Yes, I am still on this horse! As I discussed last month, there are a number of chemicals that specifically REQUIRE HRVs on liquid piping. Those are chemicals like NH3, Cl2, and LPG, to name the top three (3). But I have recently read a root cause analysis in which a flammable-liquid piping flange leaked, and it was attributed to “thermal expansion.” I called BS, and this is why…
For thermal expansion to necessitate hydrostatic relief protection, the vapor pressure or boiling point of the liquid is actually less critical than the fact that the liquid is blocked in a completely full (liquid-full) system.
The primary driver of hydrostatic relief is the liquid’s thermal expansion coefficient and its incompressibility, not necessarily its volatility. While any blocked-in liquid can cause a pipe to rupture due to solar heating or ambient temperature rises, thermal relief is most commonly prioritized for:
- Boiling Point: Liquids with a boiling point below 100°F (38°C) at atmospheric pressure
- Vapor Pressure: Liquids that exert a vapor pressure greater than 14.7 psia at the maximum expected ambient temperature.
However, even high-boiling-point liquids like heavy lube oil or water can rupture a pipe if they are trapped between two closed valves and exposed to high heat sources.
According to API Standard 521 and ASME B31 you should evaluate the need for thermal relief whenever a section of piping or a vessel can be isolated while full of liquid.
The “Liquid-Full” Condition
If there is even a small vapor space (outage) in the system, the vapor will compress and absorb the expansion of the liquid. The danger arises only when the system is 100% liquid-full. In this state, a temperature increase of just 1°F can increase internal pressure by 50 to 100 psi, depending on the fluid.
Thermal expansion protection is typically required if the blocked-in section is exposed to:
- Solar Radiation: Piping runs on a rack exposed to the sun.
- Heat Tracing: Steam or electric tracing that remains “on” while the line is blocked.
- Heat Exchangers: The cold side of an exchanger when the hot side continues to flow after the cold side is valved off.
- Ambient Temperature Swings: Large shifts between day and night temperatures.
If you are performing a relief valve study, keep these two factors in mind:
- Coefficient of Thermal Expansion: Highly “expansive” liquids like LPG (Propane/Butane) or Anhydrous Ammonia generate pressure much faster than water or heavy oils.
- Bulk Modulus: This measures how “incompressible” the liquid is. Most hydrocarbons have a high bulk modulus, meaning they do not “give” when they expand, forcing the steel pipe to stretch until it reaches its tensile limit and fails.
Summary Table: Relief Necessity
| Liquid Type | Vapor Pressure/BP Concern | Hydrostatic Relief Needed? |
| Cryogenics/LPG | Extremely High (BP < 0°F | Mandatory (Rapid pressure rise). |
| Flammable Solvents | High BP < 100°F | Mandatory (Risk of fire if rupture occurs). |
| Water/Glycol | Low BP > 212°F | Recommended (If outdoors or heat-traced). |
| Heavy Crude/Oil | Very Low | Case-by-Case (Usually only for heat-exchanger blocks). |
Pro-Tip: For long cross-country pipelines or long headers, engineers often use a “Thermal Expansion Pin” or a small 3/4″ x 1″ hydrostatic relief valve (HRV) set at 10% above the MOP (Maximum Operating Pressure) to protect the gaskets and flanges from these low-volume, high-pressure excursions.