While virtually any liquid trapped in a “blocked-in” section of pipe can cause a rupture due to thermal expansion, specific chemicals are strictly regulated by OSHA 1910.119 (PSM) and NFPA 30 because of their volatility or toxicity. The requirement for a hydrostatic relief valve (HRV) is generally triggered by the chemical’s physical state (liquefied under pressure) and its coefficient of thermal expansion.
Liquefied Compressed Gases
These chemicals are the most critical because they are stored as liquids but are gases at ambient temperatures. Even a small temperature rise causes a massive increase in internal pressure.
- Anhydrous Ammonia: Per ANSI/CGA G-2.1, HRVs are mandatory between any two shut-off valves where liquid can be trapped.
- Chlorine: Highly sensitive to thermal expansion; piping must be protected to prevent catastrophic toxic release.
- LPG (Propane/Butane): Regulated under NFPA 58, requiring relief valves on any section of liquid piping that can be isolated.
- Vinyl Chloride & Ethylene Oxide: Highly reactive and toxic; these require stringent hydrostatic protection to prevent “dead-leg” ruptures.
Flammable and Combustible Liquids (NFPA 30)
Under NFPA 30 (Section 22.11), any piping system that can be trapped and is subject to external heat (solar, fire, or heat tracing) must have a means of pressure relief.
- Refined Fuels: Gasoline, Diesel, and Jet Fuel (especially in long loading rack lines)
- Solvents: Acetone, Benzene, Toluene, and Methyl Acetate
- Alcohols: Ethanol and Methanol
Hazardous Process Chemicals
For these, the HRV isn’t just about protecting the pipe—it’s about preventing a “loss of primary containment.”
- Acids and Caustics: Concentrated Sulfuric Acid or Sodium Hydroxide. While they don’t expand as violently as LPG does, a pipe rupture still poses a severe personnel hazard.
- Refrigerants: In industrial cooling loops, trapped liquid refrigerants can easily exceed the pipe’s burst pressure during a defrost cycle or power outage.
Factors that “Mandate” an HRV
Regardless of the specific chemical, an HRV is practically required if the following conditions are met:
- Cold Chemicals in Warm Areas: Taking a chemical from a refrigerated tank into a piping run at ambient temperature.
- Heat Traced Piping: If the pipe is steam or electric traced, the risk of “cooking” a blocked-in section is 100%.
- Solar Exposure: Outdoor piping runs, especially long runs between storage and loading docks.
Summary Table: Chemical Categories & Requirements
| Chemical Category | Key Examples | Regulatory Trigger |
|---|---|---|
| Liquefied Gases | Ammonia, Chlorine, LPG | Mandatory (OSHA/NFPA/CGA) |
| Class I Flammables | Gasoline, Acetone | Required if solar/heat exposed |
| Class II/III Combustibles | Diesel, Lube Oil | Required if heat traced |
| Toxic/Corrosive | Nitric Acid, H2S | Best Practice to prevent spills |