No.1 Slight tarnish (light to dark orange)

No.2 Moderate tarnish (red, lavender, brassy gold)

No.3 Dark tarnish (magenta, red, green)

No.4 Corrosion (black, dark gray, brown)

Specifications for LP-gas products stipulate that the productmust not exceed a No.1 strip, indicating product that is notcorrosive to copper or brass fittings in the distribution system.

LP-gas from some sources may contain trace amounts of car-bonyl sulfide (COS), which alone is non-corrosive. However,COS can hydrolyze in the presence of water, either liquid orvapor, to corrosive hydrogen sulfide (H2S). The copper stripcorrosion test requires that the test cylinder be wetted withapproximately 1 ml of water prior to introduction of the sam-ple, which is intended to hydrolyze the COS into H2S and en-sure detection of corrosive product. If COS is suspected,additional product testing (See GPA Standard 2290) and treat-ment is recommended to prevent its introduction into the dis-tribution system.

Sulfur chemistry is extremely complex, and little scientificwork has been done to correlate copper strip test results, eitherquantitatively or by species, with the multitude of sulfur com-pounds that may be encountered in natural gas liquid prod-ucts. However, specific test data by Pyburn et al6 indicate thathydrogen sulfide concentrations of 0.55 ppmw or more in pro-pane almost certainly will produce a failure of the copper striptest; as little as 0.3 ppmw might fail, depending on interpre-tation of the test. These data also suggest that the thresholdlimit of elemental sulfur is approximately 5-10 ppmw, al-though concentrations as low as 2 ppmw may cause failure ofthe copper strip test.

Additional data suggest that elemental sulfur in the pres-ence of hydrogen sulfide results in a synergistic effect that willcause failure of the copper strip test with trace concentrations

Characteristics Method of Test Commercial PropaneISO-F-LPCommercial Butane

ISO-F-LBGauge vapor pressure at 104F, psig, max. ISO 42562)

orISO 8973

2251) 751)

VolatilityC2 hydrocarbons Report value3)

C4 hydrocarbons, mole %, max. ISO 7941 7,51)

C5 hydrocarbons, mole %, max. ISO 7941 0,26) 2,56)

Unsaturated hydrocarbons, mole % ISO 7941 Report value4) Report Value4)

Dienes, mole %, max. ISO 7941 0,56) 0,56)

Residual matter 5) 5) 5)

Corrosiveness to copper, max. ISO 6251 1 1Sulfur, mg/kg, max. 7) 506) 506)

Hydrogen sulfide ISO 8819 Pass PassFree water content None8) None8)

1) Certain national standards and/or regulations may prescribe other limits.2) In case of dispute about the vapor pressure, ISO 4256 shall prevail.3) Physical limitations of transport and storage facilities shall be considered for refrigerated transport; a common limit is 2 mole % maximum.4) Certain national standards may prescribe a limitation on unsaturated hydrocarbons.5) It has not been possible to take a decision on a value for inclusion in this specification because there is no suitable ISO test method and no satisfactory

correlation between the test results obtained using methods developed in different countries. Limits will be specified when a suitable test method hasbeen adopted. Until a suitable test method has been adopted, the user of this specification is referred to the introductory discussion and proposed testmethod discussed in ISO 9162-1989. Report the value for the sample concerned, and the method used to determine it, as additional information.

6) Certain national standards may prescribe other limits.7) An ISO method specific to liquefied petroleum gas is being developed. Until such an ISO method is available, ISO 4260 is recommended as the referee

method. Other methods specified in national standards, and having a valid precision statement, are acceptable for quality control purposes.8) The presence of water shall be determined by visual inspection. For shipments of refrigerated liquefied petroleum gases at their atmospheric boiling point,

it is essential that the water content is below the saturation level at this temperature as determined by a method agreed between buyer and seller.

FIG. 2-5Specifications for Liquefied Petroleum Gases

This table extracted from ISO 9162-1989

Temp., F H2O, ppmw % Saturated

100 21 880 18 1160 15 1640 11 2432 10 30

* These numbers are approximate

FIG. 2-6Maximum Water Content of Dry Commercial

Liquid Propane*

2-4