Technical Bulletin 160 Maximum Air Pressure for Diffusers by: Environmental Dynamics International Published: 11/2016
BULLETIN BRIEF In the design and application of diffuser systems, it is important to consider operating pressure and the pressure handling capability of these diffusers. There are 3 different types of diffusers that need to be considered and we will address each of those independently: Coarse Bubble Diffusers Fine Bubble Diffusers with Rigid Media Fine Bubble Membrane Diffusers The above 3 types of diffusers have different operating characteristics and different design characteristics that should be considered. The key elements for design are outlined below DISCLAIMER Environmental Dynamics International, headquartered in Columbia, Missouri, USA, presents these Technical Bulletins as a service to our customers. For more information regarding this bulletin or your specific aeration application, contact Environmental Dynamics International at +1 (573) 474-9456. 1 Copyright Environmental Dynamics International 2016 All Rights Reserved
TECHNICAL BULLETIN 160 Maximum Air Pressure for Diffusers 1. Coarse Bubble Diffusers Coarse Bubble Diffusers are designed to operate with minimum operator attention and have almost no pressure loss in the large openings in the diffuser itself. A design of coarse bubble diffusers must be concerned about the following items: a. There must be an orifice assembly on each diffuser to distribute the air across the piping. Because there is such a low-pressure loss to the diffuser, all the air may go out at the front end of the piping of the first few diffusers unless there is an orifice loss built in to distribute the air. This is an engineering designed orifice for distribution of the air at the air volumes anticipated for the piping system and the individual units. EDI does an individual pressure loss computer analysis on the system to distribute the air properly and size the orifices for each application. b. The operating air pressure on the diffuser itself is not critical. Because these are large openings the pressure loss through the diffuser is generally only a few inches (2 to 4 ) or 0.5 to 1 kpa on the diffuser itself. The orifice listed in item number 1.a is used to control air distribution. The low pressure for the coarse bubble diffusers is a not limiting situation for the compressors. c. It is important to address the issue of compressor operating pressure. Almost all aeration systems are operated low pressure air. Almost all operating systems are designed for an airflow range that will deliver a modest pressure loss across the diffuser. This modest pressure loss is designed to provide air distribution but wants to be minimize to control the amount of energy used i.e. minimize the energy. Thus, the maximum pressure that may be available to supply to the diffusers is traditionally not a concern. Main concern is to have sufficient pressure to operate the diffusers. 2. Fine Bubble Diffusers with Rigid Media Fine Bubble Diffusers with Rigid Media are traditionally ceramic aluminum oxide grains or porous Polyethylene sintered materials with beads of material stuck together. Sand and or some other plastic materials have also been used in this rigid diffuser media application. These diffusers have different operating characteristics vs. the coarse bubble described above and different design conditions that can be significant in their application. a. The Rigid Media Diffusers have a significant increase in pressure loss vs. the coarse bubble diffusers. While the coarse bubble diffusers above may have a minor pressure loss, the rigid media materials initially have a higher pressure loss for their operation that may be as much as 6 to 10 water column (1.5 to 2.5 kpa). These media have an operating pressure that is inherent in their initial clean condition and can be vulnerable to inside fouling or exterior fouling during operation that drives the pressure much higher. The structural capacity of the media is critical for these applications because it can see operating pressures that are increased and apply a major structural load on the rigid media itself with pressure loss times media area creating major upward force. 2 Copyright Environmental Dynamics International 2016 All Rights Reserved
b. The operating function for the system is not particularly compromised by the operating pressure of the media if no physical damage is available. Fine bubbles can still be generated even at high pressures and it can deliver oxygen at those higher pressures if you have sufficient pressure available in the blowers. The penalty is major increases in energy consumption and forces or stress that may be associated with the higher pressure on a rigid plate media. c. High pressure on these media are generally not a mechanical problem for the media itself; however, it can create a situation that damages the holder or causes the gasket that seals the membrane to leak and blowout, including catastrophic failure with entry of liquid and solids back into the system and blockage of the media. In summary, with the rigid media the operating pressure is typically higher than for the coarse bubble systems, is more vulnerable to increased pressure from operation and fouling, and is potentially a mechanical problem from loss of seal of the diffuser media and fouling from liquid and solids reentering the piping in the diffuser system. Typically, a high pressure or pressure increase is most likely to create problems with the blowers and overload the blowers rather than cause damage to the diffuser itself. Pressure by the blowers is limited by the pressure loss across the media rigid openings and not controlled by compressor pressure capability. 