1.8   Protection against ingress of water/steam to turbines

 

Many well documented instances of damage being caused to turbines by the ingress of water or a water/ steam mixture from the feed system have been reported [3,4,5].

There are several potential sources of water within the feed system which can flow or be induced into the turbine. The potential sources are as follows:

  • (a) High water level in an HP or LP feedheater. The high water level could be caused by a tube leak or failure of the drainage arrangements.
  • (b) High water level in a de-aerator. If there is a mismatch between inflow and outflow the vessel can flood.
  • (c) Undrained bled-steam lines. When the bled-steam is wet, the water in the steam is deposited on the pipework walls or is separated when a valve or bend is encountered. Condensate is also formed on start-up, while the lines are being warmed to operating temperatures.

In the case of items (a) and (b), if the rising level is allowed to continue unchecked, then it could flood into the bled-steam line and back to the turbine.

With regard to item (c), if there is continual fall from the points where the water is accumulating towards the extraction point on the turbine, it will most certainly flow against the steam flow towards the turbine.

The other means by which water or a water/steam mixture can be induced into the turbine is by a pressure reversal between the feed heaters and the turbine bleed points. A pressure reversal is caused by a unit trip or a sudden load reduction.

On unit trip, the HP turbine pressures decay rapidly id the IP/LP turbine pressure falls to condenser icuum almost immediately. The pressures in the feed rstem change slowly compared with the turbine, and rge pressure differentials will be created with the Dtential to cause flow towards the turbine from the ed system. The large quantity of water stored in the de-aerator tank at just below the saturation equivalent to the turbine bleed point pressure, has the potential to evolve sufficient steam to overspeed the turbine in the event of a turbine trip. To ensure that this cannot happen, a power-assisted closing non-return valve is placed in the steam line between the turbine and the de-aerator. In addition to tripping the non-return valve, the bled-steam isolating valve is also arranged to be shut on unit trip.

Reverse steam flow can also carry quantities of water from heaters and undrained low points in the bled-steam lines into the turbine and cause damage, particularly to the large LP turbine blades. These 'back flows' can induce cooler steam into hot cylinders, with the consequent risk of thermal distortion.

This is an example of how a system must be designed to allow for all conditions of operation.

Flow in both normal and reverse direction must be considered and, where provision to prevent flow in one direction is provided, care must be taken to ensure that there are no 'sneak' paths to bypass the protection. It is evident that the protection system must contain flooding and also prevent back flow of steam.

A typical feedheating plant - showing protection equipment

The following provisions (illustrated in Fig 3.15) have been made on all 500 and 660 MW units within the CEGB to prevent these possibilities:

  • (a) A non-return valve is placed in the bled-steam line as close to the turbine bleed point as practicable. For the de-aerator and HP heaters, these are power-assisted non-return valves. For the LP heater bleed points, free-acting valves are normally provided. On very low pressure heaters, the omission of the non-return valve is permissible if the conditions given in (g) have been satisfied.
  • (b) Power-operated bled-steam isolating valves are provided on each line between the turbine and a heater, as close to the heater as is practicable. Again, on very low pressure heaters the isolating valves can be omitted provided the conditions given in (g) have been satisfied.
  • (c) Feed or condensate isolating valves are provided, where appropriate, to shut off the supply of water to a heater or group of heaters.
  • (d) Duplicate level sensing devices are provided on each heater, either of which will actuate trip circuits to close the protection valves.
  • (e) In some instances the pumps which discharge to a particular heater are tripped.
  • (f) All bled-steam lines have an adequate fall towards a drainage point. Each drainage point is capable of draining by gravity to the drain receiver vessel, which is at condenser vacuum. Any pockets of water which can be formed by the closure of valves have drains.
  • (g) In the case of very low pressure heaters such as turbine moisture extraction condensers (which extract a steam/water mixture before the last blade in the LP turbine), the pressure drop caused by the isolating and non-return valves as specified in (a) and (b) may be unacceptable so, instead, duplicate unvalved drains (which can drain the heater by gravity alone) are provided between the heater and the condenser. The feedwater flow to the heater is also isolated in the event of a high water level in the heater. The protection provided is illustrated in Fig 3.16.

Drainage arrangements  for very low pressure heaters without valves in bled-steam lines

Because of the short time scale in which the commencement of water feed back to the turbine can occur, all protection measures are automatic and the hardware provisions for each type of heater is detailed in the appropriate section of this chapter. The types of protection relay and associated tripping circuits are described in Volume F, which deals with control and instrumentation.

 

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