F-class Best Practices: OPERATION AND MAINTENANCE

Smart drains help prevent piping damage

Essential Power Newington’s heat-recovery steam generators were designed with reheaters to improve overall combined-cycle efficiency. The original cold-reheat (CRH) steam piping incorporated two methods for removing collected condensate. The first two CRH locations off the HP steam-turbine exhaust relied on “smart drains.” Each incorporates a drip leg for collecting condensate, a thermowell and temperature sensing device, a temperature transmitter, and an air-operated metal-seated ball valve with feedback position switches to remove condensate.

The balance-of-plant DCS controls operation of each smart drain’s temperature control valve. At plant startup, the temperature control valves remain open until steam-turbine output reaches 20 MW. At that point, the valve control function changes to maintain 45 deg F of superheat in each dripleg. At the three other CRH drain locations, steam traps removed condensate from driplegs.

During a cold start in mid-2013, a water-hammer event occurred in one CRH header. While no serious damage was inflicted on the piping or equipment from the event, plant staff considered it a significant near-miss incident that could have resulted in equipment damage and caused injury to plant personnel. An investigation revealed the root cause as a steam-trap failure, the first such incident experienced at Newington.

Because driplegs are critical to safe operation, and the smart-drain design had proved a solid, reliable solution for removing collected condensate, the plant decided to retrofit this design on the three CRH drip legs equipped with steam traps. DCS logic was adapted from the other smart-drain installations by a plant I&C technician. Valve status and temperature indication was added to the DCS operator interface graphic for each of the replacements, with the data saved in PI.

Valves were installed during the October 2014 planned outage and have performed as expected during cold, warm, and hot starts. While the new valves increase plant heat rate slightly during startup, their benefits, listed below, far outweigh the small performance loss.

1.) Eliminate the potential for excess condensate collection in CRH system driplegs thereby reducing the likelihood of a water hammer event.

2.) New instrumentation provides continuous condition monitoring in CRH system driplegs—information previously unavailable to operators.

3.) Eliminate maintenance costs associated with replacing failed steam traps.

HRSG access improvement

Klamath Cogeneration regularly experiences failures inside the HRSG transition duct between the gas-turbine exhaust and the first set of superheater tubes. The diffuser panel (baffle wall), expansion joint, walls, and floor all require regular maintenance. To support this work, scaffolding, welding equipment, insulation, and personnel had to transit through a small 18- × 24-in. opening. This slowed down repairs, limited the size of materials, and contributed to the possibility of back and/or other injuries.

The solution was a simple: Install a larger door, but that idea proved a difficult challenge. Staff engaged several HRSG and duct manufacturers and could not find one that had completed a project like the one it was considering; however, HRST Inc was willing to work with plant personnel to find a solution.

There were many things to consider, including the following: how large to make the door, how to insulate it, how to attach it, how to install and remove it. Obviously, at about 600 lb, it is too heavy for workers to remove by hand so a trolley system was needed to carry the weight. Jack bolts were installed to pull the door out of the hole and guide rails on the top ensure that the door comes out straight, minimizing the possibility of damage to the mounting studs.

HRSG door Fig

The door is approximately 1-ft thick, including interior insulation and protective steel. Its weight causes the door to tilt forward when hanging from the removal chain fall. So a large counterweight was required on the exterior so the door would hang straight. The trolley rail is angled to allow the door to be stored to the side of the opening, permitting unobstructed movement of workers and equipment.

Today, instead of crawling through an 18- × 24-in. hole on hands and knees, personnel can comfortably walk through the new 48- ×78-in. door. The platform installed outside the door is large enough to accommodate an entire rack of scaffolding gear; the door is large enough to allow for a full sheet of steel. The handrails are removable and the stairs are wide enough that two workers can carry heavy materials side by side. Given the amount of work routinely done in this area, better access will reduce both the risk of injury and the cost of labor.

