Evaluation of Piping Loads on Nozzles
Are load considerations different between vessel nozzles and pump nozzles? In general, yes. For vessels, nozzle loads are generally calculated to determine if reinforcement of the nozzle is required for it to safely accept the loads. For a pump, design of the nozzle for the loads is not the consideration. Rather, it is the net load on the pump that is the primary concern. This net load can cause distortion of the pump that can lead to bearing and seal failures. And the strength of the baseplate connecting the driver and the pump is a key consideration. In their standards for pumps, a prominent refiner requires extra strong base plates so that the pump could carry double the loads permitted by API 610.
With the above in mind, let’s consider pressure and fluid weight loads.
PRESSURE LOADS
The internal pressure times the cross-sectional internal area of the pipe creates a pressure thrust load that results in a longitudinal stress in the pipe. This is a physical load on the nozzle as this thrust is balanced by the pressure acting on the pressure boundary opposite the nozzle in the vessel or pump. From the standpoint of stress in the nozzle, it is a load that needs to be considered. From the viewpoint of loads on the vessel that need to be carried by its foundation, it does not exist because the pressure forces are balanced at the nozzle elevation. Compare this with the pump. Although the force passes through the nozzle, since it is net load on the equipment that is the consideration (verses a vessel in which it is the load on the nozzle that requires consideration in nozzle design), the pressure thrust force is irrelevant since it does not create a net load on the pump; the pressure forces are balanced.
Then think about what happens if you put a metallic bellows expansion joint in the pipe with no tie rods or other means for it to carry pressure forces across it. In the case with no expansion joint, the pressure thrust force on the nozzle was the pressure times the internal area of the nozzle and in the case of positive pressure, is a tensile force on the nozzle. With an expansion joint, there is a pressure thrust of the bellows effective area times pressure, and the bellows effective area is the area within the mean diameter of the convolutions. This creates a compressive force on the nozzle with positive pressure. But that is actually the net force on the equipment. The force on the nozzle is pressure times the difference between the bellows effective area and the inside area of the nozzle. The remainder of the pressure thrust force is acting on the pressure boundary opposite the nozzle. So from the standpoint of nozzle stress, it is this force, but from the standpoint of designing equipment foundations, it is the total pressure thrust from the bellows that acts on it, and is required to be considered.
Further information on pressure thrust can be found in a recent blog “What is Bellows Pressure Thrust?”
From the standpoint of a pump, introducing an unrestrained expansion joint generally reduces and reverses the load on the nozzle, but can impose a very large net load on the equipment, and that is what needs to be considered with respect to pump nozzle loads.
FLUID WEIGHT LOADS
Consider now the weight of the fluid itself, with a vertical line exiting a pump nozzle. And the design of a spring at the top of that run to reduce loads on the pump. The weight of the fluid does not create a load on the pump nozzle, as it is carried by the pressure boundary opposite the pump nozzle. However, it does create a net load on the pump itself which is the essential consideration. As such the weight of the fluid in that vertical run should be included in the design of the spring, because it is creating a load on the pump, and the purpose of the spring is to bring the net load on the pump within allowable limits.