air diffusers

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Technical Bulletin

Environmental Dynamics

Published: 04/2012

Purging Diffuser System Piping

Technical Bulletin 151

Dynamics International


1

BULLETIN BRIEF

The function of fine bubble diffuser systems is greatly affected by the ability to exclude or remove liquid and solids from the piping system during normal operation conditions. In addition, it is necessary to have provisions for removing any condensate, minor leakage back into the during on and off operation or characteristics which could cause water to enter the piping. Fine bubble diffusers in general are designed to exclude those materials, however routine long term operation of fine bubble systems demonstrates the need tminor solids from the piping system with a proper PURGE system

DISCLAIMER

Technical Bulletins are presented as a service by Environmental Dynamics International headquartered in Columbia, Missouriparticular bulletin or your specific aeration application contact Environmental Dynamics International by calling +1.573.474.9456, toll free at +1.877.EDI.AIR8 (877.334email at [email protected].

Technical Bulletin 151 Purging Diffuser System PipingCopyright Environmental Dynamics International

diffuser systems is greatly affected by the ability to exclude or remove liquid and solids from the piping system during normal operation conditions. In addition, it is necessary to have provisions for removing any condensate, minor leakage back into the during on and off operation or characteristics which could cause water to enter the piping. Fine bubble diffusers in general are designed to exclude those materials, however routine long term operation of fine bubble systems demonstrates the need to have provisions to remove moisture and minor solids from the piping system with a proper PURGE system

Technical Bulletins are presented as a service by Environmental Dynamics International headquartered in Columbia, Missouri USA. For additional more information regarding this particular bulletin or your specific aeration application contact Environmental Dynamics International by calling +1.573.474.9456, toll free at +1.877.EDI.AIR8 (877.334-

[email protected].


Environmental Dynamics International 2012 All Rights Reserved

diffuser systems is greatly affected by the ability to exclude or remove liquid and solids from the piping system during normal operation conditions. In addition, it is necessary to have provisions for removing any condensate, minor leakage back into the system during on and off operation or characteristics which could cause water to enter the piping. Fine bubble diffusers in general are designed to exclude those materials, however routine long term

o have provisions to remove moisture and

Technical Bulletins are presented as a service by Environmental Dynamics International USA. For additional more information regarding this

particular bulletin or your specific aeration application contact Environmental Dynamics -2478), or via

2

TECHNICAL BULLETIN 151

Fine bubble diffuser systems can be constructed so that diffusers are mounted on the top, bottom, or near the centerline of the pipe. For all systems that are top or centerline limited opportunity to remove condensate or liquid that gets into the piping below the lowest opening in the piping. For top mounted units that means the entire piping can be filled with water and when air it turned on there is no logical place for the water to go and be purged quickly or easily. For those units where there is a diffuser mounted with holes in the center line of the pipe on the sides, that can force the water out of the diffusers themselves, to approximately thpiping, but still the remaining material is inside the piping system. For those diffusers that are mounted below the piping system there is the possibility they are the lowest point in the pipe and water can be purged through the difthat water filling the piping can create poor distribution of air in the system. Water in the piping can increase pressure losses through the system. Water in the piping that is not properor solids that accumulate can also create fouling on the backside or in the openings of the diffusers themselves. This is particularly damaging to ceramic diffuser units were not only the particulate solids that may be in the condensate or is evaporated during the passage of air through the diffuser itself can create deposits into the media as a result of evaporation and deposition of dissolved solids from this evaporation. The reneed to locate the moisture purges in a proper location for maximum diffuser system performance.

In any diffuser system, you would typically have a drop pipe, a subheader, then diffusers located as shown in figure below.


151 PURGING DIFFUSER SYSTEM PIPING

Fine bubble diffuser systems can be constructed so that diffusers are mounted on the top, bottom, or near the centerline of the pipe. For all systems that are top or centerline mounted there is little or limited opportunity to remove condensate or liquid that gets into the piping below the lowest opening in the piping. For top mounted units that means the entire piping can be filled with water and when

no logical place for the water to go and be purged quickly or easily. For those units where there is a diffuser mounted with holes in the center line of the pipe on the sides, that can force the water out of the diffusers themselves, to approximately the center line of the piping, but still the remaining material is inside the piping system. For those diffusers that are mounted below the piping system there is the possibility they are the lowest point in the pipe and water can be purged through the diffusers to clear the total piping system. It is well documented that water filling the piping can create poor distribution of air in the system. Water in the piping can increase pressure losses through the system. Water in the piping that is not properly removed and or solids that accumulate can also create fouling on the backside or in the openings of the diffusers themselves. This is particularly damaging to ceramic diffuser units were not only the particulate solids that may be in the condensate or the moisture in the piping can accumulate, but moisture that is evaporated during the passage of air through the diffuser itself can create deposits into the media as a result of evaporation and deposition of dissolved solids from this evaporation. The reneed to locate the moisture purges in a proper location for maximum diffuser system performance.

