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Safety valves are the ultimate protection device for tanks and pipelines. They prevent pressure
limits being exceeded once all automatic control and monitoring equipment has failed.
Normal safety valves initially open
proportionally up to a pressure rise of 10%. This initial phase is followed by the full opening of the
valve, allowing a large flow of medium.
Especially in the case of liquids, the wide proportional range results in a continuous operation.
Pressure rise:
above 1 bar set pressure: 10% up to required valve lift;
below 1 bar set pressure: 0.1 bar.
Proportional safety valves open almost continuously as the pressure rises.
They produce the valve lift required for draining the volume within a maximum pressure rise of 10%.
They are used where only small volumes are expected to be drained (e.g. thermal expansion) and where
the loss of medium is to be minimised.
Pressure rise:
above 1 bar set pressure: 10% up to required valve lift;
below 1 bar set pressure: 0.1 bar.
Full lift safety valvesopen instantly up to maximum lift within a pressure rise of 5%.
Because of their instant opening they are used where suddenly larger flow volumes or fast pressure
rises may occur. They are mainly used for relieving pressure in vapours and gases.
Pressure rise:
above 1 bar set pressure: 5% up to the max. valve lift;
below 1 bar set pressure: 0.1 bar.
Closing pressure difference
| compressible media: |
10% |
| below 3 bar: |
0,3 bar |
| non-compressible media: |
20% |
| below 3 bar: |
0,6 bar |
System operating pressure
The plant or system operating pressure should be at least 5% below the safety valve closing pressure
to ensure that the safety valve closes correctly.
Variable back pressure
During blow-off the variable back pressure created by blowing-off must not exceed 15% of the valve
operating pressure. If the variable back pressure is more than 15% of the valve operating pressure,
the valve capacity must be checked. For higher pressures safety valves fitted with pressure-compensating
metal bellows should be used.
External back pressure
A constant external back pressure (caused by the system) can be compensated by selecting a suitable spring.
In this case the above statements are not applicable.
Installation
Safety valves must always be installed with the spring cover at the top.
Safety valve inlet
The inlet spigots for safety valves should be as short as possible and must not produce a pressure drop
exceeding 3% of the valve operating pressure. If the pressure drop is greater, the inlet pipeline
diameter should be increased. The edge of the inlet spigot should be chamfered or rounded.
Blow-off pipeline
For vapours or gases the blow-off pipeline should rise, whilst for liquids it should be installed with
a fall. Please make sure that a blowing safety valve does not create a hazard, especially when a safety
valve with open spring cover is installed.
Draining of condensate
To keep away dirt and foreign bodies from the safety valve, the blow-off line must have a condensate
drain installed at its lowest point. In addition a drain orifice may be provided in the valve itself
(this may be necessary for special operating environments such as ships). Before commissioning the
valve the plastic plug inserted by the manufacturer must be replaced by a screwed plug.
Drain lines must have no constriction; they must have a fall away from the valve and an outlet which
is open to view; the draining of the medium must not create a hazard. If the medium is steam the above
requirement can be met by installing steam traps.
Leakage
Safety valves fitted with bellows feature a relief orifice in the spring cover. If medium leaks from
this orifice, the bellows is faulty. In the case of toxic or hazardous media you must make sure that
these are drained safely.
Insulation
If insulation is used it must not be applied to the spring cover or cooling area (if provided).
Maintenance
Safety valves must be cleaned and serviced at regular intervals. The service intervals depend on
the ambient atmosphere(corrosive, dirty) and mode of operation(occasional, continual).
Function check
From time to time the valve should be checked for correct functioning either by manual venting or
blowing off. Steam generators are subject to the regulations given in Section 6 of TRD 601 Sheet 1,
Issue 6.83.
A slight leakage can be cured by blowing-off. If this does not stop the leak the sealing surfaces
are probably damaged.
The valve should be closed by a sudden release of the venting lever. After testing the lever must
no longer be engaged with the coupling. To disengage the lever it should be pushed towards the centre
of the spring cover until the venting fork is free.
Valves free of oil and grease or silicone.
Please pay attention to order an fit only spares free of oil and grease resp. free of silicone.
Guidelines for selection, installation and operation, "Notes on Safety, Operating Instruction etc." MUST be followed.
Please consult our engineer if extreme operating conditions apply or whenever you are in doubt.
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capacity diagrams:
proportional
normal
full lift
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