(a.k.a. The Pump Guy)
Help!! I want to know the difference between a filter and a strainer. I admit I don’t know the difference. I thought I knew the difference, but I realize now that I don’t. I admit the difference may not be that important, or bring about world peace, or solve the energy crisis. Nevertheless, I am on a quest, and I’m asking for your assistance. Please read further before sharpening your pencil.
I was lecturing on pumps a while ago at a municipal water treatment plant in California. We were talking about pipe and fittings. A student in the class asked me to clarify the precise difference between a strainer and a filter. I had no clear answer. I hate it when that happens. After all, I’ve been working with pumps, pipe, and fittings most of my life.
As a maintenance manager, I’ve bought strainers and filters. As a maintenance practitioner, I’ve installed filters and strainers. I’ve changed filter screens and strainer screens. I’ve backwashed and cleaned both as well.
What I think:
• I think strainers and filters both do just about the same thing.
• I think they both separate undesirable material from desirable material, but to different degrees.
• I think that sometimes the desirable material is collected in the filter media (screens or wire, cloth, paper, or other mesh).
• I think that sometimes the undesirable material is collected in the filter media, and the desirable material is allowed to pass through.
• I think strainers and filters cause headaches and maintenance problems with adjacent pumps.
• I think so many pump problems will disappear if design engineers give more thought before locating a filter or strainer at the pump suction nozzle.
What I want to know:
• Where does one (strainer or filter) end and where does the other begin?
• What differentiates filters from strainers?
• As a plant engineer, what situation suggests installing a filter? What situation suggests installing a strainer?
Most people I talk with interchange the two terms, saying filter when they mean strainer, and saying strainer when they mean filter. I’m fine with this, but there must be a difference.
For instance, there are laptops and desktop computers. They both do mostly the same thing. One is designed principally for portability, while the other is designed principally for operation in a fixed location. This distinguishing feature separates the two designs. A buyer would ask and answer certain questions before selecting one or the other.
Most men shave every morning. Some use an electric shaver, others a manual shaver. The end result is about the same, but there are some design, operation, and ritualistic differences between the two shaving methods.
Likewise, there must be some defining feature or function that separates a filter from a strainer. A plant or design engineer must ask and answer some questions before selecting one or the other.
If a company calls itself the "So-and-So Strainer Company," the people working there must know what a strainer is, and what it isn’t. Likewise, if a company calls itself the "This-and-That Filter Company," its people must know the guidelines and definition of this device. You’d think!! Not so!!
To begin my quest, I referenced two dictionaries (Webster’s and Oxford). They both used the verb form of one to define the other. They said filters are used to "strain," and that strainers are employed to "filter." I didn’t learn much there.
I went to see Germán, a learned friend and dean of the College of Industrial Design at a prestigious university. He teaches Industrial Engineering. Germán said, "Gosh Larry, that’s interesting. I never considered it before. Let’s consult the dictionary." Hmmmm! I enjoyed listening to the Keb’ Mo’ CD on Germán’s stereo.
A couple of years ago, a student enrolled in my pump class. He was developing a new strainer for industry. His strainer is on the market now. I called him next.
"Hey Wayne, what’s the difference between a strainer and a filter?" Wayne said, "It depends on the diameter of the holes in the media screen". So I asked, “What is the dividing point?" Wayne said, "I just told you, it’s the hole-diameter size."
I sought clarification. "Well Wayne, is it 1⁄16"? If the hole diameter is greater than 1⁄16", it is classified a strainer, and smaller than 1⁄16" is classified as a filter?" He said, "Smaller." I asked, "Is it 1,000 microns?"
Wayne said, "Look Larry, it’s simple — strainers are for trapping large junk and filters are for trapping small junk." I said, "Could you define large and small?" Wayne said, "Well, a strainer would be used to trap a lost bolt or screw in a pipe."
I asked, "Would you use a strainer to trap a nanotech bolt or screw?" Now agitated, Wayne said, "Why do you want to know this, Larry? Besides, I have to go to a meeting." And he’s the president of a strainer company.
I called a filter company. The staff engineer said the principal difference between the two is that a strainer has screens that can be cleaned and reused. The filter screen is only used once and must be changed when it clogs.
I said, "How can that be the difference? Your company makes filters for residential swimming pools. You know many systems have a backwash feature that allows for cleaning and reusing the pool recirculation filter." The engineer said, "Uuuhhhhhhhh!!" The line went dead.
I called a big industrial supply catalog house (You know, the red book, about four inches thick). The application specialist said that a strainer traps the junk and let”s the good stuff go through. A filter traps the good stuff and permits the waste to pass through.
I thought, “Then why is the fuel filter on my car called a filter and not a fuel strainer?" I said, "Thanks. Goodbye," knowing this answer was not adequate.
I next contacted a company who makes those pharmaceutical grade ultra-pure filter papers. They sell little (four-inch) high-tech filter papers at 20 dollars each. The sales engineer said that a filter actually purifies the liquid or gas; a strainer just separates and traps the junk but doesn’t purify.
I was convinced that "purification" was the real difference between a filter and a strainer. I called Germán with the good news. He was convinced too. I went to the kitchen to make some coffee. As the java trickled into the decanter, I thought to myself, "If the coffee filter is a real "purifying" filter, then only pure, clean hot water should drip into the decanter below?" Damn!! Duped again by a fast talking salesman! It wasn’t the first time. Won’t be the last.
Someone else said that the strainer is a one-pass operation and a filter is a multi-pass operation. This is true with the oil filter on my car and with blood dialysis filters, but not true with fuel filters, which are one-pass operations.
Someone else said that a strainer has screens that can be changed and adjusted up or down depending on the particle size one wants to trap, but a filter”s screens are fixed and cannot be changed unless you change the filter cartridge. Uuhhhhh. More sales speak!
Next, I traded a debt for influence and got an audience with the "Guru" (or so they say) of filters and strainers. We had a good talk and it was good to peer into a different world. That exchange will appear in Part 2 of "Filters vs. Strainers," which will appear in an issue of Flow Control later this year.
Meantime, if you have any insight on the difference between filters and strainers, please e-mail me at firstname.lastname@example.org.
Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a member of ASME and lectures in both English and Spanish. He can be reached at email@example.com.