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Turbine steam seal gap adjustment and solution 2020-03-31


In the installation and maintenance of the steam turbine, the adjustment of the steam seal gap is one of the most critical processes, which directly affects the safety and economy of the entire steam turbine. During the installation and maintenance of more than 10 large domestic steam turbines we participated in, we found that many of the unit's operations were unstable due to the incorrect experience and working methods of the construction personnel. We will summarize the prone problems as follows, and I would like to share with you.

 

The purpose of steam seal adjustment is to adjust the cylinder liner and steam seal block to ensure that the gap of the steam seal is within the standard range and tends to the minimum value under the premise of ensuring safety. In this way, it is possible to ensure that steam leakage losses are reduced among the various stages of the multi-stage steam turbine, and the thermal efficiency of the unit is improved. The measurement and adjustment of the seal gap are performed after the center of the shaft system and the center of the diaphragm and shaft end steam seal sleeve are adjusted. There are usually two methods to measure the radial gap of the steam seal: one is the tape method; the other is the lead wire method. Of the two methods of measurement, the second is more accurate than the first and more realistic. For the deviation of the gap adjustment, the main problems found in the current adjustment process are:

 

(1) The effect of cat claw thermal expansion on the seal gap is not considered;

 

(2) Influence of processing and measurement deviation on adjustment

 

(3) the impact of the construction personnel's process level on the adjustment;

 

(4) Effect of rotor arc on steam seal clearance

 

(5) The influence of rotor vertical arc on the seal gap is not considered:

 

2 Analysis of existing problems and solutions

 

2.1 Effect of cat claw thermal expansion on the seal gap

 

尽管 Although the cylinders of high-pressure steam turbines are different in cylinder structure, their support is divided into two types: lower cylinder cat claw support and upper cylinder cat claw support. Cat cylinder support for the lower cylinder. The supporting plane of the cylinder cat claw is lower than the unit's center line. The temperature of the cat claw will be higher than the temperature of the bearing seat during operation. Reduce or even bump. The numerical value of the elevation of the shaft seal dimple at the cat claw support is related to the size of the cat claw, the temperature of the cat claw, and the support form. If the cat claw height H is t50m / m, and the average temperature of the cat claw is 250 ° C, the temperature of the corresponding bearing seat is 80 ° C, and the linear expansion coefficient is Q = L 2 × lo-5 / ° C. Then the elevation value of the center of the shaft seal depression is: △ H = Q HA t = 1.2 × 10-5 × 150 × (250—80) = 0.3 [m / m, that is, the lower clearance of the shaft seal depression will be reduced by 0.3lm / m, and the upper gap will increase by 0.3tm / m. Therefore, the steam-sealed dimples in the cylinder will change at any time, endangering the safe operation of the unit. Therefore, the data and drawings of the steam turbine manufacturer should be consulted, and according to the requirements of the cat paw expansion value in the "Installation Manual", the influence of the cat paw thermal expansion on the center of each part is reserved. At the end of adjusting the seal gap, adjust the center of each sleeve as required.

 

2.2 Influence of processing and measurement deviation on adjustment

 

When measuring the seal gap, pay attention to the following aspects to reduce the impact of measurement deviation on the adjustment: the back side of the seal block must be fixed with a bamboo wedge first, so that when there is a force between the seal block and the lead wire, The seal does not retreat: The result of the lead wire pressing is as much as possible for the person responsible to reduce the measurement error: when measuring the thickness of the remaining part of the lead wire groove mark, the caliper's mouth area needs to be very thin, which means that the Bottom, otherwise the measured gap is greater than the actual gap. If there is no special caliper, it needs to be ground on the spot, and the caliper mouth to be ground must be thin without damaging the mouth. When measuring with the adhesive tape method, the measurement may also cause errors: the quality of the adhesive tape is not good, the thickness is uneven, and the thickness is often less than 0.25mm, but each layer in the measurement is 0.25mm; different methods and strength of the adhesive tape Different, the same number of layers of tape will have different thicknesses: After the tape is pasted, when the rotor enters the cylinder, the rotor accidentally hits the spacer sleeve, causing false marks, which will cause judgment errors: The red lining is too thick on the tape, and it may also cause judgment errors . For the repair of steam seal blocks, it is often used to turn the lathe on a dedicated tire or use a special seal gap adjustment device for processing. It is difficult to find the correct position. In particular, when the processing volume of the two ends of the steam seal block is different, correct the added length. Many adjustment gaskets will affect the accuracy of steam seal adjustment.

