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
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 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
Colfax has pumped a variety of 'burnable' fluids within the power generation market. These fuels can range from the easily handled Distillate Oil #2 and treated Crude Oil and Bunker Oil (#6) to the more lighter, unique fuels such as Naptha, Methanol, Water-white kerosene and Jet Fuel to name a few. These fuels can be used as the primary fuel or applied in reserve with natural gas in a dual fuel gas turbine system. In a dual fuel system, the fuel pumps come online if the gas supply is interrupted allowing the plant to continue to deliver power.
In addition to the changes in fuel, pressures and viscosity can also vary with each gas turbine system. Please review the two examples below that show the effects of varying viscosity and pressures. In these examples, the delivered flow required is 150gpm. (Here's a link to a metric/english standard conversion web-site.)
Example 1: Same discharge and inlet pressure with varying inlet viscosity.
Note that the heavier viscosity liquids delivers about 9% more gallons per minute and higher volumetric efficiency. The slight improvement can be attributed to reduced slip from the heavier fluid.
Example 2: Varying discharge and inlet pressure with same inlet viscosity:
Note the increases in hp required (43%) to pump the fluid; this is because of the linear relationship between pressure and hp (Equation: (gpm * psig)/1714 = oil horsepower)
Conclusion: In the two examples, I've tried to highlight a benefit to positive displacement pump technology, which is the minimal change in gpm output that a three screw pump provides over a wide pressure and viscosity range. You will see greater changes in horsepower and efficiency when you exceed 31cst. However, this example covers some of the power generation fuel injection pump applications that we see. Each application is unique. Please consult with your field representative or our application engineering department if you have questions.
My colleague, Larry Nowakowski Power Generation Market Manager for Colfax Americas, has worked with nearly all OEM turbine and engine suppliers over his career. He provides a complimentary lunch and learn presentation where he goes into further detail on power generation systems. Also, Jim Brennan (retired product engineer for Colfax Americas) has published an excellent article on combustion gas turbine fuel pumps that you can find on the Colfax Americas - Imo Pump website.