blowdown pressure of safety valve factory

The ASME Boiler and Pressure Vessel Code does not have blowdown requirements for Section VIII (or non-code) valves. Blowdown may vary from less than 2% to more than 50%, depending on many factors including: valve design, dimensional tolerance variation, where the set pressure falls in the set pressure range of a spring, spring rate/force ratio, warn ring/guide settings, etc. Typical blowdown for most valves is 15% to 30%, but cannot be guaranteed.

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.

A rope appx. 6-7 meters with a hook one end should be attached to the valve lifting lever before starting the pressure rise. It will help in operating the lever to avoid chattering & over pressure

Safety valves blow down should be set more than required, as blow down percentage decreases as the steam temperature increases. An approximate rule is to add 0.5% of set pressure to the blow down for each 56.5 °C rise in SH steam temperature.

If a Super heater safety valve lifts at 189.5 kg/cm2 & reseats at 180 kg/cm2 at the temperature of 400 deg c, then calculate the blowdown calculation at 540 deg c

A little product education can make you look super smart to customers, which usually means more orders for everything you sell. Here’s a few things to keep in mind about safety valves, so your customers will think you’re a genius.

A safety valve is required on anything that has pressure on it. It can be a boiler (high- or low-pressure), a compressor, heat exchanger, economizer, any pressure vessel, deaerator tank, sterilizer, after a reducing valve, etc.

There are four main types of safety valves: conventional, bellows, pilot-operated, and temperature and pressure. For this column, we will deal with conventional valves.

A safety valve is a simple but delicate device. It’s just two pieces of metal squeezed together by a spring. It is passive because it just sits there waiting for system pressure to rise. If everything else in the system works correctly, then the safety valve will never go off.

A safety valve is NOT 100% tight up to the set pressure. This is VERY important. A safety valve functions a little like a tea kettle. As the temperature rises in the kettle, it starts to hiss and spit when the water is almost at a boil. A safety valve functions the same way but with pressure not temperature. The set pressure must be at least 10% above the operating pressure or 5 psig, whichever is greater. So, if a system is operating at 25 psig, then the minimum set pressure of the safety valve would be 30 psig.

Most valve manufacturers prefer a 10 psig differential just so the customer has fewer problems. If a valve is positioned after a reducing valve, find out the max pressure that the equipment downstream can handle. If it can handle 40 psig, then set the valve at 40. If the customer is operating at 100 psig, then 110 would be the minimum. If the max pressure in this case is 150, then set it at 150. The equipment is still protected and they won’t have as many problems with the safety valve.

Here’s another reason the safety valve is set higher than the operating pressure: When it relieves, it needs room to shut off. This is called BLOWDOWN. In a steam and air valve there is at least one if not two adjusting rings to help control blowdown. They are adjusted to shut the valve off when the pressure subsides to 6% below the set pressure. There are variations to 6% but for our purposes it is good enough. So, if you operate a boiler at 100 psig and you set the safety valve at 105, it will probably leak. But if it didn’t, the blowdown would be set at 99, and the valve would never shut off because the operating pressure would be greater than the blowdown.

All safety valves that are on steam or air are required by code to have a test lever. It can be a plain open lever or a completely enclosed packed lever.

Safety valves are sized by flow rate not by pipe size. If a customer wants a 12″ safety valve, ask them the flow rate and the pressure setting. It will probably turn out that they need an 8×10 instead of a 12×16. Safety valves are not like gate valves. If you have a 12″ line, you put in a 12″ gate valve. If safety valves are sized too large, they will not function correctly. They will chatter and beat themselves to death.

Safety valves need to be selected for the worst possible scenario. If you are sizing a pressure reducing station that has 150 psig steam being reduced to 10 psig, you need a safety valve that is rated for 150 psig even though it is set at 15. You can’t put a 15 psig low-pressure boiler valve after the reducing valve because the body of the valve must to be able to handle the 150 psig of steam in case the reducing valve fails.

The seating surface in a safety valve is surprisingly small. In a 3×4 valve, the seating surface is 1/8″ wide and 5″ around. All it takes is one pop with a piece of debris going through and it can leak. Here’s an example: Folgers had a plant in downtown Kansas City that had a 6×8 DISCONTINUED Consolidated 1411Q set at 15 psig. The valve was probably 70 years old. We repaired it, but it leaked when plant maintenance put it back on. It was after a reducing valve, and I asked him if he played with the reducing valve and brought the pressure up to pop the safety valve. He said no, but I didn’t believe him. I told him the valve didn’t leak when it left our shop and to send it back.

