Welcome to Process Safety Relief. A quick reference for Process Safety Management and releated items.
This is an open source project. If you would like to help out, merge requests are welcome on gitlab
Welcome
Welcome to Process Safety Relief. A quick reference for Process Safety Management and releated items.
This is an open source project. If you would like to help out, merge requests are welcome on gitlab
Nov 22 2019
Scaled Overpressure:
$$p_s = p/p_a $$
| $p$ | Overpressure (Pa) |
| $p_a$ | Atmospheric Pressure (Pa) |
Sachs Scaled Distance:
$$\bar{R} = R /(h_{combustion} \cdot V/p_a)^{1/3} $$
| $R$ | Radius ($m$) |
| $h_{combustion}$ | Heat of combustion of stoichiometric hydrocarbon-air mixture ($J/m^3$) |
| $V$ | volume of congested area (or in the case of DDT entire flammable cloud volume) ($m^3$) |
This tool does the above calculation to determine overpressure based on radius and chosen TNO curve
Find the R value on the x axis and find the corresponding $p_s$ value on your chosen TNO curve, and input here.
References
- TNO Yellow Book … etc.
Nov 04 2019
Concentration thresholds for oxygen are somewhat scarce. Here is a short list.
For personel safety in an enclosed space, US OSHA does not allow employees to enter enclosed spaces with greater than 22% oxygen.
For potential fatality, probits are generally used, however the National Insitute of Public Health and the Environment (the Netherlands) states that “An effective probit relationship cannot be worked out for oxygen” and instead suggests these concentrations for modeling potential fatalities direclty caused by oxygen exposure.
| Probability of Fatality | Oxygen Concentration |
|---|---|
| $P_{lethal} = 0.1$ | greater than 40 vol% |
| $P_{lethal} = 0.01$ | between 30 and 40 vol% |
| $P_{lethal} = 0$ | between 20 and 30 vol% |
These percentages are including the oxygen already in air, however, most modeling softwares do not take into account that air is 20% oxygen, so some calculations will need to be done to properly model these concentrations.
These equations will help you calculate the concentration of oxygen you need to model to.
Adding a percentage of pure oxygen ($\alpha$) would push out some of the atmosphere leaving a percentage behind ($\beta$).
Balancing this equation on Oxygen only, $x$ is the total oxygen in the volume (concentration of interest):
$$ x = \alpha + \beta \cdot 0.209$$
$\alpha$ is the percentage of added oxygen and thus the concentration to be modeled. Using $\alpha + \beta = 1$ :
$$\alpha = ( x - 0.209)/(1-0.209)$$
This tool does the above calculation to determine what percentage needs to be tracked in the modeling software
References
- Confined and Enclosed Spaces and Other Dangerous Atmospheres in Shipyard Employment, 29 CFR, § 1915.12(a) (1995).
- Reference Manual Bevi Risk Assessments (2009). National Institute of Public Health and the Environment (RIVM). Bilthoven, the Netherlands
Sep 17 2019
Scaled Overpressure:
$$p_s = p/p_a $$
| $p$ | Overpressure (Pa) |
| $p_a$ | Atmospheric Pressure (Pa) |
Sachs Scaled Distance:
$$\bar{R} = R /(h_{combustion} \cdot V/p_a)^{1/3} $$
| $R$ | Radius ($m$) |
| $h_{combustion}$ | Heat of combustion of stoichiometric hydrocarbon-air mixture ($J/m^3$) |
| $V$ | volume of congested area (or in the case of DDT entire flammable cloud volume) ($m^3$) |
| 2D Flame Expansion | Obstacle Density | |||
|---|---|---|---|---|
| High | Medium | Low | ||
| Reactivity | High | DDT | DDT | 0.59 |
| Medium | 1.6 | 0.66 | 0.47 | |
| Low | 0.66 | 0.47 | 0.079 | |
| 2.5D Flame Expansion | Obstacle Density | |||
|---|---|---|---|---|
| High | Medium | Low | ||
| Reactivity | High | DDT | DDT | 0.47 |
| Medium | 1.0 | 0.55 | 0.29 | |
| Low | 0.50 | 0.35 | 0.053 | |
| 3D Flame Expansion | Obstacle Density | |||
|---|---|---|---|---|
| High | Medium | Low | ||
| Reactivity | High | DDT | DDT | 0.36 |
| Medium | 0.50 | 0.44 | 0.11 | |
| Low | 0.34 | 0.23 | 0.026 | |
References
- Baker, Q.A., M.J. Tang, et al. (1994), Vapor Cloud Explosion Analysis, 28th Loss Prevention Symposium, Atlanta, GA. American Institute of Chemical Engineers.
Sep 11 2019
API 521 has provided a fire radiation model that is often used in flare and other types of fire modeling
$$ q_{absorbent} = \sigma(\alpha_{surface} \cdot \varepsilon_{fire} \cdot T^4_{fire} - \varepsilon_{surface} - T^4_{surface}) + h(T_{gas} - T_{surface}) $$
| $q_{absorbent}$ | absorbed heat flux from the fire ($Wm^{-2}$ or $Btu \text{ } h^{-1}ft^{-2}$) |
| $\sigma$ | Stefan-Blotzmann constant ($5.67\mathrm{e}{\text{-}8}$ $Wm^{-2}K^{-4}$ or $0.1713e{\text{-}8}$ $Btu\text{ }h^{-1}ft^{-2}{^{\circ}R}^{-4}$) |
| $\alpha_{surface}$ | equipment absorptivity (dimensionless) |
| $\varepsilon_{fire}$ | fire emissivity (dimensionless) |
| $\varepsilon_{surface}$ | surface emissivity (dimensionless) |
| $T_{fire}$ | fire temperature ($K$ or ${^{\circ}R}$) |
| $T_{surface}$ | equipment temperature ($K$ or ${^{\circ}R}$) |
| $h$ | convective heat transfer coefficient fo air/fire in contact with the equipment ($Wm^{-2}K^{-1}$ or $Btu\text{ }h^{-1}ft^{-2}{^{\circ}R}^{-1}$) |
| $T_{gas}$ | temperature of air/fire in contact with equipment surface ($K$ or ${^{\circ}R}$) |
References
- API STD 521 (2014), Pressure Relieving and Depressuring Systems, 6th Edition, American Petroleum Institute.
Sep 05 2019
The following is a list of topics that this site will attempt to cover. This may become a table of contents