how does a gas oven safety valve work factory

Most modern appliances have safety features built in, but your gas oven safety valve is arguably the most important. If an electrical appliance malfunctions, it can cause a fire, but a misfiring gas oven could potentially blow up your house. You don"t ​really​ need to know how the safety mechanism works to use your oven, but you may find that it gives you some extra peace of mind.

Broadly speaking, there are two ways a built-in safety mechanism can work. One option is that it remains "open" by default and to shut off if certain conditions are met. That"s how fuses and circuit breakers work in an electrical circuit: Ordinarily, the electricity is free to flow, but if the current grows too large, the fuse or breaker will blow and cut off the circulation of electricity.

The other option is for your safety mechanism to be "closed" by default and allow a device to operate only when the correct conditions are met. That"s how a gas oven safety valve works. Gas ordinarily is prevented from flowing, and if the valve is working correctly, it opens only when you want to light your oven.

Many gas stoves use what"s called a "hot surface igniter," a bar or element (similar to the ones on your stovetop) that gets hot enough to ignite the gas on contact. Gas oven safety valves on stoves with this type of ignition system take a couple of different approaches.

In one approach, a bimetallic strip operates the valve. It harnesses a simple scientific principle: Metals expand and contract at different rates when they"re heated and cooled. If you bond two suitable metals together in one strip, that strip will flex to a predictable degree as the temperature goes up and down. Wall-mount thermostats often use this principle, as do analog oven thermometers and the thermometer in the lid of your gas grill.

As appliance-repair website PartSelect explains, turning on your gas oven causes electricity to flow into the heating element of your hot surface igniter. As the igniter heats up, it warms a bimetallic strip inside your gas oven safety valve. When the igniter reaches its operating temperature, the bimetallic strip opens the valve and allows the gas to flow, igniting as it crosses the heated surface.

One intriguing thing about electricity is that a change in temperature can affect how well it passes through certain materials. For example, a lot of research revolves around ​superconductors​ – materials that offer very little resistance to an electrical current – but superconductors typically must be heavily chilled to work.

According to heating-equipment vendor Anglo Nordic, gas oven safety valves use a variation of that principle to operate. In these stoves, the flow of electrical current through the hot surface igniter becomes the control mechanism. The igniter"s bar is made of a material that offers less and less resistance to electricity as it heats. When it reaches the temperature required to ignite the gas, its resistance becomes low enough to trip the safety valve and open the flow of gas.

More modern ranges use an electrical igniter. When you turn on your oven, the gas begins flowing immediately, and it sends an electrical current to a piezo electric igniter. The current makes the igniter spark (like the manual igniter on your gas grill) and lights the oven"s burner. In this case, the safety valve works in the opposite way: An electronic sensor checks for the heat caused by ignition after a few seconds, and if it"s absent, it will close the valve and shut off the flow of gas.

It"s worth pointing out that not all gas ovens have a safety valve in the conventional sense. Older stoves simply use a pilot light, a small but constant flow of gas, which, in turn, feeds a small, candle-like flame. You essentially ​are​ the safety mechanism in this system: It"s up to you to check that the pilot is lit. When you turn on the gas manually, the small pilot flame ignites the main flame. It"s a mechanically simple system, which makes it durable, and for that reason, you"ll still see it used on commercial restaurant ranges, which must stand up to decades of heavy use.

First developed for use on early steam boilers as operating without them would cause an explosion unless carefully operated. Modern burner ovens now feature a gas safety valve to prevent an explosion when unlit. Unless ignited by an electric glow bar, pilot flame or electric spark with a gas supply, the valve will remain closed.

Safety should be you’re top priority when you’re working with gas. If you’re not careful, its extremely easy to cause a lot of damage. To prevent this modern day gas safety valves have been fitted around the world, to ensure an efficiently working system.

When your gas safety valves are on their last legs, making a repair or finding a replacement is the best thing to do. Keeping a spare in your kit is highly recommended.

The glow bar igniter and safety valve are wired in series within a glow bar system. This means that electricity can only pass through the valve after it has first passed through the glow bar. Electrical resistance from the glow bar blocks current to the gas valve, making sure that the valve closes. This decreases as the temperature increases. When the bar reaches a particular heat, enough to ignite the gas, it allows sufficient electricity through to open the gas valve.

The gas ignition source on Pilot light ignition system uses a low flame, fed by a line from the thermostat. The pilot flame will either burn constantly or be ignited electronically when the oven is switched on. More gas is fed to the pilot flame when the oven thermostat wants heat. Either excess pressure or an electric current will open the gas valve.

With some ovens the burner is directly ignited with an electric spark. An electric spark is sent to ignite the burner when the gas valve opens, this is due to the thermostat wanting heat. A sensor on the burner detects the flame and confirms ignition. The oven burner will lock out if there is no ignition after two attempts.

Pressure relief valves are usually installed in multi appliance, oil pumped ringmain systems. They are used to maintain a constant pressure on the positive side of the pump whether all appliances are in use or not.

Safety relief discharge pressurised gases and vapours to protect against overpressure. This is done by discharging pressurised gases and vapours from pipelines, this includes pressure vessels and plant components. Safety relief valves are the last line of defence and prevent explosion which could be fatal.

