36 HEALTH AND SANITATION

By Christoph Lohr

David Viola , IAPMO CEO

In the spirit of the sharing of unique experiences that shape the plumbing industries in our respective nations , the following article is the second of a threepart series looking at the formation of Hunter ’ s Curve , which attempted to predict peak water demand in a building . Written by IAPMO Vice President of Strategic Initiatives Christoph Lohr , P . E ., it originally ran in PM Engineer magazine and is the next in a regular series of similar articles that will run in this magazine . Dave Viola IAPMO CEO

“… unintended consequences that can occur from lowering flow rates without changing other practices .”

Misunderstanding Roy — Part 2

In creating Hunter ’ s Curve , the beauty of what Dr . Roy Hunter did was to take an incredibly complex problem in probability theory and make it simple – so simple , in fact , that all it required was basic arithmetic . The complexity involved a “ mixed system ” combining the calculation of instantaneous demand in a water distribution system with fixture types that differ from each other in frequency of use and flow rate . The solution resulted in a fixture list with unique unit values ( fixture units ) and a design curve predicting the 99th percentile flow rate for each fixture unit value . In order to properly size the pipes in a domestic water ( i . e ., potable water ) piping system , each plumbing fixture ( e . g ., toilet , sink , shower , etc .) had an assigned value , known as the fixture unit ( specifically , the water supply fixture unit ). All the plumbing system engineer or designer needed to do was count the number of each type of plumbing fixture inside a building or area inside the building , multiply by each plumbing fixture ’ s “ fixture unit ” value and then total the fixture units . From there , referencing the “ Hunter ’ s Curve ” chart would lead to a given instantaneous peak flow rate . Referring to Table 1.1 and Figure 1.1 you can see an example of this process .

Table 1.1 : UPC Fixture Unit Example

As a point of comparison , if you assumed the 100th percentile flow ( i . e ., everything on simultaneously ) in lieu of 99th percentile , the calculated peak flow rate would approach 250 gpm ( assuming 25 gpm per flushometer , and 0.5 gpm per lavatory ). By utilising an elegant probability model , Hunter ’ s Curve had immense benefits outside of reduced construction costs ; most notably , a drastic reduction in the time it took engineers to design plumbing systems . In fact , the simplification was so drastic that many mechanical engineers delegated the design of plumbing systems to “ drafters ” — that is the folks who would hand draw engineering plans via direction from an engineer .

The model plumbing codes are incredibly detailed in the instructions on how to size plumbing systems , which in essence “ encourages ” the design professional to follow the Hunter Curve methodology . Building codes , including the plumbing code , are considered the minimum standard of design . Deviating from code exposes the engineer to much liability — if the engineer doesn ’ t follow this methodology and reduces the size , and the system doesn ’ t work , they would have to pay an especially high amount in damages ( and potentially lose their professional engineering license ). This would be an increase in damage paid due to deviating from the code ; rather than being less severe “ negligent ”

damages that would be paid , the damages would be higher “ punitive ” ones .

Supplied by IAPMO

Figure 1.1 Hunter ’ s Curve

Additionally , most design professionals work for projects via “ lump sums ” rather than “ time and materials .” Another way to put this is that engineering firms estimate how much time a project will take and give their client a proposal for that time , rather than charging “ by the hour ” for a project . By estimating how much effort will be required for a project prior to starting the process , the challenge for the engineer is to budget their time in such a manner that the project is completed in a timely manner . Thus , the financial incentive for engineering firms is to find the most efficient way of completing projects ( rather than finding the best solution for the owner ).

www . plumbingafrica . co . za @ plumbingonline @ plumbingonline @ PlumbingAfricaOnline March 2023 Volume 29 I Number 1