SAFETY VALVES
1. Introduction to Safety Valves
2. Safety valve design
3. Basic operation of a safety valve
4. Types of Safety Valve
5. Conventional safety valves
6. Balanced safety valves
7. Pilot operated safety valve
8. Full lift, high lift and low lift safety valves
9. Safety valve options and accessories
Introduction to Safety Valves
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 primary function of a safety valve is therefore to protect life and property.
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:
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 primary function of a safety valve is therefore to protect life and property.
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:
- An imbalance of fluid flowrate caused by inadvertently closed or opened isolation valves on a process vessel.
- Failure of a cooling system, which allows vapour or fluid to expand.
- Compressed air or electrical power failure to control instrumentation.
- Transient pressure surges.
- Exposure to plant fires.
- Heat exchanger tube failure.
- Uncontrollable exothermic reactions in chemical plants.
- Ambient temperature changes.
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. Furthermore, different designs are required to
meet the numerous national standards that govern the use of safety
valves.
A listing of the relevant national standards can be found at the end of this tutorial.
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.
The ASME / ANSI PTC25.3 standards applicable to the USA define the following generic terms:
A listing of the relevant national standards can be found at the end of this tutorial.
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.
The ASME / ANSI PTC25.3 standards applicable to the USA define the following generic terms:
- 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.
This is a general term, which includes safety valves, relief valves and safety relief valves. - Safety valve - A pressure relief valve actuated by inlet static pressure and characterised by rapid opening or pop action.
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.
The European standards (BS 6759 and DIN 3320) provide the following definition:
- Safety valve - A valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a certified amount of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent the further flow of fluid after normal pressure conditions of service have been restored.
Typical examples of safety valves used on steam systems are shown in Figure 9.1.1.
Fig. 9.1.1 Typical safety valvesSafety valve design
The
basic spring loaded safety valve, referred to as 'standard' or
'conventional' is a simple, reliable self-acting device that provides
overpressure protection.
The basic elements of the design consist of a right angle pattern valve body with the valve inlet connection, or nozzle, mounted on the pressure-containing system. The outlet connection may be screwed or flanged for connection to a piped discharge system. However, in some applications, such as compressed air systems, the safety valve will not have an outlet connection, and the fluid is vented directly to the atmosphere.
The basic elements of the design consist of a right angle pattern valve body with the valve inlet connection, or nozzle, mounted on the pressure-containing system. The outlet connection may be screwed or flanged for connection to a piped discharge system. However, in some applications, such as compressed air systems, the safety valve will not have an outlet connection, and the fluid is vented directly to the atmosphere.
Fig. 9.1.2 Typical safety valve designs
The
valve inlet (or approach channel) design can be either a full-nozzle or
a semi-nozzle type. A full-nozzle design has the entire 'wetted' inlet
tract formed from one piece. The approach channel is the only part of
the safety valve that is exposed to the process fluid during normal
operation, other than the disc, unless the valve is discharging.
Full-nozzles are usually incorporated in safety valves designed for process and high pressure applications, especially when the fluid is corrosive.
Conversely, the semi-nozzle design consists of a seating ring fitted into the body, the top of which forms the seat of the valve. The advantage of this arrangement is that the seat can easily be replaced, without replacing the whole inlet.
The disc is held against the nozzle seat (under normal operating conditions) by the spring, which is housed in an open or closed spring housing arrangement (or bonnet) mounted on top of the body. The discs used in rapid opening (pop type) safety valves are surrounded by a shroud, disc holder or huddling chamber which helps to produce the rapid opening characteristic.
Full-nozzles are usually incorporated in safety valves designed for process and high pressure applications, especially when the fluid is corrosive.
Conversely, the semi-nozzle design consists of a seating ring fitted into the body, the top of which forms the seat of the valve. The advantage of this arrangement is that the seat can easily be replaced, without replacing the whole inlet.
The disc is held against the nozzle seat (under normal operating conditions) by the spring, which is housed in an open or closed spring housing arrangement (or bonnet) mounted on top of the body. The discs used in rapid opening (pop type) safety valves are surrounded by a shroud, disc holder or huddling chamber which helps to produce the rapid opening characteristic.
Fig. 9.1.3 A full-nozzle valve (a) and a semi-nozzle valve (b)
The
closing force on the disc is provided by a spring, typically made from
carbon steel. The amount of compression on the spring is usually
adjustable, using the spring adjuster, to alter the pressure at which
the disc is lifted off its seat.
Standards that govern the design and use of safety valves generally only define the three dimensions that relate to the discharge capacity of the safety valve, namely the flow (or bore) area, the curtain area and the discharge (or orifice) area (see Figure 9.1.4).
