Design pressure for Blowoff Flash Tank

[MechMad1]

Hi!
I've got to define the design pressure of a vessel which receives the intermitent blowdown from a boiler. It is an atmospheric tank, so the normal pressure would be aprox. 1,2 bara (1 + pressure losses in the vent).

I would like to know if there is a code or just good practice that states which design pressure should have the vessel.

If there is not, I will just apply a safety margen to the operation presure and set the design pressure to 1,7 ~ 2 bara.

Thank you!

 

[UtilityLouie]

Suggestions:

- Calculate the flash steam amount in the worst case
- Calculate the back pressure on the tank when that worst case flow is going through the vent to atmosphere.
- Determine what you tank design temperature is. You'll have high pressure blow down condensate, there will some of that temperature making it to your tank. Use that.
- Look at flange temperature and pressure ratings.
- Pick a pressure that matches up with 150# or 300# flanges and that temperature.

When a tank is designed and ASME stamped with a MAWP and design temperature, it's very difficult to change that and the operating cases in the future most times are hard to predict. I always try to design any tank with the highest MAWP to allow process flexibility at a later date.

 

[TD2K]

I've seen a lot of in-house pressure vessel specs that mandate a minimum design pressure of 50 psig or about 3 barg. Some have gone higher, one set minimum design pressure at 150 psig or about 10 barg. Design temperature is typically set at a minimum of 500F because for carbon steel vessels, that design temperature is essentially free under ASME Section 8, Div 1 except for flanges (can't comment on other PV codes). For flanges, I'm seeing more minimum requirements for class 300 flanges, manways are sometimes exempted from this.

I'd likely go with a minimum of 3 to 5 barg. Take a look at the change in wall thickness for the two design pressures, I doubt it will be much. The higher design pressure just avoids issues down the line. Of course, if your existing blowdown drums are all designed for 2 barg, that might be a reasonable basis for consistency as long as you think the hydraulics and backpressures fit with a 2 barg design pressure.

 

[MJCronin]

I agree with TD2K......there exists a minimum pressure for these tanks that will ensure a reasonable and robust ASME VIII design.

In my experience the commonly used design pressure (MAWP) was always 50 psig or greater.

Suggest that you consider purchase of an already designed vessel (or custom designed to your requirements) from these fine people:

http://www.pennseparator.com

On a related topic.......

Additionally, for blowdown service, you must ensure that the tank CANNOT BE ISOLATED FROM THE ATMOSPHERE, otherwise it will see the full boiler pressure and fail in a very short time.

There have been accidents where the field or maintenance personnel have installed a shutoff valve on the vent and the blowdown tank eventually overpressureized failed

Please keep us informed about your final design.......

 

[davefitz]

50 psig is the typical design pressure I have seen inthe field. The larger issue is that the supply of the tank is not from the same supplier as the vent stack + muffler- so an undersized vent may occur in cases where the vent must be routed above the adjacent plant , perhaps 100 ft long.The result is an overpressured tank.

 

[MechMad1]

Thanks to all of you, I think you've provided some good points.
The setup of the design values for the tank and the design of the vent is in my scope of supply, so I will consider a conservative routing for the vent (so in the worst case scenario, there won't be an overpressure in the tank).
The 50 psig design pressure makes a lot of sense for me, since I too have seen many atmospheric tanks with a design pressure of 3.5 barg.

OK, I agree that 50 psig is a typical design pressure for these tanks, but do we know where it comes from!? ASME VIII div.1!? Expertise!? Combination of both!?

 

[TD2K]

If you take the minimum plate thickness you would want to construct a vessel out of (1/4"?) less your CA (1/8" is a typical value for carbon steel), the material is typically good to at least 50 psig. For larger vessel's diameters, this may not be the case. High design temperatures could be another reason that "50 psig" means thicker shell material because of the material's reduced strength at the higher temperature.

 

[MechMad1]

Well, I think I've found where this design pressure value comes from.
According to API 521 (pressure relieving and depressuring systems), for knockout drums, "a minimum design gauge pressure of 345 kPa (50 psi) is suggested for KO drums in subsonic flare or other low-pressure applications." (7.3.2.4.)