These spring-loaded and direct-acting. When the opening pressure is reached, valve gives way and opens, releasing the pressure. The pressures then equalised and the automatically closes.

Relief valves can either discharge into atmosphere, or via a connected blow-off line. The opening pressure of the boiler relief valve valve is preset usually at the factory according to the customer’s requirements.

Anglo Nordic have just what you need! If you have any questions for need more information, simply call us on 0208 979 0988 or email us at sales@anglo-nordic.com

As a design engineer responsible for developing and specifying boilers, dryers, furnaces, heaters, ovens and other industrial heating equipment, you face a daunting labyrinth of standards and industry regulations. Regulatory bodies sound a bit like alphabet soup, with acronyms like UL, FM, CSA, UR, AGA, ASME, ANSI, IRI, CE and NFPA tossed about. This article will help explain a common task for many thermal processing equipment specifiers: meeting the requirements of key codes — including Underwriters Laboratories (UL), Factory Mutual Insurers (FM) and the National Fire Protection Association (NFPA) — for safety valve equipment used in process heating applications.

Key to designing safety into your fuel train configurations are familiar technologies such as safety shutoff valves and vent valves as well as visual-indication mechanisms and proof-of-closure switches.

Your design skills come into play with how you take advantage of the wide range of products available. You can mix and match solenoid and safety shutoff valves — within designs from catalytic reactors to multi-zone furnaces — to create easily installed, cost-effective solutions that comply with all necessary standards. (See table.)

Make sure, however, that you start with a good grasp of valve element fundamentals. For example, examining a proof-of-closure (POC) switch underlines how reliably modern valves can ensure combustion safety. The POC unit provides an electrical contact interlocked with the controller safety circuit. In a typical design, the switch is located at the bottom of the valve, positioned to trace the stroke of the valve disc. When the disc seal reaches the fully closed position, it triggers the mechanism to push down on the contact, closing it and triggering the unit’s visual indicator to show open or closed status. As a result, the operator can act with full confidence in situations where it is critical that a safety valve be safely closed.

To provide ease of installation, many users prefer valves with modular capabilities. For example, to reduce mounting complexity, you can choose modular gas safety shut-off valves — combining a solenoid valve with an electrohydraulic motorized valve for a compact double-valve footprint, a slow-open feature and high flow rates. An accompanying actuator can provide on/off or high/low/off firing rates as well as visual indication and proof of closure for compliance with most industry standards.

Also, you may want to look for valves that include useful features such as pipe taps, which can facilitate accurate pressure readings and leakage testing.

Knowing your valve choices — and how they meet given codes and standards — can reduce the time required for design and production while facilitating compliance. This results in safer, more efficient and cost-effective heating process installations.

Safety shut-off valves (SSOVs) are the primary automatic means of turning gas flow ON and OFF for furnaces, ovens, and in some cases, individual burners (keep in mind that a "manual shut-off valve" is always required). SSOVs are required by national codes and standards with at least two SSOVs required between a burner and the fuel supply. Besides that, they are a good way of keeping you and your factory from joining the upper atmosphere.

If your gas range is not working correctly, you should check the gas pressure regulator shut-off valve. The factory default setting for the gas pressure regulator is in the "ON" position but may have been turned to the "OFF" position during handling or transportation. When the shut-off valve is on the "OFF" position, gas will flow to the cooktop burners but will not provide a gas supply to the oven.

You can check if the shut-off valve if you can slide the range out from the cabinet. If you are unable to slide the range out, we recommend consultation with a local certified technician.

Verify the pressure regulator shut-off valve is in the open position. The pressure regulator is located at the back of the range. Make sure that the shut-off valve lever is in the "On" position (see illustration below).

NOTE: If the range is hard piped, you will not be able to slide it out from the cabinet if it connected with a flexible supply line, take care not to over-extend the supply line. The main gas valve will usually be at the end of a fixed pipe and connect to the pressure regulator with a flexible supply line. Take care not to kink or pinch this flexible pipe.

A gas stove is a stove that is fuelled by combustible gas such as syngas, natural gas, propane, butane, liquefied petroleum gas or other flammable gas. Before the advent of gas, cooking stoves relied on solid fuels such as coal or wood. The first gas stoves were developed in the 1820s and a gas stove factory was established in England in 1836. This new cooking technology had the advantage of being easily adjustable and could be turned off when not in use. The gas stove, however, did not become a commercial success until the 1880s, by which time supplies of piped gas were available in cities and large towns in Britain. The stoves became widespread on the European Continent and in the United States in the early 20th century.

Gas stoves became more common when the oven was integrated into the base and the size was reduced to better fit in with the rest of the kitchen furniture. By the 1910s, producers started to enamel their gas stoves for easier cleaning. Ignition of the gas was originally by match and this was followed by the more convenient pilot light. This had the disadvantage of continually consuming gas. The oven still needed to be lit by match and accidentally turning on the gas without igniting it could lead to an explosion. To prevent these types of accidents, oven manufacturers developed and installed a safety valve called a flame failure device for gas hobs (cooktops) and ovens. Most modern gas stoves have electronic ignition, automatic timers for the oven and extractor hoods to remove fumes.

