Cover Story: Pumps & Valves
The small size of this valve by Parker Hannifin reduces the travel distance of moving parts.
Pumps and valves do vital work in many different medical devices and manufacturing operations. Of course, their work wears them down—and eventually wears them out. But in many cases, they're able to fend off wear and tear for many years and millions of cycles.
What's being done to extend the lives of these crucial components? Using innovative designs and hardy materials, manufacturers are equipping pumps and valves for lengthy battles against wear and tear. For their part, users are promoting component longevity by making the work that pumps and valves do less damaging and less stressful. And when they work together, manufacturers and users of pumps and valves can come up with long-lasting solutions for even the most wearing fluid- and air-moving problems.
Wear-Fighting Valve Designs
This valve, by I&J Fisnar Inc., applies seals to housings of medical devices. Designing valves with only one or two moving parts can reduce wear.
Ask valve manufacturers how they minimize product wear and you get a variety of answers. One technique is to shorten the travel distance of moving parts. Inside the miniature valves made by Parker Hannifin's Pneutronics Div. (Hollis, NH), parts usually move no more than 0.02 in., reports market development manager Mark Garland.
Another wear-reducing strategy is to use simple designs. For example, most of the dispensing valves made by I&J Fisnar Inc. (Fair Lawn, NJ) have only two or three moving parts, says Vladimir Siroky, president of the company. These products are primarily used in device manufacturing operations.
In addition, some valve makers try to minimize the number of parts that come in contact with the medium flowing through the valve. These wetted parts are particularly prone to wear, according to Jim Victoria, application development manager for EFD Inc. (Lincoln, RI), which also makes dispensing valves used in medical device manufacturing. In EFD's diaphragm valves, Victoria notes, only the diaphragm and sealing head come in contact with the fluid being dispensed.
Like these EFD valves, Parker Hannifin's Liquid X valve has only two wetted parts. Pneutronics designed this miniature valve so that the diaphragm protects the stainless-steel actuator from the medium. The diaphragm isolation design helps the valve last for 10 million cycles, according to Garland.
Some of EFD's valves include packings that separate the fluid and air chambers. Made of soft Teflon material, the packings form a tight seal around the piston to prevent leakage from one chamber to the other. Teflon is compatible with more chemicals than most plastics, Victoria adds, lessening the risk of deterioration when the material is exposed to fluids in dispensing operations.
Wear can also be minimized by a valve design featuring tight tolerances and carefully balanced loads, says Ralph Buck, electrofluidic systems product manager for The Lee Co., a valve and pump manufacturer in Westbrook, CT. Valves that use a lot of force to open and close can overcome high levels of friction, so the design does not have to be highly engineered, Buck explains. But valves wear faster when excessive force is used to drive the valve seat into
the elastomer seal. “It's like hitting a nail with a sledgehammer,” he says.
The Right Material
Electronically commutated dc motors by KNF Neuberger Inc. provide speed control, allowing the pumps to be brought up to speed slowly.
A key tool in the battle against wear is material selection. Manufacturers can use a variety of metals, plastics, and rubbers to make valve components. In part, the choice depends on which material holds up best when exposed to a particular medium. For example, Siroky notes, manufacturers might choose ultra-high-molecular-weight plastics for the diaphragms of valves used to dispense aggressive cyanoacrylates, or so-called crazy glues.
Or an application may involve anaerobic fluids, which crystallize or harden when they come in contact with metal ions. Valves used to dispense such fluids must have wetted parts made of plastic, Siroky says.
Then there are UV resins, which are used in the manufacture of a number of medical devices. According to Siroky, these resins cause a reaction if they contact aluminum, so they're often dispensed using valves with stainless-steel bodies.
Polycarbonate is a popular choice for many medical device components. But lipids take a toll on many polycarbonate formulations, warns Jim Pisula, vice president of marketing for Value Plastics Inc. (Fort Collins, CO), which makes plastic tubing connectors. So for applications involving lipids, valve manufacturers must be sure to use a lipid-resistant version of polycarbonate. Another alternative, Pisula notes, is to eschew polycarbonates in favor of acrylics or other materials characterized by native resistance to lipids.
For many valves, leakage is a common failure mode. Valve leakage can often be traced to the seal, says Buck. So to lengthen valve life, manufacturers can opt for a durable seal material. For example, they might select Viton rather than silicone.
