Dear all,

our client spec says maximum blast load on the pipe system is 0.5 barg over pressure for process area. Drag force on the piping shall be calculated according to : Fb = DLFxCdxpxD,
where,
DLF = dynamic load factor (1.5)
Fb = blast load (N/m)
Cd = 1.0 drag coefficient, reflect the shape and Reynold's number (refer DNV-RP-D101)
p = Dynamic pressure (N/m2)
D = outer diameter of pipe including any insulation (m)
and,
The calculated blast stresses are the sum of the long.stresses due to pressure,weight blast pressure and other sustained loading. using wind speed for given blast pressure is:
v = sqrt((DLFxCdxp)/(0.5xCxƿ))
where,
V = wind velocity (m/s)
C = shape coefficient,0.6
ƿ = 1.225(kg/m3), the mass density of air.

My discussion is, how we considered the above one in blast analysis.

At present, we proceed with wind pressure vs elevation method. we considered as over pressure is 0.5 barg at El of 10m to corresponding elevation. is it correct.

If we follow the above condition, stresses will be fail in TEE connection. how we get the stresses are within allowable limit. someone guide me please....we are struggling to get the pass in stress particular in tee connection.

Drag pressure is drag pressure and overpressure is overpressure. please do not mix.
Then, in your velocity formula you have two time a shape coefficient - can not be.

What SIF do you use or what allowable do you use?
t is a l
Last but not least: 0.5barg is 5 tons/sqm. What do you expect?

hai danb,

thanks,
Two time a shape coefficient - can not be means formula may be wrong...

In velocity formula, v = sqrt((DLFxCdxp)/(0.5xCxƿ))
Cd = 1.0 drag coefficient, reflect the shape and Reynold's number (refer DNV-RP-D101)
C = the shape / drag coefficient, 0.6.

For SIF,we use default value of CAESAR. Allowable blast stress = 2.4xs or 1.5 sy. In tee connection, we selected as welding type.

For blast condition shall we use liberal stress allowable.

Darg pressure means dynamic pressuer right...

In Fluid Mechanics and in modern codes there is a difference between the "pressure coefficient" and the "force coefficient" (or "drag coefficient").

A pressure coefficient is related to pressure distribution over the cylindrical surface and depends on Reynolds number, surface shape and point location on the surface considered.
Some codes (ASCE for example) are giving you "pressure coefficients" in particular cases.

For engineering point of view is an advantage to work with an unique (equivalent) force coefficient related to the projection of cylindrical area, instead to consider the variable pressure around the surface. This is the force coefficient or drag coefficient. Because it is obtained as a mean value by an "integration", it depends on Reynolds number, surface shape but not to "point location". For me this coefficient is also the "shape factor".

Now you consider two drag coefficients, one from DNV and other for some books/ references related to wind. What is their physics meaning versus blast phenomenon? If you are looking only to a math equivalence, you may consider the shape coefficient as 1 and calculate the equivalent wind velocity or you may consider the shape coefficient as 0.6 and calculate the equivalent wind velocity. In the end it is important to obtain the same equivalent force- do you really need to define "velocity" instead to consider directly "pressure"?

Mariog,

As usual, cristal clear!

Best Regards

Dear Shaber,

You can use two methods:
1. Blast as uniform load (N/mm).
It is quite difficult in inputting. Why? Because you have to put load 0 N/mm at elements which parallel with the load. You will do too many ticking.

2. Blast as wind pressure - air speed
(It is easier in inputting). I would say this not wind pressure vs elevation but Wind speed (converted from dynamic drag pressure) vs radius from blast source. Ask safety for it. If they give overpressure then we need its relation to dynamic drag pressure (don't mix both of it).

Where: 1/2*ρ*v^2 = dynamic drag pressure

Hi,

Does anybody know how to apply two different Blast pressure in a single system. (pipe pass through different units with different blast pressures)

I tried to consider 2 different wind shape factors for system. But it doesn't work.
(For checking I multiplied my shape factor by 0.5 and entered as input. The results are different from when you have the original shape factor and half of Blast pressure. Although the value of equivalent wind force as per formula Fb = DLFxCdxpxD remains equal for both cases, but results are different. In the other word this force is not directly applied to elements)

Omid .....

Are you modelling two simultaneous blasts of differnet magnitudes in different areas ?

Maybe you need to do two separate calcs for the blast loads in two independent areas ?

How did you enter your blast pressure elevation profile?