how does a safety valve work supplier

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 safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds preset limits. Pilot-operated relief valves are a specialized type of pressure safety valve. A leak tight, lower cost, single emergency use option would be a rupture disk.

Safety valves were first developed for use on steam boilers during the Industrial Revolution. Early boilers operating without them were prone to explosion unless carefully operated.

Vacuum safety valves (or combined pressure/vacuum safety valves) are used to prevent a tank from collapsing while it is being emptied, or when cold rinse water is used after hot CIP (clean-in-place) or SIP (sterilization-in-place) procedures. When sizing a vacuum safety valve, the calculation method is not defined in any norm, particularly in the hot CIP / cold water scenario, but some manufacturers

The earliest and simplest safety valve was used on a 1679 steam digester and utilized a weight to retain the steam pressure (this design is still commonly used on pressure cookers); however, these were easily tampered with or accidentally released. On the Stockton and Darlington Railway, the safety valve tended to go off when the engine hit a bump in the track. A valve less sensitive to sudden accelerations used a spring to contain the steam pressure, but these (based on a Salter spring balance) could still be screwed down to increase the pressure beyond design limits. This dangerous practice was sometimes used to marginally increase the performance of a steam engine. In 1856, John Ramsbottom invented a tamper-proof spring safety valve that became universal on railways. The Ramsbottom valve consisted of two plug-type valves connected to each other by a spring-laden pivoting arm, with one valve element on either side of the pivot. Any adjustment made to one of valves in an attempt to increase its operating pressure would cause the other valve to be lifted off its seat, regardless of how the adjustment was attempted. The pivot point on the arm was not symmetrically between the valves, so any tightening of the spring would cause one of the valves to lift. Only by removing and disassembling the entire valve assembly could its operating pressure be adjusted, making impromptu "tying down" of the valve by locomotive crews in search of more power impossible. The pivoting arm was commonly extended into a handle shape and fed back into the locomotive cab, allowing crews to "rock" both valves off their seats to confirm they were set and operating correctly.

Safety valves also evolved to protect equipment such as pressure vessels (fired or not) and heat exchangers. The term safety valve should be limited to compressible fluid applications (gas, vapour, or steam).

For liquid-packed vessels, thermal relief valves are generally characterized by the relatively small size of the valve necessary to provide protection from excess pressure caused by thermal expansion. In this case a small valve is adequate because most liquids are nearly incompressible, and so a relatively small amount of fluid discharged through the relief valve will produce a substantial reduction in pressure.

Flow protection is characterized by safety valves that are considerably larger than those mounted for thermal protection. They are generally sized for use in situations where significant quantities of gas or high volumes of liquid must be quickly discharged in order to protect the integrity of the vessel or pipeline. This protection can alternatively be achieved by installing a high integrity pressure protection system (HIPPS).

In the petroleum refining, petrochemical, chemical manufacturing, natural gas processing, power generation, food, drinks, cosmetics and pharmaceuticals industries, the term safety valve is associated with the terms pressure relief valve (PRV), pressure safety valve (PSV) and relief valve.

The generic term is Pressure relief valve (PRV) or pressure safety valve (PSV). PRVs and PSVs are not the same thing, despite what many people think; the difference is that PSVs have a manual lever to open the valve in case of emergency.

Relief valve (RV): an automatic system that is actuated by the static pressure in a liquid-filled vessel. It specifically opens proportionally with increasing pressure

Pilot-operated safety relief valve (POSRV): an automatic system that relieves on remote command from a pilot, to which the static pressure (from equipment to protect) is connected

Low pressure safety valve (LPSV): an automatic system that relieves static pressure on a gas. Used when the difference between the vessel pressure and the ambient atmospheric pressure is small.

Vacuum pressure safety valve (VPSV): an automatic system that relieves static pressure on a gas. Used when the pressure difference between the vessel pressure and the ambient pressure is small, negative and near to atmospheric pressure.

Low and vacuum pressure safety valve (LVPSV): an automatic system that relieves static pressure on a gas. Used when the pressure difference is small, negative or positive and near to atmospheric pressure.

