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SteamBryan (Mechanical) (OP)
18 Jun 05 12:02
Hi,
We have a steam condenser sitting on top of a flash tank of steam and non condensables. The condenser returns the condensate back to the tank which is controlled to 5 inches of Hg by the cooling water valve of the condenser. A Vacuum pump is connected to the condenser to remove the non condensables. The vacuum pump is often tripping out. We think its because the condensate gets sucked into the pump.
Someone told me not to use the vacuum pump but to use a vent condenser, because the process steam entering the tank is above 10 psig.
Does that mean I can just remove the pump?
Please advise.  
Helpful Member!  Montemayor (Chemical)
18 Jun 05 13:22
SteamBryan:

Your query is not clear because it contradicts itself in several places.  I'm forced to ask questions to get the story straight:  You are condensing waste, flashed steam in a tubular condenser and returning it to the original Rankine (steam) cycle.  You are furnishing a partial vacuum on the condenser (and attached condensate tank) -- presumably to expel non-condensables and dissolved gases.

You don't state the type of vacuum pump device you are using (steam jet? liquid ring pump? vane pump?) but you say it is "tripping out".  I presume you mean it shuts itself down upon reaching a pre-set vacuum setting.  If it trips out for another reason, please state it.  A vacuum device is always sized with excess capacity over the normal capacity required.  This makes it imperative to control the vacuum by use of several techniques - all of them employing instrumentation.  Your vacuum device is going to cycle - unless it is a steam jet or if it employs atmospheric air bleed-in to keep the pre-set vacuum controlled.

The vacuum is primarily controlled and maintained by the capacity of the vacuum pump, not the condenser water.  The condenser cooling water can't react fast enough to do that, plus the condenser is totally unable to expel the accumulated non-condensables - which is the vacuum pump's job.  If there were no non-condensables, then the cooling water could control the vacuum.

Whoever told you to use a vent condenser has failed to tell you that you are already using a "vent condenser".  That's exactly what you have described you have.  You can certainly install yet another vent condenser downstream of the existing one, but if the existing is doing its job, why add more area and capital costs?  This doesn't begin to make sense.  The explanation about the entering steam being at 10 psig doesn't make engineering sense either.  Sure, it might be 10 psig as it approaches the vent condenser, but when it reaches that point, it is at the vacuum setting that you fix on the vacuum pump.

I would not remove the vacuum pump.  It's there for a purpose - unless you have no non-condensibles in your steam.  I would find and fix the reason your vacuum system is shutting down.  I don't think it's condensate that's tripping it out; the condensate falls inside your tank and the vacuum take-off is at the top of the condenser (I hope!).

Maybe you have more information or explanation(s) as to what you have and how you operate it, but that's what I can make out of your present description.
SteamBryan (Mechanical) (OP)
18 Jun 05 13:54
Dear Montemayor,
Thank you for the explanation. Sorry I did not explain myself clearly, I was trying to be brief. Process steam is mixed with noncondensables at 10psig. We are sending it to the flash tank. We have a steam condenser sitting on top of a flash tank. The condenser returns the condensate back to the same flash tank. The Flash tank has a pressure controller set at 5 inches of Hg. The controller moves the cold-water valve of the condenser to control the vacuum in the tank. A Vacuum pump is connected to the condenser to remove the non-condensables. The vacuum vane pump is often tripping out. We think its because the condensate gets sucked into the pump. If, the pump creates more vacuum than the condenser, can condensate flow up the pipe?
Someone told me that the vacuum pump is not necessary because the process steam we want to condense is originally at 10 psi above atmospheric pressure.
They say we only need a vent condenser. Does that mean I can just turn off the vacuum pump and open the manual valve between the pump and the condenser to vent it??
I hope I am clearer? Please advise.
Montemayor (Chemical)
18 Jun 05 14:34
SteamBryan:

The point I want to make is that the cooling water does not "control" the vacuum if you have non-condensables as you state.  The only way the non-condensables are evacuated is via the vane vacuum pump.  Once the flashed steam is cooled to 134 oF, it will condense as saturated water and drop by gravity to your condensate tank.  Your vane pump is evacuating steadily and must be capacity-controlled in order to keep the 5" Hg (2.45 psia) partial vacuum.  Otherwise, it will draw out steam and pull a deeper vacuum (if able) - or perhaps even "trip out" on overload.  The capacity of this vane pump must be regulated by some method - as I previously mentioned.  If you don't control the capacity of the vane vacuum pump, it will try to suck to a lower vacuum level and cause problems with your condensate return - especially if you are pumping the condensate out and into a boiler, as is the usual case.

