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Breaking the barrier: why seals alone will not take UHPLC to 20 kpsi and above
FEATURE
Breaking the barrier: why seals alone will not take UHPLC to 20 kpsi and above Steven Twork, Market Manager for Analytical and Laboratory Automation Products, Bal Seal Engineering Inc, Foothill Ranch, California, USA This article advises designers of pumps used in liquid chromatography instruments to look “beyond the seal” for ways to meet industry pressure demands of 20 000 psi and above. It also looks at some of the reasons behind liquid chromatography pressure evolution and advocates a more holistic approach to pump design. American novelist and winner of the Pulitzer Prize Ellen Glasgow once wrote: “The only difference between a grave and a rut is their `iÃð»ÊÃÊÌ`>Þ½ÃÊi}iiÀÃÊ}À>««iÊ ÜÌ ÊÌ iÊÃÞÀViÌ}Ê«ÀiÃÃÕÀiÃÊvÊiÝÌ}ieration ultra high pressure liquid chromatogÀ>« ÞÊ1* ®ÊÃÌÀÕiÌÃ]ÊÌ iÞÊ>ÞÊv`Ê a valuable insight into this old, but accurate idiom. For the past three decades designers of critiV>ʵÕ`ÊV À>Ì}À>« ÞÊ ®Ê«Õ«ÃÊ >ÛiÊ been witnessing a steady increase in demand for machines that can handle higher pressures. Driving this demand are end users who say they need higher pressure to achieve better sample resolution and higher throughput À>ÌiÃ]Ê>`Ê>Õv>VÌÕÀiÀÃÊiiÊÌÊÃ>ÌÃvÞÊÕÃiÀÃ½Ê needs) who are asking engineers to build in more pressure capacity with the goal of launching the most competitive, cutting-edge technology.
To shatter the 20 000 psi barrier engineers will first have to break from the “rut” of traditional design approaches and sealing technology, and adopt a more holistic perspective that may include hardware design changes. If they do not, they may find it increasingly difficult to tell the difference between a rut and the “grave” prospect of significant market-share loss.
Outpaced -Êv>À]ÊÃÊ}`°Ê }iiÀÃ]ÊÜÀ}ÊÜÌ ÊÌ iÀÊ seal suppliers, have been able to meet the new requirements, and the increasing pressures have served to propel instrument technology and performance improvements of nearly 1900% ÃViÊÌ iÊ`£ÇäðÊÃÊ>ÊÀiÃÕÌ]ÊVÕÌiÃÃÊvi improving drugs and diagnostic processes have been developed. But over the past two or three years a new problem has emerged. The race for higher «ÀiÃÃÕÀiÃÊ >ÃÊÕÌ«>Vi`ÊÌ iÊ`ÕÃÌÀÞ½ÃÊ>LÌÞÊÌÊ ÃÛiÊÌ iÊV«iÝÊV >i}iÃÊvÊ}iiÀ>Ì}Ê>`Ê maintaining them. /`>Þ]Ê Ê«Õ«Êi}iiÀÃÊv>ViÊÌ iÊvÀ`>ble task of achieving reliable, consistent operation at 20 000 psi and above. And because of limitations in material capabilities and current pump designs, they can no longer rely solely on a seal to get them there.
Pressure progression VÀi>ÃiÃÊÊ Ê«Õ«Ê«ÀiÃÃÕÀiÊ >ÛiÊLiiÊiÛlutionary, not revolutionary. Even though the ÃViViÊvÊ Ê`>ÌiÃÊL>VÊÌÊÌ iÊi>ÀÞÊ£ääÃÊ pressure was not a critical component in the iµÕ>ÌÊÕÌÊ£Çä]ÊÜ iÊÌ iÊ>ÌiÊ*ÀviÃÃÀÊ
Ã>L>ÊÀÛ?Ì ÊvÀÃÌÊÌÀ`ÕVi`ʺ* »Ê>ÃÊ an acronym for “high pressure liquid chromatography”. >ÃÌvÀÜ>À`ÊÌÊ£nÓÊ>`Ê* ÊÃÞÃÌiÃÊ were operating at 3000 psi. Almost 10 years >ÌiÀ]ÊÌ iÞÊÀi>V i`ÊÌ iÊnÊäääÊ«ÃÊiÃÌi° Further strides led to the 2004 introduction vÊ1* ]Ê>`ÊÃÌÀÕiÌÃÊV>«>LiÊvÊ}iiÀ>Ì}Ê«ÀiÃÃÕÀiÃÊÕ«ÊÌÊ£xÊäääʫðÊ`ÕÃÌiÌÃÊ to these designs enabled pumps to reach their VÕÀÀiÌÊ«iÀ>Ì}ÊVi}ÊvÊ£ÇÊäääq£Êäääʫð Now, in its continuous pursuit of increased resolution and productivity the industry has ÃiÌÊÌÃÊÃ} ÌÃÊÊÌ iÊiÝÌÊL}Ê«ÀiÃÃÕÀiÊÌ>À}iÌÊqÊ 20 000 psi and above.
“Deeper” understanding
Figure 1. A typical sealing configuration for a liquid chromatography pump, with a spring-energised flange seal and back-up device in the primary plunger position, and a similar seal seated in the wash body to isolate solvent from the main pump head.
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Sealing Technology
/ÊLiÌÌiÀÊV«Ài i`ÊÜ >ÌÊÌ`>Þ½ÃÊ Ê«Õ«Ê engineers are up against it is helpful to think about how the 20-kpsi-plus barrier equates to other applications. In an oceanic environment, vÀÊiÝ>«i]ÊÞÕÊÜÕ`Ê >ÛiÊÌÊ`iÃVi`ÊÌÊ >Ê`i«Ì ÊvÊi>ÀÞÊn°ÈÊiÃʣΰnÊ®ÊLivÀiÊ encountering this kind of pressure. The reciprocating pistons used to create force Ê>Ê Ê«Õ«ÊV>Ê>ÃÊÃiiÊÃiÊ«ÀiÌÌÞÊÀÕ} Ê treatment (Figure 1). They can be very small (in some cases, less than 2 mm in diameter), but their performance is similar to that of a piston in an internal combustion engine. In Ài}Õ>ÀÊÃiÀÛViÊÌ iÞÊ>ÀiÊiÝ«iVÌi`ÊÌÊ`iÛiÀÊÛiÀÊ two million leak-free cycles.
