Whitney Systems of Chelmsford, MA builds a wide range of customized machinery designed to enhance productivity and product quality -- everything from machines for custom automation to O-ring insertion machines to test stands such as electrical testers, flow rate testers, endurance testers, and leak testers. Unfortunately, after creating one such test stand -- a ball valve leak tester for Metso Automation -- Whitney learned that it was relying on a less than reliable test measurement device. An upgrade was needed, and that''s when Setra Systems came to the rescue.
Metso makes cast iron, steel, stainless steel, and brass ball valves that are used in a range of applications, including plumbing, air, water, gas, and oil. Metso''s metal and soft-seated ball valves are a key element in demanding, high-cycling applications. Whitney had built a machine that tested Metso''s different ball valves for pressure decay, featuring an innovative shaft/ball connection that works to assure tight shutoff, even with the lowest pressure differentials.
Whitney''s ball valve leak tester tests for pressure decay in four locations: at the stem of the ball valve handle, on either side of the ball, as well as through the ball itself. The operator puts the ball valve into a "nest" in the machine, and hits the start button, which initiates formation of a pressure seal around the ball. A pneumatic actuator is used to open and close the ball valve. The ball valve being tested gets pressurized with 100 psi of air and a pressure transducer is used to pinpoint and measure pressure drops.
Pressure Transducer Fails the Test
Unbeknownst to Whitney, a faulty pressure transducer was incorporated in the ball valve leak tester. The company that manufactured the transducer had changed its ratings without notifiying Whitney of the change. So while the machine was testing for ball valve leaks, the transducer actually ruptured -- it literally couldn''t handle the line pressure, with the machine requiring testing at 100 psi. To make matters worse, Whitney sent the transducer back for repairs and, shortly after its return, the component ruptured again.
According to Whitney Systems, locating the exact cause of the problem wasn''t as easy as it should have been. "When the old transducer broke, it took 4-6 weeks to figure out that our supplier had changed the rating on us."
Clearly Whitney needed to upgrade to a new transducer, but finding one that could meet the rigorous requirement of this ball valve tester wasn''t easy. The transducer would have to be able to measure both very low pressure differential changes as well as withstand high line pressures.
Meeting the Highest Pressure Demands
"We looked at a couple of transducers that could meet some of our needs, but they couldn''t satisfy the 100 psi line pressure requirement," Whitney''s staff said. "When we spoke with Setra, we realized that they not only had a transducer that was rated to handle our high pressure needs, but they had it available, off the shelf, without requiring any customization. Within a very short number of days, we were back up and running with Setra''s Model 235 pressure transducer in place."
Setra Systems Inc''s Model 235 differential pressure transducer is actually designed for high line pressures up to 2,500 psig. Used also in municipal gas distribution, oilfield gas recovery, coal mine ventilation, and electricity generating plants, this versatile pressure transducer was exactly what Whitney needed.
"The Model 235 is unique in its ability to handle high and low line pressure changes," according to Setra sales manager Paul Richards. "The Model 235 virtually eliminates the need to balance the high and low pressure at installation to minimize line pressure error on a differential measurement. Users can expect a minimal low line pressure effect of less than 0.1% FS change per 100 psig line pressure change."
"The Model 235''s utmost accuracy allows us to ensure that the ball valves fall within Metso''s acceptable leak rate of 0.45 cc per minute. Setra''s patented technology was a great fit for both our application and our budget," according to Whitney staff.
Unique Inductive Technological Advance
With Setra''s unique inductive sensing technology, the Model 235 does not contain a liquid-filled diaphragm and is not media-sensitive, unlike competitive transducers. This eliminates the possibility of batch contamination and thermal transient errors, as the only wetted parts exposed to media are stainless steel, alumina, ceramic, and glass.
The Model 235 is accurate to +/- 0.18% of full-scale output, with a total error band of +/- 0.5% full scale over the compensated temperature range of -20° to +160°F on 5 psig through 500 in. WC pressure ranges. There is virtually no shift in output at warmup, and a typical update rate of 25 ms makes this unit ideally suited for pressure applications that require a stable, fast response -- applications like Whitney''s ball valve leak tester.
"Since we started using the Setra 235, we haven''t had any problems," Whitney''s staff concluded. "The transducers have proven both accurate and durable. In fact, we just recently bought our fourth transducer for the Metso test stands."
With high line pressure transducers in place, the people at Whitney Systems are actually feeling quite a bit of pressure relief. And their customers, like Metso, can rest easier knowing that their ball valves are production-ready, having passed the most stringent of pressurized performance tests.