Saturday, January 10, 2009
Gland seal systems are very important to main and auxiliary turbines. Turbine shafts must exit their casings in order to couple up or connect with the unit that the turbines drive (reduction gears, pumps, etc.) The main and auxiliary gland seal systems enable the turbine to be sealed where the shaft exits the casing; in effect keeping "air out and steam in."
The purpose of gland seal system is to prevent the leakage of air from the atmosphere into turbine casings and prevent the escape of steam from turbine casings into the atmosphere (see Figure 1).
1. The pressure differential between the atmosphere and inside the main engine turbine casing will vary depending on ship's speed. Similarly, the differential between the atmosphere and inside the ship's service turbine generator (SSTG) turbine casing will vary depending on electrical load.
2. Labyrinths- Sets of labyrinth packing are employed along the turbine rotor where the rotor exits the turbine casing to maintain this pressure differential.
a. The labyrinths create many little chambers causing pressure drops along the shaft. The number of labyrinth sets depends greatly on the steam pressure possible in that area. Labyrinth packing alone will neither stop the flow of steam from the turbine nor prevent air flow into the turbine.
3. Gland Sealing Steam
a. The gland sealing system provides low pressure steam to the turbine gland in the final sets of labyrinth packing. This assists the labyrinth packing in sealing the turbine to prevent the entrance of air into the turbine, which would reduce or destroy the vacuum in the associated condenser. Excess pressure (excess gland seal) is removed by the gland seal unloader.
4. Gland Exhaust
a. Since there are times when steam escapes from the seals, a gland exhaust system is provided. The gland exhaust system consists of low pressure piping connected to the gland area between the last two outer sets of labyrinths which receives and prevents steam from escaping to the atmosphere. This system collects the steam and directs it to a condenser for further use in the steam plant.
C. Main Engine Gland Seal System Components.
1. The gland seal regulator (see Figure 2)
a. Senses system pressure on the outlet side of the regulator. The gland seal regulator valve reduces 150 psig auxiliary ("dry or wet" steam depending on the ship type) steam to gland seal system pressure of .5 to 2 psig. The valve begins to open at 2 psig and is fully open at .5 psig. The bypass valve allows the operator to maintain system pressure in the event the regulator valve is inoperable. (see Figure 1)
2. The Gland seal unloader valve (see Figure 3)
a. This valve senses the pressure of the gland seal supply piping. The unloader piping is wider in diameter than the regulator piping. The gland seal unloader "dumps" the excess gland seal piping pressure to the LP turbine exhaust trunk. The unloader begins to open at 2 psig, and is fully open at 3 psig. It has a handwheel to permit manual operation of the unloader to control gland seal system pressure during a loss of control air causality.
Piping system (see Figure 1)
a. The regulator supplies the gland seal header. This header has branch lines to each turbine gland area and a branch line to the unloader. The ahead throttle valve assembly and the astern throttle valve also have a connection to receive gland sealing steam. The reason is the same on the unloader- to prevent the possible introduction of air into the system. Any air entering the turbines or piping systems affect the vacuum in the main condenser.
b. On some ships, spectacle flanges are installed in the supply lines to the HP turbine glands to allow the gland seal and gland exhaust system to be isolated when singling up with the LP turbine operating.
c. Inputs to the gland seal system include the gland seal regulator, astern throttle leak-off, ahead throttle lifting rod leak-off, HP turbine forward and after gland leak-off, and main steam emergency throttle leak-off (on ships with singling up capabilities).
4. Main engine gland exhaust system
a. Steam leaking from the gland seal section of the shaft packing is drawn off by the gland exhaust system. Gland exhaust is drawn into the gland exhaust condenser section of the Main Engine Air Ejector.
b. The gland exhaust steam is then condensed and returned to the fresh water drain collecting tank. The air and non-condensable gases are drawn off by the gland exhaust fan.
5. Gland Seal steam system operation
a. The gland seal regulator supplies .5 to 2 psig steam to the glands in varying degrees as bells change on the main engine. When answering a low bell or all stop, the gland leak off is minimal, causing the regulator to supply the total gland sealing steam. As engine speed increases, the casing is pressurized and the increased gland leak off, along with the regulator, supplies all the gland sealing steam required by the system.
(1) As ship's speed increases, the main engine becomes self sealing. The gland seal regulator is fully shut and the unloader is functioning to maintain the system pressure between 2 3 psig, dumping the excess gland seal steam to the LP turbine exhaust trunk.
(2) As ship's speed slows, the gland seal system operates in reverse sequence. (see Figure 4,5,6)
D. SSTG Gland Seal System Components
1. These components are almost identical to the main engine gland sealing components. The major difference between the main and SSTG system is the size.
a. The Gland seal regulator
(1) Senses pressure on the drain pot (or manifold). The drain pot or manifold is similar to a header and acts as "collection area" for the system steam. This area allows for the sensing lines to accurately measure the system pressure. The valve functions by reducing 150 psig auxiliary steam to the system pressure of .5 2 psig.
2. The Gland seal unloading valve
a. Senses the pressure on the drain pot. The operating range is 2 3 psig, unloading excess gland seal pressure to the lower section of the turbine exhaust casing.
3. The piping system (see Figure 7)
a. Consists of piping to the forward and after glands from the drain pot or manifold. The inputs to the system are gland seal regulator, and the forward turbine bearing. At a 60% load on the generator, leakage from the forward end of the turbine (high pressure end) supplies the system, the regulator is closed and the unloader bleeds excess to the turbine exhaust trunk.
E. Gland Exhaust System
1. Steam leaking from the gland seal area of the shaft packing, steam leak off from the steam chest lift rods, and steam leak off from the trip throttle valve is drawn into the gland exhaust system and into the air ejector condenser.
2. The steam is condensed in the SSTG air ejector condenser. Air and non condensable gases are discharged to the atmosphere via the gland exhaust fan which maintains a slight vacuum on the auxiliary air ejector condenser.
F. Causes of System Failure
1. Since most gland seal regulators are air operated reducing valves, improper pressure settings on the air pilots for the regulating and unloading valves can cause system pressure to be too high or low, or both valves may be open at the same time. Ruptured diaphragms may occur in these air pilot controllers and air operated valves. Oil and water in the air lines to the pilots or air operated valves can cause erratic operation and deterioration of the rubber diaphragms. Upon loss of air pressure, both valves fail open and the unloader valve must be operated with the manual handwheel to control gland seal pressure.
2. Painted valve stems or improper packing installation can cause binding of the stem, restricting valve operation.
3. Improperly calibrated gages can cause the system to be improperly operated.
4. In the event of a jammed gland seal regulator, the operator should take control of gland seal pressure by using the regulator bypass valve.
G. Safety Precautions
1. Do not admit steam to the glands of an idle turbine, as varying degrees of corrosion, erosion, or a bowed rotor may result.
2. Ensure the gland seal system is in operation on the main engine before aligning the main engine air ejectors. This helps prevent dirt and debris from being drawn into the turbine glands.
3. Adjustment of components shall be conducted by qualified and knowledgeable personnel. When performing adjustments, careful coordination of involved personnel will minmize confusion of gage indication.