E-Zine July 2016
Click here to read “So Many Pumps, So Many Applications... Positive Displacement Pumps”
The peristaltic pump is used for many different pumping applications. One of its major uses is in wastewater sampling. Most wastewater samplers use a peristaltic pump as the means of metering an accurate sample volume into the sample container. Peristaltic pumps are not just used as metering pumps, and they can be used to pump many different fluids. A peristaltic pump consists of a rotating cam that repetitively squeezes a section of tubing filled with fluid. The squeezing action pushes the entrained fluid along the tubing, while creating a lower pressure to draw more fluid from the pump inlet. Peristaltic pumps are highly accurate and are regularly used in hospitals, laboratories, and other high-purity, high accuracy applications to meter reagents, medicines, and even blood and other critical fluids.
Other pump types include diaphragm pumps and double diaphragm pumps.
Pumps consist of three basic parts: the pumping chamber, the drive unit or motor, and the coupling. Some pumps are close-coupled. That is, the impeller shaft is the same as the motor's drive shaft. Many pumps are designed so that there is a coupling of some kind that joins the motor shaft to the impeller shaft. This type of design allows you to custom design a pump, selecting the pump head and the motor separately (even from separate vendors, if you so choose).
Click here to read “So Many Pumps, So Many Applications... Pumps and Flow Control”
From Flow Control (January 2002)
Part III: Pros and Cons of Mass Flowmeters for Volumetric Flow Measurement
By David W. Spitzer
E-Zine July 2016
Positive displacement flowmeters that measure the actual volume of the fluid passing through the flowmeter (volumetric flow) and flowmeters that measure fluid velocity were discussed in previous articles. These technologies measure and infer (respectfully) the volume of the fluid passing through the flowmeter. However the amount of the fluid contained in a given volume passing through the flowmeter can vary with fluid density where the fluid density is dependent upon the fluid pressure, temperature and/or composition. Gas applications can pose significant measurement challenges due to significant density changes that can occur in many applications.
Mass flowmeters can be used to measure the mass of the fluid passing through the flowmeter. Coriolis mass flowmeters use the properties of mass to measure mass flow while thermal flowmeters utilize the thermal properties of the fluid to measure mass flow. Positive displacement and velocity flowmeters that measure and infer volumetric flow can be used to infer mass flow by multiplying the volumetric flow by the fluid density.
However in many applications (especially gases), the desired measurement is the amount of fluid passing though the flowmeter that can be commonly described as a mass --- not as a volume. Stated differently, the commonly-desired measurement is the mass flow of the fluid --- not its measured or inferred volume.
It would be logical to presume that mass flowmeters should be utilized in most flow measurement installations. However cost constraints, process conditions, accuracy requirements, and relatively small operating density variations in many applications (especially liquids) allow flowmeters other than mass flowmeters to be successfully applied in most flow measurement applications.
To be continued...
This article originally appeared in Flow Control magazine.
Quiz Corner: How Do Changes in Pressure and Temperature Affect Process Gas?
By David W. Spitzer
E-Zine July 2016
The pressure of a gas increased from 4 bar to 5 bar. Its temperature increased from 25 degC to 55 degC. Approximately how much was the gas compressed or expanded?
A. Compressed by 20 percent
B. Compressed by 10 percent
C. No change
D. Expanded by 10 percent
E. Expanded by 20 percent
Commentary
Boyle’s Law and Charles’ Law can be used to estimate the effects of pressure and temperature on the gas. Atmospheric pressure is estimated to be 1 bar absolute to simplify calculations.
The gas pressure increased from 5 bar absolute (4+1) to 6 bar absolute (5+1). Per Boyle’s Law, higher pressure tends to compress gases so the gas was compressed by approximately 20 percent (6/5).
The absolute temperature increased from approximately 300K (25+273) to approximately 330K (55+273), or approximately 10 percent. Per Charles’ Law, higher temperatures tend to expand gases so the gas was expanded by approximately 10 percent (330/300).
The approximate change can be estimated by mathematically adding the individual effects. A 20 percent compression and 10 percent expansion would net a compression of approximately 10 percent, so Answer B would be the best answer.
Additional Complicating Factors
More precise calculations could entail using the precise atmospheric pressure and the Ideal Gas Law. Other techniques could also be employed to include using data obtained from tables and testing.
From Flow Control (July 2015)
ISSN 1538-5280
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