design query

Question

Hi, 
 I am working on surge project in Bentley Hammer, 
 1) I have proposed 17 Nos double acting air valve (1.7" diameter and outlet orifice size as 0.08) on the transmission mains (18" diameter and length of 20000 ft) on all peak points. It showed vacuum (-14 psi) in the pipe line. 
 I revised double acting outlet orifice size from 0.08 to 0.005 (as negligible). The results did not show vacuum. 
 Could anyone of you advise why this happened ? 
 2) In one of the transient analysis , i observed maximum negative pressures @ C= 100 instead of C = 140 (C, coefficient of friction for DI pipes).what could be the reason for this. 
 Kindly advise how to resolve. 
 Thanks, 
 Krishna

Scott Kampa · Accepted Answer

Hello Nagendra, 
 I'm not seeing vapor forming with either size for the air valves. There are air pockets forming at some air valves, which is expected when the pressure drops below zero (not that air pockets will form at any negative pressure, not just vapor pressure). See this link for more information on modeling air valves in HAMMER. 
 Regarding the pipe roughness, as Sushma mentioned, changing the roughness coefficient could change the results. The HAMMER transient solver uses the Darcy-Weisbach friction factor, f, for the frictional losses in a model. In pipes with non-zero initial flow, the Darcy Weisbach coefficient is calculated based on the headloss across the pipe. So if adjusting the C value changes the headloss in the pipe, this will mean the friction factor changes too, thus leading to different results. This link has information on the friction factor and how it is calculated. 
 I would recommend starting your modeling with a calibrated model. This will assure that accurate headlosses are calculated in the initial conditions. This will assure accurate an accurate friction factor and better overall results. 
 Regards, 
 Scott

Sushma Choure · Answer

Hello Krishna, 
 Can you please upload model files for our review? 
 Sharing model files 
 About different pressure results for different C values, pressure in pipes is directly proportional to the roughness of pipes, so it is obvious to see different results in pressure at different roughness values of pipes. I did some testing by changing C values in no protection scenario and pressure in less for low values of C - roughness coefficient.

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