That is not quite correct. You are confusing the area of the seating surface with the area based on the valve diameter. Let's say you have a poppet valve 1.00 inch in diameter at the seat (neglect the width of the seat for a moment. This yields an area of 0.785 sq. in. This is the effective area of the valve. With the valve closed, the force on the valve due to pressure is F= P*A where A= 0.785.
It could be argued whether the effective area is calculated using the inner or outer diameter of the seat. A "perfect" valve and seat contact would suggest the effective area be based on the inner seat diameter. Since nothing is "perfect", the effective area would be based more toward the outer seat diameter.
Of course the width of the seat does affect the unit pressure of the sealing surface (This is what I am talking about - K2) and narrow seats will have higher conformity and thus better sealing potential. - Correct.
And here's where I disagree, being pretty sure I am not confused. The force applied to the
sealing surface, divided by the
Area of the contact face sealing surface becomes the "EFFECTIVE PRESSURE WITHIN THE SEALING SURFACE". This must exceed the fluid pressure - otherwise molecules of fluid will pass the seal. Initially, the pressure on a poppet valve is only derived from the spring pressure, but this does increase as the fluid pressure on the "whole effective area" acts on the valve, as you explain, but unless the initial seal is formed by the spring force and valve-face to seat-face geometry, the valve will leak. Having designed equipment that had to be "hermetically sealed" and having re-designed elastomeric seals that initially proved inadequate, I had to "learn the rules" - professionally.
I apologise if my explanation was mis-leading. I was trying to explain why larger width of
lapped surface of the valve seats always leads to leakage of the valve. Oft misunderstood by seemingly "clever mechanics" who think a wider seat (lapped surface) will seal better (I have met many!). There is also a correct comment about conical misalignment - which is a root cause of leakage when valve stems and valve guides wear.
As you explained
Green twin is correct : "
"the key for the "tapping with hammer" thing to work is that the valve and seat are very close to being exactly round and concentric.
A valve or seat that is not concentric would only seal in the position where it was tapped with the hammer, and if/when the valve rotated, it would no longer seal. " - which is why I do not advocate "hitting" poppet valves in any circumstances. Any mis-alignment is likely to lead to distortion of the relationship of valve head to stem alignment. This means that when the valve turns (as it will) the valve will then naturally become mis-aligned and of course will then leak.
Of course, there are always 2 limits in design, Max and Min. We have considered "minimum seat pressure" - ergo "maximum seat area" but the materials and temperatures involved also affect the "minimum seat area", as a seat that is "too narrow" (in the case of both the poppet valve and the pressure vessel safety relief valve) will naturally deform as the yield point of the "weakest" material is exceeded, to become large enough to withstand the maximum forces on the sealing surface (forces derived from the closure spring plus pressure differential upon the valve head). (This is clearly seen when a valve acts upon an elastomeric seal in compression - the elastomer can take up a set due to material creepage with time and pressure. I saw this on equipment after many years of service where elastomeric face-seals were well distorted to the point of leaking, as a result of compression set. But that was a complex problem that needed re-designing.).
Enough said. (Most models are made, and happen to work without the engineering needed for "sustainable and profitable industry").
K2
