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Crude refineries are sensitive industrial facilities where millions of crude oil are refined to different petroleum fractions. The processing of crude oil in refineries is carried out at high pressures. For this purpose, pressure testing is carried out in the refineries to ensure a safe production process. Pressure testing is equally important for the storage and transportation of petroleum fractions. Usually, the highest chances of production failure are linked to the pipeline system and the storage tanks in the refinery. This article discusses several leak testing methods, the planning process, the test’s preparation, the pressure testing’s execution, documentation, and acceptance of the test standards.
Pressure testing in the piping system is performed according to the piping codes. It is ensured that the newly installed or the already existing repaired piping system can handle the required pressure of the system and whether the system is leakage proof.
International regulatory bodies already define pressure testing standards that ensure the quality of the pressure testing performed. In pressure testing, the tests are performed according to the piping codes defined by the regulatory bodies. The codes have been generated by implying test procedures on prototype equipment. It involves increasing the pressure to a limit where measurable yield occurs, or the prototype reaches a point of rupture. From the acquired experimental data, pressure ratings are defined according to the code specified in the system.
1.1 Piping Codes Necessary for Pressure Testing
Piping codes are numerous, with different codes relevant to different piping systems. However, two codes are usually important in pressure testing. These codes are defined by the American Society of Mechanical Engineering (ASME). These important codes are ASME B31 Pressure Piping Code and ASME Boiler and Pressure Vessel Code. These codes are usually universally applied to all piping systems; however, some piping systems need other codes defined by the design authorities and the commissioning branch of the industry. Other such codes can be AWWA standards for transmission and distribution piping and ASME B31 Pressure Piping Code. A detailed list of the piping code is given in the following table.
2 Available and Commonly Used Pressure Testing Methods for Pipeline and Tanks in the Refineries:
Different pressure testing methods are used in refineries to test leakages in pipelines and tanks. Following is the list of the commonly available method
The detail of each method is given below:
2.1 Hydrostatic Leak Testing:
This is the most frequently used method in pressure testing. Hydrotesting is preferred over other testing methods because it is relatively safe compared to the pneumatic testing method. As water is used in hydro testing, its incompressible property makes it a safer medium than air. Moreover, it is an energy-saving process, as the amount of energy spent on hydro testing is lower than air compression for pneumatic testing. In hydro testing, energy is stored in the form of potential energy that can easily be released in case a failure during the test occurs. However, during hydro testing, chances of the presence of air trapped in the pipes remain high. Therefore, the highest levels of precaution must be taken while conducting hydro testing.
2.2 Pneumatic Pressure Testing:
In pneumatic testing, the fluid medium used is compressed air or nitrogen if the source is bottled gas. However, nitrogen should be used in the pipeline only. Its testing tanks should be avoided as pressure testing involves confined space entry, and nitrogen can rapidly replace oxygen in the working environment. Workers can become unconscious and faint in a matter of seconds if the oxygen levels are not continuously monitored during pneumatic pressure testing. For some piping codes, pressure in pneumatic testing is kept lower than hydro testing for leak detection. It is because the chances of injury in pneumatic are greater than hydro testing. For instance, in the ASME B31.1 pipe system, the pressure kept less than 100psig during the entire operation.
2.3 Hydro- pneumatic testing:
In hydro-pneumatic testing, a combination of pneumatic and hydro testing is used. This method is considered safe as compared to the other two methods. In this method, pneumatic testing at low pressure, say 25psig, is performed first to indicate any major leaks in the system. This method is particularly useful in testing leakage in the pipes. In case of any leakage, required repairs are performed before moving to hydro testing. This method is not only safer but also a less time-consuming procedure. In hydrostatic testing alone, the entire system needs to be filled with water to find any leakage.
Another hydro-pneumatic is performed in which a combination of water and air system is used. It is particularly useful in the pipe and tank system in which both liquid and gaseous system is involved. In refineries, as petroleum products also travel in liquid and vapour phases, testing the system in both phases is important. Because at times, the system may not report leakage in the liquid medium, but it can report leakage in the gaseous medium.
2.4 Initial-Service Leakage Test:
The initial Service Leak testing is suitable for certain conditions only by code. ASME B31.3 limits it to category D fluid service, which is nonhazardous to humans and operated below 150 psi and at defined temperatures. Moreover, it does not allow this type of testing for external boiler piping but other piping systems. This type of testing can also be used to inspect nuclear power plants as per the ASME Boiler and Pressure Vessel Code Section XI. This test is conducted when the operation begins initially. The method gradually increases to normal operating pressure and then retains the pressure to check for leaks, as required in ASME B31.3.