3. Fine Bubble Membrane Diffusers The technology for aeration has moved rapidly to adopt flexible membrane diffuser systems. Membrane diffusers are typically flexible polymer materials such as rubber, polyurethane, silicone, etc. These membrane diffusers have demonstrated superior oxygen transfer efficiency, superior resistance to biological or mechanical fouling, and generally become the standardized product for most applications. Operation of these units is unique as operating pressure is affected differently because of the construction of the unit. These membrane diffuser systems have the following characteristics that make them unique: a. The membrane is flexible. b. The membrane has perforations that have variable orifice. This becomes important because the openings in the membrane respond to the air volume and the pressure being applied. Membranes open to reduce the pressure when air is increased opening of the orifices stabilizes or minimizes the pressure on the membrane. c. The units typically have some flexing during the operation, which can minimize any pressure build up. The pressure impact on the rubber membranes or the flexible polymer membranes are not so significant. d. There is a misconception on the amount of operating pressure that any diffuser system would create or even see. The pressure with flexible membranes can be defined as follows: i. There will be an orifice loss at the diffuser for assistance in distribution of air throughout the piping system and to limit the air loss in case one membrane gets damaged. Small losses of perhaps 1 kpa. ii. The membrane will have an operating pressure because of the airflow through the variable orifice. This air pressure is generally moderated and partially controlled by the expansion of the orifice in response to the amount of air that is being applied. 3 Copyright Environmental Dynamics International 2016 All Rights Reserved
iii. The actual pressure across a flexible membrane diffuser is generally limited to approximately 0.5 to 1.0 psi (3.5 kpa to 7 kpa). iv. The only pressure the membrane diffuser or any media sees is the pressure across the membrane itself. This is generally a modest amount of pressure as indicated above and does not typically damage the membrane. Maximum pressure of the compressor or blower is not generally a design consideration of the membrane diffuser because of this variable orifice pressure relief effect. Pressure of the compressor is not a major factor in developing system pressure and system design. v. Pressure that is applied does cause a modest expansion of the membrane diffuser unit. That expansion force and stress is limited in normal operation by the orifices as indicated above; however, if there are solvents or other physical conditions that can attack the membrane, it can attack the bonds in the membrane and weaken those structural bonds. Membranes that have been attacked by chemicals or have excess heat can stretch because of the pressure applied to the system. This is traditionally not a pressure problem it is a membrane compound compatibility problem with the wastewater that is being treated. vi. The membranes are flexible and elastic. In testing these membranes, EDI tests the amount of force required to blow the unit out of the holder. This test uses nonperforated membranes in the holder with air applied to see what structural capacity there is with the membrane and what resistance the holder has for any failures. Traditionally, the holders have not been an issue and the membranes remain in place in the holder. In addition, the application of approximately 5 psi across the non-perforated membrane will generally cause the membrane to inflate, i.e. reach the yield point of the membrane polymer. This can result in the 9 disc membrane becoming as large as a soccer ball and has obviously failed at that load. In summary pressure capability of the compressor or blower is generally not a design consideration. Pressure across the membrane to control operating energy is a major design consideration. It should be noted that all membrane or rigid media diffusers need to be designed based on their operating pressure vs. air volume curves or typically called DWP curves indicating Dynamic Wet Pressure of operation. DWP curves show the response of the membrane pressure to the airflow and the wide operating range that most flexible membrane diffuser units would carry. A sample of the typical DWP curve is attached for reference: 4 Copyright Environmental Dynamics International 2017 All Rights Reserved
This DWP curve shows 3 actual curves. The 1st curve is for the orifice loss only on the diffuser which helps distribute the air along the piping system and control the loss of air in case the membrane is damaged. The 2nd pressure loss is across the membrane itself where the variable orifices are opening in response to the airflow applied. The 3rd curve is the total pressure loss across the diffuser assembly and you can see it is comprised of the orifice plus the membrane itself. These are typical curves that would be expected with any of the diffuser units and again are modest in their actual losses themselves and do not limit the pressure applied from the compressor. The pressure from the compressor responds to the static water column and the pressure loss to the units. Regardless of that pressure capacity of the compressor the operating conditions are controlled by the static water head and the membrane losses. Even if you have a compressor capable of a 100 psi, if the water depth over the diffusers is only 10 ft. that would be the static load that the diffusers sees (30 kpa) and you would have the additional loss on a typical 9 disc diffuser unit running at 2 CFM of air (3.15 Nm3) of 0.9 kpa for the orifice plus 2.4 kpa loss for the membrane. Note that the actual losses of the membrane and the diffuser are quite small. Total operating pressure is going to be the sum of these pressures for the submergence of the diffuser, the orifice and the membrane or a total of 33.3 kpa pressure (4.81 psi) from the example. Even though the compressor can deliver 100 psi, the actual amount of pressure in the air piping is 33.3 kpa (4.81 psi). Actual pressure on the membrane is only that pressure loss across the membrane i.e. 2.4 kpa or only 0.34 psi. There is no way to build a higher pressure on the membrane, unless there is a membrane failure by fouling. Maximum pressure allowed by the air supply is not a factor. Trouble only occurs if come event causes pressure increase across the flexible media or the rigid media. 4 Copyright Environmental Dynamics International 2017 All Rights Reserved