A switch to a nickel bellows in their design has created a more versatile, more global gas flowmeter for Liqua-Tech Corporation (www.liqua-tech.com). Before the switch their LPM series of meters qualified for propane monitoring only. Now they can also handle many other gases including ammonia and the more contaminant-laden propanes found in overseas commerce.
"In other words, the flowmeter has many applications beyond the LPG market," says Ed Bruce, Liqua-Tech director of operations. The meters are found throughout the propane distribution market — from plants to fill stations and delivery trucks.
First, some background: The bellows is a key part of most propane flowmeters’ temperature compensation — or "athermalization" — mechanism. Athermalization is mandatory on flowmeters used in liquefied gas fuel distribution in the United States since the gas is bought by weight but dispensed by volume. By National Weight and Measures Bureau regulations, the standard unit of trade for propane is the gallon at 60 F.
Most scientists and instrument designers know that the most accurate athermalization device available today is the liquid-filled metal bellows. Incompressibility of the liquid ensures a positive linear displacement for the bellows with ambient temperature change. Kinematically through a linkage (Figure 1), this displacement adjusts the gear ratio between the sensing element inside the meter (usually a piston) and the readout on the register.
Delays Trigger Search
Back to the problem at hand. The bellows Liqua-Tech had been using was mechanically formed of beryllium copper and encased in a brass housing filled with glycol. When deliveries from the original bellows vendor were continually delayed, Mr. Bruce started shopping for a drop-in replacement from another vendor.
The athermalization assembly (Figure 1) works as follows: As the volume of the fluid within the thermostat bellows assembly expands and contracts with temperature changes, the change in volume is translated by the bellows into a linear movement. This linear movement is transferred by a linkage to the flowmeter’s register. The entire device acts as a transmission to offset the output of the measuring element to the register in a manner appropriate to the temperature/density of the fluid being measured.
Design engineers at Servometer (www.servometer.com) suggested several changes to the athermalization assembly that would improve the Liqua-Tech product without changing the size envelope. Headquartered in Cedar Grove, NJ, Servometer is one of the leading players in electrodeposited nickel bellows and bellows athermalization assemblies for instrumentation.
"While brass and beryllium copper may be fine for pure propane service, ammonia gas corrodes these red metals, as do many contaminants found in all but the purest propane," says Paul Hazlitt of Servometer. So he replaced the beryllium copper bellows with nickel and the brass cover with stainless steel. No more red metal. He also proposed the bellows metal be formed by electrodeposition rather than by mechanical press forming. Electrodeposition made thinner walls and finer convolutions possible, leading to a more sensitive athermalization structure and a more reliable and accurate meter.
The athermalization assembly (Figure 2) measures 3.581 inches long by 0.930 inch diameter. The push rod extends 1.544 inches out the end (at 60 F). The bellows itself measures 1.759 inches long by 0.775-inch diameter.
For safety, Servometer also recommended switching to a silicone-based fill liquid in place of the original glycol compound. "Glycol is flammable and a known carcinogen;
the silicone liquid is neither of these," says Mr. Hazlitt. "This change makes their product safer for us to manufacture and safer to use in the field. Where there’s propane, there’s always risk for fire."
The new assembly drops right into the same space in Liqua-Tech’s flowmeter bodies (Figure 3). With minimal additional changes, the Liqua-Tech model LPM 102 flowmeter, using the nickel bellows, now qualifies to meter ammonia gas and lower-purity propanes that corroded the earlier unit. The redesign also cut Liqua-Tech’s operating cost. Servometer’s assembly process includes leak tests at every stage, saving a step at the Liqua-Tech plant.
— Flow Control Staff
