Hydrodynamic calculation Spherical valve

Fig.1

The spherical valve is a relatively old and proven type of valve that is most often used to close the water dam pipeline.

In the fully open position, the spherical valve is very advantageous from a hydraulic point of view. It forms a direct continuation of the pipeline, the water flows through the spherical valve without changing the direction and without changing the flow cross section. The spherical valve in the fully open position causes virtually no energy loss.

From the spherical valve, initially, it was only required to withstand the static water pressure and seal perfectly in the closed position.

Today it is required from the spherical valve to be able to safely close the piping at maximum flow. When closing from an open position, the body resists the current flowing through not only the internal rotating body but also through the obstruction of the outside. The flow is very complex in this case and in certain opening positions the valve is extensively stressed by hydrodynamic forces. The dynamic effects of the flow are manifested by a pulsating force acting in the front of the rotating body eccentrically with respect to the axis of rotation. As a result, it acts on the rotating body of the hydrodynamic torque in the sense of closing. The resulting hydrodynamic force in the spherical valve may, in some positions, be greater than the force which causes a static water pressure on the valve in the closed position.

In order to reduce the water hammer in the piping, it is very important to solve the most appropriate course of closure of the spherical valve. A very important basis for calculating the water hammer is the characteristic flow and pressure losses in the valve.

The dynamic effects of the water stream are substantially dependent on the hydraulic ratios in the pipeline and the valve. The flow of water without cavitation is different from the flow with cavitation or aeration of the space behind the valve. In the stage of fully developed cavitation there is a strong vibration, noise, great pulsation of hydrodynamic forces and torque. Shocks and vibrations of the spherical valve can be transferred to other constructions parts (vibrations of foundations). In adverse cases, resonance of different origins can occur, and thus a serious threat to safety.

We need to know not only the mean values of the hydrodynamic forces and torque values, but also the pulsation, the maximum amplitude and the mean frequency, when designing the spherical valve and the connecting pipe. We also need to fully understand the effect of cavitation, the effect of aeration and the effect of the different placement of the valve in the pipeline on its hydraulic and dynamic characteristics.


1. Calculation of pressure on the spherical valve during its rapid closure:

To calculate the pressure on the spherical valve, we need to know the rated net head at the zero flow (closed valve). Increase pressure on water hammer must be calculated before the hydrodynamic calculation of the spherical valve. The maximum flow rate must be defined in the open position, which must be closed safely. A must be a defined aerated space behind the valve due to the under-pressure behind the valve.

To calculate the pressure on the spherical valve because of the ignorance of the piping system, a calculation for a simple serial connection of the control valves (variable and constant resistance) will be used.


Relative Flow:

Q=fp+f21-p

Q - relative flow - []

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

p - pressure parameter - []


Reduced free flow area in the throttle control system:

f=KKQmax

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

K - flow coefficient in position α - []

KQmax - max. flow coefficient - []


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*ζ+1

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

v0 - valve speed - [m/s]

g - gravitational acceleration - [m/s2]

ζ - local loss factor for open valve - []


Valve speed:

v0=4Qmaxπ*D2

v0 - valve speed - [m/s]

Qmax - flow - [m3/s]

D - valve diameter - [mm]


Flow in pipeline:

Qα=Q*Qmax

Qα - flow in pipeline in position α - [m3/s]

Q - relative flow - []

Qmax - flow - [m3/s]


The water speed in the pipeline:

vα=4Qαπ*D2

vα - the water speed in the pipeline in position α - [m/s]

Qα - flow in pipeline in position α - [m3/s]

D - valve diameter - [mm]


The pressure loss in the pipeline:

H=vα22gζα

H - the pressure loss in the pipeline in position α - [m]

vα - the water speed in the pipeline in position α - [m/s]

g - gravitational acceleration - [m/s2]

ζα - loss factor in position α - []


Loss factor:

ζα=1-K2K2

ζα - loss factor in position α - []

K - flow coefficient in position α - []


Pressure on the spherical valve:

H=H+vα22g+1-Q *ΔP+Patm

H - pressure on the spherical valve in position α - [m]

H - the pressure loss in the pipeline in position α - [m]

vα - the water speed in the pipeline in position α - [m/s]

g - gravitational acceleration - [m/s2]

Q - relative flow - []

ΔP - increasing pressure on water hammer - [m]

Patm - under-pressure behind the valve - [m]


Relative flow:

To calculate the increase in water hammer pressure.

