Simple surge tank that has orifice size bigger than pipe size

Question

Hi, 
 I would like to get advice for simple surge tank setup that has orifice size bigger than pipe size, as shown. 
 For example my pipe size is 1.8m, I noticed when I change the orifice size from 1.6m to 3m (to differentiate orifice and simple surge tank), the result is not changing. Is it because HAMMER does not recognize surge tank setup with orifice size bigger than pipe size? What is your recommendation if I want to perform such setup. 
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Scott Kampa · Answer

Hello, 
 I would review the following link for information on the setup for a surge tank: Modeling Reference - Surge Tanks . This includes information on the orifice and how it is applied. 
 It does not really make sense that the orifice size would be larger than the pipe diameter, so there may be an assumption built into the code that will limit the orifice to no larger than the pipe diameter. 
 Regards, 
 Scott

Jesse Dringoli · Answer

To add to Scott's answer - is your "pressure tunnel" circular shaped, or perhaps it is rectangular and thus able to be wide enough so that the surge tank inlet is a larger diameter than the height of the pipe? Otherwise for a circular pipe I agree with Scott that it does not seem physically possible for the inlet diameter to be larger than the pipe diameter. 
 Regardless though, if you are not seeing a different in results from changing the inlet orifice diameter, it could be an indication that the inlet headloss is relatively insignificant. Or, you may have neglected to enter the corresponding Headloss Coefficient. This is mentioned in the surge tank article that Scott linked to.