Acids, Alcohol, Solvents

Selecting a flowmeter for chemical service can be tricky. We’ll need to know exactly what the fluid is and the operating conditions. Of particular importance is the temperature and line pressure. The more information you can provide us, the better job we can do at selecting the right flow meter for your application.

Never just ask for a body material and expect the meter to work properly. The body is only a pressure vessel which contains the working mechanism. The materials in the mechanism must be specified carefully. Always review the list of materials in the specifications. If you get into a chemical compatibility problem, it will be in the working mechanism, not the body.

Never assume that a plastic bodied meter will work for you just because you are using PVC lines. PVC and CPVC can handle chemicals which will melt a plastic bodied meter in seconds. Plastic bodied meters designed for water service are made of polyacetals and polycarbonates, not PVC. They cannot handle low pH fluids and in some cases will be attacked by solvents.

Viscosity considerations and the lack of capillary sealing

Mechanical meters need to be applied with some concern given to the fluid characteristics. A mechanical meter which has an oscillating piston, nutating disc (wobble plate) or similar measuring element is known as a positive displacement meter. However, what makes it “positive” is a capillary seal between the measuring element and the side wall or the meshing components. If you are dealing with a low viscosity fluid, i.e., alcohol, toluene, MEK, etc., you lack the viscosity to develop a good capillary seal. This will manifest itself in leakage around the measuring element and generally make the meter read low.

Low lubricity and pressure drop

Positive displacement meters have surfaces in the measuring chamber which touch each other. In the nutating disc (wobble plate) the center ball rides in a shaped cup, in oscillating piston meters the center pin is in contact with the roller and the piston diaphragm rides on a hub. This means that non-lubricating fluids allow these parts to drag on each other which increases frictional loading. This loading creates higher pressure drops and increases wear on the meter.

How to meter these fluids

The best approach for this type of fluid is to use a turbine flowmeter. The turbine rotor is suspended by a ball or sleeve bearing giving it almost no drag coefficient. This allows a turbine meter to be used for low viscosity as well as non-lubricating fluids. Another choice is the oval gear meter, a positive displacement device using dual oval shaped rotors. The rotors inherently provide better sealing because their gear design traps a pocket of fluid right at the capillary sealing point, thus preventing leakage around the measuring element.