Steam Question

You are correct.There must be a trap to islolate the pressure sensor from the high temp. of the steam.If the temperature was lower at the tap, the pressure would have to drop, and this is not possible in a closed system such as you describe.

Why don't you just use a pressure sensor that can handle the high temperature? (something made for steam) There are many to choose from. I use SMAR pressure and temperature transmitters in boiler rooms that routinely see 140F ambient temps to measure superheated 600psi steam and 150psi and even 1psi. Make sure to compensate for head pressure if the sensing line will fill up with condensate.

Hi, I am not sure that I understand precisly what you are doing, but it seems to me that if you are trying to measure steam pressure with a sensor that cannot take the heat and one is suggesting that a long tube be used to take the sensor away from the measuring point - this makes sense to me. As far as I can determine there is no steam flow in the 6 ft pipe leading from the measuring point to the sensor, therefore if the pipe is thin compared to its length I would expect that no the heating would not be a problem. The pressure will be faithfully transmitted but the heat would not be because the steam is not flowing in the measurement pipe. In practice it coule be just air which is trapped in the thin pipe.

Steam by its nature is a GAS, which means that it will expand to occupy whatever volume is available. All gasses do this, which is why we have an atmosphere of Nitrogen, Oxygen, and a lot of other stuff- it's called the partial pressure principle where the pressure of each gas is based on the % of the mix and the total pressure of the mix. The tube will fill with steam, which will heat the air (and the condensate from heat transfer will fall to the main) until the air is at the same temperature as the steam. The ultimate "blend" of steam/air in the tube will be based on the mix- 99.999+% steam and almost no air- in the entire system. See partial pressure above.

Given enough time for the system to reach equilibrium, the sensor will read the pressure at the end of the tube. Give the liklihood that the conditions at the end of the tube will be different for a while due to temperature differences, condensation, fluid flow etc., the response time may not be fast enough...