| Larry Bachus
(a.k.a “Pump Guy”)
I work at a power generating station in Illinois. I am wondering if you have learned any lessons from running pumps at low flow. We are going to increase our capacity, and we will need more condensate flow. Right now we currently run three of the four condensate pumps. To get more flow, we are contemplating turning on the fourth pump. This of course will cause all the pumps to run farther back on their curves to about 75 percent of design. Our other option is to purchase new pumps that are designed for the projected flow through the existing pipes.
I was hoping I could find some articles or papers reporting experience with pump operation at about 75 percent of design to see if they we are getting any significant erosion issues. Unfortunately, I cannot find much documentation for something that seems to happen frequently.
Our condensate pumps are old Pacific-Dresser Pumps; 14×20 double-suction, single-stage pumps running at 1785 RPM. Our projected operating point is about 5,800 GPM. Best efficiency flow is around 8,000 GPM. Any help would be appreciated.
I’ll try to help. You say your system was designed for four condensate pumps and you are currently operating three of the four pumps. You say you are contemplating turning on the fourth pump to get more flow.
I gather the fourth pump is already installed and piped into the system. Now, you need your system curve.
Your piping system may be designed for four condensate pumps running in parallel. This will be evident on the pump’s system curve.
If the system is designed for four pumps, the system might be slightly throttled with a control valve, or maybe a permanent reducer or restrictor plate. This will allow for operating only three pumps at design, until the fourth pump is needed. If this is the case, you can remove that restriction and start the fourth pump.
However, your piping system may be designed for any three of the four pumps operating in parallel with the fourth pump as a standby only. This will also be evident on the system curve. If the system is designed for only three condensate pumps, the system will choke all your pumps if you start the fourth pump. Imagine four lanes of cars at highway speed, trying to squeeze onto three lanes of roadway.
Get your system curve. The system curve will show you what will happen when you start that fourth pump.
You asked about articles detailing industry experience with throttled pump operation and erosion of pump parts. Actually, there are hundreds of articles on the problems associated with throttled pump operation.
If you push too much flow (by adding another pump) through the existing pipe arrangement, the system will throttle all pumps and drag the operating coordinates (head and flow) away from best efficiency.
Among the problems you can expect with restricted flow:
1. Shaft deflection will cause strict tolerances in your pumps to abrade and erode. This degrades efficiency and leads to maintenance.
2. The shaft deflection will stress the bearings on your pumps and degrade the bearing lubricant. Again, more maintenance.
3. The shaft deflection will stress the mechanical seals. Contacting parts inside the seal will oscillate and erode from the distortion. The seal springs will have to dance constantly to maintain face contact, and, as such, will fatigue rapidly. Again, more leaks and maintenance.
4. Restricted flow adds heat to the liquid. The resistance stores energy in the pump. This energy is passed to the fluid as heat. You may know that if you heat condensate, which is already very hot, the additional heat may cause the condensate to vaporize inside the pump and flash to steam (a gas). A centrifugal pump cannot pump a gas because a gas will not centrifuge. Your condensate pumps may vapor lock and stop pumping. This is definitely not good.
5. Pumps that operate away from best efficiency will vibrate wildly, leading to overall degradation of the pump, motor, base, piping, and other equipment in the area. (However, your vibration technicians may love this. They always like to chase mysterious vibrations with their expensive, digital gadgets.)
6. Most condensate pumps are designed for low NPSH applications. Condensate is traditionally a low NPSH application because the condensate is hot and easily flashes to steam. Plus, condensate is often under a vacuum. For these reasons, condensate pumps are designed for low NPSH applications. One of the characteristics of a low NPSH pump is the fact that the pump must be operated close to best efficiency (BE). The pump operator must respect a narrow operating window around BE and not allow the pump to drift away from BE.
In your email, you state that you are prepared to purchase new pumps. Certainly purchasing new pumps is an option.
Another option is to modify your existing pumps. Sometimes these modifications are offered as spare parts (altered impellers and volutes) for your pump. You can communicate with your pump company regarding these alterations.
Another option is to modify the piping arrangement. Pipe modification is normally much less expensive than purchasing new pumps. This will make your “bean counters” happy.
Some pumps can run at 75 percent design flow without damage. Other pumps will self-destruct at 75 percent design flow. You’ll know this with the pump curve and the system curve.
You can gather a lot of information about off-design operation from your pump’s performance curve. Flow Control magazine and Larry Bachus will present a Pump Guy Seminar in Orlando later this year. Bring your curves to the lecture. For more details about the Pump Guy Seminar Series, visit FlowControlNetwork.com/PumpGuy.
Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. He can be reached at firstname.lastname@example.org.