Hi temp Differential Pressure gages 1

[Jmccray]

I have a project where I need to measure air velocity via velocity pressure. The gages that we use now (Magnehelic) have a max temperature tolerance of 140F and the air exhaust can get up to 1000F. I plan on using copper tubing to help dissipate some of the heat but I wanted to know if there was a product out there that could handle a higher temp or any suggestion on better tubing to use

 

[Compositepro]

There is no flow in the sense tubes so there is no need for the gage to be high temp. Just don't make the tubes too short.

 

[btrueblood]

And, if this is combustion exhaust, add a means to drain the condensate out of the tubes from time to time.

 

[DRWeig]

Here's a thread that covered this topic:

http://www.eng-tips.com/viewthread.cfm?qid=278809

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[moltenmetal]

Don't drain the condensate: instead, ensure that both impulse legs are filled with water and the device is zeroed prior to starting the flow. Otherwise, as condensate forms, it will accumulate unevenly in the impulse legs and affect the measurement.

 

[btrueblood]

Molten,

That might work, but then - how do you ensure the water fill doesn't boil off unevenly, which it will if there is a pressure differential between the ports, leading to an error in the measurement. Also, depending on the gage, dry or non-condensing gas may be required at the sensor face, which a wet fill won't give (I know most Magne-helic gages, at least the aluminum-bodied style, will corrode horribly if exposed to moist conditions).

 

[Compositepro]

It is common practice in steam gauges that there is a pigtail (loop) in the line to the gauge. The loop traps a constant level of water between it and the gauge. The section of line between the loop and the pipe is designed to drain back to the pipe.

 

[moltenmetal]

If the process gas contains vapours which are condensible at ambient conditions, condensate will form. Murphy's Law says that it will not accumulate uniformly, even if the dP is too small to make much of a difference between the two sides of the transmitter. Draining the condensate will only work for periodic measurements, which are all you're going to get out of a Magnahelic gage anyway, so that might not be a bad strategy. For continuous measurements, that approach will be a failure. Just be careful not to allow too much flow of hot gas through the impulse legs while draining, or else you'll cook the gage.

If you don't like wet legs and the measurement is continuous (i.e. a dP transmitter rather than a gage), you can try to heat trace the impulse legs to prevent condensation, but your gage/transmitter needs to be able to withstand the condensation temperature in that case. That route often leads to failures in practice.

A pigtail or U trap on both legs, if properly designed, can provide a dry environment to the gage while also providing a constant level of condensate in the impulse legs- by allowing excess condensate to drain back to the source piping as Compositepro mentioned. This is not an ideal option in a freezing environment unless you heat trace the U traps. Imagine, heat tracing something you want to lose heat from?! But if the ambient doesn't fall below freezing, it's relatively maintenance free.

This isn't new stuff- but it's also one of the things that people screw up most often when making DP measurements for flow or level.