Calculating Operating Pressure!!! 2

[supaman79]

Guys,

I'm pretty new to the field, and I have a simple question..I think. I have a closed chilled water loop going to cased chilled water coils...w/ a 40 ton chiller ewt=45 lwt=56...is it as simple as converting my head on my pump to psi? it seems more involved.

 

[Montemayor]

Supa:

What is ewt=45? lwt=56?

What Operating pressure are you looking for? The water pump's circuit? Where, exactly on the circuit?

Are you at the design stage (i.e., are you process designing the system)? Or is the system existing and operating? If it is operating, install a pressure gauge.

If you are in the process design stage, you have to define the system and where in that system you are interested in the "operating pressure". It might be as simple as looking at your pump curve and converting, as you suggest, your system head to psig. However, assuming you have a centrifugal water pump, you will have to generate a system curve and overlay it over the pump's performance curve to find out the pump's operating pressure under your system's definition. Once you identify the operating point, you can make the hydraulic calculations for the pressure drops involved until you get to the place in your system where you want to find out the operating pressure.

I hope you can see what is involved and what I've explained.


Art Montemayor
Spring, TX

 

[supaman79]

The system I am talking about is a design build system (brand new), the ewt and lwt...means entering water temp & leaving water temp. I guess I'm looking for my operating pressure leaving my pump. We are installing underground piping using a "Perma-pipe Econo-Gard" system, where they install concrete blocks around certain pressure points for expansion. One of the values they need to is operating pressure...it sounds like it would be my discharge pressue out of my pump. I should get more info...for you, so I don't waste your time. Thanks

 

[25362]

Would the (centrifugal) pump shutoff pressure be safe enough ?

 

[supaman79]

I don't think so...it seems like they want to know what it would be at my peak position at 42 gpm and "x" amount of head (still undetermined). I'm starting to narrow it down to discharge pressure to insure that my furthest coil gets its required gpm flow and velocity....I'm so lost lol

 

[supaman79]

I just found out from the vendor of this "perma-pipe" system that they need to know the highest operating pressure anywhere along the system that could occur so they can design for expansion and contraction. It's so critical I guess becuase it will be underground. But I have a delta of 11 F, how much can this thing expand from 45-55 degrees (no answered needed lol)

 

[Montemayor]

Supa:

25362, in my opinion, is correct in mentioning your pump shutoff pressure (maximum, "deadhead" pressure) as the value to design upon. If you hold a HazOp, that is what will come out of the discussion. It is very credible to have a blocked-in pump discharge and, as a result, attain deadhead pressure.

Therefore, go to your pump's performance curve and read off the deadhead pressure and design around that value. This is normal procedure when applying a centrifugal pump.

Art Montemayor
Spring, TX

 

[supaman79]

Montemayor,

I appreciate your time that you put in and answered my question, and I thank you. Some of the reasons why I asked this question is becuase I only have an AS in mechanical engineering, and I'm working on the rest...would fluid dynamics cover alot of these typical questions? And are you saying that my shut-off pressure can be applied to mostly all of my design water loops as a "operating pressure"? most likely not huh

 

[TBP]

Supa - Sure seems like a lot of excitment on the part of the Permapipe guys for what's probably a 2" line, full of cold water, with a very low delta-T. Anyway...

The initial fill pressure will fill the system, and push the air out. This pressure only needs to be high enough to fill the system to it's highest elevation, plus 5 or 10 PSI to make sure the lines are full. This pressure might be 20 or 30 PSI, or maybe 100 if there's a big elevation change. That's the pressure that exists in the system, even with the circulating pump is off. It's maintained by a pressure reducing valve, or small high head pump, that will make-up water lost by small leaks, or minor repairs. The circ pump will typically be a relatively large, low-head pump, because it's just moving the water around in a circle, like a Ferris wheel. The weight of water going up is the same as the weight coming down. The difference in system pressure, whether the circ pump is running or not, will likely only be a few PSI.

