I'm a bit new to control and instrumentation, so I'm reading this book called <i>"Instrumentation for Process Measurement Control"</i>. The info is a bit out of date but where I'm working is a bit out of date so not a problem.

In a section explaining the operation of a <i>"Force-Balance Pneumatic Pressure Transmitter"</i>, specifically one based on a Bourdon tube for high pressure it says, "For safety, oil or liquid, never air, should be used for high pressure calibrations." The full paragraph is below.

I'm confused slightly why it would make any difference. I'm sure there is a good explanation, but initially I would see no difference between the two. If something breaks they'll both be at the same pressure I would have thought. I'm sure someone on this forum could explain it.

Book Excerpt
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If the pressure to be measured is high, such as 5,000 psi or 35. MPa, a different sensing element is employed. The pressure being measured is applied to a Bourdon tube. This pressure tends to straighten the tube and causes a horizontal force to be applied to the lower end of the force bar. The diaphragm seat serves as both a fulcrum for the force bar and as a seal for the pressure chamber. The force is transmitted through a ftexure connector to the range rod, which pivots on the range adjustment wheel. Any movement of the rangerod causes a minute change in the clearance between the ftapper and nozzle. This produces a change in the output pressure from the relay to the feedback bellows until the force in the bellows balances the force created by the Bourdon tube. The output pressure, which establishes the force-balancing, is the transmitted pneumatic signal that is proportional to the pressure being measured. This signal is transmitted to a pneumatic receiver to record, indicate, or control. The calibration procedure for this transmitter is the same as for the low pressure transmitter, but the calibration pressure is developed with a dead weight tester. <b>For safety, oil or liquid, never air, should be used for high pressure calibrations.</b>

 

Because air can be compressed and hence store potential energy which can quickly released during a failure. Think of popping a balloon or a catastrophic failure of an inflated car tire.

Liquids are incompressible and quickly lose pressure on a rupture resulting in much less damage.

 

Up to and at the moment "something breaks" they are at the same pressure. However, immediately after that, the pressure of any liquid drops to atmospheric, because the liquid is near-incompressible. The pressure of air or another gas stays much higher for a longer time (depending on the leaking mass flow rate relative to the original gas mass of course).

 

Like they said ...
But also - never use oil if the gauge or transmitter is likely to be used on pure oxygen or with another oxidising gas: the combination of a potential fuel and high oxidising potential with high pressure as an ignition source may cause an explosion.

Bruce.

 

Thanks guys... Obviously that answers my question.

 

Because oil at high pressure won't ignite anything flammable like 5000 psi air can.

Regards

cww

 

In the actual instrument, the process itself is at high pressure, e.g. 5000psi. To calibrate the transmitter, you need the tester to develop comparable pressures. Generating that high pressure with compressed air is very difficult and would require a multistage compressor. The connecting fittings would also need to be rated at the high pressure. This is not common for any compressed air application. However, oil and water are essentially incompressible and such high pressures are easy to generate with even simple hand-held pumps. Furthermore, containing such high pressures inside process equipment is normal and can be accomplished easily.

The caution is essentially for keeping safe. You do not want your calibration piping/tubing to burst while you are running tests. A bursting air hose/pipe can be a physical danger. Bursting a high pressure liquid/hydraulic line is an inconvenience, but probably not a hazard.

 

Dear there
List bellow shows the difference between Pneumatic and Oil system:-

1-Air accessible to be compressible while oil can not be compressible.

2- Power generating from oil system is more largest than Pneumatic system.

3-Feedback pipes not needed in the oil system while it needs in the pneumatic system.

4-external leakage in the oil system will lead to critical situation while internal leakage in the pneumatic system will lead to critical situation.

5-Air lacks lubricating property and always contains water vapor. oil functions as a hydraulic fluid as well as a lubricator.

6-the normal operating pressure of pneumatic system is very much lower than that of hydraulic system.

7-Accuracy of pneumatic actuators is poor at low velocities, whereas accuracy of hydraulic actuators may be made satisfactory at all velocities.

8-Normal operating temp. for pneumatic system is 5 to 200 C, while normal operating temp. for hydraulic systems is 20 to 70 C.

9-Pneumatic systems are fire and explosion proof,whereas hydraulic systems are not.

I hope that notes above make the difference clear to you.

Best Regards
Mustafa R. Ahmed
Control and System Eng.
[email protected]