Using Agilent MS-40+ To Replace Multiple Single Stage Rotary Vane Pumps: 25-40 m3/h

TECHNICAL OVERVIEW

May 2017

Vacuum Solutions

ABSTRACTThe Agilent MS-40+ single stage rotary vane pump (RVP) probably represents the best value medium/large size RVP in the analytical instrumentation industry today. A mass spectrometer design and support company approached Agilent with an opportunity to try the MS-40+ as a replacement for two competitor’s single stage rotary vane pumps: a 25 m3/h pump and a 40 m3/h pump.

This technical overview describes a test protocol that can be used to determine MS-40+ operating conditions that met the customer’s goals but could also be extended to allow the MS-40+ to duplicate the performance of virtually any RVP in the 16–40 m3/h class.

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BACKGROUNDSupporting a population of instruments requiring multiple models and sizes of rotary vane pumps (RVPs) is costly and inefficient. An ideal solution would be a single, reliable, inexpensive pump whose performance could be ‘tuned’ to replace multiple pump models.

For a typical LC/MS application, a large RVP is used to evacuate a high gas load interface region, while also ‘backing’ the high vacuum turbomolecular pumps.

Instrument manufacturers will typically select a specific pump model, and then optimize the instrument parameters to maximize performance based on the individual characteristics of the chosen pump. This can present a challenge for customers and third-party support organizations as it can limit their ability to substitute an RVP, which may have significant improvements (cost, reliability, audible noise, etc.) over the original manufacture’s pump. The subtleties of vacuum pump designs mean that even pumps with the same nominal pumping speed can have surprisingly different performance at a particular operating pressure.

Since its introduction, the Agilent MS-40+ single stage, frequency inverter rotary vane pump has become the pump of choice for many of the world’s largest instrument manufacturers. The pump’s compact size and low weight, onboard oil mist eliminator, and excellent reliability make it the premium performer in the 40 m3/h category. The pump’s high performance frequency inverter presents the opportunity to operate the MS-40+ at reduced rotational speed, allowing the pump to mimic the pumping characteristics of smaller pump models.

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MS-40+: THE PROVEN CHOICE FOR LC/MSWhen Varian Vacuum, now part of Agilent, introduced the HS-602/HS-652 pumps with built-in frequency inverters, the world of RVPs changed forever. The longstanding 25 m3/h upper limit for a ‘single-phase power’ pumps vanished: The HS-602/652 pumps delivered pumping speeds of 32 m3/h, almost 30 % greater than the previous upper limit.

The HS series pumps achieved this using a frequency inverter (to convert 50 and 60 Hz wall power to variable speed three-phase power). The pumps became lighter (despite the increased pumping speed), and delivered universal performance whether connected to 50 or 60 Hz wall power. For global customers, avoiding having to compromise performance specifications to accommodate the pumping speed losses associated with 50 Hz power was an instant benefit.

Agilent’s proprietary frequency inverters also eliminated the massive current spike on start-up experienced by all fixed frequency pumps (current spikes > 60 A are not uncommon). Some fixed frequency pumps require that customers install special time-delay circuit breakers to compensate.

Finally, the Agilent’s frequency inverters allow end users (and OEMs) the ability to communicate intelligently with their RVPs. Remote start/stop capability and the ability to access critical pump parameters were just some of the benefits. Using simple RS-232 communication protocols or Agilent’s T-plus software, manufacturers could include pump parameters in their instrument diagnostic software, thereby improving the ability of Field Service Engineers’ to diagnose not only vacuum issues, but instrument problems as well.

Since then, Agilent has expanded its line of frequency-inverter pumps to include the MS-40+ rotary vane pump as well as several models of oil-free scroll pumps. The MS-40+ pump not only surpassed the previous 25 m3/h limit by 60 %, but also introduced an onboard oil mist eliminator and oil return to the instrumentation world - removing the need for costly external accessories.

RVP OPERATION ON LC/MS: WHY THE DETAILS MATTERLC/MS instruments often present a difficult challenge for development and support personnel. The differential pumping design drops pressure from atmosphere to rough vacuum to high vacuum through multiple stages, resulting in fairly complex vacuum systems, with interdependent operating regimes.

