How much do VSP's/VFD's to help reduce the number or size of hydropneumatic tanks in a system? How large should my hydropneumatic tank be?
If you have storage, you probably don't need either a hydropneumatic tank (except for transient control) or a variable speed pump. Therefore, this answer assumes this is a closed/dead end pressure zone with no floating (elevated) storage. 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."
Configuring pumps for a small closed system PDF
Storage needed downstream of a VFD forum post