Spring Operated Pressure Relief Valves Research Articles

Nonlinear dynamics and safety aspects of pressure relief valves

Direct spring operated pressure relief valves are regular elements of high-pressure hydraulic systems; however, valve vibrations may still occur endangering the valve and the protected system. The damping of these vibrations is a crucial issue, whereas the external viscous damping at the valve cannot be increased boundlessly based on safety aspects. The vibrations emerge via Hopf bifurcations of either super- or subcritical type, and the nonlinear characteristics have an important role in the valve dynamics. Thorough nonlinear analysis is needed to guarantee safe operation. The linear stability analysis is extended by local and global bifurcation analysis to reveal the stable domains bounded by subcritical Hopf points and the possible bistabilities that exclude the safe operation. Afterwards, the effect of a downstream pipe is analysed, which can also carry safety risks based on the literature; however, the time delay originated in the wave propagation in the pipe may also have stabilising effects. The nonlinear analysis of the presented system compares analytical, semi-analytical and numerical results and assigns the safe parameter domains of the valve with respect to a wide range of relevant parameter variations.

An implicit system of delay differential algebraic equations from hydrodynamics

Direct spring operated pressure relief valves connected to a constantly charged vessel and a downstream pipe have a complex dynamics. The vessel-valve subsystem is described with an autonomous system of ordinary differential equations, while the presence of the pipe adds two partial differential equations to the mathematical model. The partial differential equations are transformed to a delay algebraic equation coupled to the ordinary differential equations. Due to a square root nonlinearity, the system is implicit. The linearized system can be transformed to a standard system of neutral delay differential equations (NDDEs) having more elaborated literature than the delay algebraic equations. First, the different forms of the mathematical model are presented, then the transformation of the linearized system is conducted. The paper aims at introducing this unusual mathematical model of an engineering system and inducing research focusing on the methodology to carry out bifurcation analysis in implicit NDDEs.

Estimating the opening time of a direct spring operated pressure relief valve in the case of multiphase flow of fixed mass fraction in the absence of piping

This paper addresses the effect of non-flashing multiphase flow on the dynamic behaviour of direct spring operated pressure relief valves (PRV). We employ DIER’s ω method to describe the mass flux through the valve for different xg gas mass fractions, which is combined with the one-degree-of-freedom mechanical model of the PRV and the vessel pressure dynamics, providing a simulation tool for predicting venting scenarios. We emphasise on the proper description of the fluid momentum forces on the valve disc. Three typical valve geometries are considered: a ’reaction-like’ geometry with large deflection angle reversing the flow, a simple disc valve with purely radial outlet and a conical geometry, for which the direction of the flow through the valve does not change significantly. It was found that by merely varying the mass fraction of the liquid and the gas, the valve opening time changes drastically, over a range of several orders of magnitude. By using a simplified mechanical analysis and assuming frozen mixture (i.e. fixed mass fraction), a formula was developed providing an order-of-magnitude estimation of the valve opening time.

Prediction of quarter-wave instability in direct spring operated pressure relief valves with upstream piping by means of CFD and reduced order modelling

This paper focuses on modelling the dynamic instability (flutter/chatter) of gas-service direct spring operated pressure relief valves (DSOPRV) due to the acoustic coupling between the valve body dynamics and the upstream piping. Previous studies have shown through reduced order models that there exists a critical inlet pipe length beyond which self-excited vibrations occur due to the presence of a quarter standing wave in the upstream piping. However, to allow analytical computations and simple design equations, these reduced-order models relied on quasi-steady assumptions for the estimation of the discharge coefficient and the fluid force acting on the valve body. This paper proposes two CFD-based methods for the analysis of the fluid forces and valve stability: firstly, steady-state CFD runs were performed to verify the assumptions of the reduced order models and also to increase their accuracy. Secondly, dynamic CFD simulations using deforming mesh technology were conducted, with which the valve response can be resolved with high fidelity including also transient fluid–structure interactions. Comparing the results of the two approaches verify the use of the reduced-order models for stability predictions.