Gas stoves are a significant source of indoor air pollution,methane emissions from gas stoves in the United States were equivalent to the greenhouse gas emissions of 500,000 cars.gas venting and unintended fugitive emissions throughout the supply chain results in natural gas having a similar carbon footprint to other fossil fuels overall.

The first gas stove was developed in 1802 by Zachäus Winzler (de), but this along with other attempts remained isolated experiments.Northampton, England in 1826 and opened a gas stove factory in 1836. His invention was marketed by the firm Smith & Philips from 1828. An important figure in the early acceptance of this new technology, was Alexis Soyer, the renowned chef at the Reform Club in London. From 1841, he converted his kitchen to consume piped gas, arguing that gas was cheaper overall because the supply could be turned off when the stove was not in use.

A gas stove was shown at the Great Exhibition in London in 1851, but it was only in the 1880s that the technology became a commercial success in England. By that stage a large and reliable network for gas pipeline transport had spread over much of the country, making gas relatively cheap and efficient for domestic use. Gas stoves only became widespread on the European Continent and in the United States in the early 20th century.

Early gas stoves were rather unwieldy, but soon the oven was integrated into the base and the size was reduced to fit in better with the rest of the kitchen furniture.

Gas stoves today use two basic types of ignition sources, standing pilot and electric.pilot light) under the cooktop.flame failure device for gas hobs (cooktops) and ovens. The safety valve depends on a thermocouple that sends a signal to the valve to stay open. Although most modern gas stoves have electronic ignition, many households have gas cooking ranges and ovens that need to be lit with a flame. Electric ignition stoves use electric sparks to ignite the surface burners.Auto reignition is an elegant refinement: the user need not know or understand the wait-then-turn sequence. They simply turn the burner knob to the desired flame size and the sparking is turned off automatically when the flame lights. Auto reignition also provides a safety feature: the flame will be automatically reignited if the flame goes out while the gas is still on—for example by a gust of wind. If the power fails, surface burners must be manually match-lit.

Also stoves with electric ignition must be connected with gas protection mechanisms such as gas control breaker. Because of this many manufacturers supply stoves without electricity plug.

One of the important properties of a gas stove is the heat emitted by the burners. Burner heat is typically specified in terms of kilowatts or British Thermal Units per hour and is directly based on the gas consumption rather than heat absorbed by pans.

Often, a gas stove will have burners with different heat output ratings. For example, a gas cooktop may have a high output burner, often in the range 3 to 6 kilowatts (10,000 to 20,000 BTU/h), and a mixture of medium output burners, 1.5 to 3 kW, and low output burners, 1 kW or less. The high output burner is suitable for boiling a large pot of water quickly, sautéing and searing, while the low output burners are good for simmering.

Some high-end cooktop models provide higher range of heat and heavy-duty burners that can go up to 6 kilowatts (20,000 BTU/h) or even more. These may be desired for preparing large quantities or special types of food and enable certain advanced cooking techniques. However, these burners produce greater emissions and necessitate better ventilation for safe operation.

In the last few years, appliance manufacturers have been making innovative changes to the design and layout of gas stoves. Most of the modern cooktops have come with lattice structure which usually covers the complete range of the top, enabling sliding of cookware from one burner to another without lifting the containers over the gaps of cooktop. Some modern gas stoves also have central fifth burner or an integrated griddle in between the outer burners.

Usually, there isn"t much of a style difference in between them. Slide-in come with lips on their either side and controls over the front along with burner controls. Freestanding gas range cooktops have solid slides and controls placed behind the cooktop.

Many stoves have integrated ovens. Modern ovens often include a convection fan inside the oven to provide even air circulation and let the food cook evenly. Some modern ovens come with temperature sensors which allows close control of baking, automatically shut off after reaching certain temperature, or hold on to particular temperature through the cooking process. Ovens may also have two separate oven bays which allows cooking of two different dishes at the same time.

Many gas stoves come with at least few modern programmable controls to make the handling easier. LCD displays and some other complex cooking routines are some of the standard features present in most of the basic and high-end manufacturing models. Some of the other programmable controls include precise pre-heating, automatic pizza, cook timers and others.

A built-in Japanese three burner gas stove with a fish grill. Note the thermistor buttons protruding from the gas burners, which cut off the flame if the temperature exceeds 250°C.

Modern gas stove ranges are safer than older models. Two of the major safety concerns with gas stoves are child-safe controls and accidental ignition. Some gas cooktops have knobs which can be accidentally switched on even with a gentle bump.

Gas stoves are at risk of overheating when frying oil, raising the oil temperature to the auto-ignition point and creating an oil fire on the stove. Japan, South Korea and China have regulated the addition of electronic safety devices to prevent pan overheating. The devices use a thermistor to monitor the temperature close to the pan, and cut off the gas supply if the heat is too high.

The U.S. Department of Energy (DOE) ran tests in 2014 of cooktop energy transfer efficiency, simulating cooking while testing what percentage of a cooktop"s energy is transferred to a test block. Gas had an efficiency of 43.9%, with ±0.5% repeatability in the measurement. This level of efficiency is only possible if the pan is big enough for the burner.

