E-Zine May 2016
Click here to read “So Many Pumps, So Many Applications... Pumps and Applications”
There are many different types of pumps, each designed for a particular niche.
Centrifugal pumps are pumps with an inlet, outlet and an impeller with curved flights that rotates in a curved casing. There is substantial clearance between the impeller and the side of the casing, which permits bypassing of fluid when the pump is turning at a speed below its minimum rated pumping speed, or when the pump is "dead-heading" or pumping into a closed system or shut-off valve. There are probably more centrifugal pumps made than any other pump type. There are many different types of centrifugal pumps, including recessed impeller types for pumping fluids with entrained solids; self-priming units with a fluid reservoir on the suction side of the pump and multistage pumps, with more than one impeller moving in specially constructed casings.
Turbine pumps are similar, but have straight vanes and the ability to be throttled to a specific flow rate without bypassing. Turbine pumps are often more efficient than centrifugal pumps for the same horsepower expenditure. Like centrifugal pumps, turbine pumps can be "staged" and are often used as deep well water pumps with as many as 10 to 15 bowl and rotor combinations.
Another type of rotary pump is the rotary vane pump, which is often used for applications with high head requirements, but low flows.
Click here to read “So Many Pumps, So Many Applications... Positive Displacement Pumps”
From Flow Control (January 2002)
Considering the Pros and Cons of Volumetric Flow Measurement
By David W. Spitzer
E-Zine May 2016
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?
One dimension of the answer is to consider what each technology measures in the context of what you want to measure. Phrasing the problem in this way may seem a bit strange because you know that you want to measure flow… so it would be logical to use a flowmeter.
Some flowmeters measure the actual volume of the fluid passing through the flowmeter. As such, they measure the volumetric flow of the fluid. Positive displacement flowmeters repeatedly entrap fluid and are the only flowmeters that measure the actual volume of the fluid. There are many variations (geometries) such as helical gear, nutating disk, oval gear, piston, and rotary positive displacement flowmeters. Selecting the appropriate positive displacement flowmeter geometry is dependent on the actual application.
Measuring the actual volume of the fluid passing through the flowmeter can pose measurement issues when the fluid density varies with pressure, temperature and/or composition. Measurement accuracy can be adversely affected because the volume of a given amount of gas can vary significantly with seemingly minor pressure and temperature variations. Therefore, most positive displacement flowmeters are applied to liquid applications. In liquid applications where precise measurement is required, positive displacement flowmeters often compensate for temperature variations that affect the flowing density.
Despite these apparent drawbacks, positive displacement can and do measure the actual volume of the fluid accurately and are commonly applied in many applications. Consideration should be given to what positive displacement flowmeters measure (actual volume) and how they perform that function.
To be continued...
This article originally appeared in Flow Control magazine.
Quiz Corner: How Much Pressure Can a 150 lb. Flange Withstand?
By David W. Spitzer
E-Zine May 2016
A. 60 psig
B. 150 psig
C. 180 psig
D. 270 psig
Commentary
It might seem logical that a 150 lb. flange would be capable of withstanding 150 psig (Answer B) or maybe 20 percent more (Answer C). However the “150 lb.” in 150 lb. flange is its name --- not the pressure it can withstand (pressure rating).
Perhaps more basic is that the stated question is incomplete because the flange material and operating temperature are not defined. In general, carbon steel flanges have different pressure ratings than stainless steel flanges. Further, the pressure that a given flange can withstand decreases as its temperature increases.
The pressure ratings for 150 lb. carbon steel and stainless steel flanges are approximately the same and will be treated as such in this article. As a side note, the pressure ratings of 300 lb. and 600 lb. carbon steel and stainless steel flanges are significantly different at different temperatures.
Below find a table of approximate pressure ratings for 150 lb. flanges at different temperatures.
Ambient 270 psig
400 degF 180 psig
500 degF 150 psig
800 degF 60 psig
Given the vagueness of the question, it would appear that all of the answers are correct.
Additional Complicating Factors
The pressure ratings of flanges manufactured per standards are published for some materials. Pressure ratings for flanges made from different materials and other dimensions may need to be either calculated and/or tested.
From Flow Control (May 2015)
ISSN 1538-5280
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