Hydrodynamic calculation Gate valve (through conduit)

Fig.1

The gate valve is a relatively old and proven type of valve that is most often used to close pipeline.

From the gate 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 gate valve to be able to safely close the piping at maximum flow. When closing from an open position, the sliding plate resists the current flowing. 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. The resulting hydrodynamic force in the gate 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 gate 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. Shocks and vibrations of the gate 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, but also the pulsation, the maximum amplitude, when designing the gate 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 gate valve during its rapid closure:

To calculate the pressure on the gate 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 gate 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 gate 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:

Qp=frp+fr21-p

Qp - relative flow - []

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

p - pressure parameter - []


Reduced free flow area in the throttle control system:

fr=KQKQmax

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

KQ - 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:

Q=Qp*Qmax

Q - flow in pipeline - [m3/s]

Qp - relative flow - []

Qmax - flow - [m3/s]


The water speed in the pipeline:

v=4Qπ*D2

v - the water speed in the pipeline - [m/s]

Q - flow in pipeline - [m3/s]

D - valve diameter - [mm]


The pressure loss in the pipeline:

HL=v22gζ

HL - the pressure loss in the pipeline - [m]

v - the water speed in the pipeline - [m/s]

g - gravitational acceleration - [m/s2]

ζ - loss factor - []


Loss factor:

ζ=1-KQ2KQ2

ζ - loss factor - []

KQ - flow coefficient - []


Pressure on the gate valve:

Hv=HL+v22g+1-Qp *ΔP+Patm

Hv - pressure on the butterfly valve - [m]

HL - the pressure loss in the pipeline - [m]

v - the water speed in the pipeline - [m/s]

g - gravitational acceleration - [m/s2]

Qp - 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
[%][]
01,000
100,892
200,734
300,559
400,398
500,269
600,169
700,088
800,030
900,001
1000

2. Guideline for the use of the hydrodynamic characteristics of the gate valve:

In the annex section of Fig. 2 to 5, charts of dimensionless coefficients are constructed. These graphs are the basis for constructing the hydrodynamic characteristics of the gate valve for the projected water dam.


σ=10-0,1+h0-HLHv

σ - cavitation number - []

h0 - rated net head - [m]

HL - the pressure loss in the pipeline - [m]

Hv - pressure on the spherical valve - [m]


Forces on sliding plate in axis x:

Fx=πD24*ρ*g*Hv*Kx

Fx - forces on sliding plate in axis x - [kN]

D - valve diameter - [mm]

ρ - density of liquid - [Kg/m3]

g - gravitational acceleration - [m/s2]

Hv - pressure on the gate valve - [m]

Kx - coefficient of hydraulic force on a sliding plate in the axis x - []


Forces on sliding plate in axis y:

Fy=πD24*ρ*g*Hv*Ky

Fy - forces on sliding plate in axis y - [kN]

D - valve diameter - [mm]

ρ - density of liquid - [Kg/m3]

g - gravitational acceleration - [m/s2]

Hv - pressure on the gate valve - [m]

Ky - coefficient of hydraulic force on a sliding plate in the axis y - []


Forces on valve in axis x:

Fbx=πD24*ρ*g*Hv*Kbx

Fbx - forces on valve in axis x - [kN]

D - valve diameter - [mm]

ρ - density of liquid - [Kg/m3]

g - gravitational acceleration - [m/s2]

Hv - pressure on the gate valve - [m]

Kbx - coefficient of hydraulic force for the valve in the axis x - []


Forces on valve in axis y:

Fby=πD24*ρ*g*Hv*Kby

Fby - forces on valve in axis y - [kN]

D - valve diameter - [mm]

ρ - density of liquid - [Kg/m3]

g - gravitational acceleration - [m/s2]

Hv - pressure on the gate valve - [m]

Kby - coefficient of hydraulic force for the valve in the axis y - []


3. 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=βQα according to research results.

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

otherwise Qair=maxQmax-Qα;βQα

The aerated hole must be placed on the top behind the valve seat at a distance of 1.5D from the valve axis.


Coefficient of under-pressure of aerated hole:


Coefficient of under-pressure of aerated hole:

s/smax[%]20304060
β[]0,20,30,40,6
f2[]6,92416,7830,92476,342

Effective closing time factor:

cef=minlimα900,1Qps-Qps+10

cef - effective closing time factor - []

Qps - relative flow in position n - []


Under-pressure behind the valve:

P2air=min1*105;f2*v22g*ρ +1-Qp *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]

Qp - 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 gate 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.


4. Conclusion:

The gate 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.

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 are reduced.


Literature:

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

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Hydrodynamic calculation Gate valve (through conduit).pdf


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