I've gotten away from my series on diagnosing pump problems. Hopefully, you've been able to read and digest previous posts. For this post, I'll discuss the issue of low discharge pressure. I look forward to your emails and continued discussion on these topics.
Low discharge pressure can be caused only by loss of flow. Pump discharge pressure is caused ONLY by the system's resistance to the flow provided by the pump. Either the pump is not providing the flow expected, or the system is not offering expected resistance to that flow. It is possible that flow into the pump is being restricted (cavitation or suction starvation). This phenomenon is usually accompanied by noise and vibration. Or, it could be that the pump is not producing its rated flow (pump worn or damaged), or that the pump flow is bypassing rather than being delivered into the system as intended (open, improperly set, damaged or worn discharge system valve). If the pump is relatively new and not being used in abrasive service, it is most probable that discharge flow is bypassing. The most likely paths for such unwanted bypass are the system pressure relief valve (sometimes built into the pump), a bypass pressure regulator leaking (typical of a fuel oil burner system), an inadvertently open bypass valve, or any of these valves having worn valve seats, incompletely closed stems, incorrect signal control or broken springs.
Many pumps can receive a quick, though incomplete, inspection in place without disturbing piping or pump alignment. If the pump does not turn over by hand or with a little leverage assistance and in a smooth manner, the pump itself may be the problem. If one or more of the pumping elements can be visually inspected without major tear down or pump removal, do so. Enough wear to cause a pressure reduction (flow loss) should be readily visible.
It is sometimes difficult to determine if a valve is bypassing when it shouldn't, especially if the valve is built into the pump. It is probably best to remove the valve, do a partial valve dis-assembly and examine the mating valve seat surfaces or seat seals for wear or damage. Check any spring to be sure it is not broken. Work the valve mechanism manually if possibly to detect any binding or galling.
If the problem has still not been identified, be sure the pump river speed is being achieved and that the pump shaft is actually rotating at is correct speed. These conditions must be met, especially in a new system start up.Sean McCandless Industrial Market Manager Colfax Fluid Handling
I had previously posted a link to a white paper written by Rob Jordan in which he talks about some of the unique benefits and operating characteristics of 'screw pumps'. Hopefully you've had a chance to read this article and learn a little bit more about the products that we offer.
Today, I'd like to provide with readers a quick reference guide on when to use the products that Colfax Fluid Handling offers. This summary is referenced from the 2008 American National Standard for Rotary Pump types and Nomenclature, Hydraulic Institute. Note, a timed multiple screw pump is also known as a two screw pump and an untimed multiple screw pump is also known as a three screw pump.
|Progressing Cavity||Timed Multiple Screw Pump
||Untimed Multiple Screw Pump
|Self Prime Dry||Y||N||N|
|Self Prime Wet||Y||Y||Y|
As always, a thorough discussion with an application engineer is recommended if your application falls outside of these parameters.
There's a lot more to it then this and we're encouraging readers to contact us and discuss a lunch and learn session. What's better than a free lunch and a little education?Sean McCandless Industrial Market Manager Colfax Fluid Handling
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
In our continuing series on Rotary Pump Troubleshooting, we'll take a look at the condition of "Excessive Power Usage". As always, consult the factory or your local service representative for additional assistance.
Excessive power consumption can be caused by either mechanical or hydraulic problems. For rotary pumps, the pump power requirements are directly proportional to pressure and speed. If either has increased, the required input power will increase. Power required will also increase if the fluid viscosity has increased. This can happen if the liquid has been changed to something new or the liquid operating temperature has been reduced. Some liquids (grease, for example) are shear sensitive and can become more or less viscous with shear (pumping action) as well as undergo permanent viscosity change from shear over time.
Mechanical causes of high power usage include bearing wear out, pumping elements rubbing (a situation that can lead to pump failure), very bad shaft alignments and poor pulley alignments for belt drive arrangements.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.
If you’ve worked in a steam turbine driven power plant you know how difficult it is to keep water out of your machinery and lubrication systems. When water finds its way into your lube oil system, it increases the risks of corrosion, wear and premature failure. There are many articles available that discuss water’s impact on bearing life performance and system reliability including Water – The Forgotten Contaminant by Mark Barnes from Noria Corporation. It provides a good overview of the harmful effects water can have on a process.
So if you have water in your system, what can you do to remove it? In almost all cases, the answer is “it depends” as there are multiple solutions available and we recommend that you consult with manufacturers to understand the tradeoffs that you will make. Also, filtration technologies should not take the place of a complete system analysis to diagnose the root cause. In the rest of today’s post, we’ll focus on the Colfax PurLubeTM system comparing its features and benefits to alternative water filtration technologies.
