2 identical parallel centrifugal pumps ??? hello everyone i have a very confusing problem with a cold water system sonsists of 2 identical centrifugal pumps their position is under the tank so there is always a +ve suction head then those pumps takes water from the tank and deliver it to a very simple pipes network to acheive the required processes with the water such as cooling gases via heat exchangers and this water is used also as feed water to some other water systems but to make cooling """other words """"""""" closed circuit so the problem now is that some times when i operate one pump i found out that there is no water at all in the circuit althoug i can see the vortex of suction in the tank also all of the heat exchangers are okay no mixing i dont know where does the water go ??? and when i work both of them everything is okay but each pump is 100 hp and 196 m3/hr and has a head of more than 100 mt so can this be an impeller problem of the pump ??? or whats the problem i dont know ??????