Clean Compressed Air Brochure - Norgren - #16

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PROTECTING SYSTEMS,
operate when the regulated pressure is exceeded and so need to be set higher than the regulator. There will be a tolerance on the relief valve setting and on the regulators outlet setting, depending on its flow andregulation characteristics. A commonproblem is a relief setting too close to the system operating pressure. The conse- quence of this is to have the relief valve operating and venting air during normal system operation, which is an expensive waste of air.Once the relief valve setting pressureand acceptable level of over pressure are checked the flow capacity of the relief deviceand that of the system can be considered. The relief device must be able to match or exceed the amount of flow through the part of the system being protected without the system pressure rising above the acceptable over pressure level.Several methods can be used toachieve this:-The relief device has a flow capacityin excess of the compressors free air delivery capacity - in systems where no receiver exists - i.e. flow out of system is greater than flow in.The relief device has a capacity inexcess of the flow through the smallest flow passageway upstream of the equipment being protected. Tables of orifice flow exist to determine the flow at different pressuresthrough differing sizes of orifice. Thesmallest bore is acting as a restriction to the flow into the downstream system and unless the upstream pressure can be increased the flow will be choked through this area and therefore limited. This is important since a mains distribution system can be of very large volume with pipes of large bore and compressors of high capacity, but the device being protected could be fed by 1/8" nominalbore tubing. So a small low cost device onlyis required and not one large enough to cope with the full system capacity.In areas where no such flow restric-tion exists, one should be created in order to reduce the cost of the relief valve to be employed, ensuring of course that the restriction does not cause excessive pressure drop in the course of normal operation.Legislation Reference:BSEN9835.1.2 TYPES OF RELIEF VALVES PERSONNEL AND THE ENVIRONMENT
Several types of relief valves exist to achievedifferent levels of performance with respect to the flow capacity and over pressure limita- tions. The most common is the ‘pop’ type,followed by the diaphragm type. For betterperformance use pilot operated valves withthe integral pilot operated type being the most compact and cost effective (figure 32).An“in-line” type of relief device hasrelief port at 90°to the direction of flow and in normal operation flow passes through the body of the device, without interfering with normal upstream operation. A common useof this type of device is with machinebuilders, where all the control equipment/protection devices are in one discrete position, aiding both installation and scheduled servicing.The in-line device differs from thepop or diaphragm type of relief valves which are connected into the system on a tee-piece. Flow through these devices only occurswhen in operation and air vents to atmos-phere.In both cases the exhaust flow canbe piped away to an area where the noise and flow will not cause disruption or harm to the environment or the operators. Exhaust silencers may be required to reduce noise levels in high flow exhaust applications where piping away to less sensitive areas is not possible. Safety in the workplace is essential and isemphasised via the Machinery Directive, the Pressure Systems legislation and the Provision and Use of Work Equipment Regulations (PUWER).The following section can helpmachine designers and others using pneu- matics by illustrating those air line products which, when correctly applied, can be used to ensure safe pneumatic systems.In it we have cross referencedrelevant documents. Norgren strongly recommend that all who are involved withmachine and system design should becomefamiliar with these and other relevant safety documents. OVERPRESSURE PROTECTION
The components in pneumatic systems will often have a pressure rating lower than that generated at the compressor and pressure regulators are used to reduce this pressure to safe efficient levels. In the event of a fault the components can be exposed to excesspressures leading to mis-function or inextremes failure of the pressure containing envelope.To protect against this excessiveover pressure situation several solutions can be employed the most common being a relief valve. Selecting a relief valve is not a simple process, and detailed consideration of the system or element of the system is required.In general all pneumatic componentsand equipment will have a Safe WorkingPressure (SWP) and over pressure limit of10%. The designer of the pneumatic system can use regulators to run the system at pressures below the SWP and use the 10% safety factor to be the limit of over pressure that the system can experience with the relief valve in operation.A relief valve is defined as a devicewith its outlet so connected to a pressuresystem to enable the system pressure to beheld at a constant level. This constant level would then be at or below the stated SWP + 10% over pressure allowance.Relief valves need to be set to only 16