Palletised Cask Filling System
Cask Variability
There are many different types of cask; they differ mainly in dimension but also whether made from American or European oak. Casks are further classified based on their previous use. Second-hand casks can be used for the maturation of whisky unlike bourbon, the prior use of a cask has an impact on the final whisky; hence casks are classified in this way.
Casks are often referred to as barrels within the whisky industry, however strictly speaking a barrel is a specific type of cask; the ‘American Standard Barrel’ (ASB). ASBs originate from use in the bourbon industry. As demand for whisky increases so does the need for casks, since bourbon producers cannot reuse the ASBs they are often shipped to Scotland for whisky maturation and as a result ASBs are one of the most common types of cask in use.
ASBs can be shipped from America intact or flat packed on to pallets by removing the steel rings or hoops which hold them together by compression alone. The ASBs are broken-down into their individual staves; staves are the individual timber planks which make up a cask. Once in Scotland the staves can be reassembled into ASBs or used to make hogsheads which are a traditional whisky cask and have a larger diameter than an ASB. The approximate dimensions of both hogsheads and ASBs can be seen here.
Cask Staves, used/weathered on left and new/fresh on right
All casks have a bunghole, this is a tapered penetration drilled either in the bilge or one end of a cask through which the cask is filled and emptied. The bunghole is sealed using a bung which is a tapered wooden cylinder. The bung is hammered into place to create a seal.Cask dimensions vary from cask to cask even within the same type. Hogshead volumes range from 240 to 270 litres and ASBs from 198 to 207 litres. This is not only due to the variability of the wood used to construct them but also each cooperage can have slightly different specifications [3] [4].In order to make the overall maturation process as efficient as possible it is necessary to fill each cask as close to its own individual maximum as possible. This requirement to maximise the filled volume and the varying cask capacity is not common among filling applications [5]. Glass bottles vary in capacity slightly and are filled to level, however the final fill volume is less than the maximum volume of the bottle and the variation in bottle volumes are significantly less than that between casks; the variation in small containers can be less than 1% [6] whereas casks can vary by over 6%. The large variability of casks presents significant challenges not faced in other applications filling similar volumes such as beer kegs which are manufactured to close tolerances’. The filling of other large vessels such as intermediate bulk containers is done volumetrically [7].
ASB bunghole immediately after disgorging
Volumetric filling alone is not suitable due to the large cask variation. A volumetric system with a set point greater than the volume of the smallest possible cask would over fill such a cask which is an unacceptable health and safety hazard for operators as it results in loss of containment of spirit. Avoiding this hazard would require the volume set point to be lower than the volume of the smallest possible cask which would result in unused capacity in all other casks. This in effect would lead to the warehousing of excess air within casks and the use of more casks in order to mature the same volume of spirit than if the system were to fill each cask to its maximum. For example a volumetric system filling hogshead with a set point which will fill the largest cask full (270 litres) will overfill the smallest hogshead (240 litres) resulting in the loss of 30 litres of spirit. Alternatively if such a system was set so as to never overfill the smallest hogshead it would result in the largest cask being under filled by 30 litres. This under filling would not affect the maturation of the whisky. The issue with under filling is the higher cost of each litre of whisky matured. Each cask, warehouse space and cask transportation cost is fixed regardless of whether a cask is 75% full or 100% full. A system which under fills casks will require a greater number of casks in order to fill a batch of spirit than a system which fills each cask to its maximum filling the same size batch. There is not only a cost associated with the purchase of the additional casks but also their placement in warehouses, the warehouse space they occupy, and the cost of removing and disgorging a cask after maturation. Disgorging is the process of emptying casks after maturation.