If there is a problem with a safety valve, 99% of the time it is not the safety valve or the company that set it. There may be other reasons that the pressure is rising in the system before the safety valve. Some ethanol plants have a problem on starting up their boilers. The valves are set at 150 and they operate at 120 but at startup the pressure gets away from them and there is a spike, which creates enough pressure to cause a leak until things get under control.

If your customer is complaining that the valve is leaking, ask questions before a replacement is sent out. What is the operating pressure below the safety valve? If it is too close to the set pressure then they have to lower their operating pressure or raise the set pressure on the safety valve.

Is the valve installed in a vertical position? If it is on a 45-degree angle, horizontal, or upside down then it needs to be corrected. I have heard of two valves that were upside down in my 47 years. One was on a steam tractor and the other one was on a high-pressure compressor station in the New Mexico desert. He bought a 1/4″ valve set at 5,000 psig. On the outlet side, he left the end cap in the outlet and put a pin hole in it so he could hear if it was leaking or not. He hit the switch and when it got up to 3,500 psig the end cap came flying out like a missile past his nose. I told him to turn that sucker in the right direction and he shouldn’t have any problems. I never heard from him so I guess it worked.

If the set pressure is correct, and the valve is vertical, ask if the outlet piping is supported by something other than the safety valve. If they don’t have pipe hangers or a wall or something to keep the stress off the safety valve, it will leak.

There was a plant in Springfield, Mo. that couldn’t start up because a 2″ valve was leaking on a tank. It was set at 750 psig, and the factory replaced it 5 times. We are not going to replace any valves until certain questions are answered. I was called to solve the problem. The operating pressure was 450 so that wasn’t the problem. It was in a vertical position so we moved on to the piping. You could tell the guy was on his cell phone when I asked if there was any piping on the outlet. He said while looking at the installation that he had a 2″ line coming out into a 2×3 connection going up a story into a 3×4 connection and going up another story. I asked him if there was any support for this mess, and he hung up the phone. He didn’t say thank you, goodbye, or send me a Christmas present.

Pipe dope is another problem child. Make sure your contractors ease off on the pipe dope. That is enough for today, class. Thank you for your patience. And thank you for your business.

The Pressure Safety Valve Inspection article provides you information about inspection of pressure safety valve and pressure safety valve test in manufacturing shop as well as in operational plants.

Your pressure safety valve is a direct spring-loaded pressure-relief valve that is opened by the static pressure upstream of the valve and characterized by rapid opening or pop action.

When the static inlet pressure reaches the set pressure, it will increase the pressure upstream of the disk and overcome the spring force on the disk.

Your construction code for pressure safety valve is API Standard 526 and covers the minimum requirements for design, materials, fabrication, inspection, testing, and commissioning.

These are:API Recommended Practice 520 for Sizing and SelectionAPI Recommended practice 521 Guideline for Pressure Relieving and Depressing SystemsAPI Recommended Practice 527 Seat Tightness of Pressure Relief Valves

For example in the state of Minnesota the ASME Code application and stamping for pressure vessel and boiler is mandatory which “U” and “S” symbols are designated for stamping on the nameplate.

For example if there is pressure vessel need to be installed in the state of Minnesota then the pressure vessel nameplate shall be U stamped and pressure vessel safety valve shall be UV stamped.

National Board Inspection Code (NBIC) have own certification scheme for pressure safety valves and using NB symbol. The NBIC code book for this certification is NB 18.

There are some other standards and codes which are used in pressure safety valve such as:ASME PTC 25 for pressure relief devices which majorly is used for assessment of testing facility and apparatus for safety valvesBS EN ISO 4126-1, 4126-2 and 4126-3 which is construction standard similar to API STD 526.

This API RP 527 might be used in conjunction of API RP 576 as testing procedure for seat tightness testing of pressure safety valve for periodical servicing and inspection.

These are only important points or summery of points for pressure safety valve in-service inspection and should not be assumed as pressure safety valve inspection procedure.

Pressure safety valve inspection procedure is comprehensive document which need to cover inspection methods to be employed, equipment and material to be used, qualification of inspection personnel involved and the sequence of the inspection activities as minimum.

You may use following content as summery of points for Pressure Safety Valve Inspection in operational plantDetermination pressure safety valve inspection interval based API STD 510 and API RP 576 requirementsInspection of inlet and outlet piping after pressure safety valve removal for any foulingInspection of pressure safety valve charge and discharge nozzles for possible deposit and corrosion productsTaking care for proper handling of pressure safety valves from unit to the valve shop. The detail of handling and transportation instruction is provided in API RP 576.Controlling of seals for being intact when the valves arrived to the valve shop.Making as received POP test and recording the relieving pressure.