April 2012
FEATURE
Looking beyond the seal 7 iÊÌ iÊVÀi>Ã}Ê«ÀiÃÃÕÀiÃÊvÊ Ê >ÛiÊiÛiÀÊ been a simple challenge, the new requirements >ÛiÊÃÌÀiÌV i`ÊiÝÃÌ}ÊÌiV }ÞÊÌÊÌÃÊÌð According to Michael Binder, an application engineer specialising in analytical equipment Ãi>}Ê>ÌÊ >Ê-i>Ê }iiÀ}]ÊÌ Ê,>V Ê California, it is becoming clear that the industry V>ÊÊ}iÀÊiÝ«iVÌÊÌÊÀi>V ÊÌÃÊ«ÀiÃÃÕÀiÊ}>ÃÊ by focusing only on seal designs or materials. `iÀ]Ê>ÊÓÎÞi>ÀÊÛiÌiÀ>ÊvÊ ÊÃi>Ê`iÃ}]Ê advises fellow engineers to consider changes that will enable the pairing of pump and seal to break the 20-kpsi barrier. In the course of his ÜÀÊÜÌ ÊÃiÊvÊÌ iÊÜÀ`½ÃÊL}}iÃÌÊ* Ê >`Ê1* ÊÃÌÀÕiÌÊ>iÀÃ]Ê iÊ >ÃÊVÀi>Ìi`Ê a comprehensive checklist of items for designiÀÃÊvÊ Ê«Õ«ÃÊÌÊVÃ`iÀ°Ê/ iÊÀi>`iÀÊvÊ this article summarises the contents of this list, including various recommendations.
Shaft diameter, material and surface finish When sealing for ultra-high pressures, Binder says the primary requirement is meeting accurate flow rates. This goal is what drives the plunger diameter, stroke length and speed. ‘This is an area where sealing and pump «iÀ>Ì}ÊÀiµÕÀiiÌÃÊÜÊV«iÌi]½Ê iÊÜ>Àð ‘There is a very fine line for best sealing and plunger reactions when building cylinder «ÀiÃÃÕÀi°½ "iÊ>ÀÊV>ÕÃiÊvÊi>ÀÞÊÃi>Êi>>}iÊÃÊ mechanical plunger side-loading (Figure 2). This happens when the plunger movement ÌÀ>ÃviÀÃÊÌÃÊ>`Ê>}>ÃÌÊÌ iÊÃi>½ÃÊÃ`iÊÃi>ing surface. To avoid this, Binder advocates a design that enables the plunger ferrule, linked to the drive system, to provide optimal alignment (Figure 3). In time, side-loading will prematurely wear the seal material and increase Ì iÊÃ`iÊÃi>}Ê`>iÌiÀ°Ê- ÀÌi}ÊÌ iÊ stroke will not always reduce side loading if the plunger is not aligned in the linkage properly. Binder also points out that plunger size plays an important role in seal life, performance and response to rapid pressure changes. ->iÀÊ«Õ}iÀÃÊiÝ«iÀiViÊÜiÀÊ>`ÃÊ>ÌÊ ultra-high pressures, but the seal also must be small, and must be capable of withstanding Ì iÊ«ÀiÃÃÕÀi°Ê/ ÃÊi>`ÃÊÌÊÌi`ÊviÝLÌÞ]Ê iëiV>ÞÊ`ÕÀ}Ê>ëÀ>ÌÊÛ>VÕÕ®Ê>`ÉÀÊ transition to the pressure stroke. Maintaining ÃiÊviÝLÌÞÊÃÊ«ÀÌ>ÌÊÊÀ`iÀÊÌÊiÃÕÀiÊ proper sealing contact stress (Figure 4 on page 10) along the entire stroke.
April 2012
For a plunger material, the application engineer recommends sapphire, a mono-crystalline material. ‘It is the optimal choice for contact with thermoplastic seal material, because it allows for a betÌiÀÊÃÕÀv>ViÊvà Ê>`Ê >À`iÃÃ]½ÊVÌÕi`Ê `iÀ° ‘This is critical when you are trying to meet i>>}iÊÀ>ÌiÃÊÊVÀÌÀiÃÊÀÊ>ÌÀið½ He adds that the adhesion factors of a soft thermoplastic and very hard sapphire material will minimise this condition. Binder also notes that zirconium ceramic (TZP) has emerged as the material of choice vÀÊÃiÊÜÊÌÊ`iÀ>ÌiÊ«ÀiÃÃÕÀiÊ Ê«Õ«Ã°Ê Although its hardness is similar to that of sapphire, its variability can present challenges Ü iÊ>««i`ÊÊ* °ÊÌÊÃÊ>Û>>LiÊÊ>ÞÊ different types, and each type varies in grade, grain, physical structure and size, which can affect stability.
Recommendations UÊ -«iVvÞÊiÝÌÀiiÞÊÃÌ ÊÃÕÀv>ViÊvà iÃÊ iÃÃÊÌ >Ê£Ê,>®° UÊ -«iVvÞÊ >À`iÃÃÊ}Ài>ÌiÀÊÌ >ÊÇäÊ,V° UÊ ÃiÊ«ÕÀiÊÃ>«« ÀiÊ«Õ}iÀð UÊ iÃ}Ê>Ê«Õ}iÀÊÜÌ Ê>Ê`>iÌiÀÊvÊiÃÃÊÌ >Ê Ó°£ÊÊä°änÓÊV ®°
Drive mechanism/linkage When designing the drive mechanism, Binder notes that it is crucial to consider how the plunger will be attached and to compensate for pulsation during the pumping operation. The
Figure 2. Side loading, a condition caused by poor plunger guidance, can lead to uneven seal wear and, eventually, catastrophic failure.
speed and stroke length will vary plunger-side loading, which can cause premature wear on the seal. -ÞV ÀÃ>ÌÊvÊÌ iÊ«Õ«Ê>}iÊÃÊ>ÃÊ critical, as the winner in the battle between variable speed and pulsation will be determined by Ì iÊ`ÀÛiÊiV >ðÊ-ÞV ÀÃ>ÌÊLiViÃÊ >ÊVViÀÊÞÊvÊÌ iÊ«Õ}iÀ½ÃÊÀiÌÕÀÊëÀ}ÊÃÊ insufficient because of seal friction.
Recommendations UÊ iÃ}Ê>ÊÃÕvvViÌÊ`ÀÛiÊÃÞÃÌiÊÌÊV«isate for seal friction. UÊ ÃÕÀiÊ«ÀiVÃiÊ>ÌÌ>V iÌÊLiÌÜiiÊÌ iÊ «Õ}iÀÊviÀÀÕiÊÌÊ«Õ}iÀÊÕÌÊqÊ>ÜÊÌÊ float (for centring purposes) as well as firmly withstand any vertical and or horizontal ÛiiÌÃÊ>ÃÊÃiiÊLÝʼ/ iÊ«iÀviVÌÊ «Ã̶½ÊÊ«>}iʣ䮰 UÊ VÕ`iÊ>ÊÃÌÀ}Ê«Õ}iÀÊÀiÌÕÀÊiV >ÃÊ to compensate for seal friction during the aspiration stroke.
Figure 3. “Anatomy” of a high pressure liquid chromatography pump.
Sealing Technology
9
FEATURE UÊ Ý«iVÌÊÛ>À>LiÊVÌ>VÌÊÃÌÀiÃÃÊÜ iÊÕÃ}Ê high-viscosity media.
