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thought2007 (Aerospace) (OP)
29 Sep 09 11:29
Hi,

how much seating force required to control leakage for following requirements.

Allowable leakage when closed condition is 0.5cc/min.
The seating material used is Teflon
Operating pressure is 60psig
Differential pressure when closed condition is 70psid

Earlier i were trying with yeild stress value of Teflon for seating load(250psi)168lbf force,however one of articale from emerson defines 40lbf is enough to control 0.5cc/min at 50psig.More the force more the actuation torque required to actuate the ball.

would appreciate your valuable comments

Thanks
John


 
bcd (Mechanical)
29 Sep 09 12:08
First, focus on contact stress.  In a perfect world, if the contact stress equals the pressure trying to seal, then you get a seal.  The world is not perfect, not perfectly round, surfaces are not perfectly smooth, so you need a little more contact stress than the pressure you are trying to seal.  Since ptfe is resilient, 25 to 50% more contact stress will suffice in most cases.

Now you know how much contact stress you need.  Now figure out the contact area, which will allow you to calculate the required force.

The leakage rate you mention is a value selected to account for variations in the contact surfaces.  It is not be part of your analysis on determining how much force you need to get a seal.  Without in depth knowledge of the range of ball roundness, surface finish variability on the ball, and ball seat and dimensional tolerances, you will never be able to get a consistent leak rate of 0.5 cc/min for a given force.  Way to many variables that are outside your control.  You need to shoot for tight shut-off to ensure you are always under the allowable.

Ultimately, you will have to test the final result to validate your calculations.
JimCasey (Mechanical)
30 Sep 09 12:07
If you actually test comparable ball valves from 50 different vendors, you'll get 50 different results.  
Seat material, seat profile, seat preload, surface finish of the ball are >some< of the variables that affect this. USUALLY there is some preload built in to the assembly stackup so that the valve seals at zero differential when new.  Virgin PTFE seats are bad to cold-flow so they may seal when new, but if you take the valve up to pressure and then relax the pressure, the valve may leak at low differntials.    
thought2007 (Aerospace) (OP)
23 Oct 09 8:25
bcd & JimCasey

Thanks for your valuable posts.
according to bcd,working on contact stress requirement using following empirical formula

fc=Sqrt of (P*E/2.88*As)

where

P= Radial load
E=Youngs modulus
As = Contact surface area

Once  i finish will get back to you

Thanks once again

John

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