When throttling back on the discharge valve of one of two centrifugal condenser water pumps running in parallel, what is the most likely effect?

When throttling back on the discharge valve of one centrifugal pump operating in parallel with another, the flow rate to the condenser is likely to decrease. This occurs because a throttled valve creates additional resistance in the flow path, which reduces the effective flow rate from that particular pump.

In a system with two parallel pumps, the total flow rate is the sum of the individual flows from each pump. If one pump’s discharge is throttled, its ability to contribute to the overall flow into the condenser is diminished. The other pump may compensate for some of the reduced flow, but it cannot fully make up for the loss depending on the system's characteristics and the nature of the pump curve. Consequently, the overall flow to the condenser will decrease, reflecting the reduced output from the throttled pump.

Thus, throttling back on the discharge valve of one pump will lead to a reduction in the total flow rate to the condenser, making it the most accurate choice in this scenario.