Stroke length -ÌÀiÊi}Ì ]ÊÃ>ÞÃÊ `iÀ]ÊÃ Õ`ÊLiÊÃi`Ê and well-guided to ensure the plunger is concentric to the wash body and pump head bore throughout the pressure and aspiration stroke. This is especially important during the pressure stroke when the plunger is most prone to sideload the seal back-up and wash body seal. When shortening the stroke length to meet a desired flow rate, he says that the plunger speed must increase. This, in turn, increases the system PV. To maintain long seal life, he recommends keeping system PV within the limits of the seal material PV. Figure 4. Contact stresses (shown here in red) significantly impact seal performance in high pressure liquid chromatography applications. Experienced seal manufacturers will use failure modes and effects analysis models like this one to predict stress prior to production.
Heat dissipation Friction heat created under pressure can dramatically affect sealing performance. Because of this Binder advises pump engineers to design with dissipation in mind. He said: ‘Pressure created in the forward stroke, or by the seal and its energiser, can cause sealing contact stress to the plunger surface.
ÝÌÀiiÊivviVÌÃÊVVÕÀÊÊÌ iÊvÀÜ>À`ÊÃÌÀiÊ under ultra-high pressure, and in that moment the sealing contact stress is so great that seal material shear stress is overtaken, causing wear or shedding. As a result, material can sometimes be seen in the frit (inline fluid debris filter), the ÌÕL}ÊÀÊiÛiÊ>ÀÕ`ÊÌ iÊÃi>ÊiiÀ}ÃiÀ°½
Recommendations UÊ 1ÃiÊ>Ê>VÌÛiÊÜ>à ÊÃÞÃÌi° UÊ >Ì>Ê>ÊÃiÀÛViÊ«ÀiÃÃÕÀiÉÛiVÌÞÊ*6®Ê within seal material limits. UÊ -«iVvÞÊ>ÊÕÌÀ>ÃÌ Ê«Õ}iÀÊÃÕÀv>ViÊ và ÊiÃÃÊÌ >Ê£Ê,>®°
Plunger guidance Ê>ÀÊiV >V>Ê`Ã>`Û>Ì>}iÊÌÊ } pressure sealing is plunger side-loading. This vv>ÝÃÊÌÀ>ÛiÊvÊÌ iÊ«Õ}iÀÊÕ`iÀÊÌ iÊvÀÜ>À`Ê ÃÌÀiÊ« ÞÃV>ÞÊV >}iÃÊÌ iÊÃi>½ÃÊÃ`iÊvÀ]Ê and can lead to catastrophic failure. ÃÌÀV>Þ]Ê Ê«Õ«ÃÊÜiÀiÊ`iÃ}i`ÊÜÌ Ê a large plunger-size to long stroke-length ratio (for instance, 3:1). With the advent of ultrahigh pressure pumps these plungers caused early seal and back-up support failure. According to `iÀ]Ê>ÊÃ`Ê>}iÊÌÞ«iÊ>`ÉÀÊ«Õ}iÀÊÃâiÊ to stroke, which is closer to 1:1, should be used 10
Sealing Technology
in order for the seal back-up to absorb some of the remaining micro-side load. He suggests using a solid linkage, and notes that it is important to keep it concentric to the wash body and pump head.
Recommendations UÊ 1ÃiÊ>Ê`iÃ}ÊÜÌ Ê>Ê£\£ÊÀ>ÌÊ«Õ}iÀÊ`>eter to stroke length. UÊ ÃÕÀiÊ>ÊÃÌÀ}ÊÃÕ««ÀÌ}Ê>}iÊiÝÃÌÃÊ between the plunger ferrule and drive. UÊ }ÊÌ iÊÃÞÃÌiÊ*6ÊÜÌ ÊÌ iÊÃi>Ê>ÌiÀ>Ê PV limit.
Recommendations U ÃÕÀiÊÌ} ÌÊVViÌÀVÊ}Õ`>ViÊiÝÃÌÃÊ between the wash body and pump head (less Ì >Êä°äxÊ®° UÊ 1ÃiÊ>Ê } Ê`ÕÕÃÊL>VÕ«ÊÃÕ««ÀÌÊÀ}° UÊ 1ÃiÊ>Ê } Ê`ÕÕÃÊ}Õ`iÊÀ}°
Media viscosity Currently, most pumps are designed to handle >Êi`>ÊÌÞ«iÃÊqÊvÀÊÜÊÛÃVÃÌÞÊVÃÛiÌÃ]Ê such as methanol, to high viscosity media, such as disodium phosphate buffers, and everything else in between. Pumps are not typically designed to perform optimally under any given media, as during testing and approval most ÞÊiÝ«iÀiViÊVÃÛiÌð Binder says seals perform better in low viscosity media, and he advises fellow engineers to iÝ«iVÌÊ`vviÀiÌÊÀiÃÕÌÃÊÜ iÊÕÃ}Ê } iÀÊÛÃcosities. He also notes that degassing apertures can play a key role in the efficiency of very small diameter plunger seals. ‘Any trapped air starting within the seal-bore area and travelling downstream towards the flow restrictor will affect pump pressure read}Ã]½Ê iÊÃ>`°
Recommendations UÊ >>}iÊÃi>}ÊVÌ>VÌÊÃÌÀiÃÃÊLÞÊÕÃ}ÊÜÊ viscosity media.
The perfect piston? Designers of pumps for ultra high pressure liquid chromatography instruments know that there is seldom a solution for all seasons. Quite often an engineering approach that works under one condition can cause the same or similar problems under another. For instance, the use of a “floating piston” `iÃ}ÊqÊiÊÌ >ÌÊi>LiÃÊÌ iÊ«Õ}iÀÊviÀÀÕiÊ linked to the drive system to float for cenÌÀ}Ê«ÕÀ«ÃiÃÊqÊÃÊ>Ê`i>ÊÃÌÀ>Ìi}ÞÊvÀÊ«Àiventing side-loading, a condition that occurs when the plunger movement unevenly transviÀÃÊÌÃÊ>`Ê>}>ÃÌÊÌ iÊÃi>½ÃÊÃ`iÊ`>iÌiÀ° Unfortunately, while the floating piston `iÃÊ>ÊiÝViiÌÊLÊiÃÕÀ}Ê«À«iÀÊ«Õ}iÀÊ alignment at installation, break-in, and even operation at 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 liquid chromatography ®Ê«ÀiÃÃÕÀiÊ`i>`ÃÊÃÊÌÊ`iÃ}Ê>ÊvÝi`Ê drive system with a very short plunger stroke length and a smaller shaft diameter. This VL>ÌÊÜÊ i«Ê>ÝÃiÊÃi>ÊviÊ>`Ê «ÌÃiÊ Ê«Õ«ÊÕÌ«ÕÌ°
April 2012
FEATURE
Peripheral components It is important to consider the effects of periphiÀ>Ê >À`Ü>ÀiÊqÊvÀÊiÝ>«i]ÊV iVÊÛ>ÛiÃ]ÊVÕÃ]ÊvÌÌ}ÃÊ>`ÊÌÕL}ÊqÊ`ÕÀ}ÊÃÞÃÌiÊÌiÃÌing for sealing performance, as these elements can cause false positives during testing. Check valves not rated for proper pressure, weak tubing connections, underrated columns >`ÉÀÊ«ÀiÃÃÕÀiÊÀiÃÌÀVÌÀÃÊ>ÀiÊÕÃÌÊ>ÊviÜÊ«Ìitial causes. Binder suggests verifying fittings and tubing requirements carefully, as start-up leaks typically originate in peripheral components.
