Vacuum Steam Troubleshooting - Failed traps suspected but need confirmation of vacuum fundamentals

Zoso

I am having a very difficult time finding any good design information on vacuum steam systems. I'm not trying to design one, but rather I'm trying to troubleshoot one and don't know what is right or wrong. Having confirmation that I know the fundamentals would help. I appreciate any comments.

As I understand it, a vacuum condensate return removes air from the condensate lines and in turn the steam lines, thus drawing a vacuum. In my mind, the only way a vacuum would be drawn in the steam line is through any thermostatic traps or elements in F&T traps while those elements are open (i.e. before the hot steam closes them). Is that correct? The vacuum return system crosses over into the steam supply through open thermostatic vents? Or does the vacuum system connect to the steam supply some other way?

The few piping diagrams I have found show an "equalizing line" from the condensate to the steam header. There is a check valve in the line. I am assuming that check only opens if the steam header were to fall under a deeper vacuum than the condensate header. So in other words, it's a "deep vacuum breaker" to put the steam header under the same "more shallow vacuum" as the return header. This equalizing line also had a U-Trap in it. Why???

So if I am correct so far, I think I understand the basics. Now, let's say I have some failed traps in the system that are always open. So now, I am constantly sucking steam through the coils, traps, etc all the way back to the vacuum pump. The problem I am facing does have symptoms of steam discharging from the vacuum pump. This is combined with poor heat transfer from coils, so I am thinking that I may be pulling steam so fast through the coils that it isn't condensing. So the coils are essentially sensible heat transfer devices with hot vapor inside the tubes (that doesn't condense) and cold air outside that doesn't heat well. Sound possible?

The other possibility I wondered is at what temperature do the thermostatic elements close? Is it possible that I have 15" Hg vacuum steam at 180 degrees hitting the thermostatic elements, and the vents remaining open because they aren't hot enough to close?

The system is unique in that the boiler makes 10 psig steam. Some of that steam goes off on it's merry way like any other low pressure, gravity return system. But then there are a couple of zones with control valves right off the 10 psig steam header. These valves modulate to maintain 15" Hg vacuum on the other side. The vacuum return tank is around 20" Hg. Is this a normal way to control? Let's assume for now that the atmospheric and vacuum returns do not criss-cross.

The overall problem is that the steam header gets "sucked dry" under certain conditions, the heat of the steam doesn't get properly transferred at the coils, and steam comes back to the vacuum pump. That suggests to me that traps are failed open. Any thoughts on that or the overall system? 1-line diagram attached. multiple zones exist for both the 10 psig and vacuum zones, but i only showed one of each.

Steamhead

You've got the idea. There should be no connection whatsoever between the steam side and the return side except thru a trap. If steam is getting back to the vac pump, you have bad traps. Fix them immediately before the steam kills the pump.

To find the bad traps, get yourself an infrared camera such as a FLIR, and check for heat in the return lines. When you find it, you can follow it to the bad traps.

That dip in the equalizing line is there to keep the check valve wet, so it would seal more reliably.

If some parts of the system are slow to heat due to insufficient air venting, you can install larger crossover traps. Here is an extreme example that worked quite well:

http://forum.heatinghelp.com/discussion/142217/the-king-of-all-crossover-traps

Pumpguy

That equalizer line does just that; it equalizes the pressure between the steam side and return side. This vacuum does not need to be broken, just equalized. With this pressure equalized, there is no pressure differential to stop the flow of returning condensate, which then flows by gravity to the lowest in the system.

In lieu of the dropped check valve in the equalizer line, you can use a 3/4" thermostatic trap an the end of an uncovered cooling leg coming off the top of the steam header. The downstream side of this trap drops down to a return line. With steam up, the trap is closed. With no steam, the trap opens and equalizes the pressure between the steam and return side.

Do you have an equalizer just downstream of each steam zone valve? You should have. Whenever a zone valve closes, the steam that is downstream will condense, causing a high induced vacuum due to the large change in volume. Unless equalized, condensate will not flow out of this part of the system until the zone valve opens again, raising the pressure.

I don't think you have 15" Hg. vacuum with 180 degrees F condensate. According to the tables for PROPERTIES OF SATURATED STEAM, this is the maximum attainable vacuum at this temperature. A typical vacuum pump for a condensate return application producing its maximum vacuum will track this table, minus about 5" Hg. vacuum.

Is there a reason you're operating with 10 PSI steam pressure at the boiler and main steam header? Typically, you should be able to operate down around 2-3 PSI.

Keep in mind that the higher the steam pressure, the higher the mass of steam will be. Any leaking trap will pass a greater mass of steam into the return line. Just lowering the steam pressure will lower the mass of steam leaking into the returns, and may reduce the temperature at the vacuum pump. Ideally, you would want the mass of steam entering the heat exchanger to equal it's condensing rate. Because the heat output of the exchanger will vary with the weather, we need a valve to modulate this flow, so a trap is installed.

We may need more discussion to get a better understanding of your problem and solution.

