Introduction

The concept of carrying out a reliability-centered maintenance (RCM) analysis on new assets is certainly not new. The airline industry, where the RCM process was first developed, has been using this process for years to develop maintenance programs for new aircraft.

In this article, I will draw on my experiences conducting RCM analyses for Dofasco Inc., a large North American steel manufacturer with over $5 billion in capital assets. The majority of RCM analyses conducted at Dofasco have been on older, existing assets, such as the nitrogen compressor plant cooling tower. But before we examine the analysis, let's review some RCM basics.

The RCM Process

"Reliability-centred maintenance," says John Moubray of Aladon Ltd., "is a process used to...ensure that any physical asset can continue to do what the user wants it to do, in its present operating context." This is accomplished by first establishing the asset's functions and performance criteria. Next, one should determine how and why the asset fails, and whether such failure truly matters. Finally, one should ask whether anything can help predict or prevent failure. What, for instance, should be done if a suitable preventive task cannot be found?

Such questions are answered by bringing together a qualified RCM facilitator along with a review group made up of people who best know the equipment, such as operations supervisors, maintenance supervisors, operators, specialists, and craftspersons.

By analyzing each new asset, one develops a base maintenance program. Key tasks are identified and implemented during commissioning. Also, one notes critical procedures that might otherwise be overlooked. Appropriate failure-finding tasks can be set up for a protective device before it's even up and running. In this manner, one can accumulate in-depth knowledge about the equipment while reducing commissioning time. Ramp-up time to full operation may also be diminished, and operating practices can be identified during startup; from this, necessary maintenance procedures and key performance variables are encouraged to develop. This may lessen the likelihood of "infant mortality."

RCM Analysis on the Nitrogen Compressor Plant Cooling Tower

The new counterflow tower replaces two 30-year-old crossflow towers, which have since been dismantled. Its main purpose is to cool the service water used on the nitrogen plant compressors and No. 4 ASU scrubbers from a maximum temperature of 100°F down to 82°F or lower. Two compressor systems can be found inside the plant. The 45 psi Centac nitrogen compressors supply 12 psi purge nitrogen to the blast furnaces and steel-making and coke plants. Loss of this supply could result in dangerous conditions. For that reason, the nitrogen supply would have to be switched manually to an alternate offsite source when the cooling water temperature reaches 85°F. This would significantly increase the cost of supplying low-pressure nitrogen.

The 500 psi Bellis and Morcom and Ingersoll Rand XLE - ESV nitrogen compressors feed nitrogen to the Steelmaking KOBM process as an inert gas. This stirs the contents of the furnace while steel is being made. Loss of this nitrogen supply could potentially shut down the steel-making process, which would result in costly production losses. Sufficient nitrogen cannot be fed from the offsite source as an alternate supply to the 500 psi compressor.

The main components of the cooling tower system are as follows: hotwell sump and pumps, system piping, a two-cell counterflow cooling tower, cooling tower fans and fan motors with VS drives, a chemical treatment system, and a fire-protection system.

Hotwell Sumps and Pumps

This system prevents the sump from overflowing, running dry, or cycling on and off. The cooling tower hotwell stores 100°F service water, which is fed to the cooling tower. Three sump pumps, designated as duty, trim, and standby models, exist in the hotwell sump. These vertical types have a capacity of 2,000 gpm per pump and are driven by 60 hp motors. A bubbler type level system controls the pumps.

System Piping

The hotwell pumps convey hot, clean water through the carbon steel interconnecting piping to the feed water inlet at the cooling tower. A standpipe vent controls the feedwater supply to the distribution header system at a maximum pressure of 2.4 psi.

Counterflow Cooling Tower

This is a two-cell counterflow cooling tower fabricated from California redwood with stainless steel fasteners. The water distribution header system, which is located at the top of the tower just below the fan deck, is fabricated from PVC piping. The distribution system is made up of a series of piping headers, with nozzles along the bottom of each header, to distribute the water evenly over the honeycomb fill. As the water passes through the fill, a thin film of water is formed. This maximizes the surface area that will come into contact with the cooling air. The cooled water (82°F) then falls into the cold-water sump at the bottom of the tower.

Cooling Tower Fans and Drive System

The counterflow ambient air, used to cool the hot water falling through the tower, is drawn into the bottom via two 10-ft diameter, five-blade fans. The fans are enclosed in a fiberglass stack, which helps induce the required draft inside the tower and prevents access to the rotating blades. Each fan is driven by a 40-hp totally enclosed, fan-cooled, severe-duty motor. Power to each of the two motors feeds through a VS drive. In auto mode, the speed is regulated based on the cold well sump water temperature. As the temperature in the cold well increases, the fan picks up speed; as the temperature decreases, the fan slows down. The direction of the fan motor can also be reversed manually to prevent icing inside the cooling tower during the winter. If either one or both of the VS drives fail, the power to the motors can be fed from the conventional starters at a constant speed of 1,800 rpm (either forward or reverse) through a three-position transfer switch. Also, a special switch mounted on each motor housing will automatically shut down the motors in the event of high vibration.

