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High operating to set pressure ratio, high back pressure or low total height and weight
|Pilot Operated Safety Valve
Supplementary Load System
Food & Pharma applications
Clean Service Range
Agressive / corrosive applications
Critical Service Range
Liquids, steam or gas applications, with low capacity in relation to valve size
Medium-sized capacity safety valves
Steam, gas or liquids application with high capacity
High capacity safety valves
Valve Type (by blow down action)
- Full lift safety relieve valve
- Normal lift safety relieve valve
- Proportional relieve valve
- Straight type
- Angle type
|Straight type||Angle type|
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|Threaded||Flanged / Full nozzle||Flanged||Clamp|
- Spring loaded
- Supplementary Loading System (SLS) Pneumatic Actuated Safety Valve
POSV - Pilot Operated Safety Valves
POSV’s are pilot operated by means of the process medium. Thanks to this special operating principal, these valves can function within very close ratio to the set pressure. This special feature provides a maximum of efficiency and hence increaes the productivity of your installation.
Advantages of the POSV
- POSV’s are especially useful in applications with tight shut-off requirements. Tight shut-off can be guaranteed up to 97% of the set pressure.
- Normal safety valves: When the system pressure is decreasing and approaching the set pressure valve, the closing force also decreases. With the decreasing of the closing force, the risk of leakage is now also increasing, especially when approaching the set pressure valve. POSV's work just the opposite: The closing force increases when the system pressure is decreasing. So on approaching the set pressure value, the closing force will also increase. This higher closing force will now minimise the risk of leakage!
- LESER POSV’s can also be deployed in appications where there is a substantial higher back pressure ratio (= ratio between back pressure and set pressure) of up to 70%. With traditional safety valves this ratio is only 10% (With a compensation bellows: 35% acc. to DIN and 50% acc. to API).
- With the limits of it's specified operation, the LESER POSV opens and operates independent of the back pressure. The set pressure is also not affected by whatever type of back pressure: constant, variable or superimposed.
- Long length op piping between the process and the POSV may lead to high loss of inlet pressure. For applications where the loss of inlet pressure can reach ≥ 3% of the process pressure, the POSV can be fitted with remote tubing, to measure the pressure in the main supply line.
- Standard available in WCB, CF8M and LCB.
Operating principle of the POSV
- During normal operation, the system pressure on the inlet side (seat) of the valve is transmitted through the pressure pickup (2), via tubing to the pilot valve (1), and via internal passages into the dome (3) inside of the main (piston) valve (4).
- The pressurized durface of the piston in the dome is larger than that of the opposite valve seat. Hence on the piston in the dome there effectively is a higher downward force, then the upward force on the seat of the main valve, resuting from the inlet pressure.The extra downward force on the main (piston) valve will keep it closed.
- When the pilot valve senses that the set pressure is reached, it becomes deactivated. The pilot valve now blocks the pressure transmission to the dome while venting the dome.
- When the pressure in the dome is removed, the closing force breaks down, and the main valve becomes forced open by the inlet pressure. Hence the system pressure is relieved to the outlet of the main valve and can reach a safe level again.
- If the system pressure is decreasing to below set closing pressure, the pilot valve is activated again, hence the dome will become pressurized, and the main valve will close again. Either rapidly and complete (Pop Action) or gradually following system pressure (Modulate Action).
LESER POSV’s are available in two versions:
- Pop Action (series 810)
- Modulate Action (series 820)
Features Pop Action (serie 810)
- Fully opened within 1% of over pressure.
- Especially suitable in applications where the full capacity must be blown down very quickly
- Blow down: adjustable between 2% and 15% of set pressure (API requirement: 3% - 7%)
- Exclusively suitable for gases
- Medium above the dome is exhausted to open air
Features Modulate Action (serie 820)
- Fully opened within 5%-10% of over pressure
- Opens proportional to the over pressure, resulting in venting loss of only minimum volume of medium, required to prevent pressure build up
- Blowdown: between 2 and 7%
- Suitable for steam and gases
- Also suitable for toxic gases, as in this version the medium in the dome is exhausted through the valve outlet
Features of both Pop Action and Modulate Action POSV's
- Designed according to API526
- Valve sizes: 1” x 2” until 8” x 10” (DN25 until DN200)
- Orifices: D up to T
- Pressure class: #150 up and until #600
- Materials: Availalble in WCB, CF8M and LCB as a standard
- Extended orifices (also known as Full Port or Full Bore Nozzle): These offer a maximum capacity to valve size ratio
- High alloy materials to handle agressive gases.
