8 replies to this topic

[premjagan]

In manufacturing plant, Steam temperature measurement at various pressure is very important since it is used for various purposes like turbo drives, heat exchanging media etc. so exact value of the steam header temperature is required. But steam input to main header from various units like boiler, turbine exhaust at different temperatures. To nullify the temperature variation from various unit, Is there any general rule to locate the temperature measuring device to show the exact value of the steam header?

[jrtailor09]

Premjagan,

 

Steam temperature in plant calculated by thumb rule.

 

Just do 2 times square root of absolute pressure in barg & then multiply by 100. This will give steam temperature.

 

For  example suppose your LP steam pressure is 3.5 barg then steam temperature =  sqrt (4.5 ) =2.12 again sqrt(2.12) =1.456*100 =145.6 °C.

 

I hope it will help.

 

Regards,

Jatin Tailor

 

Secondly you can use the instrumennt infrare temperature measurement to know exact temperature.

 

find below link .

http://www.omega.com...ook=Temperature

 

 

[ankur2061]

premjagan,

 

Temperature measurement for steam is best done using Pt 100 RTDs mounted inside a thermowell. Thermowells can be mounted perpendicular to the pipe, at a 45° angle to the pipe or in a pipe elbow.

 

Mounting a thermowell in a pipe elbow gives the advantage of allowing a high immersion length of the thermowell in the process system and thorough mixing due to turbulence in case of temperature measurement of mixed vapor or gas streams where the individual gas or vapor streams have different temperatures.

 

Thermowell installation guidelines can be found from the links below:

 

http://bhelpssr.co.i...I book/1_12.pdf

 

http://www.bapihvac....s-Explained.pdf

 

http://www.flowcontr...e-asme-standard

 

Hope this helps.

 

Regards,

Ankur.

[joesteam]

If the steam is saturated, a pressure measurement can be made that will give you the steam temperature by default

[kkala]

1. SQRT(SQRT(P))*100 (P in bar) is simply a rough approximation for steam saturation temperature (oC) in usual operating limits.
2. Steam of various temperatures enters the header. Query is understood to ask how far downstream (of last steam entry) total steam will get adequately homogenized to have practically uniform temperature. Of course measured temperature at that point has to be precise, but main concern is to specify the proper point of measurement, which may depend on conditions.
-Nothing practical has been found. Probably the "Efficient use of steam" <http://books.google....AAJ&redir_esc=y> has some guidelines, but I do not have access to the book now.
-Simple static mixers could promote mixing, as also swirl flow, <http://web.anl.gov/P..._03-98_0266.pdf>, but never heard of something similar  for steam.
-A pipe elbow or something to increase flowing steam pressure drop would promote mixing downstream, but quantitative result is not known (to me).
Probably Instrument Engineers can give practical advice, concerning distance of Ti for representative measurement.
3. Steam in headers is superheated in local Refineries, e.g about 40 barg-400 oC, 12 barg-285 oC, 5 barg-220 oC.

Edited by kkala, 21 February 2013 - 01:21 PM.

[Pilesar]

Steam header temperature is not necessarily the same at every point. There is no ideal mixing in the header and there is no need for it. Steam will be hottest near the letdown station from the higher pressure header or immediately after a superheater coil. If steam temperature is important for a particular service, then measure the temperature at the point of use. For example, a TI on the reboiler supply line may be handy to evaluate superheat. Steam will be always at the saturation temperature or higher. Usually that is good enough if the traps are working properly.

[kkala]

Thanks. Pilesar, for the practical information. Do you have some picture of the steam temperature variation in a steam header? I conclude from some available steam data (avg 43 kgf/cm2 g - 375 oC) that temperature can vary about ±25 oC (and pressure ±2 kgf/cm2) from generation BL to consumption BL (without reducing stations). This may not concern only the header.

Edited by kkala, 21 February 2013 - 02:39 PM.

[Pilesar]

Headers may have multiple inlets and multiple outlets. Steam temperature in the header depends on the source. At the letdown station it will be some approach to the temperature of the higher pressure header. Piping specs have both a pressure constraint and a temperature constraint so sometimes the letdown stations will have a desuperheater which adds a bit of water to make more steam and reduce bulk superheat. Superheated steam is good to drive tubines and saturated steam is best for exchangers, but in a plant you generally source the steam according to proximity in the plant layout. There is heat loss in the piping to consider and usually the focus is on removing condensate reliably. I have seen problems with superheat in exceeding pipe specs and also with reduced heat transfer coefficients in exchangers. Typically the goal in a processing plant is to have just enough superheat so the steam will stay dry. The steam temperature is monitored (usually at one, maybe two locations on the header) so that the plant runs steady and the operators get a feel for the equipment operating as it is supposed to. Operations controls superheat at the highest level but for lower levels they control steam pressure and let the temperature fall where it will.

[kkala]

Thanks for the useful info on steam headers. In local refineries high pressure (HP) steam is superheated by about 150 oC, probably because most part of it is let down through back pressure turbines (post no 5). Firing rate of boilers is controlled by  HP steam header pressure, which reacts fast to HP steam "demand".