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26.12.2011
LFM-02-e
Rev Nr: 7
2.4 Hydraulic part of LPG measuring systems
The hydraulic part of an LPG measuring system is a closed system. Unless permitted by (local)
regulations, there is no venting to the atmosphere. Overpressure in the system should be relieved
back to the vapor space of the supply tank through a vapor return line. Relief valves should be
installed in situations where gas can be trapped.
The vapor return line should not be connected to other return lines or by-pass lines.
The pressure line is connected from the pump to the dispenser inlet. Typically a 20-micronic filter (or
better) should be placed before the vapor eliminator. The filter is a critical part in preserving the meter
accuracy. Depending on the gas quality the filter should be replaced regularly. Especially in the first
month of operation the filter should be checked (and replaced) at least twice a month.
1. Nozzle
2. Break-away coupling
3. Differential valve
4. Meter
5. Pressure relief valve
6. Check valve
7. Check valve
8. Gas eliminator
9. Ball valve
10. Filter
11. Ball valve
12. Pressure gauge
Figure 3: Typical hydraulic set for 1-nozzle, 1-pump systems
The vapor eliminator separates vapor and LPG as only LPG should enter the metering unit. Back
check valves at the outlets of the vapor eliminator prevent re-entering of LPG. From a weights and
measures point of view this is especially important for the connection to the meter.
Systems with two or more nozzles (two single nozzle dispensers or double nozzle dispensers)
operating on a single supply line should avoid unregistered deliveries. In other words, with one nozzle
in operation, the other nozzle should not be able to deliver product. This can be prevented with a
