Baghouse safety: best practices for hands-on maintenance
Baghouse maintenance can be quite complex, involving many trades from energy engineers, mill builders, electricians, instrumentation technicians, and contractors. The results of proper maintenance can lead to a well-functioning, efficient, and safe particulate matter collection system (baghouse) that improves plant productivity and uptime.
Based on personal experience of inspecting facilities across North America, many different aspects of maintenance are often overlooked. The following list highlights the low hanging fruits of opportunity:
1. Quality and quantity of the water source for fire protection are very important. A good fire safety maintenance program, closely monitored by a company’s insurer, will trap foreign body contaminants such as shrimp, frogs, mud, tinder, and leaves. This foreign material can clog fire valves, solenoid valves, strainers and sprinkler spray nozzles in baghouses. Fire ponds are particularly bad for this. Removing nearby trees or shrubs will help alleviate this problem. Alternatively, the installation of an upstream filter helps to keep contaminants out of the fire protection water. Monitoring and recording the extinguishing water levels is also very important to ensure that in the event of a major fire, a plant does not get stuck with its pants down and does not have enough water, which could drastically affect the handling of an insurance claim.
2. Regular inspections of fire protection systems: Examine the spark detection sensor eyes, which may be dirty, worn, or improperly installed resulting in a spark not being seen at all, which could eventually get into your baghouse and cause a fire or explosion.
3. Mechanical inspections of the baghouse system: Baghouses have a number of components that make up their functionality. One such item is a spherical blower that continuously builds air pressure via a rotary piston blower. The regulations require that every positive displacement blower has a pressure relief valve (PRV). Over time, the PRV’s springs can become weak, and when the pressure in the receiver builds to around 9 psig, the PRV begins to lift prematurely, affecting the efficiency of the bags, if they are “puffed” or “cleaned” at all. The result is a fairly rapid rise in the differential pressure of the bag chamber system and a loss of suction at the critical material pick-up points and can lead to a fire at the pick-up point or the saws. Another point is the timing of the solenoid valve in the baghouse HMI programming. There have been cases in which setting up the magnetic timer was neglected during commissioning. Typically they are set to cycle when the blowing pressure reaches 9 psig.
The buffer fan also has an inlet filter that must be checked and cleaned regularly. I have often found that the inlet filters are clogged or the mounting nut has completely disappeared, as in the example in the picture below. When the filter becomes clogged, it reduces the airflow at the nozzle inlet and extends the pressure time up to nine psig, causing the bubble to vent or “blow” the air into the bags at a lower pressure. If the filter is bypassed, as in the case of the missing mounting nut, dust can be drawn into the airflow and cause the solenoid valve that activates the bellows to fail and no puffing at all. The result is a loss of suction at the material pick-up point and a potential fire.
4th. Differential pressure photohelics in baghouse: Calibration checks should be performed at least annually. Over time, the sensor lines can become clogged or the lines can become brittle and tear. I also saw these lines accidentally disconnected and never reinserted. These photohelics are often tied to alert thresholds that alert a control room operator that the bags in the baghouse may become clogged. When an alarm comes in, it is often muted, ignored or a field service technician is asked to adjust the alarm threshold upwards, and then it is often left for the next shift or completely forgotten – until the bags clog, the suction at the pick-up point and a fire breaks out, which leads to production losses.
5. Monitoring the pressure in the baghouse: The best scenario is to electronically monitor the baghouse differential pressures with alarm thresholds and set up key point indicators (KPIs) that are reviewed every shift by the management team.
6. Baghouse lock feed: This often overlooked device can run a long time before anyone notices that the sluice vanes are worn out, creating overpressure over the rotary valves that can bridge and clog the bag discharge. A common precursor to this is a power failure – the baghouse often shows a plug indicator immediately afterwards and requires manual disconnection of the baghouse. This is a very uncomfortable and potentially unsafe practice as many of my colleagues can attest. The consequences are usually prolonged and unnecessary production downtimes.
7. Baghouse anti-implosion damper: Bag houses often have implosion dampers in the inlet of the main fan to enable a second stage of Grecon spark detection, which activates a fire damper, diverts the potential sparks away from the bag house and activates an emergency shutdown safety interlock. This implosion damper protects the lines from the baghouse back to the pick-up point from collapsing inwards due to too much negative pressure. In the case of the following picture, this implosion damper opens when the main fan disks of the baghouse have been replaced due to wear and tear or in an effort to accelerate the fan in order to generate more suction (often a plaster solution) – and this is inadvertently reduced at the receiving point suction. Hence the extra weights and bungee cords. With this approach, the suction line often implodes over time due to the detection of the second stage and cancels events.
8. Poorly developed applications: In the following case, the main fan of the baghouse pushes the airflow into the baghouse instead of pulling it through. The problem with this application is that there is no way to accurately measure the differential pressure across the bags in order to determine and plan a bag change. In this application, only the inlet pressure into the baghouse is measured, which is not as accurate as measuring the differential pressure. The Grecon fire protection sensor eyes and deluge nozzles are also not as effective due to their poor placement.
9. Adjustment of the suction line system: For the proper commissioning of tubular bag systems, the air balance at the many and several pick-up points is part of the process. Pickup points will be added or replaced / repaired over time. In the case of the picture below, a section was replaced due to wear and tear and the balance damper was found completely closed. This affected the entire system and resulted in multiple plugs in multiple locations for months, resulting in unwanted production downtime and very frustrated maintenance teams.
10. Baghouse Magnahelics: A best practice is to install Dwyer-branded Magnahelics in key locations on extraction points. This is a good visual representation of what normal sucking is. If the measuring device is checked regularly and thoroughly on every shift, a problem can often be identified early on before a fire or production disruption occurs. A saw line operator is gold if he checks this indicator frequently. In one case, the intake lines were arbitrarily clogged for a week before it was discovered that a cleaning door in the upstream pipeline had been left open on the previously scheduled maintenance day. That Magnahelic ad was low and no one questioned it.
11. Worn exhaust hoods: Worn hood brushes or improvised brushes will result in a loss of airflow efficiency and will not remove material properly. This leads to clogged saws and ultimately friction burns and undesirable and unnecessary production losses. Caution should be exercised here: in order to meet production goals, maintenance teams are often not given the time to properly repair these items, or maintenance budgets have been cut to meet the goals.
12. Instrumentation: Proper instrument installation can increase production reliability because stainless steel tubing will not harden, splinter, or crack over time and will last for many more years. The fittings also usually seal better.
In summary, I could write a book on the myriad of maintenance and operational issues that have come up over the years, and I haven’t even discussed the NFPA regulations. There are too many to list in this article anyway. If your company could use an inspection and audit report, please contact Cariboo Biomass Consulting Services at www.cariboobiomass.ca or call Kevin Ericsson at 250-612-1513, email [email protected].