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Sealless Centrifugal Pumps vs. Air Operated Diaphragm Pumps in Chemical Transfer

15 Oct 2020  |
While Air Operated Double Diaphragm (AODD) pumps are widely accepted and used in a variety of applications their use on clean chemical transfer can be argued as inefficient.

That being said the word clean is emphasized in this article. AODD pumps are monsters of pumping a variety of liquids that include viscous and dirty fluids. Clean chemical is defined as having particles under 50 micron in size and less than 5% by volume. Most chemicals delivered by rail, truck or tote fit into this category. 

Many facilities that have AODD pumps running on dirty or viscous processes will utilize this technology on clean processes also. The availability and familiarity of the AODD pumps makes them an easy choice for just about any application. Unfortunately what is sometimes over looked is the cost associated with running an AODD where a sealless centrifugal pump could be utilized. 

AODD pumps have three detriments, two of which I am sure you are familiar with while the third may be a surprise. These are maintenance, pulsation and air consumption. 

AODD pumps require preventative maintenance on their diaphragms and other wear parts. The wear items need to be replaced before a stated number of strokes occur. To replace the wear parts you must remove the pump from the system. This may lead to downtime if you do not have a spare pump on hand, although having a spare pump on hand is an added cost. Once the pump is removed there is a cost associated not only in the replacement parts but also the labor to rebuild the unit. Once rebuilt the pump will require additional labor to have it reinstalled. Rebuilding of the pump is not a onetime process, it must be rebuilt again and again every time you approach “X” number of strokes. 

AODD pumps are positive displacement pumps. Because of this pulsation occurs when the diaphragm reciprocates. This pulsation causes pipe stress and vibration that needs to be monitored. Although pulsation dampeners can be installed to help alleviate the surges this adds an expense to the process. 

Finally, AODD pumps consume air not electricity. The cost to generate and maintain air is increasing drastically. Plant air is not free!! Air compressors are very expensive pieces of capital equipment. They need to be maintained and can easily be maximized by other processes in the plant. Air compressors are also inherently inefficient. Added to this inefficiency is the fact that air lines from the compressor to the pump can leak. The AODD pump is also adds inefficiency to the process. Add these factors together and the AODD pump is one of the most expensive pumps to operate in a plant. As compared to an electrically powered pump, an AODD can cost as much as 75% more to operate on similar services. 

Enter sealless technology. Magnetically driven pumps have been around for over 50 years. As magnet technology has improved the cost to manufacture magnetic drive pumps has dropped. Developments in radial bearing materials have addressed the Achilles heal of dry running in mag drive pumps. Many manufactures offer chemical pumps that are low in cost, can withstand dry run conditions and will provide years and years of service without costly preventative maintenance. 

Magnetic drive pumps come in a variety of materials including metal, plastic and Teflon. Depending on the fluid, temperature and service conditions the user has a variety of different options and price points 

to choose from. Standard electrical motors mount to most designs providing an unlimited selection of voltages, speeds and enclosures. 

When comparing an AODD pump to a magnetic drive pump you will typically find a higher price associated with the magnetic drive option. Some users may have “sticker shock” when they see a magnetic drive option that can be as much as 50% higher than the AODD option. Most of this cost delta is associated with the rare earth magnets that power the mag drive pump. 

As an example let’s assume an AODD Pump cost $1,000 and an equivalent performance magnetic drive pump cost $1,300, a $300 or 30% difference. The user should consider the Life Cycle Costing (LCC) of the pump as opposed to just the sticker price. The AODD pump will require a rebuild after “x” number of strokes. If this rebuild occurs once a year and the cost of the rebuild kit is $300 than after one year the LCC of the AODD pump is now $1,300. After two years the LLC is now $1,600 and so on. The LCC does not take into account the labor and downtime associated with rebuilds. 

Let’s take this example one step further and compare the electrical costs associated with producing air as opposed to powering an electrical motor. An AODD pump moving 35 GPM at 10 PSI will need approximately 70 cfm to power the pump. 35 GPM at 25 ft. TDH (~10PSI) can be moved with a centrifugal pump with approximately a 1horse power motor. Theoretical power to produce the pressure and volume of air is more than double the horsepower (2.5 hp) needed to run a Mag Drive pump. Assuming you will run the pump 30 hours a week (6 hours a day, 5 days a week) and cost of $0.12 kWh the AODD pump will cost you approximately $1,000 annually. The centrifugal pump will do the same service for approximately $150. If you multiply this $850 savings by a population of AODD pumps the cost savings is substantial. 

Considerations that need to be taken into effect with AODD pumps also include noise. OSHA continues to tighten noise limits in manufacturing areas and the exhaust dB levels of AODD pump can sometimes exceed these levels. Another consideration is the stalling tendency of AODD pumps. While manufacturers continue to address this issues in their valve designs they cannot control the quality of air supplied, humidity in the operating area and temperature changes. Sealless centrifugal pumps have much lower dB ratings and no mechanisms to stall. 

In the unfortunate instance where there is a pump failure the sealless design of magnetic drive pump is more likely to contain fluid in the pump end as opposed to a ruptured diaphragm. A sealless pump will also give a warning sign of failure (diminished or no flow) where an AODD pump gives little to no warning at all. 

While sealless centrifugal pumps are not an across the board replacement for AODD pumps they do serve a purpose in clean chemical applications. Sealless centrifugal pumps come in both flooded suction and self-priming designs in a variety of materials and motor options for almost any service requirements. As always, work with a pump professional on any retrofit or design projects. 

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