Electromagnetically transparent GRP enclosures see spike in demand
Until recently, process industry take-up of WSN technology has been relatively slow, possibly because competing wireless standards have made systems designers wary of making the wrong choice. However, for mainstream industrial process applications, the field has now narrowed to just two standards – WirelessHART and ISA100.11a. Both standards define secure, self-healing wireless mesh technology operating in the 2.4 GHz ISM band, using IEEE 802.15.4 standard radios.
Intertec has long been aware that one of the many benefits of its GRP composite material is electromagnetic transparency, which produces a wide range of frangible environmental protection cabinets and shelters for radar and instrument landing systems at airports which are designed not to reflect or interfere with radio signals. The housings are also used extensively for cellular and satellite communications applications, as well as satellite navigation systems; the GRP material causes no measurable loss of received or transmitted RF signals, so the equipment antennas can be contained within the enclosure. However, until now, Intertec had not actively promoted the fact that the non-metallic construction of its products is also ideally suited to field-based 2.4 GHz wireless sensor networks.
Full article here
We would like to avoid the cost of separate valves by using existing modulating valves for safety shutdown. Is this an acceptable practice under any circumstances? If so, where and how can it be done?
--From January 2003 CONTROL
Just Dont Do It
There is not an acceptable practice under any circumstances. Each circumstance has to be reviewed based on the safety integrity level required. Please refer to ISA Standards and Technical Reports, ANSI/ISA-84.01, TR84.00.02 parts 1 to 5, TR84.00.03. Also refer to past articles published in CONTROL magazine such as "The Complete Safety System," December 2000.
(From Safety Systems to Operational Integrity)
...ISA S84.01 paragraph 126.96.36.199: "A control valve from the BPCS [basic process control system] shall not be used as the only final element for SIL-3. A safety review shall be required to use a single BPCS control valve as the only final element for SIL-1 and SIL-2. For additional information see B.1.6."
...Even if you meet the ISA S84.01 requirements, you need to validate that your complete design meets the particular Safety Function SIL requirement. This is dependent on the specific equipment failure data, the levels of redundancy used, and test frequencies. The validation calculation is done for all of the safety function components as a group. Individual component SIL ratings do not ensure system compliance.
more about is on http://www.controlglobal.com/articles/2003/446/
IEC61508 / IEC61511
The authors of the IEC standards re-examined the basic requirements that need to be satisfied to achieve safety integrity1 and risk reduction and defined four main measurement criteria that systems must achieve in order that the Safety Integrity Level (SIL) is considered compliant with the levels defined in the standards and now expected by the industry in general. These are · Hardware safety integrity which refers to the ability of the hardware to minimise effects of dangerous hardware random failures, and is expressed as a PFD (probability of failure to danger) value.· Behavior of the system following the detection of a fault condition. Safety-related systems need to be capable of taking fail-safe action, which is a system’s ability to react in a safe and predetermined way (e.g.shutdown) under any and all failure modes. This is usually expressed as the Safe Failure Fraction (SFF) and is determined from an analysis of the diagnostic cover the design can achieve (see below). · The new important parameter introduced is Safe Failure Fraction (SFF) which is a measure of the cover and effectiveness of the diagnostics in the system. In order to accommodate earlier system designs based on high levels of redundancy and lower levels of diagnostic cover, the standard considers the complete
system architecture in the assessment of the SIL achieved. Maximum SIL rating is related to Safe Failure
Fraction (SFF) and Hardware Fault Tolerance (HFT), according to Table 1 shown below.
· Systematic safety integrity refers to failures that may arise due to the system development process, safety
instrumented function design and implementation, including all aspect of its operational and maintenance
lifecycle safety management.
Understanding Ground vs Neutral
The grounding of electrical equipment is probably one of the least understood aspects of electricity. As the characteristics of electrical equipment changes from linear to non-linear, the nature of grounding expands from the task of insuring the safety of personnel to insuring that one type of electrical equipment does not interfere with other types of electrical equipment. One point for confusion rests with the often interchanged terms of Neutral and Ground. Many articles have been written concerning the problems with 3rd harmonics overloading the neutral conductor. Many articles have been written concerning the problem of electrical ground noise. Even with all these articles, there still exists confusion concerning whether equipment should be connected to the neutral or connected to a ground.
It may be possible that a simple rule would clarify the differences between Neutral and Ground.
It can be stated that Neutral can be grounded, but Ground is not neutral.
A Neutral represents a reference point within an electrical distribution system. Conductors connected to this reference point (Neutral) should, normally, be non current carrying conductors, sized to handle momentary faults (short circuits) occurring in electrical equipment. However, with the introduction of non linear loads, such as computers, electronic lighting, TVs, VCRs and other switchmode power conversion equipment, the requirements for the neutral conductor has changed (increased).
