3 replies to this topic

[ChemGeek]

G'day!

I came across an atmospheric tank design and I have a few questions that I hope you all can help clarify. Below are the tank's data;

Design pressure: 20mbarg / -2.5mbarg
Max. Inflow rate: 500cbm/hr
Max. Outflow rate: 500cbm/hr
Tank Dimension: 9.5m(D) x 17m(HT)

Current design is with a 20" Emergency Relief Valve (ERV) set at 18mbarg, and two 8" Pressure Vacuum Relief Valve (PVRV) set at 15mbarg / 2.2mbarg and 13mbarg / 2mbarg respectively.

(1) How were the set point of the ERV and PVRV determined in the first place? According to API 2000 section 4.6.2.2; the set pressure should be lower than design pressure of the tank to allow for adequate flow capacity of the device. It however, did not say how low. Are there any form of calculation available to determine it?

(2) The set point of the PVRV is different. From what I know, during the sizing of PVRV, the designer will usually set aside a safety or redundancy to provide an extra number of PVRV; in this case, 1+1. Will the PVRV pop on normal ocassion during tank filling due to the lower set pressure of one of the PVRV?

Hope to get some feedback on the matter.

Cheers!

[fallah]

ChemGeek,

1- There isn't a general method to adjust the set pressure for safety devices on the storage tanks. You should consider a margin between setpoints to avoid any interaction.

2- Single or multiple PVRV? A limitation forced us to consider multiple is PVRV size. The other reason behind breaking one to multiple is saving against loss of inventory.

Anyway with such low design pressure of your tank seems your setting was not performed properly. If there are overpressure/blowdown in devices operation the probability of interaction seems to be high. you need to recheck the setting.

Fallah

[ChemGeek]

Dear fallah,

I do not quite understand what you mean by single or multiple PVRV. Can you elablorate more?

This is actually a settling tank for the wastewater system where the AOC (water rich liquid) will be pumped to before the slop is skimmed and discharge to the slop tank and the rest will be sent to a tilted plate interceptor.

I've included the drawing for the tank

Attached Files

•  settling tank.xlsx   183.67KB   208 downloads

[fallah]

ChemGeek,

You yourself mentioned: "...and two 8" Pressure Vacuum Relief Valve (PVRV) set at 15mbarg / 2.2mbarg and 13mbarg / 2mbarg respectively." Then i did guesss it might be broken from one PVRV to two ones with nonequal set points due to, let say, size limitation or...

For checking the rightness of safety devices setting you need to know overpressure/blowdown of each device which should already be reported by relevant vendor(s).

Fallah

Edited by fallah, 24 October 2012 - 04:45 AM.