E-Zine September 2016

Click here to read “So Many Pumps, So Many Applications... Pumps and Flow Control”

One of the most important issues for users of pumps for metering is the throughput. There are hundreds of thousands of small chemical metering pumps (usually of the diaphragm type) sold annually. These pumps are designed to meter accurate volumes of liquid each time the diaphragm strokes. But if the diaphragm pump doesn't have a full chamber, the output of the pump is not accurate at a given stroke length and stroke frequency.

For example, if the fluid being pumped is gassy, or has lots of entrained air, that air may be trapped inside the pumping chamber, and each stroke will therefore not produce a full volume of fluid. In large applications, or extremely critical ones, this problem is solved by putting a flowmeter in the injection line downstream of the pump that is capable of measuring the output of the pump and is not affected by the pump pulsation or the entrainment of gas (two-phase flow). In most common applications, however, this is extremely difficult to do, not because of the technology available, but because of the add-on cost.

Take a boiler control application, for example. Here a metering pump is feeding a corrosion inhibitor based on the input signal from a controller. Typically, such a metering pump would sell for less than $500. The least expensive flowmeter/controller packages for this application sell for more than the cost of the pump itself. In some cases, the flowmeter/controller sells for more than double the price of the controller. Many companies are working on flowmeters designed to serve this very critical application.

Click here to read “So Many Pumps, So Many Applications... The Future of Pumps and Controls”

From Flow Control (January 2002)

Part V: What's the Best Flowmeter to Measure the Flow of Raw Materials?

By David W. Spitzer

E-Zine September 2016

Positive displacement flowmeters that measure the actual volume of the fluid passing through the flowmeter (volumetric flow), flowmeters that measure fluid velocity, flowmeters that measure mass flow, and differential pressure flowmeters that measure inferentially were discussed in previous articles. These technologies measure volumetric flow, infer volumetric flow, measure mass flow, and measure velocity head (respectfully).

This conversation started with, “There are many technologies that can be applied to measure the flow of raw materials such as positive displacement, turbine, vortex shedding, differential pressure, and Coriolis mass flowmeters. Which technology would you select assuming that all of these technologies will operate reliably in the service?”

Raw materials react with each other on a mass basis so it would be advantageous to measure their mass flow --- not measure their volumetric flow, velocity or velocity head. However, raw materials typically represent the largest single cost associated with production. Therefore accurate flow measurement is desirable to ensure that the correct amounts of raw materials are fed to the process so as to reduce waste.

All of the cited flowmeters are used to reliably measure the flow of raw materials. However Coriolis mass flowmeters not only directly measure mass flow (the desired process variable) but they can also measure mass flow accurately in many liquid applications and in some gas applications. On the other hand, Coriolis mass flowmeters are often significantly more expensive than the other flowmeters.

Other process constraints, such as size, physical properties, chemical properties, and process conditions may dictate the use of other flow measurement technologies. Feeding the correct amount of raw materials to the process is not only more economical --- it also can reduce waste and reduce the impact of the process on the environment.

This article originally appeared in Flow Control magazine.

Quiz Corner: How Does Temperature Affect PD Pressure Drop and Meter Sizing?

By David W. Spitzer

E-Zine September 2016

How would a decrease in temperature of a liquid with a viscosity of 1000 cP tend to affect the pressure drop and size of an existing positive displacement flowmeter?

A. Pressure drop increases and size increases
B. Pressure drop increases and size decreases
C. No pressure drop or size changes
D. Pressure drop decreases and size increases
E. Pressure drop decreases and size decreases

Commentary
Decreasing the liquid temperature tends to increase its viscosity. Increasing viscosity tends to increase the pressure drop across the flowmeter. Therefore Answers C, D and E are not correct.

The pressure drop across positive displacement flowmeters is limited because operating with a high pressure drop across the flowmeter tends to wear and potentially damage its bearings. In a given application, a larger flowmeter exhibits a lower pressure drop.

Sizing positive displacement flowmeters involves selecting a flowmeter large enough that its pressure drop is within the pressure drop limits of the flowmeter. Therefore increasing viscosity will tend to increase the pressure drop across the flowmeter which can potentially increase the size of the flowmeter. Answer A is correct.

Additional Complicating Factors Lower temperature tends to increase viscosity, increase the pressure drop across the flowmeter, and potentially increase its size. Whether or not the existing flowmeter is still applicable or requires replacement requires more detailed investigation.

From Flow Control (September 2015)

ISSN 1538-5280