Galvanised Pressure Vessels and Air Receivers
Do you need a galvanised vessel ?
We make over 1000 vessel every year and very few are galvanised or painted inside.
The vessel itself is often inherently designed to act as a dirt trap as well as a buffer of whatever it is in it.
As a result we have little or no problems with internal corrosion causing problems further down the line.
All vessels are built with a corrosion allowance, and outlive the equipment they are intended to work with by some considerable margin !
If however you really do want a vessel that not only performs structurally/ mechanically, but also have excellent corrosion resistance, we can build pressure vessels and air receivers to a standard range or to your specific requirements.
It is however important to note that the galvanised finish of pressure vessels and air receivers is not always the pretties sight, this is due to a number if reasons.
Abbott and Co.(Newark) Ltd was established for over 140 years and has ISO9001.
We take great pride in our products and strive to exceed our client’s expectations on all levels. We have worked closely with galvanizers with respect to ideal vessel design, bath sizes, skimming the bath and steel types, and continue to do so.
Despite that, the question of galvanizing quality arises 3 or 4 times a year and has done for as long as I remember.
Whilst galvanizing offers excellent corrosion protection it doesn’t necessarily make it look nice.
Galvanising pressure vessels has a number of things against it that makes it neigh on impossible to get a great overall look, here are the reasons:
Steel Type: Pressure vessel steel has a slightly higher silicone content than structural steel- this silicone content causes the zinc coating to build up faster and less evenly than usual – you end up with plenty of zinc and great protection from corrosion, but the uneven crystalline formation of the zinc is not attractive. The vessel may also have different steels on different parts further adding to the non uniformity of appearance
Dip Speed. We commonly see galvanised fence panels or structural steel these dip in and out of the bath quickly and cleanly. There is little time for thick zinc to build up, the zinc runs off fast, no surface flux or debris is left on the product – generally leaving a ‘spangly’ shiny finish.
With a vessel, best efforts to position lifting lugs vents and drains strategically on the vessel still mean the vessel has to be dipped VERY slowly – the like jumping in or out of the bath too fast at home but worse still, the vessel is hollow, and the spilt liquid very hot and very heavy. (for example a 6 cubic meter vessel weighing 2 tonnes bare will have over 40Tonnes of zinc in it)
This means that the FIRST part of the vessel in is in for a long time (all the time collecting zinc), and the LAST part in is in for a comparatively very short time – this again leads to inconsistencies in the finish.
Finally on Dip Speed, as the vessel is very slowly withdrawn from the molten zinc at 450 Deg C, the operator(s) have to try to skim the flux and any debris or oxide off the surface, this is very difficult and the same time as the vessel is being lifted stage by stage with a crane. This results in tide marks of oxide as the crane lifts and stops to allow the vessel to drain, it also lets particles sit on the undersides of flanges where debris clings to the vessel.
Iron oxides sitting on the surface of the bath can end up sitting on top of the zinc coating, these can turn brown but is superficial staining.
Welds / Sanded areas. Pressure vessels have long lengths of exposed welds, and this weld metal often picks up zinc at a different rate from the rest of the vessel. This can give the impression of weld defects such as cold lapping, undercut and porosity. Areas where the surface has been sanded can pick up weld a faster rate than others.
Galvanising bath Contents: Reputable UK galvanising companies frequently will have clients like Abbott and Co. that need to meet or exceed the requirements of BSENISO 1461 :1999. This means that they cannot water down the zinc with aluminium – this technique employed by some, greatly thins the coating saving time and money, and gives a far superior shiny finish – but doesn’t off the same corrosion resistance or compliance with the code.
Some continental vessel manufacturers use structural steel and ‘thinned’ zinc to save costs and improve appearances, they then use the fact it is galvanised to reduce the material thickness with no corrosion allowance, resulting in a vessel with marginal material thicknesses, being a good example of ‘you get what you pay for’.
Despite all of the above pitfalls in galvanizing pressure vessels, galvanizing a vessel gives it excellent longevity, and excellent corrosion resistance and renders it almost maintenance free. when done properly however it doesn’t always look pretty !
Abbott and Co. take the minimum possible action to dress a galvanized pressure vessel as to do so removes the very zinc you want on there to protect the vessel – the zinc thickness varies, so cutting too deep into the surface will expose naked carbon steel – which will rust.
Because we are aware of these idiosyncrasies with galvanized air receivers and pressure vessels, we state in our specifications “the galvanizing may not be uniform in thickness and appearance”
To achieve a more aesthetically pleasing finish to a galvanised vessel it is possible to give it a two pack paint finish – this additional corrosion protection would allow the fettling to be undertaken in the knowledge that the paint finish would protect any possible over fettled edges, without the same risk.
The end result is an excellent protection against corrosion, but it is expensive. ……
Recent Comments