maximal head

[YuriB]

Can someone please assure me that the no flow point on a head flow diagram for a centrifugal pump correspond indeed to the maximal head ? The reason I ask is because I want to replace HUPA 25-7.0 U 180 with HGPA 25-7.0 U 180. For the both the no flow (maximal head on the diagramm) is 7 m, only the second is 3 times more powerfull and, for instance, if at the head 5m the first produces about 2 m3/h, the second at the same head pumps about 4 m3/h. Is there I am missing something and my more powerfull pump could burst the system (produce the head more than the mentioned 7m), if a valve get closed?

 

[Artisi]

A not unusual belief, check Eng-tips search, head loss for diameter change has been discussed on a number of occasions.

Head increases ^2 for flow change / same pipe diameter, change of diameter ^5

as a quick and ready proof using a standard Darcy formula friction loss chart

600 GPM 6" Diameter pipe = 2.7 ft Hl/ 100
600 GPM 3" diameter = 75.7 ft Hl /100

(6\3)^5 x 2.8 = 86

Certainly look like d^5 not d^2 to me.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

Ok,I'll buy that. I knew there were other terms but not sure whicch way they made the relationship go. Had a quick check on this and to me looks like the bigger the difference in ID (one half the size) it goes towards to ^5, but smaller ratios and smaller flows(say 6" to 5") tends more to ^4.5.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Artisi]

Never 4.5 or other always ^5. Don't forget to use actual pipe diameters and not nominal diameter, ie 6" 4" etc.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)