Automate water injection for emissions control

Water injection is used on Monroe Power’s two 501F gas turbines for NOx control. The original WDPF control system relied on an f(x) function block to adjust injection flow based on unit output. This approach did not take into account ambient conditions, or bias based on actual NOx values.

The control system was upgraded in 2009 to Emerson Process Management’s Ovation™ and the process was carried forward as designed in the WDPF system. The operations staff corrected for major changes in ambient temperature (for example, summer to winter) by entering a new f(x) value in the control logic and then during a normal operating day made minor adjustments when the NOx values were not at the desired value.

With the system in place, an out-of-range value could be added into the f(x) function block and cause large swings in the water-injection rate. A possible outcome if this occurred: A significant load swing, possibly even loss of flame and unit trip.

This method of operation took a great deal of hands-on time, requiring an extra person in the control room to monitor and maintain balance-of-plant operations.

New logic was written and installed to control water flow based on load and NOx level. To adjust for ambient changes from day-to-day and season-to-season, logic also was added to allow the CRO to trim water injection and stay within a specified band, thereby preventing over- and under-spray. The new logic also includes a manual function to give the CRO direct control of flow, if necessary.

Today, a single CRO can adjust NOx water in three ways without the need to go into the logic and change the f(x) function block. This eliminates the need for an additional operator in the control room while units are in operation to adjust water injection during ambient changes. The automatic control reduces instantaneous NOx excursions caused by changes in load as well as total emissions.

Replacement of station batteries, plus ventilation upgrade

Essential Power Newington originally included a valve-regulated lead-acid (VRLA) station battery string to supply critical loads on the 125-Vdc/120-Vac plant system. The inherent short lifespans and premature failures of VRLA batteries, plus requirements set forth in IEEE 450-2010, mandated that plant personnel conduct load tests every two years.

Newington implemented a complete “in kind” cell replacement in April 2009 when load tests indicated significant signs of capacity deterioration. Testing following the retrofit revealed similar deterioration rates for the new battery string; the plant opted to abandon VRLA technology in 2013.

First step in resolving the issue was to engage an engineering firm, in February 2014, to assist with battery sizing, bid specifications, and storage-room code compliance review for converting to a traditional flooded lead-calcium battery system. Equipment bids were received by late June and C&D Technologies Inc’s LCR-31 product was selected.

Also purchased: Two-tier seismic storage racks for use in UBC Zone-2B locations according to requirements set forth in the site’s design basis; EnviroGuard spill containment system with neutralization/absorbent pillows; two ventilation fans with duct-pressure switch; Sensidyne H2 monitors; and dual-channel controller to enable operator notifications via the DCS.

The ventilation ductwork, fans, fan controller, duct pressure switch, and redundant H2 monitors/controller were installed as an additional upgrade not required by code for this installation but installed as a best practice. Ventilation fan controls and feedback are integrated to the DCS along with continuous H2 concentration feedback to the DCS via a 4-20-mA loop.

The new battery string was installed during planned shutdown in October by a local contractor and tested. All results were acceptable, with rated capacity greater than 100% according to OEM and IEEE 450-2010 guidelines. Ventilation fans and ductwork had a full air balance to comply with the National Environmental Balancing Bureau.

Benefits of the retrofit, in addition to an increase in plant reliability and safety, include these:

1.) Eliminate the potential for premature cell failures and loss of battery power because of failed cells not identified until load is applied. The expected life of traditional flooded lead-calcium acid batteries is approximately 20 years which far exceeds the nominal five-year life experienced with VRLA technology.

2.) New positive ventilation system for the battery storage area provides a slightly negative draft in the room to mitigate any hydrogen accumulation. With the redundant hydrogen gas sensors installed and wired to provide feedback to DCS for operator awareness, this is a significant safety enhancement.

3.) Eliminate costs associated with testing VRLA batteries every two years; tests of the new system are conducted at five-year intervals.

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