Recommendations UÊ ÃÊÞÕÀÊÛi`ÀÊÌÊ«ÀiVÕÌÊÌÕLiÃÊqÊ`ÊÌÊ cut them yourself. UÊ 1ÃiÊ«À«iÀÊÌÃÊÌÊ>ÌÌ>V ÊvÌÌ}Ã]ÊÜÌ Ê proper torque values, or delegate this task to service professionals. UÊ 1ÃiÊ>Ê«À«iÀÊÀ>Ìi`ÊVÕÊ>`ÊvÜÊÀiÃÌÀVtor to test pressure characteristics.
Seal installation Binder says that by setting and following an established seal installation process engineers can avoid seal damage and ensure proper seal alignment inside the pump head. This is critical because start-up failures and leaks can result vÀÊ«À«iÀÊÃÌ>>ÌÉÃi>Ì}ÊvÊÌ iÊÃi>ÊÊ the housing.
Figure 5. A technician in Bal Seal Engineering’s test laboratory with a line of high pressure liquid chromatography pumps.
UÊ ÃÌ>LÃ Êi>>}iÊLÞÊÃiÃÀÃ]ÊÛÕiÌÀVÊ changes and pressure drop programmatically, and minimise manual data collection.
ÜÌ ÊVÃ`iÀ>LiÊiÝ«iÀiViÊÊ* ÊÃÞÃÌiÃ°Ê Doing so, he says, will streamline the design process and eliminate frustration.
Test criteria
Recommendations
There are many ways to establish and correlate test criteria that simulate field conditions. An accelerated test to determine overall pump performance will not necessarily represent actual field use, Binder says, but it may induce plunger behaviour that will shorten seal life through side-loading or increased frictional heat. Also, establishing back pressure to simulate normal pump runs through a column will skew performance requirements.
UÊ ÃiÊ>ÊÃi>}Ê«>ÀÌiÀÊÌ >ÌÊ >ÃÊÃÌÀ}Ê engineering and sales support. UÊ ÃÊ>LÕÌÊiÝ«iÀiViÊÊ* ÊÃÞÃÌið UÊ i>`ÊÌiÀ>ÊÌiÃÌ}ÊV>«>LÌiÃÊÌ >ÌÊÜÊ enable product verification (Figure 5).
Recommendations UÊ 1ÃiÊ>««ÀÛi`ÊÃÌ>>ÌÊÌð UÊ 6iÀvÞÊÃi>Ê«ÃÌÊ>`ÊÌi}ÀÌÞ°
Failure criteria Understanding how much leakage is occurring and where potential leaks could originate is most crucial to establishing failure criteria for pump heads, check valves, fittings, pressure transducers, the wash body, sensors and tubing, among other areas, says Binder. ‘It is difficult to discern cause when you are working with firmware, multiple conditions iÝVii`}Ê£ä®Ê>`Ê >À`Ü>ÀiÊVL>ÌÃ]½Ê iÊ commented. ¼"vÌi]ÊÜ >ÌÊÜ>ÃÊÌ>ÞÊ`iÌvi`Ê>ÃÊ>ÊÃi>Ê v>ÕÀiÊÌÕÀÃÊÕÌÊÌÊLiÊ>Êv>ÃiÊ«ÃÌÛiÊÀÊi}>ÌÛi°½
Recommendations UÊ ÃÌ>LÃ ÊÕÌ«iÊv>ÕÀiÊV`ÌÃÊvÀÊi>V Ê i`>ÊÌÞ«iÊqÊ>ÃÊÜiÊ>ÃÊvÀÊi>V ÊvÜÊÀ>ÌiÊÀÊ speed.
April 2012
Recommendations UÊ 1ÃiÊV>«>ÀÞÊÌÕL}ÊvÀÊL>VÊ«ÀiÃÃÕÀi° UÊ Û`Êii`iÊÛ>ÛiÃÊqÊÌ iÞÊÌÞ«V>ÞÊv>Ê or create false negative and positive above 10 kpsi. UÊ /iÃÌÊÜÌ Ê>ÊVÕ]ÊvÊÌÊÃÊ«ÃÃLi]ÊÌ Õ} Ê it may be hard to find anything rated above £Ç°xʫð
Selection of a seal supplier ¼ iÜiÀÊ* Ê«Õ«Ê`iÃ}Êi}iiÀÃÊ>ÞÊÌÊ realise how critical seals can be to overall pump «iÀvÀ>Vi]½ÊÃ>`Ê `iÀ° ‘They may assume the seals are like elastomer "À}Ã]ÊÜ V Ê>ÀiÊÛiÀÞÊVÊ>`Ê >ÛiÊ>Ê simple geometry. Thermoplastic sealing is a whole different animal in terms of deflection, VÌ>VÌÊvÀViÃÊ>`Ê>ÌiÀ>Ê« ÞÃV>Ê«À«iÀÌið½ Because of this, Binder advises pump designers to seek out a well-informed seal supplier
Powerful partnerships 7Ì ÊÌ iÊÓäÊäää«ÃÊ«iÀ>Ì}ÊÌ>À}iÌÊÃÌÊÕÃÌÊ ÕÌÊvÊÌ iÀÊÀi>V ]ÊÃiÛiÀ>Êi>`}Ê ÊÃÌÀÕiÌÊ" ÃÊ>ÀiÊ}Ê>ÌÊÌ iÀÊVÕÀÀiÌÊ«Õ«Ê designs and considering many of the changes that Binder (and other engineers) have recommended. They are also inviting seal manufacturers, once relegated to problem-solving and failure consultation, to participate in the pump development process as active “innovation partners”. Acknowledging the challenges of reaching >`ÊiÝVii`}ÊÓäÊ«ÃÊqÊ>ÃÊÜiÊ>ÃÊÌ iÊii`ÊvÀÊ >ÊV>LÀ>ÌÛiÊÀiÌ }ÊvÊ«Õ«Ê`iÃ}ÊqÊ>Ê i}iiÀÊ>ÌÊ>Ê«ÀiÌÊÃ>Ê1* ÊÃÌÀÕment maker recently commented: ‘We were not getting any closer using established methods, so we started involving our seal supplier at a deeper level and, together, we began looking at the whole system. That is when we started to >iÊ«À}ÀiÃð½ Contact: Steven Twork, Bal Seal Engineering Inc, 19650 Pauling, Foothill Ranch, CA 92610, USA. Tel: +1 949 460 2160, Email: [email protected], Web: www.balseal.com
Sealing Technology
11
Breaking the barrier: why seals alone will not take UHPLC to 20 kpsi and above Steven Twork, Market Manager for Analytical and Laboratory Automation Products, Bal Seal Engineering Inc, Foothill Ranch, California, USA This article advises designers of pumps used in liquid chromatography instruments to look “beyond the seal” for ways to meet industry pressure demands of 20 000 psi and above. It also looks at some of the reasons behind liquid chromatography pressure evolution and advocates a more holistic approach to pump design. American novelist and winner of the Pulitzer Prize Ellen Glasgow once wrote: “The only difference between a grave and a rut is their `iÃð»ÊÃÊÌ`>Þ½ÃÊi}iiÀÃÊ}À>««iÊ ÜÌ ÊÌ iÊÃÞÀViÌ}Ê«ÀiÃÃÕÀiÃÊvÊiÝÌ}ieration ultra high pressure liquid chromatogÀ>« ÞÊ1* ®ÊÃÌÀÕiÌÃ]ÊÌ iÞÊ>ÞÊv`Ê a valuable insight into this old, but accurate idiom. For the past three decades designers of critiV>ʵÕ`ÊV À>Ì}À>« ÞÊ ®Ê«Õ«ÃÊ >ÛiÊ been witnessing a steady increase in demand for machines that can handle higher pressures. Driving this demand are end users who say they need higher pressure to achieve better sample resolution and higher throughput À>ÌiÃ]Ê>`Ê>Õv>VÌÕÀiÀÃÊiiÊÌÊÃ>ÌÃvÞÊÕÃiÀÃ½Ê needs) who are asking engineers to build in more pressure capacity with the goal of launching the most competitive, cutting-edge technology.
