what is safety valve in stock

Safety valves are designed to automatically shut in the flow of a well in the event surface controls fail or surface equipment becomes damaged. They are classified according to the location from which they are controlled – surface or subsurface. In this article, subsurface safety valve types, operating systems, working principle, setting depth, and selection process are presented.

It is advisable, and in most cases mandatory, to have a secondary means of closure for all wells capable of natural flow to the surface. The installation on of a sub-surface safety valve (SSSV) will provide this emergency closure capability.

Operating systems may be either remotely operated on a fail-safe principle from surface (SCSSV) actuated from a control panel located on surface, or will be a subsurface controlled (SSCSV), designed to close automatically when a predetermined flow condition occurs in the well (actuated by the pressure differential/flow velocity across the valve).

In case of SCSSV, a 1/4″ inch stainless steel control line is attached to the outside of the tubing string and installed when the tubing is installed. Depending on the wellhead pressure, it may be necessary to keep as much as 4000 to 5000 psi on the control line to keep the valve open.

The differential type subsurface controlled subsurface safety valve senses pressure drop across a flow bean. There are several variations of the differential type SSCSV. Although they employ different sealing devices, such as a flapper or ball, they all are controlled with a flow bean and spring tension.

As shown in the following video, when hydraulic pressure is applied down a control line, the hydraulic pressure forces a sleeve within the valve to slide downwards. This movement compresses a large spring and pushes the flapper (in case of flapper type SCSSV) or the ball (in case of ball type SCSSV) downwards to open the valve. When hydraulic pressure is removed, the spring pushes the sleeve back up and causes the flapper (or the ball) to shut. In this way, it is failsafe and will isolate the wellbore in the event of a loss of the wellhead.

The location of the downhole safety valve within the completion is a precisely determined parameter intended to optimise safety. There are arguments against it either being too high or too low in the well and so the final depth is a compromise of all factors. MMS regulations state that the valve must be placed no less than 100′ below the mudline.

Of all the challenges you face keeping your customers’ plants operating at full capacity, safety and relief valves shouldn’t be one of them. NASVI’s job is to give you the confidence that your valve supply chain is rock solid regardless the pressure it’s under.

Did you know that NASVI has 35,000 safety & relief valves ready to ship at a moment’s notice? Or that warehouse is so huge (63,000 square feet!) we have a 24-foot fan to keep it cool. We ship over 200 valves a day – that’s 1,000 valves a week and like a million shipping labels!*

Need a safety valve? We have 35,000+ valves in stock. And ship over 200 valves a day!  That’s 1,000 valves a week. And like, a million shipping labels.

Safety valves are used in a variety of applications, including air/gas, vapor, steam and liquid service.  Flotech has been approved by the National Board of Boiler and Pressure Vessel Inspectors to perform safety and relief valve testing, repair and certification.

Our valve experts will focus on getting your valves tested, repaired and quickly set to the exact specifications.  We evaluate the repair condition of every valve and will recommend the right solution to manage your maintenance program.

Surface-controlled subsurface safety valves (SCSSVs) are critical components of well completions, preventing uncontrolled flow in the case of catastrophic damage to wellhead equipment. Fail-safe closure must be certain to ensure proper security of the well. However, this is not the only function in which it must be reliable—the valve must remain open to produce the well. Schlumberger surface controlled subsurface safety valves exceed all ISO 10432 and API Spec 14A requirements for pressure integrity, leakage acceptance criteria, and slam closure.

Through decades of innovation and experience, Schlumberger safety valve flapper systems are proven robust and reliable. The multizone dynamic seal technology for hydraulic actuation of subsurface safety valves is a further improvement in reliability performance when compared with traditional seal systems in the industry.

The multizone seal technology is currently available in the GeoGuard high-performance deepwater safety valves, which is validated to API Spec 14A V1 and V1-H.

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During the forecast period, the global safety valve market size is estimated to reach USD 13.2 Billion by 2030 and is expected to exhibit a significant growth rate of 9.20% CAGR.

Safety Valves are precautionary valves that automatically actuate when the preset safety valve pressure and temperature are exceeded. These safety valves can be used to protect the critical equipment from damage by controlling excess pressure without any electrical support. For protecting equipment from unsafe pressure these mainly operate at a predetermined pressure. Additionally, these valves protect the employees around the plants and the environment around them. Safety valves are used in various applications like pharmaceutical, construction, oil & gas industries which foster the growth of the market.

