Flow and Pressure

 

A satisfactory water pressure does not necessarily mean a satisfactory water flow and vice versa.

 

Think about that most annoying of household problems, a dripping tap. If you block the outlet with your finger, yet cannot hold back the water the pressure is high and constant. But then the drip returns as soon as you remove your finger. So the flow is very inadequate. The pressure may be good but the flow is much less than is required in litres per minute.

 

Now think about a small brook where the opposite situation holds. The flow is great with the water moving at a speed of you doing a strolling walk. But the pressure even in the deepest part of the brook is negligible. Probably less than the dripping tap.

 

These two illustrations show the two extremes in the nature of plumbing. Good pressure but little or no flow and excellent flow with little or no pressure.

 

Static and Dynamic Pressures: 

 

Static pressure, as the name suggests, is the pressure when there is no flow. It is the pressure within the system. Dynamic pressure is the level that exists when some taps are open and there is little to hold back the water, save the resistance of the pipe distance itself.

 

Every extra centimetre of pipe adds to the resistance of the flow. It is like friction. Every new fitting and every piece of plumbing equipment added to the system increases the resistance to flow.  So the faster the flow the more the pressure is reduced.

 

With dynamic pressure and flow, the bigger the plumbing system, the more energy the water has to give up to overcome the resistance. The energy loss is felt and seen as water pressure reduction.

 

Less 'pipe drag’ equals lower dynamic pressure and higher flow rate.

 

Higher resistance equals lower flow rate.

 

All taps, bends and valves hold back the flow. This resistance rises as the flow rate rises. There is a number of fittings through which pressure losses are not too noticeable. There is also a number where as flow rate rises the loss of pressure becomes so great that the system has a problem.

 

The ‘design flow rate’ for any valve or fitting should fall within the range of the pressure drop graph that is satisfactory.  That range is before the flow rate line begins to drop away.

 

For every pressure drop through a bend, tap or valve there is a fixed flow rate drop. The pressure drop becomes greater as the flow rate does. But there inevitably comes a point where each fitting yields less and less extra flow. The flow rate reaches an unchanging point. This is where the loss of pressure across any valve goes on declining but does not affect the flow.

 

The pressure at the start of a system minus the pressure at the end of a system is the total amount of lost pressure for all of the pipe work and fittings in that system for any given flow rate.