Nozzle load Basics

[USP4]

Hello dear members, could you please help providing material, links or explanation about why we need Nozzle loads and how they are generated? I already Googled it, but would it be beneficial if subject matter experts or experienced members can share the details.

 

[Krausen]

Because your pressure design code only addresses the design of your vessel due to internal & external pressure. But in reality, the nozzles of your vessel will also typically have some kind of external nozzle loads applied to them during operation. These external nozzle loads will result in local stresses on both the nozzle neck & vessel shell (and face of flange, if using a flanged nozzle) that need to be evaluated, similar to nozzle loads on a pump for example.

 

[RVAmeche]

Well you could go the route of most equipment providers these days and just claim zero allowable nozzle loads.

Kidding aside, it's needed because as already mentioned, the external piping connecting to the nozzles can impart forces on the vessel which need to be taken into account during the vessel design. If you slap on a 8" nozzle with minimal reinforcement and the piping is actually imparting serious forces, you can be in a bad situation.

If equipment was just designed in a vacuum and never connected to anything, I suppose nozzle loads would not be required.

 

[USP4]

Thank you @RVAmeche and @Krausen for response. Could you be able to provide example of forces that will be applied to nozzle. In uploaded image there is duct (having pressure less that 15 psig inside it). The flange of that is connected to gas pipe (that too have pressure less that 15psig)and its OSBL. What are some of the typical forces that will be acting on Duct flange/nozzle (not seeking precise list of force but some examples).

 

[RVAmeche]

The external forces heavily depend on the piping/duct arrangement, materials, temperatures, etc. Duct is generally more flexible, so if the service can allow a fabric expansion joint or something maybe minimal forces will be imparted. Some companys/vendors have a "standard allowable nozzle load table" that is derived from some conservative conditions that I haven't really seen defined.

Generally vendors do FEA calculations on the nozzle/vessel to determine the allowable loads. These loads representing loads that result in acceptable stresses, deflections, etc with the appropriate safety factors applied.

 

[Snickster]

There will be a possiblity of 3 force loads and 3 moment loads on any nozzle Fx Fy Fz Mx My Mz using a three dimensional coordinate system at the nozzle to vessel connection. The forces will be developed by weight and thermal expansion of pipe pushing against the nozzle. Also there is a pressure thrust force of the internal pressure pushing on the elbow at the pipe directed away from the vessel.

For instance in your diagram:

Since the pipe is coming to the nozzle from right to left, if there is appreciable thermal expansion of the horizontal run then this would create a horizontal force on the elbow to the left at the elbow position. This would create a horizontal shear force on the vessel at the connection point of the nozzle, say Fx if the pipe is orientated along the x-axis. Since this force is at a distance from the nozzle connection then this will also create a moment on the nozzle connection of Mz consdering that the z-axis is in the horizontal and perpendicular to the x-axis. The weight of the pipe itself will put a force Fy down on the nozzle centerline considering that the y-axis is vertical. Furthermore considering pressure thrust which is the force cause by internal pressure P x A at the elbow this will produce a force Fy directed away from the nozzle at the connection point. Since the pipe assumingly makes more turns downstream then there will also be forces and moments developed in other directions at the nozzle connection due to that piping also thermally expanding.

These forces and moments at the nozzle to vessel connection procuces local stresses in the vessel and nozzle wall at the connection, above that which is produced by internal pressure which must be evaluated. This can be done by FEA or a more common method is using methods described in WRC 107 and WRC 297. Allowable stresses are provided in ASME B&PV Code or other industry standards and practices.

 

[JStephen]

Generally, these nozzles loads are going to come up when there is some sort of stress analysis in the attached piping to either generate required nozzle loads, or to assume limited nozzle strength not to be exceeded in the piping design.
BUT, this is not uniformly done in all fields. For example, municipal water piping is never analyzed in this way, and so tanks and vessels connected to that piping normally don't have specified nozzle loads. API-650 includes limited provisions for nozzle loading, but the great majority of API-650 tanks do not require nozzle load evaluations; ditto for most oilfield tanks. So don't just automatically assume that this load analysis is something that ought to be done in every case.

 

[Snickster]

Correct JStephen the cases I did were all on ASME pressure vessels in the petrochemical, chemical, refining, and oil and gas industries. Also on API 650 tanks in the same industries. Not that it is not a real thing in other industry vessels and tanks but usually you are not dealing with high pressure systems.