Carbon steel tank exposed to above 775F

[91RepairGuy]

We have three carbon steel steam drains tanks at our combined cycle power plant. High energy drain lines are connected to the tank including main steam and hot reheat drains. The drains are attemperated before entering the tank. The tank is atmosperic and is designed for 800F at 50 psi. We have experienced many problems with the tanks including thru-wall cracking and structural unsoundness. The plant became operational in 2003.

Just recently, we obtained historical data which indicated the tank design temperature had been exceeded for several years. The tanks are only used during start-up and shutting down the units. The tanks seem extremly volatile when there is an issue preventing a normal start-up and the high energy drains continue to be directed to the tank for 2-3 hours. It was also noted that the condensate pumps were shut off to early (condensate supplies attemperation to the drains before being directed to the tank).

I'm looking for advice if it is safe to continue using the steam drains tanks. With the main steam at 1006F and hot reheat at 1020F, what type of testing is required to determine the fitness-for-duty of the tank? The header connecting to the carbon steel tank is p11 material. Do you think the tank should have been manufactured with alloy material?

I'm particularly concerned with the dependency of the condensate attemperation to protect the tank from exceeding design temperature.

Is there a reference Code that addresses the design temperature of the tank being exceeded? Your advice is greatly appreciated.

Regards

 

[rmw]

First off, what is the temperature of the individual drain steam streams when it reaches the tank(s)? Since you didn't give the operating pressures, I can't look that up for myself in the steam tables.

rmw

 

[rmw]

Another thought hit me after I sent the previous post. Tanks, vessels and equipment (think condensers) subjected to this type of service are usually pretty beat up in the process.

I don't understand your use of the word "volatile" but if it means what I think it means I would use the word 'violent' to describe what parts expsed to these applications undergo.

There is a tremendous amount of energy being expended in this service, and equipment involved usually suffers in the process. I've seen some mighty strong looking metal parts bent like pretzels by these drain streams.

We still need to know the actual temperatures that the tank has been exposed to in order to be able to offer an opinion regarding your question.

rmw

 

[91RepairGuy]

The operating condition of the steam drains tank was predicted to be 220F @ 5 psi. The main steam drip leg drain operates at 1004F @ 1690 psi at the drip leg. The hot reheat steam operates at 1020F at the drip leg. The high energy lines are combined in a single 8" S80 P11 header and attemperated prior to entering the tank. The predicted temperature of the header connected to the tank is 5psi @ 220F. However, as mentioned previously the tank has seen temperatures up to 875F. There is currently no pressure indication available at the tank. Hope this info helps in evaluating the condition of the tank.

Thanks

 

[metengr]

For starters, what I would recommend is the following;

1. I would have some TC's placed on the tank to evaluate actual metal temperature. By the time the steam reaches this tank you might be surprised as to the actual metal temperature.

2. During a scheduled outage I would have internal visual and NDT inspections performed of the tank girth, seam and nozzle attachment welds using wet fluorescent MT. In addition, I would have thickness testing performed. These tests would provide enough information to evaluate the condition of the tank.

3. Based on the results of 1) and 2) above, you can decide on an action plan going forward. Like adding attemperation to reduce steam temeprature to the tank or internal baffles, etc.

 

[Guest102023]

I don't have any experience with this equipment, but was the cracking mechanism investigated?