Domestic Water Elevation Pressure Drop 1

[doopedeng]

Hi All, I'm a junior mech'l designer and have not had the opportunity to work on a bldg of more than 3 stories, until now. I have a 9 storey condo building to design and have the following question: When determining the requirement for a domestic water booster pump, do I simply look at 1psi drop for every 2.31 ft in elevation or do I have to worry about pipe friction as well? Any other tips/advise would be appreciated.

Project Data: 87psi at main, height to highest fixture outlet is 110ft.

TIA

 

[TD2K]

The 2.31 ft/psi is simply the pressure loss due to elevation. Piping hydraulic losses when flowing are on top of those.

That's all I can help you with, domestic water system design is outside my area.

 

[Montemayor]

doopedeng:

No matter where you are in the USA (assuming you are in the USA) there will most probably be a means to protect the people who will populate the 9-story condo building you are working on. I have no objections or arguments against you "designing" the piping, but sooner or later - before the design is approved for construction - the blueprints or drawings will have to be "stamped" and approved by a professional, registered engineer. If it was my building project and my stamp, I would have detailed pressure drop calculations made on all the water circuits involved in such a Condo environment. I am sure that is the situation in your project as well. If not, you are in a dilemma if you are not an experienced engineer capable of making all the fluid flow calculations required to ensure that there will be enough drinking, flushing, and fire-fighting water available at all levels of the 9-stories involved PLUS the roof - and at the correct and acceptable pressures for each of the specific services.

Of course you have to "worry" about the pipe friction. That's what it's all about - higher flow rate requirements necessitate a bigger pipe and probably more pressure. You should have a design that allows for sufficient water being available at the correct pressure and flow rate for ALL credible situations. Otherwise you are going to have a lot of irate occupants and fire marshalls on your back. You should have the detailed calculations and specifications on the size, type, layout, and quality of piping and fittings (as well as critical valve and their specific locations) in your hands as prepared and specified in detail by a professional engineer (if you yourself are not an experienced, degreed engineer). If this is not the existing situation, then in my opinion you have a very serious problem and the general contractor does too. I cannot add further advice until I can at least know that the above, minimum requirements are in hand.

 

[PEDARRIN2]

As an engineer (although not a PE) who does design plumbing systems, you must take into account the friction losses of the piping as well as the static losses due to the elevation difference.

Another thing you will need to consider which a lot of designers miss, is that to have sufficient pressure at your most remote fixture on the top floor, you may have too much pressure for the fixtures near the pump. That is why a lot of booster pumps are located on a middle floor. If that cannot be done, then a split system is typically used where the pumped water is supplied to the upper floors and the non pumped water is supplied to the lower floors.

The plumbing codes typically limit pressure to fixtures at 80 psi.

In the US, the IPC, which I am familiar and I believe it is true of the UPC, before the contractor can get a permit, the drawings have to be reviewed by the AHJ. And likewise, the AHJ requires that plans to be reviewed are stamped by a licensed professional engineer or architect.

Now I have seen drawings stamped by an architect/engineer which I would not wish on my worst enemy (their area of expertise is not in piping/plumbing). So in order to save yourself a lot of problems, have a PE who is familiar with piping and flow designs review both your design layout and the calculations.

Good luck.

 

[doopedeng]

Thanks for the input folks, and sorry the question wasn't clearer. The question is not how to design the piping, or how to size the piping. The question is do I need a booster pump? I have a very long developed length and it appears as though my piping will have to be very large to produce anything close to the required pressure at the top fixtures. When is the decision made to go to pressure booster versus just installing the larger pipe sizes?

Thanks
Engineer

 

[Montemayor]

doopedeng:

I'm afraid there is a general lack of understanding here between yourself and ourselves - especially in the terms used. The answer to whether you need a booster pump or not (and where) is resolved just as I and PEDARRIN2 have pointed out: you must define what your basic data or requirements are. There are legal, local, mandatory limits placed on the minimum pressure required for a fire hydrant or fire-fighting source, for example. There are similar minimum limits placed on drinking water and toilet water supply. You have to conform to these. When these are identified and established, then you do the customary pressure drop calculations and determine any pump needs - if you run out of pressure.

