Q. We have an old wastewater pretreatment system for our metal finishing operation. The system contains two large air diaphragm pumps that transfer the wastewater from a holding tank to the flocculation tank before the clarifier. Since we are renovating the pretreatment system to extend its life, our team is arguing over whether to replace or keep these pumps.
Since one of the pumps is for operating and the other is for standby, when they do need to be serviced we can keep the system in operation. I propose that we replace them with centrifugal pumps due to the diaphragm pumps’ high maintenance, replacing diaphragms and check valves.
Others say that we should keep them because we already have compressed air and do not need to bring additional electrical power. What is your recommendation regarding this situation? S.D.E.
A. In terms of energy costs, compressed air is the most inefficient utility used in metal finishing operations. Add more maintenance, and in our experience, a well-specified and selected centrifugal pump wins hands-down over an air operated diaphragm pump (AODP).
Compared with a centrifugal pump, an AODP has a number of moving parts: air handling system, diaphragms, and check valves. If the AODP is extensively used, which apparently the pumps you described are, then these pumps will have higher maintenance.
Furthermore, the compressed air system can be plagued with water, dirt, and lubrication issues unless it too is well maintained. While there have been great strides by AODP manufacturers to improve the reliability of the pump’s air handling system, especially with oil-free designs, poor compressed air quality will increase AODP maintenance frequency. Also, for those of us who experience cold winters, AODP can “freeze up” due to excessive moisture in the compressed air. Not only does one need to evaluate replacement costs but also labor costs and wastewater pretreatment system downtime on production.
An industrial rule of thumb is that an AODP will use about three times the electrical energy of an efficient centrifugal pump to move the same volume of water, and I have seen claims of up to seven times the energy. This energy differential does not take into account leaks and line losses in the compressed air system, which makes the AODP even more energy wasteful.
For example, a 2 hp, 230-volt, 3-phase electrical centrifugal pump will use about 5,000 kWh/year of electric power if operated 8 hrs/day, 250 days/year. Assuming electrical rates of $0.07–$0.10 per kWh, the annual electrical costs are $350 and $500, respectively.
An AODP doing the same amount of work has an additional cost of $700–$1,000 per year or more, and this does not include any maintenance costs for the air compressor and system.
Our rule of thumb is that for electrical motors of one hp or higher, it is definitely more favorable to choose a three-phase motor over a single-phase motor. Not only are three-phase motors typically more efficient, but there is also typically no cost for step-down transformers.
Based on how your wastewater pretreatment is set up, we also recommend that your anionic polymer feed be injected next to the intakes of the proposed centrifugal pumps. The high shear that is experienced inside the centrifugal pump’s casing does an excellent job of dispersing the polymer within the wastewater so that it can work more effectively in your flocculation tank.
AODP are selected over centrifugal pumps with electric motors for reasons other than energy conservation. These reasons include:
- Pumping of high solids, abrasive waste streams that would quickly wear away a centrifugal pump’s impeller and seals,
- Pumping of large solids that would clog a centrifugal pump,
- Pumping of thick, viscous fluids, such as polymers or polyelectrolytes,
- Pumping of flocculated particles to a wastewater clarifier, sludge holding tank, or filter press since the centrifugal pump would tear apart these floc-culated particles to the detriment of the down-stream process unit,
- Pumping of wastewter from sumps, tanks, or pipes with insufficient storage volume, resulting in frequent and very short pumping cycles. Our plumbing designers insist on a 3 min (but prefer a 5 min) minimum pumping cycle for centrifugal pumps in order toprotect the electrical motorfrom a short life, and
- Pumping of concentrated waste streams (acid, caustics, oxidiz-ers, hexavalent chrome, cyanide waste) where small, controllableand easily adjustable flow ratesare needed to introduce these waste streams into a wastewater pretreatment system.
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I hope these factors assist your team in evaluating which is the best pump for your application.