Table of Contents
Introduction to Pressure testing
Pressure testing is a critical aspect of ensuring the safety and reliability of various systems and components. It involves subjecting equipment to high pressure to verify its integrity and ability to withstand operational stresses. Pressure tests can be conducted using liquid or gas, depending on the specific requirements of the test. The two most common types of pressure tests are hydrostatic testing and pneumatic testing.
In this article, you will gain a comprehensive understanding of the essential guidelines for hydrostatic and pneumatic testing, including their key differences, preparations, required equipment, safety considerations, and step-by-step procedures.
Understanding Hydrostatic testing (Hydro Testing)
Hydrostatic testing is commonly used for testing pipelines, pressure vessels, boilers, and other equipment that operates under high pressure. The primary goal of hydro testing is to identify any leaks, verify pressure integrity, and ensure that the equipment meets the necessary safety and design requirements. In addition, hydrostatic testing can be used to assess the effects of corrosion, erosion, and other damage on the equipment’s pressure-holding capacity.
One of the main advantages of hydrostatic testing is its ability to detect even the smallest leaks, thanks to the incompressible nature of liquids. Moreover, in the event of a test failure, the release of liquid is less hazardous than the release of gas, making hydro testing a safer option in many cases.
Understanding Pneumatic testing
Pneumatic testing is another type of pressure test that uses compressed air or another gas, such as nitrogen, as the test medium. Like hydrostatic testing, pneumatic testing involves pressurizing the equipment to a specified test pressure to evaluate its integrity and performance. Pneumatic tests are commonly used for testing gas pipelines, valves, and other equipment that operates with gaseous media.
While pneumatic testing shares some similarities with hydrostatic testing, it also has some unique features and benefits. For instance, pneumatic testing can be particularly useful for detecting leaks in gas systems, as gas molecules can easily escape through even the tiniest openings. Additionally, pneumatic tests allow for quicker pressure changes and shorter test durations, as gases are more compressible than liquids.
However, pneumatic testing also has some inherent risks due to the compressibility of gases. In case of a test failure, the release of compressed gas can result in a rapid expansion and potentially dangerous consequences, such as explosions or projectile hazards. As a result, pneumatic testing requires strict adherence to safety guidelines and precautions.
HYDROSTATIC Vs PNEUMATIC TESTING
Although pressure testing using a liquid as the pressurizing medium (usually referred to as hydro-testing) is not without risks, it is by far the safer method and should be used wherever practicable.
Pressure testing using air or gas as the pressurizing medium (usually referred to as pneumatic testing) is potentially more dangerous because of the higher energy levels involved. For example, the energy released during a total failure of pressure equipment containing compressed air at pressures frequently used in pressure testing is more than 200 times the energy released by the same volume of water compressed to the same pressure. Therefore, carrying out pneumatic testing in the refinery shall be highly discouraged.
A disadvantage of hydro testing is the introduction of water in systems that must be water-free after the testing. Pneumatic testing should only be carried out when hydro testing is not practicable, for instance where the interior of the pressure equipment will be contaminated by the hydro test medium, or when the pressure equipment supports and/or foundations are not capable of supporting the weight of the equipment filled with the medium.
Pneumatic testing may also be considered to test large diameter lines where temporary supports are not practical and also for refractory lined piping systems. Brittle materials shall not be subjected to pneumatic pressure tests. Some commonly used brittle materials are glass, cast iron, and most high-strength alloys.
The decision to apply a pneumatic test instead of a hydro test is restricted to the following situations:
- The pressure system is designed or supported in a manner that unquestionably cannot be safely filled with liquid.
- The configuration of the pressure system is such that it cannot be dried, and traces of the test medium cannot be tolerated.
- A hydro test would damage linings, internal insulation, or other equipment.
- Operations Department in consultation with Inspection & Corrosion Division and Engineering & Services Division shall decide whether to go for a Pneumatic or Hydro test.
Pressure Testing Hazadrs: Understanding the Risks
The Dangers of Stored Energy Release
During pressure testing, the primary hazard is the unintentional release of stored energy. In pneumatic testing, this can lead to a dangerous blast wave and projectiles that can be lethal. However, with hydro testing, the blast carries less energy, and it’s assumed that all expansion energy transforms into projectile energy.
The release of stored energy can occur due to:
- Rupture of pressure equipment caused by brittle fracture
- Rupture of pressure equipment due to ductile fracture
- Detachment or removal of blanking plates, clamps, attachment bolts, screwed plugs, isolation valves, etc.
- Detachment of temporary welds on plugs, pipe ends, and nozzles
- Gasket failure
GENERAL SAFETY PRECAUTIONS FOR PRESSSURE TESTING
The following section deals with the general precautions to be followed while doing pressure testing. The precautions are applicable for both hydro and pneumatic testing unless specifically mentioned, whether for hydro or pneumatic.
