Hydrotest

In Austria, as well as all over Europe, the first and repetition tests of all pressure equipments including steam drums are required for security reasons within fixed time intervals. These repetitive inspections are done differently in the most European countries, but most time these inspections include, according to the European Pressure Equipment Directive" and the specific national law any kind of over-pressurisation (e.g. hydrotest) and visual inside inspection. 

For the repetitive inspections the required hydrotest can only be performed for a limited number of the small cylinders, and even then the drums have to be removed from the line and the cylinders will be supported in defined distances for the weight of the water and the pressurisation. For the new and long cylinders even this is impossible, because they loose due to the additional weight of the water and the over-pressurisation their roundness and balances. Therefore the law in the most countries within and outside of the EU accept as a replacement of the hydrotest an additional application of different NDT methods, which were often done by an ultrasonic measurement of the wall thickness of the cylindrical part and a MT of the flat covers. 

During a hydrotest the additional weight of the water superimposes the normal operational stress by steam pressure. The big advantage of the pneumatic test is now, that it is the best imitation of the normal service condition. 

How it was declared, the new steam drums cannot undergo a hydrotest, because they loose their roundness and balance due to the additional weight of the water, even if the cylindrical part will be supported. 

It was pointed out, that the periodical inspection of the steam drums has been become an absolute must especially under the circumstance, that the economical pressure results in smaller wall thickness, higher steam pressure and higher rotation speed. The conventional periodical inspection (hydrotest and visual inside inspection) is on one hand time consuming and therefore expensive and on the other hand the results of the hydrotest are doubtful and can result in a seriously damage of the roundness and balance of the steam drum. 

Even the replacement of the hydrotest by a NDT method is only sufficient, if you perform much more than it is usual today e.g. a complete ET of the cylindrical part, which requires the possibility to rotate the drum during the shut down and is therefore much more time consuming and expensive. 

Serious stress-corrosion-cracldng failures have occurred when chloride-containing hydrotest water was not promptly removed from stainless-steel systems. Use of potable-quality water and complete draining after test comprise the most reliable solution to this problem. Use of chloride-free water is also helpful, especially when prompt drainage is not feasible. 

If a gas leak test is called for, the test can take place shortly after the hydrotest, especially on the reciprocator cylinder or centrifugal or rotary compressors, where the gas is tested without end seals. 

The first guideline is often referred to as the "two-thirds rule." The basis of this rule is that if the low pressure side is designed for two-thirds of the high pressure side design pressure, the exchanger hydrotest pressure will not be exceeded due to a tube rupture. 

Maximum Allowable Working Preaeure or Design Pressure (Hydrotest at 150)... 

Use materials that are applicable over the full range of operating conditions such as normal, startup, routine shutdown, emergency shutdown, and draining the system. For example, carbon steel may be acceptable for normal operating conditions but may be subject to brittle fracture at low temperatures under abnormal conditions (as in the case of a liquefied gas). Cold water, of less than 60°F, during hydrotest may cause brittle fracture of carbon steel. 

At high points in piping, vent valves are required to remove air for hydrotesting and for purging the system. At low points, drain valves are required to drain liquids out of the system to perform maintenance. Normally, vent and drain valves are 14-in. or i-in. ball valves. 

Industry literature typically cites concern with open air explosions when 4,536 kgs (10,000 lbs.) or more of flammable gas is released, however, open air explosions at lower amounts of materials are not unheard of. When the release quantity is less than 4,536 kgs (10,000 lbs.), a flash fire is usually the result. The resulting fire or explosion damage can cripple a hydrocarbon processing facility. Extreme care must be taken to prevent the release of hydrocarbon from vessels resulting in vapor releases and resultant blast overpressure. Measures such as hydrotesting, weld inspections, pressure control valves, adequate pressure safety valves, etc., should all be prudently applied. 

For piping and pipelines made from materials that may fracture at a pressure lower than the hydrotest pressure when in service, all of the following are required ... 

Any areas which are machined after hydrostatic testing shall be identified on the hydrotest report. 

A567 Surfactant hydrotest SURF HYDRO c 1 1 yes (true) 0 no (false)... 

The additional differential pressure developed at trip speed is normally a momentary excursion to be absorbed by the hydrotest margin. 

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