The next section on our series on diagnosing or understanding pump problems, will focus on rapid pump wear.
Rapid pump wear is caused by either abrasives in the liquid or operation under conditions for which the pump is not suitable, such as excessively low viscosity or excessively higher pressure or temperature. If abrasives are a normal condition of the pumping application, as in slurry pumping, then pump wear will be a fact of life, and the best that can be done will include pump and drive speed selection that provides the best economic evaluation over the pump life cycle. While requiring bigger displacement and more expensive pumps, slower operation on abrasive service often pays back far beyond the initial purchase cost differential.
Wear due to abrasives in the liquid is a function of speed raise to a power usually betwen 2 and 3. If the abrasives are deliberately introduced, as when fuel oil additives intended to reduce boiler corrosion are brought into a system, they should be injected downstream of any liquid recirculation to insure that they do not go through the pump. Obviously, if abrasive foreign material is not supposed to be present, strainers or filters should be employed wherever possible and practical.
Rapid wear is sometimes not wear in the sense of a non-durable pump, but rather a catastrophic pump failure that occurs very quickly. Looking at the pump internal parts alone can frequently not provide much help in setting a direction to search. So, it is important to know what was occurring in the time period immediately preceding the detection problem.
Often, pump manufacturers offer a checklist designed to help you understand potential causes to failure. Contact your supplier today to discuss your issue and what supporting information they have available.
I'm also pleased to announce that our IMO pump brand has an updated Application Data Sheet. The data included in the online tool enables our engineers to answer your questions and projects request quickly and accurately. Moreover, you can consider products from the entire Colfax product portfolio.Sean McCandless Industrial Market Manager Colfax Fluid Handling
The Colfax Fluid Handling team was on display at the Power Gen show in Las Vegas. If you attended the show, hopefully you had a chance to come by and say hello. Overall, I thought the show attendance was greater than the 2009 Las Vegas show, but less than the 2010 show in Orlando.
The quality of leads generated at the show was, however, solid and the Colfax team was fortunate to talk with people who -
- Had problems with water in their lubrication Oil (recommend the ThermoJet or PurLube)
- Was interested in pumping sulfuric acid within the environmental system of his plant (recommended the Zenith metering pump)
- Was interested in using a progressing cavity pump in a vertical configuration to save space in his sump (recommended the Allweiler branded progressing cavity or Emtec pump)
- Needed to understand how to size three screw pumps for a fuel oil plant that they were building in the Middle East (recommended the IMO or Allweiler branded three screw pump)
These were only some of the applications that we discussed with show attendees. These leads also show the diversity and flexibility of the Colfax portfolio and the global coverage that we offer our customers.
Finally, we always welcome the opportunity to conduct a lunch and learn seminar for your associates. We offer topics such as the basics of centrifugal vs. positive displacement pumps, design and considerations for lubrication oil systems and three and two screw pumps benefits and design considerations. If you're interested, let me know.Sean McCandless Industrial Market Manager Colfax Fluid Handling
As many or you know, in the 1920’s the Swedish Engineer Carl Montelius (the MO in IMO) established the mathematical design for the 3 screw pump as we know it today. Those of us that do selections of 3 screw pumps here at Colfax are very familiar with the WIPS program which provides us with a quick selection and full detail of performance data.
There are multiple criteria that must be evaluated in the program to assure selection of a proper and reliable pump. One value that is figured into every selection in the program is called the Sommerfeld Number, named for the 19th century Prussian theoretical physicist Arnold Johannes Wilhelm Sommerfeld. Nominated a record 81 times for the Noble prize, pioneer of X-ray wave theory, as well as quantum physics, I guess we could consider him the real “Sheldon” of his day for any of you that have watched “The Big Bang Theory” on CBS.
Above (L to R): Arnold Johannes Wilhelm Sommerfeld, "Sheldon"
The Sommerfeld Number is a dimensionless quantity typically used in hydrodynamic lubrication analysis (journal bearing theory). In brief, it assures us that the idler rotor OD’s are riding on a sufficient film of fluid thus avoiding direct rubbing contact with the pump housing bore. It is defined by the following equation.
S is the Sommerfeld Number or bearing characteristic number
r is the shaft (journal) radius
c is the radial clearance between the journal (shaft) and the bearing (housing bore)
μ is the absolute viscosity of the lubricating liquid
N is the rotating speed of the shaft in revs/sec
P is the load per unit of projected bearing area
For the pump to perform reliably, WIPS must calculate a sufficiently high Sommerfeld number as one of the criteria to approve a selection. Fortunately the formula is pretty straight forward, not calculus, and taking a short look at it you can see the parameters that help or hinder the pump performance
(r) The shaft radius - The larger the diameter of the idler rotor the better. This is effectively true because the surface of the screw is passing the surface of the housing bore at a faster speed for a given rpm
(c) The radial clearance – The tight clearance of the Imo pump increases the Sommerfeld number
(μ) The absolute viscosity of the lubricating liquid – The higher the viscosity the better. We do not think much about the difference between 1 or 2 cSt, but once you place the value in the formula you see that the value of 1 cuts the Sommerfeld number in half, a big difference
(N) The speed of the rotating shaft in revs/sec – The faster the better
(P) The load per unit of projected bearing area – Basically this is saying that the longer the rotor and housing (12D vs. 3D) the better.
As a practical example of hydrodynamic film theory as discussed above, just consider water skiing, and the effect of increasing speed.
At 2 mph you can not get on top of the water, 20 mph all is good, 50 mph you no longer need the skis, if you are really good and your feet are large enough.
In your company, what criterion drives your purchasing decisions? Does first cost win or do you evaluate the life cycle cost? Does the purchasing manager win with the low priced bid or does the diligent engineer who has thoroughly calculated all costs and scenarios win? The answer – it likely depends and each scenario is different. At Colfax, we focus our discussion on total cost of ownership as our products have value beyond the purchase price and are designed to last for years once properly installed.
LCC = Cic + Cin + Ce + Co + Cm + Cs + Cenv + Cd
- Cic is the initial cost or purchase price
- Cin is the installation and commissioning costs
- Ce is the energy costs
- Co is the operating costs
- Cm is the maintenance costs
- Cs is the downtime costs
- Cenv is the environmental costs (leakage losses and permit violations)
- Cd is the decommissioning costs
One area where a firm can lower its first cost of positive displacement pumps is to properly size the pump to the requirements of the systems. Frequently, we see companies oversize their pumps in an effort to plan for future expansion. Our recommendation is to be realistic in your expectations as you will likely waste a lot of energy, time and money compensating for that pump. Pump energy consumption and maintenance issues are 52% of the total cost of ownership.
If you have already installed your system, watch the bypass valve and see if it is continually lifting to return fuel back to the system. This can be an indication that your pump is oversized for the system requirements. If you do a better job of matching the flow requirements of the system with the delivered flow of the pump, you will lower the brake horsepower required to operate the pump and your energy costs.
If you’d like additional information on this topic, the Hydraulic Institute Division, Pump System Matters, offers a class on Pumping System Optimization: Opportunities to Improve Life Cycle Performance. You may also want to experiment with the free downloadable modeling tool offered by Pump Systems Matter.
Our application engineers are ready to help you answer your application and total cost of ownership questions. We look forward to hearing from you.
 Hydraulics Institute www.pumps.org