Effective closing time factor


Reduced free flow area in the throttle control system:

fr=QQmax

fr - reduced free flow area in the throttle control system - []

Q - flow coefficient - []

Qmax - max. flow coefficient - []


Relative Flow:

Qp=frp+fr21-p

Qp - relative Flow - []

fr - reduced free flow area in the throttle control system - []

p - pressure parameter - []


Pressure parameter:

p=Δhh0

p - pressure parameter - []

Δh - theoretical pressure in the closure at full opening - [m]

h0 - rated net head - [m]


Theoretical pressure in the closure at full opening:

Δh=v022g*ξ+1

Δh - theoretical pressure in the closure at full opening - [m]

v0 - valve speed - [m/s]

g - gravitational acceleration - [m/s2]

ξ - local loss factor for open valve - []


Valve speed:

v0=Q0π*D024

v0 - valve speed - [m/s]

Q0 - flow - [m3/s]

D0 - valve diameter - [mm]


Effective closing time factor:

cef=minlimn900,1Qpn-Qpn+10

cef - effective closing time factor - []

Qpn - relative flow in position n - []


Flow characteristics:

The flow characteristic is the dependence of the actual flow rate on the position of the control actuator of the control system.

The flow characteristics of Fig.1-7 are the dependence of the actual flow rate on the position of the valve of the control system.


Note: The flow characteristics (depending on the valve manufacturer) that are dependent on the position of the valve must be recalculated for dependence on the position of the actuator. Because before the actuator may be a member (crank mechanism, ...) that is not linear.

Fig.1 Flow characteristic Lattice disc Butterfly valve
Fig.2 Flow characteristic Spherical valve
Fig.3 Flow characteristic Knife gate valve
Fig.4 Flow characteristic Needle valve
Fig.5 Flow characteristic Holow-Jet
Fig.6 Flow characteristic Howell-Bunger
Fig.7 Flow characteristic Tainter gate

Example:

We have to determine the difference between the effective shut-off time for the flow characteristic of the valve and the flow characteristic of the control actuator (hydraulic cylinder) according to Fig.8 shut-off butterfly valve DN300 with the following parameters:

h0 = 33m; Q0 = 0,314 m3/s; ξ = 0,106

0102030405060708090100
Q10,9510,8850,7330,5310,3730,2490,1830,1230,0730,0350,000
Q20,9510,8880,7550,5830,4330,3060,2180,1550,1020,0470,000
Fig.8 Flow characteristic

Q1 - flow characteristic of the valve

Q2 - flow characteristic of the control actuator

Valve speed:

v0=Q0π*D024=0,314π*0,324=4,44m/s

Theoretical pressure in the closure at full opening:

Δh=v022g*ξ+1 =4,4422*9,81*0,106+1 =1,11m

Pressure parameter:

p=Δhh0=1,1133=0,034
0102030405060708090100
Qp11,0000,9970,9890,9640,9180,8270,7290,5770,3850,1960,000
Qp21,0000,9980,9900,9730,9410,8790,7870,6670,5050,2590,000
Fig.8 Relative flow characteristic p=0,034

Qp1 - relative flow characteristic of the valve

Qp2 - relative flow characteristic of the control actuator

cef1=0,511[]
cef2=0,386[]

cef1 - effective closing time factor (valve) - []

cef2 - effective closing time factor (control actuator) - []

x=100-cef2cef1*100 =100-0,3860,511*100 =24,5%


Literature:

F. Strohmer: Investigating the characteristics of shutoff valves by model tests. Water Power & Dam Construction 1977

By G. L. Beichley, M. ASCE and A. J. Peterka, F. ASCE: Hydraulic design of Hollow-Jet valve stilling basins. Journal of the hydraulics division 1961

Stanislav Kratochvil: Vodní nádrže a přehrady. 1961

Б. И. ЯНЬШИН: ЗАТВОРЫ И ПЕРЕХОДЫ ТРУБОПРОВОДОВ. 1962


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