Steam thermodynamics

Tushar Puranik · Jul 8, 2022

Hi All

I am working in Design dept. in a company supplying Desuperheater & PRDS Station.
In recent past, we have supplied a PRDS of 160TPH capacity. Steam at 540 deg. C & 98 bar is getting reduced to 220 deg. C @ 20 bar pressure (at outlet).
Temperature sensor is installed @ 18 mtr distance from the DSH.

This steam is entering into a common header at about 4 Km.
There are steam traps installed @ certain distances from PRDS. But None of the Trap is passing the condensate.
The temperature sensor is installed @ 600 mtr & directly on common header.

Now I am facing a strange issue.
With the Desuperheater the temperature is getting controlled (at around 220 deg. C) till outlet of PRDS.
But at temperature sensor installed at 600 mtr., the temperature is senses as 230-235 deg. C
Further @ header the temperature is further rising to 250 deg. C.

Here we have to note, when the steam is being supplied from PRDS all the steam entering points in the common header are closed.

This is a very strange behavior observed. Normally when the steam is travelling such distance the temperature is reduced by 2-3 deg. C But here the temp. is rising by about 30 deg. C.

Need your experts advise.

Tushar

MortenA · Jul 21, 2022

or one (or more) of your instruments are off or installed in a poor way that gives a misreading?

--- Best regards, Morten Andersen

danw2 · Jul 22, 2022

A thermocouple sensor (as opposed to an RTD) requires cold junction compensation.

Cold junction temperature is the temperature difference between the thermocouple connections at the temperature's analog input and the freezing point of water, 0°C

If the temperature sensing device is a typical thermocouple transmitter in an environment/area where it can get up to 30°C so the connections are 30°C, then failure of the cold junction compensation would cause the transmitter to report a temperature 30°C (or whatever the cold junction temperature is) lower than actual temperature.

Some thermocouple analog inputs or thermocouple temperature transmitters can allow the internal cold junction measurement to be turned off, or are configured for remote cold junction measurement, which can produce the same result - a reported temperature lower than actual.

This is a long shot; it's not a common problem but it can happen.

Tushar Puranik · Jul 22, 2022

Thanks for the reply.
But what we observed is the temperature at the sensing point (18 mtr distance from spray point) is okay & is as per the set point.

But after certain distance, may be around 400-500 mtr. the rise in temperature is reported. (off course there is not temp. sensor in between hence this reporting is at such long distance)

So the point of worry is how the steam temperature is increasing automatically ? without addition of any high temp. steam.

Pls. advise.

georgeverghese · Jul 24, 2022

"Here we have to note, when the steam is being supplied from PRDS all the steam entering points in the common header are closed."

If the sensors are all temp transmitters, then they would be reading right. But not if one or more of them were temp guages.

If they are all TT, then would suspect this assumption is not correct - one or more of these other steam entry points into this header is probably leaking a lot.

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