blowdown ring in safety valve manufacturer

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Unless you are dealing with a Power Boiler, chances are you have a one ring design safety-relief valve. bill_bill_2000"s commnets were germain to a two ring design Power Boiler Safety Valve. On compressible fluids, the adjusting ring in a one ring design Safety-Relief Valve, must perform three functions. First it needs to be close enough to the disc to direct flow against the disc & disc holder in order to create a reaction force which combines with the inlet force to overcome spring force and cause the valve to pop open. Second, it must be far enough from the disc to allow fluid an escape path so that the valve does not remain open below normal system operating pressure, typically 10% below set pressure. Long blowdown can cause other problems in the system. Thirdly, the ring provides a cushioning effect as the disc reseats. Acting like a retro-rocket, the reaction forces keep the disc from slamming into the nozzle and damaging the seating surfaces.

Blowdown Ring is a specific Part Nomenclature for Farris because it controls reseat. It is referred to as a Nozzle Ring by Crosby for its location (threaded onto the o.d. of the nozzle. Dresser calls it an Adjusting Ring and Knukle refers to it as a Warn Ring. Warn is an older term for simmer. The PRV simmers just prior to pop, therefore simmer is also called warn. SO, 4 major manufacturers with 4 different trems for the same part. The blowdown ring is an important part of PRV Maintenance and Testing. It is what sets PRV repair apart from other mechanical disciplines. DIsassmbly, inspection, machining, lapping and reassembly are common to all valves, pumps, etc. But Blowdown Ring Adjustment is unique to Pressure Relief Valves. Many PRV manufactuerers give ring setting positions in their maintenance manuals. Liquid Settings are typically set very close to the disc during reassembly and require no further adjustment during testing. However, for air/gas/vapor service, the Manufaturers Settings will usually yield long blowdown, because short blowdown may result in insufficient capacity and in a two ring design in chatter. You do not typically see ring setting induced chatter in a one ring design. Most of the time, chatter is a result of an installation problem, i.e. reduced inlet piping, extremely long inlet piping, excessive pressure drop at the PRV inlet. I hope this is helpful

A spring-loaded relief valve can be thought of as a spring /mass system which is why relief valves chatter. Researchers have found significant differences in the stability of relief valves based on the design of their internals. One recent study found that with 6 feet of inlet piping, valves from Manufacturer X were stable in 50% of the tests while valves from Manufacturer Z where stable in 100% of these tests.¹ Smith & Burgess Laboratory research has confirmed these findings. However, relief systems designers tend to downplay (if not ignore) the importance of the mechanical design of relief valves which is important to stability. Therefore, this article discusses the fundamentals of the design parameters for the internals of a relief valve. The intent is to provide design considerations and general operation information for use by relief systems designers, specifically assisting with the understanding of the effects of valve design on stability.

Modern relief valves are wonderfully modular.The internal parts for a relief valve (valve disc,disc holder, blowdown ringandspring) can be interchanged for ones with a different design to customize valve performance based on the application, fluid service, and set pressure.Valve disccan be metal-to-metal or soft seats. Soft seat designs use an elastomer to create a better seal between thevalvediscand thenozzle. Relief valves with elastomer seats have limitations and can only be used in certain applications.Disc holdersare generally designed to allow thevalve discto float which provides an angular movement that reduces seat leakage from minor misalignments (ensuring that thevalve dischas 360 degrees of contact with thenozzle). Thedisc holderoutside diameter, shape and thickness plays an important role in determining the valve performance by defining the shape of thehuddling chamber. Thehuddling chambercan also be defined by theblowdown ring(s). Thering(s)can also be swapped to different sized and shapedringsto adjust performance based on the expected relief fluid.Springsare selected to keep the valve closed and must fit inside thevalve bonnet. The force thespringexerts is an important design criteria for a relief device and varies depending on the relief fluid, valve size and set pressure.

Spring loaded relief valves are known as "pop action" relief valves as they typically pop open at their set pressure. Initially, the pressure differential across thevalve discthat creates the force to over come the spring force and open the valve.The pop action occurs because mosthuddling chambersare designed with an area that is approximately 10%-30% larger than thevalve seat(as thedisc holderis bigger than thevalve disc). Once the pressure under the seat is enough to lift thevalve discoff thenozzle, there is a step change in the upward forces on thespringand the valve "pops" open. The shape of thehuddling chamber(created by the shape and size of thedisc holder), the position and shape of theblowdown ring, and the characteristics of the fluid being relieved together determine the initial opening force and the initial lift of the valve.

