Dear Scott Mayeux

I am doing mounded pressure Vessel design for the first time.
Our pressure vessel has foll specs
1. ID 3100 mm
2. Length Tangent to tangent 16000 mm
3. 2:1 Ellipsoidal Head.

During the analysis i found following points
(assuming shell thickness as 14.5mm)
1. for the assumed shell thickness, shell was failing under external pressure.
2.Adding stiffening rings certainly helps & 14.5 mm was found satisfactory for abt 8 rings.
3. But stresses at horn & wear plate tip were vey high.
This could be brought under control by increasing shell thk to say abt 29mm or by providing ring in way of saddle.
I opted for the later option.

But my question is why did the circumferential stresses vanished on providing ring??
Also i am not aware of the assumptions made for ZICK ANALYSIS.Can u pls help me knowing them...

4. Is there any other method to reduce circumferential stresses at horn & wear plate tip apart from incresing shell thickness & providing ring.

5. Also i referred Dennis Moss. The factors K1 to K6 for saddle design/ZICK Analysis used by PVELITE doesnt match with those listed in Dennis Moss.

Regards
maheshpatelgg@yahoo.com

Dear Mahesh,

In large horizontal vessels, a bending stress will be generated at the tip of the wear plate and at the horn of the saddle. Sometimes it is necessary to add a stiffening ring(s) at the saddle to reduce those stresses. When a ring is added, it has to take the load. Consequently, the ring has to be sized adequately to keep the stresses within certain stuctural allowables. This is why the stresses "vanish" in the shell.

You should read the paper "Stresses in Large Horizontal Cylindrical Pressure Vessels on Two Saddle Supports" by Leonard P Zick. It will provide the insight regarding the assumptions surrounding this analysis.

You may also be able to reduce the stress by changing the saddle position or reducing the weight load on the saddles. I realize of course this may not always be possible.

We are quite confident about the saddle factors used in the analysis. PV Elite computes these stress factors using formulas.

Mr. Mahesh

By providing the ring at the location of saddle, Z i.e. section modulus increases and becomes equal to pi*r^2*t (section modulus of whole X-section of shell), which otherwise equal to section modulus of only that portion of shell section corresponding to contact angle of saddle.
As per general expression
Bending Stress = M/Z
where M = Bending Moment
and Z=Section Modulus
Thus as Z increases by providing ring these bending stresses worked out to be vanishes.

Other method to reduce these stresses are
i) increase saddle contact angle but you cann't increase it more than 168 degree.
ii) increase saddle width.

Saurabh Gajwani