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Flow Meter Selection Guide The Do's and Don'ts of Oval Gear Meter Installations...

We've covered the importance of selecting the right meter for your application in our "Flow Meter Selection Guide". Now that your Oval Gear Flow Meter has been engineered, purchased, received and is ready to install... Knowing the Do's and Don'ts of Oval Gear Flow Meter Installations is just as important to the life of your meter as was the selection criteria used to choose it.

To avoid any unintentional damage to your new oval gear flow meter, just prior to installing would be a great time to double check on several crucial parameters that could have been overlooked during the selection process.

Flowrate, system pressure, system temperature and chemical compatibility are the most likely culprits when it comes to unintentional damage to your flow meter at start-up. Other things to check are fluid viscosity, particle content in the line, fluid state and any regulatory requirements that need to be met if the installation is in a hazardous area.

Some of the more common installation mistakes include:

  • Incorrect orientation;
  • Insufficient support, Oval Gear Meters (especially the larger ones) can be very heavy. Piping on both sides of the meter needs to be supported independent of the meter to avoid placing any additional weight/stress on the meter flow meter body;
  • Over spinning the rotors as a result of air not being bled from the pipe on installation;
  • No straining before the meter;
  • Incorrect electrical connections.
ORIENTATION

Failure to mount your Oval Gear meter in the correct orientation (see figure 5-2 below) will cause the weight of the rotors to bear down on the thrust bearings, and on the floor of the measuring chamber. Short term effects of incorrect mounting orientation will be a loss in accuracy. Long term effects ranging from significantly reduced lifespan (for very small meters) to significant damage to bearings, rotors, and measuring chamber (for the largest meters). Liquids can flow in a horizontal direction, or a vertical (up) direction, but in each case the rotors shafts must be in a horizontal plane. This is achieved by mounting the meter so that the terminal cover, or integral instrument display, is facing in a horizontal direction as shown below.

Meter Mounting Orientation

FLOW DIRECTION

Pulse-Type Oval Gear meters are bi-directional and will measure flow in either direction, without distinguishing the actual direction of the flow.

Mechanical-Type Oval Gear meters can only measure flow in one direction and that direction will be indicated on the body of the meter. Failure to adhere to the orientation on a mechanical flow meter will cause the mechanical counter to operate in reverse. Reverse operation for an extended period of time may cause permanent damage to the mechanical counter mechanism.

PIPING CONSTRUCTION

To avoid damage to your flow meter it is recommended that the following points be considered when designing/constructing your piping system:

  • Never install a medium or large capacity Oval Gear meters on the suction side of a pump. Systems can be designed to safely and accurately operate a small Oval Gear meter on suction (3/8" size or smaller meter).
  • System pressure and temperature must stay within safe limits. Install Oval Gear meters upstream of a flow control or shut-off valve. The back pressure provided by the valve will be beneficial to system accuracy.
  • For vertical installations the liquid should travel from bottom to top. This will ensure that the flow meter remains full of liquid, and will stop air entrapment in the meter. Piping should be designed so that the flow meter is full of liquid at all times. The inlet and outlet piping for the flow meter should be higher than all surrounding piping (see figures 5-3 & 5-4).
  • All piping surrounding your flow meter should be well supported where the piping joins to the flow meter. This is very important with 2" and larger flow meters.
  • Operating a flow meter directly discharging to atmosphere is not advisable, and will be detrimental to accuracy.
Piping Construction
If your flow meter is installed between two shutoff valves, it will be necessary to install pressure relief valves, or thermal expansion joints to protect from high pressure due to temperature change. It is always important to provide a bypass line (see figure 5-5). This provides isolation of the flow meter and strainer from the main process line. This allows quick and easy cleaning of the strainer. This allows easy maintenance and repairs on your flow meter.
Bypass Diagram
FILTRATION AND STRAINING

You must be sure your liquid is clean and 100% free of foreign particles to properly protect an Oval Gear meter. Solid particles to be present in a system from many sources:

  • Dust settling on an unsealed storage tanks
  • Wear on upstream mechanical devices such as pumps or mixers
  • Large metal particles from cutting or welding on new or modified installations

You should install a filter or strainer on the inlet side of any Oval Gear meter.

  • Small Oval Gear meters, 1/8" to 3/8" (4 mm to 8 mm), require a 200 mesh strainer.
  • Medium Oval Gear meters, 1/2" to 2" (15 mm to 50 mm), require a 100 mesh strainer.
  • Large Oval Gear meters, 3" to 4" (80 mm to 100 mm), require a 40 mesh strainer.

The Y-type strainer is the most common choice as it is the lightest, smallest, and most economical of the strainer types available. It also produces minimal pressure loss.

