A properly selected filter bag in terms of its fit with cage, tubesheet and material of construction, can yield a longer life and give an good operating experience, provided the bags are installed carefully and the commissioning is done in proper manner, writes Rajendra Kumar Yadav.
The pulse jet bag filter (PJBH) is physical filtration equipment which uses filter elements made of woven/non- woven fabric to separate the suspended particulate matter from the air or from flue gases. These particulates can be dust from different processes used for manufacturing cement. PJBH is an equipment of choice for air pollution control or product recovery for various processes in a cement plant and has gained prominence over other equipment like ESP and RABH due to its high efficiency, competitive installation cost and ease in operation. Filter bags are a consumable part in the PJBH and need replacement at regular intervals to keep the equipment running smoothly and continuously.
In the last two decades of my association with air pollution control equipment, I have come across more than 1, 000 PJBH installations. Most of these installations were either non-performing or under-performing to the ever increasing expectations of the users. There are usually three major demands of the users of PJBH namely, Filtration Efficiency – The filter must not emit more than desired levels of suspended particulate matter. For a product collector, it is maximum collection of product, letting nothing out of the chimney.
Pressure Drop – The filter must run at the lowest possible pressure drop across the inlet and outlet of the filter unit. It is directly related to the power consumed/ throughput of the production equipment. The higher the DP, the more the power consumed. Similarly, the lower the DP, the more the possibility of throughput.
Life – running hours of the filter bag before it is discarded – There are two modes of the end of useful life for a filter bag. It has either lost its permeability to a level where the differential pressure drop is more than the acceptable limit or it is mechanically damaged.
Most of the users had a common observation. When they bought the unit new, the filter bags lasted longer than when they changed the filter bags. Most of the times the reasons attributed to shorter life were found to be as detailed below:
1. The duct work, filter housing and process connections develop holes over 24 or more months as they handle dust- laden gases. Normally this happens because the equipment is manufacturing using a lower thickness of the steel sheet. This results in a lot of undesirable leakage of atmospheric air at different locations. This air may cause localised cooling and condensation of the water vapour present in the flue gases, which ends up blinding the filter bags or the chemical degradation of fibres used for making these felts.
2. The cages are rusted during use with the first/previous set of bags. This results in increased roughness of the wire cage. Even when they are repainted or repaired, the roughness is not taken care of and it causes increased abrasion with the filter bag surface during each cleaning pulse, resulting in early failure.
3. Excessive pulsing of the filter bags is a very common phenomenon. Most of the units are not fitted with an on-demand kit. These units keep pulsing the filter bags at regular intervals irrespective of the pressure drop across the filters. Each filter has a flexural strength (resistance to failure due to flexing of filter materials in opposite directions) and it will fail earlier than expected if cleaned excessively. The pulsing air pressure being more than required/recommended also damages the filters prematurely.
4. The quality of the compressed air used plays its own role in a filter’s useful life. Compressed air expands from 4-5kg/cm2 to almost atmospheric pressure and all the water vapour carried in it will instantly condense. If the compressed air is not sufficiently dried before being introduced into the filter unit, it may get the filters wet and cause higher DP. It may also cause cementing/hardening of the interstitial dust, thereby blocking pores permanently.
5. Poor fitment of the filter bags with the cages is another prominent reason for early failure. This problem is more severe when brittle fibres like glass and PTFE are used for making filters. Pulsing more frequently or pulsing with higher intensity pressure results in premature failure of filter bags made of these fibres. And if the fitting is loose, the filter may fail within days. For other fibres too, loose fit results in the failure of the filter prematurely. Also, very tight fitting will result in inadequate cleaning.
6. Wrong selection of filter media for a given application results in premature failure. Some fibres offer temperature resistance, some are good against moist heat and yet others are good against abrasion. Any mismatch will result into a shorter bag life.
7. There are other faults like improper installation of bags with cages/tubesheet, improper length of filter vis-a-vis the cage. Inadequate cleaning system design, leaking rotary air lock valve, etc, are other factors that can cause premature failure of filters. Uncontrolled process parameters like flow, temperature, dust load and size of particles also impact filter life.
8. The start -up and shutdown procedures if found to be faulty will result in the process gases remaining trapped in the casing even after the process has been stopped and the water vapour/acid fumes condensed on the filter bag will causing blinding of pores or chemical degradation.
So, the question is, what do we do to extract maximum life out of the filters?
One of my overseas associate with more than 40 years experience in PJBH design, installation and commissioning, remarked that commissioning a new set of filter bags was like flying a plane. A big checklist of pre- flight activities and a smooth take- off are essential and then you can go on autopilot.
However, before we discuss the checklist we must understand a few things. What is the total number of filter bags installed in the bag filter unit? Should it be a single compartment or a multi-module bag filter? Whether it is online pulsing or offline pulsing? These are basic facts one must collect before working out any strategy.