If the POP pressure is higher than the set pressure the test need to be repeated and if in the second effort it was near to the set pressure it is because of deposit.If in the second effort it was not opened near to the set pressure either it was set wrongly or it was changed during the operationIf the pressure safety valve was not opened in 150% of set pressure it should be considered as stuck shut.If the pressure safety valve was opened below the set pressure the spring is weakenedMaking external visual inspection on pressure safety valve after POP test. The test need contain following item as minimum;the flanges for pitting and roughness

Making body wall thickness measurementDismantling of pressure safety valve if the result of as received POP test was not satisfactoryMaking detail and comprehensive visual and dimensional inspection on the dismantled valve parts (after cleaning)Making special attention to the dismantled valves seating surfaces inspection e.g. disk and seat for roughness, wear and damage which might cause valve leakage in serviceReplacing the damaged parts in dismantled valves based manufacture recommendation and API RP 576 requirementsMaking precise setting of the pressure safety valve after reassembly based manufacture recommendation or NB-18 requirements

Making at least two POP test after setting and making sure the deviation from set pressure is not more than 2 psi for valves with set pressure equal or less than 70 psi or 3% for valves with set pressure higher than 70 psiMaking valve tightness test for leakage purpose after approval of the setting pressure and POP tests. The test method and acceptance criteria must be according to the API RP 576.The API RP 527 also can be used for pressure safety valve tightness test.Recording and maintaining the inspection and testing results.

An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP).

Many electronic, pneumatic and hydraulic systems exist today to control fluid system variables, such as pressure, temperature and flow. Each of these systems requires a power source of some type, such as electricity or compressed air in order to operate. A pressure Relief Valve must be capable of operating at all times, especially during a period of power failure when system controls are nonfunctional. The sole source of power for the pressure Relief Valve, therefore, is the process fluid.

Once a condition occurs that causes the pressure in a system or vessel to increase to a dangerous level, the pressure Relief Valve may be the only device remaining to prevent a catastrophic failure. Since reliability is directly related to the complexity of the device, it is important that the design of the pressure Relief Valve be as simple as possible.

The pressure Relief Valve must open at a predetermined set pressure, flow a rated capacity at a specified overpressure, and close when the system pressure has returned to a safe level. Pressure Relief Valves must be designed with materials compatible with many process fluids from simple air and water to the most corrosive media. They must also be designed to operate in a consistently smooth and stable manner on a variety of fluids and fluid phases.

The basic spring loaded pressure Relief Valve has been developed to meet the need for a simple, reliable, system actuated device to provide overpressure protection.

The Valve consists of a Valve inlet or nozzle mounted on the pressurized system, a disc held against the nozzle to prevent flow under normal system operating conditions, a spring to hold the disc closed, and a body/Bonnet to contain the operating elements. The spring load is adjustable to vary the pressure at which the Valve will open.

When a pressure Relief Valve begins to lift, the spring force increases. Thus system pressure must increase if lift is to continue. For this reason pressure Relief Valves are allowed an overpressure allowance to reach full lift. This allowable overpressure is generally 10% for Valves on unfired systems. This margin is relatively small and some means must be provided to assist in the lift effort.

Most pressure Relief Valves, therefore, have a secondary control chamber or huddling chamber to enhance lift. As the disc begins to lift, fluid enters the control chamber exposing a larger area of the disc to system pressure.

This causes an incremental change in force which overcompensates for the increase in spring force and causes the Valve to open at a rapid rate. At the same time, the direction of the fluid flow is reversed and the momentum effect resulting from the change in flow direction further enhances lift. These effects combine to allow the Valve to achieve maximum lift and maximum flow within the allowable overpressure limits. Because of the larger disc area exposed to system pressure after the Valve achieves lift, the Valve will not close until system pressure has been reduced to some level below the set pressure. The design of the control chamber determines where the closing point will occur.

When superimposed back pressure is variable, a balanced bellows or balanced piston design is recommended. A typical balanced bellow is shown on the right. The bellows or piston is designed with an effective pressure area equal to the seat area of the disc. The Bonnet is vented to ensure that the pressure area of the bellows or piston will always be exposed to atmospheric pressure and to provide a telltale sign should the bellows or piston begin to leak. Variations in back pressure, therefore, will have no effect on set pressure. Back pressure may, however, affect flow.

A safety Valve is a pressure Relief Valve actuated by inlet static pressure and characterized by rapid opening or pop action. (It is normally used for steam and air services.)

A low-lift safety Valve is a safety Valve in which the disc lifts automatically such that the actual discharge area is determined by the position of the disc.

A full-lift safety Valve is a safety Valve in which the disc lifts automatically such that the actual discharge area is not determined by the position of the disc.