In most countries, industries are legally required to protect pressure vessels and other equipment by using relief valves. Also, in most countries, equipment design codes such as those provided by the ASME, API and other organizations like ISO (ISO 4126) must be complied with. These codes include design standards for relief valves and schedules for periodic inspection and testing after valves have been removed by the company engineer.

Today, the food, drinks, cosmetics, pharmaceuticals and fine chemicals industries call for hygienic safety valves, fully drainable and Cleanable-In-Place. Most are made of stainless steel; the hygienic norms are mainly 3A in the USA and EHEDG in Europe.

The first safety valve was invented by Denis Papin for his steam digester, an early pressure cooker rather than an engine.steelyard" lever a smaller weight was required, also the pressure could easily be regulated by sliding the same weight back and forth along the lever arm. Papin retained the same design for his 1707 steam pump.Greenwich in 1803, one of Trevithick"s high-pressure stationary engines exploded when the boy trained to operate the engine left it to catch eels in the river, without first releasing the safety valve from its working load.

Although the lever safety valve was convenient, it was too sensitive to the motion of a steam locomotive. Early steam locomotives therefore used a simpler arrangement of weights stacked directly upon the valve. This required a smaller valve area, so as to keep the weight manageable, which sometimes proved inadequate to vent the pressure of an unattended boiler, leading to explosions. An even greater hazard was the ease with which such a valve could be tied down, so as to increase the pressure and thus power of the engine, at further risk of explosion.

Although deadweight safety valves had a short lifetime on steam locomotives, they remained in use on stationary boilers for as long as steam power remained.

Weighted valves were sensitive to bouncing from the rough riding of early locomotives. One solution was to use a lightweight spring rather than a weight. This was the invention of Timothy Hackworth on his leaf springs.

These direct-acting spring valves could be adjusted by tightening the nuts retaining the spring. To avoid tampering, they were often shrouded in tall brass casings which also vented the steam away from the locomotive crew.

The Salter coil spring spring balance for weighing, was first made in Britain by around 1770.spring steels to make a powerful but compact spring in one piece. Once again by using the lever mechanism, such a spring balance could be applied to the considerable force of a boiler safety valve.

The spring balance valve also acted as a pressure gauge. This was useful as previous pressure gauges were unwieldy mercury manometers and the Bourdon gauge had yet to be invented.

Paired valves were often adjusted to slightly different pressures too, a small valve as a control measure and the lockable valve made larger and permanently set to a higher pressure, as a safeguard.Sinclair for the Eastern Counties Railway in 1859, had the valve spring with pressure scale behind the dome, facing the cab, and the locked valve ahead of the dome, out of reach of interference.

In 1855, John Ramsbottom, later locomotive superintendent of the LNWR, described a new form of safety valve intended to improve reliability and especially to be tamper-resistant. A pair of plug valves were used, held down by a common spring-loaded lever between them with a single central spring. This lever was characteristically extended rearwards, often reaching into the cab on early locomotives. Rather than discouraging the use of the spring lever by the fireman, Ramsbottom"s valve encouraged this. Rocking the lever freed up the valves alternately and checked that neither was sticking in its seat.

A drawback to the Ramsbottom type was its complexity. Poor maintenance or mis-assembly of the linkage between the spring and the valves could lead to a valve that no longer opened correctly under pressure. The valves could be held against their seats and fail to open or, even worse, to allow the valve to open but insufficiently to vent steam at an adequate rate and so not being an obvious and noticeable fault.Rhymney Railway, even though the boiler was almost new, at only eight months old.

Naylor valves were introduced around 1866. A bellcrank arrangement reduced the strain (percentage extension) of the spring, thus maintaining a more constant force.L&Y & NER.

All of the preceding safety valve designs opened gradually and had a tendency to leak a "feather" of steam as they approached "blowing-off", even though this was below the pressure. When they opened they also did so partially at first and didn"t vent steam quickly until the boiler was well over pressure.

The quick-opening "pop" valve was a solution to this. Their construction was simple: the existing circular plug valve was changed to an inverted "top hat" shape, with an enlarged upper diameter. They fitted into a stepped seat of two matching diameters. When closed, the steam pressure acted only on the crown of the top hat, and was balanced by the spring force. Once the valve opened a little, steam could pass the lower seat and began to act on the larger brim. This greater area overwhelmed the spring force and the valve flew completely open with a "pop". Escaping steam on this larger diameter also held the valve open until pressure had dropped below that at which it originally opened, providing hysteresis.

