HYDROSTATIC PRESSURE TESTING

Hydrostatic (Hydro) Testing is a process where components such as piping systems, gas cylinders, boilers, and pressure vessels are tested for strength and leaks. Hydro tests are often required after shutdowns and repairs in order to validate that equipment will operate under desired conditions once returned to service.

The hydrostatic test is a common procedure to test piping equipment, like pipes, valves, and fittings (i.e. all products that shall withstand pressure when in operation.

When this type of test is not possible or recommended, the pneumatic test is a suitable alternative.

The hydro test is executed by skilled personnel by using chlorinated water (or other colored fluids) to check the existence of leakages in pressure-containing equipment. Before starting a hydrostatic test, the staff should pay attention to the testing fluid chemistry and how it may impact the equipment.

PNEUMATIC TEST

A pneumatic test uses compressed air or inert gases to test a piece of equipment. Pneumatic tests are generally executed on low-pressure equipment or small parts or when a hydro test cannot be executed for whatever reason.

Furthermore, hydrostatic testing cannot be performed during normal operations and cannot monitor equipment for leaks after the test has been performed. On-stream equipment integrity is best managed by an effective fixed equipment mechanical integrity program.

Hydro testing of pipes, pipelines and vessels is performed to expose defective materials that have missed prior detection, 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 constructed system. ASME B31.3 requires this testing to ensure tightness and strength.

Equipment:

  1. Pressure gauges [1.5 to 4.0 times of test pressure]
  2. Water tanks
  3. Potable tanks
  4. Pumps for water filling and pressure
  5. Pressure gauges and temperature recorder

Although hydrostatic testing is considered to be a nondestructive testing method, equipment can rupture and fail if the inspection exceeds a specified test pressure or if a small crack propagates rapidly.

Hydrostatic testing is a type of pressure test that works by completely filling the component with water, removing the air contained within the unit, and pressurizing the system up to 1.5 times the design pressure limit the of the unit. The pressure is then held for a specific amount of time to visually inspect the system for leaks. Visual inspection can be enhanced by applying either tracer or fluorescent dyes to the liquid to determine where cracks and leaks are originating.

Water Jacket Method

In order to conduct a this method, the vessel is filled with water and loaded it into a sealed chamber (called the test jacket) which is also filled with water. The vessel is then pressurized inside the test jacket for a specified amount of time. This causes the vessel to expand within the test jacket, which results in water being forced out into a glass tube that measures the total expansion. Once the total expansion is recorded, the vessel is depressurized and shrinks to its approximate original size. As the vessel deflates, water flows back into the test jacket.

Sometimes, the vessel does not return to its original size. This second size value is called permanent expansion. The difference between the total expansion and permanent expansion determines whether or not the vessel is fit-for service. Typically the higher the percent expansion, the more likely the vessel will be decommissioned.

Direct Expansion Method

The direct expansion method involves filling a vessel or cylinder with a specified amount of water, pressurizing the system, and measuring the amount of water that is expelled once the pressure is released. The permanent expansion and the total expansion values are determined by recording the amount of water forced into the vessel, the test pressure, and amount of water expelled from the vessel.

Alternative Methods

Some equipment may not be designed to handle the loads required for a pressure test. In these cases, alternative methods such as pneumatic testing should be employed. Pneumatic testing is another type of pressure test that involves pressurizing the vessel with a gas such as air or nitrogen instead of water. However, special caution should be used when performing pneumatic testing as gaseous mediums have the ability to be compressed and contained in larger amounts compared to hydrostatic testing.

Inspection for Quality Assurance and Quality Control

Quality Assurance

The purpose of QA is to inspect the process that manufactures products. In other words, QA is a process that looks to improve the product development in order to eliminate defects in manufactured products. Inspection relating to QA does not typically involve NDT nor mechanical testing but rather, uses qualitative methods to identify gaps and anomalies in the process.

Quality Control

QC is carried out after QA and involves inspection of the product. The QC process involves activities such as NDT to ensure the quality of the product will operate as intended and for a specified number of cycles (if applicable) before failure. Mechanical testing may also be performed on one part per batch or per “X” number of batches. The number of batches is up to the company and individuals involved in the manufacturing process. The purpose of QC is to detect flaws and to determine if the defect is acceptable or rejectable for service.

Codes and Standards

Relating to NDT and Mechanical Testing

Since there are hundreds of standards relating to NDT and mechanical testing, it is best to list the organizations that develop and publish standards pertaining to inspection. The following standards bodies are well known in the inspection community: