Self-operated pressure control valve is a kind of without any external driving energy, rely on is the pressure transfer medium itself as a power source, when adjusted medium pressure changes according to the predetermined set values of energy-saving automatic control device. It sets the detection, control, to perform many other functions in one, in an independent instrument control system. Because the Self-pressure regulating valve is a use of energy-saving products is simple and convenient, so the world regulating valve companies have introduced various types of products, mainly divided into leveraged self-operated regulator, the command type self-regulating valve, spring type self-operated regulating valve (the following spring type Self-pressure regulating valve for example).
The work principle and structure characteristics
Spring type Self-pressure regulating valve working principle, with the common type valve type as an example (see Figure 1), the initial position of the valve core is open, the valve after the pressure P2, the pressure guide tube membrane chamber input in actuator diaphragm valve position, its force and spring force is balanced decisions opening of the valve, and the pressure control valve. When the valve after the pressure P2 increases, the force P2 acting on the diaphragm also increased. At this time, force greater than the spring force the diaphragm is set, so that the valve position movement toward the closed, valve opening decreases, P2 decreases, force until the diaphragm and the spring force balance date, so that the reduced set value P2. Similarly, when the valve after the pressure P2 decreases, and the direction of movement in the opposite direction. The initial position of self-operated pressure control valve before the valve regulating the valve is closed, the principle and the valve force balance after the same (see Figure 2).
Self-pressure regulating valve has the advantages of simple structure, small maintenance workload pressure set point can be adjusted and wide range, convenient for the user to adjust the setting range of continuous valve using the pressure balance mechanism, so that the valve is sensitive, accurate control, allowing a large pressure difference modulated medium is corrosive, has low mobility of light oil air, water, etc., also can control the temperature in the gas, non corrosive below 350 ℃ steam etc.. For the high temperature, high degree of media need to configure the cat condenser, the isolation tank and other accessories. Regulation and control under the condition of continuous media using this product widely used in natural gas production and transportation, city heating and metallurgy, petroleum, chemical industry, electric power industry. Control but the product of media in the valve after reducing the amount, quantity, or useless or intermittent dosage and how will it according to user response phenomenon is the valve will leak, leading to differential pressure valve before and after the completely equal when using the closed again after the start using the medium, the valve and restore the decompression function, at the same time there is caused by instantaneous high impact pressure after the open, extremely has the possibility of damage to the device. The following to our company the product application example in an enterprise to the spring type Self-pressure regulating valve (hereinafter referred to as the valve) for the design and application of this kind of special conditions.
Special process parameters
The control system to stabilize the pressure after the valve, medium for nitrogen, initial pressure pipeline system is 2800KPa, the maximum flow rate after section needed for 2000NM3/h. Process requirement is after section using the medium 10 min after cessation of the use of 1 hours, and in this manner recycling; normal use valve after the pressure stabilized at 700KPa, stop using the valve after the pressure shall not be higher than 800KPa. The valve after the pipeline length of 800 to 1000 meters (Fig. 3).
Statistical analysis
According to the technological requirements, in order to ensure the normal use of the control system of medium pressure, can choose a valve pressure can also choose two, but considering the after section completely close the special requirements of medium, choose to use the two valve double step-down can set pressure decomposition better, reducing load of a single valve., in an optimal way to meet the process requirements.
(1) design in valve parts, seal form two sets of valves are used in soft sealing form, in order to improve the level of valve leakage, increased pressure decreases due to leakage caused by valve.
(2) two valve set pressure need reasonable allocation. The key task of the first valve to reduce the pressure before the valve system when most of the normal use of medium, the valve after the pressure is stable at a moderate pressure value can cut off the media and the control in cutting medium pressure after the valve only increased slightly to second valve. Second valve task in addition to the normal pressure, should be able to play better the role of cut off the medium, and the first valve due to other unknown reasons can not be normal shut down until the two cut off the media role, and ultimately ensure valve after the pressure is less than a few Tian Gang, to meet the process requirements.
(3) in the design of the control element key one one spring, take full account of the two valve role. Relatively speaking, change second valve shells Zambia under pressure is bigger, especially the two valve is closed when the valve after the pressure medium, will have some changes, only for second sets of valves, the valve before the pressure and the pressure after the valve has changed, according to the analysis of the force balance system, Effects of elevated pressure after the valve spring is far greater than the amplitude of the pressure before the valve, so that the spring load bearing is relatively large, so the calculation of the spring should give full consideration to the factors in the design.
(4) regulating valve pitch control system at the end of pipeline 800 ∼ 1000 meters, instantaneous pressure generated when the control system terminal closed again after the opening will slowly consumed in the transportation process, so the pressure in this condition can be ignored.
(5) this process requires two valve must be opened and closed frequent, so the key components of valve material also should choose reasonable holding, and suggest users update the wearing parts, with good performance of security products.
The selection and calculation
(1) pressure distribution calculation
To ensure that the valve after the pressure within the set range, the valve before the pressure and the pressure after the valve shall meet the reduction ratio (10:1 ~ 10:8) with numerical relationship, see table 1:
Two sets of valves, valve before the valve block on the reasonable after media pressure should first ensure second when the valve is closed the valve after the pressure is less than 800KPa, then the second valve before the pressure should meet more than the minimum reduction ratio, so the choice of the first valve after the pressure stabilized at 1000KPa left and right is more appropriate, the pressure as the pressure before the valve second valve, can satisfy second valve after the pressure stabilized at 700KPa. So the pressure distribution of two sets of valves are as follows: dedecms
The first valve: the valve before the pressure 2800KPa, the valve after the regulator 1000PKa, voltage range of 900-1100PKa:
Second valve: the valve before the pressure 1000KPa, the valve after the regulator 700KPa, voltage range of 600-800KPa.
(2) the caliber of computation
Control valve selection is mainly based on the parameters of the valve before the pressure, providing the valve after the pressure and the maximum flow rate through the calculation to determine the coefficient of discharge valve (Kv value), the main parameters of aperture. Calculation formula of Kv value has many kinds, in Table 2 of the formula for gas, calculation formula involving physicochemical parameters are less.
Flow lines Cv two valve is calculated by the given conditions.
The known initial system pipeline pressure is 2800KPa, the maximum flow rate after section needed for 2000NM3/h. Set the first control valve flow in 4000NM3/h, valve after the pressure stabilized at 1000kPa; second valve flow is known as the 2000NM3/h, the valve before the pressure 1000kPa, the valve after the pressure stabilized at 700kpa. Medium for nitrogen, normal temperature. The first valve: P1 ≥ 2P2, so the choice of formula (1) calculate the value of kV, second sets of valves: p1<2p2, so the choice of formula (2) to calculate kV values:
Substituting the known conditions to calculate kV, then according to the formula (3) corresponding to the size of the calculated Cv selection, the first valve diameter of DN25; second valve diameter of DN40.
This set of self-operated pressure control valve through the actual use of the certificate after the pressure control valve can be within the specified scope, with stable, and when the scene process medium full closed fully embodies its sealing characteristics, good to meet the requirements of system process, won the user's praise. Through this example can be concluded under certain conditions by means of special design, self-operated pressure control valve can have the dual function of both stable set pressure and cutting medium, expand more wide application fields for this kind of product.
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