Why my close-loop CHW pipework has 87 psi close to the highest point? 3

[Jeff1988]

Hello ladies & gents,

I'm tendering on a mechanical plumbing job for this building with the air-cooled chillers & all other equipment located on rooftop.

My client's engineer told me "the required test pressure on the roof will be approx. 925kPa". Considering we test 1.5 times of working pressure, the working pressure will be about 600kPa (87psi).

I wonder if 87 psi is a normal pressure for pump head + expansion pre-charge?

Not sure if this information is useful, but the farthest cooling coil is 130m away from the rooftop plant. (100m elevation & 30m horizontal pipe run)

 

[Compositepro]

A test pressure of 87 psi sounds reasonable. That does not means normal pressure. What problems do you have with it?

 

[Jeff1988]

Thank you Compositepro.

I want to learn how to estimate approx pump head, so that I can estimate the working pressure of a system and figure out what material/fitting to be used for piping.
Looks like I need to find some books first. All good. Thanks mate.

 

[EnergyProfessional]

Pressure at lower locations will be higher than on the roof. Based on fluid density. For water, every 2.31 ft below the high point of 87psi you need to add 1 psi.
Pump head will be highest at discharge of the pump and zero at suction side.

 

[LittleInch]

It sounds about right,

In a closed loop system with pressure > atmospheric at the top for the pump head all you need to worry about is frictional losses. The elevation pressures balance each other out between flow and return. This differential head is often quite small.

Actual pressure of a fitting does depend on elevation from the high point where you add the static pressure at the high point ( your 6 bar) plus elevation difference converted to pressure and depending on where you put your pressure / expansion vessel the differential pump head.

So it is common for the static head at the highest point to be about 5 bar to cope with changes of temperature, inpact of flow from the pump and other losses to maintain a pressure > 0 barg at the highest point.


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

 

[QualityTime]

[ol ]
[li]The engineer is advising the required test pressure on the roof will be approx. 925 kPa. That is equal to 134 psi[/li]
[li]You have advised that there is a 100m elevation difference in the piping. That is equal to 142 psi pressure at the lowest point in the piping[/li]
[li]Therefore the piping, at its lowest point will be subject to 142 psi static pressure at all times. I would clarify with the engineer if you are to pressure test at 1.5 x 142 = 213 psi to be achieved at the lowest point in the piping. I would also clarify with him what should the flange ratings and the pipe pressure ratings should be[/li]
[/ol]

 

[Jeff1988]

Thank you EnergyProfessional,
"Pump head will be highest at discharge of the pump and zero at suction side" is brand new information for me and it's making perfect sense.
I just need to get some books to make up for my ignorance about pump. Cheers.

 

[Jeff1988]

Thank you LittleInch,

That's one thorough explanation of how this thing works.
To calculate the pump head required, I assume I need to estimate total frictional losses? How do I do that? Any books/manual/webpage from which I can learn something about it?

 

[Jeff1988]

Thank you QualityTime,

Yeah we are using steel pipes for the low level because the cheaper option copper pipe definitely won't hold. And it's amazing that the fittings hold better than the pipe material... I'm just trying to figure out if the weakest link -- copper pipe will hold on the high level.

What puzzles me is that we've been using 200ø Type B copper pipes in several Chiller plants and had no problem passing pressure tests. But Type B copper is said to have a "Safe Working Pressure" of 720 kPa (104 psi). I'm a bit confused how a "Safe Working Pressure" of 104 psi got us through pressure tests that are normally at least 130 psi.

Anyhow, that's another story of mystery for me to deal with some other day.

 

[QualityTime]

....The engineer is advising the required test pressure on the roof will be approx. 925 kPa. (That is equal to 134 psi)....

Going back to my last post, just to let you know...this statement by the engineer does not make sense. If you have 134 psi at the very top that would mean you have 276 psi (134 + 142) at the lowest elevation. Something is cockeyed here. BTW why are you looking at the pipe and the fittings? The engineer should be specifying the pipe and fittings. That is their job

 

[Jeff1988]

QualityTime,

Thanks mate for showing interest in my situation here. What you said is exactly how the numbers blow my mind.

