DESIGN AND CONSTRUCTION OF PRESSURE SAFETY VALVE CALIBRATION RIG CHAPTER ONE INTRODUCTION 1.1 BACKGROUND OF THE STUDY A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds preset limits. Pilot-operated relief valves are a specialized type of pressure safety valve. A leak tight, lower cost, single emergency use option would be a rupture disk. Safety valves were first developed for use on steam boilers during the Industrial Revolution. Early boilers operating without them were prone to explosion unless carefully operated. The function of a pressure relief valve is to protect pressure vessels, piping systems, and other equipment from pressures exceeding their design pressure by more than a fixed predetermined amount. The permissible amount of overpressure is covered by various codes and is a function of the type of equipment and the conditions causing the overpressure. A pressure Relief Valve is a safety device designed to protect a pressurized vessel or system during an overpressure event. An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP). The primary purpose of a pressure Relief Valve is protection of life and property by venting fluid from an overpressurized vessel. Many electronic, pneumatic and hydraulic systems exist today to control fluid system variables, such as pressure, temperature and flow. Each of these systems requires a power source of some type, such as electricity or compressed air in order to operate. A pressure Relief Valve must be capable of operating at all times, especially during a period of power failure when system controls are nonfunctional. The sole source of power for the pressure Relief Valve, therefore, is the process fluid. 1.2 PROBLEM STATEMENT Over-pressure in industries can lead to accident and at the same time causes loss of life. One of the most critical automatic safety devices in a pressure system is the pressure safety valve. The primary purpose of a pressure safety valve is the protection of life, property and environment during an over-pressure event in a pressurized vessel or equipment. An over-pressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure. A pressure safety valve is designed to open and relieve excess pressure from vessels or equipment and to re-close and prevent the further release of fluid after normal conditions have been restored. 1.3 AIM OF THE PROJECT The primary aim of this work is to build a safety valve that is used for protection of life, property and environment. A safety valve is designed to open and relieve excess pressure from vessels or equipment and to reclose and prevent the further release of fluid after normal conditions have been restored. 1.4 OBJECTIVES OF THE PROJECT Objectives of this work are: i. To prevent damage to equipment ii. To avoid injury to personnel iii. To eliminate any risks of compromising the welfare of the community at large and the environment. Proper sizing, selection, manufacture, assembly, test, installation, and maintenance of a pressure relief valve are critical to obtaining maximum protection. 1.5 APPLICATION OF THE PROJECT This device can be used In the petroleum refining, petrochemical, chemical manufacturing, natural gas processing, power generation, food, drinks, cosmetics and pharmaceuticals industries. 1.6 LIMITATION OF THE PROJECT Pressure relief valve does not control or regulate the pressure in the vessel or system that the valve protects, and it does not take the place of a control or regulating valve. 1.7 i. DEFINITOIN OF TERMS Relief valve (RV): an automatic system that is actuated by the static pressure in a liquid-filled vessel. It specifically opens proportionally with increasing pressure ii. Safety valve (SV): an automatic system that relieves the static pressure on a gas. It usually opens completely, accompanied by a popping sound iii. Safety relief valve (SRV): an automatic system that relieves by static pressure on both gas and liquid. iv. Pilot-operated safety relief valve (POSRV): an automatic system that relieves on remote command from a pilot, to which the static pressure (from equipment to protect) is connected v. Low pressure safety valve (LPSV): an automatic system that relieves static pressure on a gas. Used when the difference between the vessel pressure and the ambient atmospheric pressure is small. vi. Vacuum pressure safety valve (VPSV): an automatic system that relieves static pressure on a gas. Used when the pressure difference between the vessel pressure and the ambient pressure is small, negative and near to atmospheric pressure. vii. Low and vacuum pressure safety valve (LVPSV): an automatic system that relieves static pressure on a gas. Used when the pressure difference is small, negative or positive and near to atmospheric pressure. CHAPTER TWO 2.0 LITERATURE REVIEW 2.1 INTRODUCTION This section presents the conceptual and related literature from various entities where the study anchored upon to provide evidence and significant review of the topic grounded with the current study. The coherence and consistency of this review of related literature illustrated from empirical studies. 2.2 OVERVIEW OF PRESSURE SAFETY VALVE A relief system is an emergency system for discharging fluid during abnormal conditions, by manual or controlled means or by an automatic pressure relief valve from a pressurized vessel or piping system, to the atmosphere to relieve pressure in excess of the maximum allowable working pressure. Pressure relief valve is the term used to describe relief device on a liquid filled vessel. For such a valve the opening is proportional to increase in the vessel pressure. Hence the opening of valve is not sudden, but gradual if the pressure is increased gradually. When pressure rises above maximum allowable working pressure the clip breaks and overpressure generated inside the equipment is relief through the nozzle, so pressure inside the equipment reduce. Pressure relief valves must be designed with materials compatible with many process fluids from simple air and water to the most corrosive media. They must also be designed to operate in a consistently smooth and stable manner on a variety of fluids and fluid phases. 