LP Gas Systems: Tanks, Regulators, and Underground Piping

LP Gas vs. Natural Gas: Key System Differences

Liquefied petroleum (LP) gas—most commonly propane—is stored as a liquid under pressure in a tank on the customer's property. Natural gas arrives as a vapor through a utility distribution main. Because LP gas is delivered and stored on-site, the plumber who works on LP systems must understand tank installation requirements, pressure regulation, and underground piping in addition to the interior work covered in natural-gas lessons.

Propane is heavier than air. If it leaks, it settles into low spots—floor drains, crawl spaces, basements—where it can accumulate to a flammable concentration without being noticed. Natural gas is lighter than air and tends to disperse upward. This difference drives several design and safety choices in LP systems.

System Pressure Stages

Natural gas is typically delivered at a single, relatively low pressure that a single-stage regulator steps down for appliances. LP systems commonly operate at much higher tank pressures—vapor pressure inside an LP tank varies with temperature and can be well over 100 psi on a warm day. Appliances, however, require a low delivery pressure similar to natural gas appliances. Two-stage regulation bridges this gap safely.

First-stage regulator – Mounted at or near the tank, the first-stage regulator reduces the high, variable tank pressure to an intermediate pressure. This intermediate pressure is still above appliance operating pressure but is stable and consistent regardless of temperature swings or liquid level in the tank.

Second-stage regulator – Located near the building (often at the meter set or just inside the entry), the second-stage regulator steps the intermediate pressure down to the low operating pressure that appliances require, typically close to the pressures used on natural gas systems.

Using two stages instead of one produces a more stable delivery pressure, reduces the mechanical stress on each regulator, and provides an extra layer of overpressure protection. Overpressure protection is required because a failed regulator in a high-pressure LP system can expose interior piping and appliances to pressures far beyond their rated limits, creating a serious explosion and fire hazard.

Tank Setback Requirements

NFPA 58, Liquefied Petroleum Gas Code, establishes minimum distances—setbacks—between LP storage containers and structures, property lines, and ignition sources. The IFGC also references these requirements. Setbacks exist because a tank leak or relief-valve discharge releases flammable vapor; distance reduces the chance that vapor reaches an ignition source before it disperses.

Setback distances vary with tank capacity. Smaller tanks (common for single-family residential service) have smaller minimum setbacks than large commercial or agricultural tanks. Always consult the current edition of NFPA 58 and the locally adopted code for the exact distances that apply to a specific installation—the numbers are table-driven and jurisdiction-specific adoption may modify them.

General principles that appear consistently:

• Setbacks are measured from the nearest point of the container to the nearest point of the structure, property line, or ignition source.
• Relief valve openings must also have clearances from openings into buildings.
• Tanks must be placed on firm, level, non-combustible surfaces and must not be placed in locations where vehicles could strike them without adequate protection.

Underground PE Piping from Tank to Building

For the buried run between the LP tank and the building, polyethylene (PE) pipe is the approved material in most jurisdictions when properly installed. PE is resistant to corrosion and is flexible, making it well suited for underground use. It cannot be used above ground or inside buildings; a listed transition fitting is required where the pipe transitions from underground to above ground or enters the building.

Correct installation sequence:

1. Depth (cover): The pipe must be buried at the minimum cover depth required by the adopted code. Adequate depth protects against physical damage from digging, landscaping, and frost.

2. Tracer wire: Because PE is non-metallic and cannot be located with standard electromagnetic pipe locators, a continuous metallic tracer wire (often yellow-jacketed) is installed alongside the pipe for its entire buried length. The wire is brought up at each end so future locating equipment can find the pipe.

3. Warning tape: A detectable warning tape—typically yellow and marked with a gas-line legend—is placed in the trench above the pipe. It alerts anyone digging in the area before they reach the pipe itself.

4. Transition fittings: A listed transition fitting (sometimes called a service-entrance fitting or PE-to-steel transition) is used wherever the PE pipe changes to a metal pipe material—at the tank outlet connection and at the point of building entry. These fittings are factory-made and listed for gas service; field-fabricated joints between PE and metal are not permitted.

Safety Highlights

• Leak testing is mandatory before backfilling and again before placing a system in service. Use only test methods approved for gas piping—never use an open flame.
• Propane sinks—always ventilate low areas when working on LP systems and never ignore the odor of mercaptan odorant added to LP gas.
• PPE and lockout/tagout: When working on any pressurized gas system, shut off and lock out supply valves, verify zero pressure, and wear appropriate PPE including safety glasses. Coordinate with the gas supplier before working on tank connections.
• No smoking, no open flames in any work area where gas piping is open or being tested.
• Tracer wire and warning tape are not optional—omitting them creates a buried hazard for future workers.