Japanese gas flames are angled upwards towards the pot to increase efficiency.heatsink-like fins.Jetboil manufactures pots for portable stoves that use a corrugated ribbon to increase efficiency.

Carbon monoxide, formaldehyde, and nitrogen dioxide from gas stoves contribute to indoor air pollution.asthma.The Lancet Respiratory Medicine found "no evidence of an association between the use of gas as a cooking fuel and either asthma symptoms or asthma diagnosis".

Cooking, especially high heat frying, releases smoke (measured as fine particulate matter), acrolein and polycyclic aromatic hydrocarbons.air purifier.U.S. Consumer Product Safety Commission is investigating reducing the health effects of gas stoves, including emissions and ventilation standards.

Gas stoves are often run on natural gas. The extraction and consumption of natural gas is a major and growing contributor to climate change.methane) and carbon dioxide, which is released when natural gas is burned, are greenhouse gases.methane emissions from gas stoves in the United States were equivalent to the greenhouse gas emissions of 500,000 cars.

Some places, such as the Australian Capital Territory, have curtailed installation of gas stoves and appliances in new construction, for reasons of health, indoor air quality, and climate protection.

The primary purpose of a safety valve is to protect life, property and the environment. Safety valves are designed to open and release excess pressure from vessels or equipment and then close again.

The function of safety valves differs depending on the load or main type of the valve. The main types of safety valves are spring-loaded, weight-loaded and controlled safety valves.

Regardless of the type or load, safety valves are set to a specific set pressure at which the medium is discharged in a controlled manner, thus preventing overpressure of the equipment. In dependence of several parameters such as the contained medium, the set pressure is individual for each safety application.

There is a wide range of safety valves available to meet the many different applications and performance criteria demanded by different industries. Furthermore, national standards define many varying types of safety valve.

The ASME standard I and ASME standard VIII for boiler and pressure vessel applications and the ASME/ANSI PTC 25.3 standard for safety valves and relief valves provide the following definition. These standards set performance characteristics as well as defining the different types of safety valves that are used:

ASME I valve - A safety relief valve conforming to the requirements of Section I of the ASME pressure vessel code for boiler applications which will open within 3% overpressure and close within 4%. It will usually feature two blowdown rings, and is identified by a National Board ‘V’ stamp.

ASME VIII valve- A safety relief valve conforming to the requirements of Section VIII of the ASME pressure vessel code for pressure vessel applications which will open within 10% overpressure and close within 7%. Identified by a National Board ‘UV’ stamp.

Full bore safety valve - A safety valve having no protrusions in the bore, and wherein the valve lifts to an extent sufficient for the minimum area at any section, at or below the seat, to become the controlling orifice.

Conventional safety relief valve -The spring housing is vented to the discharge side, hence operational characteristics are directly affected by changes in the backpressure to the valve.

Balanced safety relief valve -A balanced valve incorporates a means of minimising the effect of backpressure on the operational characteristics of the valve.

Pilot operated pressure relief valve -The major relieving device is combined with, and is controlled by, a self-actuated auxiliary pressure relief device.

Power-actuated safety relief valve - A pressure relief valve in which the major pressure relieving device is combined with, and controlled by, a device requiring an external source of energy.

Standard safety valve - A valve which, following opening, reaches the degree of lift necessary for the mass flowrate to be discharged within a pressure rise of not more than 10%. (The valve is characterised by a pop type action and is sometimes known as high lift).

Full lift (Vollhub) safety valve -A safety valve which, after commencement of lift, opens rapidly within a 5% pressure rise up to the full lift as limited by the design. The amount of lift up to the rapid opening (proportional range) shall not be more than 20%.

Direct loaded safety valve -A safety valve in which the opening force underneath the valve disc is opposed by a closing force such as a spring or a weight.

Proportional safety valve - A safety valve which opens more or less steadily in relation to the increase in pressure. Sudden opening within a 10% lift range will not occur without pressure increase. Following opening within a pressure of not more than 10%, these safety valves achieve the lift necessary for the mass flow to be discharged.

Diaphragm safety valve -A direct loaded safety valve wherein linear moving and rotating elements and springs are protected against the effects of the fluid by a diaphragm

Bellows safety valve - A direct loaded safety valve wherein sliding and (partially or fully) rotating elements and springs are protected against the effects of the fluids by a bellows. The bellows may be of such a design that it compensates for influences of backpressure.

Controlled safety valve - Consists of a main valve and a control device. It also includes direct acting safety valves with supplementary loading in which, until the set pressure is reached, an additional force increases the closing force.

Safety valve - A safety 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. Note; the valve can be characterised either by pop action (rapid opening) or by opening in proportion (not necessarily linear) to the increase in pressure over the set pressure.

Direct loaded safety valve -A safety valve in which the loading due to the fluid pressure underneath the valve disc is opposed only by a direct mechanical loading device such as a weight, lever and weight, or a spring.

