【Weekly Question 20090727】Problems in instrument installation

Support our moderators. I once worked on a project where the control cabinets and console bases were ready for installation. Both the designers and the person in charge of installation instructed that they be welded to the steel embedments after ensuring they were level. However, I discovered that this system had its own separate grounding system for the DCS, separate from the electrical system; welding them in that way actually meant connecting them to the electrical grounding network. I immediately advised the person in charge, which drew their attention to the issue, and as a result, the plan was changed – the embedments were removed and the concrete floor was raised.

1. It is best to choose a thermometer sleeve made of the same material as the pipe, to facilitate welding. 2. According to the books, rotameters should only be installed vertically, but I have seen horizontal ones in practice. 3. The time period before and after the installation of the flow meter is not necessarily fixed; it is also required to be 20D before and 10D after. It depends on what type of flow meter it is, and whether there are valves, elbows, expansions, or contractions before and after the pipe it is installed in. 4. For ultrasonic level gauges, attention should be paid to the range of the radiation angle, and sufficient space must be provided at the installation location. 5. When welding control valves, it is essential to prevent weld slag from getting inside the valve, as its presence can impair the valve’s performance, especially if it lands on the valve core.

I have encountered situations where, during the installation of new systems, the design assumed that it was merely a simple flow measurement task; factors such as the medium being measured and the ambient temperature were not taken into account, and isolation tanks and heat tracing were not included in the design. In the end, design changes were made and additional materials were added before it could be fixed.

During an instrument installation, while determining the location for a pressure measurement point at the top of the tower, the design indicated that it should be placed on the extraction line at the top of the tower. Given that the tower is 54 meters high and the design features many straight stairs, maintenance seemed inconvenient. Therefore, I decided to place the pressure measurement point on the horizontal pipe at the lowest point of the extraction line, which is located on the third tray level. This was done to facilitate maintenance, and it turned out to work well. Could experts tell me if this approach is correct?

It’s hard to determine whether it’s correct without seeing the structure of the tower. However, in general, the values differ between each layer of trays; changes to the pressure sampling points need to be made carefully. If maintenance is difficult, the position of the transmitter can be adjusted to facilitate operation. 5# Wasting time needlessly

In a project I was involved in, the grounding busbar in the on-site junction box was originally intended for shielding purposes and fell under the category of working grounding. However, whether due to requirements from the design firm or a problem on the part of the manufacturer, the busbar was connected to the enclosure of the junction box; as a result, it functioned as a protection grounding system, which does not comply with the guidelines for grounding design. In the end, this bus bar was removed, and the shielded wires were connected together using crimping.

Let’s talk about shielding and grounding: in every large-scale project we work on, we encounter probes on compressors, and the cables of these sensors must be shielded and grounded. We’ve encountered issues such as interference and instability when grounding wasn’t done, but those problems disappeared once grounding was implemented. I hope everyone will pay attention if they encounter similar problems.

In our project construction, the design institute failed to provide sufficient straight pipe sections for the flow meters during the design phase; we therefore asked the designers to lengthen these straight pipe sections so that they meet the requirements； There are also many cases where the bypass height of control valves is insufficient; we specified that the bypass height should be at least 300 mm, and these have all been modified now. In short, there are many aspects of the designs produced by design institutes that satisfy owners not, and many of these issues stem from a lack of communication between different departments within the design institute.

When installing thermocouples in the furnace tubes of heating furnaces, many construction companies forget about the factor of thermal expansion and contraction; the designs also lack specific guidelines and no allowance is provided, as a result of which the thermocouples get broken after long-term use. By making two loops before the thermocouple leads exit the furnace wall during installation, account can be taken of the thermal expansion coefficient, and such problems can be avoided.

For radar level gauges, especially those installed without a guide wave tube, attention must be paid to interference reflections and multiple reflections. I have encountered situations where it was installed at the center of a container, and this should be avoided as it causes multiple reflections; similarly, installation at the center of a manhole cover should also be avoided (multiple reflections) ； Installed in areas with interfering objects at the bottom (interfering reflections). Recommended installation location: at a distance from the pipe wall of >1/10~1/5xH (height), but not more than 1/3xD (container diameter), with no obstructions below.