Seals alone won't take high-pressure pumps to the next level
For the past three decades, there has been a steady increase in the demand for liquid-chromatography (LC) pumps that can handle higher pressures. Users say higher pressures improve sample resolution and throughput rates, while manufacturers are keen to launch morecompetitive, cutting-edge devices.
Until recently engineers, working with seal suppliers, have been able to satisfy these requirements, and the increasing pressures have led to instrument and performance improvements of nearly 1,900% since the mid- 1970s. As a result, countless life-improving drugs and diagnostic processes have been developed.
But over the past two or three years, the race for everhigher pressures has outpaced the industry’s ability to solve the complex challenges of generating and maintaining them.
Today, LC engineers face the formidable task of operating pumps reliably and consistently at and above 20,000 psi. And due to limitations in material capabilities and current pump designs, they can no longer rely solely on a seal to get them there.
The pressure progression
Increases in LC-pump pressure have been evolutionary, not revolutionary. Even though the science of LC dates back to the early 1900s, pressure wasn’t a critical component in the equation until 1970, when the late Professor Csaba Horváth of Yale Univ. first introduced high-pressure liquid chromatography.
Fast-forward to 1982, and HPLC devices were operating at 3,000 psi. Almost 10 years later, they reached the 8,000-psi milestone. Further strides led to the 2004 introduction of UHPLC and instruments generating pressures up to 15,000 psi. Adjustments to these designs let pumps reach their current operating ceiling of 17,000 to 19,000 psi.
Now, in pursuit of even higher resolution and productivity, the industry has its sights set on the next big pressure milestone: 20,000 psi and above.
To better comprehend what today’s LC pump engineers are up against, it’s helpful to think about how the 20K+ barrier equates to other applications. In a subsea environment, for example, you’d have to descend nearly 8.6 miles before encountering this kind of pressure.
The reciprocating pistons that generate this pressure in an LC pump can also see some pretty rough treatment. They can be small (less than 2 mm in diameter), but their performance is similar to that of pistons in internal-combustion engines. In regular service, they are expected to deliver over 2 million leak-free cycles.
Beyond the seal
While the increasing pressures of LC have never been a simple challenge, the new requirements have stretched existing designs to their limits. The life-sciences industry can no longer expect to reach its pressure goals by focusing only on better seal designs and materials. Engineers must also consider changes to the pump and seal together. Some items being looked at include:
• Piston (plunger) diameter, material, and surface finish.
• The connection between the plunger and its drive mechanism.
• Plunger alignment during travel.
The number-one requirement is delivering accurate flow rates, and this goal is what determines the plunger diameter, stroke length, and speed. This is an area where sealing and pump-operating requirements compete. There’s a fine line between conditions for best sealing and the plunger performance needed to build cylinder pressure. Here’s how to address some key considerations.
Pistons are typically driven by a rotating cam or linear actuator. These drives push the piston forward within the pump’s cylinder, building up pressure. When designing the drive mechanism, it’s crucial to consider how the plunger attaches (floating or fixed), and to compensate for pulsations during pumping. Speed and stroke length will vary plunger side loading, which can cause premature wear on the seal.
Synchronizing the pump linkage is also critical. In rotary- cam driven plungers, a cam follower rides along the periphery of the cam. The cam’s high spot (lobe) pushes the follower which, in turn, moves the piston forward. To retract the piston, a return spring may be used. If the plunger doesn’t return freely due to low spring force and high seal friction, the follower loses synchronization.
• Ensure the drive compensates for seal friction, and include a strong plunger-return mechanism to compensate for seal friction during aspiration strokes.
• When attaching the plunger to the mount, pay close attention to alignment.
A major cause of early seal failure is side loading on the plunger. This happens when the moving plunger transfers its load to the seal’s inside sealing surface. This off-axis plunger travel during the forward stroke physically changes the seal’s inside form, and can lead to catastrophic failure.