Blowdown ringsare adjustable rings with a design shape that modifies the effluent flow path andhuddling chamberbased on the position. For process valves, a singleblowdown ringis typically threaded onto thenozzleand can be adjusted vertically up or down. Manufacturers will specify a recommended position relative to contact with thevalve disc. The position of theblowdown ringis fixed with a locking screw. The position of theblowdown ringchanges the blowdown (or reseat) pressure. For valves with a singleblowdown ring, the closer theblowdown ringis to thenozzle, the lower the pressure in the system will need to be for the valve to close (more blowdown). Other relief valves have multipleblowdown rings. Each manufacturer designs a uniqueblowdown ringto compliment other aspects of the relief valve design. Smith & Burgess" testing confirms that position and design ofblowdown ring(s)affects valve stability.

Relief Valve manufactures generally select aspringthat is designed for the set pressure of the valve. Thespringthat is selected will have a pressure range that thespringcan be applied. In many cases, there may be more than onespringthat can be used with each relief valve each having a different spring constant. The stifferspringmay have a range that is higher than the softerspringbut still meet the overall requirements for set pressure. The selection of thespringwill affect stability as the specific spring influences the natural frequency of the valve and can also affect the blowdown.

As soon as mankind was able to boil water to create steam, the necessity of the safety device became evident. As long as 2000 years ago, the Chinese were using cauldrons with hinged lids to allow (relatively) safer production of steam. At the beginning of the 14th century, chemists used conical plugs and later, compressed springs to act as safety devices on pressurised vessels.

Early in the 19th century, boiler explosions on ships and locomotives frequently resulted from faulty safety devices, which led to the development of the first safety relief valves.

In 1848, Charles Retchie invented the accumulation chamber, which increases the compression surface within the safety valve allowing it to open rapidly within a narrow overpressure margin.

Today, most steam users are compelled by local health and safety regulations to ensure that their plant and processes incorporate safety devices and precautions, which ensure that dangerous conditions are prevented.

The principle type of device used to prevent overpressure in plant is the safety or safety relief valve. The safety valve operates by releasing a volume of fluid from within the plant when a predetermined maximum pressure is reached, thereby reducing the excess pressure in a safe manner. As the safety valve may be the only remaining device to prevent catastrophic failure under overpressure conditions, it is important that any such device is capable of operating at all times and under all possible conditions.

Safety valves should be installed wherever the maximum allowable working pressure (MAWP) of a system or pressure-containing vessel is likely to be exceeded. In steam systems, safety valves are typically used for boiler overpressure protection and other applications such as downstream of pressure reducing controls. Although their primary role is for safety, safety valves are also used in process operations to prevent product damage due to excess pressure. Pressure excess can be generated in a number of different situations, including:

The terms ‘safety valve’ and ‘safety relief valve’ are generic terms to describe many varieties of pressure relief devices that are designed to prevent excessive internal fluid pressure build-up. A wide range of different valves is available for many different applications and performance criteria.

In most national standards, specific definitions are given for the terms associated with safety and safety relief valves. There are several notable differences between the terminology used in the USA and Europe. One of the most important differences is that a valve referred to as a ‘safety valve’ in Europe is referred to as a ‘safety relief valve’ or ‘pressure relief valve’ in the USA. In addition, the term ‘safety valve’ in the USA generally refers specifically to the full-lift type of safety valve used in Europe.

Pressure relief valve- A spring-loaded pressure relief valve which is designed to open to relieve excess pressure and to reclose and prevent the further flow of fluid after normal conditions have been restored. It is characterised by a rapid-opening ‘pop’ action or by opening in a manner generally proportional to the increase in pressure over the opening pressure. It may be used for either compressible or incompressible fluids, depending on design, adjustment, or application.

Safety valves are primarily used with compressible gases and in particular for steam and air services. However, they can also be used for process type applications where they may be needed to protect the plant or to prevent spoilage of the product being processed.

Relief valve - A pressure relief device actuated by inlet static pressure having a gradual lift generally proportional to the increase in pressure over opening pressure.

Relief valves are commonly used in liquid systems, especially for lower capacities and thermal expansion duty. They can also be used on pumped systems as pressure overspill devices.

Safety relief valve - A pressure relief valve characterised by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening pressure, depending on the application, and which may be used either for liquid or compressible fluid.

In general, the safety relief valve will perform as a safety valve when used in a compressible gas system, but it will open in proportion to the overpressure when used in liquid systems, as would a relief valve.

Safety valve- A valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a quantity of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent further flow of fluid after normal pressure conditions of service have been restored.

