Dear Experts,

In what case the safety valve of the boiler drum will lift prior to the main steam line safety valve?

Thanking you

 

These questions are covered in the applicable codes. At issue is whether the main steam line can be isolated from the boiler and the pressure rating of the piping/equipment. Typically such a valve is there to protect equipment that can be isolated from the boiler.

 

The question is unclear.

Traditionally the safety valves downstream of the boiler (or drum) will be set to lift at a lower pressure than those on the boiler/drum. This is to allow continued steam flow through the superheater and other piping for cooling.

If the downstream safety cannot release enough steam to lower the pressure, then the next "stage" of protection will occur as the boiler/drum safety lifts.

As mentioned, applicable design codes for your jurisdiction will explicitly state pressure relief requirements.

 

Sir,

It had all happened with me when the plant was running normal at BMCR.
It was found later that the setting of main steam valve and drum safety valves was ok and there was no problem with the working of same.

Even then the drum safety valve lifted prior to that of main steam line safety valve.

Kindly Explain, its confusing!

 

What is the pressure setting of each safety valve?

Get back to basics. What makes a safety pressure relief valve open? Answer: Pressure!

If safety valves are all set at EXACTLY the same pressures--and tested/calibrated to confirm the setting is correct, then they should open at the same pressure.

Mechanical binding, corrosion, and poor maintenance practices can cause trouble. But lots of things could cause what you are experiencing.

If there is a valve or restriction between the drum and the downstream safety valve, you may have different pressures at each point in the system.

 

There is a pressure drop across the superheater. The higher the steam flow, the higher the pressure drop. Normally, the drum safety valve will have a higher pressure setting than the superheated steam safety valve setting because the design pressure of the drum is higher than the design pressure of the superheated steam piping.

If the high pressure condition is caused by a stoppage in the steam flow (steam turbine trip, for example), and you do not have an active bypass, the steam safeties would normally open before the drum safeties, and should prevent the drum safeties from opening.

I suppose if the high pressure condition occurs during a high steam flow condition, the pressure drop across the superheater could make the drum safeties open first, although this would be an unusual circumstance.

If you have the steam and drum pressures and steam flow recorded in a historian, you might try looking at the data. It is certainly possible that the setting of the steam safety is too high or the drum safety setting is too low, but the code requires testing of these valves. How long since they have been tested?

One other possibility is if a very high water level in the drum is what precipitated all this, you may have an interlock that closes a motor operated isolation valve on the superheater outlet which would make the drum safeties open, but this should also have tripped the heat source for the boiler.

 

After reading your answer I just looked for the previous readings of Drum Pressure, Steam flow and Super Heater O/L Pressure. Later on analysis took me to the decision as you said that, due to higher steam flow the pressure difference between the Drum Safety Valve and Super Heater O/L Safety Valve increased so much.

It all happened this way that at higher steam flow greater than BMCR the pressure difference between Drum and Super Heater O/L increased.
So, at higher steam flow than the rated for any boiler we have to maintain a higher drum pressure to get the designed Super Heater O/L Pressure.

For rising SHO pressure we have to rise the drum pressure more than the normal running parameters and this can lead to this problem.

 

I agree there is a pressure drop when steam is flowing. However, when no steam is flowing, the pressure is equalized throughout the system. At lower flows like transients, the pressure drop is much less (lower flows).

But no need to argue, let's go straight to the horse's mouth. From the B&W Steam book ed 41 ch 25-2.

"For drum boilers with superheaters, The Babcock & Wilcox Company (B&W) follows the Code procedure in which the safety valves are set such that the superheater valve(s) lift first at all loads. This maintains a flow of steam through the superheater(s) to provide a measure of overheat protection. This method permits the piping and valves downstream of the superheater to be designed for a lower pressure than other methods. This method is required on hand controlled units, stoker or other fuel-bed-fired units, and brick
set units. Another method may be used for all other types of boilers that permits the drum safety valves to lift first. This method could allow a reduced flow condition to occur in the superheater while the boiler is still at a high heat input level. As a result, some superheater materials can exceed temperature limits."

 

Thank You very much sir, may I know where do you work.
I am working in 2x600 MW super critical Thermal Power Plant at Essar Power in India.

Regards

 

There has to be an opinion or two on this forum

 

If drum safety lifts before line safety, there will not be sufficient steam inside super heater so that starvation may occur due to hot flue gas outside the SH tubes.