Air pressurisation system

[Krallu]

Hi there everyone.
Merry Christmas to you all.

I have a little question when reading on my own in the ATPL Jeppesen manual about the air pressurisation system and it's valves.

So far if I understand this right, there is a safety valve if the diff press is too high. Dump valve to manual dump the press if needed. Outflow valve that are partially open to be able to circulate air. A Duct relief valve to prevent over press in the ducts. And on safety valve (don't remeber the name exaclty) to prevent a negative diff press.

But then it is also spoken about a discharge valve. I can't find any good text where it is said what it actually are. Is it the same as an outflow valve just a different name? Or can someone explain to me te difference of an outflow valve and a discharge valve?

And please correct me if any of the information above is wrong.

Thanks!

Krallu

[Compass Call]

Krallu

As far as I understand it, the OUTFLOW Valve and the DISCHARGE Valve are one and the same. Two different names for the same valve. They are set by the pilot to maintain the required cabin pressure. If the valve should fail to operate aoutomatically, it can be operated manually. Alternatively a manual dump valve can be operated to maintain the required cabin pressure.
The SAFETY Valve will open automatically if the cabin pressure rises above the fuselage pressure limits laid down by the aircraft manufacturer.
The INWARD RELIEF Valve operates if a negative pressure situation arises inside the fuselage.
These two valves can be combined as a SAFETY & INWARDS RELIEF Valve.

I hope this is of some help and stand to be corrected if I am wrong. :ok:

CC

[Keith.Williams.]

The JAA also ask about the DITCHING valve. Its purpose is to close all valves below the waterline prior to ditching in water. This is intended to maximise the time available for escape by reducing the rate at which water comes into the cabin.

You should also be aware of the PACK valves which control the flow of compressor bleed air into the air conditioning packs.

[Flight Detent]

Hi all,
May I try and clear the air with all these valves, I will refer to the Classic B747 with reference to the system descriptions:

a/ the pressurization safety relief valves, there are two in the '74 because of the large volume of cabin, set at slightly different differential pressures, located on the port side, just forward of the left wing landing lights. If one or both of these open, the number 2 airconditioning pack is auto'd off, the FE will then take care of it.

b/ There is no dump valve as such, all the other valves take care of pressure control.

c/ The cabin outflow valves are the primary controllers of cabin pressure, they are located aft, underneath, just below the rear (#5) doors each side, and are electrically controlled by the cabin pressure controller on the FEs panel, they also have a standby DC power supply, in case of an emergency. They can also be manually controlled individually by the FE should that be necessary.

d/ All the duct overpressure/overtemperature sensors are within the bleed air/conditioned air systems, and are used to warn the FE of any malfunction within the system, but are not directly involved in controlling cabin pressure.

e/ There are four negative pressure relief valves, two located on each cargo door, used to do two things, first they will not allow the outside air pressure to exceed that inside the cabin by more than a very small amount, and second, they automatically open when the cargo door opening is required, to make the opening easier, if the inside pressure is very slightly negative.

f/ There is a discharge valve located on the underside, forward, port side, just below the #1 entry door. It is there to exhaust the warm air being drawn from behind the flight deck instrument panels and the electronic control boxes compartment below the floor, just aft of the nose wheel comparment. That discharge valve works automatically in that it is always open on the ground and at low altitudes to discharge this unwanted warm air. At other times, the warm air is ducted around the forward cargo compartment to keep it slightly warm.

There is a 'ditching' selection on the FEs panel that will close all these lower valves to exclude water in the event of a ditch.

Hope this helps,
Cheers
:ok: