SYSTEM HEAD CURVES: THE FORGOTTEN ASSET (PART 1)

This is the first in a 2-part series. Read Part 2 here.

Over the past year the profile of delegates attending courses offered by JTA have ranged from professional engineers right through to pump operators. During the courses, many were exposed to the features and benefits of systems head curves for the first time. The level of interest shown in this particular subject has been amazing. Most graduate engineers say something like "yes we did this at university but very briefly and we were not shown where and how to apply this". When the theory associated with system curves was related to real life case studies then the light bulbs really started coming on! For a trainer to see eyes lighting up and comments such as "so THAT's why that station is giving us so much trouble!" coming out, the rewards are hard to express.

What is a system head curve? Simply put, it is a graph which shows the total dynamic head (TDH) in a pipework system at various flow rates. Often this is superimposed on the performance curve of an appropriate pump and the duty point will be at the intersection of the system curve and the pump's performance curve. If the static head varies for any reason, a second system curve can be drawn which shows the increased TDH. The two intersection points show the range of flows and heads the pump will "see" as the static head changes.

Where does this all originate from? Thanks must go to a colleague who took the time to take this trainer through the process (too) many years ago. At that time the light went on and, yes, somebody was found to be at home. The lesson stuck and whenever the suspicion arose that the system was the culprit, out came the graph paper (this was before Excel and other dedicated software) and the friction loss charts. Once the plotting process began the story unfolded before some eager eyes. Yes the pipe was too small or in some cases too big!

Once all the plotting was complete, there it was, in black and white or in colours, the whole sad story. No more arguing or debate. The solution(s) became pretty self evident and the feeling of achievement and satisfaction were palpable.

All that trouble to modify the baseplate to accommodate a overmount belt drive arrangement, the bigger motor with the significant increase in energy consumed, all for a very modest and inadequate increase in flow rate. The two possible solutions? the first was replace the existing pipe with 2500m of one with a larger diameter. The second option was to use the existing line and build a bigger sump in order to accommodate the peak flows and give the pumps time to handle the temporary peak flow. The client was, understandably reluctant to purchase new pipe, remove the existing line and install the larger pipe in it's place. I must admit that his eyes did light up when I offered the larger sump solution so this poor pump salesman never got the order but later events showed he gained a trusting client!

This is the first in a 2-part series. Read Part 2 here.