By: Rodney Johnson
Technologies for metering water and sewage flows have
developed rapidly over the years since the formation of DWSD in the mid-1800s.
Meeting ever-increasing metering needs has meant keeping up with developments
in metering and trying out new methods. Today, we have several basic types,
including Venturi meters, mechanical (turbine type) meters and magnetic
flowmeters.
In recent years, for both water and sewage applications, we have
been upgrading many outmoded meters with the ABB MagMaster brand of magmeter.
Current contracts will bring the total number of installations up to 45. Some
35 of these are water meters, ranging in size from 8 to 24 inches?part of
the 278 master meters used for wholesale billings.
Magmeters have performed very well for us and offer several
basic advantages. They can be factory-calibrated to have an accuracy of up to
0.15 percent of flow rate; provide a linear scale; have a rangeability of
1,500:1; come in sizes from less than an inch up to 120 inches; have zero head
loss with no flow obstruction; and are unaffected by changes in fluid density
or viscosity. In larger sizes, they are very heavy, but if we can check
calibration in-line, once they are installed, weight is not a problem.
Checking Meter Calibration In-Line
For its MagMaster line, ABB spent considerable time and
effort developing an effective, electronic method of inline (in situ) verifying
and certifying the calibration of its magmeter system. Introduced in the U.S.
in the late ?90s, the new system confirms the entire magmeter
system? transmitter, sensor and interconnecting cable. The basic hook-up
at the meter site is shown as a schematic in Figure 1.
DWSD was one of the first utilities to take advantage of
this new development. Customers have the option of purchasing the services of a
trained technician to perform onsite validations at scheduled intervals.
However, since DWSD meter sites are numerous and widely scattered throughout
the system and we wanted them visited at least at six-month intervals, we opted
to purchase three of the systems. Training of our technicians by an ABB expert
at our facility was included. This training gave us the flexibility of handling
routine and emergency calibration checks, as needed by ourselves.
We have been using the systems for more than three years and
have found a number of benefits from their use.
We
obtained ISO certification of the complete flow metering system without having
to remove the magmeter's primary sensor from its underground location in
a vault. This is no small matter of expense, especially with larger water
meters that currently range up to 24 inches. For metering sewage flows,
magmeters can run much larger (up to 78 inches).
Keeping
the meters online also greatly reduces downtime that would be involved in
taking the meter and its converter to the flow calibration facility. Even using
a replacement meter would still mean a lengthy shutdown at the metering site.
By
verifying that the original calibration data are still valid, the system can
help satisfy the ever more stringent requirements of maintenance and
calibration set up in our department. Such data also can be useful to answer
queries from our wholesale customers. A printed verification document for proof
of calibration is provided by the system.
For
each meter site, periodic calibration and performance checks provide us with a
valuable tool for diagnostic and condition monitoring. CalMaster stores all
previous validation data and thus can plot trend curves for various meter
components. If such a curve shows a gradual deterioration in a meter parameter,
preventive maintenance steps can be taken to avoid complete meter failure.
For example, in one case, a scheduled check showed a nick in
the cable to the sensor. This eventually would have allowed water to leak into
the sensor and cause a total meter failure. With this early warning, near-term
maintenance was scheduled to replace the damaged cable.
In
our central meter calibration facility, we use several different sized
MagMasters as a standard to check the calibration of various types of meters
against (up to a 12-inch size). These test meters are periodically validated
against one of the CalMasters.
ABB
stands behind the CalMaster certification for its magmeters, as long as the
meter is installed properly and the CalMaster tool is maintained and annually
certified by ABB.
Scope of the Metering Task
Today, DWSD is the third largest water and sewerage utility
in the United States. Under contracts, it services more than four million
customers with drinking water in Detroit and 126 neighboring communities. It
operates five water treatment plants with a combined capacity of 1,500 mgd and
serves the City of Detroit through some 250,000 retail and 30,000 commercial
accounts. The utility recently has completed deploying automatic meter reading
(AMR) for 278 wholesale water meter facilities.
On the sewage side, DWSD operates the world?s largest
single-site wastewater treatment plant. It has a primary capacity of 1,750 mgd
and serves almost three million people in Detroit and 78 suburban communities.
There are approximately 50 wholesale sewage metering sites.
This article describes two typical meter sites for drinking
water, where a technician would go with the validation equipment. For both
water and sewage measurements, these sites are spread out across the
approximately 1,000 square miles in DWSD service area. These areas reach out as
far as 70 miles from our central facility, emphasizing the problem of
maintaining meters and keeping them in calibration.
Verification at a Meter Site
As shown in Figure 1, there are just two basic components of
the verification system: a portable PC that contains the software to carry out
a pre-programmed verification procedure and the Control Box. Two cables come
with the system for connecting the PC to the control box and connecting the box
to the magmeter transmitter.
As shown in Figures 2 and 3, at the meter site the
transmitter is mounted in a freestanding cabinet and permanently connected by
cable to the MagMaster sensor that is installed underground in a vault (Figure
4). The technician places the PC and control box at convenient spots near the
transmitter and then makes the cable connections as required.
Once the technician initiates the procedure, within a matter
of minutes the system has carried out a rather complex verification procedure.
The technician then can review results on his PC display to confirm that they meet
the set standards. A certification of validation for the tested meter can be
printed out at the meter shop.
Tie-In of Meters with New SCADA System
On its master water meters, DWSD has just completed a new
SCADA system that ties in with the current 278 meter sites that measure
drinking water to wholesale customers. A complete description of this system is
beyond the scope of this article.* Briefly, the system receives flow
information and other data at each remote site and, through a remote terminal unit
(RTU) at the site, sends a packet of data via radio transmission to both a
systems control center and commercial operations at the main office in downtown
Detroit. The Meter Operations and Information Systems (I.S.) Divisions, located
at the central service facility, also receive the same information.
The SCADA system reports total flow at five-minute
intervals. Also included in its data packet are pressure readings for incoming
and outgoing water as well as various alarms (intrusion, flooding of vault,
cabinet temperature, low battery, etc). The same system now will be added to
some 50 sites for wholesale sewage metering.
One unique feature of this system is that DWSD not only
collects all of this information but also simultaneously transmits it to each
community. This allows the DWSD wholesale customers to see their flow
information before the Commercial Division prepares the bill.
To tie in with the SCADA system, the ABB MagMasters connect
to a Control Microsystems RTU along with other instruments via a HART loop. The
RTU is shown in Figure 3. Having an accurate metering system is the critical
starting point for all this new SCADA technology.
Figure 5 shows an overview of our calibration facility where
all types of meters are checked. For many of these, we have nine MagMaster
meters that we use as a standard of comparison. These are installed in
8", 6", 2" and 1" test runs.
Figure 6 shows a test setup with an 8" magmeter
installed upstream from a Venturi meter to be checked. The d/p cell, shown
pipe-mounted, detects the differential pressure created by the tube as a
measure of flow rate. This measurement is compared to the flow rate measured by
the magmeter.
* Described
fully in a presentation made at a recent seminar of the Automatic Meter Reading
Association (AMRA) by Dennis L. Green, P.E, Head Water System Engineer, DSWD.
All photographs by Ray Warrior.
About The Author: Rodney Johnson is assistant superintendent of meter operations at DWSD. He has been with the City of Detroit for 24 years, 21 of those with DWSD working primarily on the water side. He graduated from Wayne State University with a BA degree.