To shatter the 20 000 psi barrier engineers will first have to break from the “rut” of traditional design approaches and sealing technology, and adopt a more holistic perspective that may include hardware design changes. If they do not, they may find it increasingly difficult to tell the difference between a rut and the “grave” prospect of significant market-share loss.
Outpaced -Êv>À]ÊÃÊ}`°Ê }iiÀÃ]ÊÜÀ}ÊÜÌ ÊÌ iÀÊ seal suppliers, have been able to meet the new requirements, and the increasing pressures have served to propel instrument technology and performance improvements of nearly 1900% ÃViÊÌ iÊ`£ÇäðÊÃÊ>ÊÀiÃÕÌ]ÊVÕÌiÃÃÊvi improving drugs and diagnostic processes have been developed. But over the past two or three years a new problem has emerged. The race for higher «ÀiÃÃÕÀiÃÊ >ÃÊÕÌ«>Vi`ÊÌ iÊ`ÕÃÌÀÞ½ÃÊ>LÌÞÊÌÊ ÃÛiÊÌ iÊV«iÝÊV >i}iÃÊvÊ}iiÀ>Ì}Ê>`Ê maintaining them. /`>Þ]Ê Ê«Õ«Êi}iiÀÃÊv>ViÊÌ iÊvÀ`>ble task of achieving reliable, consistent operation at 20 000 psi and above. And because of limitations in material capabilities and current pump designs, they can no longer rely solely on a seal to get them there.
Pressure progression VÀi>ÃiÃÊÊ Ê«Õ«Ê«ÀiÃÃÕÀiÊ >ÛiÊLiiÊiÛlutionary, not revolutionary. Even though the ÃViViÊvÊ Ê`>ÌiÃÊL>VÊÌÊÌ iÊi>ÀÞÊ£ääÃÊ pressure was not a critical component in the iµÕ>ÌÊÕÌÊ£Çä]ÊÜ iÊÌ iÊ>ÌiÊ*ÀviÃÃÀÊ
Ã>L>ÊÀÛ?Ì ÊvÀÃÌÊÌÀ`ÕVi`ʺ* »Ê>ÃÊ an acronym for “high pressure liquid chromatography”. >ÃÌvÀÜ>À`ÊÌÊ£nÓÊ>`Ê* ÊÃÞÃÌiÃÊ were operating at 3000 psi. Almost 10 years >ÌiÀ]ÊÌ iÞÊÀi>V i`ÊÌ iÊnÊäääÊ«ÃÊiÃÌi° Further strides led to the 2004 introduction vÊ1* ]Ê>`ÊÃÌÀÕiÌÃÊV>«>LiÊvÊ}iiÀ>Ì}Ê«ÀiÃÃÕÀiÃÊÕ«ÊÌÊ£xÊäääʫðÊ`ÕÃÌiÌÃÊ to these designs enabled pumps to reach their VÕÀÀiÌÊ«iÀ>Ì}ÊVi}ÊvÊ£ÇÊäääq£Êäääʫð Now, in its continuous pursuit of increased resolution and productivity the industry has ÃiÌÊÌÃÊÃ} ÌÃÊÊÌ iÊiÝÌÊL}Ê«ÀiÃÃÕÀiÊÌ>À}iÌÊqÊ 20 000 psi and above.
“Deeper” understanding
Figure 1. A typical sealing configuration for a liquid chromatography pump, with a spring-energised flange seal and back-up device in the primary plunger position, and a similar seal seated in the wash body to isolate solvent from the main pump head.
8
Sealing Technology
/ÊLiÌÌiÀÊV«Ài i`ÊÜ >ÌÊÌ`>Þ½ÃÊ Ê«Õ«Ê engineers are up against it is helpful to think about how the 20-kpsi-plus barrier equates to other applications. In an oceanic environment, vÀÊiÝ>«i]ÊÞÕÊÜÕ`Ê >ÛiÊÌÊ`iÃVi`ÊÌÊ >Ê`i«Ì ÊvÊi>ÀÞÊn°ÈÊiÃʣΰnÊ®ÊLivÀiÊ encountering this kind of pressure. The reciprocating pistons used to create force Ê>Ê Ê«Õ«ÊV>Ê>ÃÊÃiiÊÃiÊ«ÀiÌÌÞÊÀÕ} Ê treatment (Figure 1). They can be very small (in some cases, less than 2 mm in diameter), but their performance is similar to that of a piston in an internal combustion engine. In Ài}Õ>ÀÊÃiÀÛViÊÌ iÞÊ>ÀiÊiÝ«iVÌi`ÊÌÊ`iÛiÀÊÛiÀÊ two million leak-free cycles.
