Pressure Safety Valve Sizing for Fire Case Validation by Dynamic Modelling
Authors: Kazankapov N. , Mukhanzhanova R. 
Published: September 2024

Objectives/Scope
Pressure Safety Valve sizing methodology is well established and described in the widely recognized international standards and recommended practices such as API 521 and API 520 Part I. However, PSV sizing Fire Case approach is not considered as common and varies based on individual Company standards and guidelines, particularly the calculation of latent heat of vaporization that dictates the mass rate of reliving fluid, hence, the PSV orifice size.
This paper presents a comparative analysis of three widely used steady state latent heat of vaporization calculation methodology against dynamic modeling approach that allows to take into consideration the fluid physical properties change as the external heat is applied while excess pressure is released over time.

Methods, Procedures, Process
In general PSV sizing for Fire Case (liquid vaporization) consists of the following main steps:
Calculate external heat duty using API 521. This will require estimation of vessel wetted area and environmental parameters. Calculate the thermo-physical properties of the fluid at relieving conditions. In particular, the latent heat of vaporization is required to calculate relieving fluid peak mass rate. Apply the calculation methodology described in API 520 Part I to estimate the required orifice size and select a standard size orifice described in API 526.
The Step II, is explored in detail in this paper using different methodologies including transient conditions output. Specifically, latent heat of vaporization was calculated by commonly used multiple flash calculations as well as by a dynamic simulation of external heat input to a pressure vessel leading to temperature increase and liquid expansion in the vessel. Also, liquid composition was varied to study if difference in obtained results remains constant as the molecular weight of the liquid increases.

Results, Observations, Conclusions
Numerous cases were developed to assess the differences in obtained results of latent heat of vaporization estimated by a steady state multi-stage flash calculations and a dynamic modeling of direct heat input. It was observed that deviation between both methodologies vary significantly and results in different selected orifice size. A direct correlation of molecular weight increase leading to increase in calculation results differences was not observed. The dynamic simulation results in lower required PSV orifice size mainly due to reduction of liquid level hence the vessel wetted area and heat absorption, while steady state calculations assumes the constant heat input with the reference to initial liquid level. Nonetheless, it was observed if liquid contains aromatics and polar components the dynamic simulations results indicate slightly higher calculated PSV orifice area.

Novel/Additive Information
It is imperative to emphasize the criticality of accurate PSV sizing for both plant and personnel safety. Additionally, recognizing the importance of avoiding unnecessary design contingencies and oversizing is essential to mitigate high capital costs during the design phase of a hydrocarbon processing facility. The findings presented in this paper contribute to more accurate PSV sizing practices, aiming to enhance safety, reliability, and cost-effectiveness in the design and operation.