Continuing our discussion from last week, we're talking about dosing chemicals that off gas. There are pumps with degassing heads, pumps with internal sensors that initiate a degassing sequence when they deem it necessary, some pumps utilize timers, and some pumps simply pass gas. The nature of the design of a peristaltic tube or hose pump lends itself well to just passing the gas. We're gonna talk pros and cons of that design this week. I have borrowed a page from a pump manufacturer's site that explains some of the design features of a peristaltic pump.
"Peristaltic pumps provide excellent problem solving pumping solutions especially when the product being pumped is particularly abrasive, corrosive or viscous. Their lack of valves, seals and glands makes them inexpensive to maintain the only maintenance item is the hose or tube. Peristaltic pumps also have a gentle pumping action, ideal for shear sensitive polymers and fragile cell cultures. Lastly, the only part of the pump in contact with the fluid being pumped is the interior of the tube or hose, making it easy to sterilize and clean the inside surfaces of the pump.
Seal-less design
Peristaltic pumps have no seals avoiding issues such as leaks of corrosive chemicals and ongoing maintenance.
Low maintenance costs
The only replacement part is the hose or tube a relatively low cost item that can be easily changed in a short time.
Dry running and self-priming
Peristaltic pumps do not require pumped fluid to be continually present pumps can run dry, without costly downtime or repairs. The recovery of the hose or tube creates a powerful self-priming action and allows the pumps to move liquids containing entrapped air or that can off-gas.
Gentle pumping action
Peristaltic pumps have a gentle, low shear pumping action, ideal for shear sensitive products including flocculants and broths
High suction lift
The powerful suction provided by the recovery of a re-enforced hose gives hose pumps suction lift capabilities up to 9.5m or 31 ft of water.
Abrasion resistant
Hose life is not related to a product’s abrasive qualities. The hose only fails due to fatigue or chemical action.
Solids handling
Hose pumps can pump slurries containing up to 80% inorganic solids or 15% organic sludge.
Reversible
Peristaltic pumps are reversible and can be used to empty lines or clear blockages
No slip
The pumps have no internal backflow giving accurate dosing without slip
Accurate dosing
The pumps are accurate in dosing; they have a repeatability of ± 1% and metering capabilities of ± 5%."
What the manufacture doesn't discuss:
Peristaltic pumps are designed to fail. The act of compressing the hose over and over again, causes hysteresis fatigue and eventual failure. Unfortunately, it's a guessing game as to when the failure will exactly occur. Knowing you have a pump that fails, ideally you would like to change hoses just prior to failure and at your convenience. But as most of us know, it tends to happen over the weekend, when manpower is minimal, and if you don't buy all the bells and whistles that are available to alert you of your failed hose, you may have learned only after draining the tank of your chemical. While you can't predict exactly when your tube/hose will fail, once it does, provided your application really doesn't change much, you have a pretty solid benchmark timeline of when you should schedule your maintenance. I have outfitted some pumps with counters to count revolutions to account for the number of times the hose has been compressed and assisted with creating maintenance schedules based on that.
Peristaltic pumps are limited in the available hose and tube options and the compatibility with your chemical. While some tubes offer great chemical resistance, they are limited in your flow rates. And the larger the pump, the fewer and fewer options you have for material choices.
Dosing volume will change over time. Due to wear of the tube or hose and the hysteresis it will experience, there will be a change in volume pumped per revolution. If dosing accuracy is a concern, this will require a VFD to be able to adjust the speed to maintain consistent flows over time.
I have heard sales people suggest that pulsation dampeners are not required with peristaltic pumps. That really depends on your system and what size pump we're talking about. Peristaltic pumps are not free from pulsation. They can pulsate as bad, if not worse, than many AODD pumps or diaphragm style metering pumps. To achieve less pulsation, you have to run the pump much slower, which means you may be over sizing your pump to achieve the reduced pulsation. Once again, depending on your system, you could be spending more for the larger pump, than you would for the smaller pump and pulsation dampener.
Peristaltic pumps have pressure and temperature limitations as well. The max pressure any hose pump can do is limited to 232 psi or 16 bar. And the hose is limited to temps up to 180*-210* depending on what material is selected. While 232 psi is the max any hose pump can accomplish, many max out in the 60 to 116 psi range. You may think that is not really a concern when pumping sodium hypo, the example we have used from the start while discussing liquids that gas-off, you'd be surprised as more and more municipalities are having to dose at their booster stations in order to meet residual requirements in the lines furthest from their treatment facilities. And some times, a requirement like that, dictates the need to look at a diaphragm style metering pump. We'll discuss more about that next week.
Peristaltic pumps can be an excellent choice for you when handling liquids that off-gas, just be sure you do your due diligence to determine whether it's right for you! If you are contemplating whether it's the right choice for you, feel free to call me @ 815-412-5683 to help review your application, look over your system, and determine what available options you may have.
Steve Shapiro
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