【Weekly Question 20090825】Project startup and commissioning
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This post was last edited by baodingzhai on 2009-8-27 at 11:25. Many of the friends in Shanghai Chuan are in the engineering field; let’s share any good experiences regarding instrument calibration before starting a project Most of the record forms provided by installation companies are misleading; what issues should be taken into account during the actual calibration process? Which instruments require individual calibration? It can cover various aspects such as the grounding of the instrument cables, etc. For example, when calibrating control valves, it is necessary to ensure that the 4MA valve is fully closed, the 12MA valve is at half position, and the 20MA valve is fully open. Additionally, attention must be paid to whether the range is appropriate; at least 5 points need to be calibrated. In some control systems, 0% corresponds to a current of a few milliamps, while 100% corresponds to a current of slightly over twenty-one milliamps. If an intelligent locator is used on site and it requires a current level higher than that at 0%, then the system’s default current setting must be changed. Just trying to stimulate discussion – everyone, please participate actively! Note: For multiple accounts sharing the same IP, only the first one to respond counts as a valid participant!
In our company, there’s no need for such precise verification; it’s sufficient to adjust it to full open or full closed position, with an upward or downward error of around 20-30 percent
The calibration for ours doesn’t need to be that precise either
During calibration, attention should be paid to the size of the backlash; it is generally necessary to follow the standards. It is also important to note whether the scale indicates the full range of movement or the percentage of opening.
Strictly speaking, all newly installed instruments need to undergo individual calibration, but I place more emphasis on system calibration, as it is the most effective way to identify problems – it checks both the systematic errors of the instruments and the operation of the circuits. Five-point calibration is also carried out during system calibration. Furthermore, when calibrating control valves, it is not sufficient to carry out a five-point calibration alone; attention must also be paid to checking sensitivity. In other words, it is possible for the valve to activate at 25%, but fail to respond between that level and 0%, meaning there is an excessive dead zone, which is not acceptable.
As a project, before the entire project begins, all instruments should be individually calibrated, and relevant records should be kept. Many installation companies omit this step, but during the bidding process, the commissioning cost is included. At the end of the project installation, all instruments must be jointly tested. For this task, the owner side must have professionals involved to ensure that the installation company does not engage in any fraud. However, reputable installation companies have highly skilled instrument tuning technicians who arrive at the construction site towards the end of the project to get ready for the tuning work. However, many installation companies do not have professionals in this field. Therefore, for the installation of instrumentation systems, it is best to engage professional installation companies for the task. There are many automation installation companies today that specialize in the installation of electrical and instrumentation systems. They handle everything, from single-unit testing to joint debugging, including installation; they do it professionally. Even if the client doesn’t have professional instrumentation engineers, that’s not a problem.
Let me share some insights as a client: In fact, the debugging prior to instrument installation is merely a procedural step. For many of the measurement and control instruments available on the market, the installation companies do not have the capability to carry out debugging; it is understood that aside from pressure measurement instruments and control valves, most installation companies simply lack the ability to perform such debugging (they can only conduct calibrations). As for the debugging of pressure measurement instruments and control valves, relevant standards are followed, and the record-keeping format is also based on those standards. Whether the installation company actually carries out the proper work or fabricates data to deceive the client depends on the client’s methods of supervision. We used a random inspection approach – that is, we randomly selected some of the already debugged instruments from time to time and tested them ourselves to see the difference between the values obtained and those recorded by the installation company. If the difference was significant, it was clear that the other data was also inaccurate (in other words, the installation company was fabricating data to mislead the client). As for the rest, one can only try to urge them to carry out the proper work; if they are indeed unable to do so, there’s no point in forcing them, and the cost can simply be deducted during the final settlement.
I think the points to note are: 1) During the five-point calibration of the control valve, apply a negative value (-5%) at zero to check whether the valve operates properly. Starting from zero, add another 3% or 5%. In my opinion, the valve opening is rarely above 90%; a lower opening level allows for testing the sensitivity of the valve. Before this, the valve must undergo pressure testing and leakage testing. Make sure the gas is turned on and off properly. Measure the current when the valve opening is at zero (this also applies when the shutoff valve is in the zero position)