In general, the length of a pipe does not directly affect the pressure within the pipe, assuming the pipe has a constant diameter and there are no additional factors influencing the pressure. According to Bernoulli's principle, in the absence of energy losses due to friction or other factors, the total energy (which includes pressure energy) of a fluid flowing through a pipe remains constant along a streamline.
However, it's important to note that pressure drop can occur in a pipe due to factors such as friction losses, changes in elevation, or flow restrictions. These factors can lead to a decrease in pressure along the length of the pipe. Friction between the fluid and the pipe's walls causes a loss of energy, which manifests as a decrease in pressure. The longer the pipe, the more opportunity there is for friction to affect the pressure.
Additionally, if the fluid flow rate is not maintained or if there are additional components in the pipe such as valves or fittings, these factors can contribute to pressure losses. So while the length of the pipe itself does not directly affect pressure, the overall configuration, fluid properties, and flow conditions can influence pressure changes along the pipe.