On a washer job we had a Gusher Vortex Pump back where the prime level was right at the high level alarm, not by design but because required water levels were not maintained. So we had a situation where NPSHa was scary close to NPSHr. No kidding, if the weather was cloudy and the barometric pressure went down slightly, the pump would lose its prime. Sunny day; no problem. Of course, we did not know this right out of the gate.
Instead, if the pump did not prime, we would over flow the washer cabinet onto the plant floor and get a frantic call from our customer. We were baffled and even had our Gusher EVP drive 4- hours to the site. Of course, all went well when he was there so we had to endure hearing about his wasted day.
At lunch we discussed pump modifications. We decided to alleviate all entrained air and lower NPSHr by increasing the inlet size, installing a large inducer in the intake and reducing the vane height on the back side of the impeller. But then the skies clouded up over lunch and we got another call from the customer, “Come back over. We have problems again.” No kidding, the prime level was so close that any change in weather - NPSHa - would either cause this pump to prime or not prime, a lesson we all learned the hard way.