A fitting is any part used in a piping system to change direction or function and joined to that system by some mechanical means or by welding. The simplest way to change the direction of a pipe would be to bend it.However, besides restricting the flow, this method often distorts the shape of the pipe and reduces the wall strength. Another method of altering direction is with mitered joints, I.e. straight piecesof pipe cut for the required turn and welded together. This type lowers flow efficiency considerably by creating greater turbulence. A single mitered bend has about six times the resistance of a smooth elbow. Fittings are preferable for most piping systems where rate of flow, internal pressure and protection against a corrosive environment are primary design considerations
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Effect of front guide vane adjustment on pump performance and usage control
Abstract: In the second phase project of water diversion from the upper reaches of Huangpu River in Shanghai, 12 large-scale vertical mixed flow pumps were used. Inlet Vane Conttrol Device VR manufactured by German KSB company was installed in front of the impeller in order to realize In a wide range of regulating pump performance. Keywords: guide vane pump performance guide vane angle I. INTRODUCTION Shanghai Huangpu River upstream water diversion project of the second phase, the use of 12 large vertical mixed flow pump, the impeller are equipped with German front KSB company manufactured guide vane device (Inlet Vane Conttrol Device VR), the purpose is to achieve a wide range of pump performance adjustment. Pump structure as shown in Figure 1, the parameters are: flow Q = 6.5M3 / S, head H = 15.5M, speed n = 297rpm, the number of comparison ns = 353, efficiency η = 0.80, shaft power P = 1400KW. Pre-guide vane device (referred to as VR device) is currently not much use on the pump at home and abroad, in this regard, few technical information and reports. To this end, the author based on the past three years on the pump operating conditions and the relevant experimental data and technical information on the use of VR devices to adjust the performance of the pump to do some analysis and analysis of the reasons in order to VR devices more objective and correct understanding , So as to put forward some suggestions for the actual control of such pumps. Second, the front guide vane device on the performance of the pump We use the VR device is circular, guide vane straight leaf type, a total of 17, leaf length 500mm, device diameter 1300mm, see Figure 2. The device is driven by a motor and transmits the torque to the input shaft of the device through a multi-section transmission rod with a universal joint. Then, the blades of the device are rotated synchronously through the gear system in the device to adjust the angle of the guide vanes. KSB company set to VR device guide vane perpendicular to the horizontal plane 90 º, when the blade rotation tilt direction and pump impeller rotation direction for the same angle (ie, the angle becomes smaller); when the blade tilt direction and pump impeller rotation direction opposite Increase angle (ie angle becomes larger). Below the first installment of the pump installed with no guide vane, the performance of the pump for some analysis. (A) Comparison of pump performance without front guide vane and front guide vane with blade angle of 90 ° According to the information provided by KSB Company and the relevant tests conducted by our researchers, And when the blade is at 90º, it is compared with the characteristic curve of the pump without front vane device, as shown in Fig.3. From the figure, we can draw conclusions: 1. The Q-H curve of the pump without front guide vane and front guide vane is basically two parallel curves with Q- -H curve slightly lower, which is due to the addition of the front guide vane, the import flow resistance loss caused by the increase of the head down reason. 2. From the Q-η curve shows that the two curves are basically close, and there is a coincidence point on the left of this point, there is a front guide vane Q-η curve than the front guide vane Q-η curve slightly higher 3 On the right side of this point, the Q-η curve of the front guide vane is slightly lower than the Q-η curve of the front guide vane, which coincides with the optimal operating point. This shows that in the best conditions, the front guide vane resistance loss is minimal for the pump, does not cause any impact; and in the small flow, due to influent pipe less flow, uneven flow, plus the front After the guide vane, since the diversion effect, so that the liquid flow inlet uniformity of enhancement, the resulting efficiency has improved than the original; and in the large flow, the role of diversion disappeared, on the contrary due to the increase of the guide vane, the resistance loss increases, Resulting in lower efficiency. It can be seen that when the vane position is at 90 º, the performance of the pump is basically similar to that of the pre-vane pump without any effect on the pump characteristics. Second, take a look at the guide vane at different angles when the pump performance changes. (B) of the VR device guide vane at different angles on the performance of the pump 1 on the Q-H performance curve Figure 4 is a VR device pump in a variety of guide vane angle performance curve. As can be seen in Figure 4, when the current guide vane is adjusted to less than 90, the resulting performance curve is clearly to the left and is substantially parallel to the performance curve at 90 degrees (in the continuous operating limit range Inside). This is because at this time, the flow direction of the front guide vane exit is consistent with the rotation direction of the impeller. The flow has a forward pre-rotation Vlu in front of the pump impeller inlet (Vlu flow at the impeller inlet at the circumferential speed of the absolute velocity ), So Vlu> 0 (front guide vane is 90º. When, Vlu = 0). From the Euler equation: HT = (u2v2u-ulvlu) / g that when the guide vane