A Relief Valve is a pressure relief device actuated by inlet static pressure having a gradual lift generally proportional to the increase in pressure over opening pressure. It may be provided with an enclosed spring housing suitable for closed discharge system application and is primarily used for liquid service.

A safety Relief Valve is a pressure Relief Valve characterized 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 may be used either for liquid or compressible fluid.

A conventional safety Relief Valve is a pressure Relief Valve which has its spring housing vented to the discharge side of the Valve. The operational characteristics (opening pressure, closing pressure, and relieving capacity) are directly affected by changes of the back pressure on the Valve.

A balanced safety Relief Valve is a pressure Relief Valve which incorporates means of minimizing the effect of back pressure on the operational characteristics (opening pressure, closing pressure, and relieving capacity).

A pilotoperated pressure Relief Valve is a pressure Relief Valve in which the major relieving device is combined with and is controlled by a self-actuated auxiliary pressure Relief Valve.

A poweractuated pressure Relief Valve is a pressure Relief Valve in which the major relieving device is combined with and controlled by a device requiring an external source of energy.

A temperature-actuated pressure Relief Valve is a pressure Relief Valve which may be actuated by external or internal temperature or by pressure on the inlet side.

A vacuum Relief Valve is a pressure relief device designed to admit fluid to prevent an excessive internal vacuum; it is designed to reclose and prevent further flow of fluid after normal conditions have been restored.

Many Codes and Standards are published throughout the world which address the design and application of pressure Relief Valves. The most widely used and recognized of these is the ASME Boiler and Pressure Vessel Code, commonly called the ASME Code.

Most Codes and Standards are voluntary, which means that they are available for use by manufacturers and users and may be written into purchasing and construction specifications. The ASME Code is unique in the United States and Canada, having been adopted by the majority of state and provincial legislatures and mandated by law.

The ASME Code provides rules for the design and construction of pressure vessels. Various sections of the Code cover fired vessels, nuclear vessels, unfired vessels and additional subjects, such as welding and nondestructive examination. Vessels manufactured in accordance with the ASME Code are required to have overpressure protection. The type and design of allowable overpressure protection devices is spelled out in detail in the Code.

The following definitions are taken from DIN 3320 but it should be noted that many of the terms and associated definitions used are universal and appear in many other standards. Where commonly used terms are not defined in DIN 3320 then ASME PTC25.3 has been used as the source of reference. This list is not exhaustive and is intended as a guide only; it should not be used in place of the relevant current issue standard..

is the gauge pressure at which the lift is sufficient to discharge the predetermined flowing capacity. It is equal to the set pressure plus opening pressure difference.

is the cross sectional area upstream or downstream of the body seat calculated from the minimum diameter which is used to calculate the flow capacity without any deduction for obstructions.

is the calculated mass flow from an orifice having a cross sectional area equal to the flow area of the safety Valve without regard to flow losses of the Valve.

the pressure at which a Valve is set on a test rig using a test fluid at ambient temperature. This test pressure includes corrections for service conditions e.g. backpressure or high temperatures.

is that portion of the measured relieving capacity permitted by the applicable code or regulation to be used as a basis for the application of a pressure relieving device.

is the value of increasing static inlet pressure of a pressure Relief Valve at which there is a measurable lift, or at which the discharge becomes continuous as determined by seeing, feeling or hearing.

is the maximum allowable working pressure plus the accumulation as established by reference to the applicable codes for operating or fire contingencies.

Because cleanliness is essential to the satisfactory operation and tightness of a safety Valve, precautions should be taken during storage to keep out all foreign materials. Inlet and outlet protectors should remain in place until the Valve is ready to be installed in the system. Take care to keep the Valve inlet absolutely clean. It is recommended that the Valve be stored indoors in the original shipping container away from dirt and other forms of contamination.

Safety Valves must be handled carefully and never subjected to shocks. Rough handling may alter the pressure setting, deform Valve parts and adversely affect seat tightness and Valve performance.

When it is necessary to use a hoist, the chain or sling should be placed around the Valve body and Bonnet in a manner that will insure that the Valve is in a vertical position to facilitate installation.

Many Valves are damaged when first placed in service because of failure to clean the connection properly when installed. Before installation, flange faces or threaded connections on both the Valve inlet and the vessel and/or line on which the Valve is mounted must be thoroughly cleaned of all dirt and foreign material.

Because foreign materials that pass into and through safety Valves can damage the Valve, the systems on which the Valves are tested and finally installed must also be inspected and cleaned. New systems in particular are prone to contain foreign objects that inadvertently get trapped during construction and will destroy the seating surface when the Valve opens. The system should be thoroughly cleaned before the safety Valve is installed.

