Pressure Equipment Directive

The PED is a general law (not a design code) and does not directly consider specific design and safety factors for components operating under creep regimes, although creep is mentioned and regarded as a non-negligible issue in designing.

From: Creep-Resistant Steels , 2008

Terminology

Marc Hellemans , in The Safety Relief Valve Handbook, 2010

3.6.1.5 All cases except fire

The maximum accumulation on the equipment must be no higher than 10% above MAP, even in the case of multiple safety valves.

See PED Annex I, clause 7.3.

Pressure limiting devices, particularly for pressure vessels: The momentary pressure surge referred to in 2.11.2 must be kept to 10% of the maximum allowable pressure.

In the case of multiple valves, only one valve needs to be set at no more than MAP. The others can be set up to MAP +5% (inclusive). In any case, the required capacity must always be relieved at no more than MAP +10%. As taken from the harmonized standard EN 764-7 paragraphs:

To support the PED, many European (EN) standards are now 'harmonised' with the PED. The PED has been introduced as the pressure vessel code in each and every country of the European Union. Likewise, the new EN standards have been introduced and they replace all the local standards. For example, the standard for safety valves, EN 4126, is now the German standard DIN EN 4126, or the French standard NF EN 4126, etc… This harmonised standard has an annex (called 'Annex ZA') that lists the paragraphs which address the requirements of the PED. By following these harmonised standards, one is sure therefore to comply with the PED clauses supported by the paragraphs listed in the Annex ZA. (However, we may need to follow several harmonised standards to cover all the possibilities!)

This Annex ZA is reviewed and approved by the PED experts of the European Commission, and so can be considered as an official interpretation about how to comply with some of the PED requirements.

It is always important to remember that these harmonised standards remain 'standards': they are not compulsory. Only because they support the PED, they give useful guidelines to comply with the PED.

Other of such standards which are useful are the EN 764-7 and EN 12952-10: Water-tube boilers and auxiliary installations – Requirements for safeguards against excessive pressure. Although its Annex ZA shows that it supports the PED clauses 2.11 and 7.3, this standard does not give any indication on the set pressure and the overpressure of the Safety Relief Valves, in which case one can refer back to the above.

According to PED, Pressure Equipment, Part 7 – Safety systems for unfired pressure equipment, its Annex ZA lists the paragraphs which address some of the PED Annex I clauses.

PED Annex I clauses 2.11.2 & 7.3:

Supporting EN 764-7, paragraphs:

6.1.4 Pressure limit: Pressure limiting devices shall be effective at a pressure such that the pressure in the equipment is prevented from exceeding 1.1 times the maximum allowable pressure PS with the exception of external fire (see 7.2).

6.2.2.1 Safety valves shall have a set pressure not exceeding the maximum allowable pressure PS of the equipment, except as permitted in 6.2.2.2 or 6.2.2.3.

6.2.2.2 If the required discharge capacity is provided by more than one safety valve, only one of the valves needs to be set as specified in 6.2.2.1. The additional valve or valves may be set at a pressure not more than 5% in excess of the maximum allowable pressure PS providing the requirements of 6.1.4 are met.

6.2.2.3 Alternatively the safety valve set pressure may be above the maximum allowable pressure PS providing that:

–

the valve(s) can attain the certified capacity at 5% overpressure or less; and

–

the requirements of 6.1.4 are met; and

–

an additional pressure limiter is fitted to ensure that the permitted maximum allowable pressure PS is not exceeded (including peak values) during continuous operation.

PED Annex I clause 2.12:

Supporting EN 764-7 paragraph:

7.2 External fire: Where there is a potential risk for external hazards, such as fire or impact, the pressure equipment shall be protected against them in order to keep the equipment within safe limits.

Note: Protection against over-pressurization during external fire should be based on a detailed thermal response evaluation similar to the risk evaluation. Pressures higher than 1.1 PS can be permitted depending on the damage limitation requirement. Following fire attack the equipment should not be returned to service without a thorough review of its fitness for service.

