|Download this article in .PDF format
This file type includes high-resolution graphics and schematics when applicable.
With apologies to Shakespeare, “To use a suction screen, or not to use a suction screen, that is the question.”
We don’t come across many controversies in fluid power, and this is the biggest controversy I’ve seen in my 30 years (as of this month) at Hydraulics & Pneumatics. (It’s also the first time I’ve used that bad of a cliché in anything I’ve written.)
This is one of those arguments that may never reach a consensus. One side of the argument holds that you should always use a suction strainer to keep the pump from sucking in large objects like screws, washers, nuts, and any other foreign object that finds its way into a hydraulic reservoir. Suction strainers are inexpensive, so you’d be foolish not to make this small investment to protect the hydraulic system’s most vital—and expensive—component.
That’s a strong argument. But the opposing view says that anyone who doesn’t take the little effort to keep stuff like screws and other foreign objects out of the reservoir is the same type of person who never goes through the trouble of removing, cleaning, and replacing the strainer. A clogged strainer will restrict inlet flow to the pump, eventually causing cavitation and premature failure. Worse yet, if a technician replaces a failed pump and doesn’t clean the strainer before starting up the new pump, it will also fail prematurely from cavitation.
I’d like to propose a compromise. The argument against using a suction strainer is that it can become clogged over time, eventually causing premature pump failure if no action is taken. However, many strainers have a bypass option that redirects hydraulic fluid right into the pump if a negative pressure (partial vacuum) exists in the suction line. The bypass often is used if the pump will be started when the hydraulic fluid is cold. Cold fluid can be too viscous to freely flow into the pump; the bypass allows fluid to pass directly into the pump until the fluid warms up.
The same thing happens when a clogged strainer prevents fluid from flowing freely into the pump. The restriction to flow caused by the clogged screen creates a partial vacuum in the pump suction line. The differential pressure between the suction line and the static fluid pressure in the tank causes the bypass valve to open, thus avoiding cavitation.
This being the case, the suction strainer becomes a life insurance policy for the hydraulic pump. No one says you must have life insurance, but you probably do. And most states require all drivers to carry car insurance. That’s because tremendous amounts of money can be involved when the unexpected happens. Hmmm, sounds similar to what transpires when a hydraulic pump suddenly breaks down.