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Vacuum Pressure Occurence With Gravity Flow Line

siphon vacuum

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#1 cat

cat

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Posted 01 February 2016 - 03:27 AM

Dear Sirs,

 

I am currently running to vacuum issues with one of our small system and would appreciate your insights on this. I have attached an excel file for better illustration of the description below.

 

System:

A closed loop system with 54wt% water and 46wt% ethylene glycol used for cooling the process side: a pump takes in -20C liquid from a tank (both at ground) and supplies the liquid to 2 heat exchangers (HE1, HE2) located approximately +12m above ground. The outlet fluid (-14C) is used to jacket accumulator tanks (T1, T2) located below the exchanger (located at ~8m above ground). The exiting fluid from these T1/T2 (at -12C) flows to a refrigeration system located at ground to cool back the fluid to -20C. This goes back to the tank (N2 blanketed), via dip tube (all the way below the tank bottom).

 

Observations and Data:

 

Observation 1. By design, the flow rate should be approximately ~18,000 kg/hr and discharge pressure at pump should be ~320 kpag. However, the flow we are getting is ~25,000 kg/hr and discharge pressure is 230 kpag. We tried to throttle the valve outlet of the pump, and we are still experiencing high flows as compared with the theoretical (even if the valve was nearly 80% closed).

 

2. We conducted pressure profiling of the system and was surprised by appearance of vacuum  (no pressure gauge during this time, our hands were sucked when we covered the drain valve) at certain points (at elevation of 7.4m). However, at top point (HE1, HE2), pressure was ~100 kpag. Temperature during this time was <-20C.

 

3. The vacuum pressure was -35 kpag. Fluid temperature at that time we got that value was 10C (ref unit was just shut down). When we opened the points, we experienced a drop in flow rate (from 27,000 to ~25,000 kg/hr).

 

4. At fluid temperature 35C, we opened up the drain points again, we experienced alternate vacuuming and liquid draining (every 5s) for system 1 (T1) and continuous discharge for system 2 (T2). The flow dropped again to ~25,00 kg/hr (from 27,000 kg/hr), discharge pressure increased to 220 kpag (from 200 kpag). We then tried to throttle the pump discharge valve while both drain valves were still opened to atmosphere. We did not see any changes in the discharge pressure and flowrates.

 

5. We isolated the ref unit and lined up the other (ref unit 2, which is at the same elevation, but located approximately 5m away from the other), and conducted pressure profiling. We got vacuum pressure (-10 kpag) at the tie-in point (+3m above ground). Temperature during this time was still 35C.

 

6. In all tests, we do not observe any pump vibration.

 

Questions:

1. What cause/s the vacuum pressures in the line?

    - My initial guess is that siphoning effect is occurring. And such, the higher flow rate. On theoretical calculations (assuming allowed design DP across the heat exchangers), I should only get ~1 bar dP across the system, but my pump is designed for ~3bar dP, hence higher flow. Is it correct to assume that my line is "fully filled" and the equations I used are valid?

    - I back calculated from discharge point (where I used back P = static head of liquid) and got some vacuum pressure value, and should not pose for any vaporization at low temperature, VP is almost 0 barg.

 

2. Should I be concerned with the appearance of vacuum pressures in the system if my system is attaining more flow? Aside from material/piping concerns.

 

3. How would I know if there's "boiling/vaporizing" that is occurring within my system? I am quite puzzled that with increase in temperature I did get some vacuuming and discharging actions on the drain valves which I have not experienced with the cold temperature. I checked on the vapor pressure and at 35C is -0.95 barg. But my flowrate was higher (which on reading references, higher flowrates should "flush out" the vapor in the system, if any). Is there any relation to viscosity changes (Nre at low temp <10,000 vs Nre 35C >50,000)

 

Thanks in advanced to everyone.

Attached File  for share - vacuum.zip   2.03MB   66 downloads


Edited by cat, 03 February 2016 - 09:50 PM.


#2 cea

cea

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Posted 12 February 2016 - 04:10 AM

Well cat, I am just putting my best guess after studying the scenario.

 

My first impression is that your all & especially return lines are oversized & also the pump.

Further, as it appears predominantly that return lines must have been sized as self venting lines. In that case, ideally vent valve at highest elevation shall be always open. However, your pump calculation might have been done, considering all lines with pump discharge pressure (and accordingly velocity in pipe).

 

Your all queries can be answered with above justification, as below.

 

1) More flow due to line oversizing.

2) High velocity in return lines due high pressure drop available in pipe, thereby "feel of vacuum" when drain valves are opened. (Not other way round, such as high flow due to vacuum)

3) Further, as you observe alternate vacuum & pressure when temp is 35ºC, this may be because of slight vaporization due to vacuum @ 35ºC & thereby localized pressurization causing again pressure. However, you will not get this phenomenon when temp. is ~-20ºC.

 

Remedial Conclusion: Keep both vent valves on exchanger outlet return line open. With this arrangement, pump discharge pressure will also come down & accordingly flow (as per pump curve) You will not see vacuum in any of the line or in the system.

 

Please do share your observation on attempt.

 

(P.S. Please ensure that lines are really oversized as guessed by calculating velocities in line. Secondly, arrange to get motor rpm reduced either by replacing motor or by installing VFD. Otherwise, fluid will start coming out of the vent. Changing impeller can also be an option. You may have to draw system resistance curve against pump curve to get values for revised rpm or impeller diameter.)

 

Hope, this will help.






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