Ullage
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Ullage or headspace is the unfilled space in a container, particularly with a liquid.


Etymology

The word ''ullage'' comes from the
Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
word ''oculus'', used by the Romans to refer to a barrel cork hole. This word was, in turn, taken in medieval French as ''oeil'', from which a verb ''ouiller'' was created, meaning to fill a barrel to full capacity. Around 1300, the word ''ouillage'' was created by the
Normans The Normans (Norman language, Norman: ''Normaunds''; ; ) were a population arising in the medieval Duchy of Normandy from the intermingling between Norsemen, Norse Viking settlers and locals of West Francia. The Norse settlements in West Franc ...
to refer to the amount of liquid needed to fill a barrel to capacity.


Alcoholic beverages

In winemaking, ''ullage'' came to refer to any amount by which a barrel is unfilled, perhaps because some of the contents have been used. It is also applied to the unfilled air space at the top of a bottle of wine, which in this case is essential to allow for expansion of the contents as the temperature changes. By further extension, in brewing and beer retail, ''ullage'' is the residue of beer left in a barrel that has been emptied. When calculating tax returns and the like, licensed premises owners, landlords or managers can factor in the duty on ''ullage'', or unavoidable barrel wastage. ''Ullage'' therefore has come to be used as a general term, in the licensed trade, for waste beer whether at the barrel or at the bar tap or pump. However, what customers leave in their glasses does not count as ullage, because it has been sold.


Rocketry

Liquid propellant rockets and
spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...
store their propellants in tanks.
Cryogenic In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th International Institute of Refrigeration's (IIR) International Congress of Refrigeration (held in Washington, DC in 1971) endorsed a univers ...
tanks are never completely filled, to prevent severe pressure drop in the tank after engine start. On the ground, or in the continued gravitational field of Earth during rocket-propelled ascent, the space between the top of the propellant load and the top of the tank is known as "ullage space". Ullage pressure is a critical measurable during powered rocket flight, because it affects tank structural integrity and engine net positive suction head (NPSH). In the weightless condition in space without engine thrust, empty space occurs in partially-filled tanks, and the ullage space becomes distributed across much of the tank in a heterogenous mixture of masses of
liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...
amidst many gaseous regions. Under these conditions, liquid floats away from the engine intake, which is undesirable for stable engine operation. The application of a small force, from a small rocket engine for example, is sometimes used to provide sufficient acceleration to reaggregate (settle) the liquid propellant at the bottom of the tank near the engine propellant inlet prior to ignition of the main engine(s). Engines devoted to this single purpose are typically called ullage motors.
Reaction control system A reaction control system (RCS) is a spacecraft system that uses Thrusters (spacecraft), thrusters to provide Spacecraft attitude control, attitude control and translation (physics), translation. Alternatively, reaction wheels can be used for at ...
thrusters are also often used to settle propellants prior to reignition of a liquid propellant engine in space.


Industrial use

''Ullage'' is also widely used in industrial or marine settings to describe the empty space in large tanks or holds used to store or carry liquids or bulk solids such as
grain A grain is a small, hard, dry fruit (caryopsis) – with or without an attached husk, hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and ...
. In accordance with IMO regulations, the
Code of Federal Regulations In the law of the United States, the ''Code of Federal Regulations'' (''CFR'') is the codification of the general and permanent regulatory law, regulations promulgated by the executive departments and agencies of the federal government of the ...
, and the ABS Rules for Steel Vessels, certain pressurized tanks on steel ships may not be filled greater than 98% full, although there are exceptions. This is so that the pressure relief valve is always in contact with a gas or
vapor In physics, a vapor (American English) or vapour (Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R ...
. Certain pressure relief valves for chemical use are not designed to relieve the pressure when in contact with liquids. In some cases, the ullage in a ship's hold can be relevant to stability; liquid or dry
bulk cargo Bulk cargo is Product (business), product cargo that is transported packaging, unpackaged in large quantities. Description Bulk cargo refers to material in either liquid or granular, particulate (as a mass of relatively small solids) form, ...
in a part-filled hold can shift asymmetrically towards one side as the ship heels to one side and the other, reducing the margin of stability when compared with a full hold. Excessive ullage in a tank may contribute to the free surface effect. When referring to the free surface effect, the condition of a tank that is not full is described as a "slack tank", while a full tank is "pressed up".Ship inspection maritime guide.
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References

{{Reflist, 1


Bibliography

* ABS Rules for Steel Vessels 2007, Part 5C


External links



Rocket propulsion Packaging