Once-through, partial recirculation or closed loop — which seal-water system should I run?
Pick by two numbers: how much water you can afford to discharge, and how warm the water will get. Once-through is the simplest and gives the best vacuum because the liquid is always at supply temperature, but it consumes the most water. Partial recirculation returns most of the liquid through a separator and tops up; closed loop returns all of it through a heat exchanger and consumes almost none.
The vacuum consequence is the part that gets missed. A liquid ring pump cannot pull below the vapour pressure of its own seal liquid: water at 15 °C floors you at about 33 mbar, at 30 °C around 42 mbar, at 40 °C near 74 mbar. Every step toward a closed loop raises the liquid temperature unless the heat exchanger is sized for it — so a plant that switches to recirculation to save water and then complains the pump lost performance has usually undersized the cooler, not worn the pump.
Choose by process, not only by water cost. Once-through suits clean duties where the discharge can go to drain. Partial recirculation is the usual compromise in chemical and pharmaceutical plants. Closed loop is mandatory where the seal liquid is a solvent, where the gas is toxic, or where discharge is regulated — and there the seal liquid is often not water at all but the process condensate itself, which changes the vapour-pressure arithmetic entirely.
What we see when systems are retrofitted: the separator is reused from a smaller machine and carries liquid over into the discharge, or the level control is set so the pump is starved at low load. Both look like a worn pump on the gauge. If your vacuum got worse after a piping change rather than gradually, look at the liquid circuit before you open the machine.
Tell us the pump model, the seal liquid and the temperature you can supply it at, the duty pressure you need, and whether discharge to drain is permitted on your site. We quote the separator, the heat exchanger and the level control as one package when it is a closed loop, because sizing them separately is how the cooling ends up short.