To avoid this, as mentioned above, use a design that allows the plunger ferrule to provide optimal alignment. Otherwise, in time, side loading will prematurely wear the seal material and increase the seal ID. And note that shortening the stroke will not always reduce side loading if the plunger isn’t properly aligned to the linkage.
Also, historically, LC pumps were designed with a large plunger size-to-stroke ratio (3:1, for instance). With the advent of ultrahigh pressure pumps, these plungers caused early seal and backup support failures. Therefore, a plunger size-to-stroke closer to 1:1 must be incorporated for the seal backup to absorb some of the remaining side load. We suggest using a solid linkage, and it’s important to keep it concentric to the wash body and pump head.
• Ensure tight concentric guidance (<0.05 mm) between the pump head and wash body. (UHPLC pumps have a high-pressure head and wash body. The wash body is the lower end of the cylinder assembly that has a chamber that “washes” the back half of the plunger.)
• Use high-modulus backup-support rings and guide rings.
Plunger size plays an important role in seal life, performance, and response to rapid pressure changes . Smaller plungers experience lower loads at ultrahigh pressures, but the seal must also be small yet capable of withstanding the pressure. This leads to limited flexibility, especially during aspiration (suction) and transition to the pressure stroke. Maintaining some flexibility is important for proper seal contact stress along the entire stroke.
Plunger diameters should be less than 2.1 mm (0.082 in.)
In addition to ensuring the plunger is well guided and concentric, minimize the stroke length. This is especially important during the pressure stroke when the plunger is most likely to side load the seal backup and wash-body seal.
When shortening the stroke, however, plunger speed must increase to meet specified flow rates. This, in turn, increases the system PV (pressure-velocity). To maintain long seal life, keep the pump’s PV within the limits of the seal material PV.
• Design with a 1:1 plunger diameter-to-stroke-length ratio.
• Ensure strong supporting linkage between the plunger ferrule and drive.
• Align pump PV with the seal material PV limit.
Materials Monocrystalline sapphire makes an excellent plunger material. It’s the best choice for contact with thermoplastic seal materials because it allows for better surface finishes and hardnesses. The adhesion factors between soft thermoplastic and hard sapphire will minimize leaks past the seal. This is critical when you’re trying to meet leakage rates in micro or nanoliters.
Zirconium ceramic (TZP) has also emerged as a choice for some low to moderate-pressure LC pumps. Although TZP’s hardness is similar to that of sapphire, its variability presents challenges when applied in HPLC. It’s available in many different types, and each type varies in grade, grain, physical structure and size, which can affect stability.
• Choose pure sapphire plungers.
• Specify extremely smooth surface finishes (<1 Ra).
• Specify hardnesses greater than 70 Rc.
Heat dissipation Frictional heat generated under pressure can dramatically hurt seal performance. Pump designs must dissipate this heat.
Pressure created in the forward stroke, or by the seal and its energizer, can elevate sealing contact stress at the plunger surface. Extreme effects in the forward stroke under ultrahigh pressure can produce contact stresses so great that the seal shears, causing wear or shedding. As a result, material can sometimes be seen in the frit (an inline fluid debris filter), tubing, and even around the seal energizer.
• Use an active wash system.
• Maintain a service PV within the seal material limits.
• Specify an ultrasmooth plunger surface finish (<1 Ra).
Currently, most pumps handle all media types — from low-viscosity cosolvents such as methanol to high-viscosity fluids such as disodium phosphate buffers, and everything in-between. Pumps aren’t typically designed for the same performance with every media, as most see only cosolvents during testing and approval. Seals perform better in low-viscosity fluid, so expect different results at higher viscosities.
Also note that degassing apertures can affect smalldiameter plunger seals. In HPLC, degassing of solvents is essential for proper operation. Otherwise, dissolved gases lead to problems like pressure fluctuations, inaccurate readings, and clogged components. Degassing using techniques like ultrasonication or filtration removes entrapped air bubbles from solvents.
• Control sealing contact stress by using low-viscosity media.
• Expect varying contact stress with high-viscosity media.
