design query


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.