Detection Analysis of Leaks in Gas Processing Plant

The aim of this study is to analyse the detection of fluid leaks and measures that can ensure safe operation of the gas processing facilities. This will mitigate the operational, environmental and financial risks associated with gas processing activities. The study used Hydrostatic Pressure test to analyse newly manufactured pressure systems. Existing pressure systems were also pressure tested. The analysis directly surveyed, identified and quantified the leaks. Data from previous leaks were also investigated to identify the components that are most likely to leak and profitable to repair. The experimental analysis showed that a larger percentage of gas emissions are from smaller irregular cuts in components such as Pipelines, Valves, Heat Exchangers, Connectors, Compressor seals, Open-ended lines, Flanges, Crankcase vents, Pressure relief devices, and Pump seals. 
  
Index Terms— Leaks; Pressure test; Detection; Data; Gas Emission.


I. INTRODUCTION
Gas processing plants and their booster stations releases approximately 36 billion cubic feet of methane gas yearly [5].More than 70% of these gas losses in the Natural gas plants comes from leaking Compressors, Valves, Flanges, Exchangers, etc [3].Studies have shown that several casualties had occurred in history, which are related to gas production at different locations.In June 2000, gas leak from LNG pipeline resulted in an explosion that killed 50 workers at the Kuwait National Petroleum Company Refinery.The Company suffered a financial loss of $840million from damaged facilities, downtime and rebuilding of facilities [1].At Bhopal, an estimated 3800 persons died from an exposure to a leaking Methyl Isocyanate gas in 1984 [1].Leaks from gas plants and their associate compressor booster stations negatively affects the company's production, cost, workers and environment.Today, there is an increased National and International regulations regarding Environmental preservation.
Early fluid leak detection provides the most appropriate approach in preventing the aforementioned problems.This concern is attracting new technological advancement in designing sensors that works with physical, chemical and biological principles.Flange leakage analysis is an important check on major equipment in the gas plant.Inspite of proper bolting sequence and torquing between flanges, there are still bending moments acting on the flange joint from fluid pressure and thermal growth [9].It is therefore Published in June 27, 2019.I. A. Green is presently with the Department of Mechanical Engineering Technology, Federal Polytechnic of Oil and Gas Bonny.Rivers State.Nigeria.abba.green@yahoo.comU. K Ugoji is presently with the Department of Mechanical Engineering Technology, Federal Polytechnic of Oil and Gas Bonny.Rivers State.Nigeria.ugojikc@yahoo.comimperative to calculate the Flange leakage to verify the capacity of the mating Flanges withstanding external force and internal fluid pressure.In addition, Non Destructive Test (NDT) should be performed on Plant equipment after a Post Weld Heat Treatment to check for defects.
Leaks from equipment in gas processing plants should be checked and experimentally analyzed using Hydrostatic test.This type of pressure test is conducted by using water as the test medium.It is done under controlled conditions as contained in an approved test plan and properly documented for reference purposes.To mitigate leaks in a gas processing plant, pressure test is required after a hot work or Plant Turnaround Maintenance activities.A well conducted Hydrostatic test will give an assurance of the mechanical integrity of the system.

II. LITERATURE REVIEW
There are reports on leak detection and prevention in literature.Studies have shown that there are two types of leakages (internal and external) associated with the hydraulic system [12].An example of external leak is the drips and puddles on plant floor.Depending on the quantity and type of exposure, external leaks can be harmful to humans and environment [10].Internal leaks are created to provide lubrication to the equipment components such as piston, seal, etc.This is done to mitigate the negative impact of friction on these equipment parts [4].However, unintentional or excessive internal leaks can lead to equipment failure or inefficiency because of its negative impact on the system variables such as pressure, temperature, flow and level [12].
Standards such as American Petroleum Institute (API), Fluid Control Institute(FCI), Manufacturers Standard Society(MSS), etc have been adopted by Stake holders in the Petroleum Industry to quantify the amount of leaks that can be accepted for different applications [6].
The causes of leakages in the gas processing plant are Mechanical failure, Operational failure, Corrosion, Natural disaster and Sabotage [14].When stress in the equipment exceed its allowable limit, mechanical failures occurs [15].This is as a result of defect or poor material composition in the manufacturing of the equipment.30% of current documented leak cases in the gas processing plant comes from mechanical failures [16].Operational failures involve human errors such as operating the equipment without standard guidelines [8].Operational failures contribute to 3% of current recorded leaks in the Petroleum Industry [14].Corrosion of Pipelines can be categorized as internal and external [1].Internal corrosion occurs when corrosive materials are in contact with water.External corrosion occurs when the coating is not properly done [14].Basically, corrosion could occur when the pipelines are not in use [9].Natural disasters such as earthquakes, flooding, landslides etc. can result in a massive damage of gas plant [14].In the Detection Analysis of Leaks in Gas Processing Plant Green A. Ibim and Ugoji U. Kelechi design of gas plants, it is therefore imperative to make provision that will mitigate the effect of natural disaster.Sabotage contributes the highest percentage of leakages in a gas processing plant and its booster stations [14].Some of these cases occur accidentally by individuals erecting structures on pipelines caring gas.Surveillance and inspection routines are seriously required to mitigate this problem [13].Early detection and prevention of leaks from pipelines can be achieved by using designed robotic system [5].Tracing of leaks location along the pipeline of a water distribution facilities can be done by using optimum sensor device that works on the principle of genetic algorithm [15].Knowledge of transient fluid pressure signals can be used to detect leaks from a degraded pipeline thinning wall [2].Measurement of acoustics pressure, velocity and acceleration can be used to detect leaks in buried plastic pipes [11].Leaks of high molecular weight gas into a single gas system can be measured with an Ultrasonic instrument [7].