SQ
[%][]
00,978
100,896
200,715
300,561
400,42
500,3
600,212
700,138
800,078
900,036
1000

2. Accuracy of measurements:

Line pressure was measured on vertical U-tubes with fill, limit relative error in pressure measurement:

δp±1%

When measuring water flow using a calibrated Venturi tube and using a calibrated Thomson Overflow, the limit relative error was not higher as:

δQ±1,5%

When measuring the flow of air through a calibrated gas meter, the limit relative error was not higher as:

δQa±2%

The surface of the pipeline was made with a limit relative error:

δF=±1,2%

Limit relative medium speed error ν:

δν=δQ+δF=±1,5+1,2=±2,7%

Limit relative error of the flow coefficient μ:

δμ= δQ+δF+12δν =±1,5+1,2+12*1=±3,2%

Limit relative error of cavitation factor σ:

δσ=±2,7%

Pressure diaphragm sensors and electrical devices that were used to measure hydrodynamic forces and torques were the limit relative error:

δFx=δFy=δFbx=δFby=δm=±2%

Limit relative pipe diameter error:

δD=±0,625%

Limit relative torque coefficient error km:

δkm=δm+3δD+δp= ±2+3*0,625+1 =±4,9%

Limit relative error of the hydrodynamic force coefficient kx, ky,kbx, kby:

δkx=δky=δkbx=δkby= δFx+δF+δp= ±2+1,2+1= ±4,2%

Limit relative error of maximum amplitude measured by pulses:

δA=±8%

Limit relative error of the coefficients of maximum amplitudes of pulsations of hydrodynamic forces:

δax=δay=δabx=δaby= δA+δF+δp= ±8+1,2+1= ±10,2%

Limit relative error of the max. torque amplitude factor:

δam=δA+3δD+δp= ±8+3*0,625+1= ±10,9%

Limit relative error of the most frequent frequencies N measured by pulsation:

δN=±0,5%

Limit relative error Strouhal number Sh:

δSh=δN+δD+δν= ±0,5+0,625+2,7= ±3,8%

Limit relative error of Froude number Fr:

δFr=2δν+δD= ±2*2,7+0,625=±6%

Limit relative error of aeration factor β:

δβ=δQa+δQ=±2+1,5=±3,5%


3. Guideline for the use of the hydrodynamic characteristics of the spherical valve:

In the annex section of Fig. 2 to 15, charts of dimensionless coefficients are constructed. These graphs are the basis for constructing the hydrodynamic characteristics of the spherical valve for the projected water dam.


Angle between pipe axis and hydraulic force:

Φα=tan-1KyαKxα*180π

Φα - angle between pipe axis and hydraulic force in position α - [°]

K - coefficient of hydraulic force on a rotating body in the axis y in position α - []

K - coefficient of hydraulic force on a rotating body in the axis x v in position α - []


Cavitation number:

σα=10-0,1+h0-HLαHvα

σα - cavitation number - []

h0 - rated net head - [m]

H - the pressure loss in the pipeline in position α - [m]

H - pressure on the spherical valve in position α - [m]


Forces on rotating body in axis x:

Fxα=πD24*ρ*g*Hvα1±δkx*Kxα ±1+δax*axα
Fx90°=πDs24*ρ*g*Hv90°*Kx90°
δkx=0,042
δax=0,102

F - forces on rotating body in axis x in position α - [kN]

Fx90° - forces on rotating body in axis x in position 90° - [kN]

D - valve diameter - [mm]

Ds - max. diameter seal on the rotating body - [mm]

ρ - density of liquid - [Kg/m3]

g - gravitational acceleration - [m/s2]

H - pressure on the spherical valve in position α - [m]

Hv90° - pressure on the spherical valve in position 90° - [m]

δkx - Limit relative error of the hydrodynamic force coefficient - []

δax - Limit relative error of the coefficients of maximum amplitudes of pulsations of hydrodynamic forces - []

K - coefficient of hydraulic force on a rotating body in the axis x in position α - []

Kx90° - coefficient of hydraulic force on a rotating body in the axis x in position 90° - []

a - the amplitude of the hydraulic force to the axis x rotating body in α - []


Forces on rotating body in axis y:

Fyα=πD24*ρ*g*Hvα1±δky*Kyα ±1+δay*ayα
δky=0,042
δay=0,102

F - forces on rotating body in axis y in position α - [kN]

D - valve diameter - [mm]

ρ - density of liquid - [Kg/m3]

g - gravitational acceleration - [m/s2]

H - pressure on the spherical valve in position α - [m]

δky - Limit relative error of the hydrodynamic force coefficient - []

δay - Limit relative error of the coefficients of maximum amplitudes of pulsations of hydrodynamic forces - []

K - coefficient of hydraulic force on a rotating body in the axis y in position α - []

a - the amplitude of the hydraulic force to the axis y rotating body in position α - []


Forces on rotating body:

Fα=Fxα2+Fyα2

Fα - forces on rotating body in position α - [kN]

F - forces on rotating body in axis x in position α - [kN]

F - forces on rotating body in axis y in position α - [kN]


Forces on valve in axis x:

Fbxα=πD24*ρ*g*Hvα1±δkbx *Kbxα ±1+δbax *abxα
Fbx90°=πDs24*ρ*g*Hv90°*Kbx90°
δkbx=0,042
δbax=0,102

Fbxα - forces on valve in axis x in position α - [kN]

Fbx90° - forces on valve in axis x in position 90° - [kN]

D - valve diameter - [mm]

Ds - max. diameter seal on the rotating body - [mm]

ρ - density of liquid - [Kg/m3]

g - gravitational acceleration - [m/s2]

H - pressure on the spherical valve in position α - [m]

Hv90° - pressure on the spherical valve in position 90° - [m]

δkbx - Limit relative error of the hydrodynamic force coefficient - []

δbax - Limit relative error of the coefficients of maximum amplitudes of pulsations of hydrodynamic forces - []

Kbxα - coefficient of hydraulic force for the valve in the axis x in position α - []

Kbx90° - coefficient of hydraulic force for the valve in the axis x in position 90° - []

abxα - amplitude of the hydraulic force to the valve in the axis x in position α - []


Forces on valve in axis y:

Fbyα=πD24*ρ*g*Hvα1±δkby *Kbyα ±1+δbay*abyα
δkby=0,042
δbay=0,102

Fbyα - forces on valve in axis y in position α - [kN]

D - valve diameter - [mm]

ρ - density of liquid - [Kg/m3]

g - gravitational acceleration - [m/s2]

H - pressure on the spherical valve in position α - [m]

δkby - Limit relative error of the hydrodynamic force coefficient - []

δbay - Limit relative error of the coefficients of maximum amplitudes of pulsations of hydrodynamic forces - []

Kbyα - coefficient of hydraulic force for the valve in the axis y in position α - []

abyα - amplitude of the hydraulic force to the valve in the axis y in position α - []


Hydraulic moment without eccentricity:

Mα=D3*ρ*g*Hvα1±δkm*Kmα ±1+δam*amα
δkm=0,049
δam=0,109

Mα - hydraulic moment without eccentricity in position α - [kNm]

D - valve diameter - [mm]

ρ - density of liquid - [Kg/m3]

g - gravitational acceleration - [m/s2]

H - pressure on the spherical valve in position α - [m]

δkm - limit relative torque coefficient error - []

δam - limit relative error of the max. torque amplitude factor - []

K - hydraulic torque coefficient in position α - []

a - amplitude of the hydraulic moment in position α - []


Moment from eccentricity:

Meα=Fe1α+Fe2αe

M - moment from eccentricity in position α - [kNm]

Fe1α - force parallel to the axis of the rotating body in position α - [kN]

Fe2α - force perpendicular to the axis of the rotating body in position α - [kN]

e - eccentricity - [mm]


Force parallel to the axis of the rotating body:

Fe1α=Fαsin90-α-Φαπ180

Fe1α - force parallel to the axis of the rotating body in position α - [kN]

α - the position of the rotating body from the open position - [°]

Φα - angle between pipe axis and hydraulic force in position α - [°]


Force perpendicular to the axis of the rotating body:

Fe2α=Fαsin90-α-Φαπ180

Fe2α - force perpendicular to the axis of the rotating body in position α - [kN]

α - the position of the rotating body from the open position - [°]

Φα - angle between pipe axis and hydraulic force in position α - [°]


Hydraulic moment:

