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Discharge to Atmosphere Elements - Rating Curve

 I have a query about how Hammer deals with the Rating Curve option.  In the Hammer Help file, the following information is all that is provided:

  • Rating Curve - releases water from the system to atmosphere based on a customizable rating curve relating head and flow. Below a certain value of head, the discharge is zero; in stage-discharge relations, head is equivalent to level for which the discharge increases with increasing level.

It seems to me that this element is dealing with how the complete pumping/piping system behaves and therefore is making a real pump curve redundant for the analysis?  That would explain why discharge is zero below a certain head - this is just the same as the pump shut-off head.  Maybe I've got that wrong, but the explanation in the Hammer Help file is quite brief and vague.

However, there is another angle on having a rating curve for the discharge element, which does not appear to be covered by Hammer.  If the discharge manhole is being used as a balancing storage (i.e., the downstream gravity main is too small for the peak pumping flow), the water level rises in the discharge manhole, and there is a small but potentially significant  back pressure that would vary with flow rate through the rising main.  That influences the duty point of the pump and therefore the magnitude of waterhammer.

Please comment.

 

 

 

A sewerage rising (forced/pumped) main is discharging to a sealed gravity main.  So the end is open to atmosphere, but there is a small backp

  • Frank,

    The rating curve element is not intended to replace a real pump curve. The intended behavior is more like the second point you mentioned - it allows you to specify a rating curve that defines how flow leaves the modelled pipeline. The rating curve element could be used, for example, in place of the orifice-type discharge to atmosphere in the case where the discharge didn't follow a typical orifice-equation relationship.

     

    Regards,

     

    Mal Sharkey

    Product Manager
    Bentley

          

  • When the downstream manhole is flooded, it acts as a tank and should be modeled as such.