Continuous Blow Water Pipe Break at joints while passing through MW ta 1

[mufasa1]

In our boiler plant the temperature of continuous blow water drained by steam drum is near 160 C degrees. We decided to pass this continuous blow water through the Make-up Water Tank via 4" SS pipe. Our objective was to cool down the continuous blow water and send it back to Demi Unit, besides we warmed the Make-up Water too.

Volume of Make-up Water Tank is 500 m3 and temperature of water in the tank is 20 C degrees. The 4" pipe makes many u-turns at the bottom of the tank and continuous blow water exits the tank at 90 C temperature. At turns 90 degree elbows were used and joints are welded joints.

The problem is every 2-3 months the welding joints at the elbows near u-turns comes loose and blow water mixes the make-up water then too much work. We think it is due to vibration while hot continuous blow water travels through the pipe. At some points we used u-bolts to control the vibrations but the piping still breaks at some joints.

Do you have any suggestions?

Regards

 

[metengr]

If I understand your problem, the stainless steel piping in the make-up tank that is being used to cool the blow down steam is failing at the 90 degree elbows. What is the stainless pipe material?

The failures along the welded connections sounds to me like stress corrosion cracking, if immersed in relatively pure make-up water (DI).

To confirm the damage mechanism, you should remove one of the failed welded connections and have it analyzed by a reputable metallurgical/materials lab.

 

[chicopee]

Analyze reaction forces at the 90 deg elbows from flowing boiler water and determine if welded brackets are not overstresses. You have to determine velocity of boiler water to determine reaction forces.

 

[mufasa1]

Thanks alot for the comments
The piping is laid out at the bottom of the tank like heating coil with several 180 returns.
The pipe material is A312 TP304, Sch10S
Elbow material is A403 WP304, Sch10S

Is it possible that the vibration and breaks might be due to water hammer?
Does increasing the pipe schedule help to reinforce the welding joints?

 

[metengr]

As I stated in my previous post, you need to confirm the failure mechanism! Trying to solve a problem without understanding the root cause is not the correct approach.

 

[chicopee]

Also check on possibility of thermal stresses at joints. This is problematic when units are turned on and off particularly where you have sharp anchored turns.

 

[rmw]

What is the flow rate of the boiler continuous blow down? As you come across the blowdown control valve, you will have some of the boiler water flashing to steam, which gives you two phase flow in this piping.

You can go to

http://www.processassociates.com

and look in their tools section for a place that will help you see what two phase flow regime you are in. I suspect slug flow.

Then, as you are getting any heat transfer, you are condensing some of the steam portion of the two phase flow, but the condensation boundary is always changing due to the nature of the two phase flow, which moves the boundary line, and gives water hammer as the steam re flashes, and/or recondenses.

You can expect some very serious pressure fluctuations in this piping, as well as some strong vibrations.

With slug flow, you can have high velocity slugs of water hitting the return bends, enough to break the welds.

The only solution I can see for the present arrangement is to elevate the inlet end of the serpentine header by some 1/2 meter or so in order to help the condensate in the pipe be able to drain, and not 'pocket' in the serpentine.

I don't really like the idea of this serpentine arrangement in this two phase flow application. It is problematic at the best. But, I do like the idea of the heat recovery.

rmw

 

[mufasa1]

guys,
thanks alot for your help.
The flow rate is at max. 40 tons/hour.