Fire Testing of Wedge Gate Valves

[BVMan]

We have recently carried out a Fire Test on Behalf of a Client on a 'Stock' 2" ANSI 150RF Wedge Gate Valve. Around half way through the burn period there was a 'Pop', the Stem Thermocouple dropped slightly and there was a slight 'Roar' from the fire. The Pressure remained constant and temperatures recovered, without dipping below the required minimum, allowing the full burn to be completed. However, after cooling there was a significant leakage from the bonnet joint which resulted in the valve failing the test.

On post test examination the 304/Graphite Oval Spiral Wound Bonnet Gasket (Flat Faced - Non encapsulated) was found to have failed and the bonnet bolting (B7M/2HM) required only low torque to unfasten.

It seems that the Seating functioned quite well but the failure was due to trapped water in the Bonnet area vaporising. We had not fitted a Relief Valve/Gauge to the bonnet so cannot confirm that this was the case.

My issue is that even if out assumption is correct and a Relief Device were fitted and opens during the test this would result in the test being aborted. Meaning that if the valve seats function correctly during the burn the fluid trapped in the bonnet will expand resulting in either failure of the Packing or Gasket, or where fitted the Relief device will operate, either way the Valve Fails.

Obviously a Cavity Relief Hole in the Wedge is one option but this makes the Valve uni-directional and there is no evidence of other manufacturers going down this route. I have seen test reports stating that the Valve Seats were completely destroyed but the drawings state that the seats tested were A105 + Hardfacing.

Does anyone have any experience, thoughts or comments?

 

[bcd]

Metal seated gate valves are not in the scope of API-607. But, this is why the equivalent ISO standard (ISO 10497)includes it in the scope. It is a struggle to get the bonnet joints on gate valves to pass the test. You need to add the cavity relief valve or monitor the cavity pressure, otherwise the test is not valid. Typically, the bonnet bolts are heating up faster than the surrounding parts. They lose strength, stretch, and allow cavity pressure to blow by the gasket. An enclosed gasket with metal-to-metal contact outside the seal works better in a fire test. Bolts can be tightened more and are more isolated from the heat by the flange.