Safety valve, also known as pressure relief valve and safety relief valve. The valve has the most important role in steam applications. The safety valve is able to exhaust unnecessary high pressure from the boiler. The steam boiler usually has a high pressure inside. All boilers have a limit to handle high pressure. After crossing the limit of high pressure, they explode very dangerously. It can damage the assets and as well as workers, too. To prevent these unnecessary accidents and explosions, the safety valve comes into action. Safety Valve releases additional pressure from the boiler and maintains the required pressure.
Basic spring-loaded safety valves, known as “standard” or “conventional”, provide overpressure protection with a simple, reliable design.
Basically, the design is a right-angle pattern valve body. Mounted on the pressure-containing system with the valve inlet connection or nozzle. For connection to a pipe discharge system, the outlet connection may be screw or flange. The safety valve will not have an outlet connection in some applications. Such as pneumatic air systems. So the fluid vents directly to the atmosphere.
There are two types of valve inlet (or approach channel) designs: the full-nozzle and the semi-nozzle. In a full-nozzle design, the entire ‘wetted’ inlet tract is molded in one piece. Normal operation exposes only the approach channel to the process fluid. The disc is not exposed unless the valve is discharging.
Safety Valve Design
Safety valves with full-nozzles are typically used in high-pressure and process applications, especially when the fluid is corrosive.
As opposed to this, the semi-nozzle valve has a body fitted with a seating ring. The top forms set the valve seat. The advantage of this arrangement is that the seat can easily change. Changing the seat is possible without having to replace the entire inlet.
When the nozzle seat is pressed against the disc (in normal operation), the disc is held in place by a spring. Which is fitted into a spring housing arrangement (or bonnet) mounted to the top of the body. In rapid opening valves (pop type), the discs are enclosed in shrouds, holders, or chambers of huddling. Which causes the valve to open quickly.
The spring, typically made of carbon steel, provides the closing force on the disc. By adjusting the spring adjuster, the amount of compression on the spring can be adjusted to alter the amount of force required to lift the disc off its seat.
Safety valve standards typically only prescribe the 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).
- Flow area – An inlet’s narrowest point, at which its cross-section is the smallest. The diameter of the flow area is represented by dimension d.
- Curtain area – An opening is created between the seating surfaces when the disk is raised above the seat. D1 represents the diameter of the curtain area.
- Discharge area – The flow through the valve is determined by the lesser of the curtain and flow areas.
Dimension
As the name implies, full lift valves are those in which the flow area determines the capacity and not the curtain area. Compared to low lift or high lift valves, these valves will be able to handle a greater flow rate.
The primary elements of a conventional safety valve are similar, but the details can vary greatly. In general, ASME style valves are more complex, with a fixed skirt (or hood) arrangement, whereas DIN style valves are simpler, with a fixed skirt (or hood) arrangement or two adjustable blowdown rings. By adjusting the position of these rings, the valve can benefit from fine-tuning its blowdown and overpressure values.
There may be different sizes of inlet and outlet connections for a given orifice area, as well as body dimensions like centreline to face. Moreover, many competing products, especially those of European origin, have different dimensions and capacities for the same nominal size.
One exception is found with steel ASME specification valves, which invariably comply with API Recommended Practice 526, which specifies the dimensions of the centrelines and orifices as part of the valve design. A letter is assigned to each orifice area series. This practice is common.
Different sizes of inlet and outlet connections for valves with the same orifice letter.
For instance, 2″ x J x 3″ and 3″ x J x 4″ have the same orifice size (‘J’), but their inlet and outlet sizes are different as indicated before and after the orifice letter, respectively.
The 2″ x J x 3″ valve would have a 2″ inlet, a ‘J’ size orifice, and a 3″ outlet.
The Basic Operation of a Safety Valve
Lifting
When the inlet static pressure rises above the safety valve’s set pressure, the disc will lift off its seat. However, as soon as the spring starts to compress, the spring force will increase; this means that for significant flow through the valve to occur, the pressure would need to continue to rise.
In order for a safety valve to discharge at its rated capacity, it must rise to additional pressure. Depending on the standards follow and the application, overpressure will allow. A typical range is 3% to 10% for compressible fluids and 10% to 25% for liquids.
