The tube shown is of uniform cross- section. Liquid flows through it at a constant speed in the direction shown by the arrows . The liquid exerts on the tube

Momentum of liquid flowing through the tube changes at each bend because walls of  tube exerts force on water at each bend. By Newton's third law of motion, the liquid also exerts equal and opposite force F on the tube at each corner as shown in figure. From the diagram it is clear that the forces F produce clockwise couple on the tube.