Pumping Duration VS Pump Life

[torq6]

I work for a water utility which predominantly supplies water to storage reservoirs via gravity flow, however, for about 10% of our systems we pump water from a tank at low elevation to a reservoir at a higher elevation. The pumps used are typically vertical multistage centrifugal pumps such as the CRN series manufactured by Grundfos and pumping is done daily to match the total daily demand for the area to be served.

Most of these systems, however, have been designed so that only about 4 hours of pumping is necessary to fill the reservoirs. As you know larger pumps mean higher capital costs including larger pipes to accommodate the flow rates. For some of our new designs we've discussed using smaller pumps and longer pumping times.

My question is what would be the trade-off, if any, with regard to the life of the pumps and frequency of maintenance given that over a specific extended period the smaller pumps would have run for a much longer time. I know this is not something that's easy to calculate/simulate, so I'd like to hear from anyone who's had experience with both scenarios. I also realise that it would also depend on the quality of the pumps with regard to materials, bearing design, lubrication, etc. Thanks.

 

[SteveWag]

Smaller pumps have a lower initial cost and a lower demand charge. Bigger pumps may have better efficiency. Typically there is not a dedicated fill line to the tank, that is to say 95% of the pump/tank jobs I have had the fill main as part of the distribution system. In this case, running the pump 4 hours a day means that the tank is serving the system 20 hours a day. This results in maintaining “fresher” water in the tank. The longer the pumps run, the greater the average age of the stored water. Having larger pumps allows faster recovery after a main break or fire event. I think 4 hours is a pretty good number.

 

[BigInch]

I think a good average to try for would be about 85% ON time in a system with relatively stable average flowrates, but reaching a target like that would be highly dependent on your min, average and maximum demands.

It is normally supposed that a high number of starts and stops decreases motor (if not pump also) lifetime. Many pump manufacturers give a recommended maximum number of starts/stops per hour for their lighter duty pumps.

http://virtualpipeline.spaces.msn.com

 

[cvg]

water districts I have worked for obtain lower rates for electricity at night during low-demand times. pumping of wells to replenish reservoirs was designed to occur primarily at night to save on energy costs. In addition, the reservoirs fill faster at night because there is much lower demand by users - most of the water pumped into the transmission main actually gets to the reservoir.

 

[chip1998]

Pump manufacturers guarantee potable water supply pumps for continuous pumping without negative impacts on pumps. Standard practice is to supply the water demand over a maximum 0f 16-20 hours, to allow for maintenance. This is facilitated by sufficient operational storage , which allows constant inflow and variable outflow. Mass flow diagrams balancing inflow and outflow can determine this minimum size.

As pointed by others, large pumps draw high start up curents and high maximum demand. Power tarriffs are usually based on the actual units consumed plus peak demand. The later is charged regardless of your unit consumption. As such, your power costs may be significantly higher for short duration pumping using large pumps.

Your curve of capital, plus maximum demand costs , plus power consumption costs , plus maintenance costs will have a minimum value which is the optimum level.

 

[BigInch]

"Pump manufacturers guarantee potable water supply pumps for continuous pumping without negative impacts on pumps."

That's because frequent starts are more severe than continuous duty.

http://virtualpipeline.spaces.msn.com

 

[SteveWag]

Since this is a potable water system we are talking about, I think limiting the discussion to the economics of pumping water is short sighted. Very many of my clients have over-sized water storage tanks, either for fire service or piece-of-mind. It is important to draw and fill as much as possible in order the maintain fresh water in the tank(s).As mentioned above, pumping at night is a very good idea but that means pumping from 12 to 5, or five hours pumping per day.

Steve Wagner

 

[cvg]

I think that sizing the pumps for night time pumping does help to maintain fresh water as you say. However, we typically assume more than 5 hours per night. More like 8 hours per night. Tanks are drawing down for 16 hours during the day. Unless you have a dedicated well and transmission main for each tank, then you will probably have to fill every night unless you have two full days storage in all your tanks. Wells / pumps that are sized for night time pumping can take advantage of night time replenishment to save substantially on energy. If there is a fire that drains the tanks down, the larger pumps and transmission mains have sufficient capacity to replenish the tanks, even during the day when demand is high. This provides some redundancy in capacity especially during hot summer months or if you have two fires in one day.