 

2.3 Impact of construction personnel's process level on adjustment

 

Adjusting the gap of the steam seal is a delicate task. In addition to requiring the construction personnel to have a good technical level, they should also have a strong sense of responsibility. Special attention should be paid to the following items: In the process of installing the upper partition and the partition sleeve, if the horizontal mid-plane bolt is not tight enough, it cannot guarantee that the mid-plane gap is not 0.05mm feeler, and the upper steam seal gap is measured. The value will be too large: the crimping and burrs existing in the installation of the steam seal block should be corrected, and the joints of each section of the steam seal tooth should be smoothly transitioned, and there should be no misalignment. The gap between the bottom pin of the sleeve is too large, and whether the contact of the mounting ears of the partition is good will also affect the accuracy of measuring the gap of the steam seal.

 

2.4 Effect of rotor arc on steam seal clearance

 

During the overhaul of the steam turbine, the rotor is placed on the rotor support frame for a period of time. Due to the rotor's own weight, a vertical arc will occur. Before measuring the seal gap, periodically rotate 180 degrees on the temporary support, and eliminate the vertical arc before measuring. In the maintenance of a certain power plant, the two measurement results were very different and rework occurred.

 

2.5 The influence of cylinder weight and running state on the seal gap

 

When adjusting the seal gap, we must also consider the installation state of the cylinder, and consider the vertical arc of the cylinder, that is, how much the diaphragm is installed in the cylinder. Generally speaking: increase the weight, increase the vertical arc of the cylinder, and buckle the cylinder. Later, the number of cylinder bolts is large, and the clearance of the joint surface is small, which increases the rigidity of the cylinder and reduces the vertical arc. The thin cylinder and poor rigidity of the steam turbine cause large static deflection of the cylinder. When adjusting the seal gap, the cylinder state during operation should be considered. If the clearance adjustment of the semi-solid cylinder is performed directly according to the normal procedure, the gap of the seal may be exceeded in the state of the full cylinder. The following is a test of the specific impact situation. The low-pressure cylinder is first trial-fitted with an empty cylinder, and the water on the steam side of the condenser is filled to about lm higher than the normal operating water level (because of the amount of circulating water pipes) to try to simulate the cylinder state during operation . A bearing block at both ends of the cylinder is used to fix an I-beam through the cylinder. A dial indicator is installed on the I-beam, and the pointer points to the partition grooves and shaft seal dimples at various levels. Observing the dial indicator changes, it is found that the cylinder There are different degrees of sinking. The maximum sinking amount is about 0.30ram. After closing the half inner cylinder, after tightening the face bolts in the cylinder, each baffle groove is lifted to a certain extent, which is consistent with the deflection curve of the cylinder. The maximum lift is about 0.80ram in the middle of the cylinder. An approximation is obtained. Based on the deflection curve of the seal gap correction value curve, and according to the correction value in the semi-solid cylinder state, adjust the seal gap.

 

3 Concluding remarks

 

The adjustment of the steam seal clearance of steam turbines is an important and meticulous task, especially for units that have been repaired after many years of operation, and there are more influential factors to be considered. At the same time, different types of units have different influencing factors. The above-mentioned steam seal adjustment method has been tried out in several overhauls. After the overhaul, the thermal test has determined that the thermal efficiency improvement effect of the steam turbine is very obvious. It proves that the above analysis of the process improvement of the steam seal adjustment from the theory and practice is reasonable and correct, and it helps to improve the thermal efficiency of the steam turbine.


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