May-June 2019 NPJ

26 NuclearPlantJournal.com Nuclear Plant Journal, May-June 2019 I&C Commissioning... ( Continued from page 25) change if/when it is removed from its thermowell and re-installed. Research involving hundreds of nuclear grade RTDs has shown that the response time of the same RTD in the same thermowell can change by as much as 50 percent depending on how well the sensor is seated in its thermowell. For this reason, any new RTD is LCSR tested upon installation at cold shutdown to verify that it is reaching the end of its thermowell and therefore properly seated. The LCSR tests at cold shutdown do not provide the response times of the RTDs; but they must be performed as a part of commissioning tests to verify proper installation. After the cold shutdown tests and during initial plant start up, the LCSR tests are repeated at normal operating conditions at hot standby or during power operation to obtain the actual “in-service” response times of the RTDs accounting for the effects of process temperature, pressure, and flow on the response time. In particular, the process flow rate can affect the response time significantly. 2) Unlike RTDs, the response time of pressure, level, and flow transmitters does not depend on process conditions but response time testing is still required for any new transmitter. This is because transmitter manufacturers normally provide only generic values for transmitter response times and also because a transmitter response time may change from when it is manufactured until it is installed in the plant. Therefore, it is important for new transmitters to be response time tested using the ramp test technique performed on the bench or at the site of the installed transmitters in the field. Another factor that affects the response time of a pressure sensing system is blockages and voids that may form in the sensing lines or impulse lines that bring the pressure information from the process to the transmitter. However, there is no way to measure the effect of sensing lines until after the plant starts operation. During operation, the process fluctuations (noise) can be used as an input to a transmitter to measure the response time of the transmitter and its sensing lines together in the same test. The test method is referred to as the noise analysis technique and has been developed, validated, and used in nuclear power plants worldwide. However, for commissioning tests in a new plant, sensing line testing is not important because sensing lines in a new plant are normally cleaned and/or purged with nitrogen to ensure that there are no blockages or residues that can slow down the transmitters. 3. Sensor Response Time Testing Techniques 3.1 RTD Response Time As mentioned above, response time testing of RTDs are performed using the LCSR method. It is based on sending a small electrical current (40 to 80 ma) to the RTD using a Wheatstone Bridge. The current causes the sensing filament of the RTD to heat up and result in a temperature transient inside the RTD that is referred to as the LCSR transient. Figure 1 shows the LCSR transient for one of the RTDs tested at an AP1000 reactor in China together with the LCSR equipment set up that was used to generate this data. The LCSR transient results from a step change in temperature inside the RTD, while the response time of an RTD that is installed in a plant results from a step change in process temperature outside the RTD. Therefore, the raw LCSR transient cannot yield the response time of RTDs. Rather, the LCSR transient must be analyzed to produce the response time. The author and others developed and validated the LCSR analysis in the late 1970s through a detailed heat transfer modeling effort. This work proved that the internal temperature transient resulting from a LCSR test can be converted through a mathematical procedure to a transient that is equivalent to one that would be obtained if the temperature outside the RTD went through a step change. This mathematical procedure has been implemented in a software package referred to as the LCSR analysis software. Typically, AMS provides LCSR testing to nuclear plants on a services basis whereby a team of two engineers are sent to the plant to collect the LCSR data onsite and subsequently analyze them to obtain the RTD response time. The tests are performed remotely from the control room area where the RTD field leads reach plant’s instrumentation cabinets. In addition to providing testing services, AMS supplies LCSR test equipment and training to utilities who prefer to perform the work in-house. The AMS equipment for LCSR testing consists of two units referred to as ERT and ELC. The ERT unit contains a Wheatstone bridge to perform the LCSR test and the ELC unit contains the software to acquire and analyze the LCSR data. 3.2 Transmitter Response Time The ramp technique to measure the response time of pressure, level, and flow transmitters is rather simple. It involves a pressure ramp generator to produce the test signal that is directed to the transmitter under test and simultaneously to a high-speed reference transmitter. The outputs of the two transmitters are recorded as the test is performed and Figure 1 LCSR Transient and Equipment Set Up for an AP1000 RTD Tested in China.