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A Canadian production site uses high-volume blowers to propel a fine powder in one of the production processes. The powder is very fine and becomes abrasive in high concentrations.
There are five blowers distributed into two separate nitrogen draft systems. One blower operates in standby for one of the two systems. Nitrogen pushes the powder from a storage bin through a series of pipes to the production process, where the powder is separated from the nitrogen. The filter houses capture any residual powder, keeping it from passing through the blowers. Heat exchanger blockage is closely associated with the bearing and blower failures.
Figure 1. Sample port (valve with red handle) before in-line filter.
The blowers are 250 HP, 3,600 RPM, 2,082 CFM units. The blower sumps (two per blower) are lubricated with an ISO 150 polyalphaolefin (PAO) synthetic lubricant. Each sump holds approximately 10 gallons. If the powder enters the lobed area, it passes through the polymer seals, abrades and destroys the oil seals, and proceeds to abrade and destroy rolling element bearing surfaces. If the bearing fails in service, the blower must be replaced. If the conditional (seal) failure is recognized, the blower can be removed from service for a seal replacement only. This requires frequent testing for the presence of the powder in the gas stream and the lubricant.
A new blower costs $24,400 USD, and in-house labor for replacement is an additional $4,600. Seal repair per blower, providing the bearing and lobes are not damaged, costs $4,870. Heat exchanger repair adds $813 to each blower failure. During the past four years, the site has experienced three catastrophic failures and identified required seal replacement on two other occasions.
Figure 2. End view of in-line filter with sample ports before and after filter.
The average annual cost for blower maintenance based on the preceding inputs is $26,826. This does not include production opportunity losses. The site requires a method to assess the existence of the powder in the gas stream and oil sump with a high degree of confidence.
The problem of particulate contamination in the fluid stream can be tracked, regardless of the nature of the fluid. Nitrogen, being inert, can be handled relatively easily. To address the risk, the author adopted an air sampling technique to detect contaminants in the nitrogen gas (fluid) stream with a similar approach to that which might be applied to a hydraulic oil (fluid) stream.
Air sampling ports have been installed before and after gas stream in-line filters. A specially designed probe that prevents a release of nitrogen is inserted into the sampling port. The probe is connected to a preweighed filter cartridge connected to a high-volume air sampling pump. The sampling pump is necessary to eliminate any effect of a possible pressure differential around the sample probe inlet.
Figure 3. Taking air sample with sampling pump.
Each air sample is drawn for a fixed period of time, then submitted to a local environmental lab for analysis of total particulate.
Using the average in-stream velocity as measured with the velocity probe, and based on the inside diameter of the pipe, the flow rate of nitrogen in the system at the time of sampling is calculated. The flow rate is then used to determine the concentration by weight of particulate in the nitrogen transfer system at the time of sampling.
Leakage problems with filters in the bag houses can be detected. Additional sample ports will be installed at individual gas supply and return lines in time. When possible, air sampling will be handled in a similar manner as the existing oil analysis program. Results will be trended and alarms set to indicate further maintenance actions.