Heii power plant engineers how are you I hope all is well today we discussed about a very important topic related to a turbine or we can say a very important question which is generally asked by interviewer which is what is the skid –pin system in a steam turbine? And why the skid-pin system is used in a steam turbine. In this article we discussed turbine skid –pin system what is the working principle of skid –pin system?. What are the problems will occur when we did not use skid –pin system
? What is the importance of skid pin system in the steam turbine?
WHY SKID-PIN SYSTEM REQUIRED IN STEAM TURBINE
When we start to stop the steam turbine and changes load, the temperature of various parts of the turbine cylinder changes greatly and it will expand or contract along with several directions of the length, width, and height with the change of the temperature due to thermal expansion of steam turbine cylinder all take place in various direction.
Steam turbine operates at high temperature under the working of high temperature the rotor and casing of a steam turbine expands. Turbine rotor and casing expands/contracts at a different rate due to the difference in their mass. As casing wall is thick and its mass more than rotor so it expands and contracts at a slower rate than rotor as clearance between fixed blade and moving blade inside the turbine is less so expansion of casing and rotor is required to be uniform otherwise there is highly chance of rubbing of fixed and moving parts of turbine . Special care to be taken for smooth expansion of casing and rotor during startup and shutdown of steam turbine and also during load variation and shut down of the turbine.
IMPORTANCE OF SKID PIN SYSTEM IN STEAM TURBINE
Thermal condition in the steam turbine changes during starting and stopping and load change. Turbine startup curve should be followed during turbine starting for smooth expansion of the casing and rotor. Proper time should be given for the expansion of these parts. This is called as socking of the turbine. More time is given to bring the turbine into full speed from a cold condition. In hot start up the turbine is already in hot condition so the time required is less.
In most of the chances, the rear side pedestal of the turbine and hence casing is rigidly fixed to the base plate. But the front side pedestal is fixed in such a way that it can expand towards the front. The temperature of the rotor varies according to steam flow. so it is subjected to frequent thermal expansion and contraction and it is first in comparison to the casing. For smooth operation of the turbine, it is required to monitor the expansion of the casing and rotor. For this differential expansion of casing and rotor is measured.
Eddy current type measuring probes are normally used to measure rotor expansion and contraction. Through these probes the gap voltage between the probe tip and rotor parts measured. The rotor axial thermal growth with respect to turbine foundation is measured. This is the relative measurement of rotor axial thermal growth with respect to the casing.
As the rise of the base station board’s of steam turbine temperature is below the cylinder’s, if the cylinder and the base station board are fixedly connected, the cylinder will not freely expand. If the cylinder cannot expand and contract reasonably, according to thermal expansion the following problems will be caused:
Here two major problems are discussed below when the turbine cylinder cannot expand and contract reasonably.
(1) the metal thermal stress of the cylinder is too large, resulting in cylinder deformation and crack, which leads to the increase of the steam turbine vibration, cylinder steam leakage, and even the cylinder cannot work.
(2) The axial or radial gap change inside the cylinder sometimes some serious faults such as gas seal, the dynamic and static friction, will occur.
In order to ensure the free expansion of the cylinder in a fixed direction, maintain the cylinder and the rotor have the same center, and to avoid the undue stress and unit vibration because of the non-smooth expansion, a set of the skid-pin system must be set. There should be equipped with skid pin between the cylinder and the base station
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