In order to study a reverse osmosis plant, I need to represent linear headlosses (the flow in the membrane cartridge is laminar).
How to do so in WaterGems and Hammer ?
A general purpose valve may work for what you are trying to model. This allows you to include a headloss curve where you can enter a table of flow and headloss, which allows you to control the headloss at the valve.
For information on how valves operate in HAMMER, please see this link: Model Reference - Valves. You can include an operating rule to change how the valve operates, but the valve headloss is assumed to be held at the value calculated during the initial conditions calculation.