The Safety Pressure relief Relief Valve
What is a safety valve?
A pressure safety valve (acronym PSV) is an automatic device having an inlet and an outlet, generally perpendicular to each other (at 90°), capable of decreasing the pressure within a system.
The image on the left represents a stylised drawing of a safety valve, used as a symbol in the engineering diagrams of thermo-hydraulic systems.
Safety valves are emergency relief devices for pressurised fluids, which automatically operate when the set pressure is exceeded. These valves are governed by specific national and international standards. Our valves have to be sized, tested, installed and maintained in accordance with current regulations and as prescribed in our manuals.
Besa® safety valves are the result of a great deal of experience, since 1946 to the present day, in various fields of application and largely satisfy all the requirements of the latest pressure device defence. They are perfectly capable of not exceeding the maximum pressure increase allowed, even if all other autonomous safety devices installed upstream have failed.
The main components of the safety valve are shown in the figure:
Note on the application and use of the disc lever
The disc lift lever is an accessory that a safety valve can be equipped with, that allows the manual partial lift of the disc. Usually, the purpose of this manoeuvre is to cause – during valve operation – the escape of the process fluid in order to clean the surfaces between seat and disc, checking for any possible “sticking”. The manoeuvre of manually raising the shutter, must be carried out with the valve correctly installed on the system in operation and in the presence of a certain pressure value, in order to be able to benefit the pressure exercised by the process fluid to reduce the manual operator effort.
History of safety valve
Many years ago, in the streets of ancient Asia, puffed rice was used to be produced using hermetically sealed pots in which rice grains were placed inside along with water. By rotating the pot over the fire the pressure inside it increased due to the evaporation of the trapped water. Once the rice was cooked, the pot was wrapped in a sack and opened, causing a controlled explosion. This was a very dangerous method, because without a safety valve, there was a risk of the whole thing to explode unintentionally. This technique was mostly replaced after World War II by more efficient machines capable of producing continuously puffed rice.
The first safety valves were developed in the 17th century from prototypes by the French inventor Denis Papin.
Back to those days, safety valves operated with a lever and a counterbalance weight (which still exist today) although, in modern times, the use of a spring instead of a weight has become popular and efficient.
What is a safety valve for?
The main safety valves' aim is to protect people's lives by preventing any system, operating at a given pressure, from exploding.
This is why it is important to guarantee that safety valves always work, as they are the last devices in a long series that can prevent an explosion.
The following pictures show the devastating results of an incorrectly sized, installed or regularly maintained safety valve:
safety valve function
Where is safety valve used?
Everywhere the maximum operating pressure risks to be exceeded, safety valves must be installed. A system can go into overpressure for several reasons.
The principal reasons concern an uncontrolled temperature rise, causing expansion of the fluid with the consequence of a pressure increase, such as a fire in the system or a malfunction of the cooling system.
Another reason, for which the safety valve kicks in, is a failure of the compressed air or power supply, preventing a correct reading of the sensors at the control instrumentation.
Critical are also the first moments when starting a system for the first time, or after it has been stopped for a long time.
How does a safety valve work?
- The pressure applied by the fluid inside the valve body acts on the surface of the disc, generating a force F.
- When F reaches the same intensity as the spring force (the spring is mounted inside the valve and previously adjusted by compression to a predetermined value), the plug begins to lift out of the sealing area of the seat and the process fluid begins to flow (this is not, however, the maximum flow rate of the valve).
- At this point, normally, the upstream pressure continues to increase, causing, with an increase of about 10% (called overpressure) compared to the set pressure, the sudden and complete lifting of the valve disc, which releases the process medium through the valve’s minimum cross-section.
- When the capacity of the safety valve is equal to the flow rate to be discharged, the pressure inside the protected equipment remains constant. Otherwise, if the capacity of the safety valve is higher than the flow rate to be discharged, the pressure inside the equipment tends to decrease. In this case, the disc, on which the spring force continues to act, begins to reduce its lift (i.e. the distance between the seat and the disc) until the passage section of the valve closes (generally a decrease – called blowdown – equal to 10% less than the set pressure) and the process fluid stops flowing out.
How many types of safety valves are there?
In the context of pressure relief devices (acronym PRD), a fundamental distinction can be made between devices that close again and those that do not close again after their operation. In the first group we have rupture discs and pin operated devices. In contrast, the second group is divided into direct-loading and controlled devices. Safety valves are part of the devices that close again after their operation actuated by one or more springs.
