×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Pumping Duration VS Pump Life

Pumping Duration VS Pump Life

Pumping Duration VS Pump Life

(OP)
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.

RE: Pumping Duration VS Pump Life

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.

RE: Pumping Duration VS Pump Life

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

RE: Pumping Duration VS Pump Life

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.

RE: Pumping Duration VS Pump Life

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.


 

RE: Pumping Duration VS Pump Life

"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

RE: Pumping Duration VS Pump Life

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

RE: Pumping Duration VS Pump Life

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources