What are precaution while using Union Purchase?

The following criteria must be complied with at all times:

  • The minimum operating angle of either derrick should be not less than 150 to the horizontal, and it is recommended that the angle be not less than 300.
  • The maximum included angle between the cargo runners must not exceed 1200.
  • The derrick should be topped as high as practicable consistent with safe working.
  • The cargo sling should be kept as short as possible so as to clear the bulwarks without the angle between runners exceeding the safe limits (90-120).
  • The derricks should not be rigged further apart than is absolute necessary.
  • The tensions should taken off the slewing guys when the rig is set up. Only the preventer guys should be taken into account in the calculation for forces in the rig.
  • Code of Safe Working Practices recommends that “where derricks have not been marked with SWL :
    • Loads should not be excess of one-third of the SWL of the derrick
    • Load diagrams for the union purchase rig should be consulted
    • Maximum  SWL of the rig should never exceed the SWL of the cargo runner and  a good safety factor would be to lift a max weight equal to 75% of the runner SWL.
  • Unduly long slings should not be used by stevedores.
  • The standing guy (outboard) of the boom which is over the side should not be placed too far back as this will increase the possibility of the boom “jack-knifing”.
  • The winch operators should wind in pay out in step, otherwise dangerous tensions may develop in the rig.
  • Narrow angles between derricks and outboard guys and between outboard guys and the vertical should be avoided.
  • Inboard derrick guys and preventer should be secured as nearly as possible at angle of 90 degree to the derrick.
  • The standing guy should not be secured to the same eye plate on the spider band to which the slewing guy is secured.
  • Runners should not permitted to rub against hatch coamings or other structural members as it will quickly render them unfit for service.
  • Eyes on shackles lug should mouse with seizing wire to prevent lug from working loose.
  • Heel block secured additionally with light chain or wire so that block does not drop when the load is taken off the runner.


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