Manufacturers can also boost seal durability with a coating. Halkey-Roberts Corp. (St. Petersburg, FL) makes valves that go into sterilizers for colonoscopes. These valves are exposed to peracetic acid, which makes silicone seals sticky and gummy over time, explains Lew Lecceardone, the company's vice president of medical marketing. So the seals are coated with a special oil that protects them from peracetic acid. This helps to keep the valves operating efficiently in sterilizers for years.
Some EFD valves also have coated components. For example, Victoria notes, the company reduces corrosion inside its spool valve by coating wetted parts with titanium. Similarly, Garland says, Pneutronics uses passivation treatments to prevent corrosion of stainless-steel valve components.
In addition to valves, some suppliers offer extras that help OEMs reduce wear in fluid- and air-carrying systems. For example, EFD provides feed lines and fittings made of materials that are compatible with customers' media. The company has also introduced a device that controls the temperature inside dispensing equipment to minimize fluctuation of fluid viscosity. According to Victoria, the ProcessMate 6500 reduces the frequency of manual valve adjustments to compensate for viscosity changes, thereby reducing valve damage and wear caused by operator handling of the components.
KNF Neuberger Inc. uses stainless-steel or aluminum heads to make corrosion-resistant pumps.
Pump Makers Battle Wear
Like valve manufacturers, pump makers use a variety of techniques to battle wear. Inside WOB-L pumps made by Rietschle Thomas (Sheboygan, WI), the longer the stroke, the more a pump's connecting rod will wobble. This makes the pump work harder, which increases wear and tear on the piston seal, explains David Droege, the company's miniature-products support manager. Most pumps manufactured at the firm's Sheboygan facility are custom made. So, depending on the application, the company could design a WOB-L pump with a shorter stroke length to reduce wear, Droege says.
At KNF Neuberger Inc., based in Trenton, NJ, pump diaphragms are designed using finite-element analysis. The computer program “helps us develop the most robust design possible,” says David Vanderbeck, the company's business development manager. For example, Vanderbeck says, the program showed designers where to add structural elements on the underside of a diaphragm to boost strength without too much of an effect on the flexibility of the component.
Reinforced diaphragms are also used in miniature pumps made by Parker Hannifin's T Squared Pumps unit (Fairfield, NJ). In some cases, the firm molds webbing or matting into rubber diaphragms to boost their resistance to tensile loads. In high-pressure applications, the reinforced elastomer diaphragms last longer than a standard diaphragm would, notes Len Prais, engineering manager at T Squared Pumps. But in low-pressure applications, he adds, a standard diaphragm might last just as long as a reinforced one and also operate more efficiently because it would produce less internal friction.
Besides reinforcing diaphragms when necessary, Prais and his colleagues try to eliminate restrictions in their pumps that cause parasitic losses. These losses can add loads to the diaphragm and other components, thereby shortening pump life. According to Prais, one way to reduce harmful restrictions is to use special valve designs that minimize backpressure inside the pumps.
In many cases, pump life can also be lengthened by effective cooling. This is true in oxygen concentrators that use Rietschle Thomas WOB-L pumps. Enclosed in a case, these WOB-L units are cooled by two fans, one on either side of the pump. Each application is different in how and where the pump is mounted, as well as in the amount of room available to move air around the pump. So Rietschle Thomas engineers work with each OEM to produce a cooling design that will maintain the proper ambient temperature around the pump, Droege says.
Rietschle Thomas can work with OEMs to design a pump, like this miniature diaphragm dc compressor, that will maintain the proper temperature to avoid wear.
In addition, Droege notes, the company's engineers pay attention to the material used to make piston seals in the pumps. The seals include a Teflon substrate that can be supplemented with proprietary polymer materials that lessen wear and boost seal life.
To make pump heads, KNF uses a variety of materials, with the choice depending in part on the medium. Stainless steel is fine for many applications, but others might call for a plastic material. KNF has had good luck with Ryton, an expensive plastic that offers good chemical compatibility, Vanderbeck says.
According to Droege, heat generated by pumps produces moisture that will quickly degrade aluminum pump heads and valve plates. So many manufacturers treat these parts with dichromate. However, Teflon-impregnated parts last longer in high-humidity environments than those treated with dichromate, Droege explains. So sometimes the company impregnates metal components with Teflon. Some firms also use Teflon to help preserve pump diaphragms. For example, diaphragm pumps are used in diagnostic equipment that handles many different reagents. To prevent these reagents from attacking rubber diaphragms, Vanderbeck says, the elastomeric components can be coated with a protective Teflon layer.
The User's Role
Broadly speaking, there are two major factors that affect the life span of pumps and valves: how they're made and how they're used. No matter how well a component is made, its life will be short if users don't do their part to minimize wear and tear.
