Understanding Intermediate Pressure

[WhiteSands]

Thanks for the excellent explanation. I have some more questions if you don't mind.

With diaphragms, it's exactly the opposite scenario, although it's a little trickier to visualize. The air pressure from the tank is pushing the valve closed. This means with a full tank there is more pressure from the tank trying to close the valve, already working against the big spring. This means the amount of air pressure in the IP chamber-also pushing against that spring- that is needed to close the valve is lower. As the tank empties, there's less air pressure pushing the valve closed, meaning you need higher IP to close the valve.

I fully understand what you are saying. With a full tank at 3000 PSI, the supply air is already pushing down on the diaphragm, overcoming the big spring pressure, by quite a lot. So only a small amount of incremental pressure is needed to close the valve.

With a near empty tank at 300PSI, the supply air is weaker so the diaphragm is not pushed down as much. So the valve has a longer way to travel (against big spring pressure) in order to close. So a much larger incremental pressure is needed to close the valve.

However, my question arises because of my understanding of what IP is:

Assuming the IP is read with the 1st stage on land (no water pressure working against the diaphragm) at 3000PSI and again at 300PSI tank pressure:

- What changes is the incremental pressure required to close the valve, when varying tank pressure from 3000PSI to 300PSI

- The total pressure required to close the valve does not change, whether tank pressure is 3000PSI or 300PSI. This is because if the tank has 3000PSI, we need smaller additional pressure to close the valve. If the tank has 300PSI, we need bigger additional pressure to close the valve.

In the end, the total amount of pressure needed to close the valve remains the same because it is the same spring we have to work against.

- I thought that IP refers to the total pressure inside the IP chamber. I.e. initial pressure from tank supply + incremental pressure required to close valve. This is also the pressure read by the IP Gauge as it is connected to the LP hose.

- To my understanding, what varies between 300PSI and 3000PSI tank supply is the incremental pressure needed to close the valve, but the total pressure (i.e. IP) should not change.

Where have I gone wrong with my understanding?

PS: In thinking about this more, I think there is more physics to this than meets the eye, that the book goes into in this section.

The initial high pressure 3000PSI blast from the tank that moves the diaphragm and compresses the spring may not build up a high pressure in the IP chamber. The spring's force curve will also be slightly different as the "preload" is different.

Versus if the supply pressure is at 300PSI and the air molecules has to slowly pack the IP chamber and pressurize it to push the diaphragm down and overcome spring pressure.

I'm thinking of a similar analogy to loosening a stuck bolt. Using a quick jerk of the wrench (similar to tank at 3000PSI) requires less force (similar to pressure) to break it free, then using a slow, consistent pull of the wrench (similar to tank at 300PSI).

Let me know if I'm going the right way.

 

[halocline]

The only thing you really need to understand is that there is NO WAY the miami heat are coming back in this basketball game. GO SPURS!!!

I'll have to work on your questions later, but at first glance I noticed you mentioned something about tank pressure pushing on the diaphragm; that's not the case. Supply pressure on the diaphragm would result in shredded rubber....IP presses on the diaphragm. The seat is upstream of the IP chamber, and WHAT KIND OF A BS FOUL ON DUNCAN WAS THAT, REF!!! Sorry, the seat seals off supply pressure from the diaphragm.

I'll have a look tomorrow, but lots of others could probably OH NOOOOOOO ITS DOWN TO 9 POINTS.....answer your questions.

---------- Post added June 11th, 2014 at 08:53 AM ----------

With a full tank at 3000 PSI, the supply air is already pushing down on the diaphragm, overcoming the big spring pressure, by quite a lot. So only a small amount of incremental pressure is needed to close the valve.


The initial high pressure 3000PSI blast from the tank that moves the diaphragm and compresses the spring may not build up a high pressure in the IP chamber.

I think your confusion is in assuming that supply pressure ever reaches the diaphragm or is pushing against it. It doesn't. As I mentioned last night when the Spurs were methodically dismembering the miami heat the HP air encounters the seat/orifice valve prior to the IP chamber. Although it's a little more tricky to visualize the air movement in a diaphragm regulator, the principle is the same; when pressure beyond the seat builds to IP (135 PSI more-or-less) it closes the seat/orifice valve, not allowing any more pressure from the tank to enter the IP chamber. That's basically a bumper sticker explanation of the job 1st stages do. The supply pressure pushes on the closed seat, not the diaphragm beyond. That's where it influences IP, but since the exposed surface of the seat is small, the amount of supply pressure assisting the valve-closure (and thus lowering the IP necessary to keep it closed) is small.

Since we're talking basketball, maybe a timely illustrative description will help.

Think of the supply air as Lebron James blasting out of the tank wanting to wreak havoc on anything in his path. Unfortunately (for him) he soon encounters the HP seat, aka Kawhi Leonard. A little of Lebron gets past the seat, but some pre-determined point (set by the designer/technician, in this case Gregg Popovich) that seat shuts and most of Lebron is stuck in the HP chamber where he is peppered with questions from the media.

Now, at some point the seat (Kawhi) is going to wear out and the ferocious supply air (Lebron) is going to start leaking past, scoring at will and causing the IP (and Popovich's blood pressure) to spike. That's when a new soft seat in the persona of Boris Diaw is installed and all is well with the universe again....at least until Thursday.

 

[LeadTurn_SD]

Lovely explanation, which will probably remain a little mysterious to many of our friends "across the pond" due to the basketball references... Perhaps we'll see a football (soccer) analogy from followers of the World Cup soon,

Best wishes.

 

[ams511]

Lovely explanation, which will probably remain a little mysterious to many of our friends "across the pond" due to the basketball references... Perhaps we'll see a football (soccer) analogy from followers of the World Cup soon,

Best wishes.

He said the same thing last year and in the end the Heat won the championship.