Someone has preset process conditions for this condenser as 5" Hg partial vacuum.  They did it for a reason - probably to evacuate all non-condensables and dissolved gases out of the steam system.  I cannot visualize the condensate being "sucked into the pump", unless you have run the condensate level up to the bottom of the condenser - and I don't think you'd allow this to happen.  I don't know what pipe you mean - unless there is a dip pipe draining the condensate into the bottom of the tank.  This pipe, if existant, should operate under a "vented" condition and it should be impossible to suck condensate out, up and into the vacuum pump - if designed and operated correctly.

If you don't employ a partial vacuum on the condenser, you will have more dissolved gases in the condensate returning to the boiler and re-entering the steam cycle.  If that's OK, then you can by-pass or get rid of the vacuum pump.  As I said, I wouldn't do that unless I knew intimately more details about your operation and the scope of work - which I don't.
SteamBryan (Mechanical) (OP)
18 Jun 05 16:39
Dear Montemayor,
Thank you for your response. The application is for the dryer section of a paper machine Steam is used to dry the paper. When we are finished with the steam it is at 10psig.with very little non condensables.
The tank is at 5" of neg pressure.(12.3 psia) How do we control the capacity of the vacuum pump? Is it with a vacuum breaker? If so, Is there a special type? What should be the setting on it? 12.3psia? or more ? or less?
We are condensing the steam because the boiler is 1,500 feet away. We pump it as liquid and heat water used in another process via the condenser. (We use lots of hotter water.)
The tank has a level control and pumps the excess to boiler house.
The pipe connects the vacuum pump to the condenser. There is a manual valve on the pipe between them that can empty the line.
rmw (Mechanical)
18 Jun 05 20:42
Is your vent condenser truly a condenser, or just a shell and tube heat exchanger used in a condensing application.

Condensers, among which are shell and tube type, are specially designed to sweep the non condensables through the coldest section tubing, normally with internal baffling to isolate this zone from the rest of the condensing zone, in order to minimize the amount of steam (water vapor) carried over to the air (non condensable)removal system.  Otherwise, this excess carried over water vapor with the non condensables is just excess load for the vacuum pump.

This air cooling zone, as it is called, is also isolated from the condensate level in the bottom of the condenser to the maximum degree possible.

If you are using just a shell and tube HX, then all bets are off as to how this thing would operate.

If your steam flow to the flash tank is not steady state, then you could have pressure pulses that prevent the vent condenser from draining to the flash tank.

How do you drain from the vent condenser operating at a slight vacuum to a positive pressure flash tank?  I am asking how it is done in your case.  You have a 22.53 psi differential between the condenser and the flash tank, which would require about 50 ft of static head to overcome the internal pressure of the flash tank so that a vessel operating under a vacuum could drain back.

Go to www.graham-mfg.com and find a pdf paper on the proper draining of condensers.

If your vent condenser is not this much higher than the flash tank level, then I could see condensate being pushed into the air off take piping connection and choking the pump.  Or, that is to say, without the proper height on the drain leg, the negative pressure in the condenser will 'hold' the level in the condenser until it floods all the tubes, and then either carries over to the vacuum pump, or loses the vacuum due to blockage of the heat transfer surface.

Another potential problem I see is the water control to the vent condenser.  Since any controlled circuit has lag time, there may be times when the controller does not respond quickly enough and the vacuum in the condenser is not the 5" neg that you think it is.  It could be deeper, Because the pump will pull deeper, I assume, or less vacuum.  If, due to controller hysterisis, the vacuum goes deeper, and then overcorrects to go higher, then you could be slugging the vane pump this way too.

