Conduit drains? 1

[oldfieldguy]

As is often the case, we had a little problem and I got asked a question.

We had a low voltage circuit catch fire and burn at one of my sites. The wiring in question was in conduit, installed in a Class I Div. I area and was contained except for some smoke which unfortunately triggered smoke and flame detection and killed some equipment. Naturally management got excited.

Upon opening the conduit in question, they found evidence of some rather small amount of water in the conduit. Normally these conduit systems have drains and vents installed.

The management question is: "Are we supposed to be inspecting these drains?" I've been around this stuff for thirty years and never have had that question asked of me. My quick answer is "Nobody does that."

So, do any of you have programs to periodically inspect drains on conduit installations?

old field guy

 

[rcw retired EE]

Of course, every day we pump the conduits full of water and verify all the water drains out. Be sure to put in a differential water relay to alarm if the water coming out is less than the water going in.

Seriously, good question and I agree with your first response - no one does it. In the refinery and the aluminum plants I worked at we never inspected the drains unless there was some reason or evidence to do so. Except for causing rust on the equipment, there was not a big perceived hazard if the drains did not work. I guess if the drain is to remove accumulated hydrocarbons it would be a hazard to have gasoline in the conduit.

 

[waross]

Water should not damage the wire insulation. The only time I see a plugged drain or no drain as a problem is when it results in a flow of moisture into switchgear or a transformer. I do make an effort to drill small drain holes in the low spots of buried conduits. I never check unless there is an objectionable flow of water.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

rcwilson--

Are you channeling my boss or what?

He saw the pictures and knew immediately that we should Do Something. He's a mechanical engineer. He lives and dies for PM's and such on large reciprocating engines and gas turbines.

That water differential relay, though, I'm thinking that would be just the thing. Let's see... My congressman can talk to OSHA, making the relays mandatory, and I could get rich.

My controls techs don't have time to pick up another task walking around inspecting conduit drains. Maybe it's time for a contract drain inspector...



old field guy

 

[EEJaime]

So are these drains Class I, Div I listed? That would be an interesting device.

Waross,

You drill small holes in buried conduit? If there is ground water it's an invitation for water to get in, low point or not. We slope them to a vault or pullbox if they are site conduit. I have never seen anything but a fully sealed conduit system in a Class I, Div I area.

Regards,
EEJaime

 

[davidbeach]

By code all underground conduits are assumed to be full of water anyway. Weep holes could mean that it is full of water for less time than otherwise. Never assume that you can keep water out of an underground conduit.

 

[rcw retired EE]

Crouse-Hinds and other manufacturer's make breathers and drains for conduit systems that are listed for use in a Class 1 Division I, Group B, C, D classified area. They are designed with a labyrinth path for the water to drain out. The long path cools gasses from an internal explosion to prevent an igniting any gas outside the conduit.

The drains look like a pipe plug with a hole in the middle and have bug screens and other features.

Typical use is at the bottom of long vertical runs to prevent condensation build up. Or they are used on NEMA 7 motor starters and control panels to prevent condensation.

I don't know how you would inspect them except by removing them, blowing them out and then putting them back wrench tight.

 

[oldfieldguy]

I'm thinking that inspecting drains would be right up there with changing the air in my tires as a useful pursuit. I've worked for a bunch of clients over the years in and around petrochemical plants and even the most anal-retentive of them didn't have a PM inspection for conduit drains.

And to follow up my original post, I got a phone call on the way home. Apparently the investigation at the site uncovered that a fire was started by grease and oil under the engine base when a spark plug blew out and dropped hot debris. The resulting fire set off the building flame detectors, shutting down several thousand horsepower of reciprocating engines. The fire also roasted this poor little conduit and melted the insulation off the wires inside.

Had those incidents not taken place, nobody would have ever known about the 100 cc's of water inside one fitting and everybody would have been sleeping peacefully.

old field guy

 

[resqcapt19]

EEJaime,

I have never seen anything but a fully sealed conduit system in a Class I, Div I area.

Conduit couplings have straight threads and do not provide a seal that will keep water out. If you install conduit vertically in a wet location, it will fill with water. The water runs down the conduit and sits on top of the coupling and works it way around the threads and over time will fill the conduit unless there is some provision to permit the water to drain out.

I too have never heard of the drains being checked, but have seen conductors damaged when the water in the conduit froze and damaged the insulation. In one case, a 3/4" conduit was split and the #14 wire pushed out the split. When the water was frozen the circuit worked, but went it warmed up the ice melted and the conduit closed tight on the conductor causing a short and blowing the control fuse.

