System Questions

[AtomicSheep]

Hi,

I've got a couple of questions about this system. The way I see it, this system has two sections, a high pressure and a low pressure section. Before the engine driven pumps in green, the system is low pressure and high pressure thereafter.

I guess this is why the shut off valves in red are called low pressure (because they are located int eh low pressure section and not due to their nature of operation) but correct me if I'm obviously wrong.

Before I go into the questions I just want to point out that the whole point of this system is to drive the components in the blue box.

The first question that I have is... what happens to the components in the blue box if you switch switch the LP Shut off Valves in red to off? The way I see it, this hydraulic system is like power steering, so with the LP valves turned to off, you effectively lose power steering? So effectively, controlling those components becomes very difficult?

The second question that I have is a bit basic, but I'm finding it hard to grasp how you can have a high pressure state after the engine driven pumps and a low pressure state before the engine driven pumps. Do the pumps create a suction which then starts a circulation of fluid through the system first from the pumps then through the components in the blue box, then this fluid is pushed at high pressure back into the main cell and then the process repeats?

 

[btrueblood]

Do you see the boxes near the top left and right labelled "LP Warning Switch" and "High Temp Warning Switch"? I.e. these two sensors shut down the pumps in event of apparent fire or massive leak causing loss in pressure. The "Emergency Hand Pump" is then used (having its own supply tank and piping) to move the flaps and landing gear to their emergency deployed positions.

As to your second question, pressure created by the pumps is consumed at the four devices in your blue box, and/or any excess flow is consumed by the pressure relief valves. The return flow into the reservoir is at low pressure, and the air pressure in the reservoir is controlled by a pressure relief valve and positive feed from the "de-ice system".

Really, these are pretty basic questions. Please tell me you are not on a design team for safety-critical hydraulic systems like aircraft...

 

[AtomicSheep]

Thanks for the response.

Don't worry I'm not working part of a design team for aircraft, just trying to learn other areas to those that I studied at uni.

As far as I know, the LP Warning and High Temp Warnings are just sensors passing on indications to the indicators and not switches. I don't understand why they are marked as "LP warning switches" to be honest. I don't see anything on the diagram indicating what the effect of them would be if they were indeed switches. But perhaps you are correct in suggesting they shut down the pumps but I don't see the connections from the switches to the pumps so not sure.

From the manual this is what is stated about the switches:

When the output pressure of the hydraulic pump decreases to 1550 psi the LP warning switch operates. The L.P warning switch will cause the following indications:
- A CAP (amber) caption
- A lower centre panel PUMP. (amber) caption.
The caption goes off when the output pressure of the hydraulic pump rises above 1825 psi.
​

I think I also now understand point 2 now... if the pumps are not working, then there's simply not enough pressure to operate the 4 systems.

Am I also correct in assuming that the emergency pump pumps oil from the emergency reservoir into the relevant components but once it does this, there is no going back? i.e. the emergency reservoir is drained and the relevant mechanism is activated?

 

[btrueblood]

That is my understanding too (one way, one time, emergency use).

 

[AtomicSheep]

Was trying to edit the original picture but got locked out in the process, so here it is again.

What's the point of the segments of the system which I outlined in orange? Is this to redirect any excess hydraulic fluid back into the reservoir?

 

[Bumblyari]

Those are the case drain lines ie. they drain the internal leakage that takes place within the pump back to the reservoir.
It's not always recommended to put a filter in case drain lines but in a multi-pump system such as this if one pump destroys itself it stops all the debris being blown back to the reservoir only to destroy the other pump.

 

[AtomicSheep]

Thanks Bumblyari.