1. Flow area - The minimum cross-sectional area between the inlet and the seat, at its narrowest point. The diameter of the flow area is represented by dimension 'd' in Figure 9.1.4.
Standards that govern the design and use of safety valves generally only define the three dimensions that relate to the discharge capacity of the safety valve, namely the flow (or bore) area, the curtain area and the discharge (or orifice) area (see Figure 9.1.4).
1. Flow area - The minimum cross-sectional area between the inlet and the seat, at its narrowest point. The diameter of the flow area is represented by dimension 'd' in Figure 9.1.4.
Equation 9.1.1
2. Curtain area - The
area of the cylindrical or conical discharge opening between the
seating surfaces created by the lift of the disk above the seat. The
diameter of the curtain area is represented by dimension 'd1' in Figure
9.1.4.
Equation 9.1.2
3. Discharge area - This is the lesser of the curtain and flow areas, which determines the flow through the valve.
Fig. 9.1.4 Illustration of the standard defined areas
Valves
in which the flow area and not the curtain area determines the capacity
are known as full lift valves. These valves will have a greater
capacity than low lift or high lift valves. This issue will be discussed
in greater depth in Tutorial 9.2.
Although the principal elements of a conventional safety valve are similar, the design details can vary considerably. In general, the DIN style valves (commonly used throughout Europe) tend to use a simpler construction with a fixed skirt (or hood) arrangement whereas the ASME style valves have a more complex design that includes one or two adjustable blowdown rings. The position of these rings can be used to fine-tune the overpressure and blowdown values of the valve.
For a given orifice area, there may be a number of different inlet and outlet connection sizes, as well as body dimensions such as centreline to face dimensions. Furthermore, many competing products, particularly of European origin have differing dimensions and capacities for the same nominal size.
An exception to this situation is found with steel ASME specification valves, which invariably follow the recommendations of the API Recommended Practice 526, where centreline to face dimensions, and orifice sizes are listed. The orifice area series are referred to by a letter. It is common for valves with the same orifice letter to have several different sizes of inlet and outlet connection. For example, 2" x J x 3" and 3" x J x 4" are both valves which have the same size ('J) orifice, but they have differing inlet and outlet sizes as shown before and after the orifice letter respectively. A 2" x J x 3" valve would have a 2" inlet, a 'J' size orifice and a 3" outlet.
This letter series is also referenced in other standards, for example, BS 6759 part 3, which deals with valves for process type applications and NFE- E 29-414.
Although the principal elements of a conventional safety valve are similar, the design details can vary considerably. In general, the DIN style valves (commonly used throughout Europe) tend to use a simpler construction with a fixed skirt (or hood) arrangement whereas the ASME style valves have a more complex design that includes one or two adjustable blowdown rings. The position of these rings can be used to fine-tune the overpressure and blowdown values of the valve.
For a given orifice area, there may be a number of different inlet and outlet connection sizes, as well as body dimensions such as centreline to face dimensions. Furthermore, many competing products, particularly of European origin have differing dimensions and capacities for the same nominal size.
An exception to this situation is found with steel ASME specification valves, which invariably follow the recommendations of the API Recommended Practice 526, where centreline to face dimensions, and orifice sizes are listed. The orifice area series are referred to by a letter. It is common for valves with the same orifice letter to have several different sizes of inlet and outlet connection. For example, 2" x J x 3" and 3" x J x 4" are both valves which have the same size ('J) orifice, but they have differing inlet and outlet sizes as shown before and after the orifice letter respectively. A 2" x J x 3" valve would have a 2" inlet, a 'J' size orifice and a 3" outlet.
This letter series is also referenced in other standards, for example, BS 6759 part 3, which deals with valves for process type applications and NFE- E 29-414.
Basic operation of a safety valve
Lifting
When
the inlet static pressure rises above the set pressure of the safety
valve, the disc will begin to lift off its seat. However, as soon as the
spring starts to compress, the spring force will increase; this means
that the pressure would have to continue to rise before any further lift
can occur, and for there to be any significant flow through the valve.
The additional pressure rise required before the safety valve will discharge at its rated capacity is called the overpressure. The allowable overpressure depends on the standards being followed and the particular application. For compressible fluids, this is normally between 3% and 10%, and for liquids between 10% and 25%.
In order to achieve full opening from this small overpressure, the disc arrangement has to be specially designed to provide rapid opening. This is usually done by placing a shroud, skirt or hood around the disc. The volume contained within this shroud is known as the control or huddling chamber.
The additional pressure rise required before the safety valve will discharge at its rated capacity is called the overpressure. The allowable overpressure depends on the standards being followed and the particular application. For compressible fluids, this is normally between 3% and 10%, and for liquids between 10% and 25%.
