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Why
do we need a sample probe to extract sample instead of a sample
tap from process lines? |
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Answer
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As the process line has the higher fluid velocity at the center, by installing a sample
probe to at least one third of the pipe diameter will have a turbulent mixing. Such
way of sampling will likely to give a more representative process data and rapid
responses to the process changes to the analyzer. With a sample probe, it also
eliminates the possibility of suspended solids from the sample, which tends to
travel from the pipe walls.
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What
is Pressurization? What are the types of pressurization available
to protect the electrical equipment in hazardous locations? |
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Answer
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Pressurization is a process of supplying an enclosure with a protective gas with or without
continuous flow at sufficient pressure to prevent the entrance of a flammable gas or vapor, a
combustable dust or an ignitable fiber. Basically, there are three types of pressurization as below:
(i) Type Z Pressurization:
To maintain a positive pressure within an individual protected enclosure and shall be detected by an
alarm or indicator, but not necessary to de-energized the protected equipment.
(ii) Type Y Pressurization:
Basic Requirement as type Z pressurization must be satisfied except that the protected enclosure shall be
approved for Division 2 locations.
(iii) Type X Pressurization:
Basic requirement as type Z pressurization must be satisfied and in addition, a cut off switch shall be
incorporated to de-energized power automatically from all circuits within the protected enclosure not
approved in Division 1 upon failure of the protective gas supply. The cutoff switch shall supply an alarm
signal output from the protected enclosure.
(Information extracted from NFPA496 "Standard for Purged and Pressurized Enclosure for Electrical Equipment)
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When will be the best time to put the analyzer on-line during commissioning? |
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Answer
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Normally prior to commissioning phase, a pre-commissioning activity is carry out to demonstrate that all
analyzers are calibrated and providing reliable data with the certified test standards. During the plant initial
start-up, the process sample should not be introduced immediately into Sample Conditioning System since it
may not cope with unstable plant conditions. When the plant condition is reasonably stable, before putting
the sample into the Sample Conditioning System. it is good practice to commission the fast-loop system first
to flush the sample back to the return lines. Then commissioned the system to prevent the possibility of
analyzer failure and downtime.
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Why most of the times the
analyzer and the laboratory analysis result are different? |
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Answer
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The Plant Laboratory has always been the Quality Control benchmark point and it is difficult to convince people to change their mindset. It is unfortunate that operations team is likely to believe the laboratory results even though the on-line analyzer may be correct. In such circumstances, the following uncertainty are to be evaluated:
(i) location of the laboratory sample point should be taken where the representative of the process sample being monitor by the analyzer.
(ii) personnel taking the laboratory sample must follow the correct procedure such as ASTM method
(iii) adequate flushing of sample bomb or container to rid of residue before taking new sample
(iv) the injection of the sample to the laboratory analyzer is consistent and should not effect the operation of the analyzer.
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How often must the analyzers
be calibrated to give a reliable reading? |
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Answer
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As analyzer calibration is one of the criteria for maintaining a high reliability and confidence to the operation personnel in operating a process plant; a calibration schedule is usually established after plant start-up. For a initial start-up, analyzer may be calibrated with test standards
at least twice a month and drop to once a month when operation is confident on the reliability of that analyzer.
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Why the pressurization of
the Analyzer shelter from the HVAC system is not achievable? |
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Answer
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Usually the HVAC pressurization fans are sized to achieved the required shelter pressure as specified. A common problem is due to heavy leaks of pressurization through the analyzer doors and panels. This leak is normally significant for small shelters and needs rectification
immediately before pressurization is achievable. The second common problem lies in the over-sizing of the shelter louver, which can be eliminated by adding
weights to limit the air flowing out of the shelter. Precaution must be taken to ensure that the required Air Changes inside the shelter is maintained.
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