How is the turbine coastdown time determined?
Thread Content
This post was last edited by dychem on 2010-1-13 at 14:49. I would like to ask the TYs for their advice regarding the issue of the turbine’s downtime. Thank you in advance. 1. Is the turbine trip time a calculated (empirical) value or an actual test value? 2. If it is a calculated value, is there a formula? 3. If it is a test value, is it the figure obtained by testing the turbine alone after its installation, or is it the value obtained when the turbine is operated under load (for example, with a centrifugal compressor)? 4. Assume that in the event of a failure in the unit (turbine + centrifuge), high-level oil tanks are used for lubrication, and the volume of these high-level oil tanks is determined by the time it takes for the unit to stop operating. If it is a test value, then how can the volume of the upper fuel tank be determined before that test value is obtained? (Do we have to install and debug the unit first, determine its coasting time, before designing the high-level oil tank?) ）
This is a measured value. What is usually observed is the coasting time with a centrifuge present; this coasting time refers to the period from when the automatic main steam valve and control valves are closed until the rotor comes to a complete stop. However, there are many factors that affect the coasting time. For example, the rate of vacuum drop – if the vacuum drops too quickly, the coasting time decreases. The lubricating oil pressure also plays a role: if this pressure is low (which may occur when the rotor speed drops and thus the oil pressure at the outlet of the main oil pump falls, with the backup oil pump not starting), the oil film is disrupted, and the coasting time shortens. Excessively high or low lubricating oil temperatures can also damage the oil film. A faulty valve can increase the coasting time, and so on. As for conducting actual tests on your machine, keep the fuel level at its highest level; there’s no need to change any other settings. Just perform a normal shutdown and measure the time taken. Try to keep all other conditions unchanged during the first measurement, so that there is a basis for comparison later on, which will aid in making judgments in the future.
The idling time for the air pressure unit is 5 minutes, while that for the main fan unit is 8 minutes
Could you provide more details? Will the coasting time of a turbine under different loads be the same?
Hehe, it refers to the period from when the quick-shut valve is closed, and the entire unit proceeds with the shutdown procedures, until the turbine comes to a complete stop following the shutdown command. It’s the idling time; if the oil pump also stops, the capacity of the high-level oil tank should be sufficient to meet the oil requirements for lubricating the engine during this period
By the time the unit is installed and its coasting time is determined through testing, the volume of the high-level oil tank must already be fixed and the tank itself must have been installed. Do we really have to wait until on-site testing by the crew determines the drop time before designing and installing a high-mounted fuel tank? Run the oil flush again.
There is no standard for the idle time; it is generally measured on-site. In particular, the time required for the machine to start up for the first time should be recorded as this represents the machine’s idle time. During subsequent shutdowns, this time can be used to identify potential problems such as issues with the bearings or the gas seals on the turbine side.
In my opinion, it mainly depends on the time it takes for the steam quick-shut valve to close and the compressor to stop operating; additionally, by referring to the oil level in the high-level oil tank and the amount of lubricating oil required per unit time by the unit, an estimation can be made.
The time it takes for the air-pressure unit (driven by a turbine) to go from having its shut-off valve closed to its speed dropping to zero is 5 minutes. Turbines with a power rating of less than 5000 KW take 5 minutes to come to a stop, while turbines with a power rating greater than 5000 KW take 8 minutes
The high-level oil tank and the turbine unit were installed simultaneously, but this still does not explain how the volume of the high-level oil tank was calculated; the manufacturer’s staff also said that the volume is an estimated value with a margin of safety included. Hehe.
But how do manufacturers make such estimates? There must be some method for estimation after all