The best strategy to change the bags in a PJBH is to change the whole set at one go. However, if changing the entire set of bags is not possible then at least 50 per cent should be changed at one time. If even that it not feasible, then all the bags in a single module must be changed. Never change a few bags in a single module of PJBH as these new bags with very high air permeability can handle the flue gases and usually fail very soon. Instead, install new bags and keep the old but good bags for a change later, in case of failure.
Once you have chosen from the above three options, what follows is:
1. Buying the filters from a reliable source. Find out correct dimensions (inner diameter * effective length) and the fitting method of the filter bag with the tubesheet. Your original equipment manufacturer (OEM) must provide you with this information. Insist on a spare parts drawing from your OEM when you are buying the PJBH.
2. Providing complete information to the vendor from whom you are buying the filters; this includes operating parameters like flow, temperature, dust load, dust type and filter life obtained from last set of bags. Concealing information or passing on wrong information only deprives you if there is any problem in your existing system. Sometimes OEMs do make mistakes in the selection of filter bags as they are not fibre technologists.
3. Seeking installation and operation procedures from the bag supplier or sharing your installation and operation procedures with them to reach a consensus. Many filter suppliers offer a warranty on filter life.
4. When the filter units are bought as composite equipment with other machineries, the focus is always on production machinery. This could result in the selection of inadequate equipment. Take advantage of a PJBH expert when you decide to change your set of bags as this is the most cost- effective time to make any modifications in the equipment.
5. Every day there are new technologies emerging in the field of fibre materials, felt manufacturing and surface treatments. They offer huge benefits over the conventional media in terms of operating pressure drops, better collection efficiency and useful life. Do enquire about these upgrades from your filter supplier.
6. Insisting on getting a sample bag from the filter manufacturer and checking it for a perfect fit with the tube sheet. If you are using a snap band type of fitting, check whether the filter can be rotated in the tube sheet using bare hands. It is not desirable to have a fitting where the bags are rotating in the tubesheet. Also check the fitting with the cage if you intend to use existing cages. Use the guidelines provided by your OEM about bagtocage fit. Ask for whatever is not provided already.
7. Checking the condition of the cage. A thorough inspection of cages can do a lot to save your filter from damage due to use of bad cages. Avoid cages which are corroded, bent, have become oval, have broken rings, have a vertical wire missing or a horizontal ring missing. Check if the top collar and bottom cup are in good condition. If it is fitted with a ventury, then the ventury must be in good shape and should not have holes. Check the wire surface and ascertain that it is not rough. Please note that often the cage is only a fraction of the cost of the filter but can damage the filter bags really fast. Act prudently, replace the cages with a new set if required.
8. Last but not the least, checking the filter bags once they arrive in bulk and obtaining the required documentation like inspection reports/ test certificates and packing lists to ensure that they are same as ordered by you in size and material of construction.
Now that the filter bags are with you, make a checklist of all you need to do when putting the filter bags in the unit. A general or specific list can be provided by your OEM /bag supplier if you ask for it. Planning the filter bag change before actually changing the filter bags helps you organise the essential resources before the unit is actually opened and saves the plant some expensive downtime. Unfortunately, most Indian industries are not in habit of changing filters during a regular shutdown but always work on a philosophy of `change when required.` Having planned and got ready your checklist, you can change the filter bags with a new set of filter bags for the entire unit or a compartment. When restarting the unit, take care to run it on partial load for a few hours before the bags have formed a primary cake on the filter surface. It usually takes about 2-12 hours for the formation of a primary cake on the filter surface. Since it is the primary cake which does most of the filtration throughout the life of filter bags, be careful that it is not formed at full load as that would be very packed and will have a higher residual pressure drop. A primary cake formed at gentle velocity is more permeable and enhances the filter’s life and lowers the operating pressure drop. You may also seek the help of a professional commissioning engineer who can help you in installation of filter bags, cages, pre-coating, pre-commissioning and actual commissioning and setting up the most efficacious cleaning cycle.
It is not only a good filter bag which gives a good useful life and trouble-free operation but the entire process of the procurement, inspection, installation and commissioning which ensures that your filters give you total value for money. Often, they are the heart of your production system and govern productivity. Environmental compliance is being implemented in a stricter manner and failure to contain emission may result in stiff penalties. The best strategy is to learn the finer aspects of filter bags change and reap benefits. There are organisations which specialise in these services and you can always avail their services.
Having worked out the most suitable strategy for changing the filter bags, you must monitor their life by recording the dates on which you changed the filter bags, when you had to induce extra cleaning and how much life you then attained. Sharing this information with your supplier will get you better results the next time you change the filters.
PJBH is increasingly being used for different applications in cement plants. The proper selection of a filter bag in terms of its fit with cage, tubesheet and material of construction can yield a longer life and give a good operating experience, provided the bags are installed carefully and the commissioning is done in a proper manner. Pre-coating the filter bags before commissioning may give good protection against condensation on them.
Rajendra Kumar Yadav, is Managing Director, Gulmohar Filtech. rajendra.yadav@gulmohargroup.com