We use Condensing pots to measure steam flow. Recently we noticed a decrease in the steam flow. Based on the thermography, we found high side condensing pots at ambient temperature while the low side condensing pot was at steam temperature (400 deg F).

We suspect non-condensables in the low side. We normally do not Vent the condensing pots but after this issue, we vented the condensing pots. After the venting, no change in the flow was noticed but both Condensing pots were measured at steam temperature (400 deg F). Within a week, the high side condensing pot again returned to ambient temperature (105 deg F). We are not sure if there is any water in the water leg and how to check it. Any help will be greatly appreciated. Also, if there is any literature that I can look at to get better understanding.

General description of the setup is: Two taps on the steam line. One upsteam of the flow orifice and one downstream of the flow orifice. Both taps go to the vertically mounted Condensate Pots. The drain at the bottom for both pots go to the transmitter.

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Here is a link to another thread about condensate pots on control.com:

http://www.control.com/thread/1026245169

From my university days, the purpose of a condensate pot was to prevent hot steam from reaching the sensing diaphragm(s)s of a transmitter or similar device. The large condensate pots were there to ensure a water leg of known volume/height is against the sensor element. So, the purpose of the condensate pot is cool the steam to ambient temp.

If that's the case, then it would seem there is a leak somewhere in the low side tubing/piping system. That leak is allowing the water to run out of the condensate pot and steam to flow into it, raising the temperature of the condensate pot.

The condensate pot should be condensing steam and maintaining the level in the pot. I seem to recall that there was some kind of drain line from the pot to keep the water level against the sensing element constant, so that the effect of the water level could be "calibrated out".

If the water in the condensate pot leaks and there is insufficient "residence time" of the steam in the pot to condense, then it would seem that the pot temperature would increase up to steam temperature. If there is no "path" for the steam to flow through the pot, then the temperature will not rise up to steam temperature. But, since the condensing pot temperature is at steam temperature, there must be steam flowing through the pot so fast that it's not condensing and forming or maintaining the liquid "seal" necessary to protect the transmitter diaphragm.

I believe that condensate pots are usually filled for initial operation, but as long as there is no "path" (leak), or even if there is a small leak, the condensate pot should be able to maintain the liquid level. So, it would seem there is at least no liquid in the condensate pot; hopefully there is some liquid on the transmitter low side diaphragm. And, it would seem there must be a steam leak somewhere allowing the condensate pot temperature to be as high as steam temperature. That is also a safety hazard which should be corrected as soon as possible.

But, it's been nearly three decades since university.

 

Your condensate pots should both be above ambient.

The steam goes up into the pot and the level builds up to the point where it runs back into the line so fresh steam is entering the pot all the time condensing and running back.

The condensate pots should not be insulated so they radiate heat.
How much superheat do you have? If the pot is above saturation temperature that might do it.

My first thoughts were you are losing level in the upstream pot through a leak, either to outside or through the 3 way valve (zero) to the LP side

If you have significant superheat perhaps moving the pot away from the line with a longer nipple might help.

Hope this helps

Roy Matson

 

Thanks for your responses. The condensing pots should be at saturation temperature (assuming no leaks). Is there a company that have expertise in this matter. We would like to use an expert as a consultant for troubleshooting. We need to resolve this issue soon as this may have an impact on the plant operation. Thanks.

 

And I should have consulted one of my old texts before posting.

The condensate pot temperature should not be ambient temperature if it's constantly condensing steam.

It's still my impression, though, that the temperature of the fluid (water) that's against the transmitter diaphragms should be much less than saturation temperature which might be high enough to damage the transmitter diaphragm(s). The temperature of the "bottom" of the condensate pot should where liquid should be should be less than the top of the pot, where steam should be.

Sorry for any confusion I may have created.

 

I don't think you need an expert, it's pretty basic. Any instrument tech should be able to fix it for you.

CSA and I both think you are losing water from the HP side. Take a good look around the transmitter if it's leaking there you will see some staining on the ground.

Do you have blowdown valves; they could be leaking. I would suspect the zero valve as well, try closing each of the three way valves, it should hold it's pressure for a while.

 

Normally, temperature in both high and low leg should be little above of ambient temperature. Purpose of condensing pot is to condense steam to water. Since pipes are exposed to ambient, they will cool down and maintain almost ambient temperature, since, normally there shouldn't be a flow in the transmitter pipes.

