Pressurization of high rise building

[AVDesign]

Hello

I have a hotel building, approximately 75 meters (246ft) in height, with heat generating equipment at the lowest level, pumping up the building to equipment on the roof.

Max operating pressure of the weakest component in my system is 10bar gauge (145Psi), therefore I estimate the safety valve setting to be approximately 0.5bar below @ 9.5bar gauge (138Psi).

The low temperature hot water circulation pump is selected to deliver 3bar gauge to overcome the system frictional losses, and the pressurization unit has been selected to overcome the static height of the building 7.35bar gauge plus a small pressure margin of 0.3bar to expel air from the system.

It has been suggested to me the the safety valve setting should be static height (7.36bar gauge) + circulating pump pressure (3bar gauge) making the maximum operating pressure 10.36bar gauge, which he's rounded up to 11bar gauge.

However, in my mind, the pressure developed by the circulating pump is separate to the pressure developed by the pressurisation unit and its not a case of adding these values together. they do not have a cumulative impact on one another, can anyone confirm this or correct me? I know this is fundamental so hoping it should be easy for someone?

Thank you in advance.

AVD

 

[LittleInch]

In one sense the circulation pressure is separate from the static pressure in that you are only calculating and providing differential head/pressure to overcome friction around the closed loop.

However when it comes to total pressure that the pipe sees then yes, it is a summation of static pressure plus differential discharge pressure of the pump at the discharge of the pump.

Now it does start to matter where your weakest point is as this pressure will vary by height above the pump, but if its on the discharge side of the pump in the basement then you would over pressure the system once you start the pump.

Your max pressure at the bottom is actually 7.35 + 0.3 bar pressurisation at the highest point + 3 = 10.65

BTW the differential pressure across the pump is just bar not barg

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

 

[bimr]

For the domestic water supply, your building will probably require 2 pressure zones. Plumbing codes usually limit the high water pressure to 80 psi. Codes often limit the low water pressure to 20 psi, unless there are fixtures such as flush valves that require greater pressures.

I would suggest you review the typical design requirements for high-rise buildings before you begin design:

high-rise-water-distribution

DesignForHigh-RiseBuildings

 

[AVDesign]

Gents,

Thank you for the responses, most useful indeed. Its not on the domestic water system so not overly concerned about that, its just on the heating circuit.

The pressurisation unit and exp. vessels are connected to the pump return in the basement so its dealing with the entire static height of the system above it and based on LittleInch's response - shouldn't therefore over-pressure the system. Trying to get my head around the idea that the maximum pressure observed by the system will be at the highest point of the system, in this case on the roof.

If the pump generates a maximum 3 bar to overcome frictional pipe losses, surely the maximum pressure would be observed at the pump outlet and as the water moves around the system the pump generated pressure is reduced or 'used up' so-to-speak, to overcome resistance offered by the pipe and its fittings until it has moved that body of water around the system and back to the pump inlet?

I've attached a very crude sketch......

https://files.engineering.com/getfi...fd69-484d-bbaf-064d6da454ee&file=IMG_2356.JPG

 

[itsmoked]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

AV design,

Not sure how you got to the idea that the pressure was highest at the highest point?? Can't see that in either response.

Any way your third para is correct. Highest pressure as per your sketch is immediately D/S of the pump when the pump is located at the low point of the system. And the pressure seen by the pressurisation vessel is really just seeing the static head.

BTW your pressure gauges don't make sense if they are supposed to be barg.

If your PG was a single differential guage then it would read 3 bar, but if not it would read nearly 8 and 11 respectively...

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

 

[AVDesign]

Hello LittleInch

Thanks again for your input. Apologies, I interpreted your comment "Your max pressure at the bottom is actually 7.35 + 0.3 bar pressurisation at the highest point + 3 = 10.65". Thank you for clarifying I've interpreted it incorrectly.

The pressures on the gauges were just demonstrative really, but point taken

If you're saying the pump inlet gauge would be 8bar and the outlet gauge would read (8+3) 11bar, this is a simple rounding-up from 10.65bar?

So, to summarise, given the information above, the answer to the original query is that the maximum pressure operating pressure experienced in the system would involve at least the addition of the pressure due to the static height and the pressure developed by the circulation pump?

Cheers

AV

 

[LittleInch]

Yes I just rounded both up to 8 and 11 bar

In answer to your last question - Yes because your pump is at the lowest point. If your pump was half way up or at the top of the building you would have a different answer.

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

 

[georgeverghese]

Developed head at the pump would be higher when the fixed speed pump operates at min flow. If you don't have a pump curve to work from, use say 20-30% more. So max discharge pressure=1.3*(3+0.3)+7.35=11.65barg. 30% rise in dp at min flow to be confirmed upon receipt of pump curve.

 

[AVDesign]

Gents,

Thank you for this information, most useful!

Kind regards

AV

 

[TBP]

Just wondering why the mechanical equipment isn't in a penthouse? Most are - you can have a very commonly available hot water boiler with a relief valve that lifts (typically) at 30 PSIG. Having everything in the penthouse also allows you to have the lowest pressure available, with the hottest water, which enables the best possible point to remove dissolved air.

As far as the pump goes, it just needs to move the water. An example I read years ago equated the circ pump to the drive for a ferris wheel. The ride operator arranges his customers such that the weight being raised is about equal to the weight coming down the other side. This means you don't require very much horsepower to make it work. Hot water heating systems operate exactly the same way.

Find some Bell & Gossett (or whoever owns them now) literature - it covers all of this much better than I can.