During the lockdown, the global safety valve market is negatively impacted. Not only the safety valve market but the whole world was also affected drastically by this pandemic. To control the prevalence of the coronavirus, the government has imposed stringent regulations like lockdowns, maintaining social distance, covering the face with masks, manufacturing industries shut down, and transportation bans.

Even though at the primary stage of the pandemic, the safety valve market has fallen. Developing the innovations in the safety valve system and growing awareness regarding the benefits of the safety valve market by the key players are increasing the growth of this market.

Growing demand for safety valves in the oil & gas industry, the rise in nuclear energy generation, the growing importance of safety valves in industrial processes are the major driving factors of this market. The continuous need for safety valve replacement and the use of 3D printers in manufacturing lines are boosting the growth of the market. The safety valve market is highly dependent on investments in manufacturing facilities.

Some of the numerous factors that drive the safety valve market are rising demand for water & power, pollution control regulations, and rapid growth of process industries are supposed to escalate the growth of the safety valve industry during the assessment period. Growth in the construction of nuclear power plants is fueling the growth of the market. The increase of accidental incidences and soaring demand for safety valves in several industrial sectors are increasing the growth of the global market.

The constant growth of oil & gas exploration in few parts across the globe is restraining the market. The fabrication of safety valves are very expensive which is hindering the market growth

To increase the growth of the safety valve market industry integration of safety valves into the Internet of Things (IoT) environment is creating the opportunity. The innovations in the safety valve systems are anticipated to increase the strong growth of the market.

To provide a strategic profile of the prominent key players in the market, analyze their core competencies, forecast statistics, and draw a global safety valve market growth landscape.

The global safety valve market based on material is sub-segmented into steel, alloy, cast iron, cryogenic, and others. As the steel safety valves are durable and don’t leak in hot or cold temperatures, the steel segment is expected to dominate the global market.

It is segmented into less than 1”, 1” to 10”, and 11” to 20”, and 20” & above. Among these, during the review period, the 1” to 10” segment is projected to grow at the significant CAGR for the safety valves market for the benefits behind this size range like controlling the flow and pressure of liquids, gases, and slurries within different end-use industries.

The global safety valve market industry is divided into oil & gas, energy & power, food & beverage, chemicals, water & wastewater treatment, and others. In the global safety market, the oil & gas segment is expected to hold the largest share, because the oil & gas industries are the most significant revenue-generating industries which need almost all types of valves like gate, globe, ball, check and butterfly. Some of the products include a safety valve air compressor, safety valve boiler, and safety valve heater.

Asia-Pacific, Europe, North America, the Middle East & Africa, and South America are the main geographies included in this market. Due to the rapid urbanization and growing industrialization Asia-Pacific holds the largest safety valve market share.

The global safety valve market region-wise is divided into Asia-Pacific, Europe, North America, and the Middle East & Africa. Out of these regions, Asia-Pacific holds the largest market share for its growing infrastructural developments, rise of investments in various industries like oil & gas, construction industry, and drastic urbanization. Growing demand from mining, chemical, and municipal industries is expected to propel market growth in this region.

Safety valves are used in the application of the construction industry to control liquid flow in firefighting systems, water supply systems, and piping systems. The rising construction industry propels the market growth in this region. North America is accounting as the second-largest market for its growing investments in the construction industry.

As per the report of BP Statistical Review of World Energy 2018, the refinery output in 2017 in North America is registered as 2.13% and is predicted to grow during the forecast period.

Naples, Italy, Baker Hughes launched a new steam test facility in November 2018, ASME Section I safety valves that serve better to the European aftermarket with a rapid response for steam applications. The future development of the current aftermarket is launched as the new aftermarket plant which is expanded by the product scope and capacity of the plant. To fulfill the range of Masoneilan control valves and consolidated safety valves ranging up to 2000 psi test pressure.

In October 2018, Emerson Electric Co. to help the LNG marine transportation consumers developed low-pressure pilot operated pressure relief valves (POPRVs) by reducing their size which helps to reduce the investments by 25% and protects the end-users from overpressure by offering them extra profit margin.

