PLUMBING : WATER SUPPLY MAX VELOCITY 2.4M/S OR 1.5 M/S? 2

[Taha Mustahsan]

Hello, Good Day!

I am working on a Plumbing design for accommodation units. I am assuming the max water velocity is 2.4 m/S (8 fps). However, the client insists to assume 1.5 m/s(4fps) as max allowed velocity. I tried to convince him based on ASHRAE Fundamentals Chapter 22 Pipe size table 22. He is countering it with the statement given on pages 22.25 and 22.26 about plastic pipes' velocity. Are there any references I can use to convince him of the max velocity of 2.4m/s?

Also in IPC 2018, chapter 6 table 604.10.1 has given the size for both cases but he is not convinced, please help me.

thanks and regards,

 

[HVAC-Novice]

Are you mixing HVAC and plumbing? Use Plumbing design guides for plumbing. HVAC design guides for HVAC.

Let the client design and stamp the design since they seem to know everything. Sounds like they have no faith in your capabilities.

 

[ProcessHVAC]

Here in Canada, the code allows you to design down to 1.2 m/s. I would, advice the client that with 1.5 m/s itwould make the pipes bigger, increased capital cost, keep pressure higher than needed in many locations of the system, etc. If they would still want to proceed, then so be it. Usually the request is to undersize the pipes, where pipe velocity is too high, erosion, noise and the system performs poorly during high demands.

 

[Taha Mustahsan]

@HVAC Novice, no i am not mixing HVAC and plumbing in the chapter pipe size there is a part for service water/water supply. Even the IPC gives reference to Ashrae at some places. At the end of the client is a client and he is paying for everything we as a designer has to satisfy the client and as a designer we also need to save money where we can. A better helpful answer would have been appreciated even more but thank for your time.


@ processhvac: exactly my concern, the pipe size will increase and cost is also a factor. Most important thing is it is a modular building and we have limitations with wall thickness as well. Client is more concerned about noise than anyother thing.

 

[HVAC-Novice]

I'm sorry, I didn't read the ASHRAE part and wasn't aware it also includes plumbing....

I'm just starting with Plumbing design (I'm HVAC experienced). But I recommend to mainly look into your Plumbing code (duh!) and the ASPE design books. I have 6 of the ASPE books (they are somewhat redundant in some aspects) and they would be much more appropriate than ASHRAE books. And most of the stuff in ASHRAE books is 50 years old.

I don't know if you are in my situation (just starting) or experienced. But ultimately if the client wants larger pipes, they are paying for them. Maybe they want gold-plated faucets -it's their choice! Just make sure they pay you before they get the bill from the plumber

 

[Taha Mustahsan]

I am MEP (mechanical part) in the field 10 years. Working on NEOM project in saudi Arabia and my client. May be you have heard of the line project. We do have these codes but sometimes one code contradict the other. And my design reviewer (client) is fifty plus and you might understood it is difficult to make them listen to you. His point is from the part where they explain the velocities of chilled water pipes. My point is from the service water section. And even the table is given in IPC 2018 with 8 fps manifold and gpm. We have 5000 plus buildings. Agree for 1 means multiply by 5000. Cost. Saving. All related to each other.

 

[nuuvox000]

May I ask why you're arguing with him about this? If it were me, I think I'd say, "I believe you would be fine with these smaller pipes but if you want bigger pipes to reduce the velocity, it's your money." Bigger pipes are only going to help, aren't they?

 

[ProcessHVAC]

How about trying to negotiate a balance? Small pipe diameters to 1.5 m/s as per his request, larger branches to 2.4 m/s simply because they will not fit in the walls. Then you show him you are doing things "his way" but also doing your due diligence...that usually goes a long way with ol guys ;-)

 

[Artisi]

I agree with nuuvox000, why argue - is it just to win a point, remember the client is paying your bill. Recommend the pipe size you want, if the client wants otherwise so be it, just do it - end of story.

And telling a poster a more helpful answer would be better is impolite.

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.)

 

[PEDARRIN2]

Our general rule of thumb is 2.5 m/s for cold water and 1.5 m/s for hot water.

 

[HVAC-Novice]

Pedarrin: I like to know the reasoning. If there is a difference between hot and cold, I would have picked the hot to be faster since that would reduce water wasted waiting for hot water (even with recirc pump, you still have a long leg from the recirc loop to the fixture).
Only reason to oversize hot water I can think off would be reduce pressure drop fluctuation when people shower and most of the flow is hot water (only very little cold water blended in, if any).

Serious question since I'm just starting with Plumbing Design.

 

[LittleInch]

I don't know if it's true for where pedarrin2 is located, but previously in the UK, hot water systems were predominantly gravity flow from a tank in the loft so often a very low head between tank and outlet with cold supply mainly mains pressure of 2-3 bar. So you needed bigger HW pipes to reduce pressure drop for hot, but not so much for the cold supply.

Now many systems are all pressurised supply but try telling your local plumber why you want 22mm pipes both hot and cold to even out pressure changes and they look at you as though you've come from Mars.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PEDARRIN2]

HVAC-Novice

The rule of thumb is from copper industry guidance.

https://copper.org/publications/pub_list/pdf/copper_tube_handbook_print_press.pdf

Most of the time, the velocity is not just an issue for just the hot water supply but becomes more of an issue on the hot water recirculation system, mainly due to not sizing the pump correctly. Often times, the pump is oversized which causes the pump to flow more than what the piping system heat loss is designed so the flow velocity is higher.

 

[MedicineEng]

Just to add a little more information to the discussion, if the copper pipes are for hot water service, DIN EN12502-2 recommends following DIN1988 to keep flow velocity in circulating lines as low as possible and limited to a 0.5m/s regardless of diameter.
This is to reduce erosion corrosion effects, which typically happen in hot water service.