10.2 Control

 

The flow of drains water in those drains which are normally closed during operation of the plant, is usually controlled by the master and martyr valves. At start-up, the master valve opens first followed by the martyr valve. In this way, possible deterioration of the valve seals from throttling the drains water is confined to the martyr valve. The master valve is then always able to provide tight isolation.

Each drain line is provided with a pressure gauge (calibrated to show saturation temperature) at the drain source and a temperature gauge. At a temperature of approximately 30°C above the saturation temperature corresponding to the line pressure, it is reasonable to assume that only dry steam will be passing through the drains and the motorised martyr valves can be closed. Alternatively, the valve can be closed automatically from a load or temperature signal. The master and martyr valves are usually opened after shutdown or there may be provision for automatic opening when the load falls below a value of say 10%.

When the drain valves are shut during on-load operation, a column of water will collect in the drain line above the valve due to cooling of the stagnant steam in the pipe. If a turbine trip is initiated or rapid unloading occurs, the pressure in the turbine decreases and this water tends to reflux into the plant: this could cause unacceptable chilling of the plant components or pipes. To minimise this refluxing, the martyr valves are sometimes replaced by an orifice plate which provides a permanent opening for the condensate to escape. The small diameter of the orifice allows an adequate flow of water but limits steam flow.

In other cases, the valves are opened automatically on reducing load or steam traps are used in place of the martyr valves. The steam trap is designed to allow water to escape, but not steam. There are various types of trap such as ball float, inverted-bucket and thermostatic traps. The traps are similar in operation and construction, although detailed differences are needed to meet the various operating conditions.

As an example, the thermostatic trap consists of a forged-steel body with a removable cover. Inlet and outlet ports are divided by a vertical wall. A ball valve situated on the downstream side of the valve seat is activated by a bimetallic element which, on a rise in temperature, deflects against the line pressure and closes the valve. The force of the ball valve increases in relation to the steam temperature and pressure.

The trap usually discharges water after a certain amount has collected. The trap then closes and is ready for refilling. However, some traps are able to differentiate between steam and condensate by sensors which can detect a temperature drop in the condensate. These traps are set so that only condensate, air or wet steam will pass through to the drains vessel. As soon as steam at saturation temperature (or above) enters the trap, the ball valve is held tightly on its seat, thus preventing the escape of live steam to the drains vessel. The controlling temperature can be constant, or can be varied depending on conditions in the steam system.

Most steam trap installations incorporate a by-pass, so that the traps can be isolated for inspection and maintenance. This by-pass permits the blowdown of debris or contaminated condensate during commissioning or subsequent run-ups, and prevents undue contamination of the trap strainer.

 

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