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
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.