If your condenser is designed right, and your vacuum equipment is functioning, all it is going to do if it has some overcapicity is to pull more water vapor off with it.

To answer your question about running without the pump to remove the air in a negative pressure situation, the answer is yes, it will work, just that the vent condenser will operate at a pressure somewhat above atmospheric, because flash tank pressure will have to 'push' the air through it and out, rather than having the vacuum pump 'pull' it out.  This would result in condensate leaving the vent condenser at 212°F instead of 133°F.  As I understand it, the condensate of the flash tank is at saturation for 10 psig anyway, right?

rmw
SteamBryan (Mechanical) (OP)
19 Jun 05 7:49
Dear rmw,
Thank you for your response. It is most informative.
Yes, the steam is saturated when it enters the flash tank at a steady rate. The condenser sits on top of the tank and dumps the condensate back into the same tank (differential is zero). The tank is maintained at 12 psia. by controlling the condenser cooling water flow. The condenser was designed for a vacuum pump to remove the non-condensables. So it was not designed as a vent condenser?
I am wandering if I stop the vacuum pump and open (to atmosphere) the line going from the condenser to the pump, would non-condensables come out? I am afraid the air will be sucked into the condenser??
I am told that similar plants do not use a vacuum pump because the steam is at 10 psig. They only use a "vent condenser".
I looked at a few of the articals from the Graham site (Great site,Thanks), but did not find exactly what I was looking for.
Someone told me I could replace the Vacuum pump with a device they think has an orfice and something else?
If I operate the flash tank at 16 psia with the pump removed would I need another device to replace the pump?
If I stay with what I have, How can I stop the vacuum pump from tripping out? Is there a special vacuum breaker I should use?
Please advise.
rmw (Mechanical)
19 Jun 05 22:34
SteamBryan,

I need to clear up a little confusion.  You are using the terminology "flash tank" which connotes to me a tank receiving hot condensate from a higher pressure source, and flashing it down to a lower pressure, with the condensate being pumped from the bottom, and the flash steam leaving from the top.

Is that what you mean by a "flash tank"?  I have seen plenty of flash tanks in paper machine rooms taking condensate from 600 and 150 psig steam systems and flashing it down to 15 psig steam for paper machine roll drying use.

I percieve from your remarks that you are referring to something more like a 'spent' process steam receiver tank, which has a vent condenser to change the state of the spent process steam, and recover the condensate.

Please clear this up for me.

Your last post leads me to believe that your 'flash tank' is operating at 12.3 psia as determined by the vent condenser.

If so, where does the pressure drop from the stated 10 psig to 12.3 psia?

To try to answer your questions.

If you stop the vacuum pump, with the vessel at 12.3 psia, atmospheric air will back flow into the vessel until it reaches atmospheric pressure.  The vessel will then have to operate at some pressure above atmospheric, let's use your 16 psia as a theorhetical operating point, (it might be 15.5, or 17 psia, depending upon the pressure drop of non condensables and some carried over steam through your condenser.)

I am curious as to why you maintain the tank at 12.3 psia, which corresponds to 134°F.  If you are pumping it back to the boiler, then you have to use energy to reheat it to a temperature where it can be used as boiler feed water.

Plus, you are having to reject this heat somewhere, as in a cooling tower, cooling pond, lake, river, or whatever.  In most plants in my area of the country, at least, good cool cooling water is a premium commodity about this time of the year.

Condensing this process steam at 16 psia would result in 216°F condensate and reduce the energy costs necessary to reheat the water for boiler use.

Condensing at 10 psig would give you 239°F water, and would reduce steam required by the DA at the boiler house.

Since you are bothering to recover it, you might consider recovering it at the highest possible temperature, reducing reheating costs.