Exception to the above: It is permitted to have a single valve or all safety valves set higher than MAP (but still not more than MAP +5%) if all valves are certified with a maximum overpressure of 5% or less and that the sizing is done at MAP +10% and there is an additional pressure limiter (can be a control valve, etc., but only installed for this unique purpose) that ensure MAP is never exceeded (from standard EN 764-7 harmonized paragraphs).

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Specifications for creep-resistant steels: Europe

G. Merckling , in Creep-Resistant Steels, 2008

3.2.2 The pressure equipment directive

The PED 9 deals with all relevant safety issues in the design, manufacture and use of components designed to resist stresses induced mainly by pressure. The directive is aimed at simplifying intra-European trade and exchange of pressure handling equipment and harmonising safety prescriptions in order to allow intra-European exchangeability and serviceability of pressure devices. There are many types of boilers, pipelines, furnaces, heat exchangers, incinerators, chemical reactors, etc within Europe, and all have to conform to the directive. Explicitly excluded are all equipment regulated by other directives (e.g. pressure equipment for transportation on rail or road, 'simple pressure equipment', etc), and all types of rotating machines and motors (e.g. combustion engines, turbines and pumps, etc.).

The PED is a general law (not a design code) and does not directly consider specific design and safety factors for components operating under creep regimes, although creep is mentioned and regarded as a non-negligible issue in designing. For dimensioning, including equipment operating in creep regime, PED allows the use of any internationally recognised standard and accepts fracture mechanics approaches and finite element simulation, but for materials, the rules are more strict:

•

All materials must have a guaranteed minimum impact energy at the lowest possible service temperature that the equipment for which they are used may encounter under pressure, e.g. including hydraulic tests.

•

All materials must demonstrate 'sufficient' ductility in service conditions to guarantee 'leak before break'.

•

Materials need to be selected from harmonised standards, i.e. material standards that in their intention and scope consider and include the safety criteria as set forth by the PED. This requirement strongly enhanced and accelerated the production of EN standards incorporating Europe-wide agreed strength values. If materials cannot be selected from EN standards (typically because they come under non-European design codes, e.g. where ASME Boiler and Pressure Vessel Code or API standards are used), they must either pass a general acceptance procedure conducted by an accredited, notified body, or they must undergo a 'particular material appraisal' to check their suitability for the application and their conformity to the PED safety criteria, on a case-by-case basis.

•

A final, and in the creep regime very relevant, PED requirement is that all material producers involved in the production chain of a given component must declare and take responsibility that the material supplied (possibly with provisos relating to particular handling, assembly or design operations) is suitable for the intended application and its operating conditions. Alternatively, in a somewhat milder interpretation, producers must certify that the material is indeed fully compliant with the purchase specifications and the design-relevant material properties so that it can be guaranteed for safe service. For materials and welds in the creep regime this requires the material, shape, fitting and so on producer to guarantee:

–

long term strength of the base material

–

long term strength of the construction welds

–

long term strength of the assembly welds

–

long-term strength of any repair (if allowed by the design code) of any of the items listed above.

The easiest way to comply is to select materials that conform with the harmonised standards, for which long-term strength values are indicated in the material norm. Materials subjected to the 'particular material appraisal' (typically ASTM or API grades, sometimes Japanese materials) have to demonstrate the points listed above, which can be achieved by testing the actual material or by presenting previous data for the same grade and producer. Owing to the expected long service duration of modern plants (generally 200 000–250 000   h, i.e. 25 to 35 years), these demonstrations often become extremely complex for welds, repairs and multiple repairs.

Another route that is applied by some notified bodies and/or end user inspectorates is to qualify material producers as PED-compliant using stringent procedures when they first supply materials under the aegis of this notified body. This first-time qualification should include creep strength verification and demonstration through a reduced creep campaign, which, depending on the notified body/inspectorate, the type of material and the scope of supply, may include 2–3 isotherms made or composed from of three to five points each and durations of generally not less than 10 000   h, and often not less than 30 000   h.

