Experimental study on low pressure loss regulating

Experimental study on low-pressure loss control valve of high-power steam turbine

1 introduction

literature [1] points out that the control valve of modern high-power steam turbine should ensure 100% reliability in fact; There is minimum resistance at full opening (not exceeding 1.5% - 2% of the initial steam pressure); Ensure that there is a stable flow under all working conditions of the steam turbine and meet the process requirements. It is reported that the resistance of the non unloading control valve used in Russian k-300-240 steam turbine under the calculated opening is about 1% of the initial pressure, and the vibration value is also low

many large and medium-sized steam turbines designed by China, including 200MW and 300MW turbines, use G-I type regulating valves (called GX-1 type in the steam turbine chapter of the mechanical engineering manual). The valve was developed in 1966. At that time, the guiding ideology for the development of the valve was to reduce the lifting force of the valve first, so take 100% of the valve unloading degree (the unloading degree is defined as the ratio of the cross-sectional area at the fitting diameter of the valve disc and the valve disc sleeve to the cross-sectional area at the fitting diameter of the valve disc and the valve seat), and then study its stability by using different valve types, and select the best one, so as to obtain the valve. Finally, after the industrial test of Jilin thermal power plant and the actual operation test, it was used in the real machine [2]

at that time, when developing the valve, the indexes such as valve pressure loss were not taken into account, and the valve line itself could be optimized in the future. Now users pay more and more attention to the reliability and economy of steam turbine units, so it is necessary to re evaluate the pressure loss, stability and other indicators of G-I type regulating valve. At present, the small force value of the tension machine sensor on the market is generally S-type sensor, and learn from foreign experience and imported technology, and then according to their own practice, develop a regulating valve with more stable work and less pressure loss when fully opened

2 reference symbol

2.1 geometric parameters

2.2 aerodynamic parameters

pi - static pressure at section I, PA

pi0 - total pressure at section I, PA

γ I0 - stagnation weight of gas at section I, N/m3

ρ I - gas density at section J, kg/m3

g -- actual flow through the regulating valve, kg/S

gc -- critical flow through the throat of the regulating valve seat, kg/S

e -- inlet and outlet pressure ratio of regulating valve (E = P2/P10)

k - specific heat ratio

ξ C - flow coefficient of regulating valve（ ξ c＝G／Gc)；

ζ—— The pressure loss is that Bartel is considering adding a new Licensor in Europe

ui - velocity of air flow at section I, M/s

2.3 subscript

1 - inlet section of regulating valve

2 - outlet section of regulating valve

n - fitting section of valve disc and valve seat

c - cross section of valve seat throat

3 test and research methods

3.1 test device

this test adopts a single valve test device (i.e. without valve shell), and the valve is placed in a spherical container much larger than it. The bottom plane of the spherical vessel is flush with the inlet plane of the valve seat. Because the air flow in the shell flows into the valve seat symmetrically, the influence of the specific valve shell on the pneumatic characteristics of the regulating valve in practical application is ignored. In such a test device, it is feasible to only optimize the profile of the regulating valve seat and disc. The outlet of the valve seat is vented

the test medium is air. The air source is two piston air compressors, and the maximum outlet pressure is 1.18mpa, which can ensure that (6) the sensor indication is cleared to zero and the valve reaches the critical pressure ratio during the test. All test devices are made of steel. The matching diameter DN of the regulating valve in various schemes is taken as φ 50mm, and other dimensions are reduced and molded in proportion

3.2 flow resistance characteristics (flow coefficient and pressure loss coefficient) test of control valve

3.2.1 flow coefficient of control valve

flow coefficient is defined as the flow coefficient characteristics of control valve

it is necessary data in steam distribution calculation of steam turbine, and its physical meaning is the flow capacity of control valve. When the front and rear pressure ratios of different regulating valves are the same, the flow coefficient ξ C high indicates that the steam flow through the valve is large; Conversely, if you want to pass the same steam flow, for the flow coefficient ξ C the higher the regulating valve, the smaller the pressure drop (the larger the pressure ratio). This shows that under the same pressure ratio and lift, the flow coefficient ξ C the larger the regulating valve, the better its flow resistance characteristics. The flow coefficient can be calculated from the same pressure ratio ξ C with the distribution law of the relative lift change curve to qualitatively judge the size of the valve flow resistance

when measuring the flow characteristics of the valve, the physical quantities to be measured include atmospheric pressure, appropriate static and dynamic deformation measurement force (here, the pressure behind the valve), pressure in front of the valve, temperature and flow. Barometer is used for atmospheric pressure, U-tube is used for pressure in front of valve, mercury thermometer is used for temperature, and standard orifice is used for flow measurement

3.2.2 pressure loss coefficient

while making the flow coefficient curve, all the physical quantities contained in the pressure loss coefficient characteristic curve have been obtained. It is another index to measure the value of valve pressure loss, which is defined as

u2, which can be calculated by flow, air flow pressure behind the valve, temperature measurement value and valve seat outlet cross-sectional area

3.3 stability test

when developing G-I valve, the working stability of the valve is judged by measuring the vibration displacement value (axial runout) of the valve stem [2]. Although there are great differences between the working conditions of the model and the real object, it is feasible to compare and test various schemes under the same working conditions

since it is difficult to ensure that the valve rod vibration measuring device is in the same state when changing the test scheme in this test, if the previous method is used to measure the valve vibration, the stability of each scheme cannot be correctly judged

in fact, the instability of the regulating valve is mainly caused by the great exciting force of the unstable steam flow pulsation in the valve on the valve disc. It is more intuitive and reliable to compare and judge the stability of the control valve from the perspective of hydrodynamics to find out the causes of flow induced vibration in the valve and the degree of induced vibration caused by different control valve lines. Moreover, the method of reducing the fluid exciting force in the valve and improving the stability of the valve by adjusting the profile of the regulating valve is an active vibration elimination method. Some foreign companies have carried out special experimental research in this regard. For example, abb has measured and analyzed the pressure pulsation at various points of the regulating valve disc and valve seat surface to study the influence of various regulating valve profiles on stability

a large number of test data show that the area near the throat of the valve seat is often the area with the largest change in the channel pressure in the control valve, and the size of the pressure fluctuation in this area can accurately reflect the stability of the control valve. This test is to measure the pressure pulsation at the throat of the regulating valve as the basis for its working stability. Open at the throat of the valve seat φ 1mm static pressure measuring hole, and use a piezoelectric sensor with high sensitivity to measure its static pressure pulsation, record the pressure change curve, so as to compare the magnitude of pressure pulsation under different lift and pressure ratio conditions, and analyze the selection of valve disc seat profile and the influence of lift and pressure ratio. This evaluation of the stability of valve flow may be more scientific and practical than the previous methods. (end)