You need to inspect lower leg, because there you have same temperature as steam. I suspect problem is in leg with steam temperature, not the one which gets ambient temperature.

Normally, condensing pots are used when steam flow is measured by differential pressure transmitters. In that case there must be water columns in both legs to protect the sensor and also enable reliable measurement. Don't forget that dp transmitters measure difference in pressures between high and low leg. If the medium in both legs are not the same, then erroneous measurements are to be expected.

Inspect low leg for leaking. Water somehow disappears, and that is possible only if there is a flow in the transmitter leg, which shouldn't be.

Hope this helps.

 

The low side is obviously not dead headed, it has steam flowing through it, hence the high, near steam temperature.

At first I thought it was steam is escaping from some crack/opening in the piping between the flange union impulse tube tap/port and the condensate pot head.

> Recently we noticed a decrease in the steam flow. Based on the thermography, we found high side condensing pots at ambient temperature while the low side condensing pot was at steam temperature (400 deg F).<

But, how could a low side leak leading to the low side wet leg fill loss cause a DECREASE in flow? Any decrease in head pressure of the low side wet leg would increase the apparent DP at the transmitter, appearing to be a greater flow rate, not a decreased flow rate.

That leads me to believe that the low side condensed water leg must still be intact from the condensate pot downwards (still filled with condensate), with the steam escaping through a crack or fitting in the low side impulse line from between the flange union port/tap and condensate pot head.

But even that scenario does not fit a decrease in flow. A low side steam leak will not develop full low pressure on the wet leg head, leading to a higher than expected DP & flow.

So I'm guessing that the high side and low side designations are inverted.

Not a solution, but it's bothered me for a couple days now, how can a low side loss appear as a lower DP/flow?

 

Check the equalizing valve (on the transmitter manifold), as mentioned before there is a leak somewhere. If the leak were from the LP impulse line to the outside, the difference in pressure between the HP and LP impulse lines would increase and would be seen as an increase in flow.

The fact that your flow has dropped off means that somehow the pressure is equalizing. If the equalizing valve is leaking then it would be logical that you will have flow from the HP side to he LP side in which case both sides would be at ambient temperature or just above. It may be that there is some excessive flashing off of the water seal, or a combination of problems. You should look at the location of your seal pots.

Was the installation working properly before or is it a new installation?

 

You should also check your instrument calibration. Despite the claims, the new generation of transmitters have been known to drift. The transmitter may have problems also if it was not placed in service properly after the last instance instance when it was serviced or during the last outage, the legs may not have been filled initially.

 

Yes, something doesn't make sense.

The upstream side of the orifice plate should connect to the HP tap on the transmitter, The downstream orifice tap connects to the LP tap on transmitter.

You must have a steam leak on the pot that is hot, no ifs, ands or buts IMHO.

I suspect the pot you call the downstream pot is actually the upstream.

 

David,
you're right. I have overlooked that part of the information about decreasing the flow. Now, I wonder if we're faced with the correct description of the problem.

If flow is measured with an orifice plate (or some other similar device that provides differential pressure), then total flow can be calculated roughly as Q = k*sqrt(deltaP).

In normal situation, steam is condensed into water on both legs (high and low) and the condensate pots are doing just that. In both legs, water columns should be at approximately ambient temperature and even if the medium is hot, these pipes to the transmitter should be much colder and at ambient temperature.

Now, if low leg has higher temperature, that means there is a flow in that leg and since steam is lighter than water, deltaP would increase and thus flow would also increase (assuming the transmitter is not faulty).

So if the low leg is at high temperature this indicates that problem is in the low leg, but if the flow is decreased, then, problem is in high leg.

That is really confusing. Is the transmitter working correctly?

First, if you have any other steam flow application, please check the temperatures of both the high and low legs so you have a good reference point.

Is your deltaP (flow) oscillating?

I have never seen such contradictory behavior, so, I'm afraid I cannot help further. I need better description of the problem.

 

I thought I had posted another thought.
Is the downstream pressure fluctuating rapidly? Perhaps the condensate is flashing off, It's a long shot I know.

I still say it's a leak

Condensate pots should not be insulated, should be at the same level.

Please be sure to let us know what you find, it's an interesting problem.

Roy