Storage Needed Downstream of VFD

Hi nkg2,

Can you provide a bit more background on the system you're working on? Are you considering the hydropneumatic tank for transient protection? Are you conducting a steady state or EPS in WaterCAD or WaterGEMS, or a transient analysis in HAMMER?

You might find some general information in the book Advanced Water  Distribution Modeling and Management

In HAMMER, there are many factors to consider when sizing a hydropneumatic tank for surge protection, some of which are listed in this article:

Modeling Reference - Hydropneumatic Tanks

Hi Jesse - Sorry I did not know you had replied or I would have replied sooner.

VFD/VSD is supposed to help reduce the number of bladder tanks since water and pump continuously move. I am trying to figure out what is the rule of thumb in regards to what that reduction is? Is it 1/3 your peak demand flow?

So for example, lets say that I have a standard pump and controls that have a set point of 40/60 psi and I want a minimum rest time of 30 minutes for the pump after shutdown. I then specify X volume of nominal volume from Y number of bladder tanks. In the case of VFD/VSD, the electricity to the windings are constantly there and can handle the varying frequencies applied therefore a reduction in volume is okay. The volume in the bladder tank is there to help keep a smooth HGL downstream of the pump so that it does not wind up and down so fast that it create a harmonic like condition. I just didn't know if there was an AWWA standard or generally accepted practice on that.
Oops forgot to say I am doing EPS and Steady State modeling of the system as a whole, don't own Hammer.

First, I'll assume that this is a closed/dead end pressure zone with no floating (elevated) storage. If you have storage, you probably don't need either a hydropneumatic tank (except for transient control) or a variable speed pump.

For a closed system, most engineers use either a variable speed pump or a hydropneumatic tank. Either will enable the pumps to run fairly efficiently. Usually the hydro tanks are best for very small systems or systems with widely varying use (e.g. camp grounds, ski resorts, golf courses...). Variable speed pumps become economical for larger systems. Variable speed pumps must have a standby generator while hydro tanks need one if you are planning for long duration power outages.

It is difficult to turn over the water in the tank if you have a pump controlled to maintain a constant head.

The one instance where you may want both a hydro tank and a variable speed pump is to protect against a power outage where it takes a while for the generator to start. The tank can supply water while the generator is starting which could be as long as a minute. If you don't have a tank, it is likely that you are putting some people out of water while the generator sense the power outage, starts up, ramps to full speed and starts the pump.

So the answer to how big should the tank be is "How long of an outage do you want to plan for vs. how much do you want to spend vs. how frequently do you want to cycle the water in the tank."