1 Introduction & Background

Pressure vessels have served as an integral component of the Age of Steam and the Industrial Revolution. They have played a critical role in various industries, from mining to water storage to oil refining. Leonardo da Vinci in his Codex Madrid I, applied the concept of pressure vessel 1495 for lifting heavy loads underwater using pressurised air. However, the first industrial application for a pressure vessel was recognised as done by Edward Somerset when he installed a steam pump into Raglan Castel. Later, Thomas Savery built a steam pump based on Somerset’s and joined forces with Thomas Newcomen, and the two built the first commercial steam engine in 1712. From there, the world saw tremendous growth in pressure-containing equipment in domestic, commercial and industrial applications.

Any metallic or non-metallic enclosure that contains fluid at a pressure above or lower than atmospheric pressure (or external pressure) is classified as a pressure vessel. In strictly technical terms, the value of max differential pressure sustained by the enclosure walls distinguishes between vessels and tanks. As per codes and industry norms, pressure tanks can sustain a maximum differential pressure of 15 PSI, while pressure vessels classification starts at a value of differential pressure exceeding 15 PSI and normally goes up to 3000 PSI. This differential pressure exceeds 3000 psi with special allowances for wall thickness and material specifications for some specialised industrial applications related to oil and gas service and power boilers. Piping and pipelines that carry the pressurised fluid are distinctly treated by industry and codes as design considerations separate from pressure vessels.

Thus, it can be safely implied that the design requirements determine configurations of pressurised static equipment, the product being stored, amount of space at the job site, type of operations needed for service delivery, and transportation needs for a pressurised fluid to geographically scattered operational sites. Following are the most common pressurised static equipment in use in industry today:

  • Cylindrical Pressure Vessels
  • Spherical Pressure Vessels
  • Horizontal Pressure Vessels
  • Vertical Pressure Vessels
  • Pressure Piping configurations
  • Power Piping configurations
  • Transport pipelines (carrying water, gas, oil, LPG, LNG or other chemicals/gases)

Safety hazards are associated with pressure containing static equipment, mainly:

  • Impact from the blast of an explosion or release of compressed liquid or gas
  • Whiplash of broken piping/pipelines due to high-pressure release and impact from parts of equipment that fail or any flying debris
  • Leakage and contact with the high pressure / high temperature / poisonous liquid or gas
  • Fire and suffocation resulting from the escape of flammable liquids or gases
  • Pulsation and Vibration

Hydrostatic testing is the method used by industry for the quality control of pressure containing static equipment, thereby minimising associated hazards. A hydrostatic test or hydrotest is a pressure test using water as the test medium. During construction and installation, the equipment must undergo hydrostatic testing before their installation and periodically during commissioning. These hydrostatic tests have been proven to provide double protection against possible hazards. The paper[1] describing two-dimensional FEA (Finite Element Analysis) utilisation method to analyse the plasticity-induced crack closure phenomenon in a cracked pipe under variable amplitude loading conditions, states:

“The application of an overload is a spectacular phenomenon and can be used as an example while testing components working under pressure. This is a destructive control method. If a critical fault exists in the pipeline concerning the proof test, the latter is destroyed. If not (i.e., not broken), the lower than critical defects will see their subsequent propagation by fatigue slowed down or even eliminated due to the effect of overload.”

When the static pressure equipment is manufactured or repaired, a hydrostatic test is generally carried out with freshwater. For offshore pipelines, due to lack of freshwater availability, seawater may be used as an alternative. However, seawater can cause severe corrosive damage to the internal surface of the equipment. Given this, the corrosion behaviour of welded carbon steel in seawater needs to be investigated before using it. Weld metal (WM) is generally more active and susceptible to corrosion than the base metal, and necessary precautions must be taken in line with codes and standards.

