Figure 4.14 Sulzer oil-free compressor - Seaworm

Liquified Gas Handling Principles
Definitions The definitions listed in this section relate to their usage within this
book. Other publications may use similar terms with different
interpretations. Absolute Pressure
The absolute pressure is the total of the gauge pressure plus the pressure
of the surrounding atmosphere . Absolute Temperature
The fundamental temperature scale with its zero at absolute zero and
expressed in degrees Kelvin. One degree Kelvin is equal to one degree
Celsius or one degree Centigrade. For the purpose of practical calculations
in order to convert Celsius to Kelvin add 273. It is normal for the degree
Kelvin to be abbreviated in mathematical formulae to 'K' with the degree
symbol being omitted. Absolute Zero
The temperature at which the volume of a gas theoretically becomes zero and
all thermal motion ceases. It is generally accepted as being -273.16°C. Activated Alumina
A desiccant (or drying) medium, which operates by adsorption of water
molecules (see 4.7.1). Adiabatic
Describes an ideal process undergone by a gas in which no gain or loss of
heat occurs (see 2.1). Aerating
Aerating means the introduction of fresh air into a tank with the object of
removing toxic, flammable and inert gases and increasing the oxygen content
to 21 per cent by volume (see 7.9.4). Airlock
A separation area used to maintain adjacent areas at a pressure
differential. For example, the airlock to an electric motor room on a gas
carrier is used to maintain pressure segregation between a gas-dangerous
zone on the open deck and the gas-safe motor room which is pressurised. Approved Equipment
Equipment of a design that has been type-tested and approved by an
appropriate authority such as a governmental agency or classification
society. Such an authority will have certified the particular equipment as
safe for use in a specified hazardous atmosphere. Auto-ignition Temperature
The lowest temperature to which a liquid or gas requires to be raised to
cause self-sustained spontaneous combustion without ignition by a spark or
flame (see 2.20). Avogadro's Law
Avogadro's Hypothesis states that equal volumes of all gases contain equal
numbers of molecules under the same conditions of temperature and pressure. BLEVE
This is the abbreviation for a Boiling Liquid Expanding Vapour Explosion.
It is associated with the rupture, under fire conditions, of a pressure
vessel containing liquefied gas (see 2.20). Boil-off
Boil-off is the vapour produced above the surface of a boiling cargo due to
evaporation. It is caused by heat ingress or a drop in pressure (see 4.5). Boiling Point
The temperature at which the vapour pressure of a liquid is equal to the
pressure on its surface (the boiling point varies with pressure) (see 1.1). Booster Pump
A pump used to increase the discharge pressure from another pump (such as a
cargo pump) (see 4.2). Bulk Cargo
Cargo carried as a liquid in cargo tanks and not shipped in drums,
containers or packages. Canister Filter Respirator
A respirator consisting of mask and replaceable canister filter through
which air mixed with toxic vapour is drawn by the breathing of the wearer
and in which the toxic elements are absorbed by activated charcoal or other
material. A filter dedicated to the specific toxic gas must be used.
Sometimes this equipment may be referred to as cartridge respirator. It
should be noted that a canister filter respirator is not suitable for use
in an oxygen deficient atmosphere (see 9.9.1). Carbamates
A white powdery substance produced by the reaction of ammonia with carbon
dioxide (see 7.2). Carcinogen
A substance capable of causing cancer. Cargo Area
That part of the ship which contains the cargo containment system, cargo
pumps and compressor rooms, and includes the deck area above the cargo
containment system. Where fitted, cofferdams, ballast tanks and void spaces
at the after end of the aftermost hold space or the forward end of the
forwardmost hold space are excluded from the cargo area. (Refer to the Gas
Codes for a more detailed definition). Cargo Containment Systems
The arrangement for containment of cargo including, where fitted, primary
and secondary barriers, associated insulations, interbarrier spaces and the
structure required for the support of these elements. (Refer to the Gas
Codes for a more detailed definition) (see 3.2). Cascade Reliquefaction Cycle
A process in which vapour boil-off from cargo tanks is condensed in a cargo
condenser in which the coolant is a refrigerant gas such as R22 or
equivalent. The refrigerant gas is then compressed and passed through a
conventional sea water-cooled condenser (see 4.5.2). Cavitation
A process occurring within the impeller of a centrifugal pump when pressure
at the inlet to the impeller falls below that of the vapour pressure of the
liquid being pumped. The bubbles of vapour which are formed collapse with
impulsive force in the higher pressure regions of the impeller. This effect
can cause significant damage to the impeller surfaces and, furthermore,
pumps may loose suction (see 4.2). Certificate of Fitness
A certificate issued by a flag administration confirming that the
structure, equipment, fittings, arrangements and materials used in the
construction of a gas carrier are in compliance with the relevant Gas Code.