A Ground represents an electrical path, normally designed to carry fault current when a insulation breakdown occurs within electrical equipment. (Note: Breakdowns can be forced by connecting (dropping) a metal tool or conductive material from a voltage potential to the steel structure within a facility.) Connections to the electrical path (Ground) are made convenient for the installation of electrical equipment. Some current will always flow through the ground path. This current will come from a number of normal sources. Capacitive coupling and Inductive coupling between power conductors and the ground path (conductive conduit, conductive structure members, etc) are the greatest sources of ground path current.
Among the many types of distribution systems, the 3 phase, 4-wire, 480/277 V system used in commercial centers and large buildings is very common. It is used since it enables 3 phase ac motors to operate at the 480 V level while 120 V fluorescent lighting operates with the primary of a stepdown lighting transformer connected to the 277 V (line to neutral) potential. The secondary (120 V side) of the lighting transformer has one of its terminals connected to ground. This grounding procedure is done to reduce the possibilities of shocks due to an internal fault in the transformer. The grounded terminal of the 120 V lighting supply is often referred to as the lighting neutral. This IS NOT the true Neutral of the distribution system. Although this point within the lighting system is grounded, it is unlikely that a short in any lighting equipment on that branch will ever see current returned to the true neutral within the distribution system.
In a 3 phase low voltage distribution system, the preferred installation should consist of a five wire system. That 5 wire system would consist of, 3 phase conductors, a neutral conductor and a separate ground conductor. In normal practice, the ground conductor is often the building ground consisting of the metallic building structure. Although this type of ground is usually suitable for 60 cycle leakage and fault currents, it is not suitable for leakage currents that exist when non linear loads such as computers, electronic lighting, variable speed drives and other equipment using internal switch mode power supplies and other types of conversion rectifiers are used. The current caused by non linear electrical equipment consisting of low amperage high frequency currents. These currents are often measured incorrectly by 60 Hertz sensors and mistakenly interpreted as higher amperage 60 Hertz values. This occurs as a higher voltage develops across the sensor whose impedance increases as the frequency increases.
The previously mentioned equipment creates rapid changes in voltage and current while transferring energy from the distribution system to the equipment load. These changes cause currents to flow through capacitive paths that exist between phase conductors and between any phase conductor and ground conductors. These currents have high frequency characteristics which results in a phenomena not unlike the results from a small radio transmitter. Very little power is required to create magnetic field which can transmit a Radio Frequency Interference types of electrical noise to other equipment. Usually other equipment, in metallic enclosures is not affected by these small radio-type signals, however, some equipment circuitry may be affected. The typical solution is to add RFI filters in the incoming power lines to the equipment causing the condition. These RFI filters, like other electrical equipment, require grounding. The normal grounding practice is to connect the RFI filters to same ground point used by the equipment causing the condition. This practice is suitable when that ground point has a high frequency low impedance path to the equipment creating the condition. It would not be necessary to install a separate conductor back to the Neutral reference point in the electrical system as long as a high frequency, low impedance path exists from the grounding point to the equipment ground.
A note on SIL Standards
Recently there was news about Ex CEO of Union Carbide Warren Anderson. The news had me thinking about the safety issues that the plant in Bhopal faced and negligence which caused the disaster. Also there was this incident abroad in Piper Alpha offshore along the North Sea which is considered one of the worst in terms of casualties and loss of industrial value.
These incidents were of the high intensity and recovery seemed out of hand only because not only that the design of these plants were bad, but the design did not take into consideration of the industry behavior in the long run, when the plant is in full production.
So the question raise that in a plant which is operational and running continuously 24 X 7 for years around what is the guarantee that the safety systems and fire and gas systems will function when need arises?
So the Industries need was given a solution with safety standards which are even now followed and have proven beneficial to all the plants.
Some of the safety standards are found to be mentioned in wiki.
The accepted safety standards such as ISA-S84.01 and IEC 61508/61511 are used to measure the acceptable level of performace of the safety systems.
A Safety Instrumented Loop is essentially a measure of the system performance in terms of Probability of Failure on Demand (PFD).Also we need to understand that Risk is Probability or the number of times the hazard might occur multiplied by its consequence.
SIL is defined in 4 categories:
SIL-1 which has risk reduction factor of 100 to 1 and PFD of 10^-1 to 10^-2
and the highest SIL rating being SIL-4 which has risk reduction factor of 10000 to 100000 and PFD of 10^-4 to 10^-5. These details can be found here .
How a classification of SIL for the instrument is given is based on analysis and calculation which also required the in-depth of process knowledge. An example is found here.
To conclude, I have taken excerpts from a website for the basics of SIL as stated:
the basic concepts of SIL are:
SIL is an indication of system reliability.
The end user (often through the analysis of a HAZOP team) determines the desired SIL level for a Safety Instrumentated Systems and Safety Instrumented Functions.
Based on a product's reliability (in essence, the reciprocal of PFDAVG), products may by suitable for use in a desired SIL environment.
Using a product marketed, for example, as SIL 3 does not necessarily mean it is suitable for use in a specific SIL 3 environment.