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|Troubleshooting Positive Displacement (PD) Meters|
What to do when your new Positive Displacement does not work
We start with the recognition that the meter was tested at the factory. This can be confirmed by noting that when the unit arrived, it had either water or oil as a residue inside. The meter worked when it left the factory.
Problem #1 - The register pointer doesn't turn.
To test this ... remove the meter and blow through it with your mouth. You should be able to hear the piston turn. If the register needle doesn't move, you probably have a jammed meter. On larger meters you can normally test it with water pressure from a hose.
a. If the installer used excess pipe dope, this excess will coat any internal filter screen as well as jamming close fitting meter parts.
b. If the installer used teflon tape, wrapped the pipe end, then pulled it to break it off, he always leaves some thin strands on the end. If these strands are on the inside of the pipe end, when the pipe is screwed in, the strand may rip off and float down into the meter. It can slip through a mesh hole and jam the piston, or wrap itself around the disk in a nutating type meter.
c. Some oils have a lot of foreign matter in them. We have had meters on diesel engined fishing boats completely jammed with carbon particulates after a refueling stop in Mexico.
Problem #2 - Register doesn't turn but flow is not blocked.
The meter was installed backwards, or somebody blew backwards through it before installation to make sure it worked.
If the meter was installed backwards, when you blow through it you will hear the piston spin but the register needle will not turn. Remove the register and check the change gears. Many of these have left hand threads to tighten the gear on its shaft under normal operation. If you run them backwards, the gear unscrews and falls off the shaft.
Problem #3 - Register doesn't turn, but flow is not blocked.
The meter can be incorrectly sized. Flow meters never work down to zero flow. Is your flow rate within the stated range of the meter?
Problem #4 - The register turns, but reads low or high.
The response curve of a positive displacement meter is not flat. Every PD meter shows a droop in flow accuracy at the very bottom end of the curve. On oil meters the lower the viscosity the greater the low flow error. Your specification sheet will show allowable, and expected, errors for very low flow operation.
If the meter reads high - most common reason is air in the line. If the oil line is free of air a accuracy test needs to be conducted. (see below)
If the meter reads low - most common cause is foreign matter which has loaded up the meter creating frictional losses, but has not yet jammed the meter. If the meter has been examined and the piston, or disc, is turning freely, a accuracy test needs to be conducted.
Field testing of meter accuracy:
For small meters, a known volume of fluid needs to be run through the meter into a tank of known volume or into a tank positioned on a weighscale.
For large meters, use a comparison meter of known accuracy in series, or the tank as described above.
For really large meters, rent a tanker and run it across truck weighscales before and after the test.
The best method is to use a platform scale with a tank on it, big enough to handle at least 2 minutes of flow at the flow rate being evaluated.
Test procedure -
a. Run a known volume of fluid through the meter.
b. Note the error in reading:
i.e. 0.85 gallons high in a 100 gallon run at 12 GPH
2.1 pounds low in a 33 pound run at 2 pounds/minute.
c. We must know (a) the error value, (b) the total quantity and (c) is the meter reading high or low.
d. Look at the change gears, these are the gears located as follows:
In the Kent Metron oil meter - located under the register.
Each gear has a factor on it, such as G+0.4. We need to know the number stamped into the gear on the magnet, plus the number stamped into the gear on the inlet shaft of the register.
In the Neptune meter, the 157 register has the gears located under the register. Lift off the register and note the number stamped into the gear on both the register input shaft and on the shaft coming out of the gear train. This number is the number of teeth on the gear.
e. Given this information we can send you a set of gears which will correct the meter reading.
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