The gaskets used must be dimensionally correct for the specific flanges. The inside diameters must fully clear the safety Valve inlet and outlet openings so that the gasket does not restrict flow.

For flanged Valves, draw down all connection studs or bolts evenly to avoid possible distortion of the Valve body. For threaded Valves, do not apply a wrench to the Valve body. Use the hex flats provided on the inlet bushing.

Safety Valves are intended to open and close within a narrow pressure range. Valve installations require accurate design both as to inlet and discharge piping. Refer to International, National and Industry Standards for guidelines.

The Valve should be mounted vertically in an upright position either directly on a nozzle from the pressure vessel or on a short connection fitting that provides a direct, unobstructed flow between the vessel and the Valve. Installing a safety Valve in other than this recommended position will adversely affect its operation.

Discharge piping should be simple and direct. A "broken" connection near the Valve outlet is preferred wherever possible. All discharge piping should be run as direct as is practicable to the point of final release for disposal. The Valve must discharge to a safe disposal area. Discharge piping must be drained properly to prevent the accumulation of liquids on the downstream side of the safety Valve.

The weight of the discharge piping should be carried by a separate support and be properly braced to withstand reactive thrust forces when the Valve relieves. The Valve should also be supported to withstand any swaying or system vibrations.

If the Valve is discharging into a pressurized system be sure the Valve is a "balanced" design. Pressure on the discharge of an "unbalanced" design will adversely affect the Valve performance and set pressure.

The Bonnets of balanced bellows safety Valves must always be vented to ensure proper functioning of the Valve and to provide a telltale in the event of a bellows failure. Do not plug these open vents. When the fluid is flammable, toxic or corrosive, the Bonnet vent should be piped to a safe location.

It is important to remember that a pressure Relief Valve is a safety device employed to protect pressure vessels or systems from catastrophic failure. With this in mind, the application of pressure Relief Valves should be assigned only to fully trained personnel and be in strict compliance with rules provided by the governing codes and standards.

During a relief scenario as you have described, the system pressure will be constantly trying to get back to its operating pressure. So theoretically the SRV will never close. If it is a spring operated SRV, then this is how chatter then develops. Without knowing all of the service application detail, you simply need a SRV with a shorter blowdown. I guess your client has a spring SRV with a blowdown of 20% or so. There are other spring operated SRV"s with short blowdown available as gguliye has suggested above.

Per ISO, only the term Safety Valve is used regardless of application or design. RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

In any event the pressure from a centrifugal pump won"t be constant but varies with flow. As flow increase through the valve then the pressure will fall.

If the relief valve is there to protect against shut in flow then the pump should trip or you need a higher margin between operating pressure and set pressure / design pressure of the system.

80 Series from Emerson has shorter blow down but the drawback here is that the customer requires metal to metal seat, which rules out all the 80 Series and pilot operated valves.

Type 81,83 & 81P & Pilot operated has only soft seats like Teflon & Buna-N...etc RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

Per ISO, only the term Safety Valve is used regardless of application or design. RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

Sounds to be a good idea, but series 800 (Spring operated) is no longer in production. Its already obsolete. RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

Per ISO, only the term Safety Valve is used regardless of application or design. RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

How can blowdown be less for liquid service? Catalog actually states max 20% for all fluids. My earlier 15 % comment on gas/vapour is for a correct "to the book" installation. Note your client application just borders on a 20% differential op/set.

Per ISO, only the term Safety Valve is used regardless of application or design. RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

Per ISO, only the term Safety Valve is used regardless of application or design. RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

Answering above qs: The trim of the valve we selected was liquid,but the fluid passing through it is gas in nature (This specific requirement needs both liquid and gas i.e two cases, where the governing case is liquid). And thus it made us confuse. Thanks for clarification. Now we are pretty clear.

Per ISO, only the term Safety Valve is used regardless of application or design. RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

The question I"ve often had with some of these low blowdown valve designs is one of configuration differences between bench testing and in service performance.

Is this truly representative of the in service blowdown performance if a special ring configuration is required for testing? RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

I don"t think that the bliwdown test is possible in the factory since as the valve opens the pressure is relieved and the valve is closed immediately. Please correct me if I am wrong RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

"Real" Blowdown can only be realised on the protected vessel. That is why, in the main, ASME designed pressure relief valves, have an externally accessible adjustable blowdown ring. The volume of flow, rate of overpressure and decay, fluid type etc., etc., will influence the blowdown. ASME VIII requires manufactures to demonstrate their PRV designs can achieve blowdown (typically 7% for compressible fluids), during the approved laboratory flow testing (to demonstrate lift and determine floe coefficient of discharge). Blowdown tests are not done during production testing (too expensive and time consuming as would be required to be done in a lab each time). During production testing different ring settings, varied by manufacturer, are used to prevent damage to seating (from seats banging shut). After testing, the ring is set to "best setting" dependant on valve size. fluid, pressure etc.

