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chaz88z
18th Mar 2019, 14:34
Technical questions I could not find in the FCOM nor in "google" with precision, for the 737-800 NG.

As par FCOM 2: Negative pressure relief valve prevents external atmospheric pressure from exceeding internal pressure.

My guess is this valve (which looks more like a hatch) is springloaded. And i'm wondering what is its PSI rating , (what would be the max negative deltaP).

Scenario : Both packs fail, over high MSA, cabin altitude goes above FL140, Cabin alt warning memory item and emergency descent done (outlfow valve MAN closed)

Emergency descent continued as soon as MSA falls back to FL100.

Would it be possible that the cabin alt would still be >FL140 while A/C is at FL100, for 8 min ?

I found this quite disturbing as there is this Negative pressure relief valve and negative pressure would most likely make all seals leak quite a lot...

A >4000ft negative differential pressure is roughly -100hPa = -1.45 PSI

IFixPlanes
18th Mar 2019, 17:31
AMM General Description:
The negative pressure relief valve is a mechanical device and operates independently. It does not interface with other airplane pressurization systems and requires no crew action.
The negative pressure relief valve is a flapper type valve. The valve hinges on its top edge and opens inward. A spring on its hinge pin holds the valve closed.
Negative differential cabin-to-ambient pressure opens the valve. The valve opens when pressure outside of the airplane is 1.0 psi more than the pressure inside of the airplane (-1.0psid).
https://s17.directupload.net/images/user/190318/temp/c5fjhd7k.jpg (https://www.directupload.net/file/u/51580/c5fjhd7k_jpg.htm)

chaz88z
18th Mar 2019, 20:29
So -1.0PSI it is thanks.
Any info on typical 737 presssurization leakage:
packs—-> X kg/s
outflow valve —-> Y kg/s
leaks = X-Y kg/s
i guess that would be advanced engineering data...
nonetheless getting max X (pack max output) as a beginning would be the easier to get

NSEU
19th Mar 2019, 00:25
So -1.0PSI it is thanks.

So you understand that the negative pressure relief valves will not assist depressurisation in a pack off descent?

packs—-> X kg/s

Pack leakage? I guess you could say it leaks 100% into the cabin/cargo/cockpit areas. There are water drains for the water separators, but since the packs are off in your scenario, check valves should retain pressure.

Other sources: Toilet flushes (not likely during an emergency descent), drain mast outflows, faulty door seals, door sill drains

outflow valve —-> Y kg/s

The outflow valve should modulate to close to maintain cabin pressure. The seals are not perfect, of course.

There are leak checks in the Maintenance Manual, but they are started at 4psi differential (set using manual pressurisation control). There is a chart which shows a 4psid to 2.5psid in 4~8 minutes, however, the pass rate appears to be only 100 seconds from 4psid to 2.5psid..

chaz88z
19th Mar 2019, 12:44
So you understand that the negative pressure relief valves will not assist depressurisation in a pack off descent?

Pack leakage? I guess you could say it leaks 100% into the cabin/cargo/cockpit areas. There are water drains for the water separators, but since the packs are off in your scenario, check valves should retain pressure.

Other sources: Toilet flushes (not likely during an emergency descent), drain mast outflows, faulty door seals, door sill drains.

In the scenario, X=0 (packs off) and let’s simplify with a perfectly closed outflow valve Y=0, as you mentioned the drains and other leaks, so as long as Pcabin >Pstatic, we should have some leakage, plus the opening of the Negative relief valve when
Pcabin-Pstatic > 1PSI
Do the drain check valves work both ways : positive (usual pressurization) or negative dP ?

I am not sure what you mean in your first sentence, as it is not a normal depressurization scenario where dP=0 from the trigger event.

Let me illustrate: you seal a bottle at altitude and come back down. When you open the bottle at sea level air will flow inside the bottle.
Now the bottle is a complex aluminium structure which happens to be a 737-800 fuselage with all openings (and outflow valve) closed, but Safety and negative pressure relief valve working properly.
Coming back at lower altitude Pstatic rises while Pcabin stays constant at a lower pressure.
Would we reach a point in a normal dynamic situation (timescale minutes) where differential pressure exceeds the rating of the negative pressure relief valve and attains -1.45 PSI difference, for several minutes?

NSEU
20th Mar 2019, 02:04
Would we reach a point in a normal dynamic situation (timescale minutes) where differential pressure exceeds the rating of the negative pressure relief valve and attains -1.45 PSI difference, for several minutes?

The negative pressure relief valve was designed for rapid descents, but Boeing words the manual carefully:

This can prevent structure damage during a rapid descent.

I assume they are designed for full gear/spoilers extended max airspeed rapid descents.

The relief valve is simply a flapper door with a spring on it (low tech). Like all springs, they will have an operating range. They are not light switches with instantaneous on/off actions. They "meter" the flow (according to the magnitude of differential pressure)

Door (rubber seals) don't work well with negative pressure, so they may let air in (small amounts)
Door sill drains do not normally vent in the air. Normally, any water present in flight will be kept in "bladders" and vented "on the ground" (i.e. at 0.5psid). The valves actually close at 2~5psid. Again, we are talking about small leakages.
Drain masts, as far as I know, do not have valves. I've seen water, cola, tea, etc pouring from these (onto unsuspecting people on the ramp) at a relatively high rate, but again, we are talking about relatively small leaks (compared to outflow valves and negative pressure relief doors. There are anti-siphon valves between the sinks and toilets to prevent toilet water backflowing into the sinks, but I'm not sure how they would react with negative pressure differentials (if at all).

I am not sure what you mean in your first sentence, as it is not a normal depressurization scenario where dP=0 from the trigger event.