The pressure below the valve must increase above the set pressurebefore the safety valve reaches a noticeable lift. As a result of the restriction of flow between the disc and the adjusting ring, pressure builds up in the huddling chamber. The pressure now acts on an enlarged disc area. This increases the force Fp so that the additional spring force required to further compress the spring is overcome. The valve will open rapidly with a "pop", in most cases to its full lift.

Overpressure is the pressure increase above the set pressurenecessary for the safety valve to achieve full lift and capacity. The overpressure is usually expressed as a percentage of the set pressure. Codes and standards provide limits for the maximum overpressure. A typical value is 10%, ranging between 3% and 21% depending on the code and application.

A fire-tube boiler can be fitted with one or more safety valves on the top of its shell, with each set to open when the boiler reaches its design pressure. Noisolation valvesor restrictions should be integrated between the safety valve(s) and boiler. If the valves are not installed directly onto the boiler shell, the pipework connecting the valves to the boiler must be kept clear of blockagesand water, and this must be confirmed by periodic testing.

Once a safety valve opens, steam is discharged via the exhaust pipe. Exhaust pipes must be designed to encounter as few bends as possible, be as short as possible, to have no reduction in pipe section (no internal pipe diameter reduction), and should lead to asafe point of discharge(typically outside the boiler house).

Water must be drained from the safety valve or exhaust pipework via a drainpipe. Drainpipes may be connected to holes drilled into the lowest section of the exhaust pipework, or, directly to drain holes in the safety valve body; these drains are not to be confused with the blowdown ring locking bolt, if one is fitted.

Where two safety valves are fitted, it is common that one is set just belowthe boiler’s design pressure. It is vital that each safety valve permits the full flow of steam produced when the boiler is operating at maximum capacity i.e. when the boiler is producing the maximum amount of steam it can possibly produce. If safety valves are sized correctly, a boiler can be firing at full capacity without the steam pressure exceeding design limits (because the safety valve(s) relieves pressure at a faster rate than it is accumulated).

There are various types of safety valve, including high lift and improved high lift valves, which use the force of escaping steam to open a winged valve plug to achieve greater steam flow rates. In addition to this, some valves integrate a pistonat the bottom of the spring chamber. The piston has a larger surface area than the valve plug, which leads to the valve opening with a definitive ‘pop’ sound.

Some boiler safety valves include a blowdown ring. The blowdown ring can raise or lower the valve seat ring and is used to control the amount of blowdown through the valve. This ring is locked by a bolt that protrudes through the valve and into the adjusting ring segments.

Boiler safety valves should be fitted with an easing gear (looks like a handle), used, when necessary, to rapidly release boiler pressure. Easing gears can also be used for testing a safety valve, ensuring the spindle has freedom of movement and that the valve operating mechanism functions as intended. Easing gear testing is often not conducted due to operators having difficulty with the valves resealing, but this is generally only the case with valves that are not tested often enough. Actuating the easing gear several times is often all it takes to dislodge debris from the sealing area and allow the valve to seal again. For safe operation, the easing gear handle is usually connected via steel cables to an area neighbouring the boiler.

Like pressure gauges, all safety valves should be stripped, inspected, and calibrated, at least once a year; maintenance usually occurs during statutory inspections. Calibration of each valve should be conducted by a competent person, and any valve adjustment (including the blowdown ring) should be approved and sealed by the authorised inspector. After testing and calibration, all valves should be correctly marked, suitable certificates issued, and accurate records maintained.

An accumulation test can be conducted to ensure a safety valve can relieve over-pressure steam when the boiler burner is operating at maximum capacity. Accumulation testing of safety valves must be repeated after any alterations are made to the boiler e.g. replacement of a safety valve, fuel change, or changes to the control system. If, during an accumulation test, boiler pressure rises by more than 10% of its design pressure, the test must be aborted. Before the boiler is re-tested, amendments must be made to either the safety valve relieving capacity, thesafety valve exhaust pipework, or the boiler’s steaming capacity, to ensure the 10% limit is never exceeded.

Reliefand safetyvalves prevent equipment damage by relieving over-pressurisation of fluid systems. The main difference between a relief valve and a safety valve is the extent of opening at the set-point pressure.

A relief valve gradually opens as the inlet pressure increases above the set-point. A relief valve opens only as necessary to relieve the over-pressure condition. Relief valves are typically used for liquid systems.

A safety valve rapidly‘pops’ fully openas soon as the pressure setting is reached and will stay fully open until the pressure drops below the reset pressure. The reset pressure is lower than the actuating set-point pressure. The difference between the actuating pressure set-point, and the pressure at which the safety valve resets, is called blowdown. Safety valves are typically used for gas or vapour systems.

