Seminars Products Services Downloads Clients Bios Contact

So Many Pumps, So Many Applications... Positive Displacement Pumps

By Walt Boyes

E-Zine June 2016

Click here to read “So Many Pumps, So Many Applications... Centrifugal, Turbine and Rotary Pumps”

There is a whole other group of pumps called positive displacement pumps. There are many different types here, as well.

The most easily understood is the piston type. In this pump, the inlet cycle pulls fluid into the chamber and the outlet cycle uses a piston sliding in the chamber to push the fluid out the outlet port. Piston pumps require very clean fluids, and can pump into very high pressures. Piston pumps, because they are positive displacement, are often used as chemical or additive metering pumps. They can be made in very small sizes, from fractional inch diameters, to pistons as large as several inches.

Another mainstay of the positive displacement pump group is the gear pump. In this pump, the interlocking motion of two gears first creates a chamber for the fluid to fill, and then pushes the fluid out the outlet as the gears rotate around each other. Gear pumps are often used in hydraulic applications, and for pumping liquids like machine oil, lubricants and other relatively high viscosity fluids.

Two of the most interesting positive displacement pumps are the progressive cavity pump and the peristaltic pump. The progressive cavity pump is constructed of a steel rotor and an elastomer stator. The rotor is long and convoluted, and the stator has the same curves in it, one revolution off. What this does when the rotor turns is to squeeze fluid from one opening in the stator to the next, until the fluid leaves the outlet of the pump. The progressive cavity pump can pump nearly any viscous liquid. It is often used for pumping sludge and highly viscous foods, such as syrups and chocolate. It is much less efficient pumping thin liquids, such as water.

Click here to read “So Many Pumps, So Many Applications... Peristaltic and Diaphragm Pumps”

From Flow Control (January 2002)

Part II: Pros and Cons of Velocity Meters for Volumetric Flow Measurement

By David W. Spitzer

E-Zine June 2016

Positive displacement flowmeters that measure the actual volume of the fluid passing through the flowmeter (volumetric flow) were discussed last month.

Some flowmeters measure the actual velocity of the fluid passing through the flowmeter from which the volumetric flow can be calculated by multiplying the velocity times the (known) cross sectional area of the flowmeter. Turbine and vortex shedding flowmeter technologies can be used to measure the velocity of liquids, gases and steam however not all designs are so suited. Magnetic flowmeter technology is constrained by its principle of operation and can only measure the flow of liquids.

Fluid density can affect flow measurements in a manner similar to that exhibited by positive displacement flowmeters. Variations in gas pressure, temperature and/or composition can cause the density of a given amount of gas to vary and affect the measurement. Pressure and/or temperature compensation is often applied to correct the flow measurement for these changes. Liquid applications generally exhibit smaller such variations that may or may not be corrected.

Whereas positive displacement flowmeters utilize moving parts to entrap fluid, some flowmeters that measure velocity have no moving parts, notable examples of which are magnetic and vortex shedding flowmeters. The elimination of moving parts generally increases flowmeter reliability by reducing the possibility of inaccurate operation and/or flowmeter failure due to bearing wear. The possibility of plugging the flowmeter is also reduced. However flowmeters that measure velocity may have other failure modes (such as velocity and conductivity constraints) that positive displacement flowmeters do not exhibit.

To be continued...

This article originally appeared in Flow Control magazine.

Quiz Corner: What is the turndown of a flowmeter that can measure accurately from 10 to 100 units?

By David W. Spitzer

E-Zine June 2016

A. 3:1
B. 10:1
C. 100:1

Page 61 of my book Industrial Flow Measurement (ISA) defines turndown as, “the ratio of the maximum flow that the flowmeter will measure within the stated accuracy, usually the full scale flow, to the minimum flow that can be measured within the stated accuracy.”

The problem statement seems to imply that the flowmeter can measure between 10 and 100 flow units. If so, the flowmeter turndown would be 10:1 and Answer B would be correct.

However some flowmeters are nonlinear. For example, orifice plate and Venturi flowmeters generate a differential pressure output signal that is proportional to the square of the flow rate. This signal can be measured by a differential pressure transmitter, after which its square root is taken. If the problem statement refers to differential pressure units, the differential pressure transmitter can measure accurately from 10 to 100 differential pressure units that correspond to approximately 30 to 100 percent of flow and exhibit approximately 3:1 turndown on flow. In this case, Answer B could be correct.

Additional Complicating Factors
Flowmeters typically have degraded accuracy specifications (expressed as a percentage of the actual flow rate) at low flow conditions. Therefore, a given flowmeter may still operate within its stated accuracy specifications at low flow rates and technically exhibit a high turndown. For example, the accuracy of a flowmeter at low flow rates could be (say) 20 to 50 percent of rate --- much higher than the (say) 1 percent of rate accuracy exhibited by the same flowmeter at high flow rates.

From Flow Control (June 2015)

ISSN 1538-5280

Spitzer and Boyes, LLC