NGL / LPG Product Specification Fundamentals

1. LPG and NGL Products

Natural gas liquids (NGLs) are the heavier hydrocarbons — ethane, propane, butanes, and natural gasoline — recovered from raw natural gas in a gas plant. After fractionation, the individual products are sold into fuel and petrochemical markets. Liquefied petroleum gas (LPG) is the subset of these products, principally propane and butane, that is liquefied under moderate pressure for transport and use as fuel, petrochemical feedstock, and motor-gasoline blendstock.

Unlike pipeline natural gas, which is sold on an energy basis against a tariff gas-quality spec, finished LPG products are sold as defined commodities. The buyer expects a barrel labeled "propane" or "HD-5" to behave consistently in their vaporizer, burner, or engine. The GPA 2140 standard provides that contractual definition: it states, for each product grade, the limiting values of the properties that matter to safe handling and combustion, and names the ASTM test method used to measure each one.

2. The Four GPA 2140 Product Grades

GPA 2140 defines four commercial product designations. The composition column is the general descriptive guide; the numeric limits in Section 3 are what a product is actually tested against.

3. GPA 2140 Specification Table

The following limits are taken from the GPA Standard 2140-97 product specifications table. A dash indicates the property is not a specified requirement for that grade.

4. Vapor Pressure and Volatility

Vapor pressure and volatility are the properties that most directly affect safe handling. Gauge vapor pressure at 100°F (ASTM D-1267) sets the design pressure for storage vessels and rail/truck transport. It is also the primary discriminator between propane and butane: propane and HD-5 may reach 208 psig, but butane — far less volatile — is capped at 70 psig.

The volatile residue requirement controls heavy ends that would not vaporize cleanly. GPA 2140 expresses it two ways: as the temperature at 95% evaporation (ASTM D-1837), or, alternatively, as a limit on the heavier-hydrocarbon fraction by gas chromatography (ASTM D-2163). For propane grades the 95% point must be at or below −37°F (a cold endpoint, proving the product is light); for butane and B-P mixtures it must be at or below 36°F.

5. Composition and the HD-5 Requirement

Commercial Propane, Commercial Butane, and B-P Mixtures are defined descriptively ("predominantly propane", "predominantly butanes", etc.) rather than by a fixed composition. Propane HD-5 is the exception and the most tightly specified grade, because it is intended as engine and high-severity combustion fuel where consistent combustion is critical.

The "HD" designation historically stands for "heavy-duty" (engine) service. A product that is otherwise excellent propane but carries 8% propylene is perfectly acceptable Commercial Propane yet fails HD-5 — a distinction the Auto grade-finder mode in the calculator makes explicit.

6. Sulfur, Copper-Strip Corrosion, and Residue

Total sulfur (ASTM D-2784)

Sulfur compounds corrode equipment and poison downstream catalysts. The total-sulfur limit tightens with product grade: 185 ppmw for Commercial Propane, 140 ppmw for Commercial Butane and B-P Mixtures, and 123 ppmw for Propane HD-5. As noted above, this limit is measured net of odorant sulfur.

Copper-strip corrosion (ASTM D-1838)

The copper-strip test detects corrosive sulfur species (such as hydrogen sulfide and elemental sulfur) that the total-sulfur number alone may not flag. A polished copper strip is immersed in the product and the resulting tarnish is rated against a standard set from No. 1 (slight tarnish) to No. 4 (corrosion). All four GPA 2140 grades require a rating no worse than No. 1.

Residue on evaporation and oil stain (ASTM D-2158)

For propane grades, after evaporating a 100 mL sample no more than 0.05 mL of residue may remain, and an oil-stain observation must pass (no persistent oil ring from a solvent-residue mixture on filter paper). These limits keep heavy ends and oil contamination out of vaporizers and regulators.

7. Moisture and Free Water

Water in LPG causes hydrate formation and freeze-off in regulators and valves, especially for propane in cold service. GPA 2140 controls water two ways depending on the grade:

• Propane grades (Commercial Propane, HD-5): must pass a dryness test — the GPA cobalt-bromide test or ASTM D-2713 (valve freeze method). Note that if methanol is present, D-2713 is not applicable for propane dryness.
• Butane and B-P Mixtures: must contain no free water by observation.

8. Worked Example — Checking a Propane HD-5 Sample

A fractionator releases a propane stream intended for sale as Propane HD-5. The laboratory reports the following on the product (sulfur net of odorant):

Verdict: ON-SPEC as Propane HD-5. Every applicable parameter meets its GPA 2140 limit, so the product may be released and sold as HD-5. Note that the two volatile-residue lines (95% evaporation temperature and butane & heavier) are alternatives under the standard's "or" — satisfying either one meets the volatile-residue requirement; here both do.

What if the sulfur were 130 ppmw?

If the same product instead measured 130 ppmw total sulfur, it would fail HD-5 (limit 123 ppmw) but still pass as Commercial Propane (limit 185 ppmw), assuming all other propane-grade limits are met. The Auto grade-finder mode reports exactly this: the sample no longer qualifies as HD-5 but does qualify as Commercial Propane — useful for deciding how to re-market an off-spec HD-5 batch without reprocessing.

9. Test Methods and Industry Standards