Determining The Setting Of Boiler Safety Valve? 1

[powering2002]

In Power plant that I works, Last 6 months, there was a failure in the Fuel Control Valve. Suddenly Main Steam Pressure Increase and Drum Level Become Very Low. The Drum Level decreased so quickly that Unit Trip (MFT) before the Safety Valve works to release the increasing pressure of steam. Our unit was tripped. My friend told me that this would not happen if the safety valve worked. At this I would like to Ask about the correct setting of Safety valve so that it will work before MFT? For information, the existing setting of Safety valve is 98 bar. Our Power Plant has a capacity of 100 MW. Main Steam Temperature is 510 C, Main Steam Pressure is 88 bar. Main steam flow is designed at 400 ton/H. Thanks for your attention

 

[powering2002]

Just for Additional Information. Our Unit is set to trip when the Drum Level NWL- 230mm. Thanks a lot

 

[athomas236]

Some points:

1. The drum level most likely dropped because of the increased pressure which caused the steam bubbles in the evaporator to collapse taking up less volume and hence the drum water to fall.

2. The design code for the boiler will give the maximum safety valve settings in relation to the max allowable working pressure which should be shown on the boiler name plate. Clearly, these should not be exceeded.

3. The boiler supplier would have selected the safety valve settings at values that would prevent unnecessary lifting of the valves and to ensure that the valves properly reseated after they had opened. Reducing the safety valve set pressures could result in unnecessary lifting of the valves and the valves not properly reseating once they have opened and hence causing leaking and increased maintenance.

4. The function of safety valves is to protect the boiler from over pressure not to overcome operational problems. They are the last line of defence and I recommend that the settings should be in accordance with the suppliers instructions.

Regards,

athomas236

 

[powering2002]

So our next question. Is it Safe to open the safety valve (operated by solenoid) manually from the Control Room to decrease the steam pressure rapidly in order to avoid Master Fuel Trip? Thanks

 

[unclesyd]

Normally there should be an all mechanical operated relief/safety valve on a boiler. The valves normally sit on top of the steam drum or very close with no valve in between the relief/safety valve and drum.
These valves have to be properly sized for your boiler to prevent overpressure.

Do you have a mechanically operated relief/safety valve on your boiler?

 

[powering2002]

Yes, we have a mechanical safety valve located at the top of The Main Drum which is set to open at pressure 110 bar. This is the highest setting of all safety valves we have. We also have Boiler PCV(another safety valve operated by solenoid) installed in the main steam line between 2nd superheater and turbine stop valve. This is set at pressure 98 bar.