Opening Safety Valves to Avoid Master Fuel Trip

[datsacorn2511]

In my power plant there are some safety valves installed in the Boiler. The SV located on the Drum has the highset setting.We also have solenoid operated safety valve (PCV = Pressure Control Valve) installed in the main steam line. One day, there was an accident causing main steam pressure increased rapidly. This was also followed by rapid decreasing drum level. We were so panic. To avoid Master Fuel Trip caused by drum level very low, we opened the PCV manually. We opened it manually because the main steam pressure was still below PCV setting but the drum level was slightly above the Trip Setting. Finally our unit was saved from Forced Outage. Our Question is:
1. Was our action (opening the PCV) in that accident correct?
2. If our action is incorrect, could you give us another solution in overcoming that accident?
3. Could you explain the calculation of the drum level drop due to the rapid increasing of main steam pressure?
Thanks a lot

 

[hacksaw]

sounds like an effective response,

some boilers (particularly heat recovery boilers) serving a dedicated load like a turbine, have the pcv on automatic (or on back-pressure control), in the event of a turbine trip.

 

[metengr]

1. Was our action (opening the PCV) in that accident correct?
2. If our action is incorrect, could you give us another solution in overcoming that accident?

What was the accident? Why would you need to manually operate this valve to avoid tripping the unit. Your response sounds like it was a hit or miss from your post. Find out the cause of the accident to prevent recurrence.

 

[rmw]

To reduce the pressure (keep the SV's from popping) and to maintain the drum level I would have done exactly what you did if I was operating a unit that had the capability to do what you did. Many a time I have wished for just such a solution and sometimes when I have had it I have used it.

That said, I concur with metengr in wondering what was the incident that precipitated the need for that action.

If your accident was of the nature of the tripping of one of the steam users the action that you took was merely to create another steam user for the steam that the boiler had available and give the controls time to react.

If your accident was a controls malfunction that caused a runaway pressure increase, this solution might have worked on this occasion but I wouldn't plan on using it on a regular basis.

rmw

 

[ApC2Kp]

datsacorn2511,

A possible answer to your question 3, would be discussion in thread 124-173249, Effects of swell and shrink phenomenom. Consider that there are steam bubbles in the riser tubes, and what effect the increase or decrease of pressure will have on the bubbles, followed by drum level changes from the bubble sizes. The response of feedwater flow control may be incorrect, depending on the actual need for feedwater versus steam demand. See comments of rmw about performance of the preferred 3 element control scheme.

A different response to low-low drum level would seem to be manual bypass of feedwater control valve to allow more flow to drum. That could lead to exceeding high level, to point of drum water getting into superheater after pressure returns to normal. Not an easy simple answer or response for every situation.

 

[rmw]

There is more to the equation than just the steam bubbles in the risers and drum. There is also the pressure temperature relationship. At some point the boiler water that has reached the boiling point for a given operating pressure is a saturated fluid and the addition of heat causes the boiling. Now then, when the pressure is raised by some event outside the drum/riser system, principally by the sudden reduction of steam flow out of the drum now the fluid is not saturated for the new (higher) pressure and the boiling ceases until enough heat is added to make it begin again and assuming that the master fuel trip was avoided here, it will soon begin to boil again at the new higher pressure.

If water is added in large quantities via a bypass it does two things. First it is colder by whatever degree (depends on the presence or lack thereof of feedwater heaters, economizers, etc but in any case still colder) than the either saturated water in the drum or the almost saturated water due to the new higher pressure in the drum so the introduction of that water quenches boiling and further contributes to the drop in the drum level.

If then, load is suddenly increased by the addition of another user, be it a device connected to the header or the opening of a bleed valve or even a safety valve (manually-discussed elsewhere) then the pressure is reduced and at some point the water becomes saturated again and begins to flash off as the pressure continues to drop. That will make the drum level rise (swell) as the boiling begins and bubbles form. Sometimes just the swell will bring the level to a level where the high or high-high level controls will want to take the boiler out. But then, the added water admitted through the bypass valve now contributes to this new swell induced level making an already bad situation worse.

The problem at the outset is not lack of water in the boiler because at the equilibrium conditions immediately prior to the accident I assume the drum level was within limits. It is the effect of the accident that has changed that and putting water in with a bypass is treating a symptom of the problem, not the problem. Opening the vent valve as proposed by the OP is treating the problem which is the pressure temperature relationship of the water in the drum and riser tubes.

rmw

BTW, apc2kp if you type it as thread124-173249 without the space after the word thread it will create a link to that thread. And thank you because I looked for that thread and didn't find it.

 

[ApC2Kp]

rmw,

Thank you for clearing up the use of thread link. It is a good feature for referring to previous items of the subject and you could probably create a FAQ on 'swell & shrink'. I have learned something today. It is difficult enough to appreciate the boiling of water, and then be able to use the functions of the website is something for a techie!

My piping work does not get involved with the day to day functions of a boiler. However, there are the steam blow procedures to clean the piping for commissioning, and then demands on the boiler are likely to over-extend the capacity of the steam drum. Usually the level control is put into manual after level is at 'high', operators closely monitor the drum, and the steam blow is stopped at low water level. It is a different situation for controls than a plant trip or upset.

 

[datsacorn2511]

So, it's not a right solution/action if we opening feedwater control valve bypass during the rapid increasing of main steam pressure?

 

[ApC2Kp]

datsacorn1,

Like many things, it depends on the situation and other conditions. If the upset could be for some time, then the added feedwater might be useful on a one-time basis. The usual level control would probably need to be reduced to keep feedwater from reaching full high level - a maximum of half or one-third of drum would allow volume for 'swell' after pressure returns to the normal (lower) value. For upset of very extended time the fuel fire rate would be expected to be reduced to minimum.

Not sure of your controls setup for steam flow, level, pressure, and temperature - plus how the settings for controls feedback proportional/ integral/ dervative PID could interface to an algorithm for response to upset condition like you saw.

There are only two units I know that were equipped with a dump condenser to take all the steam in the event of a turbine trip. It could be an option for frequent upsets, if there is the space and cooling water capacity. The more common approach might be to take the steam to the turbine's exhaust condenser - if so equipped. There would be letdown valve and some large piping.

 

[datsacorn2511]

Our next question:

Will Our action (opening Solenoid operated SV manually below its set pressure) have some effect that could bring possibility of damaging the Safety Valve Seat?