Any pipe causes pressure loss as water flows through it. This loss is based on the following factors:

1. The velocity of the water: As velocity increases, pressure losses increase. Velocity is directly related to flow rate. An increase or decrease in flow rate will result in a corresponding increase or decrease in velocity.

2. The size (inside diameter) of the pipe: Smaller pipe causes a greater proportion of the water to be in contact with the pipe, which creates friction. Pipe size also affects velocity. Given a constant flow rate, decreasing pipe size increases the waterâ€™s velocity, which increases friction.

3. The roughness of the pipe.

Regarding the second point, remember that the water along the edge of the pipe is subject to skin friction, which reduces its flow. Water in the middle experiences laminar flow (vastly less friction).

Remember from geometry that when looking at a slice of the pipe, an increase in pipe diameter causes a direct (linear) increase in the amount of water against the edge. The water away from the edge increases exponentially, so an increase in pipe diameter causes a decrease in the ratio of molecules experiencing friction to molecules just cruising along happily.

Yeah, but what about velocity? Given a pump with a flow rate of 35 GPM, hooking a 50 foot long 2' diameter pipe up to it will definitely give a slower velocity than a 1/4" pipe. But what is velocity? Just the measure of how fast the water is moving past a fixed point. The only relevant thing about velocity is that a higher velocity results in more friction. Otherwise, it can be misleading. The important thing here is flow rate.

Flow rate is the measure of the volume of water past a point in a given time period. In our scenario, the fat pipe will stay close to the pump's 35 GPM rating, as there is avery low friction/volume ratio. If we made the pipe 3' in diameter, the velocity would be much lower, but it would increase the flow rate (very slightly...).

The velocities of the various pipes moving 35 GPM would be

3' pipe = .011 ft/sec

2' pipe = .024 ft/sec

1" pipe = 14.2 ft/sec

.25"pipe = 228 ft/sec

If you had a 350 gallon barrel with a 1" drain hole in the bottom, and you needed to move the water 50 feet and spray it out a .25" inch hole, which would you rather use?

1. a 1" pipe that immediately narrowed to .25" and then ran 50 feet and then ended.

2. a 1" pipe that ran 50 feet and then ended with a cap with a .25" hole in it?

3. a 1" pipe that widened to 2", ran 50 feet and then ended with a cap with a .25" hole in it?

The 2" pipe will deliver the highest flow to the .25" hole. In all cases the initial pressure at the bottom of the tank is the same. The smaller pipe, will have a higher velocity than the others at any point besides the outlet. The outlets are the same in all three examples; a .25" orifice. At the orifice the two larger pipes will be trying to shove a much higher flow through the same size hole, resulting in a higher pressure at the hole than with the small pipe, which is wasting much of its pressure pushing on 50' of pipe wall.

An important side note is that the pump would have to work much harder to give the same outlet pressure for the little pipe. That's why firefighters that have a 1" nozzle end don't use a 1" hose. On longer hoses the pressure coming out would be negligible.