IMPORTANCE OF USING METAL SEATED BUTTERFLY VALVE IN HIGHER RATINGS

Metal seated butterfly valve are recommended to use in higher pressure rating like #300 and more rather than PTFE seated valve  .PTFE seated valves are designed for using in chemical industry and for low pressure rating and non critical applications and they are designed for class IV leakage only i..e permissible leakage as per API 598 were as metal seated valves are designed and destined to use for higher temperature ,critical application like – hydrocarbon, Sulphur fumes, steam, gases – nitrogen, oxygen, argon etc and they give stringent class VI leakage and highly preferred or can say are only recommend to use for refineries, petrochemical ,terminals , LNG,LPG etc.

As  described and explained in previous mail also PTFE seat valves are meant for low pressure rating and non-critical applications mainly for chemical , water sector  as they have leakage class IV and this amount of leakage is permitted in these non-critical sector and loss is also economical whereas metal seated valves are used for critical application and higher ratings from #150 and above were leakage rate is nonnegotiable and require only zero leakage to avoid expensive leakage loss and reaction of hazardous medium with the environment.

 So, as per our above discussion we request you to only use metal seated valves only for the Oil & Gas projects and this is not recommendation it is a requirement.

HYDRO TESTING OF VALVE AS PER API 598

   HYDRO TESTING OF VALVE AS PER API 598 

API 598 states that the hydro test pressure maximum is determined using yield strength. 

1. For body shall test pressure shall be 1.5 times the pressure rating . 

2. Where as for Disc seat Hydro test will be 1.1 times of the pressure rating. 

In Oil & Gas sector valves need to be tested as per API 598 standard only to be suitable for the stringent requirement in refinery sector. 

Testing valves at different pressure rating as per API 598 is as follows:-

 

Rating	Design pressure as per rating	Shell test pressure	Seat Test Pressure
PN10	10 bar	15 bar	11 bar
PN16	16 bar	24 bar	17.6 bar
#150	21 bar	32 bar	23 bar
#300	52 bar	78 bar	57.2 bar
#600	104 bar	156 bar	114.4 bar
#900	150 bar	22 5 bar	165 bar

 

• Shall test pressure on Body
• Seat test pressure on Disc 

As per the above table is was understood that as the rating increase design pressure also increase hence the shall test pressure and it is to be noted that soft seat will not sustain and will break away in the high pressure and are successful till pressure rating of #150 class only. As at #300 pressure rating i.e 52 bar design pressure  were the shall testing is required to be done at 78 bar which is itself a very high pressure, soft seating even PTFE will break up so , it was found that vendors to pass the valve, do shall testing at 1.1 times (i.e. 58 bar) on shall test in place of 1.5 times  (78 bar). Which is a very wrong practice and could lead to catastrophic accident in refineries and petrochemicals as due to sudden surge or break up valve disc seat will break up as soft seat is not designed to sustain and perform at such a high pressure , they are suitable only for normal pressure and medium like water mostly.

We are attaching the API 598 Table 2 & 3 illustrating the shall test and seat test pressure mark ups on the design pressure for your kind pursual and reference .

 We hope our above given justification will fulfill your requirement and will able you to take a right decision in betterment of project.