Figure 1 Optimal pumping range of LC/MS interface

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The initial stage of an LC/MS pumping system typically involves the ‘free-jet’ expansion of gas through a critical orifice. This results in a design that will only perform over a finite variation of pressure in this ‘interface’ region.

‘Overpumping’ the interface can produce detrimental effects on performance.

Instrument makers face an extra challenge: having to compromise the MS’s interface design to compensate for the variation in performance when (traditional fixed-frequency) primary pumps are powered by 50 or 60 Hz mains power.

Using a frequency inverter pump delivers the same performance in 50 and 60 Hz installations, reducing instrument-to-instrument variability, and allowing instrument designers to optimize performance of the MS interface.

COULD THE MS-40+ BE THE UNIVERSAL PUMP FOR LC/MS?A US-based LC/MS support company approached Agilent with a unique proposal: find a way for the MS-40+ pump to exactly duplicate the vacuum performance of competing single-stage RVPs with nominal pumping speeds of 25m3/h and 40m3/h on a widely used LC/MS instrument platform.

When Speed is Not Enough: Inlet RestrictorsAs described previously, the frequency inverter on the MS-40+ can be used to adjust the rotation speed of the MS-40+. The T-Plus user interface can be used to adjust the MS-40+’s speed to ‘mimic’ a line voltage from 42 to 62 Hz, with vacuum pumping speed roughly linear over this range. We expected to be able to reduce the MS-40+ speed slightly to mimic the performance of a pump in the range of 40 m3/h (the pumping speed advantages of the MS-40+ over the range of pressures interesting to LC/MS are well known to Agilent), however, it was uncertain if the minimum operating speed would be sufficient to drop the pump performance to the level of 25 m3/h.

Anticipating this, Agilent procured ‘blanks’ (inlet flanges modeled on the NW25 centering ring) with various size restricting orifices drilled in them (Figure 2). Blanks were made with drilled orifices of nominal diameters from 6 to 15 mm.

Figure 2 Samples of inlet restrictors in diameters 12.5 - 14.5 mm

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EXPERIMENT DESCRIPTION

Single Stage RVP in the 40 m3/h rangeSince the operating pressure of an LC/MS interface/backing pump is typically 1 to 5 Torr, we wanted to be sure that the MS-40+ could mimic the competitor pump’s performance over this range.

A vacuum gauge and needle valve were connected to the pump inlet. Air was used to simulate Nitrogen, which is most common carrier gas used by LC/MS manufacturers. With the competitor’s pump running at its preset speed, the needle valve was opened and the gas flow adjusted until a stable pressure of 5.0 Torr was registered on the vacuum gauge.

The gauge/needle valve assembly was then transferred to the MS-40+ (without disturbing the position of the needle valve). The vacuum pressure was recorded at multiple speed settings (‘frequencies’) from 42 to 60 Hz. This was repeated at pressures between 1.0 and 5.0 Torr, resulting in the following graph:

Figure 3 - Effect of speed reduction on MS-40+ performance 0.5 - 5.0 Torr

Our customer was able to simply measure the foreline pressure on the instrument with OEM pumps, choose the pressure curve from Figure 3 that was closest, then select the operating frequency of the MS-40+ that gave the exact pressure.

Single Stage RVP Reduced to 25 m3/hAchieving the significantly lower pumping speed required the use of a combination of speed reduction and the installation of inlet restrictors. With the 25 m3/h pump operating at its nominal speed (60 Hz), the needle valve was opened until a stable pressure 5.0 Torr was achieved. The gauge and needle valve were then transferred to the MS-40+ with the pump inlet almost completely blocked; only a 7.6 mm opening allowed gas to flow into the pump. The vacuum pressure was recorded for multiple frequencies (MS-40+). Depending on the pressure achieved, a larger or smaller orifice restrictor was then installed and the pressure recorded at various frequencies. After seeing the huge benefits in audible noise and the reduction

MS40 Frequency Adjusted to Mimic Competitor's 40m3/h pump @ 60 Hz

Pressure (Torr)

MS4

0 Op

erat

ing

Freq

uenc

y (Hz

)

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in inlet power from operating at frequencies lower than 55 Hz, the customer decided to focus on the frequency range 42–50 Hz.