2.5 Vacuum Leak Testing:
This is an effective method to ensure whether there are leaks in the system or not. The methodology is to draw a vacuum on the system and trap the vacuum in the system. A leak is probable if the vacuum trapped in the system increases the atmospheric pressure. This test is often used as a production leak test by the manufacturers of various components. Although this test depicts a leak, highlighting the leak’s location is nearly impossible. For this purpose, smoking generators are used. However, a smoking generator isn’t functional for smaller leaks as a small leak does not draw the smoke into the piping. Thus, this method is unsuitable for piping tests where the pressure is more than the operating pressure.
2.6 Static-Head Leak Testing
The static Head Leak Testing is also known as the drop test because while retaining the required pressure, a drop in the water level in the standpipe depicts a leak. The level is noted when the sandpipe and the system are added with water. The height is re-taken after a specific time, and the decrease, if any, is noted. The visual scrutiny then determines the leak location.
2.7 Halogen and Helium Leak Testing
The Halogen and Helium Leak Testing use tracer gas to detect leak location and intensity. In case of leakage detection, the system is charged by halogen gas. The detector probe senses leakages of any exposed joint. It consists of a tubular probe that draws a blend of the leaked halogen gas and air into an apparatus sensitive to a minute amount of halogen. The Helium test has two other more accurate methods in leakage detection. These methods are the Tracer mode and Closed System mode. The tracer model involves a vacuum drawn on the system with helium sprayed at the outer side to detect leaks. The hood or closed system model is one where the system is tested by enveloping it with concentrated helium. This is the most accurate method and is the only method accepted by ASME Code Section V as a quantifiable method. The manufacturers who use a hermetic seal usually need to use the hood method of leakage detection. The system is surrounded by helium in a chamber. The system is then linked to the helium leak detector, which draws the system’s internals to a vacuum close to absolute zero.
3 Test Pressures
The test method opted, and the medium for the fluid test will together establish the rules required in calculating the test pressure needed. Usually, the pressure to be applied is slightly greater than the design pressure applied for a short time, often 10 minutes. The intensity of this pressure is often 1.5 times the standard design pressure. However, it varies upon the code and the type of test to be conducted, whether hydrostatic or pneumatic.
Moreover, the limit of the test pressure should never be greater than the pressure that would result in yielding. The ASME B1 and the Boiler & Pressure Vessel Codes state the maximum test pressure as 90% of the component’s yield to be tested. Test pressure demonstrates that the vessel can endure the rated pressure. Following this period of more than design pressure, it is often acceptable to decrease the pressure to a lower value for inspection of leakages. The examination pressure is maintained for the length of time necessary to conduct
4 Pressure Equipment Failure
Pressure vessel and piping systems are used through the industrial processes and consist of great energy concentration. The design of these vessels is subject to federal, state, and local regulations and specific industrial standards. However, there are still various equipment failures.
The reasons for equipment failure include dilapidation and material damage during operation, ageing, problems during fabrication. Opportunely, continual testing and examinations increase the safety of a pressure vessel or piping system. Thus, this is vital for successful scrutiny and examination and the development of procedures for particular types of vessels. Various accidents have focused on hazards and risk factors associated with the storage and handling of fluids under pressure. Typically, vessels fail due to shell failure due to corrosive factors.
5 Pressure Testing Program
A proper pressure test program is the one that can identify issues and ageing problems, erosion, and other equipment failures and can identify
- if the vessel is stable enough to endure the same pressure
- the control and repair required to retain the same pressure
- if a lesser pressure is required for safe operation
All companies have a complete set of guidelines that often work with pressurised apparatus for testing pipes and tanks. Such procedures are prepared after thorough scrutiny and safety standards of OSHA, DOT, ASME, local, state, and other standards.
Such papers involve details of responsibilities and roles, management functioning and safety guidelines, requirements for equipment and materials, various testing methods to validate the system and its mechanisms, guidelines for tests, emergency and hazard handling, etc. These guidelines include various protection and safety standards against toxic gases, high pressures, etc.
- Where it is specified to perform a hydro test 16 bar, the pressure should be 1.5 times the design pressure that is 10.67 bar. According to the B31.3 code, the pneumatic test must be conducted at 1.1 times the design pressure and not at 16 bar, with the highest pressure being 11.7 bar only.
- The probability of the fragility of equipment and its failure is to be reviewed by a competent engineer. If the temperature is below 0 C, a thorough examination of the apparatus should ensure that the temperature isn’t below the minimum usable level for the particular material.
- A competent engineer should produce procedures for testing. The procedures need to specify the types of sections of pipes and tanks being tested, the adjustment of valves, and other devices to be fixed or detached.
- The pressure for the pneumatic test ought to be 25 psi with a prior inspection for leakages before increasing pressure.
- Moreover, the piping design specification should be inspected before leakage or pressure testing.
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