1.3 REVIEW OF RELATED WORKS Jadhav S.G. et al. (2015) designed pressure relief valves to provide protection from overpressure in steam, gas, air and liquid lines. An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure. He focused on the review on design, analysis and weight optimization of pressure relief valve by using transient finite element analysis. Aniket A. Kulkarni et al. (2014) focused on a review of a structural analysis and optimization of pressure vessel to identify the existing work made in the analysis of pressure vessel and to form a theoretical foundation for understanding the recent developments, then to gain some insight into which domains are relevant in order to position the research. Pressure vessel has several functions apart from holding the gas pressure. Also it appears that pressure vessel can be designed using experimental, analytical and numerical techniques. A.R. Champneys et al. (2014) Summarized and extended recent scientific investigations into the mechanisms of instability in pressure relief valves (PRVs) and considers their implications for practical operation. The overall aim was to develop a new comprehensive understanding of the issues that affect valve stability in operation, in order to influence a new set of design guidelines for their operation and manufacture. They focused specifically on direct springloaded PRVs in gas service, particularly considering the combined effect of the valve dynamics with acoustic pressure waves within its inlet pipe. Prof. Vishal V. Saidpatil et al. (2014) carried out detailed design & analysis of Pressure vessel used in boiler for optimum thickness, temperature distribution and dynamic behavior using Finite element analysis software. They designed a cylindrical pressure vessel to sustain 5 bar pressure and determine the wall thickness required for the vessel to limit the maximum shear stress. Geometrical and finite element model of Pressure vessel was created using CAD CAE tools. Geometrical model was created on CATIA V5R19 and finite element modeling was done using Hypermesh. ANSYS was used as a solver. M. V. Awati et al. (2014) focused on design of an emergency shut of valve. Non-linear analysis is carried out to obtain the results. Stresses and deformations are within permissible values. Additional reinforcement pad is attached to nozzle part to avoid failure. From the results we can say valve performs functionally well. Non linear analysis gives more accurate results regarding the stresses. Sushant M. Patil et al. (2013) designed “gradual flow reducer valve” with available data on field. The thickness optimization of this gradual flow reducer valve had been done by using finite element analysis. The optimum thickness of the valve was finalized as 2mm. After finalizing the optimum design, same design had been taken for the further analysis. A basic model of the valve suitable for design purposes and optimization had been developed. C. Bazsó et al. (2013) presented detailed experimental results on the static and dynamic behaviour of a hydraulic pressure relief valve with poppet valve body, with a special emphasis on the parameters influencing the valve instability. A systematic experimental study was presented on relief valve instability for slightly compressible fluid (hydraulic oil). The experimental system consisted of a positive displacement pump, a simple direct spring loaded valve and a hydraulic hose connecting them. Pressure and displacement time histories were recorded for a large number of flow rates and set pressures. Arindam Kundu et al. (2012) investigated the flow through valve at different valve opening and different pressure drop were presented. Flow through a spool type valve at different opening corresponding to pre-set pressure difference had been considered. The commercial code FLUENT was found to aptly model the complicated flow processes inside the domain of interest. That involves compressible flow with high level of turbulence. An axisymmetric 2-D formulation was found to perform reasonably well in comparison with resource intensive three-dimensional mesh. B.S.Thakkar et al. (2012) determined the performance of a pressure vessel under pressure by conducting a series of tests to the relevant ASME standard. They observed that all the pressure vessel components were selected on basis of available ASME standards and the manufactures also follow the ASME standards while manufacturing the components. So that leaves the designer free from designing the components. Qin Yang et al. (2011) conducted three-dimensional numerical simulations to observe the flow patterns and to measure valve flow coefficient and flow fluctuations when stop valve with different flow rate and uniform incoming velocity were used in a valve system. The spectra characteristics of pressure fluctuation on the flow cross section were also presented here to investigate the wake induce of the valve part. These results not only provided people with the access of understanding the flow pattern of the valve with different flow rate, but also were made to determine the methods which could be adopted to improve the performance of the valve. J. Ortega. et al.(2009) developed computational model of a direct acting spring loaded pressure relief valve. A simplified two dimension model was built based on the valve geometrical and constructive characteristics. Further, a dynamic equation, which defines the valve disc position, was implemented. From the solution of the transient form of the conservation equations, the velocity and pressure distributions were obtained, allowing the determination of the discharge coefficient versus valve opening under its transient state. Comparisons with one-dimensional integral approach model were performed to evaluate the model. 