Ring Main Design 1

[MTU1996]

Hello,

I am designing a ring main system to distribute water to nine (9) individual take-off points. The maximum (total) flowrate the ring main can provide is 12.5 m3/hour. Ring main pipe size (PVC) is DN50 with a velocity of 1.37 m/s. I've sized my pump to match the max flowrate (12.5 m3/hour). The plan is to use a VSD to control the flow through the ring main.

Questions:

1. Should I size my ring main pump bigger than 12.5 m3/hour?
2. Can I run a ring main system with multiple take-off points with only one pump or will I need booster pumps?
3. Is a VSD the best choice or would a control valve be a better option to control flow/pressure in the ring main?
4. Any "rules of thumb" on mixing requirements. The water will be augmented with chemical constiuents and I want to enure good mixing is achieved (this may related to Question #1).

Thanks!

 

[bimr]

Is this a recirculating loop?

 

[MTU1996]

Yes.

 

[bimr]

1. If all users are connected and the total user flow is 12.5 m3/hour, then your pump should have more capacity than 12.5 m3/hour.

A rule of thumb is to size the circulated supply loop for the average demand, with a peak demand factor plus the minimum circulation flows for turbulent flow. I think a reasonable minimum circulation flow would be 4.5 m3/hour. I would size the pump for 17 m3/hour.

2. You can run the system with only one pump. You may want to add a second pump for spare purposes, depending on the critical needs of the operation. It may be possible to have the second pump on the shelf to be installed when needed.

3. Your loop is not very large so you will need a backpressure control valve to maintain the pressure in the loop. The use of a VFD is optional. You can use a PID loop to control a VFD based on the pressure in the loop. The VFD would probably maintain the pressure in the loop more accurately than just a centrifugal pump with mechanical control device. The VFD will also shave a little off the utility bill as well, but not a dramatic savings.

I have noticed that there are some on this site that seem to be adamantly against VFD's for whatever reason. But there are many VFD's installed for this type of application. Many high rises have VFD's on zones.

4. A rule of thumb is if you have 4 - 90 degree elbows, the solution should be mixed.