Sorry, I was trying to understand what your biggest concern was ... achieving a breathable atmosphere (pax concern) or negative pressure differential (fuselage integrity).

chaz88z
20th Mar 2019, 13:44
That’s how I imagined the design, as we can also hear the cockpit window seals leaking when dP < 2 PSI




The relief valve is simply a flapper door with a spring on it (low tech). Like all springs, they will have an operating range. They are not light switches with instantaneous on/off actions. They "meter" the flow (according to the magnitude of differential pressure.

Then again. With a spring metered to open at -1.0PSI, by the time -1.45 PSI is attained, that valve must have opened some amount and let outside air in the cabin quite fast.

Back to that earlier scenario.
Starting data :
Packs OFF
Outflow valve Closed.
Reaching altitude 10.000ft after a rapid descent from fl200
Cabin altitude at starting point let say 18.000ft

At this starting situation, dP= -2.7 PSI

Negative pressure relief valve is opened by outside pressure flowing inside.

As it is springloaded cabin altitude vertical speed is at least proportionate if not squared with the dP.

The negative relief pressure valve is around 15cm x 25cm, I would imagine the cabin rate of descent at that starting point way above 1000ft/min. It could be determined by thermodynamic calculus but I am to rusty to take a blank paper!
From the sim experience of operating the Outflow valve manually, a very light squeeze of the OPEN/CLOSE switch (1/40 of the travelling) gives you already 500ft/min difference.
My guess of the relief valve opening at -2.7 PSI giving 1000ft/min shall be very conservative.

With this assomption, I cannot believe it would take more than 5min for cabin alt to decrease from 18.000ft to 14.000ft which relate in dP as -2.7 to -1.45 in 5min, as at -1.45 PSI negative relief valve is still opened by outside air so cabin rate of descent should still be of some fair amount...

What is your opinion on this ?

NSEU
21st Mar 2019, 06:12
Reaching altitude 10.000ft after a rapid descent from fl200
Cabin altitude at starting point let say 18.000ft

At this starting situation, dP= -2.7 PSI

Where did you get -2.7PSI from? If the cabin is at 18000 feet and the airplane is at FL200, the differential pressure will be about 0.6~-0.7psi (cabin pressure greater than ambient).
(I have a pressure/altitude graph showing an ambient pressure of 6.75psi for 20,000' and 10.11psi for 10,000', so 18000' is around 7.1psi.)
Assuming no leaks (outflow valves closed), the aircraft would have to descend to an altitude equivalent to 7.1 plus 1.0 (8.1 psi) before the negative pressure relief valve opened. 17,000'? The aircraft has to descend 7000 feet from this point.
I'll let the experts take it from here. I'd be guessing the rest. I don't know the size of the outflow valve so I can't compare it with the relief valve for pressure equalisation properties. The outflow valves on a 747-400, for example, can dump pressure on landing at (at least) 2000fpm with the packs running in high flow.

I know Boeing's credibility has been stretched of late, but I'm sure they've crunched the numbers and come up with the required size and spring strength of the negative pressure relief valve.

chaz88z
21st Mar 2019, 12:51
What I meant was to simplify everything, after the rapid descent, considering a/c after its descent at 10.000ft. While Pcab remains constant, to simplify... I would call it the static scenario.

P at 20000 = 6.75 PSI
P at 18000 ft = 7.34 PSI
P at 10000 ft = 10.11 PSI

thus Pcab (18k) - Pstatic around a/c 10k = 7.34-10.11= -2.7 PSI

I completely agree that in real situation, if Pcab=7.34 PSI, negative pressure valve will open at 7.34+1= 8.34PSI (14.900ft).

So in dynamic situation and if Boeing design is working properly (as I assume here): during descent with the same scenario from 18000ft to 10000ft dP will go from 0 PSI (Pcab=Pstatic) to -1 PSI when aircraft reaches 14.900ft,
the. The negative valve starts opening while aircraft continue towards 10000ft, and cabin altitude will descend as air flows inside the vessel. Maintaining around -1.0 PSI as the aircraft reaches 10.000ft, Pstatic=10.11, Pcabin~9.11PSI and the initial scenario is invalid because:
-Aircraft at 10.000ft
-Pstatic =10.11PSI
-dP= -1.0PSI
-Pcab= 9.11
-Cabin altitude =12.700ft.
In the initial scenario, colleagues called “masks off” as a/c was at 10.000ft, instructor called for UAS because cabin alt (in the sim) stayed above 14.000ft so dP > -1.45 PSI during 8min after masks where called off.
Instructor point is : Boeing QRH states : MASKS OFF when cabin altitude at 10.000ft or lower, which is for a pilot operating as per Boeing is correct, and was not respected in this case.

But my interrogation is and remains, what about it in pure technical considerations, considering the negative pressure valve and natural leaks,
dP > -1.45 PSI for 8min seems like a very long time...

This why I tended to simplify with a static scenario, in my last reply, where I get a fictional -2.7 PSI dP as a starting point (disregarding the descent time and equalization during descent).
And 8 min for dP to go from -2.7 to -1.45 seems as well unrealistic as:
Alt cabin with -2.7 psi = 18000ft
Alt cabin with -1.45 psi = 14000ft
8 min time gives a 500ft/min cabin rate of descent, and seems way to low IMO for a safety device, that’s why I asked advice here...

chaz88z
21st Mar 2019, 13:08
I just realized the mistake I made :
UAS is for cabin alt > 10.000ft, not 14.000ft, so if dP stays at -1.0, for a cabin alt at 10.000ft:
AltCabin= 10.000ft
Pcabin= 10.11 PSI
dP= -1.0 PSI
Pstatic = 11.11 PSI
Alt a/c = 7500 ft
So in a perfect scenario, no leaks of the fuselage, just the perfectly working negative pressure relief valve, as long as Alt > 7500ft, masks must be on... in a perfect world....