In Modern HVAC System Design, Pump Style Matters

In commercial building hydronic heating, ventilation and air conditioning (HVAC) applications that require pumps of 10 horsepower (hp) or more, pumping capacity is not the only criterion that influences pump selection. The style of pump—vertical inline (VIL) or base mounted—should be evaluated in terms of energy and operational costs, along with its ability to minimize the footprint of the total system installation. The way for engineers and facility operators to choose the best system that delivers on the building owner’s objectives—keeping costs to a minimum and leasable space to a maximum—is to consider the HVAC system as a whole and the contribution of each component. From that foundation, it is useful to examine the benefits and costs of both VIL and base-mounted pumps.

Footprint Considerations

There is a general misconception in the market that the installed footprint of VIL pumps is much smaller than base-mounted pumps. This can be true in some smaller horsepower applications, such as boiler pumps (smaller hydronic system), but it does not necessarily apply across the board. It is important to think about the footprint of the total installation, instead of solely the pump itself. Using this approach, any space advantage of installing VIL pumps can become negligible. Smaller VIL pumps mounted in the piping can create some space savings. But overall, base-mounted pumps can be the better choice for applications involving pumps larger than 10 to 15 hp. Base-mounted pumps can present a lower installation cost, energy savings, increased reliability, easier maintenance and smaller installed footprint. In fact, there are cases where base-mounted end-suction pump installations use less floor space than VIL—notably as the size of the pump increases. The VIL pump’s footprint may not be ideal for pump/drive combinations. The National Electrical Code (NEC) calls for 3 to 4 feet of clearance in front of the drive. This can eliminate the space advantage for VIL pumps in the context of the complete installation.

Maintenance Considerations

Ongoing pump maintenance is based on a number of factors, including mechanical seal replacement and an ability to service the pump without disturbing the piping. One end-suction centrifugal pump has a function that means the motor does not have to be moved for maintenance. While mechanical seal replacement in VIL pumps is a relatively easy procedure, there is more to total service. This comprises replacement of the throttle bushing, impeller and pump shaft. In most cases, the motor must be removed, requiring space and sometimes hoisting equipment, even with a relatively small pump. Depending on where the VIL pump is installed within the system, it may need to be removed and lowered from the piping at the ceiling to the ground for safe servicing. This can add more maintenance downtime. To conserve space, VIL pumps tend to be spaced close together, which can pose accessibility issues. It can be challenging, time consuming and pose a safety hazard for technicians to work on a large VIL pump between two others that continue to operate at full speed. One pump may need to be shut down, forfeiting redundancy and risking adequate capacity. Service access in a base-mount installation is straightforward because the layout of the system enables easier access.

Pump Life Expectancy

Base-mounted pumps can have greater reliability, extending product life. The mechanical seals in VIL pumps need external flushing tubes that can be vulnerable to damage and can plug over time, leading to premature seal failures and lower life expectancy. Base-mounted pumps have internal flushing capability that can pass up to three times more flow over the seal faces, helping reduce seal temperature. In addition, some base-mounted pumps have two sets of bearings—one set for the thrust and radial load, and one set for the motor, reducing wear and tear. VIL pumps have only a motor bearing, which may reduce total life. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Equipment Life Expectancy Chart shows the longevity of base-mounted pumps.

Vibration, Support Factors

The rotating elements within a pump cause vibration. Both base-mounted and VIL pumps have motors and impellers that rotate, and both pumps are designed and manufactured to the same vibration standard (ANSI/HI 9.6.4). ANSI/HI 1.1-1.5, Centrifugal Pumps, for Nomenclature, Definitions, Application & Operation cites seven possible sources of vibration in pumps:

• 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.

Installation Costs

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 1.4.6.1.3.6): “…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.