An example of the data recorded is shown in Figure 4:

Speed Reduction and Inlet Restrictors Allows MS40+to Duplicate Competitor's 25m3/h pump Performance at 5.0 Torr

MS40+ Target Frequency (Hz)

Pres

sure

(Tor

r)

Figure 4 - Using speed reduction and restricting orifice to allow MS-40+ to duplicate Competitor's 25m3/h pump performance at 5.0 Torr

From this data, we can see that the ‘ideal’ restrictor at 5.0 Torr would be between 7.6 and 9.0 mm diameter. Once the data was collected at multiple pressures, and with multiple restrictors, a clear picture of the MS 40’s+ performance became clear. The data could then be reformatted to focus on determining the ideal

Figure 5 - Choosing 'Ideal' inlet restrictor for a chosen pressure and frequency (4.0 and 5.0 Torr, 50 Hz)

Use of Restrictors Allows MS40+to Duplicate Competitor's 25m3/h pump Performance at 50 Hz

Diameter of Inlet Restrictor Orifice (mm)

Pres

sure

(Tor

r)

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aperture size to operate at a minimum frequency (customer chose 46 Hz, allowing for any fine tuning) for the MS-40+ to duplicate the competitor pump’s performance over a wide pressure range.

Figure 5 shows that an 8.15 mm orifice, and a 50 Hz frequency setting would allow the MS-40+ to exactly duplicate the performance of the 25 m3/h pump at 4–5 Torr inlet pressure. Taking this concept to the next level allowed us to recommend an ideal frequency and orifice size to allow the MS-40+ to mimic the competitor pump’s performance over the complete range of LC/MS interface/backing pressures.

RESULTSThe experiments described previously allowed us to completely ‘map’ the MS-40+’s performance against the competitor's 25 and 40 m3/h pumps using a combination of speed (‘frequency’) reduction, and inlet orifice restrictor.

MS-40+ vs a Competitor’s 40 m3/h Single Stage RVPFigure 6 shows that for all pressures in the desired range, a reduction in rotation speed (enabling lower audible noise and input power benefits) would need to be applied to the MS-40+ to mimic the performance of of the 40 m3/h (nominal) pump. At pressures below 1.0 Torr, it is likely that a combination of speed reduction

MS40 Frequency to Mimic Competitor's 40m3/h pump Performance

Pressure (Torr)

Targ

et Fr

eque

ncy (

Hz)

Figure 6 - Reduced speed MS-40 duplicates Competitor's 40m3/h pump speed

and restricting inlet orifice would be needed to reduce the pumping performance of the MS-40+ to that of competitor’s pump.

MS-40+ vs a 25 m3/h Single Stage Rotary Vane Pump Reducing the pumping speed capability of the MS-40+ to that of a smaller 25 m3/h pump required using a combination of speed reduction and the installation of a restricting orifice at the MS-40+ pump inlet.

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Orifize Size (at different Freq) to Mimic the Competitor's 25m3/h Pump Speed

Pressure (Torr)

Orifi

ze S

ize (m

m)

Figure 7 - Reducing speed and inlet to Competitor's 25m3/h pump speedThe data recorded above allowed us to map the relative performance of the MS-40+ against the Competitor's 25m3/h pump over a wide range of speed (frequency) and restrictor (orifice diameter) conditions.

The rotation speed of the MS-40+ is variable in as little as 1 Hz increments. Therefore, the customer chose to use a restrictor orifice of 8.35 mm, in combination with frequency adjustment to ‘fine-tune’ the performance to adapt to the instrument’s precise operating pressure, achieved with their existing pump.

By doing this, our customer provided the LC/MS instrument user with a completely transparent upgrade, giving them a more robust pump, and reducing audible noise and power consumption noticeably.

CONCLUSION: MS-40+ THE UNIVERSAL RVPThe methodology described above has been used to determine an appropriate speed setting (frequency) and restrictor (orifice diameter) to allow the MS-40+ to replace pumps from multiple manufacturers in sizes from 16 to 40 m3/h.

The case above involved ‘mapping’ the minute variations in pumping performance over a wide range of operating pressures; most applications we encounter do not require this degree of precision. In almost all cases, a single pressure measurement (around the operating pressure) is sufficient to determine a combination of orifice size and a ball-park frequency.

The customer chose ultimately chose to operate the MS-40 pump slightly ABOVE its at its minimum frequency (42 Hz) to give them greater flexibility to ‘fine tune’ performance if the application demanded.

For more information, please contact Agilent Customer Support.

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