In any diffuser system, you would typically have a drop pipe, a subheader, then diffusers located as



TEM PIPING

Fine bubble diffuser systems can be constructed so that diffusers are mounted on the top, bottom, or mounted there is little or

limited opportunity to remove condensate or liquid that gets into the piping below the lowest opening in the piping. For top mounted units that means the entire piping can be filled with water and when

no logical place for the water to go and be purged quickly or easily. For those units where there is a diffuser mounted with holes in the center line of the pipe on the sides,

e center line of the piping, but still the remaining material is inside the piping system. For those diffusers that are mounted below the piping system there is the possibility they are the lowest point in the pipe and

fusers to clear the total piping system. It is well documented that water filling the piping can create poor distribution of air in the system. Water in the piping can

ly removed and or solids that accumulate can also create fouling on the backside or in the openings of the diffusers themselves. This is particularly damaging to ceramic diffuser units were not only the particulate

the moisture in the piping can accumulate, but moisture that is evaporated during the passage of air through the diffuser itself can create deposits into the media as a result of evaporation and deposition of dissolved solids from this evaporation. The result is the need to locate the moisture purges in a proper location for maximum diffuser system performance.

In any diffuser system, you would typically have a drop pipe, a subheader, then diffusers located as

3 Technical Bulletin 151 Purging Diffuser System Piping Copyright Environmental Dynamics International 2012 All Rights Reserved

This drawing shows disc diffuser units and demonstrates that the diffuser is at the top of the piping system. The lowest point in the piping is the subheader which is the largest diameter pipe and located near the inlet to the piping system. The laterals are located at the centerline of the subheader. Proper purging of this system requires moisture removal from the entire system which means the lowest point in the system is the most critical location. The subheader has the lowest point in the system and the purge assembly needs to be located to accommodate the removal of the liquids and solids form the subheader!

It is critical to locate the purge adjacent to the inlet drop pipe for superior performance. This is true for the following reasons:

1. The first air coming into the system will immediately go to the purge and start pumping water from the system. 2. Any water that is removed from the system immediately drops the water level below the crown of the pipe. This allows air to be distributed throughout the entire piping system immediately and discharged from all diffusers almost immediately. 3. Water is continually removed by the purge assembly without pumping the water and or solids through the diffuser units. 4. Water that is in the air laterals will continue to flow back to the subheader and will be removed keeping the water and solids from being discharged through the membrane or the ceramic diffusers. 5. The operating pressure of the system will be immediately reduced once the water level drops just a few millimeters below the crown of the pipe. Surges of pressure from initial startup are minimized.

Traditional practice in the industry when fine bubble diffusers were first developed and introduced had a different arrangement of the piping and the purge assembly. The piping was generally designed to have the laterals be the lowest point in the system. As a result, the laterals themselves had to be purged because of the location of a purge near the subheader would not be effective. To accomplish this objective, many times the laterals were connected at the discharge ends in order to form a sump or a low spot in the piping system to allow the purge to operate effectively. See figure below.

4 Technical Bulletin 151 Purging Diffuser System Piping Copyright Environmental Dynamics International 2012 All Rights Reserved

The arrangement that has the piping installed in such a fashion creates difficulties and problems that are avoided with proper location design of the purge assembly. The following are issues that arise when you have the piping in a complete grid with the ends of piping laterals connected:

1. This is a difficult mechanical situation. Plastic piping has significant thermal expansion/contraction characteristics. In this scenario, plastic pipe will be expanding in two different directions. This puts stress on the piping and on the pipe supports that does not need to be employed when using piping with the purge assembly located at the inlet of the drop pipe. 2. Purges located at the discharge end of the piping, even in a collector pipe as describe above, means that all of the water is pushed through the diffusers until you can get air to the purge assembly or the eductor at the discharge end of the pipe. This means that the purge is only taking out the last bit of the water and or solids, and does not have any effect on the bulk of the liquid and solids The liquid being pushed through the diffusers causes some accumulation and gradual deterioration of the diffuser performance. Note: An alternative approach requires a separate airline to feed the purge. It gives similar performance as the purge located at the drop, but creates a more complicated design. 3. The use of the collectors at the discharge end of the laterals is also more expensive and creates regular maintenance as described because of the mechanical stresses and the lack of the ability of the purge to immediately become effective and eliminate water from the diffusers.

