Booster Pump Sizing vs Distribution Pipe Sizing

[EnOm]

Hi
I know that Load Unit tables can be used to size individual pipes in a building by finding the equivalent maximum probable flow rate to a specific number of Load Units.
Can this analogy be extended such that the flow to be provided by the booster pump is that corresponding to the total Load Units in the building? By simple logic I think it should, but I'm asking just in case there is a catch somewhere.
Thank you

Regards.

 

[rotw]

I don't think this is correct.

The more you add elements the more the probability that all elements are opened simultaneously reduces.
So for one apartment there is a given probability that all equipment are activated at the same time.
For a complete building, this probability reduces the reason is because when there are much more elements there are less chances that all are activated together. Following this logic If I consider a skyscraper 100 time bigger than a small building, I don't expect the booster pump system of the skyscraper to be 100 time bigger than the one of the small building.

As you are doing, I think you will overestimate the total flow because you are considering the total probable flow to be the sum of each individual probable flow.

PS: Maybe that is the reason of the difference of flow in the previous related thread between both method we referred to?

 

[EnOm]

Thank you for the response.

But I think you misunderstood my question but let me first clarify some points:
The definition of "Loading Units" according to my understanding and what I read is:
"Loading Unit": A factor which takes into account the flow rate at the appliance, the length of time in use, and the frequency of use.
The loading units have a built in simultaneity factor.
In the KSB document that you pointed me to (I managed to understand what I need using Google Translate) the simultaneity factor was added at the end after summing all the flows demanded by the individual fixtures.
In the Loading Units method the the simultaneity factor is included along with the summation.

Another point is the following: In the Loading units methods (there are several out there based on the same concept) one cannot multiply flows. Say for a small building I have a total of 200 LUs (Loading Units) and the corresponding flow is 1.7 L/s. For the a sky scraper with 2000 LUs, the flow is NOT 1.7 x 10 = 17 L/s. It will be much lower. One has to refer to the data tables. The trend the data follows is that the simultaneity factor decreases as the number of LUs goes up.

Now, the reason my flow in other thread was much higher than what you came up with was because I was using a data set based on Hunter's Curve, which was developed in 1924. It has already been established that it tends to over-estiamte flow demand, especially for relatively small applications. But it still exists in some codes so it has some credibility.

My question for in this thread is: The methods I am using are aimed at pipe sizing, including the main pipe attached to the booster pump. Now, if I get that the most probable flow for my main pipe is X L/s, does this mean that I can take my pump flow to equal to X L/s despite the fact that the calculations I used were for sizing the pipe, and not the pump. According to my logic the pump flow-rate should also be X L/s, but I'm wondering if there a catch somewhere.

 

[rotw]

EnOm,

Thanks for the clarifications. I found a document describing Hunter's curve

http://www.cedengineering.com/upload/Potable Water Plumbing.pdf

It was good to know about it.

With regard to your question, I think the size of the pipe is standardized size and should cover the max capacity of the pump, if I am correct.

While the pump in itself should be designed to the BEP (or +/- close), so you if you proceed by designing the pump so to have the BEP matching the pipe capacity, I guess you end up with a bigger pump and then the pipe will happen to be no more adapted to max pump capacity and your efficiency will drop.
what do you think ?

 

[EnOm]

rotaryworld:
Thank you for your response. I apologize for the delay in mine, been quite busy and I kept remembering it and then forgetting it again :/

To be honest I don't fully understand your last statement, it is probably because I have not done a lot of reading about pump efficiency because final pump selection is outside my scope of work. What I am responsible for is determining the design flow rate and head. The contractor selecting the pump model and installing it will be responsible for finding a pump that can meet those parameters as close to the BEP as possible. I still did not get the answer I wanted I apologize for the repition. I think it's best to explain with an example with some random number for the sake of demonstration.

Say I have a building with 2500 Loading Units, referring to my data tables I get the most probable flow equivalent to 2500 Loading Units is equal to 4.5 L/s. Now what I used here are data tables for pipe sizing

Does this mean that the booster pressure pump supplying water to the building should be selected to produce 4.5 L/s (at the required head at a reasonable efficiency)?
Or is there a completely different method of finding the required booster pump flow? A method other than one specifically aimed at pipe sizing?

According to my logic it would stand to reason to also select a pump able to produce 4.5 L/s at the required head at a reasonable efficiency. Because after all, that is the most probably flow that would be demanded by the building. And anything outside of that should fall within the margins available around the design point.

Furthermore it would also mean that my discharge pipe should be sized for this flow as well, and not the maximum pump flow rate. I do not think it is very likely that the pump would run at its maximum capacity, and even if it did it would probably be for a very short period.

I do agree that its logical that the intake and discharge manifolds should be manufactured to support the maximum flow rate, but I don't think this extends to the discharge pipe.

Thank you
Best Regards

 

[rotw]

En0m,

A pump is selected to operate 80-120% of its Bep.
You shoud define the flow range for the pump to allow for sizing the pump.
I think what you arr doing is getting the pipe size first, pick up the corresponding flow value from standard table. I think what you get then is the MAX flow capacity of the pipe.
Then you allocate that value to be the pump NOMINAL (bep) flow which seems to me inconsistent.

 

[EnOm]

rotaryworld
You do raise an interesting point. I will keep looking into it.

Thanks

Best Regards