Purified water generation and distribution system

We are in process of writing URS for Purified water generation and distributioin system. Can any one tell me what is the effect of distribution pipe diameter on the performance. As per my knowledge whether we use 1" dia or 2" dia it will not affect the system as long as required velocity/turbulance is maitained during circulation. What is significance of reynold number? do we need to cosider this? if yes how to calculate the same?

I think you are looking at the system backwards.

First work out how much water you need to satify your peak demand. Then verify how long you want to wait for it. I.e. peak demand might be filling a 1000lt vessel - do you want to fill it in to minutes or overnight.

From these figures you can work out the flow rate you need, this will then allow you to size your pumps and pipes.

Too fast a flow, is just as bad as to slow a flow, and one inch pipe is a third of the price of two inch pipe. A water system is very expensive and if you do not size it correctly, you are wasting your comany’s money.

Get your loading correct and it is quite simple to work out the rest.

Alex Kennedy


Thanks for the suggession. Could you please tell me how to decide on the size of piping once i calculate the total water requirements. How do u say too fast flow affects the system?

I think I am a bit late on this, still, would like to add something. You require to do some trial and error calculation here. Once you get your requirement, decide a tube size to maintain velocity at about 2m/s.

Decide what return line velocity you want to maintain (though 1m/s is a common practice, this becomes too high in certain cases) and calculate the flowrate corresponding to 1m/s velocity. Now add this flowrate to the average demand and you get your total required flowrate. Calculate the pressure drop in the system. There are many good calculators available out there and
has one good and precise calculator.

Though there is no clear cut guideline nor practice for correct Reynold’s number, anything beyond 40000 is good. Once you are done with finalizing flowrate and pressure drop, select a pump that suits your operating point. Here is where the head ache starts. Sanitary pumps are generally of open impeller types and thus low head devices. If you are not able to get a pump to suit your duty point, you may have to increase the pipe size. You will have to cough up extra money both interms of capital and revenue.

is the tank turnover is affecting on pipe size & pump selection?