In any case you pick, you would be pumping a saturated fluid from the tank to the boiler, so the NPSHA aspects pf the pumps are identical to any of the above cases.  The higher pump suction pressures would mean less TDH, requiring less pumping power overall.

Now, lets get into the condensing aspect of what is going on here.

Water is a different fluid when it comes to condensing considerations.  Unlike some some chemical compounds and other types of fluids which have very, very low flash points, and which will 'pull' down to deep vacuums, if you just have enough capacity in the vacuum equipment, water will not pull down much because it will flash at its saturation pressure (temperature) and just boil off the water.  Futher, as it flashes down, it will cool off.

Vacuum refrigeration works this way, but it takes huge vacuum jets to produce enough vapor removal to cool the condensate enough to produce a reasonable quantity of chilled water.  I doubt your vacuum pump has this type of capacity, so for our discussion, your vacuum pump is not going to 'pull' your vacuum down more than a fraction of a psi from what the cooling water is producing in the condenser.

I suspect that your vacuum pump is trying to do this, that is to say it is pulling excess vapor off by subcooling the condensate the condenser produces somewhat, and is tripping out on overload.

Vacuum is produced in the vessel by the condensation of the steam entering the condenser, that is to say, the collapse of the vapor from large volume of vapor to a small volume of liquid.

The condensing pressure is a function of the amount of cooling water available, and its temperature.

It is a popular misconception that the vacuum equipment, (which is more properly called non condensable removal equipment, or more commonly called air removal equipment) produces the vacuum.  It does not, the cooling water collapsing the steam produces the vacuum, the air removal equipment simply gets the air out of the condenser so that it can work properly.

As Mr Montemeyer pointed out above, if you have no non condensables in your steam, you can maintain vacuum with the cooling water alone.  It is an old vacuum equipment service engineers trick on tight systems to isolate the vacuum pumps alltogether from the condenser for short periods of time to demonstrate that the condenser can function just fine (until the non condensables begin to build up) without the vacuum (jets, pumps, whatever.)

Enter the non condensables.  These do two things.  They raise the partial pressure of the mixture being condensed, and they blanket the heat transfer surface, reducing the effectiveness of the condenser, thereby raising the condensing pressure.  If not removed, they will finally completely blanket the condenser, stopping the condensation completely.  That is why you have air removal equipment.

Your 134°F condensing temperature tells me that your cooling water could be leaving at about 124-130°F if your condenser is properly sized and has the proper approach.  Now, depending upon where your air off take is, and depending upon what your cooling water temperature is, but assuming it to be in the 85-90°F range, if the air off take is properly located, it should be pulling the air off at 95-100°F range.

At this point, the air is as dry as it can be, minimizing the load to the vacuum pump.  However, if it is pulling from the other end of the condenser, it could be pulling air off at close to 134°F, which would bring a lot more water vapor with it, loading up the vacuum pump.

I suspect that the orifice device that someone is recommending is to prevent the 10 psig steam from just blowing through the vent condenser.  Not knowing your connection sizes, I cannot say how your system would operate with the vacuum pump disconnected.

I can state one thing.  If your condenser is sized with enough surface area to condense at 12.3 psia, then it will be oversized for 10 psig, due to the difference in the specific volume of the steam at these two different pressures, so it should condense the 10 psig steam just fine.

I think the above should give you several areas to look into and maybe they will help you arrive at a decision.

rmw



SteamBryan (Mechanical) (OP)
20 Jun 05 10:03
Wow!
You certainly know your vacuum.
My knowledge about it sucks. (Excuse the pun)
The tank is better referred to as a separator. The stream coming in is a mixture of condensate and steam from a paper machine dryer section. Actually, I cannot increase the pressure of the tank above 5 psig or I might affect the dryers. The condenser is not a "vent condenser"; I want to vent it to atmosphere if I no longer use the vacuum pump. So I am calling it a vent condenser. The pressure drop occurs in the pipe from the dryers to the separator.
I think it is because someone fears we might get hammer if we send condensate to the boiler house too hot?
Thank you for your kind help! I really appreciate it.
Best regards,
SteamBryan

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