Bleed air, FCOM, questions off course

[kevinh 409]

The vague terminology in the FCOM could be masking an underlying reason (known only unto Boeing), however, it is commonly accepted as being an advisory note, highlighting an unintended and possibly an uncertified consequence (at time of design) that could result from that configuration.

 

It is also considered advisory on the basis that, normally:

- the hsv is ordinarily closed under similar circumstances I.e. A higher pressure from 9th stage downstream of the hsv will cause the hsv to close

- there are sufficient protections engineered into the bleed system to detect and prevent damage resulting from overpressure, back pressure, over temperature (to a reasonable extent)

- the air pressure fed to the hsr downstream sense line from the apu bleed wouldn't be dramatically different to the air pressure that would ordinarily be in the sense line as if the pressure were coming from the 9th stage

 

In summary, its likely that the note exists because the unintended consequence of that configuration was not part of the systems original certification or design, and it is largely inconsequential, but noteworthy.

---------

To some of your other questions - I don't see what value you'd get by simulating the above symptoms of that configuration, and I guess PMDG thought the same, which is why you probably can't replicate it, because, even if they did, there'd be negligible cockpit affects assuming that configuration. If you went outside of that configuration, well, that would just be too stupid to consider. The procedures are written not to protect the health of either the engine, or, the apu (which is still an engine), but the procedures are written to protect both.

To your question about how valves are simulated on the ngx - There's a limit to the extent in which you can expect a $60 product to perform... It isn't a real valve... It's isn't a real airplane. So obviously valves aren't going to modulate as they open and close, because they physically aren't, because it's a simulator... To emulate it would, again, be stupid for entertainment purposes.

 

Lastly, the engine performance will not suffer because of the apu bleed closing the hsv.

I haven't seen the possible damage of dual bleed automatically modelled even in a 737 full flight sim. If the instructor wants to get damage effects from improper dual bleed operation in training then they would manually select a failure the 9th stage bleed valve or the APU. So it's not at all surprising PMDG didn't model these consequences that might occur. On most modern aircraft you can use mixed bleed sources, but the 737 has a very simple pneumatic system and so is not designed for that. Hence the DUAL BLEED caution light and warnings in the manuals in case damage might occur from backpressuring the valves.

 

Although they aren't real valves, it's often useful to simulate modulating valves in a sim as a way to model their effect on flow and pressure. So failing the valve in a mid position would be easy to model. However failing the 9th stage valve fully open or fully closed is the worst case so a mid position failure isn't of much interest.

[downscc 1,327]

The 9th stage bleed modulating valve is purely pneumatic, no electrical control. The engine bleed valve (which also modulates to limit duct pressure) is DC controlled though. Perhaps you were thinking of that?

Exactly.  My problem is in trying to visualize the high stage modulating valve. What comes to mind is a PCV but with a shutoff feature and the shutoff feature is hard to visualize. In my industrial experience, there are PCV's and there are shutoff valves (typically solenoids). I've google this and didn't find an answer. I can picture a pneumatic controller that closes on loss of controlling pressure then a solenoid that shuts off the control pressure but this gets me back to solenoids. Help.

[Copper. 78]

Think of the hsv as being one valve that is comprised of two independent functioning parts.

 

One part (the slave) is placed in-line with the 9th stage bleed duct.

The other part (the master) sits on top of the slave, is linked to the slave, and controls the slave.

 

The master controls the slave by sensing pressure upstream and downstream, summing the pressures to regulate to a nominal pressure.

 

To go any deeper than that requires proprietary knowledge to be disclosed as the hsv is a unique component that serves a unique purpose (like most else in aviation) so you'll just have to imagine the mechanical makeup of it all... It should include springs and diaphragms at least, and be entirely mechanical.

[vr-pilot 11]

Pack doesn't need to be capitalised btw, it isn't an acronym.

Hi Kevin,

I thought it would stand for "Pressurization & Air Conditioning Kit" (or similar). I may be wrong though...

Regards,

Claus

[JoeDiamond 67]

Hi Kevin,

I thought it would stand for "Pressurization & Air Conditioning Kit" (or similar). I may be wrong though...

Regards,

Claus

 

I've always understood it to be slang for "air conditioning package".

[Spin737 102]

Hi Kevin,

I thought it would stand for "Pressurization & Air Conditioning Kit" (or similar). I may be wrong though...

Regards,

Claus

It's a "Backronym." A Made up acronym from a word. Uttered often enough, it passes as truth.

[downscc 1,327]

Think of the hsv as being one valve that is comprised of two independent functioning parts.

 

One part (the slave) is placed in-line with the 9th stage bleed duct.

The other part (the master) sits on top of the slave, is linked to the slave, and controls the slave.

 

The master controls the slave by sensing pressure upstream and downstream, summing the pressures to regulate to a nominal pressure.

 

To go any deeper than that requires proprietary knowledge to be disclosed as the hsv is a unique component that serves a unique purpose (like most else in aviation) so you'll just have to imagine the mechanical makeup of it all... It should include springs and diaphragms at least, and be entirely mechanical.

