How important is filtration to the overall success and efficiency of a piping system for plant operators? Consider this: Leaks caused by severe corrosion resulting from inadequate filtration can cause catastrophic problems such as fire. Fouling can significantly increase energy consumption and also lead to costly, more frequent servicing of system equipment.

Despite this, corrosion rates have increased dramatically over the last 30 years due in part to the governmental ban on the use of chromate corrosion inhibitors. That ban removed the most effective chemical protection available at the time for piping systems.

Since the ban, sand filters have been used in process piping systems to combat corrosion, mostly because of the low maintenance that these setups require. In addition, sand media typically does not need to be changed more than once a year, and, when equipped with automatic backwash, does not require daily maintenance.

Downsides do exist, however. Installation cost is one, and sand filtration cannot easily remove particles that have a higher specific gravity than the sand media itself. Because of that, iron oxide chip scale and corrosion debris that can be found in process and HVAC piping systems cannot be properly backwashed out of the sand filters. Fouled media requires sand replacement.

Bagging an Alternative to Sand Filters

For process cooling and HVAC applications, a bag filter system offers an alternative. Bag advantages include:

  • Increased available surface area that can effectively remove larger amounts of corrosion debris while also handling higher flow rates.
  • Selectable micron ratings to provide the level of filtration desired.
  • Smaller filtration equipment footprint.
  • Electricity is not required to operate a bag filter.
  • Water is not used by bag filters. A sand filter may use thousands of gallons of water per day.

Bag filters are available in a number of designs and configurations to meet the requirements of most filtration applications. Single- and multi-bag configurations can be customized for essentially any application. Also, the low profile design of most bag systems provides for safe and easy changeouts.

Filter Placement

In most process and HVAC applications, filters are not usually installed to handle the full flow of the system. Instead, they are connected in a side-stream configuration where 5 to 10 percent of the total flow rate is filtered. That usually results in a full system turnover rate of five to 20 times in a 24-hour period.

It is highly recommended that the bag filters be installed at the lowest part of the process or HVAC return piping system. This is where pipe scale and other debris tend to collect, and installing the filter there helps ensure that the greatest amount of dirt and debris is collected. This low placement can be especially important when the initial filtering takes place, immediately after the installation of a new bag filtration system when the solids are high. It is recommended that filter bags with larger micron ratings — 100 or 200 microns — be used at the start. Filters with progressively lower ratings can be introduced as the clarity of the water noticeably improves.

As an alternative, bag filter arrangements can be used — without the actual bags — as basket strainers. Also, even when bags are in use, the systems can be used as bypass or pot feeders for adding water treatment chemicals.

While sand filters have traditionally experienced problems with excessive media fouling during chemical cleaning operations, bag filters adapt much better when used with cleaners or dispersants. These systems give operators the option to use extended-life filter bags, which can reduce operational costs by lengthening the time between filter changeouts. The increased dirt-holding capacities with this choice of filters, ranging from one to 100-micron ratings, will typically result in an effective life that is five times longer than regular bags.

 Magnetic inserts are an option and are highly recommended in HVAC applications. Magnetic inserts can increase the dirt-holding capacity of bag filters because most of the debris in HVAC piping systems is composed of iron oxide particles.