While reviewing the operation of MSF desalination plants, it seems that there is a conflict in explaining how the pressure drop between each stage is achieved.

Most authors simply say that the pressure in each stage is lower than the previous and therefore once the hot brine enters it flash evaporates as the water vapor pressure will be higher than the stage pressure.

Author put more details saying that the low pressure in each stage is achieved through using steam ejectors.

But many others mention that the feed water enters each stage through an orifice which causes the sudden pressure drop and therefore flash evaporation occurs.

Any explanation of the exact mechanism by which flash evaporation occurs in an MSF process?

Examples:

"The water then enters the first stage through an orifice and in so doing has its pressure reduced." (Kalogirou, 2005)

"The thermal energy is in the form of low-pressure bleed steam (1 to 3 bars) for the feed-brine heating, and medium-pressure steam for the ejectors to generate the required vacuuming different sections of the unit." (Al-Kalagouri, 2013)