The performance data presented in this paper is taken from the Missouri Department of natural Resources publication "performance Evaluation Test Procedures for Vapor Recovery Systems at Gasoline Dispensing Facilities" dated November 1, 1995.

Performance Standards and Requirements

MO/PETP-201, Table 4, Stage I Vapor Recovery Systems, page 44, maximum allowable leak rates at 2" H2O pressure:

.380 CFH for a Drop Tube
.038 CFH for a Vapor Adapter
.380 CFH for a P/V Vent

MO/PETP-201, Table 5, Stage II Balance Systems, page 45, maximum allowable leak rate at 2" H2O pressure:

.063 CFH for a Balance Nozzle

Dispenser Leak Testing

Leak testing of a balance system dispenser with 5 hoses and nozzles can be performed by monitoring pressure decay time or leak rate. The vapor piping of the dispenser under test must be disconnected from the main system. A pressurized test gas can then be piped into the dispenser’s vapor system. This will provide the means to perform either of the leak tests.

The allowable leak rate of a 6 nozzle dispenser is: .378 CFH (6 x .063 CFH) or 10.9 in.3/min. at 2" H2O.

To perform a pressure decay test on a 6 hose dispenser, the volume of the vapor piping including hoses and nozzles must be calculated. This vapor volume is then used to calculate the final pressure after a period of time based on the allowable nozzle leak rat. For example, a total vapor volume of 500 in.3(2.16 gal.) would lose 1.22 in.3 as the pressure went from 2" H2O to 1" H2O. With the maximum allowable leak rate of 10.9 in.3 it can be calculated that it would take 6.7 seconds to leak down to 1" H2O. Therefore, if the pressure decay test of a 6 hose dispenser takes 6.7 seconds or longer, the nozzles are within MO/PETP specifications.
It becomes evident that trying to quantify a dispenser’s leak rate by pressure decay testing requires precise calculations and measurements.

Another method of determining a dispenser’s leak rate is to measure the leak rate with a flow meter. By connecting a flow meter in-line with the pressurized test gas source, the leak rate can be measured directly as the vapor system is pressurized at 2" H2O. This test method gives a direct reading of the dispensers leak rate plus allowing for diagnostics of the dispenser. If the measured leak rate was above the allowable, various methods of determining the leak sources could be utilized. Bagging a leaking nozzle would result in a change in flow which could be easily measured. The same type of flow measurements could be made when fittings were tightened or hoses replaced.

In review of the two leak testing methods, it appears that the use of a flow meter greatly enhances ones ability to measure and locate leaks in a dispenser.