These valves coincided with a change in firing behaviour. Rather than demonstrating their virility by always showing a feather at the valve, firemen now tried to avoid noisy blowing off, especially around stations or under the large roof of a major station. This was mostly at the behest of stationmasters, but firemen also realised that any blowing off through a pop valve wasted several pounds of boiler pressure; estimated at 20 psi lost and 16 lbs or more of shovelled coal.

Pop valves derived from Adams"s patent design of 1873, with an extended lip. R. L. Ross"s valves were patented in 1902 and 1904. They were more popular in America at first, but widespread from the 1920s on.

Although showy polished brass covers over safety valves had been a feature of steam locomotives since Stephenson"s day, the only railway to maintain this tradition into the era of pop valves was the GWR, with their distinctive tapered brass safety valve bonnets and copper-capped chimneys.

Developments in high-pressure water-tube boilers for marine use placed more demands on safety valves. Valves of greater capacity were required, to vent safely the high steam-generating capacity of these large boilers.Naylor valve) became more critical.distilled feedwater and also a scouring of the valve seats, leading to wear.

High-lift safety valves are direct-loaded spring types, although the spring does not bear directly on the valve, but on a guide-rod valve stem. The valve is beneath the base of the stem, the spring rests on a flange some height above this. The increased space between the valve itself and the spring seat allows the valve to lift higher, further clear of the seat. This gives a steam flow through the valve equivalent to a valve one and a half or twice as large (depending on detail design).

The Cockburn Improved High Lift design has similar features to the Ross pop type. The exhaust steam is partially trapped on its way out and acts on the base of the spring seat, increasing the lift force on the valve and holding the valve further open.

To optimise the flow through a given diameter of valve, the full-bore design is used. This has a servo action, where steam through a narrow control passage is allowed through if it passes a small control valve. This steam is then not exhausted, but is passed to a piston that is used to open the main valve.

There are safety valves known as PSV"s and can be connected to pressure gauges (usually with a 1/2" BSP fitting). These allow a resistance of pressure to be applied to limit the pressure forced on the gauge tube, resulting in prevention of over pressurisation. the matter that has been injected into the gauge, if over pressurised, will be diverted through a pipe in the safety valve, and shall be driven away from the gauge.

There is a wide range of safety valves having many different applications and performance criteria in different areas. In addition, national standards are set for many kinds of safety valves.

Safety valves are required on water heaters, where they prevent disaster in certain configurations in the event that a thermostat should fail. Such a valve is sometimes referred to as a "T&P valve" (Temperature and Pressure valve). There are still occasional, spectacular failures of older water heaters that lack this equipment. Houses can be leveled by the force of the blast.

Pressure cookers are cooking pots with a pressure-proof lid. Cooking at pressure allows the temperature to rise above the normal boiling point of water (100 degrees Celsius at sea level), which speeds up the cooking and makes it more thorough.

Pressure cookers usually have two safety valves to prevent explosions. On older designs, one is a nozzle upon which a weight sits. The other is a sealed rubber grommet which is ejected in a controlled explosion if the first valve gets blocked. On newer generation pressure cookers, if the steam vent gets blocked, a safety spring will eject excess pressure and if that fails, the gasket will expand and release excess pressure downwards between the lid and the pan. Also, newer generation pressure cookers have a safety interlock which locks the lid when internal pressure exceeds atmospheric pressure, to prevent accidents from a sudden release of very hot steam, food and liquid, which would happen if the lid were to be removed when the pan is still slightly pressurised inside (however, the lid will be very hard or impossible to open when the pot is still pressurised).

These figures are based on two measurements, a drop from 225 psi to 205 psi for an LNER Class V2 in 1952 and a smaller drop of 10 psi estimated in 1953 as 16 lbs of coal.

"Trial of HMS Rattler and Alecto". April 1845. The very lowest pressure exhibited "when the screw was out of the water" (as the opponents of the principle term it) was 34 lb, ranging up to 60 lb., on Salter"s balance.

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.

