Determination of Residues in Liquefied Petroleum Gas (LPG): Test Methods, Standards, and Quality Con

LPG residue determination is a critical quality-control test that quantifies the non-volatile materials — heavy hydrocarbons, lubricating oils, and oily contaminants — left behind when liquefied petroleum gas evaporates. Performed in accordance with recognized methods such as ASTM D2158, ASTM D1835, EN 589, and GB 11174, the test protects vaporizers, regulators, burners, and automotive fuel systems from deposits and blockages. This article explains the principle, sampling, procedures, acceptance limits, and applications of residue testing for commercial and automotive LPG.

Technical Overview

Residue in liquefied petroleum gas refers to any material that does not evaporate with the main propane and butane fractions when the liquid is allowed to vaporize under controlled conditions. In practice, the residue consists of pentane and heavier hydrocarbons (C5+), entrained compressor and pump lubricating oils, pipeline and terminal contaminants, overdosed corrosion inhibitors, and heavier sulfur-bearing or oligomeric compounds formed during storage and handling.

Excessive residue is one of the most common causes of field problems with LPG. As the gas vaporizes inside cylinders, tanks, regulators, and vaporizers, the non-volatile fraction is left behind, where it accumulates as an oily film or waxy deposit. The consequences include sluggish regulator performance, blocked burner jets and carburetor passages, fouled vaporizer heat-exchange surfaces, elevated maintenance costs, and customer complaints. For this reason, product specifications such as ASTM D1835, EN 589, GB 11174, and GB 19159 impose strict limits on both the quantity of evaporation residue and the presence of visible oil.

Residue determination therefore has two complementary components:

• Evaporation residue — the volume (mL residue per 100 mL of sample) or mass (mg/kg) of non-volatile material remaining after the volatile C3/C4 fraction has boiled off.
• Oily residue observation — a qualitative pass/fail assessment of whether a persistent oil film or oil ring remains on the vessel wall or on filter paper after evaporation.

Typical acceptance criteria are on the order of 0.05 mL of residue per 100 mL of sample (approximately 40 mg/kg) for commercial grade LPG, while EN 589 for automotive LPG specifies a gravimetric evaporation residue limit of 50 mg/kg together with a pass requirement for the oily residue observation.

Testing Samples

Commercial Propane, Butane, and Propane-Butane Mixtures

The most frequently tested materials are commercial-grade propane, commercial-grade butane, and mixed propane-butane LPG supplied in cylinders, road tankers, rail cars, and bulk storage terminals. These products are evaluated against national and international specifications, for example ASTM D1835 and GB 11174, in which the residue limit is a mandatory quality parameter alongside vapor pressure, volatility, sulfur, and copper-strip corrosion.

Automotive LPG (Autogas)

Motor-fuel LPG intended for spark-ignition and dedicated LPG vehicles is tested against EN 589 and GB 19159. Because automotive vaporizers, injectors, and mixer units contain fine passages that are highly sensitive to oily deposits, autogas specifications are especially stringent with respect to both evaporation residue and the oily residue spot observation.

Refinery, Terminal, and Blending Streams

Depropanizer and debutanizer overhead products, C3/C4 blending components, and materials transferred through multiproduct pipelines are commonly screened for residue to detect carry-over of heavy ends or lubricating-oil contamination before the product enters the distribution chain.

Sample Containers and Preservation

LPG must be sampled and transported in the liquid phase in pressure-rated stainless steel cylinders, typically in the 300 mL to 1 L size range. Accepted practice is to purge the cylinder with dry nitrogen, rinse it several times with the product itself, and fill it to 70–80 percent liquid capacity so that representative liquid can always be withdrawn through a dip tube. Samples should be tested promptly, because oily residue can be adsorbed on cylinder walls during prolonged storage.

Testing Items

A complete LPG residue determination program typically covers the following parameters, which may be ordered individually or as part of a full product conformity assessment:

• Residue on evaporation, reported in mL per 100 mL of sample (volumetric method) or in mg/kg (gravimetric method).
• Oily residue observation (oil stain / oil film test), reported as pass or fail.
• Related quality parameters frequently tested together with residue: vapor pressure, 95 percent evaporation temperature (volatility), copper strip corrosion, total sulfur content, hydrogen sulfide, free water and moisture, and composition by gas chromatography.

Testing Methods

Evaporation Residue Test (ASTM D2158 Type Procedure)

In the classical volumetric procedure, a clean, dry, pre-chilled 100 mL graduated residue tube or burette is connected to the sampling cylinder through a dip-tube connection and carefully charged with 100 mL of liquid-phase sample. The tube is then disconnected and placed in a fume hood, where the volatile propane and butane fractions are allowed to boil off slowly at ambient temperature, typically 15 to 25 °C. Evaporation slows markedly once the meniscus falls into the lower region of the tube; when vaporization has essentially ceased and the residual volume is stable, the remaining liquid is read to the nearest graduation and reported as milliliters of residue per 100 mL of sample. Because the evaporation of C5+ hydrocarbons is temperature dependent, the tube is commonly finished at a defined temperature condition to ensure reproducibility between operators and laboratories.

