Using Coriolis mass flow meters with positive displacement vane pumps

January 15, 2015 By , and    

Developed in the 1830s, Coriolis mass flow meters are hardly a new concept, but they have recently gained popularity in liquid-fuels transfer operations, replacing positive displacement (PD) flow meters.

Coriolis mass flow meters measure fluid density and the Coriolis forces (mass flow) created, which are then used to calculate flow rates. There are two measuring tubes in a Coriolis meter; they are formed into a slightly bent tube or a “U” shape. When handling fluid, the measuring tubes are expected to oscillate at their resonance frequency. Any change in the density of the fluid produces a corresponding change in the resonance frequency of the entire vibrating system.

Flow Meter

This Emerson mass flow meter is installed horizontally on a propane system.

The resonance frequency, therefore, is a function of fluid density – the higher the density, the lower the resonance frequency. The resonance frequency is then compared to the empty-tube resonance frequency, allowing the mass of the fluid in the tubes to be calculated.

This oscillation presents no problems when systems use rotodynamic pumps, which generate a smooth flow. However, PD pumps, such as piston, gear or sliding vane types, generate pulse during operation. Since mass flow meters use vibrations to measure fluid flow, it has been wondered whether PD pumps can work effectively with mass flow meters.

To determine the accuracy and effectiveness of Coriolis meters with PD pumps, pump manufacturer Blackmer performed a study, the results of which have been published in a white paper.

Why choose mass?
Coriolis flow meters provide several advantages over PD meters.

  1. Liquids pass through smooth tubes with no moving parts and no mechanical wear of the measuring elements. There are also no parts subject to abrasion, erosion or corrosion.
  2. Particulates as large as the tube diameter can pass through the meter without constraint.
  3. Coriolis meters measure mass flow directly and are, therefore, not affected by temperature or pressure fluctuations.
  4. Accuracy is typically 0.1 percent to 0.2 percent of the flow rate being measured. This is more accurate than mechanical meters, which are listed as being within 0.5 percent of full-scale flow. Because PD meters require pressure and temperature compensation, any additional error in these measurements can increase the flow error to as much as 3 percent.
  5. Because of their accuracy, Coriolis flow meters have a turndown ratio of at least 100:1. Typical mechanical meters have a turndown ratio of 10:1.
  6. There is minimal pressure drop through a Coriolis mass flow meter with lower-viscosity fluids. The only pressure losses occur during the splitting of the flow into the two measuring tubes.
  7. Coriolis meters are unaffected by changes in fluid viscosity. Orifice plates and other pressure-differential meters must be calibrated for various viscosities and the viscosity needs to be measured during operation for accurate metering.

Some disadvantages have also been reported. Pressure drop can increase significantly in higher viscosity fluids; they can take up more space than PD meters; and they are more expensive.

In practical terms, the main question Blackmer’s technicians wanted to answer was: Do the various advantages remain when a Coriolis flow meter is used in conjunction with sliding vane pumps? For the answer, the company conducted tests using Coriolis meters from two different manufacturers in LP gas- and solvent-handling applications.

In broad terms, it was found that the advantages mentioned above for Coriolis meters are replicated when used with sliding vane pumps.

Test results
The tests reported “unexpectedly smooth” flow readings, more so than with any PD meters that had been used previously. The greatest fluctuation recorded was some ±0.3 gallons per minute (gpm) on flows up to 300 gpm.

One significant aspect of their operation is that mass flow meters are much quieter than PD meters. This was especially noticeable on the LP gas system. Because there are no moving parts, no vibration is generated during operation. However, when the meters are powered and empty, they do emit a high-pitched hum. This can be eliminated by keeping the meters filled or by de-energizing them.

Engineers did find it difficult to eliminate air in one meter, which had intentionally been mounted horizontally in the test rig, while the manufacturer recommends installing the meter vertically. Air in the meter created turbulence, reduced the accuracy of the density measurement and caused the density measurement to become unstable. However, even when installed horizontally, flow rates of about 10 percent of maximum rated flow successfully removed air in less than one minute. When mounted vertically, air was eliminated from the meters with less than 1 percent of maximum rated flow. A valve can be used to throttle the outlet to increase pressure, allowing trapped air to dissolve in the fluid.

To address the question of interference between pressure pulses from the sliding vane pump and the Coriolis meter, a variety of pumps were attached with different vane frequencies. High-speed pressure and flow data was collected and minimum and maximum pressures and flows were recorded. The results showed there was no correlation between the vane frequency, pressure pulsing and measured flow at the meter.

Too good to turn down
The decision to investigate the effectiveness of Coriolis mass flow meter measurement technology with PD pumps was based on observations from Blackmer’s customer base.

“Many Blackmer customers have evaluated Coriolis flow meters and found them to be the best flow-measurement solution where Blackmer pumps are applied,” the company says. “Increased flow-measurement precision and higher reliability were common improvements cited by end users.”

EMCO Chemical Distributors, for instance, recently installed multiple pumps for railcar loading/unloading in a system using Coriolis flow meters and says this has “yielded highly reliable system performance and excellent measurement accuracy with no reported problems after 18 months in operation.”

As a result of the testing, Blackmer has installed Coriolis flow meters on its own test stands.

The full version of this white paper, authored by Jeff Sietsema and Richard E. Foster, can be accessed via the Blackmer website at www.psgdover.com/assets/blackmer/MC/atk0210-033.pdf.

Jeff Sietsema has worked at Blackmer for 27 years. As a product development engineer, he worked in the development of both military and commercial positive displacement pumps. He is currently serving as the research and development lab manager.

Richard E. Foster joined Blackmer in 1990 as a product development engineer before being promoted to engineering manager. He left Blackmer to start his own manufacturing company in 1998 before returning as director of engineering in 2011.

1 Comment on "Using Coriolis mass flow meters with positive displacement vane pumps"

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  1. What about the LPG vapor and flashing during measurement?