For starters, Victoria says, users should read all manuals and instructions that come with a component and follow the manufacturer's recommendations. This may seem obvious, but many users simply don't do it, Victoria maintains. Failing to follow instructions “is probably one of the major reasons for wear and tear in our equipment,” he says. In many cases, he notes, rapid and excessive wear is caused by easily avoidable mistakes such as incorrect setup and inappropriate use.
Some users who don't read and follow instructions make disastrous mistakes even when they are trying to do the right thing. For instance, Victoria points to people who try to clean a valve used to dispense cyanoacrylate by flushing it with water. Instead of cleaning the valve, Victoria says, flushing with water causes the glue to harden inside the component.
Although improper maintenance can lead to disaster, proper maintenance can be crucial in extending component life. In silicone-dispensing systems, for example, small amounts of material may get cured inside a valve. Eventually, Siroky says, this buildup of cured material will impede the action of the valve. So he recommends that users periodically take the valve apart, clean it, and make sure all the components are moving freely before reassembling it.
As for pneumatic systems, air containing rust or dirt particles will eventually break down valve seals. To prevent such problems, Siroky recommends filtration of air flowing into pneumatic components. Filters should be installed at several points along a pneumatic line, including the point just before air flows into a valve.
According to Garland, filtration is particularly important in systems that include miniature valves because of the vulnerability of the small components and sealing surfaces in these valves. Generally speaking, Pneutronics recommends a minimum 40-µm filtration level at the source. But Garland says that a micron rating of 10 or less is even better at reducing the risk of contamination, which is one of the main causes of valve failure. On the downside, he notes, a 10-µm filter may have to be significantly larger than its 40-µm counterpart to prevent unacceptable flow restriction in a system.
In liquid-moving systems, debris can get trapped between the valve seat and seal, Buck says. To prevent this and the resulting valve leakage, Lee provides last-chance safety screens that filter liquids flowing into valve components. Besides harming valves, Buck says, liquid-borne debris can scratch the seal or piston inside a pump, causing leakage. So users must also filter the liquid flowing into pumps. The filters must be changed regularly to prevent dirt buildup on them that will add to the load on the pump, Vanderbeck notes.
Another way pump users can reduce wear and tear is to pay careful attention to mounting. For example, Droege says, some Rietschle Thomas pumps should only be mounted in a horizontal plane. Vertical mounting of these pumps adversely affects the motion of the internal components, which can result in damage that will shorten pump life.
In many cases, pumps are mounted on shock mounts that lessen the vibration of the unit. According to Droege, the type of shock mounts a user selects can have a major effect on how much a pump will shake while in operation. So Rietschle Thomas helps customers through the process of determining which shock mounts are right for their applications.
Temperature control units, like the ProcessMate from EFD Inc., can reduce valve wear and tear by keeping a material at optimum dispensing temperature.
To minimize pump and valve wear and extend component life, users should look beyond the components themselves and consider other issues. For instance, what type of fluid is in the system? If it's prone to crystallization, it's important not to let the system dry out, Buck warns. Drying can result in the formation of crystals that lodge in components and damage them. In pinch valves, for example, crystals can make small cuts in the rubber, creating weak spots that will be prone to failure, Buck says.
To prevent damaging crystallization, Buck recommends that OEMs either keep fluids in these systems at all times or flush them out with water or some kind of cleaning solution following each use. Systems can remain dry once the crystal-precipitating liquids are cleaned out of them, he says. But don't make the mistake of trying to clean out a system with air. This will evaporate the liquid, but leave behind the
Another issue to consider is how an air- or fluid-moving system is powered. According to Vanderbeck, one option worth considering is voltage shaping. As the name implies, this involves shaping the profile of the voltage applied to the motor. Voltage shaping requires a logic-control device that varies the supplied voltage, which in turn changes the speed of the pump.
Instead of turning on the system voltage all at once, Vanderbeck notes, voltage shaping allows users to ramp up the voltage from zero to a maximum operating level, so pumps can be brought up to speed slowly. Voltage shaping also allows control of pump speed so that output matches the requirements of the system in a particular situation. “If you don't need the full output of the pump, you can slow it down so you're just putting out what the system requires—no more,” he explains.
Since the pump isn't constantly running at full speed, the pump and other components are subjected to less pressure and flow, thereby reducing wear and tear throughout the system. As a result, Vanderbeck says, voltage shaping can result in a “tremendous increase” in component life.
William Leventon is a freelance writer who contributes frequently to MD&DI.