- Offshore: Compact dimensions, low weight and no leakage prevents fire hazard
- Gas compressors: well resistant against vibrations in compressor housings
- Downstream oil and gas industry: use with back pressures rising often above 50%, caused by long exhaust pipelines
- Paper industry: typically where high productivity requires the set pressure to be as near as possible to the maximum allowable working pressure
High pressure series
The LESER high pressure POSV allows the use of set pressure up to 256 bar (3705 psig).
All LESER high pressure POSV's also have these unique features:
- extended tightness requirements
- extremely low temperatures
- special sealings to prevent Explosive Decompression (ED-design)
Unique features of LESER POSV’s
- Tubing between the pilot valve and the main valve is integrated into the cover of the main valve bosy, with benefits like resistance against system vibrations and prevention against accidental rupture of (external) tubing.
- Integrated backflow prevention comes as a standard; work under vacuum or under a back pressure above working pressure; no process media return will flow downstream the system
- The pilot valve is fully stainless steel and suitable for NACE applications
- Integrated mounting/lifting bracket can facilitate easy mounting and can absorb system reaction forces (also during blow down)
- Version with full bore available; smaller flange size possible and higher capacity against 8T10
- Compact design: smaller mounting spaces possible
LESER POSV’s can be applied worldwide, under the following approvals:
- European Community: CE-mark conform Pressure Equipment Directive 97/23/EC and EN ISO 4126-4
- United States: stamp according to ASME section VIII Divisie 1, National Board certified capacities
In relation to the design, production and marking of the LESER indirect safety valves, the following guidelines are also met:
- EN ISO 4126-7
- EN 1092 part I and II
- ASME PTC 25
- ASME Code Sec.II
- ASME B16.34
- ASME B16.5
- API Std.527
- API RP 576
Supplementary Loading System (SLS)
The Supplementary Loading System facilitates operation on a system pressure close to the safety pressure; this is achieved by means of pneumatic actuated valve control.
Features of the SLS system
With an SLS you can work well protected with higher system pressures; the working pressure may stay closer to the system design pressure and you will still be protected. The practical advantages of the SLS are in general:
- Remains tight under design pressures very close to the set pressure
- Shorter opening and closing reponse times, increased productivity
- The safety valves opens less frequently, resulting in increased lifetime.
The SLS systeem can be dployed in many applications. Commonly used application examples are:
- Highest possible ratio between set pressure and design pressure (e.g. as in industrial steam boilers)
- Yankee cilinders in paper industry
- Safeguarding of boiler and super heater in steam installations (as used in chemical industry)
- Compensation for frequently occuring pressure spikes (as seen in waste processing industries)
- Increase of productivity in older installations, where the maximum allowable working pressure is approached
Advantages of SLS over conventional safety valves
- In applications where conventional safety valves require an overpressure of 5% to 10% to reach their full certified blow down capacity, the safety valve with SLS requires only 1% of overpressure to reach it's full blow down capacity!
- Likewise is the closing reaction to a pressure decrease of a conventional safety valve:
- A conventional safety valve will close after a minimum of 7% decrease of a system pressure, below the set pressure
- A safety valve actuated with an SLS however, will fully close after a pressure decrease of only 3%
- On top of this:
- A conventional safety valve is offering tight shut-off, up to 90% of the set pressure (according to API-527)
- A safety valve actuated with an SLS however, will tight shut-off until the set pressure is actually 100% reached!
- The SLS control unit can control several pneumatic actuated safety valves.
Primary objective: Protection against over pressure hazard
Installations pressurized with gases or liquids, need to be protected against system pressures that might rise above the maximum allowable working pressure of the particular pipe line system or pressure vessel.
- Safety relief valves are facilities within pressurized installations, that prevent such unwanted build up of system pressures, rising above the design pressure of the installation.
