in college days it is known that if you want double pressure install the pumps in series if you need double flow install the pumps parallel. but in actual field i have two pumps installed in parallel in Sewer Pump Station one as on duty and the second one is assist. but while both pumps running, i never get flow rate equal to two pumps flow rate for example i have two pumps with each pump capacity 100 l/s but if both if them runing i have only 120 l/s i cant understand this ? and how to control raising main diameter to get maximam flow while both pumps are running ? it is really difficult ?