to enter the second fluid stream 68 prior to exiting the first outlet 72 a pressure vessel isolation valve 96 on each piping 92 upstream of the y joint 94 operates to isolate a respective vessel from the second fluid stream 68 as desired e g when that vessel is inoperative and depressurized for proppant recharging to prevent fluid backflow into the vessel through the screw each screw may be readily removed from the system for servicing repair or switching to a different screw size by uncoupling the flanges 53 56 at one end and at the other end by uncoupling from the isolation valve 96 the piping arrangement 60 further incorporates an u201c upstream u201d surfactant injection port 98 at the first inlet 62 for introducing surfactants from the delivery tubing 30 into the fluid stream 61 prior to its split into the first and second fluid streams 66 68 such introduction may also be accomplished further downstream after the fluid and proppant have been mixed such as at a u201c downstream u201d surfactant injection port 99 located immediately prior to the first outlet 72 both ports 98 99 may also be used concurrently and other ports may be added in.