This is a quick steady-state approach that closely follows the procedure described previously.Įstimate the fire heat according to the API 521 as (in USC units): Initially, we employed the PSV sizing tool in the “Safety Analysis” section of Aspen HYSYS. We used Aspen HYSYS V10 to assist in our calculation. The vessel is considered to be filled at 70% capacity for this scenario. As per API 521, the fire case scenario considers an allowable overpressure of 2360.7 kPag (21% overpressure). The PSV will have a set pressure of 1951 kPag. A mixture of gas, water, and oil (38º API) enter the vessel that operates at 8✬ and 278.5 kPag. A PSV must be installed for a horizontal vessel of known dimensions. If that is the case, why do some people recommend the use of transient wetted area estimation?Ī single vessel PSV sizing is considered here. Then it is reasonable to assume that the maximum area is the initial wetted area, thus the calculation always tends to overestimate the fire heat. During a fire scenario the fluid vaporizes, so the wetted area decreases over time. Assume there is a maximum heat transfer area: The area available to transfer heat to the fluid is clearly described by API 521 (wetted area).This is in fact a common case in many smaller processing facilities. Size for the “worse” case fire scenario: assume no insulation, drainage, or firefighting equipment available.
PSV SIZING CALCULATOR SIMULATOR
Should, for example, a minimum heat of vaporization be used? As the sizing procedure is often conducted with the aid of a commercial simulator, so, why not use tools in the simulator to make a more precise estimation of λ v? This question becomes more pronounced as the mixture in question has a wide boiling point range. In a multi-component mixture, the latent heat of vaporization changes as the liquid is vaporized and the composition of the liquid in the vessel changes. What latent heat of vaporization should be used to estimate the relief load? This question haunts process engineers.A reasonable question to ask is: how can we perform the calculation in a way that the risk of PSV oversizing is minimized? Around this question there are a few practices and misconceptions, for example: This estimate must consider a higher than usual fire heat flow and a minimum latent heat of vaporization. A conservative estimate of the orifice size must consider an adequate estimate of the mass flowrate at relief conditions, but still not too high that oversize the PSV. Where Q̇ fireis the fire heat flow in kJ/h (Btu/lb), λ v is the latent heat of vaporization in kJ/kg (Btu/lb), and ṁ relief Is the relieving fluid flow rate in kg/h (lb/h). In simple terms, the flowrate to be relieved is determined as, Follow the procedure described in API 520 to obtain the orifice size.This task is normally conducted using a commercial simulator. In particular, the latent vaporization heat (λ v) is required for sizing. Calculate the thermophysical properties of the fluid at relief temperature and pressure.Both approaches depend on the vessel wetted area, environmental parameters, heat transfer coefficients, etc. Estimate the fire heat duty using API 521 correlations or a more detailed heat transfer model.In principle PSV sizing should be straightforward: But if industry standards like API 521 exist and clearly state the steps to be followed, why is there so much misinformation out there? This communication aims to clarify a few points of clear concern relating to PSV sizing. Many myths and misconceptions about this task have appeared over the years, as evidenced by a simple search on the internet and see hundreds of forums available on this topic. For many in our field, Pressure Safety Valve (PSV) sizing is considered to be a relatively simple task that can be performed in a matter of minutes by process engineers with enough experience.