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Inspection of plant and equipment

John Childs IEng, FSOE, FIPlantE , in Plant Engineer's Reference Book (Second Edition), 2002

37.15.1 Pressure

The Pressure Equipment Directive 97/23/EC (PED) covers pressure equipment and assemblies with a maximum allowable pressure above 0.5 bar. The intention of the legislation is to remove technical barriers to trade between community members by harmonizing national laws regarding the design, manufacture, marketing and conformity assessment of pressure equipment.

The range of equipment covered is wide and includes items such as reaction vessels, pressurized storage containers, heat exchangers, shell and water tube boilers, industrial pipework and safety devices. This Directive is transposed into UK law by the Pressure Equipment Regulations 1999 (PER), which is essentially concerned with the supply and putting into service of pressure equipment but not its in-service use.

Pressure systems in-service are covered by the Pressure Systems Safety Regulations 2000 (PSSR) but the scope of PER and that of PSSR are not exactly the same. For example, the scope of PER does not include any fluids at or below 0.5 bar but PSSR includes steam at any pressure.

PER divides pressure equipment into categories based on the degree of danger from the stored energy (on a quantitative basis) and the danger from the release of contained fluid (on a dangerous substances classification basis). PSSR however, is concerned primarily with the release of stored energy and the scalding effects of steam.

PSSR covers design and construction of pressure systems that is not within the scope of PER and also addresses duties placed on the users and owners of systems with regard to maintenance and continued safe operation. It also requires systems containing a relevant fluid to be assessed by a competent person to determine any need for a written scheme of examination for periodic examination of the system so as to detect conditions that might lead to failure.

Relevant fluids, which are defined in PSSR, include compressed gases and fluid mixtures with a vapour pressure above 0.5 bar, pressurized hot water above both its atmospheric boiling point and 0.5 bar and steam at any pressure.

Certain systems and parts are excepted from some or all of PSSR, based on the reduced risk from little stored energy or the particular use where they are covered by other more appropriate legislation. However, owners and users of pressure systems are still likely to have duties under the Health and Safety at Work etc. Act 1974 and the Provision and Use of Work Equipment Regulations 1998.

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General Information

Robert Flitney , in Seals and Sealing Handbook (Sixth Edition), 2014

7.2.1 General

There are many standards that impact on the design of equipment, the type of seal that may be fitted and the materials that may be used. A selection of these is listed here.

7.2.1.1 Pressure Vessels and Equipment

97/23/EC: EU Pressure Equipment Directive.

ASME BPVC Section VIII: Rules for Construction of Pressure Vessels. (Known as the ASME Pressure Vessel Code.)

BS PD 5500: Specification for unfired fusion welded pressure vessels.

API 6 A: Specification for Wellhead and Christmas Tree Equipment.

Standards of the Tubular Exchanger Manufacturers Association (TEMA).

7.2.1.2 Fasteners

BS 4882: Specification for bolting for flanges and pressure containing purposes.

ISO 3506: Mechanical properties of corrosion-resistant stainless-steel fasteners.

ISO 898: Mechanical properties of fasteners made of carbon steel and alloy steel.

ASTM A 193/A 193 M: Standard Specification for Alloy-Steel and Stainless Steel Bolting Materials for High Temperature or High Pressure Service and Other Special Purpose Applications.

7.2.1.3 Limits and Fits

ISO 286: ISO system of limits and fits.

7.2.1.4 Rotary Machines

ISO 9905: Technical specifications for centrifugal pumps – Class I.

ISO 5199: Technical specifications for centrifugal pumps – Class II.

ISO 9908: Technical specifications for centrifugal pumps – Class III.

ISO 13709 and API 610: Centrifugal pumps for petroleum, petrochemical and natural gas industries.

ISO 2858: End-suction centrifugal pumps (rating 16   bar) – Designation, nominal duty point and dimensions.

ASME B73.1: Specification for Horizontal End Suction Centrifugal Pumps for Chemical Process.

ASME B73.2: Specification for Vertical In-Line Centrifugal Pumps for Chemical Process.

