Most large seawater reverse osmosis (SWRO) desalination plants built before 2002 incorporated turbine-type energy recovery devices (ERDs) that were connected with a shaft to the high pressure pump. Both the pump and the ERD had best efficiency points corresponding to specific flow rates and pressures. To obtain the high energy-efficiency necessary for cost-effective operation, SWRO plants were designed and operated at a fixed membrane water recovery rates - typically 45% for Mediterranean Sea waters and 35% for Arabian Sea waters. Most SWRO plants being designed and built today utilize isobaric ERDs. These designs emancipate the ERD and high-pressure pump making their operation independent. High efficiency can be obtained at range of membrane water recovery rates and can be changed as seasonal variations in the seawater occur or as the membrane elements age. Numerous best-efficiency operating points can be found which is a tremendous advantage for low-cost SWRO operation but can be too complex for manual design calculations. For this reason, Energy Recovery, Inc. (ERI(R): ERI SIM is a copyright and trademark of Energy Recovery, Inc. ERI, PX Pressure Exchanger, PX and the PX logo are registered trademarks of Energy Recovery, Inc.) developed the ERI SIM(TM) program. ERI SIM is an instructional computer program that simulates the pressures, flows and salinities of a SWRO process equipped with ERI's PX Pressure Exchanger(R) technology. The ERI SIM program integrates PX(R) device performance, typical pump and valve characteristics and projected membrane responses into an interactive, dynamic model. It is the first program in the public domain that treats an SWRO process as a complete, coupled system. The author demonstrates the ERI SIM program by using it to compute SWRO-process responses to changes in process variables, typical startup and shutdown conditions, and various possible process upset conditions.