The standard API 2000 discusses Pressure/Vacuum (P/V) vents. These special safety vents can automatically open to release tank vapor if the tank pressure increases, or open to allow atmospheric air inside if the pressure decreases. This helps prevent the tank from bursting due to high pressure, or sucking in like a crushed soda can due to vacuum. Remember that tanks tend to have surprisingly low design pressures, so it is deceptively easy to break a tank if you don’t have a proper P/V Vent. A little vacuum, and they can start to resemble a crushed soda can!
API 2000 lists a number of different conditions to consider and gives guidance on them:
- Liquid movement in/out: use the API 2000 equations
- Weather change: pressure (usually only a concern refrigerated tanks)
- Weather change: temperature
- External Fire Exposure
However, you should also use your judgement to consider any extra cases. Here are some examples:
- Abnormal heat transfer scenarios (ex: excess duty from equipment or heat tracing)
- Internal failure of heat-transfer devices (ex: a steam coil failing and the steam sprays into the tank)
- Pump overheating
- Vent collection/treatment system failures
- Vapor breakthrough
- Inert gas pads or purge failures
- Stream Condensation (causes vacuum)
- Utility failure
- Chemical Reactions generating gas or vapor space change
- Liquid overfill
- Control Valve Failures
- Uninsulated Hot Tanks (API 2000 suggests that uninsulated tanks above 120°F should be checked against excessive vacuum in a rainstorm. )
- Internal fire
- Internal or Explosion/Deflagration
- Mixing Products (of Different Composition than intended, perhaps causing temperature change or reaction)
- Other
Remember that you do not design the P/V vent to handle everything. Often, it makes sense to size the P/V vent for normal inbreathing and outbreathing (due to changes in flow, temperature, pressure, etc.). Liquid overfill might be better handled with a separate overflow nozzle. Other emergency cases might suggest a separate safety device., such as “frangible” roofs (fragile roofs that can break open in an explosion, prevent the whole tank from blowing up), emergency hatches that can fly open under pressure, rupture disks, etc. Some emergency venting devices are listed in API 2000 7th ed Sect 3.4.2.
Chime in with other scenarios in the comments below!
P.S. What’s with the big gap in posting? Personal life changes, and I’ve been doing more non-process work lately. I expect a slower posting rate going forward, so consider subscribing if you want to be notified.
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