The Effects of Maintenance Actions on the Average Probability of Failure on Demand of Spring Operated Pressure Relief Valves

The safety integrity level (SIL) of equipment used in safety instrumented functions is determined by the average probability of failure on demand (PFDavg) computed at the time of periodic inspection and maintenance, i.e., the time of proof testing. The computation of PFDavg is generally based solely on predictions or estimates of the assumed constant failure rate of the equipment. However, PFDavg is also affected by maintenance actions (or lack thereof) taken by the end user. This paper shows how maintenance actions can affect the PFDavg of spring operated pressure relief valves (SOPRV) and how these maintenance actions may be accounted for in the computation of the PFDavg metric. The method provides a means for quantifying the effects of changes in maintenance practices and shows how these changes impact plant safety.

Investigation of Adhesion Formation in New Stainless Steel Trim Spring Operated Pressure Relief Valves

Examination of proof test data for new (not previously installed) stainless steel (SS) trim spring operated pressure relief valves (SOPRV) reveals that adhesions form between the seat and disk in about 46% of all such SOPRV. The forces needed to overcome these adhesions can be sufficiently large to cause the SOPRV to fail its proof test (FPT) prior to installation. Furthermore, a significant percentage of SOPRV which are found to FPT are also found to “fail to open” (FTO) meaning they would not relief excess pressure in the event of an overpressure event. The cases where adhesions result in FTO or FPT appear to be confined to SOPRV with diameters less than or equal to 1 in. and set pressures less than 150 pounds per square inch gauge (psig) and the FTO are estimated to occur in 0.31% to 2.00% of this subpopulation of SS trim SOPRV. The reliability and safety implications of these finding for end users who do not perform pre-installation testing of SOPRV are discussed.

Statististical Performance Evaluation of Soft Seat Pressure Relief Valves

Risk-based inspection methods enable estimation of the probability of failure on demand (PFD) for spring-operated pressure relief valves at the United States Department of Energy's Savannah River Site in Aiken, South Carolina. This paper presents a statistical performance evaluation of soft seat elastomer spring operated pressure relief valves. These pressure relief valves are typically smaller and of lower cost than hard seat (metal to metal) pressure relief valves. They can provide substantial cost savings in certain fluid service applications providing that PFD is at least as good as that for hard seat valves. PFD is the probability that a pressure relief valve fails to perform its intended safety function during a potentially dangerous over pressurization. The research in this paper shows that the proportion of soft seat spring operated pressure relief valves failing is the same or less than that of hard seat valves, and that for failed valves, soft seat valves typically have failure ratios of proof test pressure to set pressure much less than that of hard seat valves.

The Adhesion Failure Mode in Stainless Steel Trim Spring-Operated Pressure Relief Valves

This paper addresses dangerous failures of stainless steel (SS) trim spring-operated pressure relief valves (SOPRV) due to a particular failure mode (SS-to-SS adhesion), which is not currently being included in SOPRV failure rates. As a result, current methods for estimating or predicting failure rates for SS trim SOPRV significantly underestimate these failure rates and, consequently, overestimate the safety provided by the SOPRV as measured by its average probability of failure on demand (PFDavg) or its corresponding safety integrity level (SIL). The paper also illustrates the critical importance of root cause analysis (RCA) of dangerous SOPRV failures in understanding the impacts of various failure modes. Over 1300 proof test results for both new and used SS trim SOPRV from the Savannah River Site (SRS) were identified. RCA was used on the failed valves to classify those failed due to SS-to-SS adhesions. Statistical analysis of the data convincingly demonstrates adhesions, previously assumed to be only an in-storage failure phenomenon, are also an in-service failure mode, which needs to be included in SOPRV failure rates. The paper discusses the factors which potentially influence the adhesion failure mode, and suggests a possible approach to including this mode in failure rate predictions. An example illustrates how current failure rate models overestimate SS trim SOPRV safety by 1 or 2 orders of magnitude.

Evaluating Risk and Safety Integrity Levels for Pressure Relief Valves Through Probabilistic Modeling

The probability of failure on demand (PFD) for spring-operated pressure relief valves (SORVs) is estimated by applying the Fréchet and Weibull probability distributions using proof test data from the United States Department of Energy's Savannah River Site (SRS) in Aiken, South Carolina. The data can be accessed through the Center for Chemical Process Safety (CCPS) Process Equipment Reliability Database (PERD). The probability distributions enable the evaluation of risk, estimation of ANSI/ISA-84.00.01 Safety Integrity Levels (SILs), and the impact of potential modifications of the maintenance plan. Current SRS practices are reviewed, and recommendations are made for risk-based adjustments to the maintenance plan. Subsets of valves are identified in which maintenance times can be extended and in which increased safety margins may be needed.