Some testing organisations claim to be able to measure blowdown with their in-situ kit. While a result can be obtained (as it sure can even on a bench test), it is not the true blowdown. Blowdown can only be measured on the protected vessel without artificial lift, assistance or other.

Per ISO, only the term Safety Valve is used regardless of application or design. RE: Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII)

ASME Section VIII requires us to demonstrate that the blowdown figure can be achieved, however, I feel in some cases that demonstrations requiring special ring configurations under lab conditions are not representative of in-situ(real world) performance.

I have seen a blowdown test conducted on a full size vessel between an adjustable and non-adjustable blowdown design and the blowdown certainly was not what the manufacturer had stated.

(f) Safety valves shall be spring loaded. The spring shall be designed so that the full lift spring compression shall be no greater than 80% of the nominal solid deflection. The permanent set of the spring (defined as the difference between the free height and height measured 10 min after the spring has been compressed solid three additional times after presetting at room temperature) shall not exceed 0.5% of the free height.

(g) There shall be a lifting device and a mechanical connection between the lifting device and the disk capable of lifting the disk from the seat a distance of at least 1/16 in. (1.5 mm) with no pressure on the boiler.

(h) A body drain below seat level shall be provided by the Manufacturer for all safety valves and safety relief valves, except that the body drain may be omitted when the valve seat is above the bottom of the inside diameter of the discharge piping. For valves exceeding NPS 2½ (DN 65) the drain hole or holes shall be tapped not less than NPS 3/8 (DN 10). For valves NPS 2½ (DN 65) or smaller, the drain hole shall not be less than ¼ in. (6 mm) in diameter. Body drain connections shall not be plugged during or after field installation. In safety relief valves of the diaphragm type, the space above the diaphragm shall be vented to prevent a buildup of pressure above the diaphragm. Safety relief valves of the diaphragm type shall be so designed that failure or deterioration of the diaphragm material will not impair the ability of the valve to relieve at the rated capacity.

(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) verify that each item to which the Code Symbol is applied meets all applicable requirements of this Section and has a current capacity certification for the “HV" symbol

(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:

(a) 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. The maximum blowdown for these samples shall not exceed the value specified in the following table:

The blow down for sample valves designed for use on forced flow steam generators with no fixed steam and waterline or high-temperature water boilers shall not exceed 10% of the set pressure.

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

(c) Should any valve fail to relieve at or above its certified capacity or should it fail to meet performance requirements in PG-72, the test shall be repeated at the rate of two replacement valves, selected in accordance with PG-73.4.3(a), for each valve that failed.

(d) Failure of any of the replacement valves to meet 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 t type of valve. During this period, the Manufacturer or assembler shall demonstrate the cause of such deficiency and the action taken toward against future occurrence.

PG-73.4.4 Use of the Code Symbol Stamp by an assembler indicates the use of original unmodified parts in strict accordance with the instructions of the manufacturer of the valve.

(a) An assembler may transfer original and unmodified pressure relief parts produced by the Manufacturer to other Assemblers, provided the following conditions are met:

(2) the Quality Control System of the Assembler receiving the pressure relief valve parts shall define the controls for the procurement and acceptance of those parts

(b) However, an assembler may convert original finished parts by either machining to another finished part or applying a corrosion-resistant coating to valve springs for a specific application under the following conditions:

(1) Conversions shall be specified by the Manufacturer. Drawings and/or written instructions used for part conversion shall be obtained from the Manufacturer and shall include a drawing or description of the converted part before and after the conversion.

(2) The Assembler"s quality control system, as accepted by a representative from an ASME-designated organization, must describe in detail the conversion of original parts, provisions for inspection and acceptance, personnel training, and control of current Manufacturer"s drawings and/or written instructions.

(4) The Assembler must demonstrate to the Manufacturer the ability to perform each type of conversion. The Manufacturer shall document all authorizations granted to perform part conversions. The Manufacturer and Assembler shall maintain a file of such authorizations.

(5) For an Assembler to offer restricted lift valves, the Assembler must demonstrate to the satisfaction of the Manufacturer the ability to perform valve lift restrictions. The Manufacturer shall document all authorizations granted to restrict the lift of the valves, and shall maintain records of the lift restrictions made by the Assembler. The Assembler shall maintain a file of such authorizations.

(6) At least annually a review shall be performed by the Manufacturer of an Assembler"s system and conversion capabilities. The Manufacturer shall document the results of these reviews. A copy of this documentation shall be kept on file by the Assembler. The review results shall be made available to a representative from an ASME designated organization.

