Holding Tank Pressurising

IDAMAY

IDAMAY

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A mucky problem!

The holding tank on our Trader 42 is pressurising. This appears to be happening without any additional content being added to the tank - a theoretical impossibility unless gas is being created in the tank? But the tank is empty or very nearly. We have checked that the tank vent pipe is clear and also renewed the anti-odour carbon filter in the vent. However on removing the tank inspection plate air can be heard escaping from the aperture. Operating the forward toilet causes (after a few flushes) back up into the aft toilet. Operating the macerator on the tank outlet clears the pressurisation.

Has any kind soul any ideas as to what to do next or alternatively knows the world's expert marine waste systems engineer preferably located somewhere near Chichester?

Thanks all.

Richard.
 
Thanks, Rob - that is the obvious answer but we have poured water down the vent and it reaches the tank. We have then blown through the vent to make sure that no water is left in the any possible bend in the pipe. I do agree though that it is entirely the logical source of the problem.

Richard.
 
IDAMAY said:
Thanks, Rob - that is the obvious answer but we have poured water down the vent and it reaches the tank. We have then blown through the vent to make sure that no water is left in the any possible bend in the pipe. I do agree though that it is entirely the logical source of the problem.

Richard.
Click to expand...

Removing the inspection plate to hear air escape dictates that there HAS to be a problem with the vent system, ok you pour water down and blow down but what about in the opposite direction?....IMHO it has to be this vent but I stand to be corrected.
 
I'm also with Rob (as it's v.difficult to think of an alternative!)
how about bypassing the normal vent pipe with a piece of pipe going straight through a porthole and see what happens after a day of use or so?
Maybe the routing of the vent pipe is so awkward that there's water standing siphoning the whole thing...

V.odd, plz report if you find the solution

cheers

V.
 
+1 for the vent.

The weight of the water behind any U bends when tipped down the vent will displace the water in the bend, maybe this is what you are hearing going into the tank. But the build up of pressure in the tank isn't enough to displace the water in the U bend the other way, thus creating a seal in the vent pipe?
 
even if there was a section with water in it, gas would still bubble through. I can't see there being a large column of water in the pipe to cause that much back pressure. Is there an odour trap of some description fitted... Just a wild stab in the dark....
 
Robg71 said:
even if there was a section with water in it, gas would still bubble through. I can't see there being a large column of water in the pipe to cause that much back pressure. Is there an odour trap of some description fitted... Just a wild stab in the dark....
Click to expand...

Not sure it would bubble through a U bend. The pressure will be pushing against the head of water not trying to squeeze down the sides.
 
Robg71 said:
depends on the amount of pressure involved.

take the charcoal filter off, and try without it..
Click to expand...

We already tried that. It didn't make any difference. If the vent is blocked (and I accept that is still the most likely scenario) it is below the filter. As you might imagine the connection of the pipe to the tank is in just about the most inaccessible place! When will these boat builders give the long term problems of ownership more than just a passing glance!?
 
When did you notice this pressurisation? Is it that you've inspected the holding tank before and there was no pressurisation or is it that this is the first ever time you've inspected it? Is the top of the tank below the waterline? Is the outlet below the waterline? Is there an anti syphon loop in the outlet pipe? Like the others, I would still target the vent pipe because logic dictates that if the tank was able to vent, there would be no pressurisation. There could be an obstruction in the pipe which only blocks the pipe in one direction. Not easy I know but it looks like you're going to have to blow through the vent pipe from the tank end
 
Hi Richard. A longshot maybe but,as Deleted User has said,a possible cause could be that the pipe is only allowing passage in one direction. Is the pipe one of the thick rubberised ones because,if so,the inner liner can delaminate from the outer cover and could cause a one-way blockage as has recently happened to one of the water pipes on the genny on my own T42 (and it was a real bu**er to figure out).
 
stillwaters said:
Hi Richard. A longshot maybe but,as Deleted User has said,a possible cause could be that the pipe is only allowing passage in one direction. Is the pipe one of the thick rubberised ones because,if so,the inner liner can delaminate from the outer cover and could cause a one-way blockage as has recently happened to one of the water pipes on the genny on my own T42 (and it was a real bu**er to figure out).
Click to expand...

Horrible problem, I'm sure but the delaminated pipe sounds most likely. Just in case there is another movable obstruction, here's two ideas to try:
1. Seal off the vent outlet with a compressor or air pump for the dinghy inserted and try to create enough back pressure to clear obstruction.

2. Get a low cost waterproof endoscopy camera from ebay, copy the prog. file onto a laptop with a USB port, plug and go and take a look directly. I use one for work and they work very well, LEDs' around 10mm camera head give light which is adjustable on the side of the USB plug.

Here:
http://www.ebay.co.uk/itm/2m-5m-7m-...aterials_Plumbing_MJ&var=&hash=item5d44d7c40f

Good luck.:ambivalence::acne:
RR
 
Thanks for the further ideas folks. In answer to Deleted User's questions:
1. Pressurisation first occurred after a fairly rough passage.
2. All had worked perfectly for three years or so previous to that.
3. Top of tank is above waterline.
4. Macerator and outlet is below waterline.
5. There is an anti siphon loop in the outlet. This joins to the tank vent below the anti odour filter.

Problem with the camera idea is that it would be difficult to get it past the T piece where the anti siphon joins the tank vent. Could be useful though when we get access from the tank end.

We are now working on this but i involves cutting a hole in the companionway floor. Don't you just love these boat builders!

Will let you know what we discover.

Richard.
 
I'm still going for water trapped in a u bend. Ties in with the rough weather passage also. You tipping water down the vent pipe is just displacing the water trapped in the u bend with the water you tipped down.
 
IDAMAY said:
This joins to the tank vent below the anti odour filter.
Click to expand...
Mmm, first I'm no expert on this so I could be talking bollox but if your vent pipe was blocked in the outgoing direction by delamination or whatever, then possibly the anti syphon vent isn't venting either so you could also be drawing seawater into the holding tank too as well as the tank itself not venting. Either way, IMHO you have to check whether or not the vent pipe is venting in the outgoing direction. Possibly one quick way to check would be to put your nose next to the vent pipe outlet on the hull, get somebody to flush a toilet and your nose should tell you whether the pipe is venting or not, even with an odour filter in the line. You may even feel air exiting the outlet
Just as an aside, I regularly check that all the tank vent pipes on my boat are working simply by putting a finger near the outlets when filling the tanks. I once cracked the GRP water tank on my boat by stuffing an oversized hose into the filler neck creating an airtight seal and not realising that the vent was blocked. I was told that, in the Med, it seems that creatures like to set up home in vent pipes
 
does your tank have compartments ?
we had a similar problem on our new custom made tank,
because
the partition's only had openings on the bottom corners,
so as soon as the waste water was above these openings, water moving from one to the other compartment,
air pressure was build up in the "non" vented compartment.

after we drilled openings in the partitions, on the upper edge,
problem was cured,
luckyly we had enough and big enough inspection openings for acces for this drilling
 
BartW said:
does your tank have compartments ?
we had a similar problem on our new custom made tank,
because
the partition's only had openings on the bottom corners,
Click to expand...
I can understand why large holding tanks might have baffles to help stop the waste sloshing around but why does your holding tank have compartments?
 
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