Pressure relief according API 6D

[Freddyf]

We are working on a project with fully welded ANSI 600# trunnion mounted ball valves that have double piston effect seats. These valves will be used for gas service. The endcustomer is stipulating that a safety relief valve for the body cavity relief is not permitted. For us, nothing strange about this as we have supplied already these kind of valves in the past.

The manufacturer of the valves is telling us now that they will not accept to supply the valves without the safety relief valve. According them, the valves can not be marked API 6D if the safety relief is missing.

We have even given them a pressure calculation in case the temperature would increase. In this calculation it was clear that the pressure would never exceed the 600# rating.

We have also checked with other suppliers/clients and they all confirm that PSV is only required on liquid and condensate and not for gas.

Does someone have some written confirmation that we can use to help us to make this valve manufacturer change his reason?

Thanks

 

[bcd]

Go find another supplier that knows what they are doing. They will quickly figure out they are worng when the business goes elsewhere. Sounds abrupt, but anyone who thinks cavity pressure build-up due to thermal effects is a problem in gas service hasn't a clue and is not a technically viable supplier.

 

[PeterIgg]

Para 7.8 of API 6D does state that the manufacturer is to determine if fluid can become trapped in the body cavity and, if so, to make provision for pressure relief. If the valve is to be fitted with Double Piston seat arrangement then the pressure relief would have to be via an external device. However 6D does go on to say that for gas service pressure relief shall be provided by arrangement.

I would suggest that the supplier may just be providing 'due diligence' to ensure that the purchaser is aware of the requirements that they are placing on the supplier. If you have stated, in writing, that you - the purchaser - are aware of the implications of the request then that should be the end of the matter.

One final thought: Without a drain port in the body cavity of the valve, when hydro testing the (Double Piston) seats it will be impossible to determine if each seat is sealing indenpendantly or whether there is just one working seat!