Measurement: vapour recovery: How the constituent factors of the formula are obtained
How the constituent factors of the formula are obtained
P - The VRU performance factor
The percentage of efficiency is the difference between the vapour returned to the VRU less the vapour emitted by the VRU; the formulae for calculating these factors are given in the EI guidelines (Section 4.4). The performance factor has to meet exacting standards. UK legislation requires a tri-annual compliance test which is monitored by the appropriate environmental agency. The EI trade guidance requires the copy of the testing agency’s report to be made available to Customs.
S- The VRU Service availability factor
The VRU may not be always available due to breakdown or servicing. For the effect on the operation of the formula see the EI Guidance Section 3.3.
H - Average of hydrocarbons in collected vapour
To establish the percentage of hydrocarbons in the recovered vapour the warehousekeeper will need to analyse samples drawn quarterly from the vapour line during the loading of RTW’s.
The percentage of oxygen is determined by an oxygen meter with a paramagnetic cell. As air normally contains 20.9% of oxygen, readings of less than that are caused by the presence of total organic compounds (TOCs).
Readings are taken manually every 30 seconds during the loading of the RTW and an average reading for the vapour recorded.
If the final average for the RTW is 15.9% of oxygen, a value for the percentage of hydrocarbons recovered can be determined, using the following:
20.9 - 15.9 = 5 x 100/20.9 = 23.9% hydrocarbons.
Testing RTWs over 2 x 8-hour periods, i.e. an AM and PM, should give a mean value for the percentage of vapour returned.
The test is repeated for the distillate (DERV) loading racks if the installation’s practise is to use the same RTW’s for both motor spirit and distillate. Although distillates give off little in the way of Volatile Organic Compounds (VOCs) an RTW may load DERV into a compartment previously carrying motor spirit.
A rough estimate of the amount of motor spirit expected to be recovered is one litre per 1,000 litres loaded, and for a 1,000 litres of distillates the recovery of liquid is about a quarter to a third of a litre.
A millimetre on a small tank will be equal to 30 to 40 RTWs of motor spirit vapour. Additionally the loading tank may also be the tank used for deliveries and receipts from the absorber column. Considering that the automatic tank gauge (ATG) or the automatic level gauge (ALG) accuracy requirement is 8 mm this is a fair number of RTWs. Taking all the above into consideration, bulk storage tank measurements are not accurate. The EI guidance gives detailed advice to the operator in Section 4.
M and D - Mean molecular weight and density of recovered liquids
The EI guidance (see Section 4.3.5 and Annex 3) also gives methods of drawing and testing samples of vapour for molecular weight and density which are performed in a laboratory by gas chromatography. The test will also establish the components that make up the sample (typically the various butanes, propanes and pentanes make up 70% or more of the vapour).
VT - Total volume of product loaded through bays connected to the VRU
This will include distillate where the installation’s practice is to use the same compartments.
Ful/Fl/Fsul - Fraction of the volume of motor spirit at the different grades
Only the motor spirit grades are included with the purpose of apportioning the different rates of duty charged.
Dul/Dl/Dsul - Duty rates on motor spirits
Duty rates for motor spirit grades only.