Cavity pressure relief acc API 6D

[Radasin]

Is it acceptable to produce the ball valve for gas with relief hole (pressure equalizing hole) in the upstream side of the valve ball and on the top of ball in order to avoid possibility of fluid trapping in open and close position?

 

[Danlap]

Acceptable, Yes with several additional remarks:
[ul]
[li]The process Operating temperature range somehow covers that medium might be in fluid (below condensation point) and gas (above vapor point). Trapped liquid inside the cavity when "heated" up above vapor will result expanding volume which considered as a risk for several critical medium[/li]
[li]What is Ball valve type (Trunnion or Floating?) and what is the normal Differential Pressure. Do expect for Floating and a very low differential Pressure (below 2 Psi), medium may flow from HP side through the hole and then cavity and then pass to the LP side (in other words its constantly leaking).[/li]
[li]There should be HP remarks (the side with the hole when valve fully closed), and Operation / anybody who installed the valve should aware of this.[/li]
[li]There are some minor cases, manufacturer things that by adding cavity relief will compensate wall thickness (since there are NO concern of expanding volume in the cavity). And some do make the wall thickness thinner (do not respect standards). This is a totally wrong perspective.[/li]
[/ul]

Discuss with the process department whether cavity relief hole is really required for your application. In some cases (unless anticipated a. the delta T between vapor and condensation relatively small, b. it has prevention on the downstream such as double block and bleed system, and others) cavity relief should be avoidable as well.

Regards,
MR

Greenfield and Brownfield have one thing in common; Valve(s) is deemed to "run to fail" earlier shall compared to other equipments