Marine Engineering Study Materials

During the course of surveys and inspections, both on board ships or offshore units and at manufacturers’ premises, Surveyors may be called upon to witness different types of Pressure Tests, generally known as Hydraulic Testing, Pneumatic Testing, Hydro – Pneumatic Testing and Leak Testing.

Such types of tests are required, by the Rules, Regulations and applicable Standards, to be carried out on pressurized tanks, pressure equipment and pressure systems. Such types of testing are normally required after manufacture (initial testing), at the time of fabrication of components, during construction of ships and after repairs or modifications. These tests are also required as part of a periodical inspection regime for compliance with Class or Statutory requirements, including testing of safety valves or other pressure relieving devices (PRDs).

What are the common types of pressure tests?

Burst Pressure Test

Burst Pressure can be considered as the maximum pressure which a component can endure before a catastrophic failure occurs. It gives an indication of the “factor of safety” employed in the design, particularly when this cannot be easily determined by calculation.

Proof Pressure Test 

Proof Pressure Test is similar to a burst pressure test. It is used to prove that a component or pressure equipment is capable of withstanding a pressure greater than the standard Pressure Test without distortion or failure, based on an agreed (design) or defined (statutory) ‘Factor of Safety’.

Standard Pressure Test

Standard Pressure Test is carried out initially after manufacture, subsequently after modification/repairs and also as a periodic test. The test medium may be hydraulic or pneumatic and test pressure is normally between 1.25 and 1.5 times the design pressure, depending upon the design code used and the age of the equipment or item being pressurized. Standard Pressure test is also normally used to verify pressure relief systems.

Leak (or Leak-proof-ness/Tightness) Test 

As the name implies, is designed to check for leaks in systems, components or pressure retaining parts.

Functional Test 

It is used to check operation of a system including tightness of isolating valves, mechanical joints and functioning of moving parts, if any.

Safety Valve Testing

Safety valves (pressure releasing devices) are tested at the shop floor, following their manufacture, after reassembly following routine maintenance and during surveys of ships in service. The safety valve is required to be checked for lift (popping up) pressure as well as reseat pressure.

What are the factors to be considered before performing pressure test?

1. Purpose
2. Scope
3. Responsibilities and qualification of key Personnel (Test Supervisor, Test Operator, Class Surveyor, Safety Director, etc.)
4. Pressure Test Hazards and their controls (Checklists, Fittings, Pressure Sources, Test Area, Barricades, cordoning and marking of test area and other appropriate means to restrict access at the time of pressurization, during the test process and depressurization.)
5. Test Procedures (Flow diagram and test equipment, Position, specification of test gauges, safety valves and other means of pressure relieving (e.g. U-tube for certain air leak tests), Position of isolating valves and test medium supply line, Sequence of opening and closing vent valves, Test pressure, medium and time for the gradual pressurization / depressurization where applicable, Instrumentation to record timing, pressurization and depressurization when deemed necessary to carry out the test under remote presence of the Surveyor)
6. Pass/Fail Criteria
7. Reporting
8. Reference

What are the difference between Hydraulic and Pneumatic Pressure Testing?

In most cases pressure testing is carried out using water as the test medium, but there may be occasions where a liquid other than water (e.g. hydraulic oil, or kerosene) is used. All such testing is collectively known as hydraulic testing.

There are occasions, on the other hand, where it may be necessary to resort to what is collectively termed pneumatic testing (e.g. using air, steam, nitrogen or other inert gas as the test medium), primarily where the interior of the pressure equipment may be contaminated by water or other liquid, or where the containment system or its supporting structure is not designed to withstand the weight of the volume of liquid required to carry out a hydraulic test.

The safety hazard involved in pneumatic testing is that the stored energy available in the event of a catastrophic failure of the pressurized fluid that could result in injury or death from blast waves, as well as pieces or portions of material that may become projectiles.

In case of hydraulic testing, it is easier to detect both the leakage in the system (because of sudden pressure loss as the liquid is almost incompressible) as well as the location of leakage.

Hydraulic testing, in some cases, also facilitate the use of colored dyes for detecting location of defects.

On the other hand, pneumatic testing can be carried out with much reduced efforts as compared to hydraulic testing.

What are the general precautions related to pressure testing?

In order to reduce the risk associated with the pressure testing, appropriate risk assessment should be carried out considering the test items, equipment, medium, procedure and ambient conditions.

• Necessary personal protective equipment, as applicable, should be worn by the surveyor and the persons conducting the test.
• Test pressure should be applied gradually to avoid shock loading of the item under test.
• Couplings and connections of flexible hoses subjected to the pressure should be adequately secured to prevent injury due to “flailing” in the event of failure.
• Where leakage is detected in joints or fittings, whether on the item under test or on the test equipment itself, pressure should be reduced to atmospheric pressure prior to rectifying the leak.
• No ‘hammer test’ while the system is under pressure.
• Where safety valves are being tested (floated), the venting arrangements should lead away from the work/test area. In case of setting a boiler safety valve, drainage arrangements and escape piping should be particularly examined for blockages, appropriate support and damage.
• The test procedure should specify precautions to safeguard against hazards resulting from the possible expansion of the test medium, during the test. If a pressure test is to be maintained for a period of time, during which the test medium in the system is subject to thermal expansion, precautions should be taken to avoid excessive pressure.

