How to Reduce the Nozzle Loads in START-PROF

Read about START-PROF pipe stress analysis software

If the equipment load is greater than the allowable values as specified above, first try to understand the reason of the increased load. The increased load may be caused by:

• Force-controlled loads: Weight loads, Pressure thrust forces, etc. Can be obtained if operating temperature is equal to ambient temperature. Weight multiplier should be 1.0
• Displacement-controlled loads: Thermal expansion, Support displacement, etc. Can be obtained if weight multiplier in operation mode editor is zero.

If high loads are caused by force-controlled loads then:

• If the load is due to the weight of the piping system, provide additional vertical supports, variable spring hangers and supports
• If the load is due to the expansion joint thrust forces then piping design is wrong, remove the not pressure balanced expansion joints from the model or add anchors to resist the thrust forces

If high loads are caused by thermal expansions then try to apply any of the following methods to reduce the nozzle loads:

• Try to reduce the nozzle load by adding flexibility to the piping system

• Add more bends as bends are more flexible than the straight pipe
• Reduce the pipe and bend wall thickness if possible. The more thin pipe you use, the more flexible piping system you have
• Reduce the diameter if possible. The smaller diameter you use, the more flexible piping system you have
• Add the nozzle flexibility using WRC 297/PD 5500, API 650 or Nozzle FEM
• Add the pipe support and support structure flexibility using custom restraint with flexibilities

Let's show it on the simple example of the straight pipe between two anchors.

Without considering the anchor flexibility the axial force must be calculated by the equation:

If we consider right nozzle flexibility λ, then the equation is

For example, pipe 219x6 mm, l= 6m, dT=100 degrees, α=0.000012, E=2000000 kg/cm2, A=40.1 cm2 we have

kgf

if we consider the nozzle flexibility λ=0.14 mm/tf (very stiff nozzle) then the nozzle load reduces for 3 times!

kgf

Here are the complete equations to determine support loads, internal forces, stresses for restrained, unrestrained, and restrained pipe with flexibility

• Add gimbal or lateral expansion joints, or pressure-balanced axial expansion joints

Using of the unbalanced expansion joints should be careful. The thrust force may transfer the huge axial load on the equipment

Here are the possible correct and wrong variants of pump piping with unbalanced expansion joint

• Reduce the temperature difference. Reduce the operation temperature or raise the Ambient temperature. Remove unnecessary reserve
• Try to direct the thermal expansion away from the equipment (for example to the U-shaped loop) by providing proper restraints (guide or directional anchors). Also don't forget about the huge friction forces in such anchors, that may raise the big axial force in 1-2 pipe.

• If the load is high because of friction then try to use Teflon/graphite/polished stainless steel plates to reduce friction forces
• Hot modulus of elasticity can be used to calculate equipment nozzle loads (option "Use Eh for support loads" in Project Settings)
• Cold spring can be applied if no other way. But note, that cold spring don't reduce the expansion stress range
• Move the equipment to the half of the piping thermal expansion value from installation to operating mode

Even after all trial and error if it is not possible to reduce the loads within allowable limits then forward the actual load values (increased by at least 20% if all piping data is not final) to vendor for FEA analysis and their acceptance.