When it came back, I laid it down on the outlet flange and looked up the inlet. There was a 12″ welding rod with the tip stuck between the seat and the disc. That rod was from the original construction and didn’t get blown out properly and just now it got set free. The maintenance guy didn’t believe me and came over and saw it for himself (this was before cell phones when you could take a picture).

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.

In the process industry, both terms refer to safety devices, which generally come in the form of valves, cylinders, and other cylinders that protect people, property, and the environment. Safety valves and relief valves are integral components of process safety. However, they are used for almost identical purposes. Their main difference lies in their operating mechanisms.

In the event of an overpressure, a safety valve or pressure relief valve (PRV) protects pressure-sensitive equipment. It is recommended to strip down relief valves regularly and prevent serious damage due to backpressure. Pressure relief valves are a crucial part of any pressurized system. In order to prevent system failures, you can set the pressure to open at predetermined levels. A setpoint, also known as a predetermined design limit, is set for all pressure systems. When the setpoint is exceeded, an overpressure valve opens.

There are various types of safety valves used in several types of industries, including power plants, petrochemical plants, boilers, oil and gas, pharmaceuticals, and more. Using safety valves helps to prevent accidents and injuries that can harm people, property, and processes. Pressure builds up in vessels and systems automatically when the device is activated above a preset level. Safety valves must be configured so that their prescribed pressure is exceeded in order for them to function (i.e., relieve pressure). Ideally, excess pressure should be released either to the atmosphere or back into the pneumatic system to prevent damage to the vessel. In addition, excess pressure should be released to keep pressure within a certain range. As soon as a slight increase in pressure above the desired limit has lifted the safety valve, it opens.

Valve relief removes excessive pressure from a system by limiting its pressure level to a safe level. Often referred to as pressure relief valves (PRVs) or safety relief valves, these valves provide relief from pressure. The purpose of a relief valve is, for example, to adjust the pressure within a vessel or a system so that a specific level is maintained. The goal of a relief valve, unlike a safety valve, is not to prevent damage to the vessel; rather, it is to control the pressure limit of a system dynamically depending on the requirements. Conversely, safety valves have a maximum allowable pressure set at a certain level, which allows escaping liquid or gas whenever the pressure exceeds it, eliminating damage to the system. It is imperative that safety valves are installed in a control system to prevent the development of pressure fluctuations that can cause property damage, life loss, and environmental pollution.

The hydraulic system relies on a pressure relief system in order to regulate the running pressure. By allowing excess pressure to escape from the pressurized zone, pressure relief valves and safety valves prevent overpressure when the pressure in the system reaches a predefined limit. By venting excess pressure through a relief port, or returning it through a return line, a pneumatic system can enable the excess pressure to escape into the atmosphere. Pump-driven pressure generators and control media that cannot be vented into the atmosphere are typical examples of this type of application.

Excess pressure may be relieved from the system using relief valves and safety valves. The valve opening increases proportionally as the vessel pressure increases with the relief valve. Gradually opening the valve rather than abruptly releases only a prescribed amount of fluid. As pressure is reduced, the release proceeds at this rate until the pressure drops. By contrast, an emergency safety valve operates automatically when a predetermined pressure is reached in the system, preventing a catastrophic system failure. When the system is under excessive stress, the safety valve regulates the pressure within the system and prevents overpressure.

Defining a “setpoint” is the process of defining a pressure level which triggers the device to vent excess pressure. Setpoint is different from pressure. Overpressure is prevented by setting these devices lower than the highest pressure the system can handle before overpressure occurs. Setting the device below this pressure prevents overpressure. The valve opens when pressure rises above the setpoint. A setpoint also known as the maximum allowable working pressure (MAWP) cannot be exceeded when deciding the pressure in pounds per square inch (PSIG). The adjustment points for safety valves are generally 3 percent above working pressures, while adjustment points for relief valves are 10% above working pressures.

Pressure in an auxiliary passage can be controlled by a safety valve as well as a relief valve by releasing excess pressure. Safety valves of this type are pressure-sensitive and reliable. Safety valves can be categorized according to their capacity and setpoint, although both terms often refer to safety valves. Self-opening devices open automatically when maximum allowable pressure has been reached rather than being manually activated to prevent over-pressurizing. Contrary to relief valves, safety valves are typically used for venting steam or vapor into the atmosphere. Relief valves regulate fluid flow and compressed air pressure and gases, whereas safety valves typically regulate steam and vapor venting. Put simply, relief valves are used for more gradual pressure control requiring accurate, dynamic systems, whereas safety valves are used for one set to prevent damage to a system.