Oily Residue Observation

After the volume of residue has been recorded, the qualitative oil test is performed. The wall of the tube above the residue level is examined for a continuous, persistent oil film, and in the spot-test variant a measured portion of the residue is transferred onto clean white filter or blotting paper. After the volatile solvent fraction evaporates, any translucent ring or grease spot that persists on the paper indicates the presence of non-volatile oily material. The result is reported simply as pass (no persistent oil ring) or fail (visible oil ring), which is the form required by specifications such as ASTM D1835, EN 589, and GB 11174.

High-Temperature / Gravimetric Method

Where results must be expressed on a mass basis, as required by EN 589 for automotive LPG, a gravimetric procedure based on the principle of ISO 13759 may be applied. A known quantity of LPG is evaporated under heated conditions from a pre-cleaned, pre-weighed vessel, and the non-volatile residue that remains is determined by weighing. The residue is calculated and reported in mg/kg, providing a quantitative figure that can be compared directly with mass-based specification limits and used for trend monitoring across production batches.

Sampling and Safety Considerations

Correct sampling is decisive for a valid residue result. The sampling line and cylinder must be purged and flushed with liquid sample so that heavy material adsorbed on metal surfaces does not artificially inflate or depress the residue value. All transfer and evaporation steps are conducted in well-ventilated fume hoods away from ignition sources, with cylinders and tubing bonded and grounded to prevent static discharge. Personnel wear appropriate PPE, and vented gas is routed to a safe location, since evaporating LPG produces large volumes of flammable vapor and can cause severe cold burns on skin contact.

Applications

Residue determination is applied at every stage of the LPG value chain, including:

• Refinery quality control and product certification for propane, butane, and blended LPG prior to release to storage or shipment.
• Import and export inspection, cargo surveying, and commercial settlement, where testing by accredited third-party laboratories provides impartial evidence of specification compliance.
• Cylinder filling plants and bulk distribution terminals, for routine batch monitoring and for verifying that tanks, hoses, and pumps are not contaminating the product with lubricating oil.
• Automotive LPG programs, where low residue is essential to protect vaporizers, solenoid valves, and injectors in autogas vehicles.
• Regulatory compliance demonstrations against national and international specifications such as ASTM D1835, EN 589, GB 11174, and GB 19159.
• Failure investigation and troubleshooting, for example when customers report blocked burners, sticking regulators, or oily deposits, and the residue test is used to trace the contamination back to a specific batch, tank, or transfer operation.

FAQ

What exactly is counted as residue in LPG?

Residue is the material that does not vaporize with the propane and butane when the sample evaporates under the test conditions. It mainly comprises C5 and heavier hydrocarbons, entrained compressor or pump lubricating oil, rust and pipeline debris, and excess corrosion inhibitor or other treatment chemicals.

What are the typical acceptance limits?

Most commercial specifications limit evaporation residue to about 0.05 mL per 100 mL of sample (roughly 40 mg/kg), and EN 589 specifies 50 mg/kg for automotive LPG. In every case, the oily residue observation must also be reported as pass, meaning no persistent oil film or oil ring is visible after evaporation.

Why is oily residue judged separately from the residue volume?

A sample can show a low residue volume yet still fail the oil observation because even a thin film of lubricating oil, invisible in the volume reading, can coat regulator seats and vaporizer surfaces and cause malfunction. The spot or film observation is a sensitive qualitative check that catches oily contamination the volumetric method alone may underestimate.

What are the common causes of excessive residue?

Frequent root causes include carry-over of heavy C5+ material during fractionation or blending, lubricating-oil leakage from pumps and compressors, inadequate drainage of storage tanks, cross-contamination in multiproduct pipelines, and overdosing of additives. Storage of low-volatility material in unheated tanks during cold weather can also concentrate heavies in the liquid delivered to customers.

How should samples be taken and how quickly should they be tested?

Samples must be drawn in the liquid phase into pressure-rated stainless steel cylinders filled to 70–80 percent liquid capacity, with thorough purging and rinsing beforehand. Testing should be carried out as soon as practicable after sampling, since oily residue can adhere to cylinder walls over time and bias the result.

What should I look for when selecting a laboratory for residue testing?

Choose an accredited third-party laboratory that performs residue testing under recognized methods such as ASTM D2158 and the EN 589 gravimetric procedure, uses properly maintained pressure-sampling equipment, and reports results together with the corresponding specification limits. Experience in LPG testing and the ability to run the full parameter suite, including vapor pressure, volatility, sulfur, and corrosion, ensure a complete conformity assessment from a single sample.