Our White Papers

This paper provides a critical review of the second edition of ISO 4126-10, an international standard titled “Safety devices for protection against excessive pressure”. Part 10 of ISO 4126 focuses on “Sizing of safety valves and bursting discs for gas/liquid twophase flow”.

The process industries are primarily concerned with the reliability, availability, auditability, and maintainability of relief and flare systems data. These data are critical component of process safety information and its lifecycle must be properly managed to ensure sound process safety management and loss prevention programs.

Effective design of emergency relief systems requires accurate modeling. In particular, the PVT relation of such systems is fundamental and unique. This relation must be accurately represented during direct scale-up or computerized simulation. Variables which can significantly alter the PVT behavior of a system should be quantified, and included in the design.

Challenges associated with PRV stability issues for existing installations are not unique to any particular segment of the chemical process industry. This is an industry wide problem that has received a lot of attention from both OSHA and industry associations such as API, ACC, and AFPM.

In this paper we provide a simplified model for the assessment of PRV stability where the inlet line geometry is simple and/or where the inlet line acoustic length can be established. This simplified model has also been proposed in the 3rd ballot of API-520 part II.

How to avoid the singing PRV problem. Excitation of acoustic standing waves in a main process flow line closed side branch, such as the inlet line of a pressure relief valve (PRV), can occur due to vortex shedding generated by increased flow in the main process line. The flow velocity for process lines where pressure relief devices are mounted via a side branch should be limited to where ce is the effective isentropic speed of sound of the main process flow pipe fluid system, u is the maximum allowable fluid flow velocity in the main process line, and d/L is the pressure relief device inlet line diameter to length ratio. This limit can be very restrictive for flashing two phase flow.

An independent and accurate estimation of the speed of sound can provide an important quality check for a multitude of single and multi-phase flow applications. More recently, proposed screening methods for the calculation of pressure relief valve (PRV) stability require an accurate estimate of the speed of sound for the fluid/piping system. This paper outlines

Get a handle on PRV stability. There is general agreement that the 3% inlet pressure loss rule (IPL3) is not sufficient to guarantee PRV stability and does not work all the time. This is confirmed by recent findings from actual PRV stability measurements and dynamic modeling. IPL3 only considers irrecoverable pressure loss. IPL3 assumes that the fluid dynamic pressure is ultimately recovered at the disk surface as the PRV is closing.

Ever since OSHA implemented their National Emphasis Program in 2007, facility’s pressure relief systems design basis have come under increasing scrutiny. Recognizing that they may not be fully compliant, many companies are conducting audits of their relief systems design basis to determine their current state, identify gaps, and establish a path forward for compliance.

The rapid introduction of a fluid from a high pressure source into a lower pressure receiver can result in rapid heating of the mixed fluid in the receiver. The extent of temperature rise will depend on the initial conditions of the source and receiver fluids including temperature, pressure, composition, phase, and the rate of mass inflow.

Because the potential hazard of pressure relief valve instability (chattering) is already recognized, relief systems design basis documentation must demonstrate expected stable pressure relief valve (PRV) operation and performance for a multitude of credible scenarios. Historically, expected stable pressure relief valve performance has been demonstrated by showing that the irrecoverable inlet line pressure loss is less than or equal to 3% of the pressure relief valve set pressure (3% rule).

This paper presents a general method for the estimation of flammability envelopes for chemical mixtures containing gases, liquids, and/or solids based on chemical equilibrium. The impact of mixture initial temperature, the presence of diluents and elevated system pressures are implicitly accounted for. The performance of this method is tested against much of the experimental data reported in the literature for systems containing a wide range of chemicals from CHNO compounds to compounds containing sulfur, phosphorus, silicon and halogens.

Direct minimization of the Gibbs free energy can be used to calculate phase equilibria, chemical equilibria, and/or simultaneous physical and chemical equilibria. This method may be preferred for systems where multiple liquid phases can coexist and/or where retrograde phase behavior is possible during depressuring or pressure relief.

Traditional pressure relief sizing calculations have typically used steady state equations to determine both the required relief rate and the pressure relief system capacity, for every applicable overpressure scenario. These equations are well accepted and understood and are available in API Standards 520 and 521.

Many petrochemical companies are currently engaged in flare systems review and upgrade projects. They wish to ensure continuing safe operations, to maximize the use of their existing flare systems, and to minimize the need for modifying existing flare structures or building new ones.