A safety relief valve may open fully, or proportionally, once the pressure setting is reached. SRVs may be used for any fluid system (gas, liquid, or vapour).

In 1986, the NRC issued the Information Notice (IN) 86-05 "Main Steam Safety Valve test failures and ring setting adjustments". Shortly after this IN was issued, the Code was revised to require that a full flow test has to be performed on each CL.2 MSSV by the manufacturer to verify that the valve was adjusted so that it would reach full lift and thus full relieving capacity and would re-close at a pressure as specified in the valve Design Specification. In response to the concern discussed in the IN, the Westinghouse Owners Group (WOG) performed extensive full flow testing onmore »PWR MSSVs and found that each valve required a unique setting of a combination of two rings in order to achieve full lift at accumulation of 3% and re-closing at a blowdown of 5%. The Bopp and Reuther MSSV type SiZ 2507 has a "fixed blowdown" i.e. without any adjusting rings to adjust the "blowdown" so that the blowdown is "fixed". More than 1000 pieces of this type are successfully in nuclear power plants in operation. Many of them since about 25 years. Therefore it can be considered as a proven design. It is new that an optimization of this MSSV type SiZ 2507 fulfill the requirements of part NC-7512 of the ASME Section III although there are still no adjusting rings in the flow part. In 2000, for the Qinshan Candu unit 1 and 2 full flow tests were performed with 32 MSSV type SiZ 2507 size 8"" x 12"" at 51 bar saturated steam in only 6 days. In all tests the functional performance was very stable. It was demonstrated by recording the signals lift and system pressure that all valves had acceptable results to achieve full lift at accumulation of 3% and to re-close at blowdown of 5%. This is an advantage which gives a reduction in cost for flow tests and which gives more reliability after maintenance work during outage compared to the common MSSV design with an individual required setting of the combination of the two rings. The design of the type SiZ 2507 without any adjusting rings in the flow path is presented. The stable performance depends on the interaction of flow force and spring force. The optimization of the flow path to create a suitable flow-force-curve was managed by Computational Fluid Dynamics (CFD) and flow-force-characteristic-measurements at a model 1: 2.5. The method of the flow-force-characteristic-measurement permits systematic dimensioning of valve spring forces by means of measurement of the fluid mechanical forces occurring on the valve spindle during flow. A special procedure was established to verify a spring force versus lift curve with an accuracy of 1% for each production valve. This gives high reliability at required stable performance and this can not be influenced by wrong setting of any adjusting ring during maintenance work. (authors)« less

The National Board strives to keep information in the hands of website users. Provided here are more than 70 Technical Articles previously published in the National Board BULLETINand/or from the proceedings of past General Meetings.

(k) The set pressure tolerances, plus or minus, of safety valves shall not exceed 2 psi (15 kPa), and for safety relief valves shall not exceed 3 psi (20 kPa) for pressures up to and including 60 psig (400 kPa) and 5% for pressures above 60 psig (400 kPa).

(l) Safety valves shall be arranged so that they cannot be reset to relieve at a higher pressure than the maximum allowable working pressure of the boiler.

(e) Material for valve bodies and bonnets or their corresponding metallic pressure containing parts shall be listed in Section II,except that in cases where a manufacturer desires to make use of materials other than those listed in Section II, he shall establish and maintain specifications requiring equivalent control of chemical and physical properties and quality.

(g) No materials liable to fail due to deterioration or vulcanization when subjected to saturated steam temperature corresponding to capacity test pressure shall be used.

(a) A Manufacturer shall demonstrate to the satisfaction of an ASME designee that his manufacturing, production, and testing facilities and quality control procedures will insure close agreement between the performance of random production samples and the performance of those valves submitted for capacity certification.

(c) A Manufacturer may be granted permission to apply, the HV Code Symbol to production pressure relief valves capacity certified in accordance with HG-402.3 provided the following tests are successfully completed. This permission shall expire on the sixth anniversary of the date it is initially granted. The permission may be extended for 6 year periods if the following tests are successfully repeated within the 6 month period before expiration.

(1) Two sample production pressure relief valves of a size and capacity within the capability of an ASME accepted laboratory shall be selected by an ASME designee.

(2) Operational and capacity tests shall be conducted in the presence of an ASME designee at an ASME accepted laboratory. The valve Manufacturer shall be notified of the time of the test and may have representatives present to witness the test.

(3) Should any valve fail to relieve at or above its certified capacity or should it fail to meet performance requirements of this Section, the test shall be repeated at the rate of two replacement valves, selected in accordance with HG-401.3(c)(1), for each valve that failed.

