Available Static Head is Sufficient or Pump is required

[patrickraj]

Dear friends,
Cooling water return header pressure is 1.2 kscg (at ground level). From this point, a tapping is to be taken to take 200 M3/hr of water to a distance of 650 mts(for further treatment). Following is the piping layout:

90°C bend - 35 Nos.
45 °C bend - 5 Nos.
Gate valves - 2 Nos.
Tees - 2 Nos.
Check valves - 2 Nos.
Exit - 1 No.
Entrance - 1 No.

At the discharge point alone, the elevation is 4.0 mts from ground level. Total head loss calculated to be 31.9 ft water column inclusive of elevation. Now the head available is 7.6feet(39.5-31.9)water column. This 7.6 ft head is sufficient or I have to take a tapping from CW pump discharge (pressure is 3 kscg) for this application.

Please advise.

 

[Guidoo]

Patrick,

Do you want to control the flowrate at 200 m3/hr? In other words, are you going to install a flow meter and a control valve in the line?

 

[Guidoo]

By the way, what linesize did you use to calculate the head loss? 8" or 10" ?

 

[patrickraj]

Thanks for your reply Guidoo.
Line ize is 12 Inches,HDPE
I want to control the flowrate at 200 m3/hr
My concern is whether I will get 200 m3/hr for the available head of 7.6 feet.

Thanks in advance

 

[Guidoo]

Are you sure the head loss is 31.9 ft water column? I also did the calc and found a head loss (incl elevation difference of 13.1 ft) of only 18.4 ft water column, resulting in a pressure at the pipe end of 39.5-18.4=21.1 ft. Please note that with a 12" pipe, water velocity is less than 0.8 m/sec. I estimated a pressure drop of only 0.016 bar/100 m.

With following guidelines for control valve pressure drop:

Delta P Control Valve >= 0.25 x Delta P Static
Delta P Control Valve >= 0.33 x Delta P Friction

Using your numbers, this means that pressure drop over control valve must be minimum 0.25 x( 39.5-13.1) = 6.6 ft water column. Since you have available 7.4 ft water column, you have 7.4 - 6.6 = 0.8 ft water column left for your flow measurement.

 

[Guidoo]

Just found that other guideline for control valves is:

Delta P control valve >= 0.7 bar at maximum flow.

0.7 bar equals about 23 ft of water column, so this will be difficult if your numbers are correct.

 

[patrickraj]

Dear GUIDOO,
Thanks for your replies.

(1)The line dia is 9.7" -HDPE, not 12"
(2)I am quite sure,line loss is 31.9 ft WC
(3)Now the available head is(39.5-31.9)=7.58 feet
(3A)WC left for flow measurement=7.58-.25x(39.5-13.1)=.98'WC
(4)I think the following definition is self understandable
Del P Control Valve >=.25x(available head-static head)
(5)Could you please give the second equation also in the same above format.
(6)Is it correct:
Del P Contl Valve>=.33(avalble head - tot line frict loss)
(7)>=oes it mean greater than or equal to

Please give your comments

 

[Guidoo]

(1) With a 9.7" pipe, a 31.9 head loss may be possible.

(5) Second Equation is:
Delta P >= 0.33(total line friction loss)
So in this case:
Delta P >= 0.33x(31.9 - 13.1)=6.2 ft WC
Which is less stringent than the first equation, but most stringent "rule" is that Delta P >= 0.7 bar!

(7) >= indeed means greater than or equal to, but since we are dealing with "rules of thumb" here, I could as well have written just >

Note that these are just rules of thumb. Maybe you should check with an instrumentation vendor to see what is possible. In your case it may be more economic to spend some extra money for a special valve/flow measurement to save the pump. It may also depend on whether you want to tightly control this flow of 200 m3/hr, or whether some less perfect control would be acceptable.

 

[TD2K]

1.2 kg/cm2g (at grade) is equivalent to 12 m water pressure. The new line discharges (I assume to atmospheric pressure) 4 m above grade.

You state the 31.9 ft line loss is inclusive of elevation so that is taken cae of. 31.9 ft is 9.7 m which is less than 12m available so you will get the flow you need. In fact, if you don't have a control valve or some means of controlling the flow, the actual flow will be about 222 m3/hr.

Note, if the 31.9 m does not include the 4 metre elevation (exclusive of elevation), you have now only 8 m for line losses, less than the 9.7 m you need and the actual flow will be about 182 m3/hr.

If you need 200 m3/hr in this case, you need a larger line or a higher supply pressure.

Note, I have not checked your line loss calculations. You haven't specified if the flow through the tee is through the run or branch and more importantly, it's impossible to accurately estimate the dP through the 2 check valves you have without manufacturer's data.

 

[patrickraj]

Thanks for your responses Guidoo and TD2K.I have decided to go with the proposed taping at CW return header with 1.2ksc press. I write once the system is lined up.