ISO 10349: Petroleum, chemical and gas service industries – Centrifugal compressors.

API 617: Axial and Centrifugal Compressors and Expander-Compressors for Petroleum, Chemical and Gas Industry Services.

7.2.1.5 Hygiene

BS6920: Suitability of non-metallic products for use in contact with water intended for human consumption with regards to their effect on the quality of the water.

FDA approval: Food Contact Notifications. (FDA=US Food and Drug Administration.)

7.2.1.6 Fluid Power

ISO 6020: Hydraulic fluid power. Mounting dimensions for single rod cylinders, 16   MPa (160   bar) series.

ISO 6022: Hydraulic fluid power. Mounting dimensions for single rod cylinders, 25   MPa (250   bar) series.

ISO 10762: Hydraulic fluid power. Cylinder mounting dimensions, 10   MPa (100   bar) series.

7.2.1.7 Emissions

ISO 15848: Industrial valves – fugitive emissions – measurement test and qualification procedures.

Part 1: Classification system and qualification procedures for type test of valve assemblies.

Part 2: Production acceptance test of valve assemblies, on–off valves.

Part 3: Production acceptance test of valve assemblies, control valves.

Shell MESC SPE 77/312: Industrial valves: fugitive emission (FE) measurement, classification system, qualification procedures and prototype and production tests of valves.

EPA method 21: Method for determination of volatile organic compound leaks.

VDI Richtlinie 2440: Emissionsminderung Mineralölraffinerien.

API 622: Type Testing of Process Valve Packing for Fugitive Emissions.

API 624: Type Testing of Process Valves Leak Performance Fugitive Emissions and Seat Leakage.

7.2.1.8 Reliability

ISO 14224: Petroleum and natural gas industries – Collection and exchange of reliability and maintenance data for equipment.

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Valve actuation

Karan Sotoodeh , in A Practical Guide to Piping and Valves for the Oil and Gas Industry, 2021

Some technical requirements for actuators

The actuators in Fig. 17.49 have IP67 protection as well as pressure equipment directive (PED) marking. There is a safety integration level (SIL) associated with an actuator. SIL is defined as a relative level of risk reduction provided by safety functions that are used to specify a target level of risk reduction. In simple terms, SIL is a measurement for the safe functioning of an actuator. There are four safety levels, SIL 1 to 4. Each SIL defines a specific probability of failure on demand (PFD).

Fig. 17.49

Fig. 17.49. Tie rods on the cylinder of the actuator.

The actuator housing material could be carbon steel, cast iron, or stainless steel 316. Carbon steel and cast iron have the advantage of having higher mechanical strength than stainless steel 316. The piston rod passing from the middle of the housing should be a hard material such as hard low alloy 42CrMo4, which is 0.42% C, 1.05% Cr, and 0.2% Mo. Chromium or nickel plate gives the required hardness and corrosion resistance to the rod. The bearing around the piston rod is made of bronze with PTFE to minimize the friction and guide the rod. Fig. 17.48 shows the internals of a pneumatic actuator, including the rod.

Fig. 17.48

Fig. 17.48. Actuator internals including the piston rod.

Also, tie rods around the pneumatic/hydraulic cylinder keep and provide sufficient force for the caps on both sides of the cylinder. There can be four to as many as 18 rods, depending on the size of the cylinder. Tie rods are shown in the actuator in Fig. 17.49.

Actuators have lifting lugs for lifting purposes, and they should be used only for lifting the actuators, not for lifting the valves and actuators together. The lifting lugs on the actuators can be removed after installing the actuator on the valve. Fig. 17.50 shows the lifting of a pneumatic actuator from the lifting lug.

Fig. 17.50

Fig. 17.50. Lifting of the pneumatic actuator from the lifting lug.

Fig. 17.51 shows an actuated ball valve. Although the ball valve is smaller than 4″ and not heavy, the actuator lifting lugs are not used for lifting the valve and actuator together. A cloth is wrapped around the body flange of the valve and actuator for lifting.