Assisted safety valve -A safety valve which by means of a powered assistance mechanism, may additionally be lifted at a pressure lower than the set pressure and will, even in the event of a failure of the assistance mechanism, comply with all the requirements for safety valves given in the standard.

Supplementary loaded safety valve - A safety valve that has, until the pressure at the inlet to the safety valve reaches the set pressure, an additional force, which increases the sealing force.

Note; this additional force (supplementary load), which may be provided by means of an extraneous power source, is reliably released when the pressure at the inlet of the safety valve reaches the set pressure. The amount of supplementary loading is so arranged that if such supplementary loading is not released, the safety valve will attain its certified discharge capacity at a pressure not greater than 1.1 times the maximum allowable pressure of the equipment to be protected.

Pilot operated safety valve -A safety valve, the operation of which is initiated and controlled by the fluid discharged from a pilot valve, which is itself, a direct loaded safety valve subject to the requirement of the standard.

The common characteristic shared between the definitions of conventional safety valves in the different standards, is that their operational characteristics are affected by any backpressure in the discharge system. It is important to note that the total backpressure is generated from two components; superimposed backpressure and the built-up backpressure:

Subsequently, in a conventional safety valve, only the superimposed backpressure will affect the opening characteristic and set value, but the combined backpressure will alter the blowdown characteristic and re-seat value.

The ASME/ANSI standard makes the further classification that conventional valves have a spring housing that is vented to the discharge side of the valve. If the spring housing is vented to the atmosphere, any superimposed backpressure will still affect the operational characteristics. Thiscan be seen from Figure 9.2.1, which shows schematic diagrams of valves whose spring housings are vented to the discharge side of the valve and to the atmosphere.

By considering the forces acting on the disc (with area AD), it can be seen that the required opening force (equivalent to the product of inlet pressure (PV) and the nozzle area (AN)) is the sum of the spring force (FS) and the force due to the backpressure (PB) acting on the top and bottom of the disc. In the case of a spring housing vented to the discharge side of the valve (an ASME conventional safety relief valve, see Figure 9.2.1 (a)), the required opening force is:

In both cases, if a significant superimposed backpressure exists, its effects on the set pressure need to be considered when designing a safety valve system.

Once the valve starts to open, the effects of built-up backpressure also have to be taken into account. For a conventional safety valve with the spring housing vented to the discharge side of the valve, see Figure 9.2.1 (a), the effect of built-up backpressure can be determined by considering Equation 9.2.1 and by noting that once the valve starts to open, the inlet pressure is the sum of the set pressure, PS, and the overpressure, PO.

In both cases, if a significant superimposed backpressure exists, its effects on the set pressure need to be considered when designing a safety valve system.

Once the valve starts to open, the effects of built-up backpressure also have to be taken into account. For a conventional safety valve with the spring housing vented to the discharge side of the valve, see Figure 9.2.1 (a), the effect of built-up backpressure can be determined by considering Equation 9.2.1 and by noting that once the valve starts to open, the inlet pressure is the sum of the set pressure, PS, and the overpressure, PO.

Balanced safety valves are those that incorporate a means of eliminating the effects of backpressure. There are two basic designs that can be used to achieve this:

Although there are several variations of the piston valve, they generally consist of a piston type disc whose movement is constrained by a vented guide. The area of the top face of the piston, AP, and the nozzle seat area, AN, are designed to be equal. This means that the effective area of both the top and bottom surfaces of the disc exposed to the backpressure are equal, and therefore any additional forces are balanced. In addition, the spring bonnet is vented such that the top face of the piston is subjected to atmospheric pressure, as shown in Figure 9.2.2.

The bellows arrangement prevents backpressure acting on the upper side of the disc within the area of the bellows. The disc area extending beyond the bellows and the opposing disc area are equal, and so the forces acting on the disc are balanced, and the backpressure has little effect on the valve opening pressure.

Bellows failure is an important concern when using a bellows balanced safety valve, as this may affect the set pressure and capacity of the valve. It is important, therefore, that there is some mechanism for detecting any uncharacteristic fluid flow through the bellows vents. In addition, some bellows balanced safety valves include an auxiliary piston that is used to overcome the effects of backpressure in the case of bellows failure. This type of safety valve is usually only used on critical applications in the oil and petrochemical industries.

In addition to reducing the effects of backpressure, the bellows also serve to isolate the spindle guide and the spring from the process fluid, this is important when the fluid is corrosive.

Since balanced pressure relief valves are typically more expensive than their unbalanced counterparts, they are commonly only used where high pressure manifolds are unavoidable, or in critical applications where a very precise set pressure or blowdown is required.

This type of safety valve uses the flowing medium itself, through a pilot valve, to apply the closing force on the safety valve disc. The pilot valve is itself a small safety valve.

The diaphragm type is typically only available for low pressure applications and it produces a proportional type action, characteristic of relief valves used in liquid systems. They are therefore of little use in steam systems, consequently, they will not be considered in this text.

The piston type valve consists of a main valve, which uses a piston shaped closing device (or obturator), and an external pilot valve. Figure 9.2.4 shows a diagram of a typical piston type, pilot operated safety valve.