April 2012
FEATURE
Looking beyond the seal 7 iÊÌ iÊVÀi>Ã}Ê«ÀiÃÃÕÀiÃÊvÊ Ê >ÛiÊiÛiÀÊ been a simple challenge, the new requirements >ÛiÊÃÌÀiÌV i`ÊiÝÃÌ}ÊÌiV }ÞÊÌÊÌÃÊÌð According to Michael Binder, an application engineer specialising in analytical equipment Ãi>}Ê>ÌÊ >Ê-i>Ê }iiÀ}]ÊÌ Ê,>V Ê California, it is becoming clear that the industry V>ÊÊ}iÀÊiÝ«iVÌÊÌÊÀi>V ÊÌÃÊ«ÀiÃÃÕÀiÊ}>ÃÊ by focusing only on seal designs or materials. `iÀ]Ê>ÊÓÎÞi>ÀÊÛiÌiÀ>ÊvÊ ÊÃi>Ê`iÃ}]Ê advises fellow engineers to consider changes that will enable the pairing of pump and seal to break the 20-kpsi barrier. In the course of his ÜÀÊÜÌ ÊÃiÊvÊÌ iÊÜÀ`½ÃÊL}}iÃÌÊ* Ê >`Ê1* ÊÃÌÀÕiÌÊ>iÀÃ]Ê iÊ >ÃÊVÀi>Ìi`Ê a comprehensive checklist of items for designiÀÃÊvÊ Ê«Õ«ÃÊÌÊVÃ`iÀ°Ê/ iÊÀi>`iÀÊvÊ this article summarises the contents of this list, including various recommendations.
Shaft diameter, material and surface finish When sealing for ultra-high pressures, Binder says the primary requirement is meeting accurate flow rates. This goal is what drives the plunger diameter, stroke length and speed. ‘This is an area where sealing and pump «iÀ>Ì}ÊÀiµÕÀiiÌÃÊÜÊV«iÌi]½Ê iÊÜ>Àð ‘There is a very fine line for best sealing and plunger reactions when building cylinder «ÀiÃÃÕÀi°½ "iÊ>ÀÊV>ÕÃiÊvÊi>ÀÞÊÃi>Êi>>}iÊÃÊ mechanical plunger side-loading (Figure 2). This happens when the plunger movement ÌÀ>ÃviÀÃÊÌÃÊ>`Ê>}>ÃÌÊÌ iÊÃi>½ÃÊÃ`iÊÃi>ing surface. To avoid this, Binder advocates a design that enables the plunger ferrule, linked to the drive system, to provide optimal alignment (Figure 3). In time, side-loading will prematurely wear the seal material and increase Ì iÊÃ`iÊÃi>}Ê`>iÌiÀ°Ê- ÀÌi}ÊÌ iÊ stroke will not always reduce side loading if the plunger is not aligned in the linkage properly. Binder also points out that plunger size plays an important role in seal life, performance and response to rapid pressure changes. ->iÀÊ«Õ}iÀÃÊiÝ«iÀiViÊÜiÀÊ>`ÃÊ>ÌÊ ultra-high pressures, but the seal also must be small, and must be capable of withstanding Ì iÊ«ÀiÃÃÕÀi°Ê/ ÃÊi>`ÃÊÌÊÌi`ÊviÝLÌÞ]Ê iëiV>ÞÊ`ÕÀ}Ê>ëÀ>ÌÊÛ>VÕÕ®Ê>`ÉÀÊ transition to the pressure stroke. Maintaining ÃiÊviÝLÌÞÊÃÊ«ÀÌ>ÌÊÊÀ`iÀÊÌÊiÃÕÀiÊ proper sealing contact stress (Figure 4 on page 10) along the entire stroke.
April 2012
For a plunger material, the application engineer recommends sapphire, a mono-crystalline material. ‘It is the optimal choice for contact with thermoplastic seal material, because it allows for a betÌiÀÊÃÕÀv>ViÊvà Ê>`Ê >À`iÃÃ]½ÊVÌÕi`Ê `iÀ° ‘This is critical when you are trying to meet i>>}iÊÀ>ÌiÃÊÊVÀÌÀiÃÊÀÊ>ÌÀið½ He adds that the adhesion factors of a soft thermoplastic and very hard sapphire material will minimise this condition. Binder also notes that zirconium ceramic (TZP) has emerged as the material of choice vÀÊÃiÊÜÊÌÊ`iÀ>ÌiÊ«ÀiÃÃÕÀiÊ Ê«Õ«Ã°Ê Although its hardness is similar to that of sapphire, its variability can present challenges Ü iÊ>««i`ÊÊ* °ÊÌÊÃÊ>Û>>LiÊÊ>ÞÊ different types, and each type varies in grade, grain, physical structure and size, which can affect stability.
Recommendations UÊ -«iVvÞÊiÝÌÀiiÞÊÃÌ ÊÃÕÀv>ViÊvà iÃÊ iÃÃÊÌ >Ê£Ê,>®° UÊ -«iVvÞÊ >À`iÃÃÊ}Ài>ÌiÀÊÌ >ÊÇäÊ,V° UÊ ÃiÊ«ÕÀiÊÃ>«« ÀiÊ«Õ}iÀð UÊ iÃ}Ê>Ê«Õ}iÀÊÜÌ Ê>Ê`>iÌiÀÊvÊiÃÃÊÌ >Ê Ó°£ÊÊä°änÓÊV ®°
Drive mechanism/linkage When designing the drive mechanism, Binder notes that it is crucial to consider how the plunger will be attached and to compensate for pulsation during the pumping operation. The
Figure 2. Side loading, a condition caused by poor plunger guidance, can lead to uneven seal wear and, eventually, catastrophic failure.
speed and stroke length will vary plunger-side loading, which can cause premature wear on the seal. -ÞV ÀÃ>ÌÊvÊÌ iÊ«Õ«Ê>}iÊÃÊ>ÃÊ critical, as the winner in the battle between variable speed and pulsation will be determined by Ì iÊ`ÀÛiÊiV >ðÊ-ÞV ÀÃ>ÌÊLiViÃÊ >ÊVViÀÊÞÊvÊÌ iÊ«Õ}iÀ½ÃÊÀiÌÕÀÊëÀ}ÊÃÊ insufficient because of seal friction.
Recommendations UÊ iÃ}Ê>ÊÃÕvvViÌÊ`ÀÛiÊÃÞÃÌiÊÌÊV«isate for seal friction. UÊ ÃÕÀiÊ«ÀiVÃiÊ>ÌÌ>V iÌÊLiÌÜiiÊÌ iÊ «Õ}iÀÊviÀÀÕiÊÌÊ«Õ}iÀÊÕÌÊqÊ>ÜÊÌÊ float (for centring purposes) as well as firmly withstand any vertical and or horizontal ÛiiÌÃÊ>ÃÊÃiiÊLÝʼ/ iÊ«iÀviVÌÊ «Ã̶½ÊÊ«>}iʣ䮰 UÊ VÕ`iÊ>ÊÃÌÀ}Ê«Õ}iÀÊÀiÌÕÀÊiV >ÃÊ to compensate for seal friction during the aspiration stroke.
Figure 3. “Anatomy” of a high pressure liquid chromatography pump.
Sealing Technology
9
FEATURE UÊ Ý«iVÌÊÛ>À>LiÊVÌ>VÌÊÃÌÀiÃÃÊÜ iÊÕÃ}Ê high-viscosity media.
