Instrumentation system improvements
International Power America Inc
625-MW, dual-fuel, simple-cycle peaking facility located in Shelocta, Pa
Plant manager: Matthew Denver
Key project participants: Wes Crawford, Jim Mandella, Peter Margliotti, Bryan Miller, and Dexter Cox, plant combustion turbine specialists
“We have all the people we need all the time.” These words are almost never spoken at a GT-based powerplant in these days of a tight economy and suppressed budgets. With fewer personnel to send to check on pressures, temperatures, and conditions, the need for improved instrumentation is greater today than ever before.
The staff at the Armstrong identified its own requirements through a thoughtful, methodical, and practical approach. Regularly scheduled bi-weekly O&M meetings are held at the station to identify upcoming projects and current conditions. At this meeting a list of sensors that would yield the most improvement in operations was developed.The signals were not provided during the construction of the plant and the OEM relied on local switches and manual gages to provide protection. The list included:
- LO header pressure.
- Hydraulic system pressure.
- Lift oil pressure.
- LO sump temperature and heater control (Fig 96).
- Seal oil system supply pressure.
- Seal oil pressure (Fig 97).
- Cooling water pressure (Fig 98).
- CW inlet temp (to tower).
- CW outlet temp (from tower).
- Hydraulic accumulator pressure.
To incorporate these signals into the DCS, a controls change form was developed to track any changes made to the systems and document those changes for future reference. Sensors were selected based on a variety of factors—including sensitivity, range, and price. Because most of these sensors are used for display only—the only exception being lube oil sump temperature— low-cost sensors were selected.
There were ample spare signal wires already installed in the DCS cabinet which were routed to the appropriate junction boxes in the plant. Transducers were installed near each of the systems local indicators to provide the CRO with a remote reading as close as possible to the reading that a roving watch would report.
Sensor installation was a very simple process in most cases; some conduit work was required. After sensors were installed, screens were created for display on the DCS for display of the data, and existing and new readings were used to allow the operator to better monitor and control the plant.
Another major benefit of these additional sensors was control of the lube-oil sump heater. This heater was originally controlled by a temperature switch which had 115 Vac supplied by the LO heater control MCC bucket. This switch was physically located in the sump secondary containment which in our northern climate, can fill up with snow and ice during the winter months.
The switch would often short together, energizing the heater and overheating the oil. As the OEM had only provided a switch that alarmed only if the sump temperature was below normal, this condition could have gone undetected until a LO pump was started and the tell-tale odor of overheated oil was detected.
By installing an RTD in the sump and bringing the signal into the DCS, a control scheme was designed using the original instrument settings to maintain proper oil sump temperature during idle periods and ensure that heaters were de-energized when not required. Also, because the RTD is a low-voltage signal, a potentially dangerous voltage was eliminated from an area prone to moisture, snow, and ice.
Installation of various instrumentation system sensors and their incorporation into the plant’s digital control system allow personnel to more effectively and efficiently operate and monitor the plant’s processes from the control room.