Birmingham Water Works Board (BWWB) of Birmingham, Ala., has consistently achieved the rating of the number-five water system in the United States...
Lower maintenance cost
The primary components that lead to pump failures are bearings and mechanical seals. Excessive vibration, excessive loads and/or poor lubrication are the main causes of failure for these parts. Designing a pump with a larger shaft and bearings does not guarantee longer life. Many failures can be attributed to operator error and application factors. Operating range, impeller diameter and operating speed all have an effect on the overall reliability of a pump.
Operating range. A centrifugal pump is designed to operate most reliably at one capacity for a given speed and impeller diameter. This capacity is usually at or near the best efficiency flow. As pump operation moves away from this optimum capacity, turbulence in the casing and impeller increases. As a result, hydraulic loads, which are transmitted to the shaft and bearings, increase and become unsteady. These loads are related to the impeller diameter in a cubic manner. The severity of these loads can have a negative effect on reliability.
Impeller diameter. Impeller diameter affects reliability by the loads that are imposed to the shaft and bearings as the impeller vanes interact with the volute cutwater. These loads are also related to the impeller diameter in a cubic manner. Maximum or near maximum impeller diameters may result in a less than optimum gap between the cutwater and impeller. As each vane passes the cutwater a large pulse is produced, resulting in an unsteady deflection of the pump shaft, which can be damaging to mechanical seals. There is an optimum cutwater gap that will limit these unsteady deflections3. With larger than optimum gaps the damaging cutwater effect is minimized, but the effects of suction and discharge recirculation become more of a concern, especially if vane overlap is lost due to large impeller trims.
Operating speed. Operating speed affects pump reliability through rubbing contact and wear in seal faces, reduced bearing life due to increased loads, lubricant breakdown caused by excessive heat and wetted component wear from abrasives in the pumpage.
In addition, an increased operating speed can easily push a low suction energy pump into a high suction energy region with accompanying noise, vibration and possible cavitation damage. The onset of high suction energy levels in pumps is directly related to operating speed, suction specific speed, specific gravity and the thermodynamic properties of the liquid being pumped, as well as impeller geometry and operating point(4).
How smart pumping systems help. The smart pump controller will match pump output exactly to system head requirements. If upstream system conditions change due to a transient, the pump will either increase or decrease its speed in order to maintain a constant output. This eliminates the need to add safety margins that will oversize the pump. With variable speed systems there is a better chance of finding a selection that will operate at or near the best efficiency flow, which usually occurs at a speed less than the pump maximum design speed. In many cases a smaller pump can be selected in a variable speed system when compared to a conventional system.
Smart Pumps Monitor System Conditions. Some of the more common causes of failures are attributed to the following upset conditions:
Advanced smart pumping systems are capable of detecting all of these conditions by sensorless protection algorithms.
Protection against transients. A sensorless flow monitor will detect and distinguish between a dry running condition, operation below minimum flow or closed discharge valve condition and a runout condition (too much flow). If flow falls outside specified user settings, the response can be set to warn only, alarm and control or fault. Magnetic drive pumps can especially benefit from dry run protection(5). The alarm and control mode reduces the pump speed to a safe minimum speed until the transient has cleared and the pump can resume normal pumping operation. A protection delay can be set to distinguish between brief nuisance transients and prolonged process conditions.
Other condition monitoring safeguards can warn and protect against conditions such as overpressure, overtemperature, overcurrent, overspeed and other process variables. Setpoints can be selected to restrict operation to user-specified ranges.
Smart pumping systems can also incorporate self-diagnostic features to compare current pump performance to the new factory performance. An alarm setting advises the user when the actual performance degrades past a certain preset value. This will give ample warning to the user to schedule planned maintenance on the unit during the next outage. If a fault history and time stamp is provided, the user will be able to accurately determine system behavior at the time of the fault. This will aid in the troubleshooting and remedying of system transients.
