The ISO EN 10628-1 has this example for a basic flow chart (which block diagram may also refer to):
The blocks can be unit operations, processes, or parts of a plant, the lines can be material or energy flows.
An example process flow chart looks like this:
The "block" are no longer blocks but symbols for specific devices, amounts/flow rates should be specified at the lines. Important valves etc. may be drawn.
And the next level of detail would be a P&ID:
Individual devices inlcuding actutators, names/numbers for individual components, sensors, pipe class of piping etc.
At the end of the day, you have to ask yourself what you want to say with your drawing and what level of detail is required, either by your contract or by further work you do based on the diagram. The above hopefully helps with navigating this.