Metering
System Design Requirements
An understanding of fluid properties as well as installation requirements for metering
devices is needed for correct specification or design of equipment. Various metering
devices produce different flow characteristics - a fact that provides advantages for
intermittent dispensing, batch and continuous processes. To specify and design a
metering system correctly, engineers need to understand the requirements for a successful
installation. Along with the properties of the fluid to be metered (flow range, viscosity
and temperature) and installation requirements (pressure, power and space constraints),
they need to be familiar with the various metering devices and the effect they have on
flow characteristics and the process used.
Metering
systems can range from complex pumping systems with fully automated closed loop flow
control to simple manual additions of a certain ingredient during a time period. For the
purpose of this article, emphasis will be placed on metering devices claiming steady state
accuracy of 1% or better. This would encompass piston or plunger, diaphragm and precision
gear type devices. It should be pointed out, however, that other pumping devices in
conjunction with control systems are being applied with success, especially in higher flow
rates (above 1,200 ghp) and reduced turndown ratios.
The requirement for a metering system versus a standard pumping system is usually the
result of two important considerations. The first and foremost is the amount of fluid
being added to a process is critical to ensuring the quality control of the end product.
The second factor is the fluid being pumped. Does it have an associated cost that would
justify a metering device over a non-metering device because overdosing the product would
result in higher usage of raw materials and higher operating costs? While the control
example still remains the major justification for a metering system, the age of continuous
improvement (cost reduction and inventory control) has shifted batch processes to
continuous processes, furthering the argument for lower rate and higher accuracy flow
control. When asked to find a metering system for a fluid, develop an understanding of
whether the need is for flow control or to conserve raw material costs.
All manufacturers have a flow rate accuracy specification over turndown ratio. Accuracy,
sometimes referred to as steady state accuracy, is the ability of the metering system to
maintain and repeat the desired flow rate over the flow range or turndown ratio.
Repeatability is the ability of a device to reproduce a flow rate when returned to a set
point.
In continuous processing where production rates
can vary, a metering system's ability to repeat the flow rate accurately for a given
condition is critical to product quality. Some standards allow a metering device to have
stated accuracy based on two tests at rated or maximum capacity. These tests may not be
acceptable at the lower end of the operating range. A ±1% unit rated at 100 gph would
allow ±1gph in variation. This rated flow meets the requirements, however, when operated
at a lower flow rate a variation of ±1 gph would be greater than ±1% of the lower flow
rate.
When designing a metering system, be sure to consider
accuracy and repeatability over the entire turndown range. It is very important to specify
accuracy over the flow range desired to be assured that your conditions are met.
Three metering system devices are used and have general acceptance in
industry today. They are piston or plunger, diaphragm and precision gear. A piston or
plunger metering system consists of a movable plunger inside a cylinder, as seen in Figure
1.  The plunger is connected to a shaft that is operated in a reciprocating motion,
allowing the cavity to open and close, displacing a controlled volume on each stroke. Such
devices are well suited for lower viscosity fluids because fluids of viscous nature can
require higher inlet pressures and sometimes
booster pumps to overcome flow losses through the inlet check valve. In batch processes
requiring agitation, these units can be coupled with a stroke counter to fill the required
amount. In continuous processes, they will require a pulsation dampener or accumulator and
an inline mixer to provide a more homogeneous output of the metered fluid as the flow
characteristics are of a slug or shot feeding nature (Figure 2). 
Hydraulic or piston actuated diaphragm
metering devices are reciprocating pumps that incorporate a flexible diaphragm as the
pumping element. The diaphragm can be directly coupled to a piston or flexed by a
hydraulic fluid to increase and decrease its volume depending on the stroke position
(Figure 3).
While the operating
principles of a plunger and diaphragm pump are very similar, the diaphragm serves as the
pumping chamber and is a flexible member. Diaphragm pumps operate well in dispensing with
stroke counters and similar processes when operation is timed with the process. In
continuous processes, a diaphragm unit is also well suited for lower viscosity as its
output remains a controlled volume. Flow characteristics are still of a slug or shot feed
nature, and processes could require a pulsation dampener or accumulator and an inline
mixer. To reduce the effects of slug feeding, many diaphragm units offer multiple heads,
and the flow characteristics are dampened by synchronized displacements (Figure 4). In cases where the fluid to be metered has a containment
concern, the diaphragm type reciprocating pump has a distinct advantage since it is
sealless and the fluid is contained within the pumping and piping chamber.
