Short form of ASME Boiler & Pressure Vessel Code.
This is a comprehensive body of regulations pertaining to the manufacture and testing of pressure equipment, e.g., steam boilers, pressure vessels, nuclear components. It is published and continually updated by ASME. A new issue appears every three years. Between issues, amendments and addenda are published. Amendments and interpretations are automatically sent to purchasers. The ASME Code has a modular design, so that it is also possible to purchase individual sections. For "stamp-holders," however, it is necessary to have a complete set of the appropriate literature (see accreditation program).
ATEX is a widely used synonym for the ATEX guidelines of the European Union. The term "ATEX" is derived from the French abbreviation for ATmosphère EXplosive. This directive currently comprises two guidelines pertaining to explosion protection, namely: ATEX Product Directive 94/9/EC and ATEX Workplace Directive 1999/92/EC.
As a general rule, all products manufactured according to "New Approach" directives and brought into the market in the European Community must bear the CE marking. This includes pressure equipment, too. The CE marking is a legally valid conformity symbol, and not, e.g., a quality marking or origin marking.
When a product is marked with the CE symbol, the manufacturer or authorized agent is declaring that the pressure equipment conforms to all requirements of the Pressure Equipment Directive and that corresponding conformity assessment procedures have been carried out.
Pressure Tolerances – Set Pressure, Opening Pressure, and Closing Pressure Tolerance
EC type examination is the procedure (Module B) in which a notified body ascertains and certifies that an example of machinery satisfies the provisions of the Pressure Equipment Directive which apply to it.
Classification (I , II, III, or IV) of the pressure equipment according to the potential hazard.
The classification depends upon the fluid group, the physical state (gaseous, fluid), the max. permissible pressure (PS), the volume, or the nominal width.
The classification determines which conformity assessment procedure the manufacturer must follow.
Before placing pressure equipment on the market, the manufacturer shall subject each item of equipment to one of the conformity assessment procedures described in Annex III.
The conformity assessment procedures to be applied to an item of pressure equipment with a view to affixing the CE marking shall be determined by the category, as defined in Article 9, in which the equipment is classified.
The conformity assessment procedures to be applied for the various categories are as follows:
Module A 1
Module D 1
Module E 1
Modules B1 + D
Modules B1 + F
Modules B + E
Modules B + C1
Modules B + D
Modules B + F
Nominal Size DN
Nominal size (DN) means a numerical designation of size which is common to all components in a piping system other than components indicated by outside diameters or by thread size. It is a convenient round number for reference purposes and is only loosely related to manufacturing dimensions. The nominal size is designated by DN followed by a number.
Safety valves (also: relief valves) protect pressurized rooms or pressure chambers (e.g., steam boilers, compressed gas tanks, pipelines, shipping tanks) against excessive rises in pressure which could damage the connected pressure device. When the set pressure is exceeded, the safety valve opens and diverts a portion of the gases, vapours, or fluids and releases them into the atmosphere or into collection pipes.
The safety valve opens when, due to foreseeable malfunctions, the pressure in a pressurized tank rises more than 10% above the permitted operating pressure. When the safety valve is properly sized, the rise in pressure can be reliably controlled. After the safety valve opens and the excessive pressure is reduced by releasing it into the atmosphere or into collection pipes ("blow down"), the valve is reseated, i.e., it closes again, and the installation can continue operation.
Safety valves are categorized according to the type of back pressure / counterforce they employ to counteract the pressure against which protection is to be provided::
Safety valves for controlling non-toxic media must allow for venting (i.e., release of excess pressure into the atmosphere). Using a lever or a cap which can be loosened and which counteracts the spring force, the valve must be opened at the very latest upon reaching a pressure of 85% of the set pressure. Safety valves should be vented in order to prevent their valve seat from becoming gummed up or corroded in place. In the case of toxic or environmentally hazardous fluids, no use is made of venting. Instead, two safety valves are connected to the two outlet sides of a shuttle or changeover valve and the inlet side is connected with the pressure chamber requiring protection. This arrangement allows the safety valve to be dismantled and removed for checking while the other valve is still in operation. This enables the connected pressurized chamber to continue operation. In the case of larger liquid gas tanks, ammonia tanks, tanks for cryogenic liquefied gases, or tanks in refrigerating systems which are difficult and complicated to empty, the relevant standards require this arrangement.
Safety Valve – Norms
Prior to the implementation of the 2002 Pressure Equipment Directive, safety valves were designed according to the AD-Leaflet A2 (Arbeitsgemeinschaft Druckbehälter, Ausrüstung = working group for pressure vessels and equipment). Safety valves had to be type-tested. The tests were carried out by the TÜV technical inspectorate or similar institutes. The procedures and scope of the type-testing were listed in the VdTÜV Leaflet "Sicherheitsventil 100."
After introduction of Pressure Equipment Directive 97/23/EC, the AD-2000 Leaflet A2 can be employed as a testing basis. Alternatively, other regulations – e.g., the harmonized series of European Directives of EN 4126 – can be applied.
According to the Pressure Equipment Directive, safety valves are defined as pieces of equipment with a safety function. They are classified in the highest group, Category IV, and must be manufactured according to the established manufacturing and testing requirements under the auspices of a notified body.
Safety Valve – Overflow Valve
Overflow valves (also known as "bypass valves") control the pressure in pressurized rooms when non-permitted pressures arise in closed rooms at low pressures. This ensures that no medium enters into the atmosphere and thus that no hazard can occur.
In principle, safety valves can also be employed as overflow valves. However, the prerequisite for this is that the top of the safety valve (the spring housing and the cap) are designed so as to be sufficiently gas-tight vis-à-vis the Earth atmosphere. The valve stem guide to the spring housing is not gas-tight; thus, the pressure in the downstream side prevails in the top of the safety valve, thus allowing it to apply back pressure to the valve head. As a result, the set pressure is dependent upon the back pressure.
Bellows (see figure) can be included to compensate for the back pressure. An expansion bellow is welded to the valve head; the inside shafts of the bellow exactly correspond to the diameter of the valve opening d0. The expansion bellow encompasses the stem guide and is connected to the housing so as to be gas-tight. In this manner, the outside pressure is exerted on the cross-section A0 of the valve head from above. The set pressure of the overflow valve is thus independent of the pressure on the downstream side. The overflow valve is independent of the back pressure and/or is back pressure-compensated.
However, it must be noted that, as the back pressure rises, the momentum of the flow drops. As a result, the valve head will no longer be lifted the full stroke h. The discharge coefficient no longer reaches the value which had been determined with a pressureless downstream chamber. The effective mass flow is correspondingly reduced.