Selection Criteria for Wastewater Pumps (Part 2)

Bayard Bosserman, P.E. and Paul Behnke, P.E.

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Conventional Pumping Stations

A typical design for a conventional wastewater pumping station is depicted in Figure 4. Such a wastewater pumping station may have the following components:

A wet well to hold the wastewater and from which the pumps take the wastewater for pumping to the plants;
A dry well that contains the pumps;
A motor room that contains the electric motors that drive the pumps;
Miscellaneous features such as a standby engine generator (to serve the pumps when the normal electrical supply is interrupted), a truck bay (to allow trucks to remove equipment for repair), restroom facilities and storage rooms.

Capacities: Conventional pumping stations are used in almost any capacity contemplated. Small stations (smaller than 1- or 2-mgd) are frequently of the "packaged" type in which a steel structure containing the pumps is buried. For larger capacity stations, cast-in-place concrete structures are typically used.

Ease of Maintenance: Conventional dry well/wet well wastewater pumping stations are generally regarded by O&M personnel as requiring the least maintenance effort compared to other types.

Estimated Life Cycle Costs: For comparison with other types of pump stations, the annual O&M costs for conventional pumping stations were assigned a value of zero for this analysis. All O&M costs for other types of pumping stations are differentials above this value. Thus, the life cycle costs are the same as the construction costs presented in Table 1.

Submersible Pumping Stations

A typical submersible pumping station layout is shown in Figure 5. The design typically features a wet well with no mechanical screens although some agencies in the United States do provide grinders. A small building is provided to house the electrical and control systems. No on-site cranes are provided; instead truck-mounted cranes are used to remove the pumps for maintenance and then reinstall them. The construction costs associated with this design are shown in Table 2 and compared with the construction costs for a conventional wet well/dry well pump station.

Capacities: Generally speaking, most public agencies in the United States only use submersible pumping stations in relatively small capacities compared to conventional wastewater pumping stations. The largest submersible station among the 15 agencies contacted had a capacity of 30-mgd. Typically, those agencies that used submersible pumps extensively used them up to a maximum station capacity of about 5- to 15-mgd. Other agencies that are more accustomed to using conventional dry well pumps generally use submersible pumps in relatively small pumping station (in the range of 1- or 2-mgd and smaller).

O&M Aspects: All the O&M personnel at the agencies contacted or visited felt that it was easier to maintain a conventional dry well pump than a submersible wet pit pump in a wet well. Comparative maintenance costs were seen to be a function of the quality of the submersible pump used. If a low cost, poor quality submersible pump is installed, maintenance will be frequent and therefore difficult.

The O&M personnel's main reason for stating that it is much easier to maintain a dry pit pump is that with a dry pit pump, they can observe and inspect it and determine when mechanical failure may be imminent and repair it before the problem becomes too serious. This is impossible with submersible pumps that cannot routinely be observed and have to be removed from the wet well in order to do servicing. Consequently, there is a greater tendency to let the submersible pump run until it simply fails. Therefore, the cost of repair is usually very high.

Submersible pumps are quite variable in their requirements for frequency of maintenance. Some agencies have had to pull submersible pumps as frequently as every six months. Others have gone as long as six or seven years before having to pull the pumps for maintenance. The typical maintenance interval for high quality submersible pumps of reasonable size seems to be about every two years. Seal failure is the most common problem with submersible pumps. Less frequent failure items are broken or damaged power cables and motor winding failure.

It can take a crew of six to ten people to remove a submersible pump from a wet well, depending on how large the pump is, although agencies having personnel with extensive O&M experience in submersible pumps can use crews half this size. The average crew typically consists of the following personnel:

One or two crane operators;
Two or three electricians;
Three to five other people to remove the pump.

Most agencies have discovered that above a certain size of submersible pump (between 100 to 200 horsepower), the O&M costs become extremely high. Generally speaking, the agencies have discovered that

Submersible pumps and motors larger than 100 horsepower break down more often than smaller ones.
Removal of large submersible pumps takes a lot more effort than they originally thought it would take.
The frequency of repair of submersible pumps is greater than that for conventional dry pit pumps. One agency thinks (although exact records are not available) that submersible pumps must be repaired 20 to 40 percent more often than dry pit pumps.

A majority of the agencies send out all their submersible pumps to private repair shops to be repaired. The most common failure items in submersible pumps are: replace bearings, replace seals, and rewind motors. It typically costs $4,000 to $8,000 to repair a submersible pump. It can cost as much as $27,000 to replace the rotor and stator on a 215 horsepower submersible pump.

Chapters 24 and 25 in Reference [2] presents a detailed discussion of O&M aspects of wastewater pumping stations, especially submersible pump designs.

Pump and Motor Electrical Efficiencies: There are no gains in pump and motor efficiencies associated with the submersible pump design. In fact, the efficiency of this design in terms of electricity consumption, is usually less than that of the conventional pump system, although the efficiencies of submersible motors are improving in response to energy efficiency regulations (e.g., EPACT) as well as market demand. Table 3 is a comparison of the typical efficiencies of submersible pumps with those of a conventional dry well design at the present time.

Thus, the submersible pump motor is about 8 percent less efficient than a conventional dry well pump motor. Partially offsetting this is the fact that eliminating the vertical shafting in a dry well design improves system efficiency by zero to one percent. This leaves a net decrease in efficiency of about seven percent compared to the conventional dry well pump system.

Assuming a 50-horsepower motor operating continuously, this seven percent decrease in system efficiency indicates an increased electricity consumption of 23,000 kilowatt-hours (kwh) per year. At a cost of 6¢ per kwh, this results in an increased electricity cost of $1,400 per year. It is likely that this value will decrease over the future as submersible motor efficiencies increase.