Such certification may be issued on behalf of the administration by an
approved classification society (see 3.7.1). Certified Gas Free
A tank or compartment is certified to be gas-free when Its atmosphere has
been tested with an approved Instrument and found in a suitable condition
by an Independent chemist. This means it is not deficient in oxygen and
sufficiently free of toxic or flammable gas for a specified purpose. Cofferdam
The isolating space on a ship between two adjacent steel bulkheads or
decks. This space may be a void space or ballast space. Condensate Reliquefied gases which collect in the condenser and which are then
returned to the cargo tanks. Compression Ratio The ratio of the absolute pressure at the discharge from a compressor
divided by the absolute pressure at the suction. Critical Pressure
The pressure at which a substance exists in the liquid state at its
critical temperature. (In other words it is the saturation pressure at the
critical temperature) (see 2.12). Critical Temperature
The temperature above which a gas cannot be liquefied by pressure alone
(see 2.12). Cryogenics
The study of the behavior of matter at very low temperatures.
Dalton's Law of Partial Pressures
This states that the pressure exerted by a mixture of gases is equal to the
sum of the separate pressures which each gas would exert if it alone
occupied the whole volume (see 2.17). Dangerous Cargo Endorsement
Endorsement issued by a flag state administration to a certificate of
competency of a ship's officer allowing service on dangerous cargo carriers
such as oil tankers, chemical carriers, or gas carriers. Deepwell Pump
A type of centrifugal cargo pump commonly found on gas carriers. The prime
mover is usually an electric or hydraulic motor. The motor is usually
mounted on top of the cargo tank and drives, via a long transmission shaft,
through a double seal arrangement, the pump assembly located in the bottom
of the tank. The cargo discharge pipeline surrounds the drive shaft and the
shaft bearings are cooled and lubricated by the liquid being pumped (see
4.2). Density
The mass per unit volume of a substance at specified conditions of
temperature and pressure (see 2.16). Dew point
The temperature at which condensation will take place within a gas if
further cooling occurs (see 2.18). Endothermic
A process, which is accompanied by the absorption of heat. Enthalpy
Enthalpy is a thermodynamic measure of the total heat content of a liquid
or vapour at a given temperature and is expressed in energy per unit mass
(k Joules per 1 kg) from absolute zero. Therefore, for a liquid/vapour
mixture, it will be seen that it is the sum of the enthalpy of the liquid
plus the latent heat of vaporisation (see 2.19.1). Entropy
Entropy of a liquid/gas system remains constant if no heat enters or leaves
while it alters its volume or does work but increases or decreases should a
small amount of heat enter or leave. Its value is determined by dividing
the intrinsic energy of the material by its absolute temperature. The
intrinsic energy is the product of specific heat at constant volume
multiplied by a change in temperature. Entropy is expressed in heat content
per mass per unit of temperature. In the SI system its units are therefore
Joule/kg/K.
It should be noted that in a reversible process in which there is no heat
rejection or absorption, the change of entropy is zero. Entropy is the measure of a system's thermal energy which is not available
for conversion into mechanical work.
Many calculations using enthalpy or entropy require only a knowledge of the
difference in enthalpy or entropy at normal operating temperatures.
Accordingly, to simplify calculations, many different enthalpy or entropy
tables have been produced which have different baselines. Care should be
taken when using such tables as they do not provide absolute values (see
2.19.2). Explosion-Proof/Flameproof Enclosure
An enclosure which will withstand an internal ignition of a flammable gas
and which will prevent the transmission of any flame able to ignite a
flammable gas which may be present in the surrounding atmosphere (see 4.8).