Many thanks... I know exactly what you are talking about. I have seen such things on the industrial side too. It's a fairly common method to control a high pressure stream with a low pressure signal. Many vendors provide compound PCVs.  However, my question is not how the pressure regulation works but how the shutoff works. I have not yet seen a PCV used in a shutoff valve function therefore this has caught my attention.

It's a "Backronym." A Made up acronym from a word. Uttered often enough, it passes as truth.

I think wiki has it:

 

The term air conditioning pack or A/C pack is used differently by different sources. Boeing, Airbus, Embraer, Bombardier and Lockheed use the term as an abbreviation of package, applied to both pneumatic and non-pneumatic systems. At least one business jet supplier uses the term PAC instead on their systems control panels, said to be an abbreviation for pneumatic air conditioning. Supposedly, PACK can also be an acronym for pneumatic air cycle kit, however, this abbreviation is not used by the major manufacturers above.

[Copper. 78]

Shutoff under what conditions?

 

- Without a feed from 9th?

- With a higher pressure downstream?

- With an over pressure up or down stream?

- Afer turning off the engine bleed switch?

 

We need more details to help.

[kevinh 409]

Hi Kevin,

I thought it would stand for "Pressurization & Air Conditioning Kit" (or similar). I may be wrong though...

Regards,

Claus

As Matt and Joe said it's a "backronym". The pack has nothing directly to do with pressurisation (which is controlled by the outflow valve) and the word "kit" is not used for any other aircraft equipment.  Pack is simply a contraction of package as in "air conditioning package", which is a common term in the air conditioning industry for an air-con unit.

Exactly.  My problem is in trying to visualize the high stage modulating valve. What comes to mind is a PCV but with a shutoff feature and the shutoff feature is hard to visualize. In my industrial experience, there are PCV's and there are shutoff valves (typically solenoids). I've google this and didn't find an answer. I can picture a pneumatic controller that closes on loss of controlling pressure then a solenoid that shuts off the control pressure but this gets me back to solenoids. Help.

I don't know the details but the HP bleed modulates open to admit higher pressure air flow when the LP bleed pressure is too low. The pressure regulation is done downstream by the engine bleed valve but there's no point in releasing too much high pressure air only for it to be throttled by the regulator valve.

 

Don't forget this 9th stage valve is part of the CFM56 engine (used by Airbus as well as Boeing) but the pressure regulation in the pneumatic system is aircraft specific. Pressure Regulating Shut Off Valves (PRSOVs) like the engine bleed valve are not at all uncommon.

[downscc 1,327]

Thanks Kevin, that did the trick. Using PRSOVs as a search key I did find products that combine pilot valve regulation and shut off. This is new to me because it is not something I've run into but Jordan valve has the Mark 687 Series piloted regulated valve that I can at least now say... oh, I know what that is.  There is still a leakage, which is why I've not used this type in a refinery, but for passing air it makes sense. 

[Copper. 78]

Thanks Kevin.

 

Dan, sorry if insult was caused. I didn't know you had extensive experience with this stuff, in a refinery no less!

[downscc 1,327]

Brian, no insult occured. I am not an expert at anything.

[vr-pilot 11]

Supposedly, PACK can also be an acronym for pneumatic air cycle kit, however, this abbreviation is not used by the major manufacturers above.

 

So that is what I would have called it if I was a manufacturer... B)

(replacing "pneumatic" with "pressurization" and "cycle" with "conditioning" though)

 

Brian, no insult occured. I am not an expert at anything.

  Now that's understatement!

 

 

The pack has nothing directly to do with pressurisation (which is controlled by the outflow valve) and the word "kit" is not used for any other aircraft equipment.

 

Correct me if I am wrong but:

the variable air inlets of conditioning packs are used for (cold) outside air supply and so actually "pump up" the cabin in flight, while the outflow valves just control the overall cabin pressure (delta PSI). So IMO PACKs (or Packs) are the initiators of pressurization actually...

(And engine bleed air (about 200°C?) is just taken/added for hot air supply/mix to regulate the temperature)

[JoeDiamond 67]

The inlet doors are for the heat exchangers, they do not feed the packs. The airflow into the cabin from the packs from the bleed air is constant. All the pressurization regulation is done by the outflow valve.

[vr-pilot 11]

The inlet doors are for the heat exchangers, they do not feed the packs. The airflow into the cabin from the packs from the bleed air is constant. All the pressurization regulation is done by the outflow valve.

Well, you may try to regulate your cabin pressure via the outflow valve only, but I would suggest to stay below FL100, because otherwise you will get a CABIN ALT WARN.

If the cabin is not supplied with "fresh outside air" via the PACKs, the cabin climb rate will maintain to be approx.100 to 150 feet per minute.

So from T/O to FL360 CRZ you will get the 10,000ft CAB ALT ALERT after approx. 30 minutes, and the "OXYGEN MASK light will illuminate" at 14,000ft CAB ALT after one hour in flight.

 

It is all logical: one cannot regulate the pressure in an unclosed system just by regulating the loss. There also must be some source of gain...

 

BTW: when at CAB ALT 14,000ft turning the PACKs on will give you a CAB descent rate of approx. 1,500ft per minute bringing your PAX back to a safe life environment in about 2 minutes...

 

B)