In order to achieve full opening from this small overpressure, the disc arrangement has to be specially designed to provide rapid opening. This is usually done by placing a shroud, skirt or hood around the disc. The volume contained within this shroud is known as the control or huddling chamber.
Fig. 9.1.5 Typical disc and shroud arrangement used on rapid opening safety valves
As
lift begins (Figure 9.1.6b), and fluid enters the chamber, a larger
area of the shroud is exposed to the fluid pressure. Since the magnitude
of the lifting force (F) is proportional to the product of the pressure
(P) and the area exposed to the fluid (A); (F = P x A), the opening
force is increased. This incremental increase in opening force
overcompensates for the increase in spring force, causing rapid opening.
At the same time, the shroud reverses the direction of the flow, which
provides a reaction force, further enhancing the lift.
These combined effects allow the valve to achieve its designed lift within a relatively small percentage overpressure. For compressible fluids, an additional contributory factor is the rapid expansion as the fluid volume increases from a higher to a lower pressure area. This plays a major role in ensuring that the valve opens fully within the small overpressure limit. For liquids, this effect is more proportional and subsequently, the overpressure is typically greater; 25% is common.
These combined effects allow the valve to achieve its designed lift within a relatively small percentage overpressure. For compressible fluids, an additional contributory factor is the rapid expansion as the fluid volume increases from a higher to a lower pressure area. This plays a major role in ensuring that the valve opens fully within the small overpressure limit. For liquids, this effect is more proportional and subsequently, the overpressure is typically greater; 25% is common.
Fig. 9.1.6 Operation of a conventional safety valve
Reseating
Once normal operating conditions have been restored, the valve is required to close again, but since the larger area of the disc is still exposed to the fluid, the valve will not close until the pressure has dropped below the original set pressure. The difference between the set pressure and this reseating pressure is known as the 'blowdown', and it is usually specified as a percentage of the set pressure. For compressible fluids, the blowdown is usually less than 10%, and for liquids, it can be up to 20%.
Once normal operating conditions have been restored, the valve is required to close again, but since the larger area of the disc is still exposed to the fluid, the valve will not close until the pressure has dropped below the original set pressure. The difference between the set pressure and this reseating pressure is known as the 'blowdown', and it is usually specified as a percentage of the set pressure. For compressible fluids, the blowdown is usually less than 10%, and for liquids, it can be up to 20%.
Fig. 9.1.7 Relationship between pressure and lift for a typical safety valve
The
design of the shroud must be such that it offers both rapid opening and
relatively small blowdown, so that as soon as a potentially hazardous
situation is reached, any overpressure is relieved, but excessive
quantities of the fluid are prevented from being discharged. At the same
time, it is necessary to ensure that the system pressure is reduced
sufficiently to prevent immediate reopening.
The blowdown rings found on most ASME type safety valves are used to make fine adjustments to the overpressure and blowdown values of the valves (see Figure 9.1.8). The lower blowdown (nozzle) ring is a common feature on many valves where the tighter overpressure and blowdown requirements require a more sophisticated designed solution. The upper blowdown ring is usually factory set and essentially takes out the manufacturing tolerances which affect the geometry of the huddling chamber.
The lower blowdown ring is also factory set to achieve the appropriate code performance requirements but under certain circumstances can be altered. When the lower blowdown ring is adjusted to its top position the huddling chamber volume is such that the valve will pop rapidly, minimising the overpressure value but correspondingly requiring a greater blowdown before the valve re-seats. When the lower blowdown ring is adjusted to its lower position there is minimal restriction in the huddling chamber and a greater overpressure will be required before the valve is fully open but the blowdown value will be reduced.
The blowdown rings found on most ASME type safety valves are used to make fine adjustments to the overpressure and blowdown values of the valves (see Figure 9.1.8). The lower blowdown (nozzle) ring is a common feature on many valves where the tighter overpressure and blowdown requirements require a more sophisticated designed solution. The upper blowdown ring is usually factory set and essentially takes out the manufacturing tolerances which affect the geometry of the huddling chamber.
The lower blowdown ring is also factory set to achieve the appropriate code performance requirements but under certain circumstances can be altered. When the lower blowdown ring is adjusted to its top position the huddling chamber volume is such that the valve will pop rapidly, minimising the overpressure value but correspondingly requiring a greater blowdown before the valve re-seats. When the lower blowdown ring is adjusted to its lower position there is minimal restriction in the huddling chamber and a greater overpressure will be required before the valve is fully open but the blowdown value will be reduced.
Fig. 9.1.8 The blowdown rings on an ASME type safety valveTypes of Safety Valve
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:
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.
- Low lift safety valve - The actual position of the disc determines the discharge area of the valve.
