Why is the force on the fluid not equal to the change in momentum per unit time?

[Chestermiller]

Chet , I understand your point . Just help me understand why in the books it is written that horizontal force exerted by fluid on pipe is $\rho v^2A$ . Are there any assumptions involved or am I misinterpreting something ?

Maybe they are assuming that the fluid pressure in the pipe is just atmospheric pressure (zero gage), or maybe they just inadvertently left out the pressure force.