The protection that smart pumping systems offer should result in extended meantime between failure and improved life cycle maintenance cost. One method of quantitatively predicting these life cycle cost savings is outlined by Bloch and Geitner(3). In this method, reliability factors for operating speed, operating point and impeller diameter are assigned values between 0 –1, where higher values indicate more reliable selections. A reliability index, which is the product of the three reliability factors, can then be compared to pumps of similar design to give an indication of overall reliability. Table 2 shows the effect on life cycle maintenance savings for the cooling tower application using this method. It assumes an average MTBF for a conventional sealed pump of 18 months at an average cost per repair of $2,500.
The difference in pump life cycle maintenance savings for this smart pumping system represents a 33.5% decrease when compared to a conventional system, and MTBF is extended from 18 to 27 months.
When the total initial cost of a smart pumping system is compared to that of a conventional system, smart systems have proven to be competitive in price.
Since the smart controller is a variable speed device, there is no need for an automatic control valve in most systems. Advanced products are available which calculate pump flow by sensorless algorithms within 5% of rated flow. This is sufficiently accurate for most applications. The sensorless flow value can also be used to provide pump protection, thereby eliminating external flowmeters and/or auxiliary protective devices such as power meters, pressure switches, flow switches etc.
Additionally, smart controllers have integral starters and there is no need for a separate starter. One of the safeguards built into smart controllers protects against operation below minimum flow. Operation can also be restricted to user-specified ranges. Depending on system design, recirculation lines and valves can also be eliminated. Since added safety margins are not required when controlling with a variable speed system, in some cases a smaller pump can be used. Figure 3 shows a conventional system with pump/motor, control valve, flowmeter, isolation valves, recirculation line, DCS and starter. Smart pumping systems integrate the functionality of several of these pieces of equipment as shown in Figure 4.
Table 3 demonstrates that the life cycle initial cost savings for a smart pumping system compares favorably with that of a conventional system.
By reducing the amount of equipment in a system, both installation and maintenance costs are decreased. The installation costs associated with piping, air lines, wiring and communication lines can all be decreased by eliminating a control valve, flowmeter, separate starter and recirculation line valve and piping (in some applications).
Table 4 shows the effect on life cycle installation savings for the application shown in Table 1. In this example, pump installation costs are based on 5x initial cost. Installation costs for the control valve, flowmeter and starter are based on 3x initial cost of these components.
Although a control valve is not required, some users continue to include them in their systems due to past practices or for auxiliary process use.
The difference in pump installation savings for this smart pumping system represents a 35% decrease as compared to a conventional system.
Since the control valve and external flowmeter have been removed from the smart pumping system, maintenance for these items can also be eliminated as shown in Table 5.
Total life cycle cost savings
A summary for each of the major life cycle cost components is shown in Table 6 for a conventional and smart pumping system.
The total life cycle savings for the smart pumping system is $268,650 for this installation. This represents a 35% saving over a 15-year equipment life. The present value of these savings, assuming a 10% interest rate, is $148,300 and the difference in initial capital investment is negligible.
Smart pumping systems are those that react and adjust themselves to system changes without manual intervention. These types of systems must have the ability to recognize and safeguard themselves from operating under difficult conditions, which may reduce the amount of time between failures (MTBF). A smart pumping system should be capable of understanding when the system transient or unusual operating condition has cleared, thereby allowing normal pump operation to resume. The value of these systems to the user can be seen in reduced life cycle costs.
Smart systems utilize a variable frequency controller that can match pump output to system head requirements, thus reducing operating costs over the life of the pump. Energy consuming control valves are no longer required. The smart-control software will not permit the pump to operate outside user specified ranges or under conditions that typically cause pumps to fail. As a result, maintenance costs will decrease and MTBF will increase for these systems. Smart pumping systems can integrate the functionality of several pieces of equipment from a conventional pumping system to reduce both initial and installation cost. When comparing total life cycle cost, smart pumping systems can have an overwhelming advantage over conventional systems.