Superheater - pressure and temperature distribution

[yecko]

TL;DR Summary

how to design thermocouples and pressure gauges, in terms of position and quantity?

Quoted from the O&M manual of an electrical steam superheater:
"The Steam Superheater has eight thermocouples of the NiCr-Ni type, known as "K," attached to the heating element sheath. Four of these thermocouples are positioned near the steam outlet nozzle, where the highest temperature is expected. The other four sensors are also securely fastened and placed in almost the middle of the heating bundle. The thermocouples are evenly distributed between the two heating groups. This means that it is possible to operate the Steam Superheater with only one group active, without any compromise to operational safety."

Questions:
(1) Why are there four thermocouples at the center of the heating bundle? why not the inlet being measured?
(2) why is a pressure gauge not important in a superheater? (can't even find any pressure simulation online regarding superheaters)... but why there is a local legal requirement as below?
"a correct steam pressure gauge connected to the steam space and easily visible by the boiler attendant, ..."
(3) if I position the pressure gauge at the vent of the steam superheater, although it can fulfill the local legal requirement, is it representable in an engineering sense?
(4) why do we have a pressure safety valve on the main body of the superheater, but no need to the heating element sheath?

Thank you very much for your concern : )

 

[Gunter1977]

Hello,

I'm not a specialist, but this is what I know from a study project I did in the past. Perhaps someone else can further answer your question or correct my response.

(1) Why are there four thermocouples at the center of the heating bundle? why not the inlet being measured?
The four thermocouples in the middle of the heating bundle measure the average temperature inside. This is important because the temperature isn't the same everywhere. It's more useful to measure the temperature in the middle than at the entrance because we're more interested in the avarge temperature of the steam leaving the superheater.

(2) why is a pressure gauge not important in a superheater? (can't even find any pressure simulation online regarding superheaters)... but why there is a local legal requirement as below?
"a correct steam pressure gauge connected to the steam space and easily visible by the boiler attendant,
Not all superheaters are used in critical applications where accurate pressure measurements are essential. In some cases, the absence of a pressure gauge may be acceptable. However, it is advisable to consult regulatory guidelines as a reference; I would always recommend installing one regardless.

(3) if I position the pressure gauge at the vent of the steam superheater, although it can fulfill the local legal requirement, is it representable in an engineering sense?

Placing the pressure gauge at the steam superheater outlet may meet local legal requirements, but it might not provide an accurate representation of the steam pressure within the system. Ideally, pressure gauges should be positioned where they can directly measure pressure in the steam space or at critical points in the piping to provide meaningful data for operational monitoring and safety.

(4) why do we have a pressure safety valve on the main body of the superheater, but no need to the heating element sheath?
A pressure safety valve on the main body of the superheater provides overpressure protection for the entire system, including associated piping, akin to a hydraulic system. These valves release excess pressure to prevent catastrophic failures. While the heating element sheath may not have its own dedicated safety valve, the design and operation of the superheater should incorporate safety measures to mitigate pressure-related risks.

Have a nice weekend.

 

[yecko]

In some cases, the absence of a pressure gauge may be acceptable.

May I know in what circumstances would absence of a pressure gauge be acceptable?

t is advisable to consult regulatory guidelines as a reference

Is there any reason to have this regulatory guideline?

While the heating element sheath may not have its own dedicated safety valve, the design and operation of the superheater should incorporate safety measures to mitigate pressure-related risks.

what are the usual safety measures to mitigate the pressure-related risk of the heating element sheath?

your answer is really helpful, thank you very much!

 

[Lnewqban]

... Quoted from the O&M manual of an electrical steam superheater:


Questions:
(1) Why are there four thermocouples at the center of the heating bundle? why not the inlet being measured?
(2) why is a pressure gauge not important in a superheater? (can't even find any pressure simulation online regarding superheaters)... but why there is a local legal requirement as below?
"a correct steam pressure gauge connected to the steam space and easily visible by the boiler attendant, ..."
(3) if I position the pressure gauge at the vent of the steam superheater, although it can fulfill the local legal requirement, is it representable in an engineering sense?
(4) why do we have a pressure safety valve on the main body of the superheater, but no need to the heating element sheath?

Besides the excellent responses of post #2 above, I would like to further discuss your questions:
(1) Why are there four thermocouples at the center of the heating bundle? why not the inlet being measured?
It seems to me that the functions of the thermocouples are to prevent overheating of the dry steam, as well as damage to themselves and to the electrical heating elements.
If my assumption is correct, it is desirable to monitor the internal zones of highest temperature, which are outlet of the device and the middle area of the steam flow cross-section.
Flow velocity and heat exchange is greater there, as shown in the following diagram.

(2) Why is a pressure gauge not important in a superheater?
As the device has a more or less constant cross-section, no significant compression or expansion of the steam is supposed to happen inside of it.
A superheater utilizes only sensible heat to superheat the steam in order to increase its enthalpy and make it dryer for the following downstream process or device.
It seems to me that the pressure gauges of the upstream and downstream devices are sufficient to monitor internal pressure in the superheater.

(4) Why do we have a pressure safety valve on the main body of the superheater, but no need to the heating element sheath?
Sorry, I don't understand your concern about pressure on the heating element sheath.
Normally, thermocouples of NiCr-Ni type K with ceramic sheath are designed to withstand high fluid pressures and about 1200 °C.