Startup instrument problems

Few column startups are completed without instrument problems. Instrument problems are not unique to startups they also occur during normal operation. Below are some of the most common instrument problems encountered during column startup and operation ... 

Thermocouples and wire leads require constant maintenance due to wire breaks and malfunctioning thermocouples. In most cases, the root cause of the problem is easy to determine. When a thermocouple loses continuity the instrument panel typically reports either an open circuit or a very negative temperature. A more difficult but also common problem occurs when the control thermocouple is not located close to the heating element on the control system. In this case, it is possible for the thermocouple to be influenced by another heated control zone while allowing its control zone to operate at temperatures that can be either very hot or very cold. Such cold conditions can cause the polymer to solidify in the transfer line while a hot condition can cause the resin to degrade. During the installation and startup of new equipment, it is important that the thermocouples are matched with the proper controller and control zone. 

To invesigate whether run order influenced the results for this example, the compOBHit A concentration residuals are plotted versus the order in which the aeference values for component A were measured (Figure 5.17). The first sai Ie is the one with the unusually hi error, which may indicate problems wih the startup of the instrument used to determine the reference concentraticffi of component A. What appears to be a nonrandom pattern over time may innate instrumental fluctuations in the reference method determination. 

Instrument and control problems, startup and/or shutdown difficulties, and malfunctioning column internals are the major single... 

Trouble-free startup requires a well planned and disciplined check of field instruments. If hardware is carefully checked against design and installation specifications, later problems can usually be attributed to the control system itself. This allows ttie startup team to function more efficiently. 

Continuous and batch processes require different strategies for implementation of process analyzers. For continuous processes, startup must be controlled to quickly reach the optimal process conditions. Constant monitoring is needed for both continuous and batch processes to detect when the process is going out of control so that corrective action can be taken. For batch processes, end point detection is an important control parameter. For all analyses, the simplest method that performs the desired analysis is best. As the complexity of a method increases, changes in the instrument, materials, process, and measurement conditions can cause problems (1). 

Instrumentation (see Chapter 6) — during commissioning, problems are often encountered with the instrumentation. Every effort must be made to ensure that the instruments are functioning accurately and correctly before startup and have not been damaged during commissioning. 

A typical printout of the instrumentation performance analysis algorithm is shown in Fig. 12. The error computation is not accurate for reactor period signals, which are differential in nature and have large fluctuations around the mean value. The mean-value of the period signals is however useful for reactor operator. There have been many instances where the computer has detected a noisy instrumentation chaimel or detector, and the problem was corrected by maintenance. It was observed that by replacing the detector signal cables by double shielded cable in the startup channels, the scatter in neutron counts signal reduced by a factor of two. The time constant of the log countrate meter was also adjusted with the help of the system to obtain minimum error. 

---