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)

 

[QualityTime]

Hi Moideen, I looked closer at what you are saying. Let’s look at initial drawing that was posted and the information that was given to us and referred to by you in your example:

Given:

[ol 1.]
[li]The farthest cooling coil is 130m away from the rooftop plant. (100m elevation & 30m horizontal pipe run)[/li]
[li]The pump is rated at approximately 822,000 gal/day (570 usgpm, 36 l/s)[/li]
[/ol]

Approximate Calculations

[ol 1.]
[li]Therefore assume 6” discharge run of 439 ft (130m) and a pipe friction factor C=130. The line velocity is approximately 6.3 ft/s. The velocity head is approximately 0.62 ft. The friction loss is approximately 25 ft/1000 ft or 11 ft[/li]

[li]Therefore assume 8” suction run of 439 ft (130m) and a pipe friction factor C=130. The line velocity is approximately 3.55 ft/s. The velocity head is approximately 0.2 ft. The friction loss is approximately 6.1 ft/1000 ft or 0.6 ft[/li]

[li]Therefore at a flow of 570 gpm the total dynamic head = 12.2 ft (11 + 0.6 + 0.6). Therefore we are looking at a pump that is supposed to have an approximate condition point of 570 usgpm @ 12.2 ft TDH[/li]

[li]For a 8x6 pump at 822,000 gal/day (570 usgpm, 36 l/s) the difference in velocity head = 0.6 ft (0.62-.2). The velocity head difference is 5% (0.6*100/12.2) of the total dynamic head. This is not insignificant[/li]
[li][/li]
[/ol]

In summary, I will advise that this is a low head application. I will also advise the selection of the pump looks cockeyed to me

 

[Jeff1988]

Hi QualityTime, moideen,

Sorry gents, my question must have caused confusion and data is cockeyed. That's partly because that project I looked at was at a preliminary stage.

This is the doc I've received from my client. There's no schematic available. The only schematic available for the chillers looks like a copy-&-paste from some other project. All the pipe route and sizes are marked on preliminary drawings by the contractor. I don't think they've planned it all out yet.

Honestly I've got over that tender and moved on now.

 

[QualityTime]

What was the purpose of putting out preliminary drawings for tender

 

[Jeff1988]

Thanks QualityTime,

Thank you for explaining what's system curve. Gosh I should have asked earlier, instead of staring at it a whole morning.

 

[Jeff1988]

Budgeting.
In fact, some of project are won by the contractors based on their preliminary design, because compared to the consultant engineer's design, the contractor's design is cheaper. (for example, water-cooled is changed to air-cooled; water-based changed to refrigerant; etc)

I know the best practice is consultant engineer making the best design and the contractors just quote on it in open tenders. But not all consultant engineers are great. Maybe it's life.

 

[Jeff1988]

QualityTime,

Sorry I forgot to mention that I'm in a subbie doing piping job for the contractors. We just provide budget for our client the contractor to estimate the cost impact of different designs.