Effect of Body Bowl Choking on Pressure Relief Valve Stability

It is known that the velocity at the outlet flange of conventional PRV (Pressure Relief Valve) reaches the sonic velocity in case of larger size of PRV and higher set pressure. This phenomenon is called as body bowl choking and the pressure in the PRV body rises irrespective to the back pressure at the PRV outlet piping. This pressure rise in the PRV body could result in insufficient opening and/or unstable motion of the PRV. However there is no design guide to prevent this adverse effect of the body bowl choking for the conventional PRV. The body bowl choking would occur relating to the PRV outlet flange size which is defined by API standard 526 and several studies pointed out the size problem defined by this standard. API standard 520 Part I mentioned that the built-up backpressure should not exceed 10 % of the set pressure at 10 % allowable overpressure in conventional PRV application to prevent variations in opening pressure, reduction in flow capacity, instability, or a combination of all three. However, there is no description on the effect of the body bowl choking in it. This paper presents the investigation results of the body bowl choking effect on the conventional PRV stability. The pressure rise in the PRV body is calculated by a simplified pressure calculation formula of the one dimensional gas flow and the pressure rise ratio in the PRV body to the set pressure is investigated relating to the PRV stability from the comparison with the experimental data. As a result the pressure rise in the PRV body should be less than 15% of the set pressure to prevent unstable motion of the PRV. Using this criterion of the pressure rise in the PRV body, the adequacy of the PRV outlet flange sizes defined by API standard 526 is investigated and recommended changes in PRV outlet flange sizes are introduced. Instead of the changes in the PRV outlet flange sizes the limited lift of the conventional PRV is also effective to prevent the PRV instability. From this point of view the critical nozzle area ratio to the outlet flange area is also obtained as a function of the set pressure of PRV.