Since the end of World War II, the chronic requirement for power and yet more power has become a problem of international rather than purely national interest. Throughout the world, the increasing tempo of industrialization and the general improvements in living standards have created a situation that can only be countered by bold, far-sighted planning and adventurous engineering. Hence, the now-familiar emphasis on nuclear power stations and hydro-electric schemes. Carbon and its numerous applications in nuclear engineering will no doubt be a subject in some future edition of this publication. This article is concerned with the part carbon-graphitic glands play in the running of that essential Io a nation’s industrial and domestic power needs - the hydro-electric power station.To date, nobody has discovered, produced or invented a material for turbine glands that can better carbon. For this reason, carbon is widely employed in this capacity, except in one or two countries like Italy and the U.S.A. where soft packing glands arc preferred. Carbon glands arc reliable. can be easily fitted in comparison to other types. do not need frequent adjustments. and, very important, do not damage or wear the turbine shaft.
The glands manufactured by Assam Carbon Products Limited are highly-specialized products developed in association and collaboration with prominent turbine constructors throughout the world. They have won for the Company a substantial percentage of the world’s markets for such equipment. So far, glands have been supplied for shaft diameters up to 1170 mm. It is hoped, however. that the Company will shortly embark upon a contract for the largest. Most ambitious hydro-electric scheme in the world, where shafts of 1350 mm diameter will have to be sealed.
Every water turbine installation, big or small, introduces its own peculiarities, its own difficulties and running conditions. This article can do little more than cover the subject in general terms and, in doing so, signpost the broad principles and rulings that dictate which gland materials and gland design are best suited to a specific application.
The majority of gland rings supplied conform broadly teither one of two patterns – the tenon - type or the wedge - type. Depending upon the vagaries of the particular commitment, rings of the same type may differ in detail amongst themselves, although in an overall sense, they will generally comply with the ensuing descrpitions.
This gland consists of a ring composed of a number of carbon segments held together by a peripheral close-coiled tension or garter spring. It earns its name from the method of assembling the carbon segments, each of which is connected to its neighbour by an integral toungue fitting into a recess.
Glands of this type are self-adjusting in that the segments move radially inwards under the force of the spring as wear takes place. The tongues and recesses must, of course, be machined with sufficient allowance to permit this radial closing to occur without buckling or distortithe ring. To prevent the ring turning with the turbine shaft,each segment is located radially with respect to the housing by a pin.
To provide added protection against axial leakage, tenon rings are usually installed in pairs with the tenon joints between the upper and lower rings staggered so that thereis no direct passage for the water. This arrangement is employed with both the single tenon type already described, and the double tenon ring, which is a similar but stronger version of the gland having tenons and recesses at both ends of each segment.
Although tenon rings are by far the most common they are not recommended for shaftdiameters above 500mm and water pressures greater than 2 bar. At figures much in excess of these certain disadvantages soon become evident:
To overcome as many of these disadvantages as possible, another type of ring has been evolved in whichthe tenons are replaced by wedges. This ring – appropriately designated the wedge-type - has won an immense popularity and is likely to supersede the tenon ring in the majority of installations.
Like the tenon-type, this ring is made up of carbon segments, the actual quantity of which will vary from eight to sixteen depending upon the shaft diameter. Instead of having tongues and recesses, the ends of these segmeare chamfered to present sliding surfaces to tapered wedge pieces, one of which is interposed between ends of every pair of neighbouring segments. A garter spring seated in a groove around the outside circumference of the ring holds the assembly of segments and wedges together. Pins rooted in the outer sealing faces of the gland housing and located in slots in segments stop the gland from turning with the shaft.
Wear on the carbon is taken up by the action of the spring continually forcing the segments into contact with the shaft, this inward movement of the segments being accompanied by a corresponding outward shifting of the wedges. Thus, at all times under all conditions of wear, the gland presents a true and pressurized surface to the shaft circumference. One of the main differences between the tenon and wedgerings is the inability of the latter to seal in the axial direction of its own accord. Obviously, if the self-adjusting capabilities are to be obtained, it is not feasible to have the spring bearing against a tapered peripheral face as it does in the tenon ring. Seperate provision must be made therefore to attain adequate axial sealing.
This is usually achieved by separate compression springs housed in recesses in the gland housing and urging in the axial direction on metal support plates bearing against one face of the gland ring. Normally, two springs arc applied to each segment.
Further safeguard against axial leakage is provided by mounting two rings one above the other in much the same way as with tenon-type, the joints between segments being arranged out of coincidence.
Developed in association with a leading turbine maker, wedge-type gland rings offer very distinct advantage over their tenon-type counterparts.
Already, wedge-type carbon rings absorb a substantial proportion of current business in water turbine glands. There is every prospect that their considerable appeal will bring about a general re-appraisal of turbine sealing practices and techniques.
Steam and Gas
For steam and gas applications, non-contact gland rings, which seal by throttliAng are used. With these rings, the bore of the carbon rings is designed to match the shaft diameter at the operating temperature. For assembly reasons the carbon rings are made in segments and are held in position in L-housings and a stop pin is fitted to each ring to prevent it rotating with the shaft. Either rectangular-section rings or bevel-sections rings may be used; the segments are usually butt-jointed. A bevel-section gland ring is shown in figure3. Types of different joints can be seen in figure 4.
Carbon Labyrinth Glands
Carbon Labyrinth glands are similar in function to metal labyrinths, but because much smaller radial clearances can be used, higher pressures, through a shorter axial length can be sealed effectively. They are used as main shaft seals in gas turbines, ausiliary steam turbines, rotary compressors and blowers. A typical carbon labyrinth is shown in figure 5.