For pressure control applications that require dynamic setpoints and therefore varying pressure limits, our Electronic Relief Valve is the appropriate solution. This device accepts a control voltage to dynamically set the relief pressure setpoint. Traditional relief valves are set manually, so that a technician must adjust the relief valve and have a pressure gauge to find the accurate setpoint. The Kelly Pneumatic Electronic Relief Valve allows an electronic control system to quickly and safely command a dynamic maximum pressure based on feedback from current system specifications. The Kelly Electronic Relief Valve also has an optional feedback signal representing the current pressure in the system. This allows the control system to dynamically respond to changing conditions.

A series of anomalies occurred in the boiler room that evening. The steel compression tank for the hydronic loop flooded, leaving no room for expansion. Water will expand at 3% of its volume when heated from room temperature to 180° F. When the burner fired, the expansion of the water increased the system pressure within the boiler. The malfunctioning operating control did not shut off the burner at the set point which caused the relief valve to open.

The brass relief valve discharge was installed with copper tubing piped solid to a 90° ell on the floor and the tubing further extended to the floor drain. The combination of hot water and steam from the boiler caused the discharge copper tubing to expand, using the relief valve as a fulcrum. The expansion of the copper discharge tubing pressing against the floor was enough to crack the brass relief valve, flooding the boiler room. The damage was not discovered until the next morning, several hours after the leak occurred. Thousands of dollars in damage was sustained and luckily no one was injured.

Each boiler requires some sort of pressure relieving device. They are referred to as either a safety, relief or safety relief valve. While these names are often thought of as interchangeable, there are subtle differences between them. According to the National Board of Boiler and Pressure Vessel Inspectors, the following are the definitions of each:

• Safety valve— This device is typically used for steam or vapor service. It operates automatically with a full-opening pop action and recloses when the pressure drops to a value consistent with the blowdown requirements prescribed by the applicable governing code or standard.

• Relief valve— This device is used for liquid service. It operates automatically by opening farther as the pressure increases beyond the initial opening pressure and recloses when the pressure drops below the opening pressure.

• Safety relief valve— This device includes the operating characteristics of both a safety valve and a relief valve and may be used in either application.

• Temperature and pressure safety relief valve— This device is typically used on potable water heaters. In addition to its pressure-relief function, it also includes a temperature-sensing element which causes the device to open at a predetermined temperature regardless of pressure. The set temperature on these devices is usually 210°.

• Relief valve piping— The boiler contractor installed a bushing on the outlet of the safety relief valve. Instead of 1 1/2-in. pipe, the installer used 3/4-in. pipe. When asked about it, he answered that he did not have any 1 1/2-in. pipe but had plenty of 3/4-in. pipe. I explained and then had to show the disbelieving contractor the code that states that the relief valve discharge piping has to be the same diameter as the relief valve outlet (see 2012 International Mechanical Code, 1006.6). By reducing the discharge pipe size, the relieving capacity of the safety valve may not be adequate to properly relieve the pressure inside the boiler, causing a dangerous situation.

The code also states that the discharge material shall be of rigid pipe that is approved for the temperature of the system. The inlet pipe size shall be full diameter of the pipe inlet for the relief valve. Some manufacturers suggest using black iron pipe rather than copper tubing. If using copper, it should have an air space that allows expansion should the relief valve open to avoid the accident that I referenced above. The discharge piping has to be supported and the weight of the piping should not be on the safety relief valve. Valves are not permitted in the inlet piping to or discharge piping from the relief valve. If you are using copper tubing on discharge piping, verify that there is room for expansion.

• Installation— Read the manufacturer’s installation manual as each may have different requirements. For instance, Conbraco requires that the discharge piping must terminate with a plain end and use a material that can handle temperatures of 375° or greater. This will preclude PVC or CPVC pipe for the discharge piping. The instruction manual for its model 12-14 steam relief valve stipulates that you cannot use a pipe wrench to install it. That would be good to know.

