The expansion tank for a VFD system is used primarily to help the pumps start-up and shut-down. Therefore, tanks are much smaller than the ole style constant speed systems. And some large buildings don’t need a tank.
Typically rated to 125psi, and up to 119 gallons, these are the BEST VALUE for most booster systems.
These were originally designed for heating systems, and later incorporated into booster systems. ASME tanks are typically about 5X more expensive than non-ASME tank. If the tank has a replaceable bladder, it’ll most likely be about the same price as a non-ASME tank. And replacing a bladder is not an easy project. If the discharge pressure is above 120psi, then an ASME tank can be considered.
Drawdown is used to prevent the booster system from short-cycling. A VFD system uses two drawdown sources: expansion tank, and the VFD programming. The smaller tank has some drawdown capability. But the VFD also has a programmed electronic drawdown of 10psi. pump needs adequate water to operate.
When the VFD’s turn the pumps off, they’ll boost the pressure an extra 5psi. For example, if the setpoint is 70psi, the VFD will turn pumps off at 75psi.
A really large building may not need an expansion tank. Because the water will probably always be flowing, a tank is probably a waste of money. And if the water demand actually stops, the VFD will ramp up in SLEEEP BOOST, and turn off.