For the pipework at the bottom of the building, the engineer advised us to "clarify what jointing method you are proposing for the copper pipe (up to 40dia). This will need to be able to withstand a worst case 2430kPa (350 psi) during our pressure testing."

I think this 350 psi requirement is calculated by (87 psi pump head & expansion + 145 psi static) x 1.5 times for pressure testing.

My limited knowledge of engineering only tells me 100m elevation means 145 psi static pressure. But I had no idea if 87 psi pump head & expansion is a reasonable number. Judging from the answers I've got, I suppose it seems reasonable, considering the pump is feeding 400+ FCUs. But I really need to learn something about pump head calculation to know better.

 

[QualityTime]

For the pipework at the bottom of the building, the engineer advised us to "clarify what jointing method you are proposing for the copper pipe (up to 40dia). This will need to be able to withstand a worst case 2430kPa (350 psi) during our pressure testing."



Copper pipe cannot withstand this pressure. You need to provide a piping schematic with pipe sizes, elevations and flow. What are the pumps rated for? What is the precharge pressure?

 

[Jeff1988]

 

[Jeff1988]

"What is the precharge pressure?" -- Don't know how much is Precharge. But it shouldn't be very high I suppose?

"What are the pumps rated for?" -- What is pump rated/rating?

 

[Jeff1988]

By the way, engineering is so much fun, when you are not doing it for a living that is.

 

[Jeff1988]

QualityTime,

"Copper pipe cannot withstand this pressure." Yeah we seldom test above 2000 kPa in most of our job. But 40ø Type B Copper has "Safe Working Pressure" of 2300 kPa, and 40ø Type A Copper has 3100 kPa.
I'm not sure what this "Safe Working Pressure" means, but looks like 40ø and below sizes of copper is actually ok, lucky for me.

 

[EnergyProfessional]

you should study up on "point of no pressure change"

I also recommend reading "Modern hydronic Heating" by Siegenthaler.

 

[QualityTime]

[ol 1]
[li]Sounds like you are Australian judging from the references to the copper pipe designations that you are using[/li]
[li]A pump has a rating. It is typically mounted on a nameplate on the pump. In this case, from the drawing you submitted, the pump is rated at 36 L/s. The other part of the rating not shown on the drawing is the head on the pump measured in meters (or ft). In summary, for example, if the pump was rated for 36 L/s at 10 m head that means if the pump experiences 10 m of head it will put out 36 L/s. "But I had no idea if 87 psi pump head & expansion is a reasonable number...." The 87 psi (201 ft) value sounds way too high. The head on the pump, in this case, should only be the pipe friction losses in the closed loop system...there should not be too much friction loss because this is a small system [/li]
[li]You seem to be eager to learn about pumping systems. That is a very good thing. There is a lot of information and terminology to learn. You have asked the right questions. But you can get yourself in trouble and waste your time if you do not have the initial proper guidance. This forum is more for engineers who have some experience in the basics[/li]
[li]I would suggest that you have a meeting with the engineer or find an engineer who can take the time to explain the basics, with diagrams and examples, to you. Nothing beats face to face communication. But I will say that there seems to be a disconnect in what the engineer was purported to have said and some of the numbers you have advised here. Good luck [/li]
[/ol]

 

[QualityTime]

I am curious. Is the chilled water system a variable flow system or a constant flow system

 

[EnergyProfessional]

Keep in mind pressure ratings have a temperature associated. the higher the temperature, the lower pressure it can withstand.
I'm too lazy to look that up, but some piping equipment has a 160 psi rating at 150°F,and 120 psi at 250°F (random numbers just to illustrate).
Typically you use pressures way below the rating. One wouldn't 100 psi rated equipment when you expect 99 psi.