gag safety valve brands

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.

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.

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.

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 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).

IMO your question is more suitable for PSV forum http://www.eng-tips.com/threadminder.cfm?pid=1203 . Of course some of us in this forum have experiences in safety valve.

See some hint from some expert whom have done this http://www.eng-tips.com/viewthread.cfm?qid=180240 . Moral of the story, this is a risky job with some uncertainties, be cautious for operator to position themselves when performing this. If your safety valve is open loop, do not position any parts of someones" body in front or near the outlet

For gaging as well as adjusting it in relatively safe manner, Trevi-test is one way to do it. similar video by Tru-test https://www.youtube.com/watch?v=gAwS-tjbLeg

Be cautious that you or the commissioning personnel should correspond both safety valves" setting pressure vs the testing pressure. If its gagged (set the screw to prevent it from opening)at X Psi, then the testing pressure is not advisable to exceed 5% of X. this potentially bent the spindle. And if this happened, as per safety regulation the damage parts shall be renewed with UV stamped (read: original) manufacturer parts (if your safety valve manufacturer and your boiler are referring to ASME).

Because a safety valve is often the last device to prevent catastrophic failure under pressure conditions, it is important that the valve works at all times i.e. it must be 100% reliable.

Safety valves should be installed wherever the maximum allowable working pressure of a system or pressure containing vessel is likely to be exceeded, in particular under fault conditions due to the failure of another piece of equipment in the system.

The term “Safety Valve” and “Relief Valve” are generic terms to describe a variety of pressure relief devices. A wide range is available based on the application and required performance criteria. The different designs are required to meet numerous national standards.

The images below show the devastating results of a failed Safety valve (due to poor maintenace) or ones which have been incorrectly sized, installed or maintained.

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.

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.

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.

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

The images below show a standard Relief valve and a standard Safety valve from a well-known UK manufacturer. Each manufacturer does things slightly differently however all of the basic components and principles of operation are the same. As described previously, a safety valve differs from a relief valve in that it opens rapidly once the set pressure has been reached. For the same inlet size and with the valve in the closed position, the surface area that the pressure on the inlet side will see is the same. When the set pressure is reached and the valve starts to open, the disk on a Safety valve is larger (see the diagrams below) and hence the same pressure then sees a much larger surface area and consequently the force increases greatly causing the valve to open quickly and hence the characteristic pop action.

The image below shows the above Safety valves and Relief valves dismantled. The disk diameter on the 1" (DN25) Safety valve is only 7mm larger than on the Relief valve which doesnt sound like much, but when you calculate the areas it is an increase of 36%.

This diagram represents a Safety valve in its very simplest form. The force acting on the inlet side of the disk is acting against the force applied by the spring plus the force applied by the back pressure on the top of the disk.

The valve remains closed when(PI x Ab) < Fs + (PB x At), is in equilibrium when(PI x Ab) = Fs + (PB x At) and opens when(PI x Ab) > Fs + (PB x At) were PI = Inlet pressure, PB = Back pressure, At = Top of disk area, Ab = Bottom of disk area. Things to notice from this design are that if PB is variable and quite large relative to PI, then this will cause the pressure at which the valve opens to vary which is undesirable. The following two designs (Fig 3 & Fig 4) are available that eliminate the effect of back pressure on the set pressure.

The bellows prevents backpressure acting on the top side of the disk. In relation to the piston there is no top side within the main body of the valve hence again the back pressure cannot affect the set pressure. Bellows failure is an important concern in critical applications where a very precise set pressure is required. In these cases some mechanism to detect a leak of process medium out of the top vent would be implemented. Piston designs are not usually found in conventional Safety valves but are more common in Pilot Operated Safety valves.

API 520 Practice Guidelines: a conventional design should not typically be used when the built-up backpressure is greater than 10% of the set pressure at 10% over pressure. European standard EN ISO 4126: the built-up backpressure should be limited to 10% of the set pressure when the valve is discharging at the certified capacity.

Overpressure is the percentage over the set pressure by which the valve is fully open. The blowdown is the percentage below the set pressure by which the valve is fully closed.

The basic elements of the design are right angle pattern valve body, inlet can be either a full nozzle or a semi-nozzle type. With a full nozzle design has the “wetted” inlet tract formed from one piece (as per figure 6) with the seat integrated into the top of the nozzle. The internal bore of the nozzle and the disc is the only part of the valve that is exposed to the process fluid with the valve in the closed position. A semi-nozzle design consists of a seating ring fitted into the body.The disc is held onto the seat by the stem, with the downward force coming from the compression on the spring mounted in the bonnet. The amount of compression on the spring is adjusted by the spring adjuster under the cap.