(4) Failure of any of the replacement valves to meet the capacity or the performance requirements of this Section shall be cause for revocation within 60 days of the authorization to use the Code Symbol on that particular type of valve. During this period, the Manufacturer shall demonstrate the cause of such deficiency and the action taken to guard against future occurrence, and the requirements of HG-401.3(c) above shall apply.

(d) Safety valves shall be sealed in a manner to prevent the valve from being taken apart without breaking the seal. Safety relief valves shall be set and sealed so that they cannot be reset without breaking the seal.

(a) Every safety valve shall be tested to demonstrate its popping point, blowdown, and tightness. Every safety relief valve shall be tested to demonstrate its opening point and tightness. Safety valves shall be tested on steam or air and safety relief valves on water, steam, or air. When the blowdown is nonadjustable, the blowdown test may be performed on a sampling basis.

(c) Testing time on safety valves shall be sufficient, depending on size and design, to insure that test results are repeatable and representative of field performance.

HG-401.5 Design Requirements. At the time of the submission of valves for capacity certification, or testing in accordance with this Section, the ASME Designee has the authority to review the design for conformity with the requirements of this Section, and to reject or require modification of designs that do not conform, prior to capacity testing.

HG-402.1 Valve Markings. Each safety or safety-relief valve shall be plainly marked with the required data by the Manufacturer in such a way that the markings will not be obliterated in service. The markings shall be stamped, etched, impressed, or cast on the valve or on a nameplate, which shall be securely fastened to the valve.

(6) year built or, alternatively, a coding may be marked on the valves such that the valve Manufacturer can identify the year the valve was assembled and tested, and

HG-402.2 Authorization to Use ASME Stamp.Each safety valve to which the Code Symbol (Fig. HG-402) is to be applied shall be produced by a Manufacturer and/or Assembler who is in possession of a valid Certificate of Authorization. (See HG-540.) For all valves to be stamped with the HV Symbol, a Certified Individual (CI) shall provide oversight to ensure that the use of the “HV" Code symbol on a safety valve or safety relief valve is in accordance with this Section and that the use of the “HV" Code symbol is documented on a Certificate of Conformance Form, HV-1.

(3) have a record, maintained and certified by the Manufacturer, containing objective evidence of the qualifications of the CI and the training program provided

(1) The Certificate of Conformance shall be filled out by the Manufacturer and signed by the Certified Individual. Multiple duplicate pressure relief devices may be recorded on a single entry provided the devices are identical and produced in the same lot.

(2) The Manufacturer"s written quality control program shall include requirements for completion of Certificates of Conformance forms and retention by the Manufacturer for a minimum of 5 years.

HG-402.3 Determination of Capacity to Be Stamped on Valves. The Manufacturer of the valves that are to be stamped with the Code symbol shall submit valves for testing to a place where adequate equipment and personnel are available to conduct pressure and relieving-capacity tests which shall be made in the presence of and certified by an authorized observer. The place, personnel, and authorized observer shall be approved by the Boiler and Pressure Vessel Committee. The valves shall be tested in one of the following three methods.

(a) Coefficient Method. Tests shall be made to determine the lift, popping, and blowdown pressures, and the capacity of at least three valves each of three representative sizes (a total of nine valves). Each valve of a given size shall be set at a different pressure. However, safety valves for steam boilers shall have all nine valves set at 15 psig (100 kPa). A coefficient shall be established for each test as follows:

The average of the coefficients KDof the nine tests required shall be multiplied by 0.90, and this product shall be taken as the coefficient K of that design. The stamped capacity for all sizes and pressures shall not exceed the value determined from the following formulas:

Note: The maximum and minimum coefficient determined by the tests of a valve design shall not vary more than ±5%from the average. If one or more tests are outside the acceptable limits, one valve of the Manufacturer"s choice shall be replaced with another valve of the same size and pressure setting or by a modification of the original valve. Following this test a new average coefficient shall be calculated, excluding the replaced valve test. If one or more tests are now outside the acceptable limits, as determined by the new average coefficient, a valve of the Manufacturer"s choice must be replaced by two valves of the same size and pressure as the rejected valve. A new average coefficient, including the replacement valves, shall be calculated. If any valve, excluding the two replaced valves, now falls outside the acceptable limits, the tests shall be considered unsatisfactory.

(b)Slope Method. If a Manufacturer wishes to apply the Code Symbol to a design of pressure relief valves, four valves of each combination of pipe and orifice size shall be tested. These four valves shall be set at pressures that cover the approximate range of pressures for which the valve will be used, or that cover the range available at the certified test facility that shall conduct the tests. The capacities shall be based on these four tests as follows:

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