Precision in valve position indication28 September 2001
Pilgrim has retrofitted a measuring system for recording movement of self-contained valves onto a replacement valve. The IPIS system monitors valve position with greater accuracy and reliability than acoustic or magnetic methods.
With self-contained valves, such as plug and swing check valves and system-medium-operated valves, it has always been difficult to determine and monitor actual valve position with absolute dependability. This, of course, results from the fact that in these designs there is no external stem whose position can be positively tracked. As a result, sonographic technology has been used by the nuclear industry to “see” and keep track of valve position. But even this technology is not without its problems. For example, valve configuration and the body thickness — of 41/4 inches (100mm) at Entergy’s Pilgrim Power Station — can impede this non-invasive-testing (NIT) process.
Not long ago these difficulties produced an undesirable result in a safety-related, plug check valve at another nuclear plant in the US. Here, the associated high-pressure safety injection pump failed to achieve its required hydraulic
performance parameter due to a failed check valve. Neither the acoustic nor magnetic NIT methods employed indicated the disc was in the bottom of the valve. As a result of this incident, the US Nuclear Regulatory Commission (NRC) issued an Information Notice (NI-2000-21) to alert operators that in some cases acoustic and magnetic NIT techniques may not provide accurate and repeatable results. In addition, other plant noise sources may adversely affect the identification of the sound pattern of the disc striking the valve backstop or seat.
Pilgrim station, a 670MWe BWR commissioned in 1972, undertook an investigation of alternative methods for repeatable monitoring and accurate recording of disc position in one of the two redundant 18” (450mm) swing-disc check valves in the LP safety injection system. Flows through these valves range up to 10,000gpm (2300 m3/sec) at 900psig (62 BarG) and 450°F (230°C).
After reviewing available technologies, Pilgrim settled on an Inductive Position Indicating System (IPIS) incorporating recent technological improvements. This IPIS technology could be retrofitted onto a newly installed replacement valve. Furthermore, on-site preparation for installation could be done in advance in order to reduce personnel exposure time in an area of high radiation. In this case, installation consumed less than 30 minutes for one valve.
Inductive Position Indicating System
The IPIS is an analogue measuring system for indicating and recording movement of self-contained valves, such as check valves and system-medium-operated valves used in nuclear plants. The measuring principle is based on the change in inductance of a coil when exposed to a ferritic iron core. Essentially, this is a linear variable differential transformer (LVDT) that produces an electrical output proportional to the displacement of a separate ferritic core.
Usually these IPIS austenitic stainless-steel sensors are separated from the fluid (steam or water) by an inner tube that contains the sensor rod. This design avoids the necessity of sealing moving parts passing through the pressure boundary. Hence, the sensor can be easily removed even though the system is under fluid pressure. The digital transducer is remote and can be located a considerable distance away from the valve upon which the sensor is mounted. This digital transducer can be connected to a PC for configuration and measuring purposes.
In the case of plug-type check valves and system-medium-operated valves used in main-steam-isolation valve (MSIV) applications, the internal plug or stem moves linearly, hence the linear-motion IPIS’s ferritic sensor rod can be directly fixed to the valve’s plug or stem. In the case of swing check valves however, the disc’s rotational motion must be translated to the sensor rod’s linear motion. This can be accomplished by either a pinned hinge arrangement as used at Pilgrim or a roller mounted on the bottom of the rod that rides up and down on the check-valve disc as it opens and closes.
The remote transducer incorporates an embedded, 32-bit microcontroller and includes a sophisticated temperature-compensation circuit to handle temperatures ranging up to 640°F (340°C). Offset drifts resulting from various thermal expansions with the valve body and the sensor rod are compensated for as well. The voltage requirement of the transducer is 120/230 Vac or 24 Vdc. Standard output signal is 4 to 20mA.
The microcontroller-based transducer has a built-in data-logging function that allows automatic storage and display of up to six different valve strokes. The time frame of each stroke may be selected ranging from 15 seconds up to five days for very slow processes. This feature helps to determine the valve’s condition and can be used as a valve monitoring function. It permits year-to-year operational comparisons without the need for additional recording equipment. In fact, the transducer is capable of increasing the control of data by storing strokes in several different standard formats. This is very useful for future analysis and comparison. Furthermore, the transducer contains three, freely selectable limit values to provide limitswitch functions.
All sensors and connectors of the IPIS are LOCA proof according to the IEEE-323 standard and are type-approved by the KTA 3 (German Nuclear Technical Committee).
As a result of the successful operation of the IPIS system at Pilgrim and operational experience in the US and abroad, Pilgrim station plans to retrofit the other 18” (450mm) swing check valve in their LP safety injection system this year.