Thermal relief refers to automatic release of fluids or gases from a system to a specified level. Pressure relief systems are intended to prevent pressures in process equipment from increasing to the point where a mechanical failure or rupture might take place, automatically releasing any materials within.

What are causes of Overpressure?

There are several most common causes of overpressure.

• Exposure to fire. If a vessel is subjected to heat radiation from a fire, internal pressure may increase mainly due to thermal expansion of the contained material.
• Excessive process heat input. Most process systems require varying amounts of heat exchange. An overpressure may result from the expansion of liquid or vapor contained within the system when heat input exceeds the design condition.
• Control valves failures. Process upsets can happen when control valves or other equipment that governs process conditions fail. Pressure regulation will not be efficiently regulated once the process upset occurs, and pressure increase may result.
• Unexpected chemical reaction. Overpressures that have not been evaluated before may be produced by unexpected or uncontrolled chemical reactions that lead to heat or vapor evolution.
• Cooling water supply failure. A reduction in the flow of cooling water used to condense vapor in a vessel could result in an increase in the pressure drop through the condensers, which would raise the pressure inside the vessel.
• Isolation. Internal pressure may build up if a vessel or tank is completely or partially isolated from normal process conditions if it has no venting outlet.
• Heat exchanger tubes failure. If the heat exchanger shell rating is lower than the pressure level of the circulating medium and the internal heat exchanger tube bursts or leaks, the vessel will be overpressure.
• Introduction of a volatile material. When a material is introduced into a container at a temperature over its boiling point, the material will quickly vaporize, increasing the container’s pressure and leading to a higher vapor output.
• Introduction of a reactive foreign material. A reactive foreign substance added to the process might generate a vapor that overpressures the system.
• Failure of reflux system. The quantity of reflux determines the amount of vapor generated. If a reflux fails, it may lead to lower pressure through the condensers and vessel and result in higher pressure in the whole systems
• Thermal expansion. Contained liquid may be exposed to heat input that causes them to expand, resulting in pressure increase. The heat sources are typically direct sunlight and fire exposures.
• Accumulation of non-condensable gas. If non-condensable gas are not eliminated, the gases will blanket the surface of heat exchanger causing overpressure.
• Outflow rate exceeds inflow rates. If material is being withdrawn from a tank faster than the incoming rate, a vacuum will occur. If the tank is not strong enough to withstand the vacuum condition, it will collapse on itself.

Pressure Relief Valves

Pressure relief valves are used to handle two main conditions of the process, which are normal conditions and emergency conditions. The causes of overpressure are considered random, therefore the capability of pressure relief valves have to constantly available and automatic.

Thermal Relief Valves

Thermal relief valves are necessary in liquid piping section when it is expected that the liquid will be isolated when the piping is exposed to temperature rises from solar radiation, warm ambient air, steam tracing, fire exposures, or other external heat sources.

High temperature input to a piping will cause both the piping and the fluid contained within it to expand. Lab testing showed that increasing pressure from thermal expansion may be 70-100 psi for each ^oF in temperature increase.

Installation Location of Pressure Relief Devices

Pressure relief valves are usually installed at the following locations:

• Storage tanks, all storage tanks subject to high flow rates in or out require compensation for the displaced vapor
• Equipment susceptible to thermal expansion, such as:
  • Vessel or tank subject to ambient or thermal expansion
  • The cold side of heat exchanger may be subject to excessive heat input from the hot side, if blocked off
  • Circulation lines of a heater, where they may be blocked off
• Discharge of compressors, variable speed drivers can increase compressor discharge pressure above desired condition
• Pumps with variable speed drivers
• Heat exchangers, heat exchangers that can be blocked in or where the shell of the exchangers may be subject to high pressure if there is internal tube leak

References:

Nolan, Dennis P, Handbook of Fire and Explosion Protection Engineering Principles Edition 3, Elsevier, 2014.