I once visited Boiler Utopia as the floor was clean and waxed. All the pipes were covered and exposed pipes were painted. There were large stickers detailing what was inside each pipe as well as directional arrows. Nothing was stacked next to the boilers. Yellow caution lines were painted on the floor around each boiler. I was in heaven. As I walked around the rear of the boiler, something clicked and triggered a warning bell. The discharge of the relief valve piping was about 6 in. from the floor but instead of a plain or angled cut end, the pipe had a threaded pipe cap on the termination. I asked the maintenance person about it and he said that the valve was leaking all over his newly waxed floor and this was the only way he could stop it. When I said that the discharge pipe should not have been threaded, he explained that it was not threaded and he had to take it to the local hardware store to thread it. I informed him that the cap had to be removed. We cut the pipe on an angle to prevent this.

• Steam boiler— Most manufacturers recommend a drip pan ell on the discharge of the steam boiler relief valve to eliminate the weight of the discharge piping on the relief valve. Some codes require the discharge to be vented outdoors.

• Testing— I will ask the attendees in my classes, “How often do you test the relief valves?” Most do not make eye contact and when I follow up with, “Why are they not tested?” I often hear that opening the relief valve will cause it to leak. I suggest that you refer to each manufacturer’s directions for testing. For instance, one will recommend once a year while another recommends twice a year. One manufacturer says, “Safety/relief valves should be operated only often enough to assure they are in good working order.” I am not sure what that even means. You want to also verify the proper test procedure as some will only want the relief valve tested when the boiler is at 75% of the rated pressure or higher of the relief valve.

Years ago, it was not uncommon to read news about tragic boiler explosions, sometimes resulting in mass destruction. Today, boilers are equipped with important safety devises to help protect against these types of catastrophes. Let’s take a look at the most critical of these devices: the safety valve.

The safety valve is one of the most important safety devices in a steam system. Safety valves provide a measure of security for plant operators and equipment from over pressure conditions. The main function of a safety valve is to relieve pressure. It is located on the boiler steam drum, and will automatically open when the pressure of the inlet side of the valve increases past the preset pressure. All boilers are required by ASME code to have at least one safety valve, dependent upon the maximum flow capacity (MFC) of the boiler. The total capacity of the safety valve at the set point must exceed the steam control valve’s MFC if the steam valve were to fail to open. In most cases, two safety valves per boiler are required, and a third may be needed if they do not exceed the MFC.

There are three main parts to the safety valve: nozzle, disc, and spring. Pressurized steam enters the valve through the nozzle and is then threaded to the boiler. The disc is the lid to the nozzle, which opens or closes depending on the pressure coming from the boiler. The spring is the pressure controller.

As a boiler starts to over pressure, the nozzle will start to receive a higher pressure coming from the inlet side of the valve, and will start to sound like it is simmering. When the pressure becomes higher than the predetermined pressure of the spring, the disc will start to lift and release the steam, creating a “pop” sound. After it has released and the steam and pressure drops below the set pressure of the valve, the spring will close the disc. Once the safety valve has popped, it is important to check the valve to make sure it is not damaged and is working properly.

A safety valve is usually referred to as the last line of safety defense. Without safety valves, the boiler can exceed it’s maximum allowable working pressure (MAWP) and not only damage equipment, but also injure or kill plant operators that are close by. Many variables can cause a safety valve on a boiler to lift, such as a compressed air or electrical power failure to control instrumentation, or an imbalance of feedwater rate caused by an inadvertently shut or open isolation valve.

Once a safety valve has lifted, it is important to do a complete boiler inspection and confirm that there are no other boiler servicing issues. A safety valve should only do its job once; safety valves should not lift continuously. Lastly, it is important to have the safety valves fully repaired, cleaned and recertified with a National Board valve repair (VR) stamp as required by local code or jurisdiction. Safety valves are a critical component in a steam system, and must be maintained.

All of Nationwide Boiler’s rental boilers include on to two safety valves depending on the size; one set at design pressure and the other set slightly higher than design. By request, we can reset the safeties to a lower pressure if the application requires it. In addition, the valves are thoroughly checked after every rental and before going out to a new customer, and they are replaced and re-certified as needed.