The first step to take before carrying out any pressure test (both hydro and pneumatic) is to perform a risk assessment of the activity. It helps to list the safety measures needed to carry out pressure testing. It relies on the identification of all relevant hazards and dangers and consists of an estimation of the risks arising from them with a view to their control or avoidance.
Permit to work system
An important element of the safe system shall be the written permit-to-work system.
The safe system of work should also require that all persons involved in pressure testing are adequately and properly trained. Training shall be provided by the executing division.
Hydro Test Safety Pneumatic Test safety.
Written testing instructions and procedures
Written test procedures shall be available which include procedures for pressurizing, depressurization and venting. These should be prepared by the executing division and shall be made available to all parties involved in the testing. The procedure shall define:
- the test pressure, the test duration, method of pressurization (manual or using a pump, pump discharge pressure, flow rate, etc.) and testing medium to be used;
- specification of safety valves and pressure gauges;
- An up-to-date piping and instrumentation drawing (P&ID) or Piping Isometric drawing showing the position of all isolation valves, safety valves, non-return valves, pressure gauges, testing medium supply points, vent valves and blind plates
- the sequence for opening vent valves when more than one venting position is to be used.
PREPARING FOR PRESSURE TEST: SAFETY PRECAUTIONS
In this section, we’ll discuss the essential precautions you need to take before initiating pressure tests, including hydrostatic and pneumatic testing.
- The area where the test is being performed shall be roped off to prevent access to unwanted persons with signs (in English and local language) posted on the ropes warning about the test. Minimum essential persons should only be allowed near the test area.
- Personnel engaged in the test shall be briefed before the test, safety talks shall be conducted explaining the procedure and the hazards involved.
- The individual system documentation shall be available before any testing and shall include information such as test limits, test pressure, test medium, duration, test blinds, blind flanges, vents, and drains.
- Before testing, the Executor shall inspect the system to assure it conforms to the system layout drawing and that all appropriate safety precautions as per Risk Assessment have been taken.
- P&IDs and blind lists should be utilized to identify the locations of blinds, valves, vents, and drains. Client’s (For exmple KNPC) Blinding &Tagging procedure shall be followed.
- Testing equipment such as pumps, manifold, pressure and temperature recorders, pressure gauges shall be of valid calibration (as per company procedures).
- Pressure gauges shall be fitted at both low and high point when testing large volume systems.
- All necessary measures shall be taken to remove air from the line during filling. This includes back-pressure control, a steady controlled filling rate, use of a break tank, etc. It shall be ensured that the pump suction will not draw in the air with the water.
- Each member of the test team shall wear goggles, and ear protection whenever pressure is applied to the system being tested. This is in addition to the mandatory PPE required.
- Pressurize equipment shall not be approache for close examination until a reasonable period of time has elapsed.
- Joints in the system which may fail and flexible tubing shall be restrained during the test to prevent personnel injury.
- Equipment such as pumps, compressors, blowers, turbines, other rotating machinery and vessels, heat exchangers, towers, etc, shall be isolated from the pressure test while testing a pipeline.
- For high-pressure systems the test pressure shall be reached in stages, each stage allowed to stabilize then visually checked for leaks. The rate of pressure increase shall be mentioned in the approved procedure.
- The pressure gauge(s) shall be connected directly to the vessel or system and shall be visible to the operator throughout the duration of the test. They shall have valid calibration certificates.
- All gaskets used for hydro testing/pneumatic testing shall be of an approve type meeting the actual pipe specifications.
- All restrictions that would interfere with filling, venting or draining, such as orifice plates and flow nozzles, shall not be installed until testing is completed. Suitable spacers shall be installed for testing.
- Flexible Hoses: Flexible hoses used shall be suitable for the specific application. They shall be of armored type. Flexible hoses shall be used only when required for the connection of portable equipment or vibration isolation when no other feasible means are available. Hoses must be identified with physical marking to verify proper application and facilitate inspection and tracking. They shall have valid test certificates. All hoses that may whip and cause physical injury to personnel or equipment damage shall have whip arrest installed (anchored at maximum 5m intervals) to prevent excessive whipping action, should breakage occur. The manifold or other pressure piping shall not be used as an anchor.
- Clamps and bolts: Clamps or bolts on bolted flanges must not be loosened or tightened while the pressure equipment is under pressure. The pressure equipment shall be isolated from pressure sources, de-pressurized and vented before clamps or bolts are adjusted or removed.
- Protection against blast waves and missiles: Protection against blastwaves and rocketing fragments can be provided wherever practically possible by placing the pressure equipment behind a suitable barrier or in a properly designed pit.