AIR ELIMINATION

It's very important point to note that Oval Gear meters cannot differentiate between liquid and gas. The presence of air bubbles or vapor in your liquid will cause measurement errors. Air bubbles or vapor can also cause major damage to your flow meter.

If air or vapor in your low viscosity liquid, cannot be avoided, an Air Eliminator Strainer (AES) is recommended. The AES should always be installed upstream of the Oval Gear meters. The AES is capable of capturing and venting large amounts of air that are found with the initial filling of the system.

For large viscosity liquids (> 100cP), or where large amounts of air have to be removed from the flow stream, a large capacity AES is required. The large AES should be installed immediately downstream of a large capacity air separator tank. The air separator tank provides the necessary time for viscous liquids to release the entrained air or vapor prior to entering the flow meter.

START-UP PROCEDURES
The most common cause of damage to an Oval Gear meter is improper start-up procedure after installation or modification, or after long periods of shutdown. New or modified piping is generally full of large volumes of air.

The newly installed meter must NOT be run until the piping is completely flushed of foreign materials. The most common foreign matter that is present in new or modified piping is welding slag, grinding dust, sealing tape/ compound, or surface rust. If your meter has been installed with a bypass line it will be easy to isolate your flow meter from the remainder of the system in order to flush out the majority of the piping.

If you have not installed a bypass line around your flow meter, the best solution is to replace your meter with a spool piece for the duration of the flushing procedure. Following the start-up procedure, and during the period of initial operation, it is recommended that the inlet strainer on your flow meter be inspected regularly.

MAINTENANCE & CLEANING

Oval Gear meters are a mechanical device, and as such are subject to some wear and tear over their operational life. In order to maximize the operational availability of your meter, and reduce system downtime, a periodic maintenance and inspection program should be implemented. The amount of normal wear that the meter will experience will depend on several factors.

  • Flow Rate
  • Process Temperature
  • Fluid Cleanliness
  • Fluid Lubricity
  • Continuous Duty Run Time

Before beginning any flow meter maintenance, ensure the following items have been completed:

  • The flow line and flow meter have been shut down, de-pressurized and drained of all liquids
  • Associated alarms, control outputs, etc., have been isolated so as not to affect the process
  • Any power supply/voltage supplied to the meter have been isolated and/or shut down

Periodic maintenance and inspection should occur every 6 to 12 months, especially for installations in harsh conditions. Specifically in applications with:

  • Large Temperature Variations
  • Long periods flows greater than 80% of the meters maximum continuous flow rating

For any installations that require CIP (clean in place) it is important that the cleaning or flushing procedures do not exceed the advertised operating specifications. CIP cleaning procedures that increase system temperature at a rate greater than 50°F (10°C) per minute may damage the reed switch output. Chemical compatibility of cleaning solutions should be checked against the flow meter materials of construction.

CALIBRATION

Each flow meter is individually calibrated at the factory and then supplied with a calibration certificate showing the number of pulses per unit volume (for a pulse output meter) or the nominal reading error (for mechanical meters).

Factory calibrations are done using the master meter method, using Castrol Diesel Calibration Fluid 4113. For measurement of any liquids significantly similar to Diesel Calibration Fluid 4113, that is lubricating and with a viscosity of between 3-100cP, there will be no loss of flow meter performance when using the factory calibration factor.

For any liquids where the viscosity or the lubricity of the liquid varies significantly from the factory calibration liquid (viscosity less than 1cP, or greater than 1000cP), it is advisable to recalibrate your flow meter on the liquid that will be measured. For any field calibration of your Oval Gear meter it is important to consider the accuracy of your calibration procedure, and of your volume standard:

  • Calibration of your Oval Gear meter using a poorly controlled procedure, or using a volume standard of unknown accuracy will produce poor results.
  • Any calibration procedure should be run for at least 2 minutes in order to reduce the impact of start-up and shut-down flows on your readings

Possible volume standards that can be used for field calibrations are:

  • Scales - Although this is on the most common calibration methods, it can also be one of the least accurate. Scales must be recently calibrated to ensure accuracy, and must have fine graduations in order to achieve the required measurement uncertainty.
  • Master Meter - The master meter method is the most common volume standard used for factory calibration of PD flow meters, which is due to its accuracy and practicality.

  • Volumetric Calibration Container - This method is typically considered to be the least common, most impractical method. Ironically, it's also the most accurate.
Reference/Downloads:
OM Installation Guide OM Troubleshooting Guide
 

Flow Specialist - Bill Michie Written By: Bill Michie  
Flow Applications Specialist
Cross Company
Instrumentation Group
Phone: (866) 905-9790 (M-F, 8am-5pm Eastern)
Contact the Experts

Special thanks to the Technical Team at GPI Austrialia for preparing, publishing and giving us full permission to use the valuable information provided in this post.
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