Bulkhead Fitting for 1000L Tank and Cistern - FPM Viton Seal, 1/2-2 inch

Create an outlet on a 1000-litre tank that holds an aggressive product. This bulkhead fitting goes on entirely from the outside and its FPM Viton seal, a fluorinated elastomer, withstands solvents, acids and plant protection product concentrates. Six sizes, from 1/2 to 2 inch.

The fluorinated seal, for tanks that other elastomers cannot hold

On a 1000-litre tank bulkhead fitting, the mechanical part is almost never what gives way. The body, the ring and the clamp are made of glass-fibre filled polypropylene, a plastic that the vast majority of products leave indifferent. What decides the life of a wall penetration is the seal, and the seal alone. It is the only flexible part of the assembly, the only one that absorbs, the only one that works permanently in contact with what is in the tank.

When that content is water or fuel, the choice is simple and inexpensive. When it is a chemical product, a solvent, a technical concentrate or a mixture whose full composition you do not control, an ordinary water seal is not undersized : it is outside its field of use. It swells, softens, stops bearing evenly around the edge of the hole, and the drum ends up weeping — usually on the side you are not looking at.

FPM is the answer to that precise problem. It is an elastomer whose chain carries fluorine atoms : this bond is one of the strongest in organic chemistry, and it makes the material very difficult to attack. You will meet three names for the same material and it is better to know it before comparing two product pages : FPM is the European designation, FKM the American designation, and Viton® is a registered trademark of Chemours, which has passed into everyday language. Bucchi describes this seal as an FPM Viton.

In practice, FKM does not take up aromatic hydrocarbons, which are the carrier of many technical and plant protection concentrates. It keeps its dimensions and its hardness in chlorinated hydrocarbons, which nitrile cannot stand. It holds concentrated mineral acids when cold. A seal that does not swell is a seal that stays pressed, and a seal that stays pressed never needs a re-tightening that nobody will come and do.

Where FKM works, and where it does not

A chemical seal is not sold by promising everything. FKM has a very wide field, but it also has two or three families of products that damage it, and keeping quiet about them would amount to selling you a leak. So here, with no arranging of the facts, is what this tank fitting withstands and what it does not.

This table is a selection guide, not a certificate : it does not replace the safety data sheet (SDS) of your product, which is the only legally binding document. Concentrations, temperatures and mixtures change everything, and two products of the same family can behave differently. If your tank holds anything other than water or fuel, give us the exact trade name before ordering : we will look at the sheet with you rather than let you try your luck.

Here, the limit is not the seal, it is the plastic

FKM is almost always sold on its temperature resistance, and on a storage tank or cistern that point deserves to be put back in its place : the elastomer alone works above 200 degrees. On this bulkhead fitting, that quality will be of no use to you, and it is better to say so frankly before the purchase than to let you find out. The part that holds the wall is made of polypropylene : it sets the limit, and it is rated for 4 bar at 20 degrees and 80 degrees at most. Put steam behind a fluorinated seal fitted on a plastic body and it is the body that creeps, not the seal that gives way.

What you buy with FKM is therefore not temperature : it is chemical resistance when cold or lukewarm, within the range where polypropylene is perfectly comfortable. That is exactly what is needed for a 1000-litre tank, a storage cistern, a chemical drum or a workshop recovery tank, which work at ambient temperature all year round.

If your installation calls for both chemistry and heat, no version of this fitting will do, this one included. You need to go for a metal penetration, in stainless steel, with a flat gasket added. Tell us and we will point you to the right product even if it is not on this page : a fitting that ends up leaking costs us more than a sale we did not make.

Drilling a tank that has already held a chemical product

This fitting has no inner counter-piece : it is engaged and tightened from the outside, and that is precisely what makes it workable here. On a tank that has held a technical product, you do not put your arm inside, and above all you have no wish to have to. Fitting comes down to drilling to the right diameter, engaging the body, then tightening the clamp.

The real precaution is therefore not mechanical, it is chemical, and it takes place before the drill. Here is the order of operations we recommend :

• empty the tank completely and have it rinsed three times, with clean water or with the solvent recommended by the safety data sheet ;
• neutralise if the product was an acid or a base, then rinse again ;
• let it dry and air out in the open : a hole saw makes sparks and heats the material, which has no place near flammable vapours ;
• work outdoors or in a ventilated room, with gloves resistant to the product and safety glasses ;
• collect the rinse water in a labelled container and have it taken by the appropriate waste channel. It goes neither down the drain nor into the gutter.

Once the drilling is done, the rest needs no special tooling : the clamp is tightened by hand, and with a wrench for the large diameters. Bucchi also lists in its catalogue a tightening wrench dedicated to this fitting, size by size, useful if you are fitting several. Choose an even area of wall, away from ribs, corners and welds, and check that no frame bar runs behind before making the first cut.

Dimensions and drilling diameter

The six fluorinated references reproduce exactly the geometry of the EPDM and nitrile versions. That is not a deduction : the manufacturer states strictly identical net weights for the three variants, size by size. Only the seal compound changes. The values below are taken from the Bucchi catalogue and given in millimetres ; E is the drilling diameter, the only dimension to note before ordering, and the others describe the space taken once the part is fitted.

The table headings use the letters on the manufacturer's drawing. Here is the key :

• E — the hole to be drilled, as a diameter in millimetres. This is the one dimension your drilling has to respect.
• E1 — the outer diameter of the shoulder resting on the wall around the hole.
• G1 — the thread size of the outlet, in inches, parallel gas pitch.
• D — the clear bore: the flow follows it, not the thread.
• H — the overall length of the fitting, with its sleeve.
• K — the hexagon size across flats: the wrench opening that tightening requires.

Two notes to avoid a wrong order. One: K never exceeds the drilling E — that is the very requirement for fitting from outside : the hexagon must pass through the hole so that the fitting engages from the outside. Two: the flow depends on D, the actual bore, and not on the advertised thread size : a 1 inch passes only 24 millimetres.

The last column deserves a second of your attention, because it is the most common mistake on this type of part. It gives the maximum tightening torque of the thread published by the manufacturer for a polypropylene material : it is the limit not to exceed when screwing the tap, the valve or the fitting onto the male outlet. Beyond it, the plastic thread is marked and will never take properly again. Tighten by hand, then a quarter turn with the wrench, and stop there. If you are afraid of overdoing it, two turns of sealing tape are worth more than one more turn of the wrench.

The outlet is a male thread in parallel gas pitch, to the ISO 228/1 standard, in the size you choose. It takes, without an adapter, a tap, a ball valve, a drain spout, a hose barb and its pipe, a gauge or a vent. The fitting creates the opening ; what goes on it remains your decision, today as on the day the installation changes.

Taking FKM, or not taking it

Let us be clear : this version is about three times more expensive than the EPDM version, because the fluorinated elastomer itself costs three times more to produce. It is not a step up the range, it is a different material for a different use, and there is no point paying for it if your tank does not call for it.

If your 1000-litre tank holds rainwater, borehole water or diluted liquid fertiliser, take the EPDM version. If it holds diesel, domestic heating oil, GNR or an oil, the nitrile version is amply sufficient and will cost you far less. FKM is justified in four situations, and it covers them alone : an identified solvent or chemical product ; a plant protection product concentrate ; contents that change over the months, for example on a recovery tank or a rented tank ; and the most frequent case, a product whose full formulation you do not know.

One last point, and it carries practical weight. The three versions share the same body, the same drilling diameter and the same thread. If you start with a water seal and the use of the tank changes, you switch version without touching the wall, without re-drilling and without plugging the previous hole. That is also why we keep all three in stock rather than just one.

Characteristics