The importance of overpressure protection of installations is emphasized by legislation on the subject. Beside statutory design regulations in general, the site technician is presented with (international) regulations with regards to the deployment of installations. If no safety valves would be installed, potentially life threatening situations may exist, like explosion hazard and the release of toxic or corrosive chemicals. At least severe damage may be caused to the installation (including the instruments and equipment fitted to it).
|Valve type||Statutory regulation|
|Safety relief valve||PED 97/23/EG, category IV|
|Relief valve||PED 97/23/EG, category pressure/size dependent, of good workmanship (SEP)|
Selection of the valve type: Safety relief valve or a relief valve?
Choosing the right valve type depends on the desired application/solution to a problem.
Safety relief valve
- Designed to protect installations against a single calamity
- Will not be activated under normal working conditions
- Designed to function by continiously regulating the pressure within an installation, a vessel or similar (modulate action)
Grouping of safety relief valves by application and capacity
The intended application requires specific valve opening and closing actions, in response to the occuring system pressure behaviour and the capacity of media to be relieved in cae of a calamity. Below are the blowdown valve characteristics with their common use in application and capacities.
|Normal / low lift||Liquids||Medium|
|High lift||Gases, vapours, steam||High|
Modulate action relief valves
These valve types will open proportional to the increase of system pressure. They are installed where only a limited relieving capacity is required and the loss of media needs to be limited as much as possible (e.g. thermal expansion).
As opposed to the other relief valve types, modulating relief valves open and close frequently in normal operation.
Normal lift safety relief valves
These valves types are ideally suited to relieve medium volume capacities. Especially when used on liquids, the large proportional blowdown range facilitates quiet operation while relieving pressure surges. This type is used when the capacity of a modulating valve is too small, whereas the smallest full list relief valve is still too large.
Full list safety relief valves
These types of valves are designed to open very quickly (pop action) in response to a 5% increase in system pressure. These valves are mostly used to protect installations containing gases and vapours, especially where a maximum volume needs to be relieved as quickly as possible.
Blowdown characteristics of safety valves
The graphics below show the characteristics of the main types of relief valves.
The right choice of pressure/capacity characteritics is of great importance, when selecting the right relief valve for each application. Always consult an Econosto specialist!
The changes in the system pressure, at or during opening of the safety valve, for the main types of relief valves characteristics.
Remark: The above information is based upon the industrial standards AD 2000-A2 (issue 11/93), TRD (issue 9/90) and DIN 3320 (issue 9/84)
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Important data for determining the right valve size/bore
Medium and medium-related characteristics
- Medium phase (gas, vapour and liquid)
- Temperature (°C)
- Density (kg/m3)
- Viscosity (cSt)
- Capacity (kg/hour or m3/hour)
- Set pressure (bar)
- Back pressure (bar) - when applicable
Detailled tables to determine the valve capacity
In the valve selector selection, under the tab "Tables", you'll find pressure/capacity tables the valve figure number. These tables enable you to select the right valve dimensions to match the blowdown capacity needed. Econosto specialists will be glad to assist, making the best valve selection for you application.
Econosto has based her testing procedures on the API-527 standard. This standard for example describes how to test functional valve tightness.
Explanation to TÜV type approval
Almost every safety valve has TÜV type approval; please consult the related TÜV datasheets. The capitals D/G/F within the type approval designate respectively; Vapour (D), Gasses (G) and Liquids (F).
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- Applied to compensate the negative effects of back pressure (for stainless steel bellows only) and to protect moving parts against dirt and corrosion.
- Also available combined with additional stainless steel full nozzle.
- Benefit stainless steel full nozzle: Cost efficient against using a complete stainless steel safety valve.
Stainless steel bellows
- Material: SS 1.4571 + 1.4404 (Nozzle)
- Maximum temperate range: 450oC
- Minimum set pressure: 3 bar
- Back pressure: Max. 35% of set pressure (DIN)
- Back pressure: Max. 50% of set pressure (API-526)
- Other bellows materials on request
- Material: EPDM
- Temperature range: -50oC until 130oC
- Back pressure: Max. 3 bar.
Bonnet and cap: Open or Closed (gastight)
- The closed bonnet combined with the gas-tight cap (plus additional lifting lever), can prevent the process medium to make contact with the environment.