Chemical Treatment System

To control microbiological activity, the chemical-feed system transmits sodium hypochlorite 12% into the bulk water. An oxidation reduction potential system automatically regulates the sodium hypochlorite feedrate by measuring the mV potential in the water. To control scale and corrosion, an inhibitor is fed in at a very low continuous rate, which is manually adjusted based on four water samples collected and analyzed each day for phosphate levels. The scale level is also mechanically controlled by blowing down a portion of the bulk water based on water conductivity. Corrosion coupons, installed in the bulk water, are removed and analyzed every 90 days.

Fire Protection System

This cooling tower has a deluge-type fire protection system, which is fed by city water. In the event of a fire, an alarm is sent to the fire department and the cooling tower fan motors shut down.

Analysis Description

Kick-Off Meeting

Once preparation was complete, a meeting was set up with the review team members to "kick off" the RCM analysis. In addition to the team members, the area operations and maintenance coaches were also present, as well as the operations/maintenance manager.

The purpose of this meeting was twofold. First, the manager had an opportunity to convey his support for conducting an RCM analysis. Secondly, the operating context and performance targets were reviewed with the team, so everyone clearly understood what the analysis would cover and why it was being done.

The review team included a mechanical representative along with instrumentation, electrical, and operations reps. Also present were two representatives from the company that designed and built the cooling tower, two representatives from the company that supplied and built the VS drives, and one member of the company that supplies the chemical treatments. The review meetings were run under a compressed schedule. In other words, meetings were held every weekday for four hours each morning until the analysis was complete. This analysis was estimated to require seven meetings, but it wound up taking nine. The maintenance representatives continued with their regular duties in the afternoon, while the operations representative, who was removed from his regular duties, gathered additional information for the analysis in the afternoon.

Analysis Results

The following significant findings came to light after the RCM analysis:

1) The VS drives would vary the speed of the motors from 0-1,800 rpm based on the temperature of the water. The fan drive angle gearboxes were designed to operate at a minimum speed of 900 rpm. Below this speed, the gearbox internals would receive little or no lubrication.

2) If the pitch angle on the fan blades was set below 12 degrees, the fan motors might run at too high an amperage, causing random shutdowns or shortening the life of the motors.

3) The cooling tower fans would windmill in reverse when the fan motors were shut down. When the fan motors were started automatically by the VS drive or manually by the operator, significant damage would be done to the fan driveline system.

4) There was no audible/visual control room alarm to denote the shutdown of either of the VS drives. The only indication that either one of the drives had shut down was a local alarm light on the front of the VS drive panel. In order to get the fan motors running again as quickly as possible, the operators would need a control room alarm to determine when to switch to the conventional power feed.

5) There was no alarm circuit for high water level in the coldwell sump, which meant that water could have overflowed. The operator would have been unaware of this condition, resulting in a severe imbalance in the chemistry of the cooling tower system's bulk water.

6) The cooling tower fans could be operated in the reverse direction for an indefinite period of time. To prevent ice buildup on the stack above the fans, the cooling tower fans must not move in reverse for any longer than 20 minutes during de-icing.

7) During the winter months, the VS drive would slow the fan motors down to 900 rpm, but would not shut down the motors if the coldwell temperature continued to drop below 55°F. This could significantly increase the amount of ice buildup inside the tower.

8) If the fan motors were run in reverse during the winter months, the VS drive would vary the speed based on the coldwell temperature. The fan motors should only be run at a constant speed of 900 rpm when operated in reverse to minimize the stress on the fan motor and drive system.

9) The fan motors are tied into the deluge fire protection system, so they will shut down automatically in the event of a tower fire.

Conclusions

A comprehensive maintenance program was developed for this new cooling tower, ensuring that such tasks as checking oil levels and measuring vibration were carried out on the gearboxes and drive motors. During the commissioning stage, a critical vibration shutdown switch was found to be faulty, and a new switch was installed immediately. Had this faulty switch not been found, it could have significantly hampered operation and compromised safety.

The in-depth knowledge gained about this new tower resulted in several important benefits. Because the team was able to focus on commissioning this new equipment (rather than first learning how it works), commissioning time was reduced. Similarly, ramp-up time was abridged because there was less equipment failure attributable to infant mortality. Critical operating practices were identified during startup of the cooling tower. These practices were developed into procedures, and the required training was carried out for all the operations personnel. Also, key performance standards were developed for operating this tower during the winter months.

This validates the need to consider doing RCM analyses on the right new assets.For more information about conducting such analyses in your organization, contact Ivara Corporation at (905) 632-8000.