Fire exposure poses a severe safety risk to pressure vessels, especially those used with hydrocarbons or volatile liquids. When exposed to an extensive fire, a vessel runs the risk of major damage and in some cases, explosion. This can cause fatal injuries to operators, as well as catastrophic damage to property. To mitigate against this risk, some pressure vessels require emergency depressurisation to be built into their design, in case of a major fire, as well as fire safe precautions to guard the vessel against minor fire damage.
Fire Safety Precautions
Prevention is better than cure, so most high risk pressure vessels are equipped with inbuilt fire safety precautions that minimise the risk of fire occurring internally. These features include:
- Fluid level indicators
- Emergency heat exchangers
- An attached refrigeration plant
- Steam gauges, valves and traps
In addition, fire risk can be reduced by ensuring proper installation and maintenance, thorough product testing and comprehensive operator training.
However, sometimes a vessel is exposed to fire despite taking all reasonable precautions – perhaps due to a fire starting elsewhere in the facility. In these cases a vessel should have fire protection features to minimise exposure and limit damage:
- Fireproof insulation
- Deluge taps/sprinklers
- Drainage vents and valves
- Pressure outlet valves
With these precautions and safety features in place, a high quality pressure vessel should resist a low-temperature or short duration fire without risk of rupture or major damage. However, when exposed to a longer duration or high temperature fire – e.g. a hydrocarbon fire lasting 2 to 4 hours – explosion and noxious vapour release become imminent risks.
Depressurisation & Blowdown
To avoid explosion, a vessel needs to rapidly depressurise to 50% or less of its operating pressure. This should be achieved within 15 minutes of exposure to a high temperature fire. The purpose of depressurisation is to prevent escalation of the fire and minimise damage to the pressure vessel itself.
For vessels that operate using gases this is accomplished through an emergency outlet valve. As depressurisation releases vapours which may be flammable and/or explosive, a flare at the outlet is ignited to allow controlled combustion of escaping gases without escalating the fire.
If your vessel uses liquids, these must be removed through blowdown, which is the equivalent of depressurisation for fluid pressure vessels. Liquid blowdowns are channelled through drainage vents into a reinforced storage tank or processing area safely removed from the vessel. In these cases a flare is not used, as this could create a flammable shower of liquids that serves only to spread the fire.
Does Your Pressure Vessel Require Depressurisation Capability?
Not all pressure vessels have the same risk of explosion, and not all require rapid depressurisation capability. You should consider emergency depressurisation if your vessel meets the following criteria:
- Contains more than 900 kg of vapours or fluids
- Operating pressure is more than 10,000 psi
- Vessel uses flammable or volatile liquids – e.g. propane or butane
- Rupture would cause direct danger to staff or critical damage to equipment or facilities
- Escape of toxic fluids/vapours risks environmental damage or a hazard to human health.
The explosion risk can be reduced by setting the maximum design pressure at least 10% - 15% higher than operating pressure.
Safety By Design
Safeguarding against fire and explosion is not just a matter of workplace risk management. Adequate design also plays a major role. At Vessco we design pressure vessels for safe operation in a range of environments, including those that carry a high fire risk and use flammable/toxic fluids. To speak with one of our engineers about your requirements, please call +44 0156 750262 or email firstname.lastname@example.org