 

[waross]

Where are you located resqcapt19? In Canada electrical rigid couplings were changed to tapered couplings decades ago.
I have done many underground conduit and duct runs where the specs called for tees to be installed at the low point and turned down. In some instances a rock pit was dug beneath the tees, in some instances no pit was used. It depended on the ground conditions and the designers personal preferences.
The last time I did it, we had about 1000 feet from the sub station to the risers up the poles where the connection to the overhead distribution lines was made. The profile of the conduits was such that over 500 feet was at a higher elevation than the weatherheads up the poles. We felt that any amount of water entering the conduits would eventually exit through the weatherheads. This may not be an electrical problem but would certainly be an aesthetic issue. Add to that the possibility of freeze up and splitting. This issue was identified after the conduits were installed. The conduits were 5 inch sched 40 PVC. A worker was sent to the lowest locations with a 1/2 inch drill. We had 1/2 inch holes drilled through the conduits from the top down and out the bottom. There have been no problems related to water.
In regards to the ice issues;
Water and glass share an interesting property:
Both impure water and molten glass are conductors.
Both are insulators in the solid phase.
Water has a very high capacitive constant, ice has a very low capacitive constant.
An instance; in lumber mills, lumber with a high moisture content will appear to be dry when frozen. This will happen with either a resistance type moisture meter or a capacitive type moisture meter.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SteveWag]

Pressurize each conduit section to about 6 PSI and put a floating ball meter on the individual supply. Adjust for about 0.1 CFM of air flow per conduit section. Group all meters in one area and read once a day. Once a year go to each drain and verify flow with a cup of water. Log the number of bubbles per minute in the annual log. Investigate any readings that deviate from the calculated annual mean by 10%. If in freezing climate, provide dry air or nitrogen, argon etc. This should make him happy. CONDUITS are to be threaded with tapered threads but connectors and fittings still have straight threads.
Steve

 

[EEJaime]

resqcapt19,
Properly installed modern RGS systems are very well resistant to moisture. There is always minor amounts either from condensation or entrainment that occurs, but we are not talking about flowing water quantities. Be that as it may, would you build in fittings with unsealed openings to the atmosphere? Such as occurs with drain fittings?

SteveWag
Can you pressurize a conduit section with drain seals installed in it?

 

[resqcapt19]

Bill,
I think you will find that the opposite is true. About 20 years ago the conduit couplings used in Canada had tapered threads, but under pressure from the manufacturers they changed the rules to permit the use of straight thread couplings. The conduit threads in both countries are tapered.
PS, I am in Illinois.

 

[resqcapt19]

EEJaime,
I don't agree. Even when properly installed, vertical conduit will fill up with water via leakage at the couplings unless some type of drain is provided. The use of straight thread couplings with tapered thread conduit does not provide a watertight seal.
In non-classified areas, it is very common to find a small hole drilled in a conduit body at the low point in the run. In classified areas, drains listed for the purpose would be used. I have even seen the holes drilled in the conduit itself for underground, as Bill said in his post.

 

[resqcapt19]

Steve,
You can really get conduit to hold 6 PSI?

 

[rbulsara]

OFG:

Take your boss to some water filled manholes with 25 kV cables in them!

Rafiq Bulsara

http://www.srengineersct.com

 

[oldfieldguy]

Rafiq--

I thought about that. It would scare him to death. Fortunately our water-filled manholes are not within his area of responsibility. Not only are they water-filled, but being next to a salt-water ship channel, I'm pretty sure that it's salt water in the manholes.

Let's see: We could put in a filtration and desalinization unit to process the manhole water. that's what I need...

old field guy

 

[EEJaime]

The OP referenced a failure inside a conduit. Inside a Class I, Div I, atmosphere. I know that manufacturers such as Appleton Electric, make an explosion proof rated, combination breather/drain fitting for enclosures, (ECDB fitting). I don't believe one exists for a conduit fit. That was the only point I was trying to make-although I imagine it would be possible to run the conduit through a junction box fitted with this specific fitting. Putting it at a low point in the system would effectively drain the system.

Just a suggestion,
Regards,
EEJaime

 

[roydm]

This thread just confirmed my suspicion, running conduit to an electrical device is like running a water pipe to it.
When are you guys going to learn?
Armoured cable is the way to go!

 

[waross]

I found out years ago that often the only way to keep some cabinets dry is to drill a small drain hole. Control cabinets for car washes, (the roll-over type was the worst) and outdoor pumping station control cabinets, particularly pole mounted cabinets. I guess that today we would use an Appleton drain fitting instead of a 1/4 inch hole. If you want it dry, let the water out.

Bill
--------------------
"Why not the best?"
Jimmy Carter