In May 2019, the Mexican government announced that it is going to construct a new refinery set in the Tobasco coast, Mexico in June 2019. Hence safety valves are used in refineries to control the pressure of liquids and gases in plants.

This global safety valve market research includes the Market Overview, COVID-19 analysis, Market Dynamics, Study Objectives, Segment Overview, Regional Analysis, Competitive Landscape, Recent developments, Segmentation Table, and FAQs. The market scenario includes the safety valve market drivers, restraints, challenges, and opportunities. The safety valve forecast segments are material, size, end-use, and region.

Pressure relief valves are a type of safety valve that are commonly used to protect a system and the people operating it. Whereas pressure regulators take incoming line pressure and regulates it down to the pressure that is required by the downstream system. Pressure Regulators can be used for reasons of safety and/or cost. Both of these valves are very important to its specific application. In this article, we will discuss the difference between a pressure relief valve and regulator.

Pressure Regulators take an incoming line pressure and regulate it down to the pressure that is required by the downstream system. This may be for other instrumentation to operate effectively or simply to control the output flow of a pipe. Lower system pressures mean less risk and lower running costs and a reduced risk of air loss through a system. Pressure regulators can be used in many applications including pneumatics, compressed air and water.

There are various kinds of pressure regulators available within MGA Controls range – from general purpose units covering everyday industrial applications to more specialised precision pressure regulators, manifold regulators, pilot operated regulators and large capacity pilot operated versions. View our full range of pressure regulators in our store.

Pressure relief valves are used to control or limit pressure spikes in a compressed air system. When the system pressure increases beyond a predetermined set point, the valve opens and relieves that pressure, bringing it back in line with normal operating parameters. The main function of a Pressure relief valve is to vent excess pressure and protect other system components, all the while maintaining optimum performance.

Air systems benefit highly from pressure relief valves, however different types of pressure relief valves can be used in a wide range of industries. For example, the water industry utilises the valve to make sure water pressure doesn’t reach such a level that it will burst pipes.

The IMI Norgren Olympian Plus pressure relief valve is designed to protect compressed air systems against over-pressurisation. It has high relief capacity while being sensitive and accurate. As part of the Olympian Plus range of products, it is suitable for in-line or modular installation and is compatible with other products in the Olympian Plus range, such as the B64G Filter/Regulator and the L64 Series Lubricator. Some of the key pressure relief valve features include:

Choosing a pressure relief valve isn’t always easy, but here at MGA Controls, we specialise in helping you choose the correct valve for your application. There are six basic factors to consider before choosing your pressure relief valve:

You must also consider the physical dimensions of the application and the plant, as well as factors related to the environment in which the valve will operate.

Here at MGA Controls, pressure relief valves can be fitted to an existing system and can be specified in sizes ranging from 1/4″ to 1.1/2″. We carry a wide range of stock that is available for quick delivery in your time of need.

To speak to a member of our technical team about choosing a pressure relief valve or the difference between a pressure relief valve and regulator contact us today on 01704 898980 or email sales@mgacontrols.co.uk. To request a free quote or view our general range of products contact our team.

A safety valve is a device that serves as a last line of defense in numerous situations. It is critical to guarantee that the safety valve can function at all times and under all conditions. A safety valve is not a process valve or a pressure regulator, and it should never be used in that capacity. It should only be used for one thing: overpressure protection. The Safety Valve"s purpose is to safeguard people and property in the event of a failure to manage system pressures, i.e., it is the final line of defense before ultimate failure.

Excessive system pressures in heating and chilled water systems are generated by a variety of factors such as during the start-up of ‘plant" components, control valve failure is a common occurrence, failure of the system"s temperature and pressure sensors, or human error. The progress of any industry relies on safety valves because it is reflected not only in the system"s control, but also in the costs of construction and maintenance. All these factors are assisting the safety valve market share globally.

The global safety valve market is segmented on the basis of size, type, material, industry vertical and region. Based on size, the market is divided into Up to 1 inch, 1–6 inch, 6 – 25 inch, 25 – 50-inch, 50 inch, and above. In terms of type, the market is categorized into spring-loaded pressure-relief valves, dead-weight pressure-relief valves, pilot-operated pressure-relief valves. On the basis of material, the market is divided into cryogenic, stainless steel, alloy, cast iron. On the basis of industry vertical the market is divided into oil & gas, water & wastewater, energy & power, pharmaceuticals, paper & pulp, agriculture, metal & mining. Geographically, the market is analyzed across several regions such as North America, Europe, Asia-Pacific, and Latin America, Middle East & Africa (LAMEA).