The piston and seating arrangement incorporated in the main valve is designed so that the bottom area of the piston, exposed to the inlet fluid, is less than the area of the top of the piston. As both ends of the piston are exposed to the fluid at the same pressure, this means that under normal system operating conditions, the closing force, resulting from the larger top area, is greater than the inlet force. The resultant downward force therefore holds the piston firmly on its seat.

If the inlet pressure were to rise, the net closing force on the piston also increases, ensuring that a tight shut-off is continually maintained. However, when the inlet pressure reaches the set pressure, the pilot valve will pop open to release the fluid pressure above the piston. With much less fluid pressure acting on the upper surface of the piston, the inlet pressure generates a net upwards force and the piston will leave its seat. This causes the main valve to pop open, allowing the process fluid to be discharged.

When the inlet pressure has been sufficiently reduced, the pilot valve will reclose, preventing the further release of fluid from the top of the piston, thereby re-establishing the net downward force, and causing the piston to reseat.

Pilot operated safety valves offer good overpressure and blowdown performance (a blowdown of 2% is attainable). For this reason, they are used where a narrow margin is required between the set pressure and the system operating pressure. Pilot operated valves are also available in much larger sizes, making them the preferred type of safety valve for larger capacities.

One of the main concerns with pilot operated safety valves is that the small bore, pilot connecting pipes are susceptible to blockage by foreign matter, or due to the collection of condensate in these pipes. This can lead to the failure of the valve, either in the open or closed position, depending on where the blockage occurs.

The terms full lift, high lift and low lift refer to the amount of travel the disc undergoes as it moves from its closed position to the position required to produce the certified discharge capacity, and how this affects the discharge capacity of the valve.

A full lift safety valve is one in which the disc lifts sufficiently, so that the curtain area no longer influences the discharge area. The discharge area, and therefore the capacity of the valve are subsequently determined by the bore area. This occurs when the disc lifts a distance of at least a quarter of the bore diameter. A full lift conventional safety valve is often the best choice for general steam applications.

The disc of a high lift safety valve lifts a distance of at least 1/12th of the bore diameter. This means that the curtain area, and ultimately the position of the disc, determines the discharge area. The discharge capacities of high lift valves tend to be significantly lower than those of full lift valves, and for a given discharge capacity, it is usually possible to select a full lift valve that has a nominal size several times smaller than a corresponding high lift valve, which usually incurs cost advantages.Furthermore, high lift valves tend to be used on compressible fluids where their action is more proportional.

In low lift valves, the disc only lifts a distance of 1/24th of the bore diameter. The discharge area is determined entirely by the position of the disc, and since the disc only lifts a small amount, the capacities tend to be much lower than those of full or high lift valves.

Except when safety valves are discharging, the only parts that are wetted by the process fluid are the inlet tract (nozzle) and the disc. Since safety valves operate infrequently under normal conditions, all other components can be manufactured from standard materials for most applications. There are however several exceptions, in which case, special materials have to be used, these include:

Cast steel -Commonly used on higher pressure valves (up to 40 bar g). Process type valves are usually made from a cast steel body with an austenitic full nozzle type construction.

For all safety valves, it is important that moving parts, particularly the spindle and guides are made from materials that will not easily degrade or corrode. As seats and discs are constantly in contact with the process fluid, they must be able to resist the effects of erosion and corrosion.

For process applications, austenitic stainless steel is commonly used for seats and discs; sometimes they are ‘stellite faced’ for increased durability. For extremely corrosive fluids, nozzles, discs and seats are made from special alloys such as ‘monel’ or ‘hastelloy’.

The spring is a critical element of the safety valve and must provide reliable performance within the required parameters. Standard safety valves will typically use carbon steel for moderate temperatures. Tungsten steel is used for higher temperature, non-corrosive applications, and stainless steel is used for corrosive or clean steam duty. For sour gas and high temperature applications, often special materials such as monel, hastelloy and ‘inconel’ are used.

A key option is the type of seating material used. Metal-to-metal seats, commonly made from stainless steel, are normally used for high temperature applications such as steam. Alternatively, resilient discs can be fixed to either or both of the seating surfaces where tighter shut-off is required, typically for gas or liquid applications. These inserts can be made from a number of different materials, but Viton, nitrile or EPDM are the most common. Soft seal inserts are not generally recommended for steam use.

Standard safety valves are generally fitted with an easing lever, which enables the valve to be lifted manually in order to ensure that it is operational at pressures in excess of 75% of set pressure. This is usually done as part of routine safety checks, or during maintenance to prevent seizing. The fitting of a lever is usually a requirement of national standards and insurance companies for steam and hot water applications. For example, the ASME Boiler and Pressure Vessel Code states that pressure relief valves must be fitted with a lever if they are to be used on air, water over 60°C, and steam.

A standard or open lever is the simplest type of lever available. It is typically used on applications where a small amount of leakage of the fluid to the atmosphere is acceptable, such as on steam and air systems, (see Figure 9.2.5 (a)).

Where it is not acceptable for the media to escape, a packed lever must be used. This uses a packed gland seal to ensure that the fluid is contained within the cap, (see Figure 9.2.5 (b)).