MHα=Mα+Meα

M - hydraulic moment in position α - [kNm]

Mα - hydraulic moment without eccentricity in position α - [kNm]

M - moment from eccentricity in position α - [kNm]


The most common frequency of pulse forces:

Nα=1-δNShα*vαD
δN=0,005

Nα - the most common frequency of pulse forces in position α - [Hz]

δN - limit relative error of the most frequent frequencies N measured by pulsation - []

Sh - Strouhal number for pulse rate determination - []

vα - the water speed in the pipeline in position α - [m/s]

D - valve diameter - [mm]


4. Dimensioning aerated hole:

To reduce valve vibration, pulsation of hydrodynamic forces and erosion effects of cavitation by aerating the area behind the valve. The aerated hole should be large enough for air flow to reach Qair=0,2Qα according to research results.

It is subject to the condition L*v*ρt*cef<50000

otherwise Qair=maxQmax-Qα;0,2Qα

The aerated hole must be placed on the top side behind the valve seat at a distance of 0,0875D to 0,512D. The rotating body must be closed in the clockwise direction and the flow through the valve must be left to right, see fig. 1.


Coefficient of under-pressure of aerated hole:

α[°]15254060
f2[]0,2600,7352,1775,578

Effective closing time factor:

cef=minlimα8019*Qpα-Qpα+10

cef - effective closing time factor - []

Qpn - relative flow in position n - []


Under-pressure behind the valve:

P2air=min1*105;f2*v22g*ρ +1-Qpα *minL*v*ρt*cef;1*105

P2air - under-pressure behind the valve - [Pa]

f2 - coefficient of under-pressure of aerated hole - []

v - speed in pipeline - [m/s]

g - gravitational acceleration - [m/s2]

ρ - density of liquid - [Kg/m3]

Q - relative flow - []

L - the length of the pipeline behind the valve - [m]

t - valve closing time - [s]

cef - effective closing time factor - []


Air velocity:

Air velocity in the narrowest cross section

vair=min0,7*2*P2airρair;250

vair - air velocity - [m/s]

P2air - under-pressure behind the valve - [Pa]

ρair - air density - [Kg/m3]


Air flow area of the aerated hole:

The minimum flow area of the aerated hole is located in the shell of the spherical valve. Air flow area of the aerated hole need not be one, but there may be several. To calculate the area of the aerated pipeline the air velocity should not exceed vair=50m/s

fair=Qairvair

Qair - air flow via the aerated hole - [m3/s]

fair - the flow area of the aerated hole - [m2]

vair - air velocity - [m/s]

When calculating the aerated hole, the ability of the aerated device to assess whether it meets all the under-pressure and air flow rates. At low pressure parameters p<0,2 there may be a small under-pressure behind the valve that the aerated device may not be functional and therefore the hydrodynamic calculation must be calculated without aerated.


5. Conclusion:

The spherical valve must be hydraulically positioned in a straight diameter D. The effect of cavitation on pressure losses, flow and dynamic effects of the water stream does not occur immediately in the initial stage (when the first steam bubbles are formed), but only with a fully developed cavitation which can be defined by the critical values of the cavitation coefficient σcrit = 0,4 to 1,2.

Erosion effects of cavitation are most apparent in the pipe behind the closure at a distance of 1 to 1.3D from the sealing gasket, mainly at the bottom of the pipeline.

When aerated the water stream in the pipeline beyond the valve, the pressure loss in the valve increases, but the mean values of hydrodynamic forces and torques are reduced. With aerated, the pulsation of hydrodynamic forces, torque pulsation, and pulse pressure in the piping before and after the valve will greatly reduce.

Aerated is most effective in reducing valve vibration at flow rates σair = 0,2σwater.


Literature:

Miroslav Žajdlík: Hydraulické pomery a dynamické účinky prúdenia v guľovom uzávere 1968

Miroslav Nechleba: Vodní turbíny jejich konstrukce a příslušenství 1954

V. Kolář, St. Vinopal: Hydraulika průmyslových armatur 1963

Fig.2
Fig.3
Fig.4
Fig.5
Fig.6
Fig.7
Fig.8
Fig.9
Fig.10
Fig.11
Fig.12
Fig.13
Fig.14
Fig.15

Download PDF:

Hydrodynamic calculation Spherical valve.pdf


Social media:

Twitter Scribd Pinterest