Projected Annual O&M Costs: Based on these investigations, the annual O&M costs of submersible stations will almost certainly be greater than those for conventional dry-well pumping stations. This average O&M cost for submersible stations will be approximately $5,500 per year per pump greater than for conventional dry well pumps. This figure was derived from the following assumptions and analysis:

Remove each submersible pump from the wet well an average of once every two years for maintenance.
Maintenance performed by an outside private pump repair company at an average of $6,000 per event.
Pump removal requires a six-person maintenance crew at four hours per person.
Pump maintenance requires crane rental at the rate of $400 per day each for pump removal and replacement.
Pump installation after completion of repairs also requires a six-person maintenance crew at four hours per person.
Average labor rate of $30 per hour, including benefits.

Thus, the additional O&M cost for a submersible pump compared to a conventional dry well/wet well pump is shown in Table 3a.

Under these assumptions, it was concluded that using submersible pumps in a wet well will likely result in increased average O&M costs of about $5,500 per year per pump compared to dry pit pumps.

A tabulation of O&M costs for different values for the items in above table are presented in Table 4 to illustrate the possible magnitudes. These different values result primarily from different assumed repair intervals. The last column can be filled in by the reader to suit his particular situation in terms of pump quality and experience.

Estimated Life Cycle Costs: Table 5 summarizes the projected life cycle costs of various sizes of submersible pumping stations using different repair cycles or frequencies (i.e., different levels of pump quality). The present values of the annual O&M costs were determined assuming a period of 20 years at an interest rate of 7 percent.

Design Aspects: Those agencies in the United States that have utilized submersible raw sewage pumps extensively have developed several observations that should be considered. One agency reported that they do not allow submersible pumping station wet wells to be more than 30 feet deep. They have established this requirement because the power cables are very heavy and sometimes stretch. When they stretch, the cable insulation develops cracks. Hydrogen sulfide gas from the wet well then enters the cable and corrodes the power wires. When that happens, the power cable has to be replaced (it cannot be repaired). However, another agency has found that wet well depths up to 40 feet are acceptable.

It costs $15,000 to $20,000 to buy a replacement power cable for a 450-horsepower, 4,160-volt submersible pump. It costs $3,000 to $6,000 to buy a replacement power cable for a 50-horsepower, 480-volt submersible pump.

With most foreign manufacturers, spare power cables are not stocked in the United States (cables must be sent in from abroad). It can take four to six weeks to get replacement cables. It is easy to damage a power cable when removing or reinstalling a submersible pump. Turbulence in the wet well bends and stretches the cable, too. So, there is a need to minimize the power cable lengths.

A major problem that this same agency sometimes encountered with the submersible pumps is the discharge elbow in the wet well tearing loose from its anchor bolts. This did not happen very often, but when it happened, it required a major effort to repair. The wet wells are classified as confined spaces per OSHA regulations in the United States, so it can take up to an eight person crew to comply with all the confined space observation and supervision requirements and actually do work in the wet well. It is believed that the cause of this problem is

A pump ingests a large rock or some solid.
The pump then vibrates as it tries to expel the solid.
This vibration creates forces and displacements in the pump and its discharge elbow that are much greater than those the elbow anchor bolts and associated concrete anchoring system were designed to withstand.
Each time this event occurs, it weakens the concrete and anchoring system a little more.
Eventually, the bolts tear out of the concrete or the concrete breaks.

Most agencies prefer a maximum 1,200-rpm pump speed, if at all possible. For pumping heads greater than about 90 feet, 1,800-rpm pumps or tandem 1,200-rpm pumps are used.

Recommendations for Submersible Pumping Stations: It is suggested that a maximum pump size of 150 horsepower be permitted in a wet well arrangement. The experiences at the agencies investigated in the United States indicated that pumps larger than this tend to fail much more frequently, they also require quite large crew sizes to remove them from the wet wells, and maintenance costs in terms of labor effort and parts costs are very high. Higher quality pumps, obtainable from a limited number of manufacturers, may possibly be satisfactory in larger sizes.

There is a general feeling (supported by the experience, practice, and opinions at various agencies) that at some point in size (between 100 and 200 horsepower), increased repair and maintenance costs begin to offset the savings gained initially with the submersible pumps. There is, however, insufficient data to establish the exact point of diminishing returns. Ideally, manufacturers should be asked to provide lists of current installations of large submersible pumps. The owners and operators of those pumps could then be surveyed to define size, application, repair records, etc. The data thus gathered could be analyzed to see if there is a logical cutoff point for horsepower and speeds beyond which wet pit submersibles are not advisable as well as a second point beyond which submersibles should not be used even in dry pit applications.

It is suggested that the wet wells should be no more than 40 feet deep. It also is suggested that a maximum pump speed of 1,200 rpm be selected for pumps between 100 and 150 horsepower. For pumps less than 100 horsepower, 1,200 rpm should be the preferred maximum speed but up to 1,800 rpm may be acceptable if a suitable 1,200-rpm unit cannot be obtained.

References

Robert L. Sanks, et al, Pumping Station Design, second edition, Butterworths-Heinmans, 1998.
DG-04 Value Engineering Study, Report of Value Analysis, Contract 95MM1279A, Washington Suburban Sanitary Commission, prepared by Boyle Engineering Corporation, July 1996.
B. Bosserman, S. Collins, A. Will, "Current Concepts in Wastewater Pumping Station Design," presented at the 28th Joint Annual Conference of the Chesapeake Water Environment Association and the Water and Waste Operators Association of Maryland, Delaware, and District of Columbia, Ocean City, Maryland (July 9p11, 1997).