Sensors - February 2001 - Enhancing Overpressure Tolerance

PRESSURE

Enhancing
Overpressure Tolerance
with Bellows

How do you revamp your pressure transducer to withstand peaks of 35 X the rated range, at no added cost? Try an electrodeposited nickel bellows.

Paul Hazlitt, Servometer Corp.

By the nature of their work, pressure transducers often end up in tight and awkward locations. When one fails or deforms, replacing it can be challenging at best and usually means shutting down an operating process. Removing the broken transducer and installing a new one can take several hours-a very expensive interruption, when you consider the production time lost in a commercial manufacturing application. Furthermore, because the pressure sensor has failed, there is no easy way to trace the cause of the overpressure, meaning the new transducer may fail again, even during installation.

Photo 1. The electrodeposited nickel bellows made by Servometer and incorporated in Transicoil's pressure transducer offer increased resistance to mechanical deformation during overpressure conditions.

To help customers avoid this frustrating and costly scenario, the Transicoil division of Horizon Aerospace LLC improved the reliability of its transducers by replacing diaphragm-type pressure sensors with bellows sensors (see Photo 1) from Servometer Corp. The new electrodeposited (ED) nickel bellows components offer 35 × the resistance to mechanical deformation in overpressure conditions. Better still, the bellows do not compromise accuracy or add to manufacturing costs.

Capsule Assembly Drawbacks
Used to measure variations in pressure and changes in position, Transicoil pressure transducers convert mechanical energy into an electrical signal. Transicoil mounts a mechanical sensing element inside a standard cast aluminum NEMA 1 housing. When pressure is applied, the element's motion is directly transmitted to the ferromagnetic core of a linear variable differential transformer (LVDT). The electrical signal from the LVDT is processed by an internal electronics package to show the pressure condition.

The conventional mechanical sensing element specified by Transicoil is called a capsule assembly. It consists of hemispherical diaphragm elements welded together to form a hollow chamber. The usual material is NiSpan-C because its modulus of elasticity remains constant over a relatively wide temperature range, although sometimes phosphor bronze or plain bronze is used instead. The NiSpan-C and bronze materials are predictably elastic, so they respond as needed to mechanical pressure changes. They are generally rated to withstand peak pressures about 1.5 × the rated range without permanent deformation. In some cases, this overpressure capacity is sufficient. In many other applications, such as industrial process machinery, the pressure peaks beyond the rated margin and causes a dent or bubble in the spherical body of the capsule assembly. Once deformed, the capsule assembly is out of calibration and the entire transducer needs to be replaced.

Redesign Team
To increase the overpressure tolerance of its pressure tranducer's mechanical sensors, Transicoil partnered with Servometer to formulate a solution. The primary design constraints were to:

Provide identical static error performance
Maintain the size of the existing enclosure
Not exceed the cost of the existing unit

Photo 2. Transducers equipped with nickel bellows (left) provide up to 35 × the overpressure tolerance of transducers equipped with capsule assemblies (right).

A year of joint development work and testing culminated in the standardization of bellows-type transducers (see Photo 2) for high-overpressure applications. Transicoil's standard industrial pressure transducer Model 252C, equipped with a bellows, withstands overpressures of about 35 3 the rated measurement range, and the installed cost for the bellows type transducer was the same as the diaphragm-type device.

Because the bellows component worked so well with the 252C transducer, Transicoil also designed it into a wide zero-span differential pressure (D/P) transducer. In this product, you can adjust the range of pressure measurement to move the zero value to different parts of the rated range. Or, you can compress the calibrated range or span from 100% to 10% of the rated range. The high overpressure capability of the precision metal bellows provides similar protection with this product.

The Manufacturing Process
Servometer manufactures the bellows component by forming a mandrel to the shape of its inside surface and then depositing a defined thickness of spring quality nickel onto the mandrel. The plated material is trimmed to define the ends, then the mandrel is removed by dissolving it. This leaves behind a thin, convoluted tubular shell of plated metal-the bellows.

The electrodeposition manufacturing process provides tight control over wall thickness, high chemical purity, and excellent retention of mechanical properties. Wall thickness on the bellows components fabricated for Transicoil ranged from 0.0015 to 0.006 in. (0.038-0.15 mm). The ability of the nickel to withstand overpressure conditions is attributable to its combined yield strength (110,000 psi min.) and tensile strength (125,000 psi min.). The dynamic capability of the bellows design dissipates the excess mechanical energy.

The bellows are designed for "infinite" (100 3 10⁶) cycles of life expectancy and greater reliability. The intrinsically flexible nature of the bellows allows it to resist permanent deformation, and bellows components are widely used in many pressure responsive devices. They function as the sensing element in transducers, switches, gauges, actuators, and compensators.

Bellows Benefits
For Transicoil, one of the most important features of the bellows is the enhanced corrosion resistance the component's material offers. The transducers are often exposed to process conditions that would corrode the NiSpan-C or bronze assemblies. Nickel is more resistant to corrosion than brass or bronze, but not so resistant as stainless steel. The sources of corrosion are often unpredictable, such as in marine applications or sewage treatment plants. Corrosion can also be avoided by fabricating nickel bellows plated with gold or silver coatings.

An unforeseen benefit emerged when users of the new bellows-type transducer realized they could take advantage of its overpressure resistance to locate or diagnose problems. With the capsule assemblies, the process was almost always shut down if the transducer failed. The strength of the bellows, however, allows plant operators to continue the process while they track the source of the overpressure. And, after the event, the bellows is still working-which is probably the greatest benefit of all, considering the headaches and expense that replacing failed components entails.

Paul Hazlitt is Director of Engineering, Servometer Corp., 501 Little Falls Rd., Cedar Grove, NJ 07009; 973-785-4630, fax 973-785-0756, paulh@servometer.com.