2.4 HISTORICAL BACKGROUND OF SAFETY VALVE Function and design The earliest and simplest safety valve was used on a 1679 steam digester and utilized a weight to retain the steam pressure (this design is still commonly used on pressure cookers); however, these were easily tampered with or accidentally released. On the Stockton and Darlington Railway, the safety valve tended to go off when the engine hit a bump in the track. A valve less sensitive to sudden accelerations used a spring to contain the steam pressure, but these (based on a Salter spring balance) could still be screwed down to increase the pressure beyond design limits. This dangerous practice was sometimes used to marginally increase the performance of a steam engine. In 1856, John Ramsbottom invented a tamper-proof spring safety valve that became universal on railways. The Ramsbottom valve consisted of two plug-type valves connected to each other by a spring-laden pivoting arm, with one valve element on either side of the pivot. Any adjustment made to one of valves in an attempt to increase its operating pressure would cause the other valve to be lifted off its seat, regardless of how the adjustment was attempted. The pivot point on the arm was not symmetrically between the valves, so any tightening of the spring would cause one of the valves to lift. Only by removing and disassembling the entire valve assembly could its operating pressure be adjusted, making impromptu 'tying down' of the valve by locomotive crews in search of more power impossible. The pivoting arm was commonly extended into a handle shape and fed back into the locomotive cab, allowing crews to 'rock' both valves off their seats to confirm they were set and operating correctly. Safety valves also evolved to protect equipment such as pressure vessels (fired or not) and heat exchangers. The term safety valve should be limited to compressible fluid applications (gas, vapour, or steam). The two general types of protection encountered in industry are thermal protection and flow protection. For liquid-packed vessels, thermal relief valves are generally characterized by the relatively small size of the valve necessary to provide protection from excess pressure caused by thermal expansion. In this case a small valve is adequate because most liquids are nearly incompressible, and so a relatively small amount of fluid discharged through the relief valve will produce a substantial reduction in pressure. Flow protection is characterized by safety valves that are considerably larger than those mounted for thermal protection. They are generally sized for use in situations where significant quantities of gas or high volumes of liquid must be quickly discharged in order to protect the integrity of the vessel or pipeline. This protection can alternatively be achieved by installing a high integrity pressure protection system (HIPPS). 2.5 REASONS FOR EXCESS PRESSURE IN A VESSEL There are a number of reasons why the pressure in a vessel or equipment can exceed a predetermined limit. The most common are: • Blocked outlet • Exposure to external fire, often referred to as “Fire Case” • Thermal expansion of fluid • Abnormal process conditions (Chemical reaction) • Cooling system failure • Heat exchanger tube rupture • Pipework component failure • Control Valve failure Each of the above listed events may occur individually or simultaneously. Every cause of over-pressure will create a different mass or volume flow to be discharged. For e.g. small mass flow for thermal expansion and large mass flow in case of a chemical reaction. It is the process engineers responsibility to determine the most worst case scenario for the sizing and selection of a suitable pressure safety device. 2.6 TYPES OF PRESSURE SAFETY VALVES Spring Loaded Pressure Safety Valves Figure 1 – Spring Loaded Pressure Safety Valve In a Spring loaded Pressure Safety Valve the closing force or spring force is applied by a helical spring which is compressed by an adjusting screw. The spring force is transferred via the spindle onto the disc. The disc seals against the nozzle as long as the spring force is larger than the force created by the pressure at the inlet of the valve. Pilot Operated Pressure Safety Valves Figure 3 – Pilot Operated Pressure Safety Valve Pilot Operated Safety Valve is controlled by process medium. To achieve this, the system pressure is applied to the pilot valve (= control component for the main valve) via the pressure pickup. The pilot valve then uses the dome above the main valve piston to control the opening and closing of the main valve. Figure 4 shows all the steps in working of Pilot Operated Pressure Safety Valves. During normal operation, the system pressure is picked up at the main valve inlet and routed to the dome. Since the dome area is larger than the area of the main valve seat, the closing force is greater than the opening force. This keeps the main valve tightly closed. At set pressure, the pilot valve actuates. The medium is no longer routed to the dome. This prevents a further rise in dome pressure. Also, the dome is vented. As a result, the closing force ceases as a precondition for the system over-pressure to push the main valve open. Depending on the design of the pilot valve, this opening is either rapid and complete (Pop Action) or gradual and partial following system pressure (Modulate Action). If system pressure drops to closing pressure, the pilot valve actuates and again routes the medium to the dome. The pressure in the dome builds up and the main valve closes either rapid and complete (Pop Action) or gradual and partial following system pressure (Modulate Action).