- Suitable communication channels shall be established before commencement of test (especially if the test site extends for the large area as in the case of cross country pipelines)
- Depressurization: After the satisfactory completion of the test, the test section shall be depressurized to hydros test head plus 1 bar so that air does not enter into the test section. Pressure let-down valves shall be opened slowly and depressurizing continued at a rate that does not generate vacuum and vibrations in associated pipework.
- Do not use 90-degree nozzle for blowing down pressure – can twist off and turn it into a lethal projectile. 45 degrees are better, but the safest blow-down is straight up.
HYDRO TEST: SPECIFIC SAFETY PRECAUTIONS
In addition to the precautions discussed above, the following precautions shall be taken while doing a hydro test:
- The system shall be filled from the lowest available point; all vents and high point connections shall be open during this operation to allow the air in the system to vent off. There should be sufficient venting provisions to prevent the testing medium being trapped behind non-return valves, in dead legs or between isolation valves.
- For hydros testing, a nonhazardous liquid such as water shall be used at temperatures below 32°C and over 10°C to aid in avoiding condensation during the test.
- A high-volume pump may be used to fill large lines that provide adequate pressure to overcome the static head. Only a small capacity test pump shall be used for the pressurization of smaller systems (manual hand pumps and pneumatic pumps of capacity up to 500 PSIG can be considered as small pumps). A Pressure relief valve shall be installed on the discharge of the test pump (PRV is not mandatory for small hand pumps since the pressure is increased manually keeping a constant watch on the pressure gauge). The relief capacity of the PRV should be equal or more than the test pump discharge rate to avoid sudden pressurization of the system. The PRV shall be set at 110% of the test pressure.
- Some hydro tests are conducted with seawater (especially for tanks) and may require chemical additives to be added. Environmental considerations on disposing of such a medium contaminated with chemicals shall be addressed. If chemicals are not used, then the internal surfaces should be washed with freshwater and samples should be tested to get chloride-free water.
- Expansion bellows and spring supports shall be restrained or removed during hydro testing.
- Care shall be taken to avoid overloading any part of the pipe support system or supporting structures. Large piping designed for vapor or gas shall be provided with additional temporary supports, if necessary, to support the weight of the test fluid. They shall be designed considering the weight of the pipe after filling.
- Before hydrostatic pressure is applied, the test equipment shall be examined to see that it is tight. All low-pressure filling lines and other components of the test equipment that should not be subjected to test pressure shall be disconnected or blanked off.
- During discharge of water from the pressure tested equipment, care shall be taken to ensure that the vent is free from any debris or plastic sheets that may clog the vent and create a vacuum.
PNEUMATIC TEST: SPECIFIC SAFETY PRECAUTIONS
The following are some hazards specifically associated with pneumatic testing. The precautions mentioned under PREPARING FOR PRESSURE TEST shall be taken into consideration in addition to those mentioned here. Personnel attempting a pneumatic test of any system should be aware of these potential hazards.
- The pressure test medium if inert can pause its own hazard (eg: nitrogen atmosphere is asphyxiating).
- Fragmentation into shrapnel will result if the part under test breaks up. Since the shrapnel will travel at high velocity for long distances, the likelihood of injury to unprotected personnel or equipment is very high.
- An explosive noise will result from a large rupture, whereas noise of extended duration will result from a small-orifice failure. By reflex action from the sudden noise, personnel in the nearby area could expose themselves to injury.
- Another hazard to consider is the cutting action of a high-velocity, small-orifice air leak.
- A pressure wave or pulse could develop from a gross rupture, presenting a hazard to personnel and possible damage to surrounding equipment and structures.
- Equipment motion resulting from a gross rupture can cause whip action from a failure of a flexible pipe or hose unless the system is securely fastened. Also, a severe structural failure of a lightweight container or vessel could cause the part to act as a projectile, propelled by the resulting discharge force. Therefore, before any pneumatic test is conducted, the parts shall be securely fastened to prevent hazardous movement.
- Protection of buildings and major structures shall also be considered if they are inside the exclusion area. The restricted area shall be barricaded or patrolled to control the movement of all personnel in the area.
- Where possible, steps should be taken to reduce to a minimum, the internal volume of pressure equipment to be pneumatically tested e.g. utilizing non-compressible material. This will reduce the amount of energy stored and so reduce the consequences of a rupture. It may be possible to isolate sections of the equipment and test them separately, followed by re-assembly and leak testing.
- Large pressure systems: If the risk assessment of a pressure test indicates the pressure equipment should be placed behind barriers but it is too large or heavy to do so, then any persons in the test danger zone, shall be kept clear until the test is over.
- Restrictions for operation may be required to ensure the safe operation of the system. The restrictions may include physically isolating the system with blast walls, removing all personnel from a defined area when the system is pressurized, or placing the vessel in a remote location. Permanent signs and barriers should be considered. Other precautions may require.
- Combination hydro/pneumatic pressure tests are as hazardous as pneumatic tests; therefore, identical precautions shall be employe.