The dynamic behavior of a parabolic trough collector field with direct steam generation under varying solar conditions is analyzed using a transient simulation model. It is found that the peak water flow rates observed during transients may reach several times the steady-state design values. Taking into account these results, a method is developed for calculating the required separation efficiency of the water-steam separator between evaporating and superheating sections of the solar field. For a field with individual phase separators arranged in each collector row, the drainage system, used for transporting the separated water from the field to a central buffer tank, is dimensionally defined. It turns out that a buffer capacity of about 0.1m3 and a large-diameter drainage line have to be foreseen in order to cope with the high liquid loads under solar transients. The results are compared to a field layout with one central separation drum in terms of materials consumption and thermal inertia. It turns out that the originally intended effect of a reduced thermal inertia is not reached when transient conditions are taken care of in the design of the components.