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Description> Control Valve 1 2
3 4 5
6
VI Operational Principle
When front and back differential pressure
of the valve is a certain fixed value, the valve changes the flow
through changing flow area of the medium passing the valve. The
change of flow area is formed by relative travel (or relative corner)
produced by flap and the valve base (or sleeve) under operation
of valve pole which is controlled by executive framework.
VII Structual Features
Control valve produced by our company adopts
basically four types of structure.
First type£ºIt¡¯s a type most used, that is,
sleeve single-base sealing structure. This structure has small leakage
and linear flow features at shutoff state.
Second type£ºadopt double-valve base structure.
Thus structured flap has good balancing ability while with certain
leakage.
Third type£ºadopt flashboard structure. This
structure is simple and has small valve pole applied force and self-sealing
of effect of the medium.
Fourth type£ºunder high differential pressure,
it¡¯s easy to occur cavitation erosion phenomenon. So multi-step
adjusting structure is adopted to eliminate the hidden trouble of
cavitation erosion damaging the valve. For principle of cavitation
erosion process and how to eliminate, simple introduction is as
follows£º
As shown in chart 1, valve base functions
as throttle orifice plate. When high-pressure liquid flowing through
throttle holes, static pressure energy and dynamic pressure energy
alter into each other£ºIncrease of flow speed results in pressure
reduction. When pressure is reduced to equal or less than vaporizing
pressure Pv at inlet temperature, gas nucleus in the liquid expand
and form steam bubbles. Flowing through throttle face, flow speed
decreases at spacious lower part of the channel and the pressure
increases back. When pressure increases to P2, which is equal or
even higher than Pv, steam bubbles debacle and release huge energy,
damaging throttle components such as valve base and flap, namely,
vaporization process. Destroy force of cavitation erosion is great.
Common valve under vaporization process will experience serious
cavitation erosion after two or three months¡¯, or even one or two
weeks¡¯ operation, and leakage of valve base increase as high as
over 30% of rated flow. And the control valve losses adjusting control
function completely.
 Chart
1
Caviation is related to differential pressure.
When actual differential pressure of the valve ¡÷P is larger than
critical differential pressure of vaporization ¡÷Pc, and when outlet
pressure P2 is equal to or higher than Pv, then vaporization occurs.
Therefore, it uses the method of step reduction to make very step
different pressure of total pressure of the valve ¡÷Pi£¼¡÷Pc, to prevent
vaporization. Multi-level pressure reduction ensures no vaporization
in course of throttle pressure reduction of high-pressure liquid
on valve structure through changing flowing state of the liquid
inside the valve. It¡¯s effective method so far to deal with vaporization
resulted from high differential pressure.
Critical technology of anti-cavitation erosion
control valve is throttle assembly. According vaporization principle
of multilevel to pressure reduction, this product adopts throttle
assembly of multilevel diffluence and multilevel valve base.
It divides full opening of the valve into
a number of separate space, each set up with radial runner, throttle
holes and buffer room distributing on some valve bases according
to certain rule. Valve base is processed, moulded and assembled,
adopting taper sealing structure of upper valve base to reach absolutely
credible sealing.£¨Chart 2£©
Because water control valve of boiler is
running under conversion pressure differential pressure, this valve
should bear extremely high differential pressure at small flow£¨small
opening£©and do not vaporize. While under large flow, it demands
the valve has resistance as small as possible to reduce power source
wastage of water pump, that¡¯s to say, it demands the valve has characteristic
of conversion flow resistance. Generally, inherent flow character
of the valve is measured under constant differential pressure. When
used in working condition of conversion differential pressure, features
of its factual working flow will produce aberration£º the linear
feature changes to quick opening feature, which leads control valve
to reach saturation at 40% opening generally and fail to conduct
all-travel control. Water control valves in use now of our country
all adopt constant flow resistance structure.
Herein, this design adopts multilevel pressure
reduction for opening under 25%, and for 25%-100% opening range,
it uses ordinary sleeve structure and flow feature is revised equal
percentage£¨Chart 3£©. Cast steel straight tube of this product is
used generally as water control valve of boiler, which is able to
effectively prevent vaporization and cavitation erosion. It has
small flow resistance, saves electricity, able to complement influence
of conversion differential pressure and satisfy requirement of linear
flow features of actual operation.
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