Global Processing

Flush mount pressure sensors ease headaches for FMC

January 10, 2003
In fluid sensing applications, accurate pressure readings are essential to the process. A pressure device that offers long-term stability and reliable status indication contributes to ensuring a smooth running operation. In the case of FMC Biopolymer Corporation, based in Newark, Del., pressure detection was put to the test in a chemical manufacturing facility running 24 hours-a-day, seven days-a-week.
FMC Biopolymer Corporation markets products, systems and services to the pharmaceutical and food ingredients industries. FMC is a leading producer of functional excipients, such as microcrystalline cellulose (MCC).

MCC is a white, odorless, tasteless powder that is used as a binder in prescription medications and is included in over-the-counter pain relievers and cough/cold remedies. The product acts as a stabilizer and thickener that can replace fat and absorb moisture in food products such as frozen desserts, salad dressings, nonfat yogurt and pie fillings.
MCC and other cellulose-based products manufactured by FMC are derived from purified wood pulp. To transform paper-thin sheets of raw wood pulp into a fine microcrystalline cellulose powder, FMC uses a network of applications. One particular application example is found in the slurry processing area. Slurry is a media that is inherently difficult-to-handle and can present pressure sensing challenges. FMC''s slurry application encompasses a series of vessels and transfer pumps that process the product. The end result of processing is a fine powder that is ready to be used as an ingredient in medicines and food products. To travel to each of these vessels, the media moves through transfer pumps and pipes in the system. Maintaining proper pressure in the transfer pumps is vital to the process. To detect the media pressure throughout the application, FMC was using pressure isolators, or pressure donuts, as they were referred to on the plant floor. A pressure isolator is a section of pipe containing an elastomeric bladder that holds a fill fluid. The isolator''s elastomer element or diaphragm is the same diameter as the pipe, allowing a seamless integration into the pipe. Media travels through the pressure isolator, exerting pressure on its diaphragm. As the diaphragm flexes, it transmits pressure via the fill fluid up to the isolator''s controlling instrument, such as a gauge, switch or transmitter. In theory, pressure isolators are sufficient for controlling slurries.
However, challenges were encountered with the installation of the isolator. A section of pipe is cut and fitted with two flange adapters on either end of the pipe opening. The pressure isolator is then inserted between the two flanges that are tightened to make a clean seal.
FMC began to find problems with the pressure isolator after installation. Damage was occurring to the rubber elastomer after the flange adapters were tightened. Pressure from the flanges would pierce the elastomer, creating a tiny hole in the diaphragm. During setup, if the isolator read "no pressure," FMC''s maintenance personnel would stop the line, remove the flanges and examine the isolator.
At times, "no pressure" readings occurred for other reasons. A rupture in the diaphragm or trapped air in the fill fluid prevented the fluid from transmitting the proper line pressure.
The combination of the rubber elastomer and the media also proved problematic. If a crevasse developed in the elastomer, media could stick in the crevasse and harden, creating a possible contamination condition.
To correct these problems, the isolator was returned to the factory for maintenance or completely replaced. About this time, FMC''s electrical and instrumentation engineer, Jerry Daniels, began looking for a solution to his failing pressure isolators.
"When there''s a 50/50 chance the isolator might fail, and someone had to stand at the pump constantly checking the status, I lost confidence in the product," he said.
Daniels found a solution in ifm efector''s flush mount pressure sensors. FMC replaced the pressure isolators with the PF Series pressure sensors, which provide a 4-20mA analog and switched output in a compact 316 stainless steel housing. Incorporating the same ceramic pressure sensing technology found in high-accuracy pressure transmitters, the sensor provides high-burst pressure and overpressure protection. The sensor''s compact housing incorporates flexible film technology that results in excellent shock and vibration resistance. The first advantage the sensor had over pressure isolators was its easy installation. A hole is drilled into the top of the pipe and a stainless steel adapter is welded to the pipe. The sensor is then twisted manually into the adapter. The sensor''s ability to sit flush on the pipe makes it ideal for pharmaceutical applications. Its base is completely smooth and grooveless, preventing media from sticking and warding off contamination. Easy to clean, the sensor can be removed quickly from the pipe and its design meets 3-A, FDA and EHEDG requirements. The sensor''s visual indication was another selling point for FMC. The pressure sensors feature a three-digit numeric display that is particularly useful for immediate pressure status, especially in low-lit areas. The ability to see the sensor''s numeric pressure reading from across a room is a great benefit in troubleshooting. The display allows access to a microprocessor-based programming menu to input analog start- and end-points. Through one of these menu options, the face of the sensor can be rotated to read right side up, even where the sensor is facing the opposite direction on a vertical pipe. In the past year, FMC has replaced their pressure isolators in the slurry applications with ifm efector pressure sensors. During the last 12 months, the pressure sensors have proven extremely reliable.
"It''s a low-cost solution that''s reliable, and it was a success from the beginning," said Daniels.

For more information on the pressure sensors, contact ifm efector at 800-441-8246.