ZDM
2
FM Rev. 11/10
2.0
Description
2.1
Principle of Operation
The ZDM operates on the volumetric displacement principle. The meter moves pockets of
liquid from the inlet port to the exit port in discrete segments of known volume. These
pockets are defined by the space between adjacent gear teeth and the internal housing
wall.
As liquid enters the meter, the inlet cavity fills. The liquid pressure forces the meter’s
gears to rotate from the gear mesh-point inwards. The gear teeth cavities fill with liquid in
the inlet side of the meter and remain filled as the teeth come into near-contact with the
meter’s interior walls. The gear teeth then drag the liquid to the exit cavity at the opposite
side of the meter. This process causes the exit cavity to fill since the liquid is squeezed
out of the spaces between the teeth at the gear mesh point. The liquid then flows out the
exit port of the meter.
A proximity sensor in the housing senses the passage of individual gear teeth. Since the
passage of a tooth signals the passage of its trailing space (filled with liquid), the system
as a whole signals with an electrical pulse every time a displaced amount is emptied into
the exit port. The resolution of the ZDM is, therefore, determined by the volume between
the gear teeth.
This entire procedure is completely independent of the properties of the liquid, making the
system applicable to a wide variety of media (even liquids with dynamically changing
characteristics.) The only limitations to the system are mechanical ones. Since metal to
metal contact is involved, the liquid must have some lubricating properties in order to
prevent excessive wear of the gear teeth. If the gear teeth wear down, liquid will leak past
them excessively. This excess leakage will degrade the meter’s accuracy. The required
amount of media lubricity is very slight. For example, leaded gasoline (such as jet fuel)
will work, while unleaded gasoline will not.
Another limitation is caused by the relatively high pressure loss inherent to positive
displacement technology. The pressure loss through the ZDM becomes unacceptable if
the media viscosity exceeds 100,000 cSt.
2.2
Specifications
2.2.1
Mechanical Data
Table 1: ZDM Wetted Parts
Wetted
Component
Nominal Construction Material
Cast Iron
(ZDM-X1XX)
316 Stainless Steel
(ZDM-X3XX)
Housing
Cast Iron
316 Ti SS (DIN 1.4571)
Gears
SS* (DIN 1.4122)
316 Ti SS (DIN 1.4571)
Seals
FKM
(EPDM Optional)
FKM
(EPDM Optional)
Bearings
Carbon Steel
Tungsten Carbide
* Alloy content of DIN 1.4122 SS: X 35 Cr Mo 17. No direct AISI equivalent. Similar
to 400 series SS.