- The open bonnet protects the spring from high temperatures and prevents formation of condensate and accumulation, for example in the protection of steam boilers.
Valve position indicator
- Integrated proximity switch: To remotely indicate the valve position (open/closed)
- Test gag to be placed to perform a system pressure test, otherwise the valve has to be removed and a blind flange has to be installed before testing.
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The vibration damper option can be applied to absorb vibration effects in extreme conditions.
Benefit: Stable blowdown of the installation.
Chane-over valves are used in the event that shut-down of the installation is not feasable or undesired, either due to production or economic reasons. Using change-over valves makes it possible to switch between parallel safety valves and perform your maintenance without interrupting the process.
Cast variant DN25 - DN50
- Low loss of pressure (3%-criterium)
- Full bore available during switch-over
- Stable blowdown
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Pressure type definitions of process media.
Good notice should be taken of the different pressure definitons when selecting and installing safety valves.
Barg = Overpressure (gauge pressure)
Bara = Absolute pressure
Blowdown pressure (relieving pressure)
The blowdown pressure is the pressure at which the safety valves will reach the maximum blowdown capacity. The actual value of the blowdown pressure is defined by the opening characteristic of the specific safety valve. The statutory design regulations indicate as follows:
- For full lift safety valves for gases and vapours, the blowdown pressure must be reached at a system pressue that s 5% above the set pressure
- For normal lift and modulating safety valves for gasses and vapours, the blowdown pressure must be reached at a system pressure that is 10% above the set pressure.
The set pressure is the pressure at which the safety valve starts opening.
- For gases and vapours, this is the pressure at which gas or vapour is audibly released on the valve outlet side.
- For liquids, this is the pressure at which the first continuous flow of liquid droplets can be detected at the valve outlet side.
The set pressure adjustment is done with atmospheric pressure at the valve outlet side.
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This is the maximum allowable pressure that may be applied to a valve, instrument or installation, as defined by manufacturer and/or authorities.
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The reseating pressure is the pressure at which the safety valve is fully closed, after blow down and depressurizing of the installation. The actual value of the reseating pressure is defined by the opening characteristic of the specific safety valve. The statutory design regulations indicate as follows:
- For full lift safety valves for gases and vapours, the reseating pressure must be reached at a system pressure that is 10% below the set pressure
- For normal lift and modulating safety valves for gases and vapours, the reseating presssure must be reached at a system ressure that is 20% below the set pressure.
Visualisation of pressure types
This is the actual pressure that is present in the system or installation.
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The pressure that can be measured at the outlet side of the valve is called back pressure. Back pressure can be constant or variable. It can be caused by:
- Build up of pressure during blowdown of the safety valve
- Already existing system pressure at the outlet side of the valve (superimposed back pressure)
If the back pressure increases toa bove 10% of the set pressure, this could have an adverse effect on the functioning of the safety valve (as mentioned in the API norms). A good solution to overcome a back pressure situation can be the use of an optional stainless steel compensating bellows. Please consult our product specialists for advice.
The working pressure is the maximum system pressure at normal operation.
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Discharge capacity tables for safety valvesEach safety valve has it's own discharge capacity table for use within different process media. These tables are avaiable for download below or from the "Extra Information" tab on the product page.
|Figure numbers||Link to the capacity table|
|Figure 512, 517, 518||Download this capacity table|
|Figure 527||Download this capacity table|
|Figure 546, 548, 1542, 1547||Download this capacity table|
|Figure 562||Download this capacity table|
|Figure 564, 566||Download this capacity table|
|Figure 565, 567, 1550, 1551||Download this capacity table|
|Figure 597, 599||Download this capacity table|
|Figure 1525, 1526, 1527, 1528, 1529||Download this capacity table|
|Figure 11511, 11512, 11521, 11522||Download this capacity table|
Steam TableFurther to the discharge capacity tables per figure number, the generic table for saturated steam pressure applies, which you can find below:
|Saturated steam table|
|Absolute pressure||Temperature saturated steam||Specific volume||Enthalpie (kJ/kg)|
|Bar||MPa||oC||(m3/kg)||Specific enthalpy water Hf (kJ/kg)||Specific enthalpy evaporation Hfg (kJ/kg)||Specific enthalpy steam Hg (kJ/kg)|