FACTORS AFFECTING LINEARITY
There are many factors affecting the linearity of turbine flow meters . The following enumerates some of these factors and
their effect .
Size
The size of the meter selected is determined by the flow range required and the fluid characteristics . Standard flow ranges are
listed in the Product Data Sheet . Where range requirements fall between listed ranges, it may be necessary to use two meters
or a meter can be ordered for the specific range required . Overspeeding to meet a required flow capacity results in lowered
operating life . Going to a larger meter size to avoid overspeeding will result in the non-linear range at the lower flow rates .
Bearings
The rotor in a standard meter is mounted on ball bearings . The function of rolling friction in regard to linear operating range
is nil and can be disregarded . Where ball bearings cannot be used because of fluid characteristics, a sleeve or bushing type
bearing is available at the expense of a reduced linear range . Due to the inherent character of increased friction, the linear
operating range may be sharply curtailed in the lower capacity meters .
Pickoffs
Magnetic pickoffs affect the linear range of a meter, due to magnetic drag on the rotor . Since the turning force available is a
function of the total mass flow, the low capacity meters will be more affected at the minimum flow rate than the high capacity
meters . Replacement pickoffs should have the same part number as original equipment, otherwise the linear range can
be affected .
Fluids
There are two types of fluids: compressible and incompressible . Considering only the incompressible (liquids), there are three
factors that affect the linear flow range . They are lubricity, density and viscosity .
Lubricity
This is not a measurable quantity . It is that property of a liquid which determines the friction within the bearing and affects
the life of the bearing as well as the linear operating range . Lack of lubricity can cause erratic action, especially at the low end
of the flow range .
Density
Turbine flow meters are designed to operate over the standard frequency range with liquids of 1 .0 specific gravity (H
2
O) .
If a liquid of 1 .5 specific gravity is used, it will have a 50% increase in driving force available at a given frequency . Also, the
differential pressure of the meter is increased a like amount . This increased differential pressure can reduce the life of the
bearing . Reduction of maximum operating frequency to maintain design pressure drop will result in reduced bearing life . The
maximum frequency can be approximately calculated as shown by the following example:
𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀
𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝑀𝑀𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹
= 1200
�
1.0/1.5 = 980
𝐹𝐹𝑐𝑐𝑐𝑐
Figure 19: Calculating maximum frequency
User Manual
Page 21
May 2014
