Condensate Pot for Pressure Transmitters

S

Thread Starter

Sukat

I'm preparing a Hookup design of pressure transmitters for steam application. I checked my previous project standards and some required condensate pot only for steam more than 30bar (400degC) to protect pressure sensor with a design of 80degC max allowable temp. I checked some Steam Turbine generator (STG) and Boiler vendor's hook-ups but they not at all recommend to use condensate pot (even for high pressure steam as high as 140bar).

Now, I am more convinced that condensate pot is not necessary for pressure transmitters because of the temperature gradient effect, rule of thumb 1degC/mm at steady state (zero flow). So, if my sensor temp rating is only 80degC, using the rule of thumb, I can tap it to steam line as high as 600degC with minimum 520mm of non-insulated stainless tubing to get a temp drop of 520degC before the sensor.

Appreciate if anyone can share his/her thoughts on my understanding.

Thanks
 
F

Frank19842003

For Steam applications is necessary to use condensate pots since it helps to equalize the differential pressure transmitter (make a 0 differential pressure). the thing is that when you fill the two pipes to the + and - pressure input, the level of the fluid will always be constant at the middle of the condensate pot (if there is more water in one of the condensate pots than in the other the steam will vaporize it). with this condensate pot you guarantee that the hydrostatic pressure of the conduit is the same in the positive and negative input of the DP Tx.

i can also tell you that as a rule of thumb, you can say that per every 1m of piping you cool down the process pressure 100°C approximately. that´s true. So in other applications you can consider this in order to cool it down or warm it up. however, in this applications with steam the condensate pots are a must.

I hope it helps.
 
Hi Frank,

thanks for your reply. I am referring only to pressure transmitter (not d/p), where balancing is not necessary.

In this case do I still need condensate pots?

I have seen varying designs and opinion on this:

I have at least 3 projects where pot is not used even for high pressure steam. In one other project, the client required pot for high pressure (50bar and above) and medium pressure (6bar to 50bar) steam but not for low pressure steam. In this case I think pot was used only for safety purposes, where during commissioning you fill it with water before actually opening the isolation valve on your steam line whereby thermal shock is eliminated in the pressure cells in you PTs'.

What is your opinion.
Thanks
 
Hi there,

Seems like all the replies so far are referring to ancient steam flow measuring techniques.

Just for information the following installation standard can be used for steam flow applications as well, there is no need for these catch pots either in flow applications.

The impulse line can serve the same purpose as a catch pot so no need to use the catch pots on a steam pressure or flow installation. The only difference is that the transmitter will be a bit further away from the tapping point without a catch pot in the installation.

Think in terms of volume of liquid between the tapping point and the transmitter and you will see there is no difference if you just make the impulse lines a bit longer. The impulse line only design, will actually cool the condensed steam faster per meter than the pots due to overall volume per surface area contact with the cooler environment.

The tapping point should be made to the side (10 or 2 o’clock) or preferably the top (upper quadrant - 12 o’clock) of the process line, and fitted with a suitable rated process isolation valve.

The pressure transmitter should be positioned well below the tapping point so that the impulse line will stay filled with condensate while in service (same as wet-leg in a steam drum level application). A T-Piece must be installed at the bend in the impulse line before it goes down to the transmitter for the purpose of filling. a Suitable rated isolation valve must be installed pointing upward on the T-Piece, with a screwed plug on the open end for safety purposes.

The distance of the impulse line from the tapping point down to the transmitter should be chosen to ensure that adequate cooling occurs to prevent thermal damage to the transducer. Transmitter temperature rating as well as steam possible max temperatures needs to be considered to calculate the length and not just the "rule of thumb".

The impulse line MUST be filled with ambient temp water prior to start-up (putting transmitter online for the first time) to prevent possible thermal damage to the transducer by the live steam.

The original idea if the catch pot, a million years ago, in steam level, flow and pressure applications was to form a thermal buffer between the steam and the transmitter and to keep the impulse line to the transmitter filled to exactly the same level all the time. The T-Piece fitting instead of a catch pot, can do exactly the same job. All you need to do is install the transmitter in some applications a bit lower down or further away from the tapping points. The overall effect is exactly the same in level, flow and pressure steam installations.
 
Hello Everyone,

Pls help me. I am battling with the installation of steam flow meter. The manufacturer is confusing me. Flow Meter is using pot & is not reading 0 TPH. Initially the condensate pot was below steam line. But due to condensate Totaliser use to stop reading, so b/c of it manufacturer ask me to raise condensate pot above steam line. But after amendment it stop reading.

I checked the DPT. it was okay but b/c of manufacturer send it to lab for calibration & they also confirm the same that DPT is okay. after that manufacturer ask me bring the Condensate Pot to the same level with steam line, then also it's behaving same. I am completely screwed over. My boss is quarreling with me.

Pls help me to overcome from this situation.
 
The idea is to
- have two condensate pots, one for each impulse leg, at exactly the same elevation

- have a constant level of water in each leg, so that at all flows, the head pressure of the liquid column in the impulse tubes will be equal, and therefore will contribute any pressure to either side of the DP.

Read Sam's post above, and look at a couple of these graphic illustrations to get an idea of what the seal pots should be doing.

This one shows the contents of the condensate pot: steam in red, water in blue:
http://i43.tinypic.com/34guafc.png

This one shows the Filling Tees that do the same thing as a condensate pot, that Sam is talking about, on a horizontal steam pipe:
http://i41.tinypic.com/b97zon.jpg

Similar to the one above, but for a vertical steam pipe:
http://i40.tinypic.com/scx1ed.jpg

This one shows close coupled filling Tees above the horizontal steam pipe
http://i42.tinypic.com/206jwp1.jpg

This one shows older style piping with horizontal condensate pots:
http://i41.tinypic.com/2mwwgaq.jpg
 
A

Asok Kumar Hait

In my experience there is no technical problem if you install the pressure transmitter without a condensate pot as long as you keep sufficient impulse tube length for the cooling down of the temperature. However, use of condensate pot has some advantages. It allows steam line to be short and most part of the impulse tube handling liquid only thus reducing the chance of leakage. Also condensate pot allows for easy start up without waiting for steam to cool down.

But for DP flow measurement application condensate pot is a must. Not only that condensate pot location is also vital. The main technical reason is to avoid the unequal condensation between two lines. Otherwise you will get wrong DP and wrong flow measurement. My personal experience in one project I have seen the flow signal to be oscillating continuously.
 
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