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).