angle to less than 90º. When the direction is adjusted, because Vlu> 0, the theoretical lift of the pump HT is less than that of the guide vane. When the pump head HT. And, the smaller the value of the front vane angle, Vlu value is larger, the greater head lift, so Q-H characteristic curve to the left. In actual use, it is the use of this feature, while keeping the head is basically constant, the flow becomes smaller with the VR angle becomes smaller, so as to achieve the purpose of reducing the flow. However, when the current guide vane is adjusted to be greater than 90 °, the direction of the front guide vane flow outlet is opposite to that of the impeller, that is, reverse pre-rotation occurs, so that Vlu <0. The same Euler equation shows that at this time the head of the pump HT is greater than the front guide vane at 90º head. Furthermore, the larger the leading vane angle, the smaller the Vlu, the greater the lift of the pump, and the Q-H characteristic shifts to the right. Therefore, it is possible to increase the pump flow rate with the increase of the vane angle under a certain lift. Practice shows that the above effect is obvious. 2 on the efficiency of the pump η As the guide vanes to the 90º position on both sides of the adjustment so that the flow before entering the pump impeller, respectively, had a positive pre-rotation and reverse pre-rotation, the impeller blade inlet to produce absolute speed V1 circumference The component Vlu, therefore, changes the impeller inlet velocity delta, as shown in FIG. 5. The solid line is the velocity triangle without pre-rotation, and the dotted lines are the velocity triangles with positive pre-rotation and reverse pre-rotation, respectively. It can be seen from the figure that the relative velocities ω1 are not the same in the three conditions, ω'1 is the relative velocity when the liquid flow is positively pre-rotated, and ω1 is the relative velocity when the liquid flow is reversely pre-rotated. ω1 increases with the increase of the vane angle value.From Figure 6 we can clearly see that the front vane angle adjustment on the pump efficiency is obvious.When the guide vane at 90 o position, When the guide vane angle increases or decreases step by step, the operating efficiency of the water pump also decreases gradually, and the farther the guide vane angle deviates from 90 °, the greater and the more obvious the decline of efficiency will make the pump unable to be normal Therefore, we set the pump guide vane adjustment angle within the range of 75 º - 110 º, so that the pump can be more than 75% efficiency in the safe operation within the range of 75 º --- 110 º, the pump Of the operating efficiency changes, according to our analysis of the test data, the following rules: When the guide vanes in the range of 75 º ---- 95 º adjustment, the pump operating efficiency changes less, and more efficient; and once the guide Ye Xiangda The efficiency of the pump will decrease obviously when it is adjusted in the direction of 95º.Table 1 is the test data of the operating efficiency of the same type of pump at different guide vane angles. Table 1 Test data of pump operating efficiency at different guide vane angles Guide vane angle 75º 80º 85º 90º 95º 100º 105º 110º A Pump efficiency% 81.82 82.22 82.51 82.70 81.76 80.25 77.89 76.10 B Pump efficiency% 85.62 85.73 85.73 85.01 84.08 82.26 79.83 77.43 C Pump efficiency% 88.50 87.36 87.40 86.92 85.80 84.47 8107 79.25 Reasons for the above mentioned phenomena , Which can be analyzed by Euler's equations and velocity triangles: From the above we know that in the range of 75º-110º, when the guide vanes are adjusted to less than 90º, the pre-rotation Vlu of the flow will reduce the theoretical head of the pump HT However, the efficiency of the pump did not decrease significantly due to the decrease of the relative velocity ω1, which greatly reduced the impact loss of the liquid flow on the impeller. On the contrary, when the guide vane angle was adjusted to more than 90 °, Rotary Vlu, to improve the theoretical head HT. However, due to the relative velocity ω1 increases, so that the liquid flow on the impeller impact loss increases, If the guide vane angle is adjusted beyond the limit, it will make the flow deviate from the designed flow rate Qd, the flow angle will change, and the vortex zone will be formed on the working face of the impeller blade, causing a larger Impact loss, the pump efficiency is lower.In summary, we believe that: the front guide vane adjustment caused by the pump efficiency changes, the pre-flow of liquid and impeller impact loss is the main factor.Therefore, the front guide vane adjustment Is limited, even in the limited use of 75 º a 110 º should also be avoided long-term operation of the pump at extreme angles. 3 on the pump cavitation performance Obviously, when the current guide vane to adjust the direction of more than 90, due to anti-spin flow generated, so that the liquid flow in the pump impeller inlet relative speed ω1 increase, the flow of impeller produce Impact, as the guide vane angle increases, the impact is more serious, the pump cavitation performance adversely affected. The basic equation of the pump cavitation: NPSHr = λ1V20 / 2g ten λ2ω12 / 2g that, due to the relative speed ω1 increases, making the required NPSHr NPSHr greatly increased, so that the pump cavitation performance decreased. Therefore, in operation, according to the pump cavitation characteristic curve and the water level and head changes, adjust the guide vane angle to ensure that the effective NPSHa NPSHa greater than the required NPSHr. In addition, due to the impact of the flow on the impeller, the vibration value at the pump impeller becomes larger as the vane angle increases. Table 2 is a pump at a certain water level, the angle of the front guide vane and the value of the vibration value at the impeller. Table 2 Vane angle and vane at the vibration value of the corresponding changes in the value of the guide vane angle 75º 80º 85º 90º 95º 100º 105º 110º Vibration value (mm / s) 1.87 1.90 1.93 2.01 2.