2 Relevant Codes and Standards

Following codes provide guidance and recommendations for hydrostatic testing, and the reader is advised to consult the following to acquire adequate knowledge:

  • API Std 510 – Pressure Vessel Inspection Code: Maintenance Inspection, Rating, Repair and Alteration
  • API Std 570 – Piping Inspection Code: Inspection, Repair, Alteration and Rerating of In-Service Piping Systems
  • API Standard 6AR and Spec 6A – Specifications for Wellhead and Christmas Tree
  • API RP 1110 – Pressure Testing of Liquid Petroleum Pipeline
  • ASME BPVS relevant sections
  • ASME B 31.3 – Chemical Plant and Petroleum Refinery Piping
  • ASME B 31.4 – Liquid Transportation Systems for Hydrocarbons, LPG, etc
  • ASME B 31.8 – Gas Transmission Pipelines
  • DOT 49 – Code of Federal Regulations Parts 190 – 199
  • OSHA 29 – Code of Federal Regulations Part 1910 and Parts 1926.351 and 352 (gas pipelines welding)

3 Hydrostatic Test Equipment

Hydrostatic test equipment, whether fitted in lab or skid mounted for mobility to the site, has the following essential components:

  • Primary Pressure Gauge
  • Secondary Pressure Gauge
  • Master Pressure Gauge (with back-up gauge)
  • Hydrostatic pressure connecting piping
  • Primary Pressure Relief Valve
  • Secondary Pressure Relief Valve
  • Blank flanges with a gasket
  • Isolation valves
  • Reciprocating pump – capable of delivering required flow rate and pressure boosting
  • Emergency Shut-off
  • Digital Pressure / Flow monitoring and control system (optional)

For hydrostatic pressure testing of wellheads and offshore pipelines, additional equipment is included, which may be:

  • Chemical injection systems
  • valve actuator control
  • hydraulic cylinder actuation
  • hydraulic power generation system

For normal testing of onshore pipeline, piping and pressure vessels, it is common to accompany hydrostatic test equipment with flushing and purging equipment, which constitutes an air compressor with necessary hoses and connections. Some applications may need special flushing fluids for which air compressors are replaced with suitable equipment to provide the necessary kinetic energy and/or potential energy for flushing fluid.

4 Specific Application of Skid-Mounted Hydrostatic Testing Equipment

Most hydrostatic testing for pressure vessels is carried out in workshops/labs where skid-mounted units are not required. Skid-mounted equipment is mostly needed at plant sites or along transport pipelines during construction or maintenance works. As such, the focus of this article is on hydrostatic testing of piping and pipelines through the use of skid-mounted mobile equipment.

4.1 Code Requirements for Hydrotesting under B31.3 – Understanding Fluid Service

ASME B31.3 requires hydrostatic / leak testing of all piping systems other than Category D Fluid Service. Category D fluid service can be examined for leakage during commissioning operations, and newly fabricated/repaired piping may be put into service without a hydrostatic / leak test. Fluid service is defined in the code as the application of a piping system, considering the combination of fluid properties, operating conditions, and other factors that establish the basis for the design of the piping system. Category D Fluid Service occurs when all of the following apply:

  1. the fluid handled is non-flammable, non-toxic, and not damaging to human tissues as defined in the code
  2. the design gauge pressure does not exceed 150 psi
  3. the design temperature is not greater than 186°C
  4. the fluid temperature caused by anything other than atmospheric conditions is not less than −29°C

4.2 Code Requirements for Hydrostatic Testing under ASME B31.4 & B31.8

Oil and gas pipelines require hydrostatic testing before new commissioning and revalidation of old pipelines to establish fitness for service. The main objectives achieved through hydrotesting of pipelines are to expose defective materials that have missed Non-Destructive Testing, ensure that any remaining defects are insignificant enough to allow operation at design pressures, expose possible leaks and serve as a final validation of the integrity of the pipeline system.

4.3 Uses of Skid-Mounted Hydrostatic Testing Equipment

Buried pipelines

Buried high-pressure oil and gas pipelines span long distances, and it is impossible to have a stationary testing facility for hydrostatic testing. Skid-mounted hydrostatic testing equipment configured with a towing mechanism is used for testing long pipelines for strength by pressurising them to at least 125% of their maximum operating pressure (MAOP) at any point along their length. Since many long-distance transmission pipelines (especially in mountainous terrain with elevation changes) are designed to have a steel hoop stress of 80% of specified minimum yield (SMYS) at MAOP, the steel is stressed to SMYS above during the testing. Mobile skid-mounted hydrostatic test equipment ensures that test sections can be selected to ensure that excessive plastic deformation does not occur.