- Full lift safety valve - The discharge area is not determined by the position of the disc.
- 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.
The
following types of safety valve are defined in the DIN 3320 standard,
which relates to safety valves sold in Germany and other parts of
Europe:
- 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.
EN ISO 4126 lists the following definitions of types of safety valve:
- 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 following table summarises the performance of different types of safety valve set out by the various standards.
Conventional safety valves
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:
- Superimposed backpressure - The static pressure that exists on the outlet side of a closed valve.
- Built-up backpressure - The additional pressure generated on the outlet side when the valve is discharging.
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. This can 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.
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. This can 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.
Fig. 9.2.1 Schematic diagram of safety valves with bonnets vented to (a) the valve discharge and (b) 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:
Equation 9.2.1
Where:
| PV | = | Fluid inlet pressure |
| AN | = | Nozzle area |
| FS | = | Spring force |
| PB | = | Backpressure |
Therefore,
any superimposed backpressure will tend to increase the closing force
and the inlet pressure required to lift the disc is greater.
In the case of a valve whose spring housing is vented to the atmosphere (Figure 9.2.1b), the required opening force is:
In the case of a valve whose spring housing is vented to the atmosphere (Figure 9.2.1b), the required opening force is:
Equation 9.2.2
Where:
| PV | = | Fluid inlet pressure |
| AN | = | Nozzle area |
| FS | = | Spring force |
| PB | = | Backpressure |
| AD | = | Disc area |
Thus,
the superimposed backpressure acts with the vessel pressure to overcome
the spring force, and the opening pressure will be less than expected.
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.
Equation 9.2.3
Where:
| PS | = | Set pressure of safety valves |
| AN | = | Nozzle area |
| FS | = | Spring force |
| PB | = | Backpressure |
| PO | = | Overpressure |
Therefore,
if the backpressure is greater than the overpressure, the valve will
tend to close, reducing the flow. This can lead to instability within
the system and can result in flutter or chatter of the valve.
In general, if conventional safety valves are used in applications, where there is an excessive built-up backpressure, they will not perform as expected. According to the API 520 Recommended Practice Guidelines:
In general, if conventional safety valves are used in applications, where there is an excessive built-up backpressure, they will not perform as expected. According to the API 520 Recommended Practice Guidelines:
- A conventional pressure relief valve should typically not be used when the built-up backpressure is greater than 10% of the set pressure at 10% overpressure. A higher maximum allowable built-up backpressure may be used for overpressure greater than 10%.
The
European Standard EN ISO 4126, however, states that the built-up
backpressure should be limited to 10% of the set pressure when the valve
is discharging at the certified capacity.
For the majority of steam applications, the backpressure can be maintained within these limits by carefully sizing any discharge pipes. This will be discussed in Tutorial 9.4. If, however, it is not feasible to reduce the backpressure, then it may be necessary to use a balanced safety valve.
For the majority of steam applications, the backpressure can be maintained within these limits by carefully sizing any discharge pipes. This will be discussed in Tutorial 9.4. If, however, it is not feasible to reduce the backpressure, then it may be necessary to use a balanced safety valve.
Balanced safety valves
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:
- Piston type balanced safety valve
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.
Fig. 9.2.2 Schematic diagram of a piston type balanced safety valve
By considering the forces acting on the piston, it is evident that this type of valve is no longer affected by any backpressure:
Where:
| PV | = | Fluid inlet pressure |
| AN | = | Nozzle area |
| FS | = | Spring force |
| PB | = | Backpressure |
| AD | = | Disc area |
| AP | = | Piston area |
Since AP equals AN,
the last two terms of the equation are equal in magnitude and cancel
out of the equation. Therefore, this simplifies to Equation 9.2.4.
Equation 9.2.4
Where:
| PV | Fluid inlet pressure | |
| AN | Nozzle area | |
| FS | Spring force |
- Bellows type balanced safety valve
A bellows with an effective area (AB) equivalent to the nozzle seat area (AN) is attached to the upper surface of the disc and to the spindle guide.
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.
The bellows vent allows air to flow freely in and out of the bellows as they expand or contract. 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.
Pilot operated safety valve
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.
There are two basic types of pilot operated safety valve, namely, the diaphragm and piston type.
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.
There are two basic types of pilot operated safety valve, namely, the diaphragm and piston type.
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.
Fig. 9.2.4 A 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.
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.
Full lift, high lift and low lift safety valves
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.
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.
Safety valve options and accessories
Due to the wide range of applications in which safety valves are used, there are a number of different options available:
Seating material
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.
Table 9.2.2 Seating materials used in safety valvesLevers
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))
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))
Fig. 9.2.5 Levers
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).
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On what basis,lift formula is differentiated for three type of lifts.Kindly explain
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