Stroke length -ÌÀiÊi}Ì ]ÊÃ>ÞÃÊ `iÀ]ÊÃ Õ`ÊLiÊÃi`Ê and well-guided to ensure the plunger is concentric to the wash body and pump head bore throughout the pressure and aspiration stroke. This is especially important during the pressure stroke when the plunger is most prone to sideload the seal back-up and wash body seal. When shortening the stroke length to meet a desired flow rate, he says that the plunger speed must increase. This, in turn, increases the system PV. To maintain long seal life, he recommends keeping system PV within the limits of the seal material PV. Figure 4. Contact stresses (shown here in red) significantly impact seal performance in high pressure liquid chromatography applications. Experienced seal manufacturers will use failure modes and effects analysis models like this one to predict stress prior to production.
Heat dissipation Friction heat created under pressure can dramatically affect sealing performance. Because of this Binder advises pump engineers to design with dissipation in mind. He said: ‘Pressure created in the forward stroke, or by the seal and its energiser, can cause sealing contact stress to the plunger surface.
ÝÌÀiiÊivviVÌÃÊVVÕÀÊÊÌ iÊvÀÜ>À`ÊÃÌÀiÊ under ultra-high pressure, and in that moment the sealing contact stress is so great that seal material shear stress is overtaken, causing wear or shedding. As a result, material can sometimes be seen in the frit (inline fluid debris filter), the ÌÕL}ÊÀÊiÛiÊ>ÀÕ`ÊÌ iÊÃi>ÊiiÀ}ÃiÀ°½
Recommendations UÊ 1ÃiÊ>Ê>VÌÛiÊÜ>à ÊÃÞÃÌi° UÊ >Ì>Ê>ÊÃiÀÛViÊ«ÀiÃÃÕÀiÉÛiVÌÞÊ*6®Ê within seal material limits. UÊ -«iVvÞÊ>ÊÕÌÀ>ÃÌ Ê«Õ}iÀÊÃÕÀv>ViÊ và ÊiÃÃÊÌ >Ê£Ê,>®°
Plunger guidance Ê>ÀÊiV >V>Ê`Ã>`Û>Ì>}iÊÌÊ } pressure sealing is plunger side-loading. This vv>ÝÃÊÌÀ>ÛiÊvÊÌ iÊ«Õ}iÀÊÕ`iÀÊÌ iÊvÀÜ>À`Ê ÃÌÀiÊ« ÞÃV>ÞÊV >}iÃÊÌ iÊÃi>½ÃÊÃ`iÊvÀ]Ê and can lead to catastrophic failure. ÃÌÀV>Þ]Ê Ê«Õ«ÃÊÜiÀiÊ`iÃ}i`ÊÜÌ Ê a large plunger-size to long stroke-length ratio (for instance, 3:1). With the advent of ultrahigh pressure pumps these plungers caused early seal and back-up support failure. According to `iÀ]Ê>ÊÃ`Ê>}iÊÌÞ«iÊ>`ÉÀÊ«Õ}iÀÊÃâiÊ to stroke, which is closer to 1:1, should be used 10
Sealing Technology
in order for the seal back-up to absorb some of the remaining micro-side load. He suggests using a solid linkage, and notes that it is important to keep it concentric to the wash body and pump head.
Recommendations UÊ 1ÃiÊ>Ê`iÃ}ÊÜÌ Ê>Ê£\£ÊÀ>ÌÊ«Õ}iÀÊ`>eter to stroke length. UÊ ÃÕÀiÊ>ÊÃÌÀ}ÊÃÕ««ÀÌ}Ê>}iÊiÝÃÌÃÊ between the plunger ferrule and drive. UÊ }ÊÌ iÊÃÞÃÌiÊ*6ÊÜÌ ÊÌ iÊÃi>Ê>ÌiÀ>Ê PV limit.
Recommendations U ÃÕÀiÊÌ} ÌÊVViÌÀVÊ}Õ`>ViÊiÝÃÌÃÊ between the wash body and pump head (less Ì >Êä°äxÊ®° UÊ 1ÃiÊ>Ê } Ê`ÕÕÃÊL>VÕ«ÊÃÕ««ÀÌÊÀ}° UÊ 1ÃiÊ>Ê } Ê`ÕÕÃÊ}Õ`iÊÀ}°
Media viscosity Currently, most pumps are designed to handle >Êi`>ÊÌÞ«iÃÊqÊvÀÊÜÊÛÃVÃÌÞÊVÃÛiÌÃ]Ê such as methanol, to high viscosity media, such as disodium phosphate buffers, and everything else in between. Pumps are not typically designed to perform optimally under any given media, as during testing and approval most ÞÊiÝ«iÀiViÊVÃÛiÌð Binder says seals perform better in low viscosity media, and he advises fellow engineers to iÝ«iVÌÊ`vviÀiÌÊÀiÃÕÌÃÊÜ iÊÕÃ}Ê } iÀÊÛÃcosities. He also notes that degassing apertures can play a key role in the efficiency of very small diameter plunger seals. ‘Any trapped air starting within the seal-bore area and travelling downstream towards the flow restrictor will affect pump pressure read}Ã]½Ê iÊÃ>`°
Recommendations UÊ >>}iÊÃi>}ÊVÌ>VÌÊÃÌÀiÃÃÊLÞÊÕÃ}ÊÜÊ viscosity media.
The perfect piston? Designers of pumps for ultra high pressure liquid chromatography instruments know that there is seldom a solution for all seasons. Quite often an engineering approach that works under one condition can cause the same or similar problems under another. For instance, the use of a “floating piston” `iÃ}ÊqÊiÊÌ >ÌÊi>LiÃÊÌ iÊ«Õ}iÀÊviÀÀÕiÊ linked to the drive system to float for cenÌÀ}Ê«ÕÀ«ÃiÃÊqÊÃÊ>Ê`i>ÊÃÌÀ>Ìi}ÞÊvÀÊ«Àiventing side-loading, a condition that occurs when the plunger movement unevenly transviÀÃÊÌÃÊ>`Ê>}>ÃÌÊÌ iÊÃi>½ÃÊÃ`iÊ`>iÌiÀ° Unfortunately, while the floating piston `iÃÊ>ÊiÝViiÌÊLÊiÃÕÀ}Ê«À«iÀÊ«Õ}iÀÊ alignment at installation, break-in, and even operation at 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 liquid chromatography ®Ê«ÀiÃÃÕÀiÊ`i>`ÃÊÃÊÌÊ`iÃ}Ê>ÊvÝi`Ê drive system with a very short plunger stroke length and a smaller shaft diameter. This VL>ÌÊÜÊ i«Ê>ÝÃiÊÃi>ÊviÊ>`Ê «ÌÃiÊ Ê«Õ«ÊÕÌ«ÕÌ°
April 2012
FEATURE
Peripheral components It is important to consider the effects of periphiÀ>Ê >À`Ü>ÀiÊqÊvÀÊiÝ>«i]ÊV iVÊÛ>ÛiÃ]ÊVÕÃ]ÊvÌÌ}ÃÊ>`ÊÌÕL}ÊqÊ`ÕÀ}ÊÃÞÃÌiÊÌiÃÌing for sealing performance, as these elements can cause false positives during testing. Check valves not rated for proper pressure, weak tubing connections, underrated columns >`ÉÀÊ«ÀiÃÃÕÀiÊÀiÃÌÀVÌÀÃÊ>ÀiÊÕÃÌÊ>ÊviÜÊ«Ìitial causes. Binder suggests verifying fittings and tubing requirements carefully, as start-up leaks typically originate in peripheral components.