Unless bellows or diaphragm sealing is used, process fluid will enter the spring housing (or bonnet). 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.

A lifting mechanism is recommended to test for correct valve operation at all times where corrosion, caking, or any deposit could prevent the opening operation.

Foreign particles can lodge under the seat of the valve when it discharges. The lifting lever allows you to lift the valve and flush the obstruction. Pressure relief valves for Section VIII require a lift lever on all air, steam, and hot water valves used at temperatures over 60 degC. Typically used where periodic testing of the valve in location is desired to assure its operation. With an Open lifting lever design, when the valve discharges, fluid media will escape into the atmosphere around the open lifting lever assembly. If this is not desirable or when back pressure is present you would select a Packed Lifting Lever design.

As described above, this type is selected where leakage of the media to the atmosphere during valve discharge or during back pressure would be un-desirable. A packed lever design is a completely sealed assembly.

Under certain circumstances i.e. under the start-up conditions of a plant or to pressure test the system in a controlled environment, it may be required that the valve is prevented from opening.This is achieved by screwing the bolt (shown on the wire) into the cap which screws down onto the stem and prevents it lifting. Obviously it is important that test gags are removed prior to placing the valve into service.

The bellows is designed to cover the same area on the back of the disc equal to the seat area hence the back pressure will have no effect on the set pressure. See the previous section “Basic Safety Valve Principles”. Bellows also protects the spindle, spindle guide and spring from the process medium.

A disc is held against the nozzle by a spring, which is contained in a cast bonnet. The spring is adjusted by a compression screw to permit the calibration of opening or set pressure. An adjustable nozzle ring, threaded onto the nozzle, controls the geometry of the fluid exit control chamber (also known as a huddling chamber). The control chamber (huddling chamber) geometry is very important in controlling valve opening and closing pressures and stability of operation. The nozzle ring is locked into position by a ring pin assembly as shown in Figure 15 below.

Under normal system operation the valve remains in the closed position because the spring force (Fs) is greater than the system pressure acting on the internal nozzle seating area (PA). If system pressure increases to a point when these forces are equal, then the set pressure is reached. The disc lifts and fluid flows through the valve. When pressure in the system returns to a safe level, the valve closes.

Just prior to reaching set point, the pressure relief valve leaks system fluid into the huddling chamber. The fluid now acts on a larger area of the disc inside the huddling chamber (PAh), causing the valve to experience an instantaneous increase in the opening force. Refer to the figure 16 above to see relationship between Nozzle Area (A) and the Huddling Chamber Area (Ah). System pressure acting on the larger area will suddenly open the safety relief valve at a rapid rate.

Although the opening is rapid and dramatic, the valve does not open fully at set point. The system pressure must increase above set point to open the valve to its full lift and capacity position. Maximum lift and certified flow rates will be achieved within the allowable limits (overpressure) established by various codes and standards. All pressure relief ales are allowed an overpressure allowance to reach full rated flow. The allowable over pressure can vary from 10% to 21% on unfired vessels and systems, depending on the sizing basis, number of valves, and whether a fire condition is encountered.

Once the valve has controlled the pressure excursion, system pressure will start to reduce. Since the huddling chamber area is now controlling the exit fluid flow, system pressure must reduce below the set point before the spring force is able to close the valve. The difference between the set pressure and the closing pressure is called blowdown, and is usually expressed as a percentage of set pressure. The typical blowdown can vary from 7% to 10%, the industry standard.

The nozzle ring adjustment changes the shape and volume of the huddling chamber, and its position will affect both the opening and the closing characteristics of the valve. When the nozzle ring is adjusted to its top position, the huddling chamber is restricted to its maximum. The valve will usually pop very distinctly with a minimum simmer (leakage before opening), but the blowdown will increase. When the nozzle ring is lowered to its lowest position, minimal restriction to the huddling chamber occurs. At this position, simmer increases and the blowdown decreases. The final ring position is somewhere between these two extremes to provide optimal performance.

On liquid service, a different dynamic situation exists. Liquids do not expand when flowing across orifices, and a small amount of fluid flow across the nozzle will produces a large local pressure drop at the nozzle orifice. This local pressure drop causes the spring to reclose the valve if the fluid flow is minimal. Liquids leaking into the huddling chamber can quickly drain out by gravity and prevent fluid pressure from building up in the secondary area of the huddling chamber. Liquid relief valves are thus susceptible to a phenomenon called chatter, especially at low fluid flow rates. Chatter is the rapid opening and closing of the pressure relief valve and is always destructive.

