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In a flare debottlenecking project, a new pressure control valve (PCV) continuous releasing gases into flare header will be added. Normally the flare system is almost at atmospheric pressure or to be exact, a positive back pressure due to the fuel gas purging which is rather small and negligible. With the inclusion of new PCV, the continuous flow into the flare header will result an increase in superimposed backpressure of 0.5 bar. The exercise is to assess how far the impact to existing pressure relief system.



There are several design criteria shall be checked such as :

1. Mach No of flare header
2. Noise and Vibration level
3. Suitability of Pressure Relief Valve type (PSV) if conventional type
4. Suitability of Bellow if PSV is balance bellow type
5. PSV set pressure (Conventional type PSV spring setting)
6. Others
   

The intention of this post is discussion on the item (5), PSV set pressure. What is the impact to the PSV set pressure or PSV spring setting ?

One the critical information required is the bonnet vent. Conventional type spring loaded PSV may have bonnet vented to atmosphere or non-vented bonnet (or exact is bonnet vented to flare header). The following images show both type of conventional spring loaded PSV.
Image (a) is non-vented bonnet (or bonnet is vented to flare header)
For non-vented bonnet type,


Increase in backpressure will increase PSV opening pressure (higher than set pressure). Set pressure shall be reduced by adjusting spring setting (reduce Fs).

Image (b) is bonnet vented to atmosphere.
For bonnet vented to atmosphere type,


Increase in backpressure will decrease PSV opening pressure (lower than set pressure). Set pressure shall be raised by adjusting spring setting (increase Fs).

Another important point shall be noted is the built-up back pressure (when PSV is opened and relieved) will increase due the the increase in superimposed backpressure caused by PCV. One shall check the built-up back pressure is lower than 10% of set pressure. The post in "Several Impact of Backpressure on Conventional PSV" has discussed in detail the impact of back pressure on conventional type PSV.

Let check out the case studies
Back Pressure Affect Conventional PSV Set Pressure : Case Study #1 - Bonnect Vent to ATM
Back Pressure Affect Conventional PSV Set Pressure : Case Study #2 - Non-Bonnect Vent

Updated on July 19, 2008

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