Steam Turbine Extraction System 1

[mitraps]

Is it essential that during start up of a large steam turbine system, lowest heater is charged first and then sequentially the higher pressure heaters and for planned shut down, the reverse sequence is followed? If so,please explain the reasons and what is the risk for not building up such a logic in the control system.
For condenser neck mounted or other LP heaters without any NRV on the extraction line,how the turbine water damage protection is ensured?

 

[samuelliu]

ASME TDP-1 may answer your questions.

 

[poetix99]

The described sequence will, in a general way, minimize the thermal-mechanical transient loads on the turbine (and possibly the heaters and other piping components as well).

Looking at the start-up as an example, as the load is increased, and the heaters are "charged" from the bottom up, this will GRADUALLY increase the amount of steam that is extracted from the turbine, and ensure that at low loads there is always enough steam flowing to the last stage blades to prevent overheating of the last stage - blades and casing.

[ At low loads, and hence low flows, the tallest turbine stages are not doing any useful work because the entire pressure drop has occurred upstream. In fact the tall blades are acting parasitically and dissipating some of the shaft work back into the LP steam as heat. There must be enough steam flow at low loads to carry away the excess heat. ]

Charging the top heater(s) first, for example, could divert a large portion of the relatively small steam flow from the HP turbine to the top heaters fed with very cold feedwater. The large LP blades would overheat the exhaust casing. The thermal stresses induced in the top heaters might also become excessive if they are loaded first.

The deaerator would also not yet be operational, and could cause problems in the top heaters and the boiler drum.

 

[dcpl0341]

Cutting in heaters in reverse direction i.e from Highest pressure heaters to lowest pressure would cause a steam flow through theat heater much in access of the design flow which in turn would lower down the steam flow down stream of Turbine highest extraction.There is a steam starvation downstream of extraction resulting rise in temperature and another problem in the heater.Large steam flow would generate large heater drain flow and as the preeciding heaters are not in service this drain flow has to be handled by emergency drain only.Normally darin system are not designed to handle this flow.
In reply to 2nd part of the question,TWDPS recommends to design the system so that no single failure of equipment shall result in water enterin the Turbine. For a heater, extraction line stop valves,heater drain system and feed water valves in the heaters are the three basic equipment failure of which may result in water induction to turbine.In case of no valves in the extraction steam lines as condenser neck heater, feew water or the condensate isolation valves and drain valves from the heater has to be power actuated.

 

[marada93]

Placing feedwater heaters in service sequentially does minimize or eliminate high exhaust hood temperatures and resultant high blade path temperatures upstream. A general rule of thumb would be to place the lowest pressure heater in service, then place the next lowest pressure in service after you have a clear indication of feedwater (condensate) temperature rise exiting the heater. When you have this, you know you have flow through the extraction, and all other turbine stages as well.

This method also reduces thermal stress at the heater itself as water temperature is gradually increased through the string of heaters. Placing the highest pressure heater in first for example, would bring into contact relatively cold feedwater and relatively hot extraction steam.

If you have heater with no emergency drain to protect the turbine from water induction, the standard is to isolate the heater in the event of high-high level. In this case, the feedwater bypass valve would open, followed by the sub-sequent closure of the inlet and outlet valves.