A disc arrangement must design so that it can open rapidly under this small overpressure. In order to achieve full opening. A shroud, skirt, or hood typically fits around the disc. A control chamber or huddle chamber is the volume contains within this shroud.
Lifting the shroud begins when fluid enters the chamber (Figure 9.1.6b), which causes a greater area of the shroud to be exposed to the fluid pressure. As the lifting force (F) is proportional to the pressure (P) and the area (A) of the opening (F = P x A), the opening force increases.
Due to this incremental increase in opening force, the increased spring force is overcompensated, causing rapid opening. By reversing the flow, the shroud also increases lift, resulting in a reaction force.
The valve is able to achieve its designed lift with a relatively small percentage of overpressure when these effects are combined. Re. As the volume of incompressible fluid changes from one pressure area to another, rapid expansion is another contributing factor. As a result, the valve is able to open fully within the small overpressure limit. Liquids affect more proportionately, resulting in an overpressure that is more than 25%.
Reseating
When the pressure drops below the original measure pressure, the device will close once normal operating conditions restore. The disc won’t close, however, until it drops below the set pressure, since a large portion remains to the fluid. It’s usually expressing as a percentage of the set pressure, and it’s the difference between the set pressure and the resting pressure. Blowdown is usually less than 10% for compressible fluids and up to 20% for liquids.
When a potentially hazardous situation arrives, the shroud should have both a rapid opening and a relatively small blowdown. So that any overpressure release as soon as possible. While preventing excessive amounts of fluid from being out. The system pressure needs to reduce sufficiently at the same time in order to prevent a reopening of the valve immediately.
Many ASME-type safety valves use blowdown rings to finely adjust the overpressure and blowdown values of the valves. Often, on valves that require tighter overpressure and blowdown, the lower blowdown (nozzle) ring is a common feature. Huddling chamber geometry is generally determine by the upper blowdown ring, which is normally factory-set and essentially eliminates manufacturing tolerances.
Under certain conditions, the lower blowdown ring can also adjust to meet the decide performance requirements. When the lower blowdown ring adjusts to its top position. The valve will pop rapidly when the huddle chamber volume is maximize.
Requiring a greater blowdown to re-seat the valve while minimizing the overpressure value. If the lower blowdown ring setting to its lowest position. There will be minimal restrictions within the huddling chamber. But a greater overpressure needs before huddling can occur and the valve is fully open. But the blowdown value will be lesser.
Approval Authorities
In most countries, independent bodies will evaluate a product’s design and performance to ensure conformity with the relevant code or standard. In the case of safety-related products, third-party approval is a requirement before purchase.
Depending on the code or standard, the approval process may differ. In some cases, revalidation is necessary every few years. While in others, approval is indefinite so long as no significant changes taking to the design. In which case revalidation does not require and must seek from the approval authority. As a government agency responsible for ensuring compliance with boiler and pressure vessel codes in both the US and Canada, the National Board of Boiler and Pressure Vessel Inspectors represents these agencies.
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In India and South Asian countries, the Indian Boiler Regulations (IBR) approved safety valves are most welcomed by industrialists. The industrial product has to go through performance tests and determined parameters to get the IBR approval for the safety valves.
Codes and Standards
The format of safety valve standards varies quite a bit around the world. Many of them place within codes that deal with boilers or pressure vessels. Performance requirements, tolerances, and construction details show the result. But there will be no guidelines regarding dimensions or orifice sizes. Installation and use will also address.
Many markets use a combination of several methods together.
Several national standards and insurance companies set very specific requirements for steam boiler safety valve performance. Often, an independent authority requires, such as British Engine, TÜV, or Lloyd’s Register.
The standards associated with the Pressure Equipment Directive (PED) also apply to safety valves used in Europe. The PED regime requires the most rigorous level of assessment for ‘Safety accessories’, i.e., safety valves, as they are also famous as ‘Category 4’ equipment. The manufacturer will usually have an ISO 9000 quality system, and the safety valve design and performance will be assertive by an official approval authority calling as ‘Notified Body’.