Pumpguy

Equalizer lines file

Pumpguy

equalizer line file

Zoso

I don't know enough history on this building, and it is easy to pick apart someone else's design, but I just trying to address the problems I know of. This building apparently used to be high pressure steam - 80 psig - but some zones were still run in a vacuum. The operators claim that they could never get the heat up to the top floors (i think there are 60 stories) without vacuum.

A couple years ago, a low-pressure (8 psig) system was installed, apparently without changing traps or riser piping. It sounds like a disaster, but again, I don't know enough to be that critical. I am being asked to provide opinions to help a service technician troubleshoot.

I know there are failed traps because we opened a union after a trap on the vacuum system and steam was coming out under slight pressure. But this in and of itself was strange because the steam side is supposedly under vacuum. I though air would get sucked in, but steam came out instead under very low pressure. Maybe it was flash from the vacuum????

Aside from that, I don't think there is an equalizing line from steam to condensate. Since I have several zones on 8 psig steam and several under vacuum, I think I need an equalizing line after each vacuum zone valve, correct? I modified my sketch. Any idea how to size these lines?

I originally thought I might need vacuum equalizing lines around each coil trap (similar to the way you would install a vacuum breaker on a pressurized system), but I think the thermostatic element in the tap serves this propose.

The operators don't know what to do. Their strategy is to increase the boiler pressure and increase the vacuum, because they are out of options. I don't know how they made it through last winter, because it seems like the system is not going to heat well. Perhaps they did not have as many failed traps. I believe a trap survey is scheduled soon.

Regarding the thermostatic element on an F&T trap, how do you determine the temperature at which the element will close. A typical trap at this building is a Barnes & Jones FT2015-5. When you look up the literature on this trap, you get generic info that the traps are rated anywhere from vacuum to 125 psig. and the only thing it says about the element is that it closes at "saturated steam temperature". But saturated at what pressure???

So I probably have a combination of periods of deep vacuum in the steam line that eventually get overcome as the zone valves (or coil control valves) open. I also have some failed traps that are constantly sucking steam back to the vacuum pumps. And those coils with failed traps probably aren't condensing much steam, which of course is killing heat transfer.

Pumpguy

The equalizer shown in your sketch is correct, although IMO, the equalizer should be downstream of the coil control valve. When that valve closes and downstream steam condenses, a high induced vacuum (aka vacuum of condensation) will occur.

If this vacuum of condensation is greater than the return line vacuum, the unfavorable pressure differential will prevent condensate from draining from the coil. Condensate will only drain from the coil when the coil control valve opens again and the coil pressure goes higher than the return line vacuum.

You wouldn't want a vacuum breaker on this coil, since this will act as an air leak, and put more load on the vacuum pump.

I am a knowledgeable pump technician, but most other regulars on this board know more about steam systems than I do. I seem to recall a thread about thermostatic traps and how they work on a vacuum system, but can't locate it now. I believe the poster was Dave in QCA. Hopefully others with more trap knowledge will chime in on this.

This is off topic, but I would be interested to know where you are located, and what you have in the way of vacuum pumps?

Sailah

Zoso said:

Regarding the thermostatic element on an F&T trap, how do you determine the temperature at which the element will close. A typical trap at this building is a Barnes & Jones FT2015-5. When you look up the literature on this trap, you get generic info that the traps are rated anywhere from vacuum to 125 psig. and the only thing it says about the element is that it closes at "saturated steam temperature". But saturated at what pressure???

The traps will work identically from 25” Hg Vacuum to 250 PSIG saturated steam. The reason for this is because when we fill the elements we draw 25 “ Hg vacuum on the inside of the element to pull it up to a solid stop before sealing the fill hole. When steam surrounds it while in service then the fill on the inside of it flashes back to steam to create a “balance pressure” . This process causes the element to expand approximately .100” (+ or -) and that seals off the plug to the seat.

That is why most subatmospheric systems (VariVac) use either Mepco or B&J elements. The other manufacturers don’t do this so their elements will not work.

The B&J and Mepco element close based upon the Saturated Steam principles. Steam is steam to both elements whether it is steam at 190 degrees at 10” Hg vacuum or 230 degrees F at 5 psig.

If it would be helpful to determine if the air vent (thermostatic element) is bad, I can send you a new element & gasket to try. Or you are welcome to ship a complete trap (or head assembly to save freight) and I will test here and ship back to you.

Sailah

Just a note, we rate these traps by pressure and that is because of the float mechanism. So a FT2015-5 is a

FT - Float & Thermostatic
20 - 2000 series
15- 15 PSIG
-5 - 1 1/4"

An FT2075-4 would be 75 PSIG, 1". The last number is pipe size by 1/4" so:

3- 3/4"
4- 1"
5- 1.25"
6- 1.5"
8- 2"
10- 2.5"

Zoso

Wow, good stuff Sailah. I still need to wrap my mind around how that works with the entire range of saturation temps. If you have any literature, please share.

I'm not the owner of the building, just a contractor troubleshooting, so I'm not sure what they may want to do. The building is in Chicago.

So your comments are putting my mind at ease that the elements may all be ok. So, we may just need to verify which ones have failed open floats.

