Steam Turbine - Part III ( Turbine casing and its components)
Casing
Based on high inlet pressure and temperature the casing construction shall be either single or double shell type. The double shell construction prevents the steam contact to the outer casing joint and it will be used in case of high inlet steam conditions.
Single casing has all its stages in the one casing and all are connected to a common drive shaft. Steam expansion, controlled / uncontrolled extraction provision within casing.
Double casing turbines have the exhaust steam circulating in the annular space between the casings as in case of high steam inlet condition and also the steam within the space will be taken into controlled / uncontrolled extraction for process / feedwater heating if required.
 | |
|
 |
| Single shell casing |
Most commonly used are horizontal split type casing (except few special occasions), since it have advantage to provide reliable, leak-free operation with metal to metal joints, moisture drainage provisions, and multiple casing inspection openings than other split types.
Steam Admission sections
Steam enters the turbine at the admission section through one or more governing valves. (Either bar lift or cam lift arrangements are common).
Bar lift arrangement
It is relatively simple, a servo motor operates the lever that raises or lowers the horizontal bar that is enclosed in the steam chest. The governing valves hang loosely on the bar and adjusted for length of stem so that it will open sequentially. There are typically five valves in the design but as many seven valves have been used.
The flow capacity is limited to 273TPH (Tons per hour), special precautions should be made for high flow application to prevent the free hanging valves from spinning and wearing out in service.
Cam lift arrangement
It is used for high flow and high pressure applications, can be shaped for obtaining the best balance of lifting forces and flow travel characteristics. The spring load on top of each valve stem is to oppose the blowout force of the valve stem when valve is wide open. This spring force is also used as a safety measure to help make sure the valves close in case of emergency unloading of turbine.
Emergency stop valve
Emergency stop valve / shut-off valve / trip-throttle valve that are built in the turbine steam chest and often provided with strainer, they are typically removable without having to dismantle any pipe works.
Diaphragms
Diaphragms in an impulse turbine is located between each rotating wheel that separates the turbine into a number of separate pressure stages of successive lower pressure. It contains the nozzles that accelerate and direct steam jet into and through the blading of the next succeeding rotating wheel and also retains the packing through which passes the shaft containing the wheels of each stage. It is horizontal split type and lower half fixed on the casing by means of set screws after shimming. The top half rest on dowel, when lifting the casing - upper half diaphragms is prevented from dropping by use of stops located at the casing split. In normal fit-up, there is clearance between the stop and the diaphragms.
The size and passage area from stage to stage varies to efficiently handle the steam volume encountered at the various pressure levels throughout the turbine.
Types of Diaphragms are
1) Conventional cast diaphragms (Limitations :- temp 500F & less and stage pressure drop 25 psi)
2) Investment cast diaphragms (Limitations :- temp 750F & less)
3) Milled and welded diaphragms (Limitations :- temp 750F & can be increased further w.r.t material)
4) Spoke type diaphragms ( Preferred where higher nozzle height required)
Labyrinth Packing and Housing
Any unit in compliance with API 612 will require use of labyrinth packing for interstage sealing, it helps to reduce the interstage leakage by arresting the clearance space between the casing and blades. For gland sealing also, labyrinth type packing can be used. Brush type sealing also can be used, it has better leakage resistant that the labyrinth type, moreover both type also can be installed on the same turbine.
Adjustment collars, eccentric pin also part of the casing components, it will helps for alignment purpose.
Bearings will be discussed on next topic and other steam turbine related topics will be discussed subsequently.
Bye 👋👋... See you all in next topic
If you like the content, please share and follow us on the social media platforms.. Link below
Comments
Post a Comment