required nozzle thickness for input nozzle pressure

[PVdesigner]

Hi all
Could anybody please interpret the following error message in compress calcs? How do i get rid of it?
"Required thickness tr for the input nozzle pressure exceeds the component wall thickness" Thanks

 

[vesselfab]

That's the one you get when you specify a neck thickness that is too thin for the design criteria

Generally get rid of it by specifying a thicker neck.

If that don't work, maybe Tom will be in a little later.

 

[PVdesigner]

Thanks Vesselfab

This nozzle is designed for 645 psi plus 17.95psi liquid head and required thickness is only 0.216" including 1/8" corr. allowance per UG-45. But the nozzle being radial on the bottom head, I have chose HB with 600# Flg rating (wall thickness 2.16" )for reinforcement. Still warning appears in report!!!!!!!

 

[vesselfab]

sounds odd

that should work

 

[doct9960]

PVdesigner,

Can you give more information like design conditions, head type & diameter, material? Are you going to use Code Case 2260 in your head design? I presume your nozzle is 8" based on the barrel thickness of the Class 600 HB flange you specified.

Compress flags a deficiency when the required thickness tr for UG-37 reinforcement calculations is greater than the specified vessel wall thickness less corrosion allowance (i.e. tr > t). Check your Nozzle Report if this is the case. The area available in vessel wall A1 is most probably a negative value. I don't know why Compress flags this as a deficiency. Maybe Tom Barsh can clarify.

If you really want to get rid of the deficiency, go to
Action Menu
Set Mode Options
Nozzles 1 Tab

Check the box for "Full nozzle area replacement". Compress will not calculate for tr but will set it equal to t (i.e. tr = t).

Here's the explanation in the Compress Manual about the full area replacement option.

Full nozzle area replacement -- This option causes all area removed to be replaced; in other words excess thickness in the shell will not be credited towards reinforcement of the opening. Internally it sets the required thickness tr from UG-37 equal to the nominal thickness of the vessel wall t when performing nozzle calculations. This option is above and beyond the requirements of the ASME Code.

 

[TomBarsh]

I was out of the office most of last week. To close this issue I will copy and paste a "standard" reply I use when this question comes up (it is "standard" but discusses numerical values for a specific case).

While the discussion below is not the only explanation for this issue it is the most common. Nor is the suggested resolution the only one possible (nor is it possible in all cases).

Discussion:

The Deficiency message for the affected nozzles in your file results from a "perfect storm" of events and does not indicate an error in the calculations.

COMPRESS determines the pressure acting at the elevation where the nozzle penetrates the outside surface of the shell or head. The pressure includes the design pressure plus any static head at that location. To eliminate confusion, the same pressure is applied throughout the entire nozzle analysis (except for an attached flange) for purposes of all applicable nozzle checks, eg: area replacement, UG-45, etc.

Because the pressure used for nozzle design is taken at the outside surface of the shell it will always exceed the pressure at the inside surface of the shell. This contrasts with the MAWP (or MAP) of the shell components being based on the pressure acting on the inside surface of the shell.

In some cases the design of nozzles for the chamber MAWP (or MAP) is logically impossible due to a contradiction: the MAWP is based on the inside surface of the vessel, whereas the nozzle design is to be based on pressure at the outside surface of the vessel. The contradiction will occur when the component to which the nozzle is attached governs the chamber MAWP and yet the nozzle itself is to be designed for the chamber MAWP. In this case the required thickness "t" of the nozzle's parent shell component under the effect of the MAWP plus the small additional static pressure due to operating liquid acting over the thickness of the shell component will exceed the actual thickness of the shell component on which the chamber MAWP is based. Thus the "logical impossibility" of achieving this condition.

This problem may exist when all of the following conditions exist simultaneously (the "perfect storm"): (1) the nozzle design mode is set to "Chamber MAWP" or "Larger of MAWP or MAP", (2) there is a liquid level acting on the nozzle, and (3) the chamber MAWP (or MAP) is governed by the cylindrical shell or head to which the nozzle is attached. Under these conditions the thickness of the component required to support the nozzle design pressure (which is the chamber MAWP/MAP plus static head to outside surface of the vessel) will be greater than the actual thickness of the component (on which the vessel MAWP/MAP is based, as based on inside surface). In such a case the nozzle cannot be designed for the chamber MAWP/MAP and a different nozzle design mode should be selected.

For example, for your vessel the nozzle mode selected is "Chamber MAWP". The chamber governing value is 645.01 psi based on the bottom head. Nozzles "O3A" and "O3B" are located in the bottom head and must be designed for this pressure as well. COMPRESS also includes a portion of the liquid level acting on the nozzle, this results with the required thickness "tr" being slightly more than the actual head thickness.

Nozzles located on heads often benefit from the smaller value given in the definition of "tr" in UG-37(a). This benefit occurs only when the nozzle and all reinforcing is located within the "dished radius" defined in that paragraph. For example, nozzle O1 does not suffer this problem. The affected nozzles on the bottom head do not meet this condition. However, these nozzles have more than adequate reinforcing area and you could resolve the deficiency by specifying a user-defined limit of reinforcement that would bring all the reinforcing area to within the dished radius. I was able to resolve the problem for nozzle O3B by entering a user-defined limit of reinforcement of 6.5"; this was sufficient to bring all the reinforcing area within the dished radius, yet still provides sufficient reinforcing area.


Tom Barsh
Codeware Technical Support

 

[Bobfromoh]

Tom,
So how do you find the pressure at the outside surface?
Do you just use the code equations for the OD and solve for the pressure?

 

[TomBarsh]

The hydrostatic head at the outside surface is calculated from "statics": pressure = (density) * (depth of liquid).

The pressure to the bottom elevation (inside surface) of the component is shown in the COMPRESS report. The additional pressure due to static head existing at the outside surface will be: (density) * (shell thickness). In most cases this will be a small numerical value.

By the way, the density of the liquid will be found as the product of the specific gravity (entered by the user) and the density of water, which COMPRESS takes to be 0.03609710 lb/in^3.


Tom Barsh
Codeware Technical Support