One of the major difficulty piping stress engineers face while analyzing any piping system is to keep piping side loads or external loads (forces and moments are combinedly mentioned as loads) on equipment nozzle connection within allowable limits. All equipments to which piping system is connected is categorized in two groups. a) Static Equipments and b) Rotary Equipments.
Nozzle Loads for Static Equipments:
Most of the prevailing EPC organizations follow a project specific table as allowable values for static equipments (made of Steel, ferrous material) like pressure vessels (Columns, Horizontal vessels, Drums, Reactors, Filters, Scrubbers, sometimes Tanks which are not within API 650 scope, etc) and shell and tube heat exchangers or similar equipments. The table is generated based on the following two parameter: the nozzle diameter and flange rating. In some organizations the table value is multiplied with some factors (normally 0.75) while checking nozzle loads for shell and tube heat exchangers.
Normally mechanical department send these load tables to the equipment manufacturer indicating that the nozzle connections must be designed to resist at a minimum the table values. The equipment vendor reproduces the values in the equipment general arrangement drawing to avoid any confusion at a later stage.
For cases while the static equipment does not fall on the types mentioned in the above criteria the nozzle loads has to be obtained from equipment vendor or from some ASME B 31.3 code specified standards. Few of such type of equipment and nozzle connection is listed below for your reference:
- Jacketed nozzles connected to Normal pressure vessels: Loads to be obtained from manufacturer, in case the piping side load is more than allowables the loads has to be forwarded to vendor for FEA/vendor acceptance.
- Jacketed nozzles connected to Jacketed pressure vessels: Loads to be obtained from manufacturer, in case the piping side load is more than allowables the loads has to be forwarded to vendor for FEA/vendor acceptance.
- Pressure vessels made of non ferrous (Aluminium is more common) materials: Loads to be obtained from equipment vendor.
- Nozzles connected to Air Fin Fan Cooler: Loads are mentioned in API 661, discuss with vendor (check internal project specification) if any factor is to be used (Normally a factor of 2 or 3 is used in some organization).
- Nozzles connected to Plate Fin Heat Exchanger: Refer API 662 for nozzle loads (There are 2 tables in the standard depending on fluid service (normal service and severe service), check carefully which table to be used)
- Tangential nozzles connected to Pressure Vessels: Loads have to be taken from manufacturer.
- Nozzles whose axis is not perpendicular to Vessel axis: Obtain allowable loads from vendor.
- Nozzles connected to API Tanks with diameter more than 36 meter: Refer API 650 for nozzle loads (No standard table is provided for load values, you have to determine the loads from equations.)
- Nozzles connected to Fired Heaters: Refer API 560 for allowable nozzle loads. Sometimes a factor of 2 or 3 is used for multiplying the table values. Refer project specification for the same or discuss with vendor.
- Nozzles connected to Miscellaneous Equipments (Cold Box, Flaker system, Packaged items, Spherical Equipments, Cooling Tower etc): Arrange limiting loads from Vendor.
Nozzle Loads for Rotary Equipments:
Normally rotary equipments are designed based on some code specified standards and accordingly the limiting loads has to be taken from respective standards. Few of such commonly used equipments are mentioned below:
- Centrifugal Pumps: For pumps which are designed based on API standard, allowable loads has to be taken from API 610 (If loads are more than allowable values as specified in table 5 of the standard, perform appendix P). Allowable load values upto nozzle size 16 inch is provided in the table. For higher sizes ANSI standard is used. If the pumps are not designed as per API standard (now a days non API pumps are most frequently used due to its lower costs) obtain loads from vendor. Sometimes ANSI/HI 9.6.2 is used for nozzle loads in absence of data.
- Positive displacement (Screw pumps, gear pumps etc) pumps: Use API 676 for allowable nozzle loads. Loads can be taken from vendor.
- Reciprocating Pumps/Compressors: Obtain the allowable nozzle loads from vendor.
- Centrifugal Compressors: Use API 617 for equipment nozzle loads. Note that combined analysis must be performed for proper functioning of the compressor. Sometimes vendor permits more loads so discuss with them.
- Steam Turbine: Refer NEMA SM 23 or API 612 for allowable nozzle loads. Dont forget to perform combined nozzle load checking. Sometimes vendor permits more loads so discuss with them.
- Positive displacement compressors: Refer API 619 or manufacturer allowable loads.
- Gas Turbine: Loads to be obtained from manufacturer.
Means for Reducing Nozzle Loads:
Now if the nozzle load on equipment is found to be more than the allowable values as specified above, first try to get a feel of the reason of the increased load and then try to apply any of the following alternatives to reduce the nozzle loads:
- Try to reduce the nozzle load by adding additional flexibility in the piping system (Could be followed if the load is arising because of less flexibility)
- If the load is due to the weight of the piping system, provide additional support.
- Try to direct the thermal expansion away from the equipment by providing proper restraints (guide or directional anchors).
- If the load is more because of friction then try to use PTFE/graphite/Mirror polished SS plates to reduce frictional loads.
- In extreme situation expansion joint or cold spring (normally not preferred) can be applied.
- Sometimes hot modulus of elasticity can be used to calculate equipment nozzle loads.
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.