VFD's work great if you need them to modulate the pressure over a relatively small range. They work less efficiently if you try to control over a large pressure range.

Head tank.. Are you referring to a bladder tank? There is a recent thread here on Bladder tanks verses VFDs. You should hunt that down.

Heck here's a quick search:
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Keith Cress
kcress - http://www.flaminsystems.com

Hi Keith,

Thanks for your answer! By head tank, I mean large elevated tanks where the water is pumped to create constant pressure. They are normally used on industrial sites such as mines.

IHQ22,

It would help if you could describe the system better.

"Pumping system requiring constant pressure" is rather vague.

Where is the pressure constant? At the pump suction, discharge or at the user interface?

What is the flow range?
Large flow range then static tanks are probably better, smaller range then pumps.
What about losses in the "system"?
As soon as you turn a user on and off it impacts pressure due to frictional flow. If you want the user to have constant pressure then you will need a pumped system or some sort of pressure control from the tank.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Hi LittleInch,

Like always thank you for your contribution to my post. I am not talking about a system in particular. I am only looking for a rule of thumb or guidance as to when each type of system is most appropriate.

Thanks!

OK, thought it was rather vague.

The other thing to consider is the relative size of header or supply pipe and offtake.

Quite often if people want a constant pressure at different locations you need to really oversize the header so that the friction losses are low in the header for flows at different points.

So man possible permutations of this it's difficult to come up with any standard rules which would apply.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.