In addition, a further distinction can be made according to the operation of the valves. As we can see from the diagram, there are full lift safety valves and proportional safety valves, also called relief valves.
safety valve vs relief valve
What is the difference between safety valves and relief valves?
Pressure safety valves (acronym PSV) and pressure relief valves (acronym PRV) are often confused because they have a similar structure and performance. In fact, both valves automatically discharge fluids when the pressure exceeds the set value. Their differences are often ignored, as they are interchangeable in some production systems. The main difference is not in their purpose, but in the type of operation. To understand the difference between the two, we need to go into the definitions given by the ASME (American Society of Mechanical Engineers) Boiler & Pressure Vessel or BPVC .
The safety valve is an automatic pressure control device actuated by the static pressure of the fluid upstream of the valve, used for gas or steam applications, with “full lift” action.
The relief valve (also known as an ‘overflow valve’) is an automatic pressure relief device actuated by the static pressure upstream of the valve. It opens proportionally when the pressure exceeds the opening force, primarily used for fluid applications.
Quality over quantity
Accessories for safety valves
Safety valves with balancing / protection bellows
Bellows in a safety valve have the following functions:
1) balancing bellows: guarantees the safety valve’s proper work, cancelling or limiting the effects of backpressure, which can be imposed or built-up, to a value within valve’s specified limits.
2) protection bellows: protects the spindle, spindle guide and all the safety valve’s upper part (included spring) from the contact with process fluid, ensuring all moving parts integrity and helping to avoid damages due to cristallization or polymerisation, corrosion or abrasion of internal components, which could compromise the safety valve’s correct functioning.
Safety valve equipped with pneumatic actuator
The pneumatic actuator allows the complete disc lifting, remote controlled and independently from the working pressure of the process fluid.
Safety valve equipped with disc blocking device
Besa can equip its safety valves with the “test gag”, which consists of two screws, one red and one green. The red screw, being longer than the green one, blocks the raising of the disc, preventing the valve from opening.
Safety valve equipped with pneumatic valve equipped with lift indicator
The lift indicator function is to detect the disc lifting, i.e. the valve opening.
Safety valve equipped with vibrations stabilizer
The vibration stabilizer reduces to a minimum oscillations and vibrations which can occur during the relieving phase, causing the valve to function improperly.
Resilient seal safety valves
To obtain a better seal between disc and seat surfaces, it is possible to equip the valve with a resilient seal. This solution is carried out after Technical Department analysis and considering exercise conditions: pressure, temperature, nature and physical state of process medium.
resilient seal is obtained with following materials: viton ®, NBR, neoprene ®, Kalrez ®, Kaflon™, EPDM, PTFE, PEEK™
Safety valves with heating jacket
In case of highly viscous, sticky or potentially crystallising media, safety valve can be supplied with heating jacket, which is a stainless steel case welded on the valve body, filled with a hot fluid (steam, hot water, etc.) in order to guarantee the process media flowability through the valve.
Stellited sealing surfaces
In order to obtain a better corrosion and wear resistance of disc and seat sealing surfaces, on request or after Tech. Dept. analysis, safety valves are supplied with disc and seat having stellited sealing surfaces. This solution is recommended in case of high pressure and temperature values, abrasive media, media with solid parts, cavitation.
Combined application of safety valves and rupture disc
Besa® safety valves are suitable for installation in combination with rupture discs arranged either upstream or downstream of the valve. Rupture discs used in such applications must be guaranteed non-fragmenting, from the structural point of view. For the fluid dynamics, on the other hand, any rupture disc sited upstream of the valve must be installed in such a way that:
- rupture disc flowing diameters is larger than or equal to safety valve’s nominal inlet diameter
- the total pressure drop (calculated from the nominal flow capacity multiplied by 1.15) from the protected tank inlet to the valve inlet flange is less than 3% of the safety valve’s effective set pressure. The space between the rupture disc and the valve must be vented to a 1/4” pipe in such a way as to ensure that atmospheric pressure is properly and safely maintained. For correct sizing of discs in terms of fluid dynamics, the factor Fd (EN ISO 4126-3 Pages 12. 13) must be taken into account, and can be taken to be 0. 9.
Application of a rupture disc upstream of a safety valve can be recommended for the following cases:
- when operating with aggressive media, to isolate inlet side of the valve body from continuos contact with process fluid, avoiding the use of expensive materials;
- when the metallic seal is provided, to avoid accidental leakage of fluid between seat/disc surfaces.