III. METHODOLOGY
Experimental and analytical pressure tests were done as a unit on some piping system and associate equipment.Pressure tests were done separately on piping systems and their connected equipment, when the difference in design pressures is not less than 15% of the lowest system pressure.
Cascade pressure testing was used for piping systems that are fully welded together and having different design pressures.This was done by first carrying out the pressure test on the system with higher pressure prior to completing the welding operation.The second step is to repeat the pressure test procedure for the combine systems at the lest pressure.
Heat Exchangers, Furnaces and Pressure Vessels were pressure tested in this analysis.This was done by ensuring that the test pressure is higher than the system operational pressure.

A. Spading of lines for pressure testing
Specified sizes and ratings of carbon steel blinds were used for the spading of the flanges.Properly rated spiral wound gaskets were used in between the flanges to seal the lines.
The exact sizes of bolts and nuts were used to couple the flanges together.For Heat Exchangers, the channel head was properly coupled to the shell of the tube and shell Heat Exchanger.Proper bolting sequence and torquing were used for the coupling.

B. Workpack for hydrostatic pressure test
The test limits were evaluated and stated in the P&ID of the test work pack.The documents that constituted the test work packs are reinforcing pressure test certificate, permit to work, post treatment certificate, equipment drawings, welding reports, spade list and piping pressure test report.The work-pack also contain the following information such as: test limits, sizes of the mono-block connections, provision for entry of water into the system for pressure testing, vents, drain points and calibration of pressure test gauges.

C. Hydrostatic water condition
The pressure test analysis was done with a clean water.Filters and Strainers were used to remove diets and obstacles in the water before the hydrostatic test.For austenitic steel, chlorine of maximum 50ppm with clam potable water was used for the pressure test.

D. Testing
The hydrostatic test pressure was 50% higher than the designed pressure.For systems with test temperature higher than the design temperature, the following equation will be used to calculate the minimum pressure.
Where: Pₜ = Minimum test pressure calculated Pi = Equipment design pressure Pst = Allowable stress for test temperature Psd = Allowable stress for design temperature When Pst and Psd are the same, From (1), the hydrostatic test pressure generated a stress that is higher than the defined minimum yield strength of the test temperature.In addition, the test pressure produced a stress that is less than the defined minimum yield strength at the test temperature.

E. Hydrostatic testing of piping design
The following steps were adopted for calculating the external pressure of the piping system.
(1) The gauge pressure of 15psi was less than the external differential design pressure.(2) The external pressure of the lines was internally tested with half of the external design pressure.(3) Prior to the completion of the jacket, the external design pressure was used for pressure testing of the internal lines.(4) The jacket design conditions were used for the pressure testing.(5) The Vessel test pressure exceeded the hydroststic testing of static equipment.The following equation was used to evaluate the maximum hydrostatic test pressure.

F. Leakages from pipe cuts
The minimum cut of a pipe was evaluated by using Boolean algebraic operation.
Where: C = number of cuts Uf = minimum cut sets n =number of repeated cuts Yc(i) = sizes of cuts

G. Specified Minimum Yield Strength(SMYS)
To prevent a pipe from getting burst due to stress, the analysis evaluated he Specified minimum yield strength(SMYS) using Barlow's formula as shown in eqn 5 below.
Where: SMYS = Specified Minimum Yield Strength Sa = Allowable stress tn = Norminal wall thickness D = pipe diameter

IV. RESULTS AND DISCUSSION
The Hydrostatic test results (table 2-6) indicated changes in pressure across the boundaries of the system.The difference in pressure was used to evaluate the leak rate of each component.This was achieved by comparing the maximum allowable change in pressure to the change in percentage volume and mass per time.The system gave a known volume under pressure.Hence, the change in volume to pressure (dV/dP) was calculated theoretically (eqn 6) and compared to the practical (field) test values.
For aboveground pipe, For buried pipe, Where: V = volume of each pipe diameter Do = outside diameter of pipe E = Elastic modulus of pipe tp = pipe wall thickness C = compressibility test material The theoretical value of dV/dP was equal to the field (practical) value of dV/dP.This indicated that there was no air in the line under test.V. CONCLUSION A combination of screening and measurement analysis such as hydrostatic testing can give an accurate data of gas leaks.Identification and quantification of leaks can be efficiently obtained by using the higher volume gas sampler.Prior to installation and connection of equipment in the gas processing plant, potential leak i each component can be located, quantified and addressed.This will mitigate start-up delays and improve the efficiency of the gas processing plant.
= maximum hydrostatic test pressure Sm = maximum yield strength at test temperature E = quality factor td = difference in wall thickness and mill tolerance Do = outside diameter

TABLE I :
CAUSES OF LEAKAGES IN GAS PROCESSING PLANT FOR THE PAST 30 YEARS

TABLE 3 :
PRESSURE DIFFERENTIALS FOR TUBE AND SHELL HEAT EXCHANGER

TABLE 5 :
PRESSURE TEST STRENGTH FOR COMPACT HEAT EXCHANGER

TABLE 6 :
MINIMUM PRESSURE LEAK TEST FOR PARALLEL AND COUNTER FLOW HEAT EXCHANGER