It was supposed to be a routine job. An LPG storage tank had been taken out of service, emptied and depressurized for internal inspection. Then the readings came back. Low levels of hazardous material were present in the vapor and dust inside the tank.
Inspectors checked other tanks. They found accumulated residue there too.
The finding prompted a safety advisory from the World Liquid Gas Association (WLGA) Core Safety Group on hazardous residues in LPG storage and transfer equipment. The culprit was heavy ends: the part of the product that never boils off and never leaves.
For anyone who runs a propane bulk plant, a vaporization system or other LPG-fired equipment, that advisory is worth a closer look. "Heavy ends" are a worker safety issue when a tank is opened. Long before that, they are an equipment problem. They cost capacity, upset control and drive-up maintenance, one delivery at a time.
What are heavy ends in LPG?
Heavy ends are anything in propane or butane that won't vaporize at normal LPG operating conditions. They arrive from several directions. Pentanes and heavier hydrocarbons carry over from refining and gas processing. Compressor and pump oil gets picked up along the supply chain. Plasticizers leach out of transfer hoses and elastomer seals. Scale, rust and particulates ride in from pipelines, rail cars and transports.
The WLGA traces the hazardous residues in its advisory mainly to specific refinery crude feedstocks. It stresses this is not a common occurrence across LPG. But heavy ends of some kind are present in nearly every supply.
Why on-spec propane still leaves residue behind
Here is the counterintuitive part. Propane can pass every spec and still foul your system.
ASTM D1835 allows HD-5 propane up to 0.05 mL of residue per 100 mL evaporated, measured by ASTM D2158. On a single delivery, that's next to nothing. But propane leaves the tank and the residue doesn't. A tank that turns over its volume many times a year keeps a little more with every fill. Across the 10 to 12 years of service the WLGA points to, it becomes sludge.
Where that sludge ends up depends on how you draw product. A vapor-withdrawal system works like a still. Propane boils off, and heavies concentrate in the tank. A liquid-withdrawal system does the opposite. It sends heavies downstream into vaporizers, regulators, engines and burners.
How heavy ends damage LPG equipment
Heavy ends rarely announce themselves. A vaporizer loses a little output. A regulator starts to creep. A pilot drops out on a cold morning. Each symptom gets treated on its own, and the root cause stays in the tank.
The table below shows where residue does its damage and what to watch for.
| Equipment | What Heavy Ends Do | Warning Signs |
| Propane storage tanks |
Collect as sludge in the bottom; foul drains, internal valves and float gauges |
Sticking valves, erratic gauge readings, oily product at the drain |
| LPG vaporizers |
Concentrate as propane boils off; coat heat-transfer surfaces and cut capacity |
Falling output, higher bath or steam demand, liquid at the outlet |
| Propane regulators |
Coat diaphragms and seats; stiffen soft goods |
Pressure creep, failure to lock up, sluggish response |
| Meters and strainers |
Plug strainers; build film in meter passages |
Rising differential pressure, flow loss, measurement drift |
| Boilers, furnaces and process burners |
Plug burner and pilot orifices; upset air-fuel ratio |
Yellow or lazy flame, sooting, high CO, pilot outages, nuisance trips |
| Engines and gensets |
Foul converters, mixers and injectors; heavier fractions reduce knock margin |
Rough running, derating, tar in converters, shorter service intervals |
The bottom two rows carry the biggest price tag. When a propane-fired generator derates or a process heater trips, the cost isn't a service call. It's lost production.
LPG tank inspection: the hidden safety risk
That inspection finding shifts the conversation from maintenance to people. According to the WLGA, residue left in a tank can release hazardous vapor and dust once the tank is opened. The risk climbs with any task that adds heat, including buffing, sanding, grinding and welding. Workers can be exposed by breathing it in, swallowing it, or through skin and eye contact.
The WLGA is careful not to overstate the case. It ties the hazardous residues to specific feedstocks, not to LPG. The practical problem is history. Few operators can trace every load that went into a tank over a decade of service. The prudent approach is to treat any long-service tank as potentially contaminated until it's tested.
How to prevent heavy ends buildup in your LPG system
None of this calls for exotic technology. It calls for a routine. The operators who stay ahead of heavy ends remove them on a schedule, before they build up.
Keep a drain log. Drain tanks, low points and pipework regularly, and record what comes out each time. The log is an early-warning system. A rising volume points to a change in supply quality.
Blow down vaporizers on schedule. Follow the manufacturer's blowdown interval. If residue volumes grow, shorten it.
Filter ahead of sensitive equipment. Strainers and filters upstream of meters, regulators, burners and engines catch what the drains miss. Track differential pressure so you know when they're loading up.
Test the product. Sample incoming and stored propane for residue per ASTM D2158, especially after a supplier change.
Use LPG-rated hoses and soft goods. Components built for LPG service limit plasticizer leaching.
Plan every internal inspection. Follow the WLGA guidance. Take an initial vapor reading before anyone goes in. Apply tighter controls for hot work, buffing and grinding. Work with an occupational hygienist on a trigger plan that steps up PPE as contamination rises. Treat all dust and residue from the inspection as hazardous waste.
Frequently Asked Questions
What causes oily residue in a propane tank? Oily residue is usually heavy ends: heavier hydrocarbons, compressor oil and plasticizers that don't vaporize with the propane. They concentrate in the tank over time, especially in vapor-withdrawal systems.
Can propane that meets spec still cause residue problems? Yes. HD-5 propane is allowed a small amount of residue per delivery. Over years of tank turnover, that residue accumulates.
How often should LPG tanks be drained? There is no single interval. Drain on a regular schedule and log the volume. If it rises, drain more often and look at your supply.
Why is my propane vaporizer losing capacity? Heavy ends coat heat-transfer surfaces and reduce output. Falling capacity, higher bath or steam demand, or liquid at the outlet are common signs. Regular blowdown helps.
Is it safe to grind or weld inside an LPG tank? Only with a written procedure. The WLGA recommends an initial vapor reading before entry and higher levels of control for any work that adds heat.
Stay ahead of heavy ends with TransTech Energy
Heavy ends are a slow problem, which is exactly why they get ignored. A drain log, a blowdown schedule and good filtration cost far less than a failed regulator, a derated generator or an unplanned tank cleaning.
TransTech Energy provides LPG bulk plant maintenance and repair nationwide, with 24/7 emergency response. Our crews can help you set up a drain and blowdown routine, troubleshoot regulators, vaporizers and burner trains affected by residue, and plan around internal tank inspections.
Source: WLGA Core Safety Group, Safety Advisory Note: Hazardous residues exposure with LPG storage and transfer equipment.