The effects of peripheral hardware such as fittings, columns, tubing, and check valves are important to consider when testing sealing performance, as they can be the source of inaccurate results. (A column, a filter used to purify mixtures by separation, is the actual device used for chromatography chemistry. It’s normally a high-pressure cylinder filled with specific-size glass beads.)
Check valves not built to handle the pressure, weak tubing connections, underrated columns, and pressure restrictors are just a few potential culprits. Engineers should verify fitting and tubing requirements carefully, as start-up leaks typically originate in peripheral components.
• Get tubes precut by your vendor — don’t cut them yourself.
• Use the right tools to attach and correctly torque fittings, or delegate this task to service professionals.
• Use properly rated columns and flow restrictors to test pressure characteristics.
Set and follow an established seal-installation process to ensure proper seal alignment inside the pump head and avoid seal damage.
Pump designers should adhere to the seal manufacturer’s suggested bore dimensions and ensure all sharp corners and burrs are removed prior to seal installation. A seal must “press fit” into and seat concentric with the bore. This is critical because improper installation and seating of the seal in the housing results in start-up failures and leaks.
Use approved installation tools and verify seal position and integrity.
Understanding leakage rates and where leaks might originate is crucial to establishing failure criteria for pump heads, check valves, fittings, pressure transducers, wash bodies, sensors, tubing, and other components.
It’s difficult to discern cause when you’re working with firmware and more than 10 conditions and hardware combinations. Often, what was initially identified as a seal failure turns out to be a false positive or negative.
• Identify potential failure conditions for each media type, as well as for each flow rate or speed.
• Follow a set procedure to detect and quantify leaks using sensors, volumetric changes, and pressure drop, and minimize manual data collection.
There are many ways to establish and correlate test criteria that simulate field conditions. Accelerated tests to determine overall pump performance won’t necessarily represent actual field use, but they may induce plunger behavior that will shorten seal life by side loading or increasing frictional heat.
Also, establishing back pressure to simulate normal pump flow through a column may skew performance requirements. The column provides the restriction, creating high downstream pressure in the pump cylinder. Results may skew because current designs have pressure limits. An alternative is capillary tubing with 0.002 to 0.003-in. ID. This micro high-pressure tubing creates suitable back pressure, and this pressure can be controlled by adjusting the tubing length. The tubing will not be affected by long-term pressures changes whereas, over time, the column beads will lose their original packing.
• Use capillary tubing for back pressure. It is possible to test with a column, though it’s difficult to find ones rated for pressures above 17.5K psi.
• Avoid needle valves. They typically fail or cause false readings above 10K psi.
HPLC pump design engineers may not realize how critical seals can be to overall pump performance. They may assume seals are like elastomer O-rings, which are quite common and simple. Thermoplastic sealing required for HPLC is much more complex in terms of deflection, contact forces, and material properties.
Because of this, pump designers should seek out well-informed seal suppliers with considerable HPLC experience. It will streamline the design process and eliminate frustration.
With the 20,000-psi operating target still just out of reach, leading LC instrument OEMs and pump designers are inviting seal manufacturers, once relegated to problem-solving and failure consultation, to participate in the pump-development process.
Choose a sealing partner with strong engineering and sales support; ask about experience in HPLC; and demand internal testing capabilities to verify product performance.
The perfect piston?
For instance, the use of a “ oating-piston” design one that allows the plunger ferrule linked to the drive system to oat for centering purposes is an ideal strategy to prevent side loading. This condition occurs when plunger movement unevenly transfers its load against the seal’s inside diameter.
Unfortunately, while the oating piston does an excellent job ensuring proper plunger alignment at installation, break-in, and even operation in low pressures, it permits too much vertical and horizontal movement at high pressures. As a result, it can end up becoming the cause of side loading, premature leakage, and failure.
For now at least, the best approach to meeting skyrocketing LC pressure demands is to design a xed drive system with a short plunger stroke length and a small shaft diameter. This combination will help maximize seal life and optimize LC pump output.