What are the precautions specific to hydraulic pressure testing?

• Confirm that the test medium cannot freeze throughout the period of testing.
• Eliminate all air/gas pockets from the system under test.
• The effect of the weight of the test medium on the item under test, and any supporting structure or foundation needs to be especially considered.
• If liquid other than water is used as a test medium, e.g. kerosene, then hazards specific to that medium should also be considered.

What are the precautions specific to pneumatic pressure testing?

• Because of the potential for high levels of stored energy, the internal volume of any items to be pneumatically tested should be kept to a minimum by isolating certain sections or testing components individually.
• The test procedure should specify a suitably sized restricted zone to protect human life and properties in cases of blast waves and projectiles in the event of a catastrophic failure.
• Local chilling due to filling and emptying of the items under test needs to be controlled to avoid the possibility of local brittle fracture. This can be achieved by maintaining constant flow rates across inlets or exhaust nozzles.

Examples of some pressure systems and equipment?

• Boilers and steam heating systems
• Pressurized process plant and piping
• Compressed air systems (fixed and portable)
• Hydraulic systems such as steering gear, Windlass, V/V actuating system
• Fuel Oil circulating system
• Liquid cargo loading/unloading system
• Heat exchangers and refrigeration plant
• Valves, steam traps and filters
• Pipe work and hoses
• Pressure gauges and level indicators

What are the common hazards associated with the pressure systems?

• Impact from the blast of an explosion or release of compressed gas or liquid
• Impact from parts of equipment that fail or any flying debris
• Contact with the released liquid, gas or steam, including compressed air
• Fire resulting from the escape of flammable liquids or gases
• Failure of flexible hoses, causing unsupported length of hose to whip or snake violently

What are the principal causes of incidents associated with pressure testing?

• Defective equipment and / or system design
• Lack of maintenance of pressure equipment
• Unsafe installed systems of work leading to release of pressure
• Operator error, insufficient training / supervision
• Unsafe / worn out installation
• Inadequate repairs or modifications

What are the factors to be considered while conducting risk assessment for pressure testing?

• The pressure in the system
• The type of media, liquid or gas, and its properties
• The suitability of the equipment and pipe work that contains it
• The age and condition of the equipment
• The complexity and control of its operation
• The prevailing conditions (e.g. a process carried out at high temperature)
• The skills and knowledge of the people who design, manufacture, install, maintain thesystem, carry out the test, and operate the pressure equipment and systems

What is a “written scheme of examination” associated with the pressure system?

A written scheme of examination is a document containing information about selected items of the plant or equipment which form a pressure system, operate under pressure and contain a ‘relevant fluid’. A written Scheme of Examination is required to obtain the Statutory approval of the Pressure System in the case of an industrial application. A written Scheme of Examination normally covers all protective devices and should include every pressure vessel and those parts of pipelines and pipework which, if they fail, may give rise to danger. The written scheme should specify the nature and frequency of examinations and include any special measures that may be needed to prepare a system for a safe examination.

What types of typical pressurised systems might require a written scheme of examination?

• A compressed air receiver and the associated pipework, where the product of the pressure in bars multiplied by the internal capacity in litres of the receiver is equal to or greater than 250 bar litres.
• A fixed high pressure fire-fighting system, using gas smothering media and associated pipework and protective devices.
• A steam boiler and associated pipework and protective devices.
• A gas loaded hydraulic accumulator.
• A vapour compression refrigeration system where the installed power exceeds 25 kW.
• The components of self-contained breathing apparatus sets (excluding the gascontainer).
• A fixed liquefied petroleum gas (LPG) storage system, supplying fuel forheating in a workplace.
• Various regulations, such as Quality Management System (QMS) standards, Safety Management System standards such as IMO International Safety Management System (ISM) Code, require Owners/Users to determine a documented system of maintenance, inspection and testing of all Pressure Systems as per maker’s recommendations and other applicable requirement such as SOLAS, Flag administration regulations and class Rules.

What are the common checks to be carried out while dealing with pressure systems?

• Is the gas or liquid toxic or flammable?
• What are the process pressures and temperatures?
• What are the safe operating limits?
• Is there a set of operating instructions for all of the equipment?
• Have the operators had suitable training on the operating instructions?
• Have the protective devices been set correctly and in good operating condition?
• Audible and visual warning devices are in satisfactory condition.
• Do the fitted safety valves, bursting disc and other releasing systems discharge towards a safe place?
• Has the condition of the pressurized system(s) been found satisfactory?
• No part of the pressure system should be allowed to operate beyond the safe operating limits.
• To check and confirm that limits are not exceeded, protective devices should be correctly specified and, where applicable, adjusted to the correct setting.
• Instrumentation and measuring equipment are properly selected, maintained and calibrated.
• Ensure that the equipment is provided with overpressure protection.

What are the Personal Protective Equipment (PPE) for pressure systems?

• Eye protection – safety glasses / goggles Ear protection, when required.
• Hand Protection
• Safety Shoes
• Helmet
• Body protection/apron, where required Multi-gas meter, when required
• Safety Torch
• Safety Face Shields, where required Additional special PPE, if and as required.