Pumpguy, regarding your comment about the equalizer at the coils. Doesn't the thermostatic element in the trap serve that function? If the steam condenses and drops the coil into a deeper vacuum than the return line, the thermostatic element should open and equalize. I think i just need to add equalizers between my zone valves and coil valves.

Pumpguy

Sailah may be a better person to answer your question about about the thermostatic element opening and serving the same function as the equalizer, but IMO, I would say no. Typically, IIRC, the orifice for the thermostatic element is around 1/8" or so. With a 10" Hg. vacuum pressure differential, this results in a flow rate across the orifice of around 3 or 4 ACFM, probably less, and certainly less than the flow across a 3/4" check valve.

Also, please note he says B&J and Mepco thermostatic elements are filled and sealed when surrounded by a 25" Hg. vacuum. He goes on to say "others are not, and so their elements won't work".

The condensing steam is upstream of the F&T trap, and that's where the induced vacuum will occur. If this induced vacuum is greater (AKA a lower absolute pressure) than what is in the return line, the pressure differential across the trap will be the wrong way and the trap will not drain until the pressure on the steam side becomes greater than the pressure on the return side.

Your 2nd drawing shows an equalizer between the zone and coil valves, but IMO, is also needed between the coil valve and condensing coil. Ultimately I would think it would depend on how long the coil valve is closed. Mild weather, when the load is low and the coil valve is closed for a longer time period, would allow the vacuum of condensation to remain in the coil for a longer period of time. As it gets colder, the coil valve is open more often, so there would be less vacuum of condensation occurring. Others may present different thoughts.

I'm north of Chicago in Spring Grove, IL, so if you need me to answer more questions, or take a look, give me a call at 1-888 853 9963. I'll do all I can to help.

Sailah

Typically air vents fail way more often than floats. If someone told me their F&T trap was failed I would first check the inlet and outlet temps. You should see a gradient. Then replace the air vent and gasket. The lower half of an FT trap should be cooler than the top half because the lower half should always have condensate in it and the upper half is steam. Say a 10-20 degree differential. If you have access to a FLIR camera it allows you to look right through the trap and see the water level. If the whole trap is glowing yellow then you are blowing steam.

Zoso

Sailah, You mentioned that saturated steam at any pressure will close the vent. What if superheated steam hits the thermostatic element? Will it close?

Sailah

I don't personally know the answer but I will find out and report back. My feeling is that it will up to the temperature that the solder holding the diaphragm or bellows will hold. We do make high temp/high pressure cage units but they are designed for industrial traps running 250 PSIG and 450 degrees. Our normal cage units are not assembled with the high temp solder.

Sailah

And here is the answer from the head honcho at Barnes & Jones:

Cage units will work up to 20-30 degrees of Super heat. After that it is not recommended

Zoso

So back to my original issue. This building is taking 10 psig steam and expanding it to -15" Hg. So for argument's sake, let's say 25 psia to 7 psia. If I'm doing my thermodynamics correctly, this steam will be superheated with 40 degrees or more of superheat. Isn't it possible that the vacuum is sucking this superheated steam right through the thermostatic elements, and the elements are not closing?

Steamhead

Earlier you said there was steam in some of the vacuum returns under a slight pressure.

Did some knucklehead install a huge trap at the inlet to the vacuum pump?

If not, is there anything else that would keep vacuum from reaching that part of the return line system?

JohnNY

@Steamhead
Frank, the post you reference in this thread is really outstanding. Thanks so much for detailing that job here. Excellent work. Very smartly done.

Zoso

I will need to visit the site again to learn more. I think some of the zones valves were taken out of auto while I was there and left in a fixed position in a panic to get steam where it needed to go. That may explain the positive pressure. Now that I think about it, the control panel showed certain zones with negative inches of Hg and some with positive. I thought typo at the time, but those areas probably were under positive pressure.

On that note, a proper gauge reading of vacuum should read negative inches of Hg, shouldn't it? " Hg GAUGE is considered to be atmospheric. I know in absolute terms, a perfect vacuum would approach 0" Hg, but gauge is the reverse, correct?. It's hard to second-guess someone's control panel, but I would assume that the more negative the reading, the deeper the vacuum. and I am now assuming that a positive reading on mercury corresponds to an actual positive pressure in the pipe. Agree?

Zoso

Sailah,

Can I ask you to check with the manufacturer about superheated steam hitting the F&T elements? I'm not positive that this is occurring, but if it is, will the vents close or remain open when hit with superheated steam? Worse-case, I am expanding from 10 psig to -15"Hg. If all of that superheat is not removed in the piping or coil/radiator, it will not condense and I will still have superheated (somewhat) steam hitting the thermostatic vents.

Sailah

They should close when hit with superheated steam. But if you are running elevated temps for too long it's time to go to a vent rated for those temps. We make bellows in both bronze and stainless steel that are assembled with high temp solder to withstand vac-300 psig. But I would think that's overkill for what you are experiencing in a building vs an industrial customer running a vulcanizing line or something.

We are the manufacturer but I don't pretend to know all the answers. I will be happy to ask the question again to my boss but I think when I asked last time he told me what I wrote above, that the elements will work within 20-30 of superheat after that not recommended.

Do you want to send me an element from a suspect trap and I will test it?