- recirculation radial forces at low flows
- fluid separation at high flows
- cavitation due to net positive suction head (NPSH)
- air entrainment or aeration of the liquid
- hydraulic resonance in the piping
- solids contained in the liquid
- wear of rotating components
A closer review of the ANSI/HI standard shows that base-mounted pumps can handle more vibration (per ANSI 9.6.4-2000). Inline pumps send their vibration into the system piping and ultimately the building structure.
Base-mounted pumps, typically attached to the ground, send their vibration into the earth, which can extend its equipment life (per ASHRAE 2015 handbook and the U.S. Department of Energy).
Vibration challenges must be managed based on the location of installation and required horsepower.
Inertia bases are necessary for pumps that are not installed on the ground floor to help absorb vibration. Base-mounted pumps installed on the ground floor only require a housekeeping pad. VIL pumps can be installed without an inertia base where the mass of the pumps, piping and water is relatively small, and the building, ceiling and piping can accept the load—generally less than 7.5 hp. In larger horsepower applications—usually greater than 25 hp—a base-mounted pump is recommended with an inertia base (ASHRAE HVAC Applications Handbook, section 48.44-45).
According to the ASHRAE HVAC Applications Handbook, in addition to inertia bases being required for most large horsepower applications not at ground level (i.e., base-mounted pumps), “all inline pumps should be mounted on free-standing springs, regardless of location.” This point is worth noting, as there are mounting requirements for both types of pumps.
Initial costs of VIL pumps are typically less than base-mounted pumps. However, more equipment is often needed to for appropriate pump support and vibration mitigation.
An example of this is apparent in the ANSI Effect of Rigidity (Article 220.127.116.11.3.6): “…conventional horizontal split-case pumps (i.e., base mounted) are more rigid than vertically mounted pumps,” which means that they become part of the structure of the building.
Conversely, VIL pump installations are inherently flexible and may require additional support, such as isolation pads, flex connectors, heavier pipe flanges and piping to support the weight of the system. This additional equipment offsets the cost of the housekeeping pad for a base-mounted pump, and in some instances exceeds the cost of the housekeeping pad.