I believe PEDARRIN2 is very clear in his explanation of how - generally - a project like this is handled. But you must verify the conditions and requirements locally, under your local environment and in accordance with your project.

I hope this explains a little better what we are trying to say. Please continue to ask questions if there are any doubts.

 

[trashcanman]

As pointed out above, pressure required at ground floor = elevation loss (add 6' above top floor for shower) + friction loss + pressure req'd at fixture. For non-flush valve fixture, I use 12 psi. For flush valves, use 25 psi. Also, I include about 5 psi for a water meter and/or backflow preventer (unless I have the actual meter data available). The friction loss includes piping losses to most remote fixture (horizontal + vertical piping + fittings). Generally, add 50% of the longest run to account for fittings.
Now that you have calculated the pressure required = X, check the incoming pressure. If incoming pressure is < X, you need a booster pump. If it is at least 10 psi > X, use the city water pressure.
After completing the calculations, have it checked by a PE who has expertise in this field.

 

[doopedeng]

trashcanman,
All good info. It never fails, the architect always wants to know the space requirments before any other information is known. Hence the reason to quickly determine if I need a booster pump. I was hoping for a quick "rule of thumb" type response to avoid doing it twice (floor plans aren't even frozen yet), but it's looking like I'm not going to get it.... I suppose I have no choice but to get into the more detailed calculations for pipe friction losses and fittings while still in the preliminary stages of the project. I guess that's why they pay us the big bucks, huh!? (I wish!)

If you use a pressure of 12psi at non-flush valve fixtures (and some texts even suggest as low as 8 psi), why is it that well pump pressure switches are usually 40/60 psi? Shouldn't a high end condo project have substantially more pressure than 12psi? 12psi just seems so low. Thoughts?

 

[cvg]

you state 87 psi at the main and 110 feet to the top which will give you about 39psi at the top. however, your main pressure will fluctuate throughout the day, maybe as much as 10 - 20 psi. given that, and the fact that you will have numerous losses in the system, I would recommend a pump

 

[PEDARRIN2]

If you are in the beginning stages of the design and you are wondering if you will need a pump, box out in the mechanical room a ~7' long by ~4' wide "box" which a typical booster pump would fit and have access to.

Once you have more solid floor plans (at least where you can locate the farthest fixture), you can run the calcs to see if you need the pump. If you do then you have space. If you do not you can "give back the space. That way you are not having to get more space later when a pump is required.

Good luck.

 

[cvg]

regarding your question of 40 psi, that is usually measured at the point of connection to the main line. Once flow goes through your meter and service connection into the building, pressure will drop. The quoted pressures at plumbing fixtures should be considered minimum to operate the fixture properly.

 

[zdas04]

doopedeng,
You have probably spent more time trying to find a shortcut than it would have taken to do the calcs. Assume a generous number of independent spaces on the top floor, set you capacity for that and do the calcs. If you are near a cusp of needing or not needing a booster pump, ask the architect for a better number of units.

David

 

[trashcanman]

The 12 psi at the shower is what is recommended. Bear in mind that it might take 100 psi at the street main to get to that shower with 12 psi.

 

[doopedeng]

Thanks everyone,
zdas04 is right... I'm spending too much time worrying about it. Nice to have more people to bounce ideas around with though. Appreciate the input. It's looking as though I will be going with a booster pump.

Here's a question....maybe this should be a separate thread, but - does anyone here use COPPER victaulic (or equivalent) for larger DOMESTIC water lines 2.5, 3, 4 inch? If not, just soldered? or anyone using cast iron? What are the industry norms where you are located? Reason for asking is that we don't typically have many larger projects here, so it doesn't come up that often.

 

[bimr]

Here is an article from PM Engineer that explains how to size the booster pump:

http://www.pmengineer.com/CDA/Archives/d6bdedda89298010VgnVCM100000f932a8c0____

If the link doesn't work, search pm engineer magazine for "booster pumps".

You can probably find your water line question in the magazine as well.