NOTES: Within the requirements of PG-73.4 and PG-73.5, a manufacturer is defined as a person or organization who is completely responsible for design, material selection, capacity certification, manufacture of all component parts, assembly, testing, sealing, and shipping of pressure relief valves certified under this Section.

Anassembleris defined as a person or organization who purchases or receives from a manufacturer the necessary component parts or valves and assembles, adjusts, tests, seals, and ships pressure relief valves certified under this Section at a geographical location other than and using facilities other than those used by the manufacturer.

PG-73.5.1 Pressure Testing. Each pressure relief valve to which the Code Symbol Stamp is to be applied shall be subjected to the following tests by the Manufacturer or Assembler:

(a) The pressure-containing parts of each valve shall be hydrostatically tested at a pressure at least 1.5 times the design pressure of the parts. Parts meeting the following criteria shall be exempt from pressure testing:

(b) Alternatively, testing may be performed pneumatically at a pressure 1.25 times the design pressure of the part. Pneumatic testing can be hazardous; it is therefore recommended that special precautions be taken when conducting a pneumatic test.

(d) When the valve is designed for discharging directly to atmosphere, the valve components downstream of the valve disk are exempt from pressure testing.

PG-73.5.2 Every valve shall be tested with steam by the manufacturer or assembler to demonstrate its set point and pressure-containing integrity. The blowdown control elements of the pressure relief valve shall be set to the Manufacturer"s specifications.

PG-73.5.2.1Tests shall be conducted either on equipment that meets the requirements of PG-73.5.6, or on the boiler, by raising the pressure to demonstrate the set pressure.

PG-73.5.2.2 When the valve is beyond the production test equipment capabilities, an alternative test method presented in PG-73.5.2.2.1 or PG-73.5.2.2.2 may be used, provided all of the following conditions are met:

(a) testing the valve at full pressure may cause damage to the valve, or testing of the valve is impractical due to boiler system operational safety considerations

PG-73.5.2.2.1 The valve, with its lift temporarily restricted during the test, if required to prevent valve damage, shall be tested on steam to demonstrate set pressure.

PG-73.5.2.2.2 The valve may be fitted with a hydraulic or pneumatic lift assist device and tested on steam at a pressure less than the valve set pressure. The lift assist device and test procedure shall be calibrated to provide the set pressure setting within the tolerance of PG-72.2.

(a) A seat tightness test shall be conducted at maximum expected operating pressure but at a pressure not exceeding the reseating pressure of the valve. When being tested, a valve exhibiting no sign of leakage shall be considered adequately tight.

(b) Closed bonnet pressure relief valves designed for discharge to a closed system shall be tested with a minimum of 30 psig (200 kPa) air or other gas in the secondary pressure zone. There shall be no sign of leakage.

PG-73.5.6 Test fixtures and test drums, where applicable, shall be of adequate size and capacity to assure that the observed set pressure is consistent with the stamped set pressure within the tolerance required by PG-72.2.

PG-73.6 Design Requirements. At the time of submission of valves for capacity certification or testing in accordance with PG-69, the ASME designee has the authority to review 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.

PG-73.7 Code Symbol “V" Stamp. Each pressure relief valve to which the Code “V" symbol (see Fig. PG-105.4) will be applied shall have been fabricated or assembled by a manufacturer or assembler holding a valid Certificate of Authorization (PG-105.2) and c capacity certified in accordance with the requirements of this Section. A Certified Individual (CI) shall provide oversight to assure that each use of the Code “V" symbol on a pressure relief valve is in accordance with the requirements of this Section, and that each use of the Code “V" symbol is documented on a Certificate of Conformance, Form P-8.

(c) have a record, maintained and certified by the manufacturer or assembler, containing objective evidence of the qualifications of the CI and the training program provided

(a) verify that each item to which the Code “V" symbol is applied has a current capacity certification and meets all applicable requirements of this Section

(a) The Certificate of Conformance, Form P-8, shall be filled out by the manufacturer or assembler and signed by the Certified Individual. Multiple duplicate pressure relief valves may be recorded as a single entry, provided the valves are identical and are produced in the same lot.

(b) The manufacturer"s or assembler"s written quality control program shall include requirements for completion of Certificates of Conformance, Form P-8, and retention, by the manufacturer or assembler, for a minimum of 5 years.

The fabrication of boilers and parts thereof shall conform to the general fabrication requirements in the following paragraphs and in addition to the specific requirements for fabrication in the Parts of this Section that pertain to the methods of construction used.

PG-76.1 Plates may be cut by machining, punching, shearing, or cutting by the electric arc or gas process, providing enough metal is left at any unfinished edges to meet the requirements of PG-79.