For service where a lever is not required, a cap can be used to simply protect the adjustment screw. If used in conjunction with a gasket, it can be used to prevent emissions to the atmosphere, (see Figure 9.2.6).

A test gag (Figure 9.2.7) may be used to prevent the valve from opening at the set pressure during hydraulic testing when commissioning a system. Once tested, the gag screw is removed and replaced with a short blanking plug before the valve is placed in service.

The amount of fluid depends on the particular design of safety valve. If emission of this fluid into the atmosphere is acceptable, the spring housing may be vented to the atmosphere – an open bonnet. This is usually advantageous when the safety valve is used on high temperature fluids or for boiler applications as, otherwise, high temperatures can relax the spring, altering the set pressure of the valve. However, using an open bonnet exposes the valve spring and internals to environmental conditions, which can lead to damage and corrosion of the spring.

When the fluid must be completely contained by the safety valve (and the discharge system), it is necessary to use a closed bonnet, which is not vented to the atmosphere. This type of spring enclosure is almost universally used for small screwed valves and, it is becoming increasingly common on many valve ranges since, particularly on steam, discharge of the fluid could be hazardous to personnel.

Some safety valves, most commonly those used for water applications, incorporate a flexible diaphragm or bellows to isolate the safety valve spring and upper chamber from the process fluid, (see Figure 9.2.9).

An elastomer bellows or diaphragm is commonly used in hot water or heating applications, whereas a stainless steel one would be used on process applications employing hazardous fluids.

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.

Curtiss-Wright"s selection of Pressure Relief Valves comes from its outstanding product brands Farris and Target Rock. We endeavor to support the whole life cycle of a facility and continuously provide custom products and technologies. Boasting a reputation for producing high quality, durable products, our collection of Pressure Relief Valves is guaranteed to provide effective and reliable pressure relief.

While some basic components and activations in relieving pressure may differ between the specific types of relief valves, each aims to be 100% effective in keeping your equipment running safely. Our current range includes numerous valve types, from flanged to spring-loaded, threaded to wireless, pilot operated, and much more.

A pressure relief valve is a type of safety valve designed to control the pressure in a vessel. It protects the system and keeps the people operating the device safely in an overpressure event or equipment failure.

A pressure relief valve is designed to withstand a maximum allowable working pressure (MAWP). Once an overpressure event occurs in the system, the pressure relief valve detects pressure beyond its design"s specified capability. The pressure relief valve would then discharge the pressurized fluid or gas to flow from an auxiliary passage out of the system.

Below is an example of one of our pilot operated pressure relief valves in action; the cutaway demonstrates when high pressure is released from the system.

Air pressure relief valves can be applied to a variety of environments and equipment. Pressure relief valves are a safety valve used to keep equipment and the operators safe too. They"re instrumental in applications where proper pressure levels are vital for correct and safe operation. Such as oil and gas, power generation like central heating systems, and multi-phase applications in refining and chemical processing.

At Curtiss-Wright, we provide a range of different pressure relief valves based on two primary operations – spring-loaded and pilot operated. Spring-loaded valves can either be conventional spring-loaded or balanced spring-loaded.

Spring-loaded valves are programmed to open and close via a spring mechanism. They open when the pressure reaches an unacceptable level to release the material inside the vessel. It closes automatically when the pressure is released, and it returns to an average operating level. Spring-loaded safety valves rely on the closing force applied by a spring onto the main seating area. They can also be controlled in numerous ways, such as a remote, control panel, and computer program.

Pilot-operated relief valves operate by combining the primary relieving device (main valve) with self-actuated auxiliary pressure relief valves, also known as the pilot control. This pilot control dictates the opening and closing of the main valve and responds to system pressure. System pressure is fed from the inlet into and through the pilot control and ultimately into the main valve"s dome. In normal operating conditions, system pressure will prevent the main valve from opening.

The valves allow media to flow from an auxiliary passage and out of the system once absolute pressure is reached, whether it is a maximum or minimum level.

When the pressure is below the maximum amount, the pressure differential is slightly positive on the piston"s dome size, which keeps the main valve in the closed position. When system pressure rises and reaches the set point, the pilot will cut off flow to the dome, causing depressurization in the piston"s dome side. The pressure differential has reversed, and the piston will rise, opening the main valve, relieving pressure.

When the process pressure decreases to a specific pressure, the pilot closes, the dome is repressurized, and the main valve closes. The main difference between spring-loaded PRVs and pilot-operated is that a pilot-operated safety valve uses pressure to keep the valve closed.

Pilot-operated relief valves are controlled by hand and are typically opened often through a wheel or similar component. The user opens the valve when the gauge signifies that the system pressure is at an unsafe level; once the valve has opened and the pressure has been released, the operator can shut it by hand again.

Increasing pressure helps to maintain the pilot"s seal. Once the setpoint has been reached, the valve opens. This reduces leakage and fugitive emissions.

At set pressure the valve snaps to full lift. This can be quite violent on large pipes with significant pressure. The pressure has to drop below the set pressure in order for the piston to reseat.

The pilot is designed to open gradually, so that less of the system fluid is lost during each relief event. The piston lifts in proportion to the overpressure.

