E-Zine August 2016
Click here to read “So Many Pumps, So Many Applications... Peristaltic and Diaphragm Pumps”
There are two ways to control the flow in a pumping system. First is to use a valve on the discharge of the pump. This can range from a simple manual valve, which can be throttled closed to reduce the flow rate through the system, to a process control valve, which uses an input signal from a controller to open or close to a specific position. The second is to control the speed of the pump using a signal from a controller.
Which of these methods to use depends on the application. If you are working with a pumping system that is simply transferring fluid from one tank to another, the simplest solution is a hand-operated shut-off valve. If you are working in a system where the flow through the pump needs to be regulated based on pressure, flow or another value, then a control valve or a variable speed drive would be the better choices.
The head and flow of a particular pump are produced by a given impeller size at a given pump speed (RPM). If you need a lower or higher flow, you can either change the impeller or change the pump speed. Changing the pump speed is generally easier, and many companies now make electronic variable speed drive controllers that modulate the speed of the pump motor based on an input from a controller. When using a variable speed drive, it is essential that the motor be sized to produce sufficient torque at any point in the operating flow range to operate the pump. Sometimes this means that in a given application, the motor size will be larger for a variable speed application than for a fixed speed application.
The use of variable speed drives (or variable frequency drives -- VFDs) is complex, and they should not be applied without a clear understanding of the way they work, and how. Along with the need to pick the right pump in a flow control application, you also need to pick the right flowmeter and flow controller. Selecting the flowmeter and sizing it are just as important to the function of the pumping system as selecting and sizing the pump. If the flowmeter is oversized, the signal it sends to the flow controller and to the pump will be too coarse for good control. If the flowmeter is undersized, it can be above full scale long before the maximum pumping rate the pump was designed for is reached.
Click here to read “So Many Pumps, So Many Applications... A Critical Application”
From Flow Control (January 2002)
Part IV: How Differential Pressure Flowmeters Infer Flow Measurement
By David W. Spitzer
E-Zine August 2016
Positive displacement flowmeters that measure the actual volume of the fluid passing through the flowmeter (volumetric flow), flowmeters that measure fluid velocity, and flowmeters that measure mass flow were discussed in previous articles. These technologies measure volumetric flow, infer volumetric flow, and measure mass flow (respectfully).
Other flowmeters do not measure volume, velocity or mass but rather infer the flow through the flowmeter from other measurements. Differential pressure flowmeters are commonly used to infer the flow rate in a pipe by measuring the differential pressure produced across a restriction. Differential pressure flowmeters such as orifice plate, Venturi, nozzle, variable area, and target flowmeters generate differential pressures that are proportional to the square of the flow through the flowmeter.
The differential pressure produced by differential pressure flowmeters is proportional to the velocity head --- one-half of the fluid density times the square of the fluid velocity. Stated differently, the flow measurement is proportional to the square root of the product of the fluid density and differential pressure. Therefore, the inferred flow measurement is affected by the density of the fluid where the effect is approximately one-half that of flowmeters that measure volumetric flow or fluid velocity.
Differential pressure flowmeters have been widely applied to measure fluid flow throughout industry for decades yet it is somewhat ironic that these flowmeters infer flow and do not measure the volume, velocity or mass of the fluid.
To be continued...
This article originally appeared in Flow Control magazine.
Quiz Corner: Why Doesn't Ketchup Flow Out of an Upside Down Container?
By David W. Spitzer
E-Zine August 2016
The viscosity of a Newtonian fluid is defined by the composition and temperature of the liquid. For example, the viscosity of water at a given temperature is the same regardless of physical manipulation.
However the viscosity of non-Newtonian liquids is dependent on the temperature of the liquid and how it is stressed. Ketchup is a non-Newtonian liquid that exhibits a high viscosity at rest that becomes lower when stressed for a time. Therefore, ketchup does not flow until it is stressed for a while. This is why it does not flow out of an upside down ketchup jar. However shaking the jar (adding stress) for a while can make the ketchup flow out of the jar --- sometimes relatively quickly.
Nonetheless decreasing the temperature of the ketchup tends to decrease the ability of the ketchup to flow. Therefore, it might be advisable to remove ketchup from the refrigerator prior to use so it can warm up and flow a bit better when used.
Additional Complicating Factors
Some liquids exhibit other non-Newtonian behaviors. For example, the viscosity of some liquids is high until stressed after which it can exhibit a dramatically lower viscosity. The behavior of other non-Newtonian liquids can be baffling because increasing stress (pumping harder) can actually increase the viscosity of the liquid.
From Flow Control (August 2015)