Because of the difference in the characteristics of gases and liquids, some valve designs require a special liquid trim in order to meet ASME Code Section VIII performance criteria of full rated liquid flow at 10% overpressure. With liquids since no visible or audible pop is heard at set point, the set pressure is defined as the pressure when the first heavy flow occurs (a pencil sized steady stream of water that remains unbroken for approximately one inch).

If you have a system that is shut down for annual maintenance then this is an ideal time to remove your Safety valves and have them inspected and recertified.

For systems that can only be off for short periods of time, it is sensible to keep a spare valve to swap over and then the removed valve can be inspected and recertified.

For systems that cannot be shut down, you will need to use a changeover valve which allows you to swap between Safety valves allowing one to be removed for inspection and testing.

For larger Safety valves on systems that run continuously, you may consider using in-situ testing. This method does have some limitations however since you cannot visually inspect the inside of the valve, but it will tell you if the valve is opening at the correct set pressure.

(a) A valve passing (leaking) on the outlet side when the valve is supposed to be closed. This can happen to valves of any age (new or old) and occurs if debris contained in the medium passes through the valve at a point when the valve lifts, and the debris either traps or damages the internals of the valve. On soft seated valves, hard particles may embed themselves in the soft material causing re-sealing issues. If your valve has a lifting lever and it is safe to do so, then it is worth lifting the handle for a few seconds which will hopefully clear any debris allowing the valve to reseal correctly. If this isn’t an option or it doesn’t cure the problem, then the valve will need to be removed and returned for maintenance and recertification. The time we often see this the most is during the startup of a system and there is a pressure spike, hence this is why it is extremely important that a system is flushed out well before hand.

Test Gag is a bolt used for the spindle block of the pressure relief valve (PRV) to allow the PRV to be closed during the pressure test. The bolt should be tightened when the system has 80% of the pressure used. test (test pressure) to ensure that the PRV is closed at the test Pressure:

If you don’t know your blow down from your pop action, NASVI has you covered. Here is a handy cheat sheet on safety valve lingo and how to accurately order them.

SAFETY RELIEF VALVE:Safety relief valves are basically like pop safety valves and are primarily for liquid service where the thermal expansion in a liquid-laden vessel actuates the valve. When vapor is generated in these vessels, due to uncontrolled heat input, this valve with the huddling chamber, will give a high disc lift and discharge the expanded vapors. This valve is also suitable for gas or vapor service.

SELECTION OF VALVE:Valves should be selected for the particular installation on which they are to be used and also on the basis of the rated discharge capacity. This should be equal to or greater than the maximum output of the system.

INSTALLATION:The valve is to be installed in a vertical position, into a clean fitting, using the proper size and type of wrench so as not to damage the valve. The discharge piping, without stop valves, shall be independently supported and sloped downward slightly to drain condensate.

In the United States, use of such devices was spurred by the 1,700 boiler explosions that resulted in 1,300 deaths from 1905 to 1911. By 1915, the American Society of Mechanical Engineers (ASME) published its first boiler code, Rules for Construction of Stationary Boilers and Allowable Working Pressures, incorporating rules for construction and installation of safety valves for boilers.

The primary purpose of a pressure relief valve is to open to relieve excess pressure, reclose and prevent further flow of fluid after normal conditions have been restored (Figure 5). A secondary purpose is to minimize damage to other system components through operation of the pressure relief valve itself. A pressure relief valve designed under ASME Boiler and Pressure Vessel Code is stamped with the certification mark, and one of the certification designators: V, NV, HV, UV, UV3 or TV.

The many types of pressure relief valves that exist are based on different designs and construction. Generally, they’re classified as: safety relief valves, relief valves and safety valves.

A conventional safety relief valve is a spring-loaded pressure relief valve characterized by a rapid-opening pop action. Conventional safety relief valves are used for applications where excessive variable or built-up back pressure is not present in the system. The operational characteristics of these valves are directly affected by changes in the back pressure on the valve.

The working principle of a conventional spring-loaded safety relief valve is based on the balance of force. The spring load is preset to equal the force the inlet fluid exerts on the closed disk when the system pressure is at the set pressure of the valve.

The disk remains seated on the nozzle in the closed position when the inlet pressure is below the set pressure. The valve opens when the inlet pressure exceeds set pressure, overcoming the spring force. The valve recloses when the inlet pressure is reduced to a level below the set ­pressure.

Once the valve has opened, an additional pressure buildup at C occurs. This additional force at C causes the disk to lift substantially at pop. The valve closes when the inlet pressure has dropped sufficiently below the set pressure. The pressure at which the valve resets is called the closing pressure. The difference between the set pressure and closing pressure is the blowdown.

