Conventional spring loaded safety vs Pnuematic assisted safety valve (Controlled safety valve)

[Raai2015]

As subject, most of us knows , conventional springloaded safety valve has checknut in the bonnet which will keep the spring during operation , when it reaches the set pressure , spring will be compressed against the checknut by steam and releive it.
In the Pneumatic, air pressure will act on the diaphragm on both side, when it reaches the set pressure , lift air pressure is activated and relieves the pressure. Queries are,
1. How the construction of spring on the pneumatic operation , specifically cecknut portion? Because i received the offer which says pneumatic supplementary safety valve but it has the check nut.
2. Basic clarification. Pneumatic air is supplied with 5 bar. But the set pressure is 92bar . How does pneumatic air with 5 bar loading and lifting the 92 bar steam?

Thx and reg
Baskar

 

[Danlap]

Hi,

I think this link quite self-explanatory if you read it all

http://www.imi-critical.com/industry-sectors/Documents/IMI_BR_PI_Product_Range_43_EN_Web.pdf

Spring force (F) = kx. For simplification where seating force is spring force. And x is tweaked by means adjusting the check nut, whilst k remain constant. That is the basic concept of conventional type (btw both are conventional type)
1. x of (loaded) spring for different set pressure are different. E.g 92 bar and 91.5 requires different x, that is the reason of the check nut (regardless there is pressure assisted air on top of it).
2. If I see the brochure, I would say the 5 bar is acting as either signal (triggered by the over-pressure in the main line) and/or pressure assisting.

Therefore, your safety valve is let say set at 92 bar. When it is about to reach 92 bar, this pressure provide "signal" to the "junction box" to assist the blow-off. Thus 5 bar air goes via the red arrow. Thus 92+5 bar push the spring upwards.
And after pressure is below the set pressure, the junction box switched the air line through the blue arrow. To provide tighter seating force (extra 5 bar is relatively harmless for the Seat and/or plug), therfore seating force is the Spring force + 5 bar.

Maybe others have better explanation.

Kind regards,
MR

https://nosuchvalve.com

All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

http://www.eng-tips.com/faqs.cfm?&rat1=2&fid=376