Fig. 17.51

Fig. 17.51. Lifting of the pneumatic actuated ball valve.

There is a black color position indicator on hydraulic and pneumatic actuators (usually on the top of the limit switch box) which can be green in the open position or red in the closed position. Fig. 17.52 shows a position indicator on the top of the actuator showing that it is in the open position.

Fig. 17.52

Fig. 17.52. Pneumatic actuator position indicator.

Fig. 17.53 shows a type of position indicator on the top of an actuator that is not a good solution. This type of label is not permanent and can be scratched or removed.

Fig. 17.53

Fig. 17.53. Unacceptable position indicator solution.

After the FAT, including a valve and actuator function test (ensuring that the ball is aligned with the valve bore, torque test, etc.), the valve and actuator will be sent to the painting shop. The actuator should be disassembled from the valve and assembled to the valve after the valve painting. Disassembly and assembly of the actuator may disturb the correct positioning of the ball on the seat, which had been tested in advance. Therefore, one solution is to do the actuator function test after painting the valve and assembling the actuator. However, there is usually no need to add an additional ball positioning test since the actuators are fitted on the valves with dual pins between the valve top flange and the actuator bottom flange. The ball stop for the actuator valves is done with a limit switch, and end stoppers for the complete alignment.

An actuator control panel or control system includes different components for delivering, controlling, and cleaning of the fluid to the actuator. Typical components on the control panel are the ball valve, pressure regulator, pressure gauge and filter, check valve plus solenoids and exhaust valves, and a tube for air or oil supply to the actuator. There is a drain connection on the air filter to release the water and humidity in the instrument air to the atmosphere. The ball valve supplies the air or oil to the control panel. The oil temperature should usually be from −   30°C to 100°C, but in special cases could be −   60°C to 140°C. Fig. 17.54 shows control panels, as well as components on the panel, mentioned earlier. The first component on the left with the blue lever (gray in print version) is the ball valve. The next one is component is a pressure regulator and air filter as an integrated component plus the pressure gauge. They are connected to a check valve, shown as a piece of tubing. The check valve is connected to the exhaust and then the solenoid valve (orange, dark gray in print version). There is an exhaust valve between two solenoid valves.

Fig. 17.54

Fig. 17.54. Actuator control panel.

 Courtesy: Biffi.

The pressure regulator is a safety valve installed on the control panel to adjust the pressure to a standard value of 6.9 barg air or 180 barg oil. Overpressure fluid can increase the load on the valve and the actuator. The fluid pressure is measured by the pressure gauge.

Fig. 17.55 shows a test on the control panel in which water and soap are sprayed on the control panel components. Any leakage from the connections and components can be seen in the form of bubbles.

Fig. 17.55

Fig. 17.55. Leakage test of the control panel.

There is a bubble made of glycerine inside the pressure regulator to absorb the shock (Fig. 17.56).

Fig. 17.56

Fig. 17.56

Fig. 17.56. Bubble in the pressure gauge.

A filter separates particles from the air or oil to keep them from entering the solenoids, which can disturb solenoid valve functioning. The check valve prevents fluid from returning from the actuator when the solenoid valve closes (on the piston force). The cracking pressure of the check valve is 0.1 barg. Exhaust releases the oil or air in the fail mode away from the operator. All the components (e.g., integrated filter and air regulator, pressure gauge, etc.) on the control panel are usually made of SS316.

A tube between the actuator and the control panel supplies the oil or air to the actuator (Fig. 17.57). There is no tube in a compact control panel, to reduce the chance of corrosion and to reduce space requirements. In a compact control panel, the components are attached together like blocks.

Fig. 17.57

Fig. 17.57. Tube between the valve and actuator.

The supply tube size is usually ¼″ or ½″, or rarely ¾″ or 1″.

Pneumatic and hydraulic actuators include a solenoid function test (opening and closing) and a leak test, end stopper leak test, air or oil supply tube leak test, actuator function test (timing the opening and closing), and a limit switch test to rotate 90°. Fig. 17.58 shows the pneumatic actuator function test.

