BS 7577:1992 download free.Specification for Calculation procedures for static measurement of refrigerated light hydrocarbon fluids.
Storage and transport of large quantities of refrigerated hydrocarbon liquids [e.g. liquefied natural gases (LNG) and liquefied petroleum gases (LPG)1 is now common practice. Existing standards for the measurement of petroleum products are either not applicable to, or in some cases inadequate for, these products at low temperatures and, for these products, such standards shall be replaced or modified by the procedures in BS 7577.
Accurate measurement is essential in the sale, purchase and handling of refrigerated hydrocarbon liquids. Custody transfer agreements call for the standardization of static measurement procedures, and it is recommended that quantities be expressed in mass or energy units. It is recognized that other units are currently used for LPG transfers, but these are not covered in BS 7577.
Although the principles of calculating the quantity of a static refrigerated hydrocarbon liquid are basically similar to those for petroleum liquids at ambient temperatures, there are differences caused by the low temperature and the physical properties of refrigerated hydrocarbons. These include the following:
a) The liquid product is at or near a temperature at which bubbles of vapour are first formed within the liquid (bubble point). In a tank containing refrigerated liquid there will always he a small inward flow of heat through the insulation, which will cause a continuous vaporization of the product. The vapour will contain a higher concentration of more volatile constituents than the liquid. To avoid over-pressure, this vapour is vented from the tank and can be compressed, cooled and re-liquefied for re-introduction into the tank.
b) When a liquid product is transferred from one tank to another, additional heat inflow will occur in the pipeline and also from work done by the pump, causing additional evaporation in t.he receiving tank.
c) For custody transfers from a supply to a receiving tank, it is normal practice to provide a vapour return line linking the tanks to avoid displacement of vapour to the atmosphere. Build-up of pressure in the interlinked system is avoided by re-liquefaction.
d) After a partial filling, stratification into different temperature and density layers may occur in the liquid contents of a tank. Therefore, a number of temperature measuring points and a special sampling system may be necessary. If the filling operation is such as to ensure mixing, these needs may he reduced.
In order to implement the detailed recommendations given in this International Standard, it is essential that personnel responsible for the measurement procedures have the necessary experience and skill. At all times, scrupulous attention must he given to detail.
NOTE Use of units:
a) Temperature — Celsius temperature is used in connection with the measurement and transport of refrigerated gases and has been used in general in BS 7577: however, in some calculations the thermodynamic. i.e. kelvin. temperature scale must be used. For accurate conversion. 273,15 K 0°C should be used, but in the examples given here 273 K 0°C is sufficiently accurate.
b) Pressure — The pascal (Pa) is used as the unit of pressure in this standard, but the bar is given as an alternative unit. The bar may be substituted in calculations; the conversion 1 bar = 100 kPu should be used.
1 Scope
1.1 BS 7577 specifies the calculations to be made to adjust the volume of a refrigerated hydrocarbon liquid, such as LPG or LNG, from the conditions at measurement to the equivalent volume of liquid or vapour at a standard temperature and pressure, or to the equivalent mass or energy (calorific content). It applies to quantities of refrigerated hydrocarbon liquids stored in or transferred to or from tanks and measured under static storage conditions by tank gauges.
1.2 Using these procedures, the final quantity shall be expressed in terms of the following:
a) mass (see the note);
b) energy (calorific content):
c) equivalent volume of vapour under standard conditions.
NOTE The current practice for measurement of LPG is by apparent mass in air.
The factors in Table I may be used to convert mass into apparent mass in air.
ISO 3993:1984. Liquefied petroleum gas and light hydrocarbons — Determination of density or relative density — Pressure hydrometer method.
ISO 5024:1976. Petroleum liquids and gases Measurement — Standard reference conditions.
3 Definitions and symbols
3.1 Definitions
For the purposes of BS 7577, the following definitions shall apply. Definitions are given for those terms which have particular relevance in calculation procedures used for refrigerated hydrocarbon liquids.
3.1.1
refrigerated hydrocarbon liquids
liquids composed predominantly of hydrocarbons. which are stored in a fully refrigerated condition at pressures near atmospheric
3.1.2
liquefied natural gases (LNG)
liquids composed predominantly of methane
3.1.3
liquefied petroleum gases (LPG)
liquids composed predominantly of any of the following hydrocarbons or mixtures thereof: propane, propene. butanes and butene
3.1.4
gross calorific value (specific energy) on mass basis
the number of heat units generated when unit mass of a product in the vapour phase at standard temperature and pressure is burned completely in dry air. The gaseous products of combustion are brought to the same standard conditions of temperature and pressure but the water produced is condensed to liquid in equilibrium with water vapour
4 Volume of LPG at standard temperature
The procedure for converting the volume of refrigerated LPG to jtg equivalent volume at a standard temperature and corresponding equilibrium pressure includes the following aspects:
a) Very large factors may have to be applied for the correction of observed density to density at standard temperature, e.g. a correction for the effect of a temperature difference of 60 °C may be necessary for refrigerated propane. Provided that the LPG does not contain more than 20 % of unsaturated hydrocarbons the correction tables referred to in Iso 91 shall be used for volume corrections. However, the tables for this density range are those retained from the 1952 edition of the API-ASTM-1P Petroleum Measurement Tables (see sub-clause 3.4 of ISO 91.1:1982). If the LPG contains 20% or more of unsaturated hydrocarbons, the density shall he calculated using the method given in clause 8.
b) The equivalent liquid content in the vapour space of a container holding refrigerated LPG is significantly less than if the tank and contents are at ambient temperature. Therefore, any error in accounting for the equivalent liquid content in the vapour space will be of lesser significance.
NOTE 1 The following examples illustrate the magnitude of errors that can be introduced by using the tables referred to in ISO 91,
a) Pure butene or propene: the maximum error will be approximately 2 % for a correction from — 60 °C to + 20 °C;
b) Mixtures containing approximately 20 % of unsaturated hydrocarbons: a typical error will be approximately 0,1 % for a temperature difference of 20 °C.
NOTE 2 A condition in which a liquid has a vapour pressure significantly higher than atmospheric pressure at a standard temperature of 15 °C (or 20 °C or 60 °F) can only be considered as a pseudo-condition. and the volume of the liquid in this condition may be used only when convenient in a procedure for obtaining the density at refrigerated temperatures by means of pressure hYdrometer measurement at ambient conditions (see ISO 3993).
5 Mass
5.1 Mass of liquid phase
5.1.1 Calculate the mass of liquid (rniiq), in kilograms, using the equation.