PG-76.2 When end faces of nozzle or manhole necks are to remain unwelded in the completed vessel, these end faces shall not be cut by shearing unless at least 1/8 in. (3 mm) of additional metal is removed by any method that will produce a smooth finish.

The inspection required by this Section shall be by an Inspector employed by an ASME accredited Authorized Inspection Agency,12 that is, the inspection organization of a state or municipality of the United States, a Canadian province, or of an insurance company authorized to write boiler and pressure vessel insurance. These Inspectors shall have been qualified by written examination under the rules of any state of the United States or province of Canada which has adopted the Code.

12 Whenever Authorized Inspection Agencyor AIAis used in this Code, it shall mean an Authorized Inspection Agency accredited by ASME in accordance with the requirements in the latest edition of ASME QAI-1, Qualification for Authorized Inspection.

PG-93.1 When flat plate greater than ½ in. (13 mm) thickness is welded to other pressure parts to form a corner joint, such as in flat heads [Fig. PG-31, illustrations (g), (i-1), and (i-2)], waterlegs of firebox boilers or combustion chambers of wetback boilers [Fig. A-8, illustrations (l), (m), (n), and (p)], and the exposed edges of the plate are closer to the edge of the weld than a distance equal to the thickness of the plate, the peripheral plate edges and any remaining edges and any remaining exposed surface of the weld joint preparation shall be examined after welding by either the magnetic particle or liquid penetrant method. When the plate is nonmagnetic, only the liquid penetrant method shall be used. The requirements of this paragraph shall not apply to those joints when 80% or more of the pressure load is carried by tubes, stays, or braces, or when the exposed edges of the plate are farther from the edge of the weld than a distance equal to the thickness of the plate.

PG-93.2 Laminations, cracks, or other imperfections found during the examination required by PG-93.1 that would affect the safety of the vessel shall be repaired in accordance with PG-78. The imperfection(s) may be pursued by any suitable method (grinding, chipping, etc.). The repaired area shall be subjected to the same examination that first revealed the imperfection.

After a boiler has been completed (see PG-104), it shall be subjected to pressure tests using water at not less than ambient temperature, but in no case less than 70°F (20°C). Where required, test pressures are specified in this paragraph; whether minimum or maximum pressures, they apply to the highest point of the boiler system. When the boiler is completed in the Manufacturer"s shop without boiler external piping, subsequent hydrostatic testing of the boiler external piping shall be the responsibility of any holder of a valid “S," “A," or “PP" stamp. The safety valves need not be included in the hydrostatic test. The tests shall be made in two stages in the following sequence:

PG-99.1 Hydrostatic pressure tests shall be applied by raising the pressure gradually to not less than 1 ½ times the maximum allowable working pressure as shown on the data report to be stamped on the boiler. No part of the boiler shall be subjected to a general membrane stress greater than 90% of its yield strength (0.2% offset) at test temperature. The primary membrane stress to which boiler components are subjected during hydrostatic test shall be taken into account when designing the components. Close visual inspection for leakage is not required during this stage.

PG-99.2 The hydrostatic test pressure may then be reduced to the maximum allowable working pressure, as shown on the Data Report, to be stamped on the boiler and maintained at this pressure while the boiler is carefully examined. The metal temperature shall not exceed 120°F (50°C) during the close examination.

PG-99.3 A completed forced-flow steam generator with no fixed steam and waterline, having pressure parts designed for different pressure levels along the path of water-steam flow, shall be subjected to a hydrostatic pressure test by the above procedure (PG-99.1 and PG-99.2) based upon:

PG-99.3.1 For the first stage test (PG-99.1) a hydrostatic test pressure of not less than 1 ½ times the maximum allowable working pressure at the superheater outlet as shown in the master stamping (PG-106.3) but no less than 1 ¼ times the maximum allowable working pressure of any part of the boiler, excluding the boiler external piping.

PG-99.3.2For the second stage test (PG-99.2) the hydrostatic test pressure may be reduced to not less than the maximum allowable working pressure at the superheater outlet.

PG-99.4.1 An indicating gage, visible to the operator controlling the pressure applied, shall be connected to the pressure parts. Hydrostatic head on the gage shall be considered such that the required test pressure is achieved at the top of the boiler.

PG-99.4.2 Dial pressure gages used in testing shall preferably have dials graduated over their entire range of about double the intended maximum test pressure, but in no case shall the range be less than 1 ½ times that pressure. The spacing between graduations shall be such that the inspector and the operator controlling the test shall be able to determine when the required test pressure has been applied. Digital pressure gages having a wider range of pressure readings may be used provided the readings give the same or greater degree of accuracy as obtained with dial pressure gages.

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.