Sealing in the diesel

[Guest]

I have just had two new inspection hatches fabricated in stainless steel to replace the pathetic, leaky plastic ones that came with the boat.The new ones are designed to sandwich the sealing medium between two plates and the body of the tank. My only doubt is how to seal the nasty stuff in. The methods under consideration are to obtain (don't know where) some Nitrile 'O' rings or use some type of 'Hermatite' and a gasket. Do any of you have any ideas? I NEVER want to have another diesel leak!

Wully

 

[Guest]

O rings....

You should be able to buy O-ring rubber in lengths, and simply glue the ends together to create a seal the exact size you need. Try engineering supply companies in Yellow Pages.

 

[Guest]

Re: O rings....

Hmmm...

Never thought of that as joining 'O' rings at work is a BIG No-No. I'll try that- thanks.

Wully.

 

[Guest]

you can use 10 second glue to stick the ends of some appropriet size "o" ring to fit.

 

[Guest]

O rings work best when there is a grove for them to sit in

Our tank inspection plate have an 1 1/2 wide gasket of rubber about 1/8 " thick secured with approx 30 10mm bolts, never leaked frequently lifted for inspection. You havnt mentioned the material of the tank, as this will have an impact on the sealing method

David www.yachtman.co.uk

 

[Guest]

I'd give the 'O' Ring idea a miss. 'O' Rings are most often designed to sit in a groove or grooves opposite two machined faces with a very small gap between them. Internal pressure then forces the 'O' Ring from its groove to seal the tiny gap between the faces. A diesel tank doesn't have the pressure and an 'O' Ring is so thin that any difference between the torque on the plates will allow it to leak.

From your description it looks as if you have two flat faces to seal. My own diesel tank has a threaded 250mm inspection plate with fairly smooth faces on the male plug and the female receptor. I have sealed it successfully with a 25mm wide gasket cut from a sheet of soft rubber (an old heavy duty tractor inner-tube). It's still working OK after a year.

 

[Guest]

Hmmmm, Plenty to think about there, thanks for the suggestions. I might go for a rubber gasket or a flatened 'o' ring.

"So long diesel smells"

Wully.

 

[Guest]

Re: O rings....

Wully

O-rings are designed to work with a specific degree of compression. They work best when used rather like a piston ring, so that the distance between the bottom of the piston groove and the cylinder wall is controlled in the machining, rather than by screwing together two flat faces.

I would reckon that you would be better off using a sheet gasket, possibly with a Hermetite sealing compound.

How do you plan to secure the hatches to the tank? Don't forget that any screw holes penetrating the tank will also need to be sealed.

 

[Guest]

The clever bit.

Each hatch is made from a sheet of 4 mm stainless steel (316) over size for the opening. both has a 11 mm hole to take a bolt. The bottom plate has a slot cut almost to the hole through the center (with a nut welded in place) which allows the assembled plates to be positioned, one inside the tank, one on to. When the bolt is tightened down the plates will sandwich the sealing element. That's the plan anyway.....A cunning arangement of washers and an 'o' ring will seal around the bolt. Fingers crossed.

Wully.

 

[Guest]

Re: The clever bit.

you say "to take a bolt. " do you mean only one bolt will secure the plates? on most designs I have seen there are 11mm bolts at about 5cm centres all the way round.

david

 

[Guest]

Some points and ideas

If I understand this correctly, you are going to sandwich the tank metal between two 4mm plates, held together by a single bolt?

Not knowing the tank hole diameter makes it difficult to comment, but as it is an inspection port I guess it is 15 cm across at least? In this case, I doubt very much whether a 4mm plate is sufficiently rigid to seal all around its periphery, regardless of the seal material selected. Ports like this are usually closed with a plate that has a significant lip extruded into it, to give it rigidity, and held with a large number of small bolts all around the edge. An alternative may be to increase the thickness of the discs significantly, I would guess more like 10 mm might be required. Seal the bolt with a fibre washer, trying to get an o-ring to seal inside a tank will be very hit-and-miss. Better, why not weld the bolt in place to the inner disc, then you won't need to slot the disc and you can tighten the nut from the outside.

Cork is probably the best gasket material, as it is resilient and resists diesel. Inner tube is attacked by diesel, albeit slowly. Asbestos-type gaskets (eg Klingerit) may be too hard for sheet metal joints as they require some force to compress. Hermetite should help, but should not be relied upon. You can buy most gasket materials from a decent automotive supplies factor.

 

[Guest]

P.S.

Why not use a ring of nuts and bolts just inside the inspection port ID? You still have the basic problem that tightening this arrangement bends the plate before it applies load to the gasket, but you minimise it by putting the bolts as far out as possible.

 

[Guest]

A couple of days I removed the manhole covers on my Moody 42 for cleaning the glassfibre fuel tanks. They are 30 cm square with a LOT of bolts along the edges They have never been opened as far as I know and the yacht is built 1977. Moody used a cork gasket about 3 mm thick (similar to the gasket you´ll find under a cylinder head cover)and so far, no leaks. I´ll definetely go for the same type, it have worked fine for 24 years.

 

[Guest]

Re: P.S.

Thanks for the suggestions,I might try cork as a sealing medium. The plates I'm using are far thicker than the tank and will not bend when the nut and bolt are made up, I'm confident that the sealing medium will be compressed enough to make a good seal. (he said confidently...) I've knicked the idea from a design that is sold by a Swedish company but was to small to fit my tank, their product was made from nylon and had a single bolt securing it. I'll report back if it's a success-or not.

Wully.