At Curtiss-Wright we also provide solutions for pressure relief valve monitoring. Historically, pressure relief valves have been difficult or impossible to monitor. Our SmartPRV features a 2600 Series pressure relief valve accessorized with a wireless position monitor that alerts plant operators during an overpressure event, including the time and duration.

There are many causes of overpressure, but the most common ones are typically blocked discharge in the system, gas blowby, and fire. Even proper inspection and maintenance will not eliminate the occurrence of leakages. An air pressure relief valve is the only way to ensure a safe environment for the device, its surroundings, and operators.

A PRV and PSV are interchangeable, but there is a difference between the two valves. A pressure release valve gradually opens when experiencing pressure, whereas a pressure safety valve opens suddenly when the pressure hits a certain level of over pressurization. Safety valves can be used manually and are typically used for a permanent shutdown. Air pressure relief valves are used for operational requirements, and they gently release the pressure before it hits the maximum high-pressure point and circulates it back into the system.

Pressure relief valves should be subject to an annual test, one per year. The operator is responsible for carrying out the test, which should be done using an air compressor. It’s imperative to ensure pressure relief valves maintain their effectiveness over time and are checked for signs of corrosion and loss of functionality. Air pressure relief valves should also be checked before their installation, after each fire event, and regularly as decided by the operators.

Direct-acting solenoid valves have a direct connection with the opening and closing armature, whereas pilot-operated valves use of the process fluid to assist in piloting the operation of the valve.

A control valve works by varying the rate of fluid passing through the valve itself. As the valve stem moves, it alters the size of the passage and increases, decreases or holds steady the flow. The opening and closing of the valve is altered whenever the controlled process parameter does not reach the set point.

Control valves are usually at floor level or easily accessible via platforms. They are also located on the same equipment or pipeline as the measurement and downstream or flow measurements.

An industrial relief valve is designed to control or limit surges of pressure in a system, most often in fluid or compressed air system valves. It does so as a form of protection for the system and defending against instrument or equipment failure. They are usually present in clean water industries.

A PRV is often referred to as a pressure relief valve, which is also known as a PSV or pressure safety valve. They are used interchangeably throughout the industry depending on company standards.

From time to time, you may find it necessary to turn off the gas for a stove (or other appliance) that heats with natural gas or LP (liquid propane) gas. Several common repairs for stoves and other appliances may require that you shut off the gas. And should you ever detect a faint odor of gas in your home, shutting off the gas may be required.

Furnaces, water heaters, space heaters, stoves, ovens, and cooktop ranges can all be powered by gas, so it"s a good idea to know where the gas shutoff valves are located on any of these appliances.

It"s a good idea to make a point of knowing where the gas shutoff valves are located for your range and all other gas appliances in your home. In the event that you smell gas, don"t spend a lot of time searching for the appliance shutoff valves if you don"t already know where they are.

Building codes require the gas shutoff valve to be located within 6 feet of the appliance and in the same room. If you find an improper situation that doesn"t meet the code requirements, it"s a good idea to have a service person install a proper shutoff valve. Shutoff valves for standard (freestanding) gas stoves or ranges typically are located behind the appliance.

Any time there is a gas leak in one of your appliances, there is the possible danger of an immediate and catastrophic explosion or fire, so the official recommendation from most utility companies is to leave your house immediately if you smell gas, then call the utility company. Only if the odor is faint should you take the time to find the shutoff valve to an appliance to stop the flow of gas. Shutting off the gas to the stove yourself is safe only if the gas odor is faint and is clearly coming from the stove area.

If a gas smell permeates the entire house, or you detect a strong smell when you walk inside your home, leave the house immediately and call 911from a locationwell away from your house.

If the odor of gas is faint and is contained in a small area, you can shut off the gas yourself before calling the utility or a plumber. In many instances, the location of the gas smell will give you a strong hint about what appliance is malfunctioning or where the bad connection is located. If you smell gas in the kitchen, for example, it"s quite likely that the problem lies with the stove or range. When the smell of gas is localized in this way, it is usually safe to act quickly to turn off the gas supply to the appliance itself, but always use your best judgment, and err on the side of safety.

R series proportional relief valves provide simple, reliable, overpressure protection for a variety of liquid or gas general-industry applications with set pressures from 10 to 6000 psig (0.7 to 413 bar). They open gradually as the pressure increases and will close when the system pressure falls below the set pressure. The set pressure can be set at the factory or easily adjusted in the field. PRV series proportional safety relief valves are certified to PED 2014/68/EU Category IV, CE-marked in accordance with the Pressure Equipment Directive as a safety valve according to ISO-4126-1, and factory-set, tested, locked, and tagged with the set pressure.

Swagelok bleed valves can be used on instrumentation devices such as multivalve manifolds or gauge valves to vent signal line pressure to atmosphere before removal of an instrument or to assist in calibration of control devices. Male NPT and SAE end connections are available. Purge valves are manual bleed, vent, or drain valves with NPT, SAE, Swagelok tube fitting, or tube adapter end connections. A knurled cap is permanently assembled to the valve body for safety. Both bleed and purge valves are compact for convenient installation.