete evaporates, that leaves voids that can make the concrete permeable.

Michael

[Physopto]

Could you push blue water pipes down through and out the other end. Just keep the pipes well above the inlets at each end. Then feed your power and usb cables through these. Seal with silicon at both ends. The pier can be vented both top and bottom with several holes drilled for airflow. That should prevent condensation forming. You could also line the inside with 10mm of foam to prevent cold bridge.

I have used the water pipe fora good few years without any problems. It is extremely  tough and resistant toshovels and  garden forks 😃

[old_eyes]

Thanks for the warning. A problem I had never even thought about. My conduit comes up the side of the pier, sp I won't have hte same problem, but worth checking whether the pier is filling up!

[bottletopburly]

Probably due to the fact that you haven’t left pipe work sticking above floor a couple of inches I would be inclined to extend the pipe above floor level using some polyurethane sealant and a pipe off cut unless you can’t put a straight join in .

[tomato]

Well, after about a week there are signs of fresh water ingress, the pier is now very well isolated from the duct so it looks like the water  is getting in by some other route.
It is a 80 mm diameter  pipe and it isn’t chockablock with cables so I could try feeding a flexible inner pipe through it, thanks for the suggestion.

[tooth_dr]

I have been following your thread with interest.  I have a metal pier also, and it's dry inside, so it is quite strange to see yours soaking wet.  Do you run a dehumidifier?

[JamesF]

Well, that's a right pain :(

I've just had a look back over your build thread and it's really not obvious where any water could be getting in.  If the underground elbows are buried in concrete then even if it were to weep a little I'd not expect it to be noticeable after a week.  Not unless you were needing to paddle out to the warm room, anyhow.

Trying to feed some flexible tube containing all the cables through does seem like the simplest way out at the moment, but will that actually be possible given the need to negotiate the elbows?  And personally I'd be much happier if I understood where the water was coming from in the first place so I could be sure it wasn't likely to affect anything else.  Is it perhaps worth drying it all out and then leaving everything for a couple of weeks with the pier removed to see if there's still a build-up of water?  Given the present weather if you even missed a single night of imaging it would probably be a surprise :(

James

[discardedastro]

With my telecoms hat on, we use this stuff to seal up any ducts entering structures: https://www.comtecdirect.co.uk/product/filoseal-duct-sealing-kits/PG5771  - street cabinets, underground chambers, and the like.

It's re-enterable - you can cut it out with a knife pretty readily - but it's water and gas tight and will keep your duct ends sealed shut. So long as your elbows etc aren't leaking and the water level in the ground isn't too high then you'll never get standing water. Those seals will also work with flexible conduit to a degree though the seal will be less reliable if there's any motion. Installation takes about 5 minutes - it's literally a case of lightly sandpapering the duct/cables, shoving a (supplied) chunk of foam in to support the mastic, and squirting it in with a caulking gun. Then you use a wet sponge to smooth it over.

The seals are also rodent-proof - a concern for some more rural observatory operators!

For future builders, I'd strongly recommend solvent-welding PVC pipe bends and joints if you want them to be watertight - embedding in other stuff is a good second but chemically bonding the two halves together is unbeatable.

Once you have the ducts sealed up, maybe consider a continuous dehumidifier for general removal of water from the ambient air?

Edit: Picture of a (not-yet-set!) seal, just for an example from one of my test sites. It sets to a soft but firm solid.

Edited December 4, 2020 by discardedastro

[tomato]

Thanks for all of your replies and suggestions. I have a desiccant dehumidifier in the dome. I’m beginning to think the condensation seen inside the pier was a result of the water present at the base of the pier rather than the source of the water filling the duct.

The ducting was well sealed and well supported when installed so I’d be surprised if it has been subject to damage since construction.  I dug down a metre for the central pier support into the clay layer and there wasn’t even a little puddle in the bottom after a week prior to pouring the concrete. There is no sign of the plinth sinking or cracking after 16 months. Nevertheless the water is there so I will continue to investigate.

[Swoop1]

Some poor soul nearby is wondering why all they get when they turn the cold tap on is a stream of data.......

[tomato]

I’ve ordered 5 metres of PVC tubing which I will run through the duct then run the cables through this so if the water ingress continues the cables will stay dry.

 

[Physopto]

I would strongly suggest that when you do run the new ducting, first seal the end to stop any water ingress whilst pulling the tubing through.

[tomato]

9 minutes ago, Physopto said:

I would strongly suggest that when you do run the new ducting, first seal the end to stop any water ingress whilst pulling the tubing through.

Thanks for tip, I intend to pull all the cables through the pvc tubing, seal the end of the tubing, then pull this through the fixed ducting.

When all of the cables have been removed from the duct, I will suck out the water and blow it dry. If the weather continues to be hopeless,  ie no chance of using the observatory, I’ll leave it a couple of days and see if the water returns before pulling the tubing through.

Edited December 10, 2020 by tomato

[LongJohn54]

7 hours ago, tomato said:

I’ve ordered 5 metres of PVC tubing which I will run through the duct then run the cables through this so if the water ingress continues the cables will stay dry.

 

This is really good tubing, nice and flexible to get through any route between A and B.   However, it has a lot of internal stiction making it difficult to pass cables through it.  My tubes were about two metres in length and I spent a long time pulling and pushing and moving barely 5mm at a time.  Not fun when the bends were a fairly large radius and almost impossible with any sharp bend.   Try passing a cable through to test it before installing.

[bottletopburly]

Spray plenty of pledge on pipe the more the merrier 

[Physopto]

I have used this type of pipe for suction on a sump pump and also for waste disposal on a caravan. It is extremely stiff when cold. It will be very difficult to pull or push though any small bore pipe. So you can try, absolutely bucket loads of kitchen sink "Fairy type liquid soap" to lubricate it and make sure it is warm. Also you can try twisting it when pushing or pulling to help progress.
I did suggest blue water pipe, but this is very stiff to work with (possible problems around corners) but slips through without the same friction effects.

Best of luck however you do it.

 

[tomato]

Again, thanks for your suggestions. I plan to pull all the cables through with it laid straight out in the garage, so the tricky part will be pulling the loaded tubing through the fixed ducting. I have a nylon brass ended pull through cord to put down the pvc tubing for pulling the cables through, and of course the fixed ducting already has a pull through cord.

On the fixed ducting the two bends to be negotiated are fortunately right at each end so maybe I have a chance of success.

If I put washing up liquid into the ducting in it’s current state I’ll end up with a suds monster.😄