Nozzle Loading
Nozzle loads are the net forces and moments exerted on equipment nozzles. The loads exerted are predominantly affected by how the connected equipment and piping are supported, misalignment in the system, and thermal expansion of piping material.
Figure 1 shows the forces and moments acting on the inlet and discharge ports of a pump at any given time. Should any of these forces combine to give an excessive resultant force, damage will occur to the pump system.
Pump nozzles may not always be subject to the maximum allowable resultant force and resultant moment simultaneously. Thus an increase in either the resultant applied force or the resultant applied moment may be permitted. Expecting zero resultant forces and moments is unrealistic. Loading can be minimised with good system design but obtaining zero system loads is impractical.
Issues
A cause of excessive nozzle loading is thermal expansion. This is the change in the length of a particular metal as a result of the change in temperature of that metal. Hot fluid systems can cause pipe-work to expand with temperature, the expansion being equal to the coefficient of thermal expansion of the piping material used. The expansion intensifies nozzle loading along all three axes. It is therefore vital to select appropriate piping material when designing a pumping system.
Nozzle loads can create torsional stresses across pipe-work, pumps and supports in a pumping system. In external gear pumps, torsional stresses often lead to uneven gear wear and in severe cases contacting of the pump housing with gears. For internal gears the same result occurs as the centre housing becomes distorted from the uneven wear.
Even in less severe cases the irregular wear will affect tight clearances designed to cater for shaft deflection, therefore slip occurs and the pump needs to run faster to maintain the required flow. When pump components begin to wear, more power is required to meet the demands of the application, hence associated running costs increase and overall efficiency is lost.
Excessive nozzle loading can also lead to increased vibrations throughout the pumping system. Should the frequency of vibration of a system component reach its natural frequency then resonance will occur. This can have a dramatic effect on residual out of balance forces, exacerbating existing nozzle load issues and potentially leading to catastrophic pump failure.
Misalignment is a further cause of excessive nozzle loading, and this is discussed in detail in our Misalignment Article.
Solutions
To minimise nozzle loading, all pipe-work should be externally supported on both the inlet and discharge sides of the pump. Pipelines must be supported by external means at regular intervals to prevent them from sagging due to their intrinsic self weight and the weight of the fluids inside the pipe. If sagging were to occur then nozzle loading on the pump would rapidly increase. Where possible all pipes should be supported at the same level as the pump so as to reduce the number of bends and potential loading sites in the system.
The first piping support before or after a nozzle should be an adjustable base support or a spring. Both options provide changeable support which can be altered to meet the requirements of the pumping system on site. Springs take the weight load without imposing a reaction load on the piping, and are the preferred option.
When it is necessary to isolate a pump from piping to reduce noise, vibrations or to provide insulation; a flexible link, hose or bellows should be considered. These devices will aid in reducing moments and forces on the inlet condition. The pumping system should be designed with sufficient inherent flexibility to withstand thermal expansion without creating excessive forces at the flanges.
Summary
-
Expecting zero resultant forces and moments is unrealistic. Loading can be minimised with good system design but obtaining zero system loads is impractical.
-
All pipe-work should be externally supported on both the inlet and discharge sides of the pump. Pipelines must be supported by external means at regular intervals to prevent them from sagging due to their intrinsic self weight and the weight of the fluids inside the pipe.
-
Inlet piping should be free from any flow disturbing fittings for a given minimum length to reduce potential pressure spikes.
-
The pumping system should be designed with sufficient inherent flexibility to withstand thermal expansion without creating excessive forces at the flanges.
