RIVETED PRESSURE VESSELS

[Stanley]

RIVETED PRESSURE VESSELS

11 October 2005

I was recently asked a question about riveted vessels and realised that this knowledge is being lost. Here is a short description of riveting iron and how it was used.
Ever since man started to work metals there has been the need to join two pieces together. Adding a piece to another in order to gain length was pretty easy, the two pieces were brought up to ‘welding heat’ in the smith’s fire, this is the stage at which the metal is white hot and starts to burn, this produces white sparks. If two clean pieces of iron in this condition are brought together and hammered, or ‘forged’, they will fuse together. Properly done this is a very strong joint and ‘fire-welding’ was the forerunner of modern gas and electric welding processes.
Fire welding was, for many years, the standard method of closing the links in chain-making and iron tyre-making to name but two common applications. A less well-known application was in the production of circular fire tubes for Lancashire boilers. Large plates were flanged, formed and fire welded to produce cylindrical components up to three feet in diameter. It’s a good trick question to ask an ‘expert’ how many welds there are in a riveted Lancashire boiler. Most people will say none, they are wrong of course, every section of fire tube was welded from the earliest days on the standard Lancashire boiler. If you’re looking for them, they are usually situated at twenty to and twenty past in the flues.
However, fire welding was not practical for large structures such as the shell of a Lancashire boiler or a large metal structure such as a bridge. Screw bolts and fish plates could be used for structural steelwork but this technique was no good for steam boilers because of corrosion and the stresses of expansion and contraction. I will concentrate on the use of rivets in boiler and pressure vessel construction. Note that the same considerations applied to ship-building and bridge-building as well. The Titanic and the square tubes of Stephenson’s original Britannia Bridge over the Menai Straits were riveted.
The basic principle behind hot riveting is that a closely fitting bar of mild steel with a head already formed on one end is heated to welding temperature, inserted in holes already drilled in a joint between two plates, ‘held up’ at the head end and hammered into a head at the other end while still white hot. As the rivet is hammered it swells in the hole and then as it cools it contracts and draws the plates firmly together. Properly done this makes a perfect joint which is water and steam proof and very strong.
The rivets themselves can have a variety of different head shapes ranging from countersunk to a full domed head. The latter are the most common in boiler practice although in the early days smaller heads were favoured. The material they are made of is a mild steel containing 0.10% to 0.20% carbon. The standard test for rivet material was that it should be capable of being bent hot through 180 degrees without cracking. Boiler and pressure vessel rivets varied in diameter from half an inch up to two inches and I have helped to install rivets of the larger sizes which were up to twelve inches long.
The most usual task these days is to replace a rivet which has failed. The standard method of doing this is to remove the old rivet by burning it out with an oxy-acetylene cutter, this is a very skilled job but in the right hands is a very efficient way of clearing the hole. The alternative is drilling but there is a danger here of either enlarging the hole or leaving part of the old rivet in there. When the old rivet is cleared, the hole is reamed to the correct size with a large hand reamer and any rags cleared off the holes at each end. The new rivet is heated to welding heat, inserted in the hole from the inside and immediately ‘held-up’, usually by an extending strut powered by compressed air. In the old days a screw jack arrangement or even a strut and a wedge would be used. In ship-building the head was often held up simply with a very heavy hammer which gave the riveter something to work against. Once held up, the riveter hammers the plain end of the rivet up and forms a head at the same time. In the early days a plain hammer was used which gave a flat head, with pneumatic tools a cup shaped hammer is used which gives a domed head to the rivet. This process is very quick and takes less time that reading this paragraph.
Making a new vessel requires a different technique. The shaped and drilled components have to be held in place while the riveting is accomplished. This is usually done by using temporary bolts in all the holes and only removing them when inserting the rivet. Using this method the parts can be kept in the same relationship to each other, however, there are still pitfalls for the unwary. Joining a seam which could be say eight feet long might be done perfectly with every hole in proper alignment but once the seam is finished, it will be found that the plate edges have grown slightly, this expansion under the stresses of riveting has to be allowed for. There is also the small matter of the fact that where one riveted section is attached to another there has to be a place where the seam of one is attached to the other plate. This is accommodated by scarfing the edges of the plate at this point so that the resulting joint of three thicknesses of plate is only slightly thicker than the double seam.
Repairs on site are always done using air hammers but in the shop hydraulic riveting was used in later days. This was complicated and expensive to set up and was only used for large volumes of work but was very quick and accurate. Basically, what happened was that the rivet was squeezed from both sides using hydraulic pressure. The anvils could be set to give exactly the right travel so that the plates were not damaged.
How effective was riveting of steam boilers? The short answer is that done properly it was, and still is, the perfect way to manufacture a large pressure vessel. There are many boilers still insured for their original pressure which have given over 100 years of trouble free service. The beauty of a riveted boiler is that it has a slight amount of ‘give’ in it. Too much expansion and contraction, say in an application where big pressure differentials occur, breweries and paper making are good examples due to heavy draw-off of steam when heating large vats, can lead to problems with any boiler. Boilers used on applications like a steam engine where a constant pressure can be employed can survive almost indefinitely with proper firing and water treatment.
Welded boilers are the norm these days and even though welding has progressed to the point where it is now a science, welded boilers still fail tests due to inclusions or cracking in the welds. One major problem that was encountered in the early days was ‘brittle zone cracking’. This was failure of the plate a short distance back from the weld due to changes in the crystalline structure of the metal because of the temperature gradients during welding. Theoretically, if the right grade of plate is used this cannot happen but there have been catastrophic failures due to this cause.
Barring total failure due to misuse, bad maintenance or corrosion, the worst that can happen to a riveted boiler is a small leak on one rivet. This does not always mean that the rivet has to be replaced. Most small leaks can be cured by ‘caulking’ the rivet. This entails hammering the edge of the rivet head whilst it is cold in order to draw the rivet tighter and make a seal. A few small weeps are often encountered on a new boiler. These will almost all heal themselves up with use as a small amount of corrosion develops in the path of the leak and stops the weepage. The old boilermakers used to put a small amount of sal ammoniac [the old name for ammonium chloride, NH4Cl] in the initial water fill of a boiler to accelerate this corrosion and seal any leaks. Other just urinated in the water!
Riveting is by no means dead in other industries. It is still the favoured method of joining large plates in aeroplane manufacture and associated applications. It is not widely known that every large air-liner is actually a pressure vessel. It has to be pressurised in order to allow passengers to be carried safely at 35,000 feet plus. For over 200 years riveting served the boiler makers well up to pressures of over 500psi. It is still a good manufacturing method but has been overtaken by electric arc welding on the grounds of speed and economy. However, in terms of utility and longevity the riveted steam boiler was and still is superior for all normal applications.

11 October 2005