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Articles  >  Butterfly valves vs. Globe type valves for pressure regulation  >    

Butterfly valves vs. Globe type valves for pressure regulation


A good control valve should be able to create the requested energy loss in a controlled manner.
A valve, that small modification in its position generates large change in its hydraulic resistance- is a bad regulator, as it is very difficult to set the requested position and the expected result is "hunting" (un-stabilized pressure and flow rate). 
In order to anticipate the behavior of the designed automatic valve in a specific case, assumed:
Pipe diameter 800mm
Flow rate 400 l/s, V= 0.8m/s
Requested δ P across the valve: 10 to 50m, at 5m steps. 

Two valves were inspected:

1. A Butterfly valve model B2, D= 600mm, Kv = 4800 (L/S @ 1 bar)

2. An Automatic valve mod. 30-24, D= 600mm, Kv= 1805 (L/S @ 1 bar)

The calculations were done using the equations:
              δ P (Fully-open) = K*Q2/2gA2 = C*Q2  Where C=K/2gA2
              δ P (Throttled)  = Cn*Q^2
              Cn = C / (An / Ao)^2
(An / Ao =  Ratio of open area of the throttled valve to the area of fully- open valve)

For a Butterfly valve:   An / Ao = 1- COS Æ (Æ = Degrees of opening)
For a Globe valve:       An / Ao = Zn / Zo   (Z= diaphragm position, linear movement) 
    C of Butterfly valve   δ P / Kv^2  =  10 / 4800^2 =  4.34E-7
    C of the S-300 valve  δ P / Kv^2  =  10 / 1805^2 =  3.07E-6

Results of calculations- Expected positions of the valves at a set flow rate and changing pressure differential conditions:

δ P (m)

B2 Butterfly

Degrees open

An/Ao

% of travel

S-300 valve An/Ao

% of travel

1

5

10

15

20

25

30

35

40

45

50

 

42.567

28.097

23.556

21.257

19.77

18.684

17.845

17.165

16.598

16.113

15.691

 

0.264

0.118

0.083

0.068

0.059

0.053

0.048

0.045

0.042

0.040

0.037

 

47.297

31.218

26.173

23.619

21.963

20.761

19.828

19.072

18.442

17.903

17.435

 

0.701

0.313

0.222

0.181

0.157

0.140

0.128

0.118

0.111

0.104

0.099

 

70.086

31.343

22.163

18.096

15.672

14.017

12.796

11.847

11.082

10.448

9.9117

 

artbfl1_719

Kn change comparison for valves regulating at a set pressure differential (30 m) and changing demand conditions:

Q (L/sec)

B2  Butterfly degrees open

% of full travel

S-300 valve An/Ao

% of full travel

1

1.170

0.013

0.001

0.055

5

2.615

0.029

0.003

0.277

10

3.699

0.041

0.006

0.554

20

5.232

0.058

0.011

1.108

40

7.402

0.082

0.022

2.216

60

9.069

0.101

0.033

3.324

80

10.48

0.116

0.044

4.433

100

11.72

0.130

0.055

5.541

120

12.84

0.143

0.066

6.649

140

13.87

0.154

0.078

7.757

160

14.83

0.165

0.089

8.865

180

15.74

0.175

0.010

9.973

200

16.60

0.184

0.111

11.082

artbfl2_636

The butterfly valve will operate at a small fraction of its passage, and very small modifications of position generate large change of pressure value (especially at low flow demand conditions).
This feature creates considerable difficulties for a reliable, stable regulation. 
However, it is obvious that lower flow rate determines even smaller opening of the valves.
The selection of a motorized butterfly for regulating pressure in condition of a wide demand flow range will be an incorrect decision. 

NOTES:
Butterfly valves have much lower losses at high flow rates, though this parameter significance reduces for long systems with high pipe losses.
One must also take into considerations the fact that a butterfly valve is an inline device that can not be serviced without  having to take it out of the line.



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