 

[supaman79]

thanks tbp,

My elevation should only be around 15-25 ft. My longest run will be in the crawl space, but my shortest will be in the attic of the house. Bascially Here are my parameters:

40 ton gas fired chiller
600' to my furthest coil.
11F delta T
96 gpm
pump will be located far away from the house near the chiller.

20-30 psi would do it?

 

[TBP]

15 or 20 PSI should fill your system. I misread the 40 tons as 40 gallons of water circulating. More like a 3" line for 96 GPM. You can ballpark it with 600 feet X 0.06 to get a circ pump with 36 feet of head. This will work for a system with an average number of fittings. Go to Bell & Gossett's web site

http://www.bellgossett.com

and download the "System Syzer" software. It'll help you with a lot of this.

 

[supaman79]

tbp..you've been a great help...along with eveyone else, I appreciate your time and efforts...When I take my fluid Dynamics course, should that help alot of my dumb questions?

 

[Montemayor]

Supa:

When you design a system, such as a centrifugal pump, you have to take into consideration the credible possibility of the pump reaching it's maximum possible pressure - the deadhead pressure. That may not be the ultimate design pressure in the case where other events can also occur - such as a hydrostatic test on the entire system (in situ) or a high pressure source of fluid connected to the system.

Yes, if you're lucky and get a good, hands-on fluids prof who takes the pain to take through a side bar adventure into Fluid Transport (pumps & compressors), you should be able to really soak up all the input you will need in the future to resolve this type of problems (really, applications). Lots of luck in Fluid Dynamics.

Art Montemayor
Spring, TX

 

[imok2]

supaman79 You need not concern yourself with the elevation as the pressure regulater will take care of that; it is not a factor in sizing a pump on a closd system. You need only concern yourself with the PD in the piping, and heat x-changers,like the cooling coil, and evaporator. I can tell you that with 3" pipe and 96 GPM at 600FT the pd is~2.4#/100FT all you need to know is the PD of the HX" which comes from the mfg to determine total PD for the pump

 

[25362]

I didn't grasp the circuit configuration. Anyway, if there is a chance of the water being blocked in and heated, the pressure rise would be about ~40 psi/^oF. However, this wouldn't be an operating condition, it would be rather an abnormal situation, which should be protected against with a thermal relief valve. Any other risk that should be considered ? Please comment.

 

[quark]

I would sure go with TBP if your system operates with 3 way valves and not with the redundant shut off pressure. If you have 2 way valves with your coils, then there are chances that your system will experience shut off pressure during winter conditions.

is it as simple as converting my head on my pump to psi?

Don't ever go with name plate data. Once you install the pump in the system, you will directly get your pressure (no need for any conversion.

Regards,

 

[supaman79]

I'm starting to understand alot of what you guys are saying, but one question I still have though is the head of the pump still an independent value than operating pressure? To clarify some of you guys that don't know exactly how my closed loop consists of its a 40 ton chiller (96 gpm) going to (13) chilled water coils w/ 3-way valves. 20 psi sounds good, but perma-pipe (underground piping) asks for the highest possible operating pressure at any point of the system for expansion and contraction. To me that would sound like a shut-off pressure, or even a pressure that happens when the system stops.."water hammer"...20 psi seems low for 600+ ft...am I wrong?

 

[TBP]

There will be two "expansions" you need to consider. One will be the thermal expansion/contraction of the pipe. This will probably be of little consequence, with the temperatures involved in a tyical chilled water system. The other will be the expansion and contraction of the water in the system. It will also be small, but a little bit of temp increase on a closed system can generate some exciting hydraulic pressures. An expansion tank will give the expanding water somewhere to go. An expansion tank is an operational device, not a safety device. Expansion tanks keep the relief valve from lifting and burping out a cupfull of water every time the system water warms up. (Long term, this is bad, from a corrosion standpoint.) I believe the piping guys will want the relief valve set point, so they know what the max pressure in the system can go to, under worst-case conditions.