It should be noted that the diffuser purges can be located at any location. These purges can have some effect regardless of their location; however the optimum point for purges is at the low point of the air piping system located near the drop pipe. The initial scenario described shows the optimum location of the purge assembly.

5

TYPES OF PURGES

There are two types of purges that can be employed.

1. Airlift purges with manual ball valve or automated pneumatic valves

Originally, this style was the only type used. This mechanism uses an ejector tube with an air inlet at the top and installed at the low point of the piping system. Liquids are pumped out as soon as air hits the ejector, and sends the liquid to a discharto an air lift pump principle with the benefits outlined above from proper location. This is still a very effective purge assembly and it is routinely applied on large grids in particular.

Note these manual purges do not operate by pushing water out of the pipe. The units pump water and require air supply to operate; hence they must be located close to the drop pipe or have a separate air line to the discharge end.

2. Automatic or continuous purge

Continuous purges are becoming more common on modest size grids in particular. Continuous purge uses a fine bubble diffuser unit either tube or disc mounted on the bottom of the lowest point in the piping system. These membrane type diffusers are typicato pass considerable amount of liquid or solids quickly with minimum pressure buildup. Locating them at the bottom of this piping system allows the moisture and the condensate to be routinely pushed out of the system on a continuous basis. This is in contrast with the manual or with the automatic purge that pumps moisture to the top of the tank near the handrail. Purge valves must be opened manually on an individual basis when you want to activate the purge, or in maleft open which wastes air into the atmosphere in order to be sure the purge works when moisture does accumulate.

In any case, location of the purge and piping system with fine bubble diffusers. The locations are recommended to be at or near the drop pipe of the aeration system


wo types of purges that can be employed.

Airlift purges with manual ball valve or automated pneumatic valves

Originally, this style was the only type used. This mechanism uses an ejector tube with an air inlet at the top and installed at the low point of the piping system. Liquids are pumped out as soon as air hits the ejector, and sends the liquid to a discharge above the water level. This mechanism is similar to an air lift pump principle with the benefits outlined above from proper location. This is still a very effective purge assembly and it is routinely applied on large grids in particular.

manual purges do not operate by pushing water out of the pipe. The units pump water and require air supply to operate; hence they must be located close to the drop pipe or have a separate air line to the discharge end.

Automatic or continuous purge

Continuous purges are becoming more common on modest size grids in particular. Continuous purge uses a fine bubble diffuser unit either tube or disc mounted on the bottom of the lowest point in the piping system. These membrane type diffusers are typically high capacity diffusers designed to pass considerable amount of liquid or solids quickly with minimum pressure buildup. Locating them at the bottom of this piping system allows the moisture and the condensate to be routinely

on a continuous basis. This is in contrast with the manual or with the automatic purge that pumps moisture to the top of the tank near the handrail. Purge valves must be opened manually on an individual basis when you want to activate the purge, or in maleft open which wastes air into the atmosphere in order to be sure the purge works when moisture

is the most critical item in the long term performance of the purge ith fine bubble diffusers. The locations are recommended to be at or near the



Originally, this style was the only type used. This mechanism uses an ejector tube with an air inlet at the top and installed at the low point of the piping system. Liquids are pumped out as soon as air

ge above the water level. This mechanism is similar to an air lift pump principle with the benefits outlined above from proper location. This is still a very

manual purges do not operate by pushing water out of the pipe. The units pump water and require air supply to operate; hence they must be located close to the drop pipe or have a

Continuous purges are becoming more common on modest size grids in particular. Continuous purge uses a fine bubble diffuser unit either tube or disc mounted on the bottom of the lowest point

lly high capacity diffusers designed to pass considerable amount of liquid or solids quickly with minimum pressure buildup. Locating them at the bottom of this piping system allows the moisture and the condensate to be routinely

on a continuous basis. This is in contrast with the manual or with the automatic purge that pumps moisture to the top of the tank near the handrail. Purge valves must be opened manually on an individual basis when you want to activate the purge, or in many cases it is left open which wastes air into the atmosphere in order to be sure the purge works when moisture

is the most critical item in the long term performance of the purge ith fine bubble diffusers. The locations are recommended to be at or near the

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