The mathematical models were established for parallel pumps, outlet valves and pneumatic tank in a make-up water system to study hydraulic transients after electricity failure. The calculated results showed that the negative pressure and the water column separation in local salience could be avoided when the pneumatic tank was mounted near pumps for the raising of minimal water hammer pressure on long pipeline. The total volume and initial capacity of pneumatic bag were determined by calculations. Since the backflows generated in pumps ahead of time obviously the closing laws of outlet valves were needed to adjust correspondingly.