Certifications and approvals
Besa® safety valves are designed, manufactured and selected in accordance with the European Directives 2014/68/EU (New PED), 2014/34/EU (ATEX) and API 520 526 and 527. Besa® products are also approved by RINA® (Besa is a recognized as manufacturer) and DNV GL®.
Upon request Besa offers full assistance for the performance of tests by the main bodies.
Here below you can find our main certifications obtained for the safety valves.
Besa safety valves are CE PED certified
The PED directive provides for the marking of pressure equipment and everything where the maximum allowable pressure (PS) is greater than 0.5 bar. This equipment must be sized according to:
- the fields of use (pressures, temperatures)
- the types of fluid used (water, gas, hydrocarbons, etc.)
- the size/pressure ratio required for the application
The aim of Directive 97/23/EC is to harmonise all legislation of the states belonging to the European Community on pressure equipment. In particular, the criteria for design, manufacture, control, testing and field of application are regulated. This allows the free circulation of pressure equipment and accessories.
The directive requires the compliance with the essential safety requirements to which the producer must conform the products and the production process. The manufacturer is obliged to estimate and minimise the risks of the product placed on the market.
The organization conducts audits and controls based on various levels of monitoring of the company’s quality systems. Then, the PED organization releases CE certificates for each type and model of product and, if necessary, also for final verification before commissioning.
The PED organization then proceeds with:
- The selection of models for certification/labelling
- The examination of the technical file and design documentation
- The definition of the inspections with the manufacturer
- The verification of these controls in service
- The body then issues the CE certificate and label for the manufactured product
Besa safety valves are CE ATEX certified
ATEX – Equipment for potentially explosive atmospheres (94/9/EC).
“Directive 94/9/EC, better known by the acronym ATEX, was implemented in Italy by Presidential Decree 126 of 23 March 1998 and applies to products intended for use in potentially explosive atmospheres. With the entry into force of the ATEX Directive, the standards previously in force were repealed and from 1 July 2003 it is prohibited to market products that do not comply with the new provisions.
Directive 94/9/EC is a ‘new approach’ directive which aims to allow the free movement of goods within the Community. This is achieved by harmonising legal safety requirements, following a risk-based approach. It also aims to eliminate or, at least, minimise the risks arising from the use of certain products in or in relation to a potentially explosive atmosphere. This
means that the likelihood of an explosive atmosphere arising must be considered not only on a “one-off” basis and from a static point of view, but all the operating conditions that may arise from the process must also be taken into account.
The Directive covers equipment, whether alone or combined, intended for installation in “zones” classified as hazardous; protective systems serving to stop or contain explosions; components and parts essential to the functioning of equipment or protective systems; and control and adjustment safety devices useful or necessary for the safe and reliable functioning of equipment or protective systems.
Among the innovative aspects of the Directive, which covers all explosion hazards of any kind (electrical and non-electrical), the following should be highlighted:
- The introduction of essential health and safety requirements.
- The applicability to both mining and surface materials.
- The classification of equipment into categories according to the type of protection provided.
- Production supervision based on company quality systems.
Directive 94/9/EC classifies equipment into two main groups:
- Group 1 (Category M1 and M2): equipment and protective systems intended for use in mines
- Group 2 (Category 1,2,3): Equipment and protective systems intended for use on the surface. (85% of industrial production)
The classification of the installation zone of the equipment will be the responsibility of the end user; therefore according to the risk area of the customer (e.g. zone 21 or zone 1) the manufacturer will have to supply equipment suitable for that zone.
Besa safety valves are RINA certified
RINA has been operating as an international certification body since 1989, as a direct consequence of its historic commitment to safeguarding the safety of human life at sea, safeguarding property and protecting the marine environment, in the interest of the community, as set out in its Statute, and transferring its experience, acquired over more than a century, to other fields. As an international certification institute, it is committed to safeguarding human life, property and the environment, in the interests of the community, and applying its centuries of experience to other fields.
Eurasian Conformity mark
The Eurasian Conformity mark (EAC, Russian: Евразийское соответствие (ЕАС)) is a certification mark to indicate products that conform to all technical regulations of the Eurasian Customs Union. It means that the EAC-marked products meet all requirements of the corresponding technical regulations and have passed all conformity assessment procedures.
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Besa safety valves main fields of application
In the field with you
BESA has been manufacturing safety valves for many years, for a wide range of installations, and our experience provides the best possible guarantee. We carefully study each system during the quotation phase, as well as any special requirements or requests, until we find the optimal solution and the most appropriate valve for your installation.