A precision gear metering device operates in a similar
respect to all external gear pumps. Unlike normal gear pumps, however, it is precisely
ground to operate with running clearances of .00015 inches or less and surface finishes
less than 4 rms to negate the effects of slip or internal leakage back to the inlet. In
lieu of using inlet and outlet check valves to seal the liquid in a controlled volume,
the
device seals the fluid between the close clearances of the tips of the gear teeth and
center housing and tooth-to-tooth contact as shown in Figure 5.
These types of devices are well suited for
all viscosity ranges and pressures since they have no inlet check valves to restrict high
viscosity fluids from entering the pump or to keep the valves from seating. For very low
viscosity fluids with high pressures, a review by the manufacturer should be conducted to
assure that gear contact will be lubricated to avoid excessive wear. Precision gear
devices have continuous motion, and each gear mesh displaces an amount of fluid equal to
its rated capacity divided by the number of gear teeth. Some sets of teeth are meshing
while other sets are opening and collapsing (Figure 6).  Therefore, the gear teeth spread
the controlled volume and discharging action over a much greater number of pumping
actions.
As a result, the flow
characteristics depicted in Figure 7 produces a virtually continuous flow because of the
rotary versus reciprocation motion of the controlled volume.
 In intermittent dispensing applications, the precision gear
device can be coupled to a stepper motor to produce a shot flow characteristic. In batch
processes, this device must be coupled with a simple counting control to fill the amount
required. In continuous processes, the flow characteristics are of a continuous feeding
nature. Therefore, pulsation dampers or accumulators are not required. Because the
precision gear device produces continuous feed versus a shot or slug at is discharge, it
produces a more homogeneous mixture.
As with all pumping devices, metering devices put work or energy into the fluid to create
a pumping action. This energy can cause some shearing action as well as a temperature rise
depending on stroke or speed ranges. It is important to understand the fluid's reaction to
these conditions before considering various types of metering devices. With fluids that exhibit dilatent properties (increase in viscosity
when exposed to shear), one should have a rheology curve developed before attempting to
specify a metering device because the viscous properties could exceed the hydraulic
properties in power requirements. Fluids exhibiting Newtonian properties (viscosity
remains constant when exposed to shear) should not pose a power requirement problem so
long as the viscosity is known at the operating temperature. Fluids exhibiting thixotropic
properties (viscosity decreases when exposed to shear) should also have a rheology curve
developed so as to not oversize the power required. This can result in increased capital
cost in a system. In all cases it is important to understand the viscosity of a fluid at
its intended operating temperature.
Piston and diaphragm metering devices are normally provided with two types of flow control
adjustment designed to change the length in which the reciprocating action occurs,
therefore adjusting the volume of liquid displaced or acted on by the diaphragm. The
second is a variable speed control for motor rpm. The combination of stroke and motor
speed adjustment provides a large turndown ratio. Both devices can be provided with manual
or automated speed control, where an input signal can be provided to operate at a desired
flow rate or change with a mainstream process as product rates change.
Precision gear pumps are normally provided with a closed loop
speed control that constantly monitors input signals of desired flow rate with actual pump
speeds to hold a set rpm. While automated speed control is the norm, these pumps are
also available with manual operation. In all metering devices, simple manual controls up
to and including automated PLC control systems are available to meet the user's
requirements.
As with all process equipment, a calibration and check off should be performed under
actual process conditions to assure that the equipment specified produces the desired
result. A periodic verification schedule should also be put in place to verify expected
conditions and detect wear. Many processes have adapted sophisticated process controls
such as flow meter, rheometers and other devices to verify product quality. Depending on
your process, these additional systems may be redundant to the metering device specified
and would do nothing more than check a product that is as accurate. In depth discussions
with manufacturers on proper scheduled maintenance and inspection or calibration methods
could reduce your capital costs.
From "Pumps and Systems
Magazine"
Seal
Selection & Application
Volumetric Efficiency
of Gear Pumps
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