(a.k.a. “The Pump Guy”)
In the first installment of my “Filters vs. Strainers” series (Feb. ‘07, page 30), I exclaimed, “Help!! I want to know the difference between a filter and a strainer.” In the May issue (page 38), the readers responded, but the confusion continued. Now, as promised, I present the results of my much-hyped interview with the “Guru of Filtration & Separation” in the hope of finally shedding some light on the Filters vs. Strainers debate.
Due to contractual obligations, the self-proclaimed “Guru of Filtration & Separation” will go by the name of Ken S. here. Ken has spent his life in the filtration and separation business and writes for a major filtration publication. I opened our conversation by recalling all the partial answers I had heard over the course of my search. (For a sampling of some of the more creative responses I’ve received, see my May column.) After listening carefully, Ken said, “You’ve met some funny people.”
I first asked, “What is the exact precise definition of a filter?” Ken said, “A filter is any device that separates solids from liquids or gases by passing the fluid suspension through a barrier (the filter medium), which removes all particles above a certain size and allows the fluid and any smaller particles to pass through the medium.
Ken said a strainer is defined by its purpose—to remove unwanted large particles from a liquid suspension, usually to protect some sort of downstream equipment, such as a pump or a more delicate filter, from the damage that these rogue particles might cause.
I responded, “Ken, you just said that strainers and filters both separate solids from fluids. What is the difference? Where does one end and the other begin?” Ken said, “All strainers are filters, but all filters are not strainers. A strainer is only one type of filter.”
I asked about particle size. Ken said, “There is no hard and fast size division to define strainers from filters. I would expect them to go down to say 0.025”. They work by surface filtration. Surface filtration means that a particle sits in a hole in the medium until it is removed by cleaning. Surface filtration is only one of the mechanisms by which filtration occurs.”
|The correct design for suction filtration/straining calls for the screen to be located in the wet well.|
The main difference seems to be in resistance. He said strainers normally present low resistance to liquid flow. The pressure drop across most strainers is relatively small compared with the pressure drop across thick media filters or membrane filters.
Another difference is the function. A strainer is used to protect other downstream equipment (e.g., pumps, instrumentation) from damage by rogue junk. A filter is employed to separate particles from the fluid. This is why I say that strainers are a type of filter.
The interview was going well. Ken’s answers were good information. Then it became evident that Ken and I were from different worlds. I said, “Ken, you just said that strainers protect downstream equipment (instrumentation, heat exchangers, etc). I perceive that many process strainers and filters are installed into the suction piping leading to a pump. Why is this?”
Ken said, “Most strainers, whose function is to protect downstream equipment, are indeed mounted at the pump inlet. Their location is where that protective function is most effectively achieved, and where the pressure drop across the strainer will have least impact on the whole system, and where the strainer can easily be accessed for cleaning.
“And some filters operate under vacuum (behind the filter) rather than high pressure in front of the filter,” he said. “The adjacent pump suction provides that vacuum. But you generalize too much. Filters are employed where they are needed. All filters are not close to the pumps.”
I consulted the owner’s manual of a prominent pump company. The manual states: “DO NOT RESTRICT THE FLOW INTO THE PUMP. Do not erode the NPSHa > NPSHr margin. Never throttle a pump on the suction side.”
People, what do strainers and filters do to the flow into a pump?
The owner’s manual continues: “Strainers are not recommended on the pump inlet. If considerable foreign matter is expected, a screen installed at the entrance to the wet well is preferable.”
Install the resistance (strainer or filter) on the discharge side of the pump if you’re concerned about protecting the equipment. If the fluid entering the pump carries particles and solids, it’s better to prepare the pump internals for abrasives (improved metallurgy, rubber lining, ceramic coating, etc.), and install a slurry seal. Why do you think these options exist? DUH!! Then design the pump with additional head to handle the energy consumed by the downstream filter or strainer, and protect everything leading to the next process.
Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control. He is a pump consultant, lecturer and inventor based in Nashville, Tennessee. Mr. Bachus is a member of ASME and lectures in both English and Spanish. He can be reached at firstname.lastname@example.org or 615-361-7295.