Deciding Whether to Install Suction Strainers on Hydraulic Pump Inlet Lines
A hungry donkey enters a barn in search of hay. Much to his delight, he discovers two identical haystacks, each on the opposite side of the barn. The donkey stands in the middle of the barn between the two piles of hay, not knowing which one to choose. Hours go by and then days, but he still can’t make up his mind. Unable to decide, the donkey starves to death.
This short parable comes from the French philosopher and logician Jean Buridan’s commentaries on Aristotle’s Theory of Action, and so the story is known as “Buridan’s ass.”
When he constructed this story, Buridan presumably chose a donkey because it is not renowned for intelligence. But having grown up on a farm, I can’t imagine any hungry animal dithering in such a way - not even a donkey.
Regardless, how smart donkeys really are is unimportant to the moral of the story, which is: not deciding has consequences, and options can be a blessing as well as a curse. Had the donkey found only one haystack inside the barn, there would have been no decision to make, and he wouldn’t have starved to death. But life is not that simple.
Back in 1941, philosopher Eric Fromm wrote a book called Escape from Freedom. In it he said that people in a modern democracy are beset not by a lack of options but by a dizzying abundance of them. This is even more so today. Figuratively speaking, we’re all Buridan’s ass, completely surrounded by haystacks. The options may be nice to have, but not deciding has consequences.
This of course applies on all levels of life, both personal and professional. In the maintenance and reliability game, there are always decisions waiting to be made, such as what type of oil to use, a planned change-out to call, which proactive maintenance tasks to do (on what machines and when), a component to be declared faulty, and so on.
Often such decisions must be made in the absence of perfect information, or worse, in the presence of conflicting information. An example that comes to mind is the installation of suction strainers on hydraulic pump inlet lines.
As you may be aware, I’m an advocate of not installing suction strainers and, in the majority of applications, removing and discarding them when they are installed. As a result, I’m always interested in any new information on this issue, especially if it’s anything official from a pump manufacturer.
The Importance of Protecting the Pump
What is the "heart" of the hydraulic system? Or, asked another way, which component would shut down the system if it suddenly failed? The answer is the pump. Without the pump, the entire system is dead. As in the human body, if the heart stops working, all functions cease. Thus, it makes sense to protect the pump in order to keep the system operating smoothly and efficiently. Sump strainers or suction strainers are effective types of filtration for this purpose. They can be installed in two locations. A simple strainer can be placed on the end of the inlet line submersed in the bottom of the reservoir, while a strainer within a housing can be installed in the suction line outside the reservoir.
One thing to guard against is having too much restriction in the suction line. Too fi ne a level of filtration could cause the pump to cavitate, which might cause additional prob-lems. Therefore, the suction line must always flow freely. To prevent cavitation, it is sometimes necessary to install a bypass valve along with the suction strainer to allow flow when the strainer is indexed. Suction strainers are commonly available with a built-in bypass valve. How do you know what level of filtration is needed for any given system? To answer this, consider the following:
- The type of pump being used (gear, vane or piston pump)
- The viscosity of the fluid
- The flow rate
- The system pressure
Because the level of filtration has an indirect effect on the suction strength, strainers range anywhere from 30 mesh (595 microns) to 200 mesh (74 microns). A filtration specialist can help determine the right mesh for your system. It generally is not recommended to use filtration any fi ner than 74 microns in the suction line.
To protect the system as a whole, proper filtration should be placed throughout the system as required by the application. Some manufacturers require the pump be protected to keep the warranty in effect.
Take a proactive approach to ensure your hydraulic system's proper operation and longevity. This includes careful system design, regular monitoring, fluid analysis and preventive maintenance.
Keep in mind that a suction strainer serves one job and one job only: to protect the pump. Suction strainers are an inexpensive and effective investment that will safeguard the "heart" of your hydraulic system and, in so doing, prolong its life.
- Donald C. Krause, Flow Ezy Filters
Making Tough Decisions
In his book, Predictably Irrational, Dan Ariely describes the inefficiency of having too many options as well as the problems posed by having to choose between two appealing possibilities.
“In fact, choosing between two things that are similarly attractive is one of the most difficult decisions we can make,” Ariely writes. “This is a situation not just of keeping options open for too long, but of being indecisive to the point of paying for our indecision in the end.”
On this note, the data sheet for Nachi PVS series variable volume piston pumps was recently brought to my attention. In the document, Nachi recommends the installation of a suction strainer: “Provide a suction strainer with a filtering grade of about 100 microns (150 mesh).”
To the best of my knowledge, Nachi is in a small minority of pump manufacturers that actually recommend the use of a suction strainer. Contrast Nachi’s recommendation with Bosch Rexroth’s. The data sheet for its SV-20 and SV-25 series variable vane pumps states: “Bosch Rexroth does not recommend the use of inlet suction strainers.”
That’s one pump manufacturer for and one against. Another manufacturer, Eaton-Vickers, appears to be sitting on the fence based on its Mobile Hydraulics Manual: “(Reservoir) outlet line strainers, also called (pump) inlet filters or inlet screens are very common. This may be more traditional than functional. They are intended to keep larger solid contaminants from entering the hydraulic system. A drawback is that they are quite inaccessible for service and cleaning. If they become restricted due to excess contamination, they can cause cavitation and damage to system pumps. A more current approach is to ensure clean fluid is maintained in the reservoir, precluding the need for an outlet line strainer.”
This piece of prose may well have been written by one of the company’s in-house attorneys. It seems this pump manufacturer wants you to make up your own mind. At least it acknowledges that the tradition of always installing a suction strainer is outdated.
Then there’s this from the Contamination Control Program Manual by Stauff, a company that does not manufacture pumps but does make suction strainers: “It is advisable to check with the pump manufacturer before any type of filter is fitted to the pump inlet line ... In general, suction strainers do not contribute to system cleanliness. The difficulty associated with changing strainers, and knowing if and when they are clogged (may result in pump damage).”
So if you take Stauff’s advice and check with the pump manufacturer, one says “yes,” another says “no” and a third says “you decide.” As is the case with a myriad of other equipment maintenance decisions, this one is your call, and it’s a call you have to make - one way or the other. As Buridan’s ass discovered, not deciding can result in the very same consequences you wish to avoid.
