Pumps have long been the workhorses of HVAC systems in institutional and commercial facilities, supporting the efficient operation of chillers, boilers, cooling towers, domestic water systems and hydronic distribution systems. Unfortunately, many maintenance and engineering managers tend to overlook the pump as an opportunity to improve the performance and reliability of HVAC systems.
Ensuring pumps’ reliable, energy-efficient performance means facility managers and front-line technicians need to understand common issues that can arise during pump design and performance life and adapt their activities to prevent them.
When discussing the role pumps play in facilities, the most common application is moving water through pipes to heat and cool air or equipment. Pump types include rotary, screw, reciprocating and centrifugal, with centrifugal pumps being the most common. The popularity of centrifugal pumps can be attributed in part to their relative simplicity, efficiency and low manufacturing cost.
The main parts of a centrifugal pump are the casing, shaft, packing or mechanical seal, impeller, suction nozzle and discharge nozzle. The pump shaft is connected to a motor that spins the impeller, which imparts a centrifugal force on the water and increases the pressure of the water as it flows through the casing. This pressure forces the water through the pipe to connected devices downstream in the system.
Problems commonly associated with centrifugal pumps include seal failure, bearing malfunction or failure, decreased performance and cavitation.
Seal failures. These failures occur when the seal inside a pump degrades to the point that it begins to leak. The two main seal types in centrifugal pumps are packings and mechanical seals. Packing seals use soft material that is packed into the gap between the shaft and pump casing.
These seals inherently degrade over time and should be checked during regular pump inspections. A typical mechanical seal consists of a combination of stationary parts typically sealed with an O-ring and an interface between a rotating face and stationary face.
One common reason for mechanical seal failure is the fluid film between the rotating face and stationary face of the mechanical seal is insufficient to cool the gap, which leads to degradation of the seal. Ensuring the fluid film cooling capacity at the interface is sufficient is critical.
An easy way to determine whether a pump is suffering from a mechanical seal failure is to check for visible leakage around the shaft’s entrance to the pump casing. Technicians should be aware that fluid can travel along piping, so it does not always have to be directly at the pump casing or on the ground beneath it.
Bearing problems. Much like mechanical seal failures, bearing failures are typically the result of overheating of the bearings inside a pump. One reason for such failure is improper pump alignment, which causes excessive frictional wear on the bearing and overheating. All new pumps must be properly aligned during installation, but in facilities with existing pumps, this alignment can degrade and should be checked regularly to prevent wear from occurring.
Along with checking pump alignment, technicians also should ensure that bearings are properly lubricated. Regular alignment checks and bearing lubrication should always be included in a facility's preventive maintenance plan.
Cavitation. The degradation of pump performance can result from several factors, one of which is cavitation. This problem occurs when the temperature of the fluid being pumped increases above its boiling point, which causes microbubbles of air suspended in the fluid to rapidly expand and collapse resulting in physical damage to the pump surfaces. Cavitation decreases the performance of a pump, and it is detrimental to the pump.
Cavitation is the result of when a pump’s net positive suction head available (NPSHA) is less than its net positive suction head required (NPSHR). A pump’s NPSHR value is determined during manufacturer testing, is included in most pump performance charts and should always be considered by the engineer when sizing a pump. The NPSHA value describes the level of pressure at the inlet or suction side of a pump.
If a technician suspects that a pump might be experiencing cavitation, test it by measuring the pressure of the water entering the pump using a calibrated water meter and verifying that it exceeds the published NPSHR value on the pump performance curve provided by the manufacturer. In some cases, a decrease of overall pressure in a system results from a lack of make-up water in the system. Ensuring that any water lost from the system from blowdown, evaporation or leaks is replaced, which will help prevent cavitation from occurring.
In some cases, pump performance can be affected by its design. One such problem is an improperly sized impeller or a pump in which the wrong impeller has been installed. The impeller is the rotating wheel housed inside of the pump casing. The design and size of the impeller is critical to the performance of the pump.
An engineer sizing a pump and impeller is looking to balance the flow rate and discharge pressure of the pump. System flow and system head are calculated prior to sizing pumps and are used to guide the engineer in selecting a pump/impeller combination that achieves both.
For example, if a system requires 400 gallons per minute (gpm) at a head pressure of 75 feet, an engineer probably will select an impeller with a diameter of 9.5 inches. If for whatever reason the manufacturer accidentally installs an impeller with a diameter of only 9 inches, the pump would still be able to achieve the design flow rate of 400 gpm but would only be able to produce a head pressure of 65 feet.
This issue can be easily caught if the pump is properly tested by a certified testing, adjusting and balancing technician during system testing. But it is critical that system testing occurs and that the subsequent test report be generated and reviewed prior to systems being released to the facility to prevent such issues from occurring.
Sticking with old design and operating practices will prevent pumps from operating as efficiently and reliably as facilities need them to. Successful pump operation requires that managers and technicians continue to update their maintenance practices in order to prevent problems that hamper pump operation.
Eric Reinhard is a project engineer with FTS Technical Services, which provides testing, inspection, certification and engineering services.
