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Here you can see two of them on the B747. What are they and what's their function? 747

Source: FAA's Pilot's Handbook of Aeronautical knowledge

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The pressurization relief valves are on the left side of the fuselage and are used to provide protection (for the aircraft) against excessive differential pressures. In this case, they are set to open at a differential pressure of 63.8 KPa (with 66.9 KPa) as a backup. From ATSB Investigation Report on the depressurization of Boeing 747 VH-OJK:

The relief valves provided protection to the aircraft against excessive differential pressures, with the external latching doors providing a positive indication of valve operation. Aircraft systems documentation specified that the valves open at a differential pressure of 63.8 kPa (9.25 psi) to vent the fuselage interior to the ambient atmosphere. An additional relief setting of 66.9 kPa (9.7 psi) acts as a backup.

The same report shows the external blowout doors of the (both) relief valves open, as shown below.

Pressure relief valve

In a related note, there is a negative pressure relief valve to prevent vacuum damage to aircraft during a rapid descent. Usually, this is a spring loaded valve that opens inwards at negative pressure differential.

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The purpose of pressurization safety valves is to prevent too large differential pressure in the pressure-hull.
The safety valve(s) are normally closed, and they're set to open in a certain differential pressure. So for example in case the normal outflow valve fails for some reason and is closed, and the ac-packs continue to push pressure in the cabin, the safety valve opens thus maintaining a safe differential pressure.
They are a part of the pressure hull "safety back-ups" just like the negative pressure relief valve too.

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    $\begingroup$ Yes, and the purpose of preventing too-large pressure differential is to avoid structural damage. As an added note, the valves in the 747 pictured in the question open at a differential pressure of 63.8 kPa (9.25 psi), with an additional backup relief at 66.9 kPa (9.7 psi). $\endgroup$ – Porcupine911 Feb 5 '16 at 20:36
  • $\begingroup$ And... The photo in the question technically shows the blowout doors downstream of the valves. The doors latch open after any over-pressurization event. $\endgroup$ – Porcupine911 Feb 5 '16 at 20:53

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