Fig. 17.58

Fig. 17.58. Function test on the pneumatic actuators.

The control panel can be installed directly on the actuator (Fig. 17.59) or installed separately. Fig. 17.60 shows testing the control panel separately from the actuator since it will not be mounted directly on the actuator for operation.

Fig. 17.59

Fig. 17.59. Control panel installed on the actuator.

 Courtesy: Biffi.

Fig. 17.60

Fig. 17.60

Fig. 17.60. Testing the control panel separately from the actuator.

 Courtesy: Biffi.

The control panel can be low flow or high flow depending on the flow capacity required to open or close the actuator within a certain amount of time. A shorter time of closing or opening means that a higher flow capacity is required for the actuator. The flow capacity requirements may affect the supply tube size as well as the cost. There is no specific rule for high or low flow control panel based on material, size, and rating. The flow of the control panel can be selected based on the required opening or closing time. High and low flow control panels are the same size, but a low flow control panel has smaller size components. The size of the control panel is the same regardless of whether it has one, two, or three solenoid valves. A high flow control panel has a booster and flow regulator, but a low flow control panel does not.

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Installation and Construction

G.F. Hundy , ... T.C. Welch , in Refrigeration, Air Conditioning and Heat Pumps (Fifth Edition), 2016

11.3 Pressure safety and containment

Refrigeration systems contain pressurised fluid and there are certain safety standards and legal requirements that must be adhered to. Under the European Pressure Equipment Directive (PED) and the UK Pressure Equipment Regulations the main duties are placed on the user/owner of the system. They are a clear and practical means of legislating for safe practices in refrigeration. Responsible contractors and users will always use such safe procedures. In addition to the regulations themselves the HSE has published 'Safety of pressure systems – Approved Code of Practice' which is a clear and helpful. The regulations apply to vapour compression refrigeration systems incorporating compressor drive motors, including standby compressor motors, having a total installed power exceeding 25 kW.

Factory-built equipment will be constructed to the relevant standards and will be pressure-tested for safety and leaks prior to shipment. In cases of doubt, a test certificate should be requested for all such items. Under the PED, vessels, including compressors, are categorised, depending on the refrigerant and volume. Those falling into certain categories will be CE marked and for smaller ones, not categorised, a statement of sound engineering practice can be obtained from the manufacturer.

It is necessary to hold a Safe Handling of Refrigerants Certificate to work with refrigerants. This can be obtained through short training courses. Maintenance engineers must keep themselves updated on safety procedures and training requirements.

Site-erected pipework, once complete, must be pressure tested for safety and leak tightness. The pressure test should be carried out in accordance with the current safety standard BS EN378. The test pressure requirement is dependent on the category under the PED 97/23/EC, currently between 1.1 and 1.43 times the maximum allowable pressure, PS. The Institute of Refrigeration's Codes of Practice provide guidance.

Factory-built components and pressure vessels which have already undergone test should not be retested unless they form part of the circuit which cannot be isolated, when the test pressure must not exceed the original figure. Site hydraulic testing is considered unnecessary, owing to the extreme difficulty of removing the test fluid afterwards. However, it must always be appreciated that site testing with gases is a potentially dangerous process, and must be governed by considerations of safety. In particular, personnel should be evacuated from the area and test personnel themselves be protected from the blast which would occur if a pressure vessel exploded.

Systems should be pressure tested with dry (oxygen-free) nitrogen (OFN) or high-purity nitrogen. Nitrogen is used from standard cylinders, supplied at about 200 bar, and a proper reducing valve must always be employed to get the test pressure required. A separate gauge is used to check the test pressure, since that on the reducing valve will be affected by the gas flow.

If the high side is being tested, the low side should be vented to the atmosphere, in case there is any leakage between them that could bring excessive pressure onto the low side. It may be necessary to remove relief valves. Other valves within the circuit will have to be open or closed as necessary to obtain the test pressure. Servo-operated valves will not open on a 'dead' circuit, and must be opened mechanically.

