14.5 Gland sealing - part 2
Since the leakages through labyrinth seals are relatively high, it can be desirable to reduce the leakage flow when the pumps are stationary. This is particularly relevant to standby feed pumps.
Therefore standby seals are included which operate when the pumps come to rest. The standby seals consist of three segments which are held together on the shaft to form a closed seal when the pump is stationary. They are opened by centrifugal force at a predetermined speed to minimise rubbing contact during pump operation. For base-load feed pumps, it is possible to replace the centrifugal seal arrangement with an extended labyrinth. This reduces on-load leakage losses at the expense of higher leakage while the pump is stationary. A labyrinth bush outboard of the injection point controls leakage of water to a clean drains vessel, from where it can be recovered to the system.
Problems with labyrinth glands on some earlier feed pump designs led to a programme of conversion to mechanical seals. These reduced the risk of temperature stratification problems in the labyrinth area of these long slender shaft designs, in addition to minimising running and standby leakage. As a result of the early satisfactory performance of mechanical seals, there has been a progressive development of mechanical seal systems up to the high speed vapour lock requirements of the advanced class feed pumps.
Recent CEGB policy for new stations has been to install mechanical seals on the starting and standby feed pumps, while retaining labyrinth arrangements on the main feed pumps. For a 3 x 50% pump arrangement, mechanical seals are accepted where it can be demonstrated that the seals have a design life of at least 15 000 h.
A typical high duty mechanical seal arrangement is shown in Fig 4.61. The 'cartridge' design enables it to be assembled as a complete unit into the pump, thus minimising installation errors. A pumping ring is incorporated into the rotating part of the seal, which circulates water around a closed loop, through a heat exchanger and returning to the seal face area. This recirculation enables the correct seal face temperatures and stable face conditions to be achieved. The heat exchangers are normally positioned above the seals in order to obtain a good thermal syphon effect during hot standby conditions. An inner cooling jacket is also included to reduce the heat flow into the seal chamber. In certain designs, the effectiveness of this inner barrier eliminates the need to thermal syphoning. A simple magnetic separator is also installed into the seal pumping loop to ensure that any magnetic particles, which could abrade the seal faces, are removed.
The face materials used on early feed pump seals were tungsten carbide rotating against a stationary carbon face. While this combination proved very satisfactory on low speed applications, on certain high duty seals a selective corrosion/erosion process was found to occur on the tungsten carbide binder material. This process can lead to excessive face wear and premature failure. The reliability of feed pump seals was improved by the introduction of silicon carbide in place of tungsten carbide. Since silicon carbide is chemically inert against conventional feed-water additives, use of this material eliminates this potential failure mode.