Holding Tank - Becomes Pressurized

Jokani

Jokani

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I fitted a holding tank early in the season. A 28 litreTek Tanks tektankslimited.com/waste-tanks-42-c.asp

Ever since I have had problems with 'dirty' water accumulating in the locker and bilge, and I have been struggling to find the source of the leak.

This weekend on the boat I heard what sounded like gas escaping from the holding tank, I took a look at the tank had expanded and appeared to be under pressure. I think the noise I heard must have been gas (and I expect some water) exiting from a pressure relief valve.

I know the tank was empty the previous night, and that it was a physical impossibility that it was filled via the toilet.

A can only think that somehow the tank is being filled from the toilet water intake, is that possible. The toilet does not have an anti-syphon pipe fitted, there was not one fitted when I purchased so I presumed unnecessary to fit one now. But I'm guessing I'm wrong?

But - The tank has an air vent so I do not see how pressure could build up?

I'm totally confused!

Can someone please point me in the right direction, boating is not fun when each time starts with rubber gloves!
 
Holding tanks don't have a pressure relief valve. If air was escaping with a hissing noise, that was a leak.

I have no idea how you're managing to pressurise it. The toilet inlet, anti-siphon valve or no, should have nothing to do with the tank. And the vent should ensure that there is no pressure difference between inside and outside.

The only thing that occurs is if it's a pumped tank rather than a gravity one, perhaps the pump has been fitted the wrong way round, and when you think you're pumping it out, you're actually inflating it? But that still doesn't explain the vent not working. Even a minor blockage in it, I'd expect to be blown out by the sort of pressure you seem to be encountering.

Can you see all the plumbing, and do you understand what each bit is?

Pete
 
You should have a one inch pipe from the vent fitting to a skin fitting (without any obstruction or valve) fitted high above the water line. I see no good reason to have a pressure relief valve if you have a proper vent pipe as above, and any such valve fitted to a tank in the bilges is just asking for waste to get into the bilges. You need to make sure that the vent pipe is not blocked or kinked. I suggest you draw a diagram of the plumbing, with the position of the boat's static waterline shown relative to toilet and tank, so that others can advise you on the subject of fitting a syphon break. My boat does not have one, but the tank is well above water level.
 
Yes, the tank should be vented and you might want to have a filter between the vent and the tank (charcoal) so as not to spoil your G and T when someone uses the flush :--).
 
Sorry but the only plausible explanation to me is that you have the connections wrong, perhaps the vent is connected to the outlet, and as this has a dip pipe to the bottom of the tank it would not vent, it would perhaps also prevent the tank emptying.
I assume you are emptying the tank by opening the seacock when at sea to bottom drain the tank.
Otherwise something is very blocked.
 
I'm sure I took a picture of the final installation, but damned if I can find it.

I'll grab a picture at the weekend and post it.
 
GaryCottam said:
I'm sure I took a picture of the final installation, but damned if I can find it.

I'll grab a picture at the weekend and post it.
Click to expand...

A diagram should be sufficient and even preferred to a photo if it does not show relative levels and clearly and how everything is connected.

Why not draw it and photograph the drawing?

This tank presumably

28-litre-waste-tank-tt-4300-450-x-390-x-200-275-p.jpg
.................................. http://tektankslimited.com/28-litre-waste-tank-tt-4300-450-x-390-x-200-275-p.asp

No mention of a prv ....... but you say, "I think the noise I heard must have been gas (and I expect some water) exiting from a pressure relief valve." If you installed it you presumably know that you fitted a prv, if so where and why.


My guess is that its been overfilled and there is now "something" blocking the 1" vent.
 
Our poopmeister guru when we had a revamp of holding tank and systems on the mobo we lived, on installed an extra for us, a little replaceable weak link type safety valve in the tank to prevent it from being imploded by excessive suckout vacuum, as some marinas round here have huge suction pumps that can try to turn a tank inside out and certainly can crack them over time and cause leaks?. .
 
This is an image of the top of a Tek Tanks sent by the manufacturer with the correct fitting/use marked.

The red arrow indicates a fitting that looked like it was made of brass I presumed was a pressure relief valve, I will ask the manufacturer.

tek-tanks.png


It is possible that I may have the inlet and outlet pipe mixed up (they are the same size) but I would be 100% that the vent is correctly connected.

I think the outlet is correct - when first installed, I tested by approx half filling the tank with clean water and tissue, then operated the electric pump, and the was visual evidence of tissue beneath the boat.

I do not believe that the tank can ever have been filled more than half by normal use.

The vent may now be blocked, but if so, I think the root cause must have been the tank filling via the toilet inlet.

I guess I need to establish where the waterline is in relation to the tank? If it is lower or near the waterline then I need to:

1) Install a longer pipe on the inlet to create an anti-syphon loop

2) Unblock the vent

Is that a plan?
 
Could be you are mistaken about the tank being under pressure. I had my tank overfull earlier this year and the sound of the fluid bleeding
through various seals did sound like gas escaping.

Martin
 
I found a picture :-)

holding-tank-2.jpg


The vent (clear pipe) is connected to the smallest fitting, leads to the charcoal filter, and the on to the vent outlet.

Inlet from toilet is the older pipe

Outlet leads to the pump and on to the larger seacock

The smaller seacock goes to the toilet and this was not touched during the holding tank installation.

Seems like I have everything right there?
 
martinriches said:
Could be you are mistaken about the tank being under pressure. I had my tank overfull earlier this year and the sound of the fluid bleeding
through various seals did sound like gas escaping.

Martin
Click to expand...

I could be wrong, and often am, but I do not think so on this occasion, the tank is a tight vertical fit, I only noticed that the tank had expanded as the vent pipe was approx 5mm higher than it should be, this draw my attention so that I noticed the tank was bulging. Once I opened the outlet seacock and operated the pump the pressure was released and the tank returned to a it's normal size.
 
Looks like it's almost certain that your tank will be wholly/mainly below sea level, so water will flow in through the outlet pipe. The outlet pipe needs to loop up some way above sea level, then back down to the outlet, with an anti-syphon device at the highest point.
 
The sagging vent pipe looks like the main culprit to me, especially with the charcoal cartridge in it. Bit of soggy loo-roll shoved into the front of the filter and blockage seems almost certain.

Also the pump is mounted upside-down. Maybe you have no other option and have to accept it, but Whale's instructions say that the motor should be above the pump chamber in case of leaks.

Pete
 
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