The test pressure should be maintained for at least 15 min. If the pressure has not significantly reduced in that period the nitrogen is slowly vented until the pressure in the system has reduced to the pressure test (leak test) pressure. To determine if leaks exist, new equipment may be left pressurised at leak test pressure overnight or for longer periods, and any pressure drop noted. Pressure will change with temperature, and so this must be taken into consideration. Another option is to leave the equipment under vacuum for a period. A traditional way of finding leaks is to use soapy water. Many people discount it, but for finding leaks it is possibly the most effective method. It can be used to find very small leaks. All leaks must be fixed before equipment is put into service. Electronic leak detectors are should be checked for their suitability for different refrigerants. It is important to use a detector of sufficient sensitivity; it should be capable of detecting a leak of 5 g/year.

Reference should be made to the codes of practice and guidance notes published by the Institute of Refrigeration (see Bibliography). Leak testing is covered in chapter: Commissioning and Maintenance.

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Codes and Standards

Marc Hellemans , in The Safety Relief Valve Handbook, 2010

4.2.1 National Board approval

ASME in itself does not approve nor certify the safety devices; this is done by the National Board (NB). The NB certifies the valve's capacity and verifies the valve's compliance with the ASME code. The NB maintains and publishes the 'red book' – NB-18, which contains all manufacturers and products approved according to ASME. It also publishes the true flow coefficients as measured and approved by them. So, when in doubt, one can always consult the NB-18 document at the website: http://www.nationalboard.org/SiteDocuments/NB18/PDFs/NB18ToC.pdf

NB's method for certifying the capacity (and flow coefficient) is very similar to that of other notified bodies such as PED.

■

At an ASME-approved flow facility, a total of 9 valves of a particular valve design or range are flow tested at 10% overpressure above set pressure. They select three valve sizes and three set pressures. This way they establish the KD factor of each test valve, considering the flow conditions and each measured orifice area.

■

KD is established by dividing the flow of the test valve by the flow of a 'perfect nozzle'.

■

Then they calculate the average KD for the 9 test valves.

■

No test valve(s) KD can deviate more than +/ − 5% from that calculated average.

■

Then the ASME K flow factor of the valve is established by de-rating the KD factor with 10%: K = Average KD × 0.9.

■

The actual KD , K and A (orifice area) values are published for all code stamped relief valves in the NB-18 (red book).

Ever since this 10% de-rating rule was established in 1962, it has been a cause of confusion. Manufacturers' catalogues do not always show the same coefficients as those published in the red book, making it extremely confusing for end users, who do not know which coefficients to use without verifying the NB-18 each time for every supplier and each valve range.

To eliminate the need for new capacity tables, revised catalogues and so on, the ASME/NB allowed manufacturers to use the KD figures as K values on the condition that the relief valve flow areas would be increased by at least 10%. The manufacturer can show any K and any A (orifice area) as long as their advertised KA is equal or smaller to the certified ones.

Of course, the capacity (W) of the valve is directly proportional to the KA.

W = C K A P M T Z

Since 1962 most SRV manufacturers have overstated their K values and understated their A values.

For example a perfect nozzle has a KD = 1 and a K = 0.9. Yet some manufacturers show their K = 0.975 or 0.95, which theoretically is impossible. To compensate for this, the manufacturer must furnish actual SRV orifice areas larger than those published in their brochure.

Example: Gas service – J orifice (API A = 8.303 cm2)

National Board Vendor catalogue
K A (cm2) KA K A (cm2) KA
Consolidated spring valve 0.855 3.774 3.227 0.95 3.269 3.106
AGCO pilot valve 0.830 3.393 2.816 0.830 3.393 2.816

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Codes, Standards and Documentation

Steven E. Hughes , in A Quick Guide to Welding and Weld Inspection, 2009

European harmonised standards

European harmonised standards are those that are considered to satisfy the relevant essential safety requirements (ESRs) specified in European product directives such as the Pressure Equipment Directive (PED). Harmonised standards contain an appendix Z, which defines which directives and ESRs the standard meets. The idea of building to a harmonised standard is that it gives a 'presumption of conformity' with any relevant European directive. Products demonstrate their compliance with relevant directives by having a CE mark affixed by the manufacturer. Not every European standard will necessarily be a harmonised standard but it is worth noting that when a European standard is released it replaces the relevant competing national standard from all the countries making up the EU.

