Boiler Blowdown Vessel Design 1

[Yoth]

Hi all,

I was given a task to re-design a boiler blowdown vessel. The information that I was given with regards to the inlet condition is very limited.

I checked the standards on this iem and found out that the vessel must not hold more than 35 KPa of pressure.

I could be lazy and draw an exact replica of the existing blowdown vessel but that would be unprofeshional.

I guess my main concern is to make sure that my vessl will be able to operate at below 35 KPa. What information & calculations do I need to compute to ensure that my vessel falls within the regulation?

 

[johnnymist2003]

Hi,

I'd suggest you look at ASME Section VIII, Div.1. This is an internationally recognised code for the design and manufacture of pressure vessels. Although your vessel has a very low operating pressure, this code may be used.
John

 

[davefitz]

There was a blowdown vessel design code used in the USA until 1992- it was published by the national board of boiler inspectors. It is very conservative and might not be appropropriate for blowdowns from high pressure boilers. I recall that the vent stack was sized to ensure the max operating pressure would not exceed 10 psig, and most large utilities add a tank corrosion allowance plus specify a minimum tank design pressure of at least 50 psig. ( Tanks of design pressure above 15 psig can be stamped by ASME section VIII div 1). This older code assumed that if multiple boilers or drums were connected to it, the calculation of max casualty flow to the tank was based on the assumption that only one boiler would blowdown at one time.

Some tank vendors ( e.g. Keystone tank)still use this older code and will size the tank for you if you specify some basic information of the boiler.

Some issues that are not well covered by the older code:

a) for 3 pressure level HRSG's, the assumption that only 1 drum blows down at a time is not valid.

b) For HRSG's, with less than 10 ft gravity head from bottom headers to blowdwon tank NWL, it is possible to cause backflow into the reheater lower headers due to the reheater at vaccuum during cold startup and tank pressure at 10 psig.

c) for high presure units, you need to specifically calculate the max casualty flow through the HP superheater drains to the tank and ensure the tank wil not be overpressured and increase vent diameter accordingly.

 

[pablo02]

Be sure to consider your vent stack loads and supports along with the tank design...

We replaced the blowdown tank and vent stack on our old boilers (the original BD tank and vent stack were inside the main building -- 1950's vintage; we moved them outside) -- the welders got ahead of us and started sticking the stack up -- I had them stop and hold off until we finished the stack support design -- the vent stack was to be supported independently from the tank and to allow for pipe expansion (hung with spring supports)-- the welders just assumed the stack would be supported by the tank and a couple fixed supports along the way...

 

[PeteBennett]

Are you designing in Europe, or the USA? If its Europe then there will be PED issues, I have just done it for my company here in the UK

 

[Yoth]

I'm designing it in Australia PeteBennett.

and this is what I think I'm going to do. If I can get a boiler detail I'll ring the manufacturer and see if they can give me some detail with regards to the blowdown condition.

If the manufacturer can't help me out I'll just measure the temperature at a few different point on the existing vessel eg. at the inlet pipe, a few reading s on the actal vessel and the outlet pipe. Then the steam chart should be able to tell me the coresponding pressure at those points. By doing so I should know straight away if I need to increase the size of the vessel or not.

If any one have a better method of sizing up a pressure vessel please let me know.

PS. Don't warry Pablo2 I got the vent stack under control.

 

[bogu]

Mucielago

There is an old victorian workcover document on blowdown tank sizing and calculations etc. I can send you a copy if you wish.

Bogu.

 

[Yoth]

bogu,

It would be great if you can forward the Workcover document to me. I have been looking up current workcover site but I cant find anything helpful. My e-mail address is yothd@yahoo.com.au.

Thanks

 

[nans]

bogu,
Hi
Please send the document to nsurya@stantec.com