Monthly Archives: September 2015

Reducing Glove Box Filter Costs

Glove Box Filter

Handling radioactive materials must be done in glove box. A glove box is a clear plastic enclosure with rubber gloves attached to the sides so that an operator can handle material inside the glove box, but air and radioactive dust are not allowed to escape. The air pressure inside the glove box must be controlled so that it is always less than the ambient atmosphere. A series of risers, ducts, fans and filters connected to the glove box keeps the pressure inside the glove box lower than the outside atmosphere so that no dust can escape.

If the airflow velocity up the riser from the glove box is too high, radioactive dust is carried into the exhaust system, and these particles are trapped by a series of special filters. When a filter needs to be changed, it is very expensive to dispose of the dirty filter because it is charged with radioactive dust and dirt. The key to reducing filter disposal costs is to limit the exhaust airflow velocity so that dust is not carried out of the glove box and into the filters, while at the same time insuring that the pressure inside the glove box is always less than the ambient atmosphere.

Measurement of the air velocity in the glove box risers is accomplished with a pitot tube and a sensitive differential pressure transducer. As air velocity increases, the differential pressure across the pitot tube increases. The very low velocity needed to exhaust air from the glove box, without raising dust translates into a very low pressure drop across the pitot tube. This pressure drop is as little as 0.02 In H2O under normal operating conditions. A very sensitive differential pressure sensor, combined with a high level DC signal output, is needed to provide the air velocity signal to the system controller.

A special version of the Validyne P532 provides the low pressure measurement required for such a control system and these are used at a major North American nuclear fuel rod processing facility.

What is the Difference Between Line Pressure and Overpressure?

The over pressure and maximum line pressure specifications for differential transducers are often confused. This application note will describe the differences and give examples.

Over Pressure:

For all differential pressure transducers, over pressure is defined as the maximum differential pressure the transducer can withstand without compromising subsequent measurements. This is determined by the Validyne pressure range code of the transducer . The range code is given in the transducer model number.

For example, a transducer having a -36 range code will measure from 0 to +/-5 psi, differential
pressure. If the sensor is exposed to up to 10 psi differential, no damage will result and the transducer can make subsequent measurements of 5 psi and below accurately.

Some further examples:

-42 will tolerate a maximum differential pressure of 40 psi
-20 will tolerate a maximum differential pressure of 7 In H2O
-56 will tolerate a maximum differential pressure of 1000 psi

Note that the maximum pressure that the P55 or DP15 sensor can contain is 4000 psig! So the -64 range, with a full scale of 3200 psi differential will likely leak once the over pressure reaches 4000 psig – somewhat less than twice the full scale range of the transducer.

Line Pressure:
The line pressure specification is the maximum pressure that can be applied to both ports at the same time. The maximum line pressure for the P55D, for example, is 3200 psig, and this is the maximum pressure that can be applied to both ports simultaneously. It is often necessary to measure small differential pressures at high line pressures – as in measuring the pressure drop across a high-pressure filter. The filter may operate at 1000 psig, but have less than a 5 psi differential pressure drop across it. A P55D with range code -36 could be used to measure the actual pressure drop across the filter because the + port (upstream side of the filter) might have 1005 psig and the – port (downstream side) 1000 psig. There is a difference of 5 psid but the common-mode line pressure is 1000 psig.

There is a slight error that occurs as a function of line pressure – the zero output will shift as much as 1% per 1000 psig of line pressure. This can be corrected using a 3-valve manifold so that the differential pressure can be equalized across the sensor while the full line pressure is applied to both ports. Turning the Zero adjustment will re-zero the output signal at the operating line pressure.

For any differential transducer operating at high line pressures, care must be taken not to expose one side of the transducer to full line pressure while the other side is at atmospheric pressure – this will result in severe over pressure and require repair.

 

Pressure Transducer