Natural Frequency for Vessels - MOSS

[LaylaZ]

Dear Engineers,
For those of you who are familiar with Dennis Moss' Pressure Vessel Design Manual, for calculation procedure 3-9, exactly in page 161 where he shows the method to calcualte the Natural Period of Vibration T = (H/100)^2*SQRT(sigma (w*Delta Alpha + W*Beta/H)/sigma (E*(D/10)^3*t*Delta Gama)) for stacks. Since this is an emperical formula, can anyone please tell me what units go with what.
For example do we use D in feet and thickness t also in feet, although the author expresses it in inches in the notation part , page 157. In addition, say E = 28,600 ksi, what value would I put in the formula? putting simply 28.6 seems to be working but i have no basis for it, P.O.V. just seems to be right when using 28.6.

Thank you very much guys.

Layla Zimmerman,
EIT.

 

[dig1]

Layla,

You have to be careful w/ your units.

If you are using kips for your weights then 28,6 is the correct number because of the 10^3 in the denominator of the next term. If you are using pounds then 28 600 would be the correct value.

Be careful, because some other references do not use the 10^3 term so E would be 28 600 ksi or 28,6E6 depending on whether you are using kips or pounds.

t is in inches but D and H are in feet.

The coefficients do not make it easy because none of the "dimensioned" terms have seconds in them but you end up with seconds.

In the cases I've used this method and compared against more analytical methods the results have been fairly equivalent.

Hope this helps some.