Reduction of Environmental Impact of Emissions by Improved Design of Safety Relief Valves

1
Reduction of Environmental Impact of Emissions by
Improved Design of Safety Relief Valves Wael
Elmayyah, William Dempster Mechanical
Engineering, University of Strathclyde, Glasgow,
UK
INTRODUCTION To protect the environment for
current and future generations, we must reduce
our emissions of heat-trapping gases by using
technology, know-how, and practical solutions.
One of the ways to reduce these emissions is to
improve the performance of safety valves. Safety
valves are responsible for ensuring the safe
working of pressurised systems but do so by
releasing emissions to the environment.
Maintaining safety but reducing emissions is the
challenge!
Fig.(3) shows the air mass flow percent reduction
when water flows through the valve. It is shown
that the greater the liquid flowrate the less gas
flows out of the system. Fig. (3) shows that a
50 reduction of gas flowrate could occur for a
mixture flow consisting of 80 liquid. Since,
maintaining the pressurised system at a safe
pressure depends on removing the gas this
indicates that the designing the valves
specifically for two phase flow is critical for
safety.
OBJECTIVE The ISO-4162 code, described as the
best code for selecting safety valves, oversize
the selected valves 1. A study which included
4000 safety valves showed that more than 65 of
the safety valves are oversized, Fig.(1)
resulting in more emissions than needed. On the
other hand many plant designers prefer oversized
safety valves to maintain safety. These issues
drive us to develop better analysis tools for the
design and specification of valves. In this study
safety valves used in the refrigeration industry
are investigated and specifically how the valves
can be designed for complex flows of liquid and
vapour is the main aim of this work.
Fig. 4 Model Validation
CFD RESULTS Valve geometry has been presented by
a two dimensional-axis- symmetric model. The CFD
code FLUENT has been used with a multi- phase
mixture model to model the two phase flow through
the valve. Fig.(4) shows good agreement of the
CFD model and experimental results. Fig.(5) shows
the valve construction and the contours of the
pressure and Mach number through the valve at 1.5
mm lift and 11.7 bar
Fig. 1 Oversized valves in industry
METHOD OF INVESTIGATIONS Both a theoretical and
experimental approach was taken to understand
what happens when mixtures of liquid and vapour
flow through the valve simultaneously.
Computational fluid dynamics (CFD) has been
adopted with initial investigations carried out
to determine appropriate models and their
accuracy. Experimental work has been carried out
to validate the CFD model and to quantify general
trends. Air as an ideal gas and droplets of water
have been chosen to be the fluids flowing through
the valve.
Fig. 5 Valve assembly and CFD results
The combination of a validated flow model
combined with experimental testing can produce a
full understanding of the safety valve
characteristics and will lead to better designs
to reduce the emissions while maintaining safety.

EXPERIMENTAL RESULTS A test rig has been
constructed to create the two phase flow
conditions and to measure the flow
characteristics through the valve by measuring
the mass flow at different openings (piston
displacement), Fig (2). Characteristics at
different pressures and water flow rates have
also been measured.

• CONCLUSIONS
• Safety relief valve performance changes when
  exposed to two-phase flow. As water flow
  increases the total mixture mass flow will
  increase but results in less gas flow which means
  both more emissions and unsafe operation.
• A simplified two dimensional axisymmetric mixture
  has been found satisfactory in predicting the
  two-phase flow through safety valves.
• Data analysis and statistical surveys on
  industrial safety relief valves shows that
  optimum design of safety valves could reduce 40
  of the cases that emits more gases than expected.

REFRENCES 1 H. Derlien and L. Friedel. Accuracy
of safety valve two-phase mass flow capacity
sizing. Chemical Engineering Technology,
29(1)8796, 2006. 2 Jordan Gronkowski, RPD
2009,Dept. of Mech. Eng., University of
Strathclyde. 3 O. Koper and F. Westphal,
Database-supported documentation and verification
of pressure relief device design in chemical
plants. J of loss prevention in the process
industries, 2003.
Fig. 3 Air flow rate reduction
Fig.2 Air mass flow rate
University Research Day 2009