Leak Testing

Leak testing can be performed by mobile skid-mounted hydrostatic testing equipment by matching changes in the measured pressure in the test section against the theoretical pressure changes calculated from changes in the measured temperature of the test section.

Pneumatic Leak Testing

Code allows for pneumatic testing of pipe segments if the required test gauge pressure does not exceed 100 psi (7 bar). A minimum 1-hr hydrostatic or pneumatic leak test shall be used for piping systems to be operated at hoop stress of 20% or less of the specified minimum yield strength of the pipe. The test pressure shall not be less than 1.25 times the internal design pressure. Mobile skid-mounted hydrostatic testing or pneumatic testing equipment is good to perform leak testing under such situations.

Testing of Fabricated Items

Any localised repaired/refabricated sections such as scraper traps, manifolds, valve assemblies, etc., can be easily hydrostatically tested by mobile skid-mounted equipment all along the pipeline to limits equal to or greater than those required of the completed system. If section length and other parameters allow, these tests may be conducted separately or as part of the completed pipeline system.

Testing After New Construction

Pipeline systems are normally designed to be operated at hoop stress of more than 20% of the specified minimum yield strength of the pipe. Occasionally when pipelines are to be operated at hoop stress of 20% or less of specified minimum yield strength of the pipe, code allows for pipeline operations without a hydrostatic test – pipeline may only be subjected to a leak test by code guidelines. Mobile skid-mounted hydrostatic testing equipment is equally good to perform leak testing under such situations.

Testing Tie-Ins.

For new construction or repairs, there arises a necessity to connect complete the pipeline constituting test sections, connecting piping, and other necessary appurtenances, or to install a pretested replacement section – known as tie-in operation. Code does not specifically require tie-in welds to be tested; tie-in welds and girth welds joining lengths of pretested pipe shall be inspected by radiographic or other accepted non-destructive methods according to code if the completed pipeline is completed not pressure tested after tie-in. Mobile skid-mounted hydrostatic testing equipment may be used where the site engineer advises pressure testing.

Testing of Replacement Components.

Components other than pipe that are being replaced or added to the pipeline system need not be hydrostatically tested if the manufacturer certifies that either each component was hydrostatically tested at the factory or each component was manufactured under a quality control system that ensures each component is at least equal in strength to a prototype that was hydrostatically tested at the factory. Even in such cases tie- in butt welds are subject to the non-destructive tests as per code. Mobile skid-mounted hydrostatic testing equipment may be used where the site engineer advises pressure testing.

Pipeline Integrity Assessments and Repairs

Oil and gas pipelines designed by code should be considered for periodic integrity assessments to ensure trouble-free operations. An integrity assessment and validation require hydrostatic testing of the pipeline sections AND / OR an in-line inspection (ILI) followed by remediation of anomalies indicated by the inspection. Hydrostatic testing provides the highest confidence to the integrity assessment team for a particular section of the suspected pipeline, and mobile hydrotesting equipment facilitates such validation economically.

5 Skid-Mounted Hydrostatic Testing Equipment Use for Off-Shore Pipelines

Safe hydrostatic testing and the development of offshore pipeline hydrostatic test procedures need consideration of all the below-mentioned parameters to the extent that they are significant to the proposed test. The most critical combinations of hydrostatic test and occasional loads, acting concurrently, has to be identified to which the offshore pipeline system may be subjected.

Excluding conditions that may result from occasional incidents, loads that should be considered during offshore hydrostatic testing include:

(a) weight, including (as appropriate) the weight of (1 ) pipe (2) coatings and their absorbed water (3) attachments to the pipe (4) freshwater or seawater used for hydrostatic test

(b) buoyancy

(c) internal and external pressure

(d) thermal expansion and contraction

(e) residual loads

(f) overburden

As per code requirements, Environmental Loads During Hydrostatic Test should be estimated, including those arising due to:

(a) waves

(b) current

(c) wind

(d) tides

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