Ask the Experts

If multiple PX units are operated on a manifold is the performance of the array worse than that of individual units?

PX devices are usually installed in multiple-PX arrays. PX devices perform as well on manifolds as they do individually as has been demonstrated in many long-running multi-PX arrays. The pressure drop through a PX device (about 15 psi or 1 bar) is generally much greater than the pressure drop along the length of a PX manifold, resulting in a natural and even flow distribution. There is no need for valves or flow controls on individual devices, and in fact, ERI recommends against such designs.

PX manifold design is the same as membrane manifold design in any plant: liquid velocity should be limited to prevent excess pressure drop due to flow friction. PX arrays are similar to membrane arrays providing the operator with beneficial redundancy. In the rare instance that a PX rotor stops due to debris, flow through the stuck rotor is the same as flow through a spinning rotor. Therefore, a stopped rotor does not alter flow distribution in a PX array. In applications where several rotors are arrayed in parallel, a stopped rotor has minimal impact on RO membrane performance and the plant can typically keep running until scheduled maintenance corrects the problem. In contrast, the failure of large- or piston-based ERDs can necessitate plant shutdown.

Is flow distribution difficult to control, causing units operate at different flow rates and individual units to require flow control instrumentation?

The performance of an individual PX unit in an array can be verified with a salinity sample from a plastic valve installed on the low-pressure discharge of each unit. Based on such samples and years of operating experience, there are no reported instances of flow imbalance in a PX array in hundreds of multiple-PX installations.