The phenomenon of bonnet-overpressurization occurs when liquid is trapped in the bonnet cavity of a (cold) gate valve in the closed position, and then the valve heats up and the trapped liquid vaporizes to a pressure greater than the valve is designed for.  This is dangerous.

In my experience valve bonnets are designed for the same pressure as the rest of the valve.  For example a standard class 900 WCB gate valve is rated to 2220 psig at 100 deg F; the same figures apply to the entire valve or just the bonnet. The valve is probably tested at 1.5 times that pressure and there is additional saftey factor inherent in the rating, but don't rely on that for protection against over pressurization--to do so would be poor practice.

There are several options to prevent bonnet overpressurization:

*drain the liquid
*use a pressure relief valve on the bonnet
*install an external bypass from the bonnet to the downstream side
*use an "internal byapss" (hole through the disc or seat)

The last two options make the gate valve into a one-way valve.

I hope this helps.

Jonathan Nieuwsma

Hi, JNieuwsma: do you know which code will apply to the strength design of a bolted bonnet, looks like a flind flange with a centre stem hole?

ASME Code Section VIII Appendix 2 covers strength of bolted flanges.

JN

Hi,

Refer ANSI-B-16.34 in the Material vs Temp.ratings for a certain pressure class.

You have to also see the relevant standard in API.

Cheers!