The Colfax PurLube System
A PurLube system is commonly installed in a kidney loop off the main lube oil console. In a kidney loop, the PurLube is able to pull lubrication oil from the system and return it after treatment.
Once the oil enters the PurLube, it is heated to 180°F without causing thermal or oxidative stress to the oil. Next, a venturi draws air into the oil creating small air bubbles. When the air bubbles mix with the heated stream of lube oil, the volume and surface area of the bubbles expand causing any moisture that contacts the bubbles’ surface to evaporate into the heated air inside the bubbles. The oil stream is then passed through a settling tray allowing the bubbles to come to the surface and break, allowing the formerly entrained water to simply escape as evaporating water vapor.
The ‘water entrapment principal’ is comparable to relative humidity. The amount of water in air (grams H2O per kilogram of air) increases with temperature. For example, air will entrain 30 times more water at 180°F than it will at 80°F.
The PurLube removes all three forms of water contamination in lube oil - free, emulsified and dissolved – to lower than 100ppm. Additionally, the discharge does not contain any oil or emulsion and maintenance is relatively low because the system only has one moving part, the oil recirculation pump.
Finally, the PurLube system is an OEM component available in steam turbine lubrication systems built by a major US based turbine / generator manufacturer.
Machinery Lubrication provides a good summary of the technologies available for water filtration. There are three other types of water stripping systems available today - centrifuges, coalescing filters and vacuum dehydrators.
As outlined in the Machinery Lubrication article, the centrifugal separator is the earliest design of an oil water separation system and removes both free water and emulsified water. The principal of the centrifuge is to separate the oil’s heavier elements by spinning the oil to create “G’s”. The technology works well when you have a significant difference in specific gravity between the two fluids.
A centrifugal separator, however, does not remove dissolved water and the discharge must be treated to insure that hydrocarbons do not enter the ground water or municipal water treatment systems. Also, a centrifuge will lose on average 1 or 2 gallons of lube oil per day in the discharged emulsion. At an average price of $30.00 / gallon, this loss can represent from $10,000 to $20,000 per year.
Coalescing filtration systems represent an improvement in energy consumption and maintenance when compared to a centrifuge and there is less loss of the expensive lubricants. However, there is a significant loss of filtration and separation efficiency if there is any emulsion in the lube oil. The coalescing filter system also has no effect on the removal of dissolved water in the lube system. The most significant operational cost can be the periodic replacement of the coalescing cartridge elements. This annual cost can easily exceed the initial cost of the complete system.
A vacuum dehydrator system lowers the partial pressure and thus the temperature at which water boils. Like the PurLube, a vacuum dehydrator will remove free, emulsified and dissolved water. However, there are several factors that set these systems apart such as:
- The vacuum dehydrator is a much more complex system having a significantly higher initial cost.
- The annual energy consumption, maintenance and consumable costs for these systems can approach the initial cost of the installed system.
- The separation efficiency of these systems may be compromised with higher viscosity or synthetic oils.
Marketing Manager Power-Gen / Energy for Colfax Americas
Operators and engineering firms regularly challenge manufacturers to “boldly go where no man has gone before”, pushing the limits of their pumping technologies. In their desire to improve the effectiveness of current process designs, as well as conceptualize new cutting-edge processes, they aggressively push against the proven capacity, pressure and temperature limits of commercially available pumping equipment. In the face of this challenge manufacturers are presented with three possible responses:
- Try to find a suitable compromise between what the process demands and what the client has asked for. This approach not only stagnates the advancement of new technologies, but in the end the process effectiveness is narrowed due to the acceptance of the compromised technical solution.
- Advise the client that a proven technical solution is not available at the present time, leaving them in a dilemma with no foreseeable means to realize their process design.
- Promote a new design that is not fully proven in the field. In this case the new technology often consists of many proven elements that now need to be challenged further to handle this new application.
In several cases, response 1 is the most practical approach when the project design and construction schedule is tight, but there are instances with new, grassroots facilities where response 3 is the better course of action. In these situations there is time available to evaluate stronger technical solutions, particularly if they offer huge financial savings over the operating life of the facility.
Pushing the technology envelope is exciting and rewarding… How would you do it? What level of due diligence and commitment would you as a client need to see manufacturers exercise to overcome the dreaded FEAR of Serial Number #1? Would you be willing to invest your creativity, time, and effort to collaborate on something new? Can’t wait to see your comments!
Director, Global Oil and Gas Marketing
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