Key players operating in the global safety valve industry include Spirax Sarco Limited, Emerson Electric Co, General Electric, LESER GmbH & Co. KG, The Weir Group PLC, Forbes Marshall, Bosch Rexroth AG, Curtiss-Wright Corporation, Schlumberger Limited. These companies have adopted several strategies such as product launches, partnerships, collaborations, mergers & acquisitions, and joint ventures to strengthen their foothold in the global safety valve market.

Companies CoveredSpirax Sarco Limited, Emerson Electric Co, General Electric, LESER GmbH & Co. KG, The Weir Group PLC, Forbes Marshall, Bosch Rexroth AG, Curtiss-Wright Corporation, Schlumberger Limited

The COVID-19 pandemic has decreased the growth of several sectors, including automobiles, oil & gas, and others. It has three major effects on the global economy: directly impacting production and demand, causing supply chain and market disruption, and having a financial impact on businesses and financial markets. The COVID-19 outbreak has posed a significant strategic threat to electronics and manufacturing companies in recent months. The electronics and semiconductor manufacturing industries have been harmed by disruptions in raw material supply, temporary shutdown of manufacturing units, inadequate finance, and weak consumer demand. All these factors are hampering the safety valve market size globally

Increased awareness of the importance of safety valves in various industrial processes such as power generation, gas and petroleum, including water and sewage processing, mining, processing of oil, food manufacturing, increased demand for the product in the oil & gas industry, growth in nuclear energy generation, and increased use of 3D printers in manufacturing lines are some of the major and impactful factors drive the growth of safety valve industry. On the other hand, the ongoing requirement for safety valve replacement, as well as product integration with the internet of things environment, will generate numerous potentials for the safety valve market opportunity to develop throughout the projected period. The rise in cost of these safety valves and the companies" low profit margins limit the safety valve market growth.

Industrial valves are essential for the safe operation of oil and gas pipelines. They are in charge of flow control, supply line integrity, and other important responsibilities. The majority of the oil & gas industry"s operations, from refining to distribution, rely largely on pipeline systems. As a result, pipeline infrastructure and reliable control systems are vital to the industry"s success. Any faults or breaches in these systems could result in significant financial losses, hazardous leaks, and possibly environmental disaster. The oil & gas valve manufacturing sector has grown all throughout the world.

China has already established itself as one of the world"s major producers and consumers. The local valve industry, regardless of device or technical level, has a significant deficit in comparison to international valve technology. For high-end products, there are very few companies with independent research and development capability. A terminal actuator that controls fluid movement in a pipe is an oil and gas valve. Valve goods use modern computer books, sensor technology, network, and remote-control technologies. To keep up with the rapid advancement of information technology.

Key Benefits of the ReportThis study presents the analytical depiction of the safety valve market forecast along with the current trends and future estimations to determine the imminent investment pockets.

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.

With our partner specifications archives, six on-site lathes, technical know-how and 35,000 in-stock valves, we’ve got your valve repairs covered. Valves are quickly repaired and set following exact specifications, and if they’re irreparable, chances are we’ve got a replacement in stock.

Tired of keeping track of your valve inventory’s annual certification records? We offer complete management of your safety relief valves. With an inventory of repair parts and in stock relief valves of all sizes, we can respond to any customer emergency. We offer annual certification services as well as repair of all major brands, including Kunkle, Conbraco, Consolidated, Dresser, Apollo and more.

Notice: Item(s) that qualify will ship out the same day as long as the order is received before 1:00 PM EST (Monday-Friday), orders received after 1:00PM are not guaranteed same day shipment. Orders received after 1:00 PM EST on Friday will ship the following Monday. If you have questions regarding stock availability please call us at 1-800-277-4466 before placing an order.

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Call to Check Availability: The item is listed as available to order from one of our supporting warehouses, STOCK IS NOT GUARANTEED. If a rush item is needed, please call and check item lead time before placing an order.

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.