If pressure equipment is manufactured to a non-harmonised standard it does not automatically have 'a presumption of conformity' with the PED and will therefore need to show compliance by other means. This usually entails utilising the services of a 'notified body' (NoBo) to prove conformance. The notified body is an organisation that has been notified to the EU as having sufficient knowledge and experience to be able to ascertain compliance with the directive.

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Quality, inspection and testing

In Handbook of Valves and Actuators, 2007

15.4 Guidelines for testing and documentation

It is very difficult to generalise about quality requirements for such diverse products as valves, but Tables 15.2 and 15.3 are given for guidance. If necessary, the requirements of the PED must be fulfilled.

Table 15.2. Guidelines for valve quality requirements

Quality grade DN15 DN50 DN100 DN200 DN300 DN500 DN750 DN 1 000
Grade D 320 152 122 85 66 42 24 17
Grade C 448 220 117 135 90 67 37 26
Grade B 584 362 290 246 182 119 57 40
Grade A 920 472 377 255 249 238 88 60

Design pressures shown in barg

Table 15.3. Description of quality grades

Quality Grade Inspection requirements
Grade D Hydrotest certificate
Grade C Hydrotest certificate
Grade C Certificate of compliance, EN 10204-2.2
Grade B Hydrotest certificate
Grade B Copy of sizing calculations
Grade B Copy of Classification Body Approval certificate
Grade B Surface inspection of pressure containing parts and welds, magnetic particle or dye penetrant
Grade B Hardness certificate for HAZ and welds for NACE valves
Grade B Material certificates for pressure containing and pressure retaining parts, EN 10204-3.1.A
Grade A Hydrotest certificate
Grade A Copy of sizing calculations
Grade A Copy of pressure containment calculations
Grade A Validated Copy of Classification Body Approval certificate
Grade A Surface inspection of pressure containing parts and welds, magnetic particle or dye penetrant
Grade A Confirmation of material integrity for pressure containing parts and welds, radiography or ultrasonic inspection
Grade A Hardness certificate for pressure containing parts, HAZ and welds for NACE valves
Grade A Material certificates for pressure containing and pressure retaining parts, EN 10204-3.1.B

Table 15.3 indicates a design pressure associated with a valve diameter. The Table is based on carbon steel valves; the pressures indicated should be adjusted in proportion to the strength of the ductile material used. Valves in cast iron would use pressures approximately 35% of the values shown.

Some national governments severely restrict the use of cast iron; pressures in excess of 200 barg may not be permissible irrespective of the valve size. The quality requirements refer to pressure containing and pressure retaining parts. Other tests may be appropriate depending upon the application. The requirements shown are based on a "safe" fluid; hazardous fluids should use requirements one or two levels higher.

Valves for use with hazardous fluids may require additional tests for material, joint and seal integrity. Pressurised tests with air, methane or helium should be considered. Hygienic valves should have the relevant surface finishes checked; the purchaser should ask the manufacturer about his standard procedure.

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Energy storage for district energy systems

P.D. Thomsen , P.M. Overbye , in Advanced District Heating and Cooling (DHC) Systems, 2016

7.9.4 General design requirements (Europe)

In Europe, there is a distinction in the design criteria when designing hot water storage. If the storage unit is nonpressurized, it should be designed according to the standard EN14015. When designing pressurized tanks, the Pressure Equipment Directive (PED) applies.

Calculating the thickness of the tank shell (including roof and bottom plating), the designer should always include a minimum of 1   mm for corrosion allowance. Furthermore, it should be proven that the tank can withstand the transition zone passing any level in the tank twice a day, all year round for its entire technical lifetime (~   45 years), without succumbing to failure due to fatigue.

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