I think he’s saying that it’s 15lb rated pressure when there is 15lbs on one side and normal atmospheric pressure on the other side (14.7lbs/sq inch). If the cap has 22 lbs of pressure on the “atmosphere side” instead of the normal 14.7 then it would take 15lbs plus 7lbs (the difference between 14.7 and 22) or a total of 22lbs to open it.
I think the original math was off a little because it didn’t subtract the expected atmospheric pressure from the cap rating. In other words I don’t think the pressure rating is additive.
Joe Hull
Cozy Mk-IV #991 (preping for DAR inspection - details, details)
Redmond (Seattle), Washington
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Echo Lake Fishing Resort (Georges Boucher) Sent: Thursday, December 15, 2005 9:53 AM To: Rotary motors in aircraft Subject: [FlyRotary] Re: Radiator Expansion Tank
Hi! Todd
I'm not getting it, what stops the 15lb cap relief valve from opening at its rated pressure? Even if it dumps into the 22lb reservoir the pressure still can't be greater than 22lbs. What am I missing?
Georges B.
-------Original Message-------
From: Todd Bartrim
Date: 12/15/05 05:50:21
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Radiator Expansion Tank
Todd
Can you explain how using two pressure caps (one 22lb & one 15 lb) add up to a possible total system pressure of 37lbs. I can only see it as a max of 22lbs,
Hi Georges;
The rad caps are differential meaning that they will hold back their rated pressure above the pressure seen at the outlet. So if the the first cap produces a 15 psi increase over atmospheric pressure, then the second cap which has it's outlet plumbed to the 15 psi reservoir will further increase this by it's rated pressure (22 psi in this case) to give a total of 37 psi.
At least that's the way I see it.