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
Don't be afraid.
There are many factors when choosing a pump - pressure, flow, liquid properties, accuracy, shear and power requirements to name a few. Imo pump recently introduced one of the most complete pump specification sheets on the market. If you're not sure which pump type you need, start here and we'll work with you to choose the best product for your application.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
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