In the design of a conventional valve, an important consideration is seat leakage. This leakage can result in continuous loss of system fluid and may cause progressive damage to the valve seating surface. Based on the seating material, conventional valves are classified as:

Metal-seated valves. Metal-to-metal seats are commonly made from stainless or other hard alloy steels and are normally used for high-temperature applications such as steam and corrosive media applications for ­processing a wide variety of chemicals.

Soft-seated valve. An alternative to metal is resilient disks that can be fixed to either or both the seating surfaces where tighter shut-off is required. They are common for gas or liquid applications. These inserts may be made from a number of different materials, but Vinton, nitrile or EPDM (ethylene propylene diene monomer) are the most common.

Balanced bellows with auxiliary balancing piston. With this valve, the balanced bellows seal the body and fluid stream from the bonnet and working parts. The auxiliary balancing piston assures proper valve performance by compensating for back pressure in case the bellows fail.

The primary difference between a pilot-operated safety relief valve and a spring-loaded pressure relief valve is that the pilot-operated valve uses process pressure to keep the valve closed instead of a spring. A pilot is used to sense process pressure and to pressurize or vent the dome pressure chamber, which controls the valve opening or closing.

A pilot-operated safety relief valve consists of the main valve, a floating, unbalanced piston assembly, and an external pilot. The pilot controls the pressure on the top side of the main valve’s unbalanced moving chamber. A resilient seat is normally attached to the lower end.

At below-set level, the pressure on opposite sides of the moving member is equal. When the set pressure is reached, the pilot opens and depressurizes the cavity on the top side so the unbalanced member moves upward, causing the main valve to relieve. When the process pressure decreases to a predetermined pressure, the pilot closes, the cavity above the piston is depressurized and the main valve closes.

The valves operate bubble tight at higher operating pressure-to-set pressure ratios, allowing operators to run very close to the vessel’s maximum allowable working pressure.

Valve movement to open or close is fully controlled by a source of power such as electricity, steam or water (hydraulic). The valve may discharge to the atmosphere or to a container that is at lower pressure. The discharge capacity can be affected by downstream conditions.

Power-actuated safety relief valves are used mostly for forced-flow steam generators with no fixed steam or waterline. They are also used in nuclear power plants.

A temperature and pressure-actuated safety relief valve (also called a T&P safety relief valve) is a pressure relief valve that may be actuated by temp­erature or pressure on the inlet side (Figure 10).

Such a valve is designed for dual purposes. First, the T&P valve prevents temperature within a vessel from rising above a specified limit (generally 210°F or 98°C). Second, the T&P valve prevents pressure in the vessel from rising above a specified value.

A relief valve is actuated by inlet static pressure and a gradual lift that is generally proportional to the increase in pressure over opening pressure. Such a valve can be provided with enclosed spring housing suitable for closed ­discharge system applications.

Relief valves are commonly used in liquid systems, especially for lower capacities and thermal expansion applications. They also can be used on pump systems.

Adjustable relief valves feature convenient adjustment of the pressure setting through the outlet port. They are suitable for non-vented or vented inline applications in chemical, petrochemical and high-purity gas industries.

Electronic relief valves (ERVs) are pilot-operated relief valves that offer zero leakage. The ERV package combines a zero-leakage ­isolation valve with electric controls to monitor and regulate ­system pressure. These valves provide protection either in a capacity-relieving function or simply in an overpressure-protection application.

Safety valves are typically used for boiler overpressure protection and other applications such as downstream from pressure-reducing controls. These valves are installed wherever the maximum allowable working pressure of boilers is likely to be exceeded. Safety valves are also used for compressible gases, in particular for steam and air.

Safety valves are classified according to the lift. The term “lift” refers to the amount of travel the valve undergoes as it moves from its closed position to the position required to produce the certified discharge capacity.

Low-lift are safety valves in which the valve lifts a distance of 1/24th of the bore diameter. Since the valve has a small lift, the capacity is much lower than other types.

High-lift are safety valves in which the valve lifts a distance of at least 1/12th of the bore diameter. High-lift valves are used on compressible fluids, where their action is more proportional.

Full-lift are safety valves for which the valve lifts a distance of at least 1/4th of the bore diameter. Full-lift valves are considered the best choice for general steam applications.

Test gags are used to hold the safety valve closed while equipment is subjected to a hydrostatic test. To avoid damage to the spindle and/or seat, care is required so the gag screw is not tightened.

Lifting mechanisms are used to open the pressure relief valves when the pressure under the valve disk is lower than the set pressure. These mechanisms are available in three basic types: plain lever, packaged lever and air-operated lifting devices.

A key advantage of selecting a butterfly valve is the reduction of space and weight to a system compared with other options such as ball, check, globe or gate valves.