Boiler explosions have been responsible for widespread damage to companies throughout the years, and that’s why today’s boilers are equipped with safety valves and/or relief valves. Boiler safety valves are designed to prevent excess pressure, which is usually responsible for those devastating explosions. That said, to ensure that boiler safety valves are working properly and providing adequate protection, they must meet regulatory specifications and require ongoing maintenance and periodic testing. Without these precautions, malfunctioning safety valves may fail, resulting in potentially disastrous consequences.

Boiler safety valves are activated by upstream pressure. If the pressure exceeds a defined threshold, the valve activates and automatically releases pressure. Typically used for gas or vapor service, boiler safety valves pop fully open once a pressure threshold is reached and remain open until the boiler pressure reaches a pre-defined, safe lower pressure.

Boiler relief valves serve the same purpose – automatically lowering boiler pressure – but they function a bit differently than safety valves. A relief valve doesn’t open fully when pressure exceeds a defined threshold; instead, it opens gradually when the pressure threshold is exceeded and closes gradually until the lower, safe threshold is reached. Boiler relief valves are typically used for liquid service.

There are also devices known as “safety relief valves” which have the characteristics of both types discussed above. Safety relief valves can be used for either liquid or gas or vapor service.

Nameplates must be fastened securely and permanently to the safety valve and remain readable throughout the lifespan of the valve, so durability is key.

The National Board of Boiler and Pressure Vessel Inspectors offers guidance and recommendations on boiler and pressure vessel safety rules and regulations. However, most individual states set forth their own rules and regulations, and while they may be similar across states, it’s important to ensure that your boiler safety valves meet all state and local regulatory requirements.

The National Board published NB-131, Recommended Boiler and Pressure Vessel Safety Legislation, and NB-132, Recommended Administrative Boiler and Pressure Vessel Safety Rules and Regulationsin order to provide guidance and encourage the development of crucial safety laws in jurisdictions that currently have no laws in place for the “proper construction, installation, inspection, operation, maintenance, alterations, and repairs” necessary to protect workers and the public from dangerous boiler and pressure vessel explosions that may occur without these safeguards in place.

The documents are meant to be used as a guide for developing local laws and regulations and also may be used to update a jurisdiction’s existing requirements. As such, they’re intended to be modifiable to meet any jurisdiction’s local conditions.

The American Society of Mechanical Engineers (ASME) governs the code that establishes guidelines and requirements for safety valves. Note that it’s up to plant personnel to familiarize themselves with the requirements and understand which parts of the code apply to specific parts of the plant’s steam systems.

High steam capacity requirements, physical or economic constraints may make the use of a single safety valve impossible. In these cases, using multiple safety valves on the same system is considered an acceptable practice, provided that proper sizing and installation requirements are met – including an appropriately sized vent pipe that accounts for the total steam venting capacity of all valves when open at the same time.

The lowest rating (MAWP or maximum allowable working pressure) should always be used among all safety devices within a system, including boilers, pressure vessels, and equipment piping systems, to determine the safety valve set pressure.

General guidance on proper installation may seem like common sense to experienced installers and inspectors. A few of the most important guidelines and best practices include:

Avoid isolating safety valves from the system, such as by installing intervening shut-off valves located between the steam component or system and the inlet.

Contact the valve supplier immediately for any safety valve with a broken wire seal, as this indicates that the valve is unsafe for use. Safety valves are sealed and certified in order to prevent tampering that can prevent proper function.

Avoid attaching vent discharge piping directly to a safety valve, which may place unnecessary weight and additional stress on the valve, altering the set pressure.

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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.

Pressure relief valves (safety relief valves) are designed to open at a preset pressure and discharge fluid until pressure drops to acceptable levels. The development of the safety relief valve has an interesting history.

Denis Papin is credited by many sources as the originator of the first pressure relief valve (circa 1679) to prevent overpressure of his steam powered “digester”. His pressure relief design consisted of a weight suspended on a lever arm. When the force of the steam pressure acting on the valve exceeded the force of the weight acting through the lever arm the valve opened. Designs requiring a higher relief pressure setting required a longer lever arm and/or larger weights. This simple system worked however more space was needed and it coud be easily tampered with leadin