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Flow Meter Selection Guide The importance of chemical compatibility when specifying an Oval Gear Flow Meter

All about compatibility... The lifespan of any flow meter is totally dependent on several factors that need careful consideration during initial selection of that meter for any given application. These include, but are not limited to choosing the best metering technology, sizing the meter correctly and as important as anything else, engineering the meter using materials that are fully compatible with the fluid that will be measured.

When choosing compatible materials for an Oval Gear meter, you need to consider more than just the body material. Other wetted parts that must be compatible with the process fluid include the rotors and elastomers. For example, most petroleum products are well suited for an Oval Gear meter with an aluminum body and PPS Rotors. But what about the O-Rings? Choose the wrong O-Ring material and they may soften, harden or even swell to a point where your meter may be damaged and/or your readings become inaccurate.

Common Materials of Construction in Oval Gear Meters

Typically, the three wetted parts of an Oval Gear that need to be considered when specify ing a meter are the Body, the Rotors and finally, the elastomers.

Meter bodies are offered in:

  1. Stainless Steel
  2. Aluminum
  3. PPS (Limited Availability)

Rotors are available in:

  1. Stainless Steel
  2. PPS
  3. Aluminum (Limited Availability)

Elastomers are available in:

  1. Viton
  2. EPR (Ethylene Propylene Rubber)
  3. Teflon Encapsulated
  4. Buna-N (Nitrile)

As a starting point for choosing material combinations, we identified five common material combinations for Oval Gear Meters along with the typical applications that either work well, are acceptable, but not our first choice or should be avoided at all cost...

Aluminum Body, PPS Rotors
Good Choice Oils Fuels        
Possibly Suitable Base Organic        
Not Suitable Acids Aromatics Brines/Salt Water Oxidizer Alcohols

Aluminum Body, Stainless Steel Rotors
Good Choice Oils Fuels        
Possibly Suitable Base Organic Aromatics      
Not Suitable Acids Brines/Salt Water Oxidizer Alcohols  

Aluminum Body, Aluminum Rotors
Good Choice Oils Fuels        
Possibly Suitable Base Organic        
Not Suitable Acids Oxidizer Aromatic Water Salts Alcohols

Stainless Steel Body, PPS Rotors
Good Choice Aromatic          
Possibly Suitable Base Organic Alcohol Salts Fuels/Oils Water
Not Suitable Acids Oxidizer        

Stainless Steel Body, Stainless Steel Rotors
Good Choice Aromatic          
Possibly Suitable Base Organic Alcohol Salts Fuels/Oils Water
Not Suitable Acids Oxidizer        

Rotors Bearings, more than meets the eye...

Chemical compatibility considerations for the major components used to build an Oval Gear Flow Meter are not the end of the story. When considering which rotor material will work best, you should also give some consideration to the materials used for the rotor bearings.

Rotors have a very small (~0.1 mm) thrust bearing that allows the rotor to spin without rubbing on the meter cap or body.

  • Stainless steel meters use a carbon ceramic bearing that is self-lubricating but consequently may wear faster.
  • Aluminum rotors use hardened steel roller bearings – making them unsuitable for water applications or liquids with a viscosity like water where no lubrication is available.
  • PPS rotors are self-lubricating; requiring no other bearing material and the best choice when no lubrication is available from the liquid.

Your choice of rotor material and consequently, the respective bearing type is crucial to the trouble free operation and long life of your oval gear flow meter.

Sintered and Wire Cut Rotors

Stainless steel rotors can be manufactured by two methods:

  1. Sintering - Sintered rotors are constructed using a metal powder which is molded, compacted and heated to just below its melting point. This results in a complex process of solid state bonding occurs forming a shape that resembles a cast item.

  2. Wire Cut - Using this method, the rotor is cut from solid stainless steel billet.

The sintering process is far more cost effective with less waste material and low machining demands. However sintered rotors have a limitation with acidic liquids that 316 Stainless Steel would otherwise normally be suitable for. Due to the chemical process of solid bond formation, there are far more porosities in sintered stainless steel than cast metal. As the sintering process produces weaker bonding than the casting process, the acid has a large surface area in which to attack the weaker intergranular bonds and delaminate or reduce the sintered rotor back to the powder it was formed by.

For this reason GPI recommends sintered rotors as suitable only for applications down to approximately pH 2 - this would be similar to vinegar or lemon juice.

Wire cut rotors will have a lower corrosion rate when used in Acid Service, but are still subject to some limitations based on the actual fluid and concentrations being measured.

Some common misconceptions...
  • Alcohols are commonly considered to be compatible with aluminum, which is true if it is pure (99.8%) alcohol. But, most alcohols will absorb water from atmosphere if they are being stored in a tank and this can lead to an oxide coating on aluminum surfaces. Alcohols and particularly methanol, which are slightly acidic, will dissolve the oxide layer and continuous corrosion and pitting will occur. For this reason we recommend stainless steel oval gear meters for alcohols.

  • PPS is commonly thought of as being resistant to most organic chemicals. It's also a common misconception that it PPS can and will increase in strength on exposure to some organic chemicals.

    In fact, chemicals such as benzene, toluene and xylene will swell PPS. Given the very tight tolerances found in the measuring chambers of an Oval gear meters, any swelling of the rotors can cause them to stop rotating and at that point, the meter becomes blocked and is no longer functional.

Bottom line... Be sure to check the compatibility of all body, rotor and elastomer materials with the fluids you are measuring. Choosing the wrong materials can shorten the life of your oval gear flow meter, effect the accuracy of your process and worse yet, you may be creating a safety hazard that could be devastating to all.

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 gathering, publishing and giving us permission to use the valuable information and images provided in this post.
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