Recommendations UÊ ÃÊÞÕÀÊÛi`ÀÊÌÊ«ÀiVÕÌÊÌÕLiÃÊqÊ`ÊÌÊ cut them yourself. UÊ 1ÃiÊ«À«iÀÊÌÃÊÌÊ>ÌÌ>V ÊvÌÌ}Ã]ÊÜÌ Ê proper torque values, or delegate this task to service professionals. UÊ 1ÃiÊ>Ê«À«iÀÊÀ>Ìi`ÊVÕÊ>`ÊvÜÊÀiÃÌÀVtor to test pressure characteristics.
Seal installation Binder says that by setting and following an established seal installation process engineers can avoid seal damage and ensure proper seal alignment inside the pump head. This is critical because start-up failures and leaks can result vÀÊ«À«iÀÊÃÌ>>ÌÉÃi>Ì}ÊvÊÌ iÊÃi>ÊÊ the housing.
Figure 5. A technician in Bal Seal Engineering’s test laboratory with a line of high pressure liquid chromatography pumps.
UÊ ÃÌ>LÃ Êi>>}iÊLÞÊÃiÃÀÃ]ÊÛÕiÌÀVÊ changes and pressure drop programmatically, and minimise manual data collection.
ÜÌ ÊVÃ`iÀ>LiÊiÝ«iÀiViÊÊ* ÊÃÞÃÌiÃ°Ê Doing so, he says, will streamline the design process and eliminate frustration.
Test criteria
Recommendations
There are many ways to establish and correlate test criteria that simulate field conditions. An accelerated test to determine overall pump performance will not necessarily represent actual field use, Binder says, but it may induce plunger behaviour that will shorten seal life through side-loading or increased frictional heat. Also, establishing back pressure to simulate normal pump runs through a column will skew performance requirements.
UÊ ÃiÊ>ÊÃi>}Ê«>ÀÌiÀÊÌ >ÌÊ >ÃÊÃÌÀ}Ê engineering and sales support. UÊ ÃÊ>LÕÌÊiÝ«iÀiViÊÊ* ÊÃÞÃÌið UÊ i>`ÊÌiÀ>ÊÌiÃÌ}ÊV>«>LÌiÃÊÌ >ÌÊÜÊ enable product verification (Figure 5).
Recommendations UÊ 1ÃiÊ>««ÀÛi`ÊÃÌ>>ÌÊÌð UÊ 6iÀvÞÊÃi>Ê«ÃÌÊ>`ÊÌi}ÀÌÞ°
Failure criteria Understanding how much leakage is occurring and where potential leaks could originate is most crucial to establishing failure criteria for pump heads, check valves, fittings, pressure transducers, the wash body, sensors and tubing, among other areas, says Binder. ‘It is difficult to discern cause when you are working with firmware, multiple conditions iÝVii`}Ê£ä®Ê>`Ê >À`Ü>ÀiÊVL>ÌÃ]½Ê iÊ commented. ¼"vÌi]ÊÜ >ÌÊÜ>ÃÊÌ>ÞÊ`iÌvi`Ê>ÃÊ>ÊÃi>Ê v>ÕÀiÊÌÕÀÃÊÕÌÊÌÊLiÊ>Êv>ÃiÊ«ÃÌÛiÊÀÊi}>ÌÛi°½
Recommendations UÊ ÃÌ>LÃ ÊÕÌ«iÊv>ÕÀiÊV`ÌÃÊvÀÊi>V Ê i`>ÊÌÞ«iÊqÊ>ÃÊÜiÊ>ÃÊvÀÊi>V ÊvÜÊÀ>ÌiÊÀÊ speed.
April 2012
Recommendations UÊ 1ÃiÊV>«>ÀÞÊÌÕL}ÊvÀÊL>VÊ«ÀiÃÃÕÀi° UÊ Û`Êii`iÊÛ>ÛiÃÊqÊÌ iÞÊÌÞ«V>ÞÊv>Ê or create false negative and positive above 10 kpsi. UÊ /iÃÌÊÜÌ Ê>ÊVÕ]ÊvÊÌÊÃÊ«ÃÃLi]ÊÌ Õ} Ê it may be hard to find anything rated above £Ç°xʫð
Selection of a seal supplier ¼ iÜiÀÊ* Ê«Õ«Ê`iÃ}Êi}iiÀÃÊ>ÞÊÌÊ realise how critical seals can be to overall pump «iÀvÀ>Vi]½ÊÃ>`Ê `iÀ° ‘They may assume the seals are like elastomer "À}Ã]ÊÜ V Ê>ÀiÊÛiÀÞÊVÊ>`Ê >ÛiÊ>Ê simple geometry. Thermoplastic sealing is a whole different animal in terms of deflection, VÌ>VÌÊvÀViÃÊ>`Ê>ÌiÀ>Ê« ÞÃV>Ê«À«iÀÌið½ Because of this, Binder advises pump designers to seek out a well-informed seal supplier
Powerful partnerships 7Ì ÊÌ iÊÓäÊäää«ÃÊ«iÀ>Ì}ÊÌ>À}iÌÊÃÌÊÕÃÌÊ ÕÌÊvÊÌ iÀÊÀi>V ]ÊÃiÛiÀ>Êi>`}Ê ÊÃÌÀÕiÌÊ" ÃÊ>ÀiÊ}Ê>ÌÊÌ iÀÊVÕÀÀiÌÊ«Õ«Ê designs and considering many of the changes that Binder (and other engineers) have recommended. They are also inviting seal manufacturers, once relegated to problem-solving and failure consultation, to participate in the pump development process as active “innovation partners”. Acknowledging the challenges of reaching >`ÊiÝVii`}ÊÓäÊ«ÃÊqÊ>ÃÊÜiÊ>ÃÊÌ iÊii`ÊvÀÊ >ÊV>LÀ>ÌÛiÊÀiÌ }ÊvÊ«Õ«Ê`iÃ}ÊqÊ>Ê i}iiÀÊ>ÌÊ>Ê«ÀiÌÊÃ>Ê1* ÊÃÌÀÕment maker recently commented: ‘We were not getting any closer using established methods, so we started involving our seal supplier at a deeper level and, together, we began looking at the whole system. That is when we started to >iÊ«À}ÀiÃð½ Contact: Steven Twork, Bal Seal Engineering Inc, 19650 Pauling, Foothill Ranch, CA 92610, USA. Tel: +1 949 460 2160, Email: [email protected], Web: www.balseal.com
Sealing Technology
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