The oil, gas, water, food and beverage, aerospace, power generation, construction and pharmaceutical industries all fabricate many thousands of metres of tubes and pipes every year and all joints need to be tested for leak tightness before release for use. This is particularly so in the nuclear sector where potential release of toxic compounds presents a health hazard. It’s also a significant requirement in the aerospace industry where leaks could endanger life.
Hydrostatic testing is the safest and most common method employed for testing pipes and pressure vessels and this is normally undertaken using water. Pneumatic testing using compressed inert gas or air may be used, but only under carefully controlled conditions. Failure during testing with water releases only nominal energy because water is almost incompressible. Escape of gas during pneumatic procedures can be dangerous because it can result in the sudden release of very large amounts of energy.
In principle, tests are simple operations. The pipe or vessel joint being examined is isolated and the test medium is introduced and then pressurised. Leaks are detected either by measuring pressure fall or by examination of the surface for visual evidence.
Selecting the optimum sealing technique prior to testing requires a sound knowledge of available products. The operator needs to consider tube or pipe diameter, test pressure and possible contamination by the sealing equipment.
Products for sealing fall generally into two categories: mechanically expandable plugs and inflatable dams. Routinely available products cover diameters between 12 and 3000 mm but special versions have been developed to accommodate testing of smaller and larger pipes. Access for pressurising is through a standard fitting in all plugs and dams.
Plugs are available from manufacturers such as Huntingdon Fusion Techniques with nylon, steel and aluminium bodies. Sandwiched between each body is a flexible seal that can be expanded by applying a radial force through a manually operated machine screw/bar on the shaft. These seals are available in a variety of materials including nitrile, Viton, natural rubber and silicone.
Specially braced plugs and double disc plugs are available for use at higher pressures.
|Fig 1. A comprehensive choice of nylon bodied plugs is available covering the range 12 to 150 mm diameter. They can even be made to meet customer colour preferences.
|Fig 2. Aluminium Test Plugs are heavy duty and are available in the size range 38 to 900mm. These are suitable for arduous duties such as immersion in chemicals and/or higher temperatures.
|Fig 3. Available in the size range 38 to 1800 mm Steel Test Plugs are typically suitable for more demanding applications such as long immersion in water, for use at higher temperatures or for contact with acidic or alkaline environments for which the nylon and aluminium plugs may be unsuitable.
As the diameter of expandable plugs increases, so does the weight. For some applications where these plugs may be useful, they often become too heavy to handle with safety (the weight of an 1800 mm diameter steel plug is 178 Kg). The solution is to use inflatable dams – these are available to accommodate pipe diameters between 35 and 3000 mm.
Manufacturers such as Huntingdon Fusion Techniques have a range of dams, each having a wide degree of flexibility in diameter. The dams are capable of withstanding a temperature range between -40 to +70ºC – (up to 300⁰C with heat-resistant covers).
These devices can be inflated quickly to sealing and working pressure and are resistant to most hydrocarbon gases and fluids. The dams are manufactured from a variety of materials depending on the application.
Generally, the length of inflatable dams ensures that the sealing area is very large compared with expanding plugs and also ensures that tilting of such mechanical plugs, once inside the pipe, does not occur.
Inflation is carried out using a compressor or foot pump through an integrated valve system. An optional by-pass facility allows for admission of liquids or gases to the sealed volume following inflation. The by-pass is also used for draining after testing.
The dams can also be used to prevent build-up of debris and ingress of unwanted material or animals.
|Fig 4. General purpose inflatable dams can accommodate the size range between 25 and 2900 mm. They are available in cylindrical and spherical forms.
|Fig 5. Special purpose dams are available for service in the petrochemical industries where resistance to a wide range of chemicals is required. Sizes range from 25 to 900 mm.
|Fig. 6. Inflatable solid rubber plugs and stoppers offer a large length to diameter ratio and are particularly useful where sealing against irregular surfaces is required. They can be used to seal pipes between 38 and 2000 mm diameter.
|Fig 7. Low Profile (Pancake style) Stoppers are available when insufficient length of pipe is available for the general purpose cylindrical and spherical and solid rubber dams. These are available in sizes from 152 to 2235 mm and are heat resistant up to 90°C:
BS EN 1507:2006. Ventilation for buildings. Sheet metal air ducts with rectangular section. Requirements for strength and leakage.
BS EN 8301:1985 BS 8301:1985. Code of practice for building drainage.
Detailed technical information is available in a publication Inflatable Rubber Plugs from Huntingdon Fusion Techniques.
Huntingdon Fusion Techniques. Technical notes. Expandable Plugs (TN 1) and inflatable stoppers (TN 2).
By Michael Fletcher, PhD. Metallurgy
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