Changes to a vacuum condensate return system 1

[BronYrAur]

I have a building fed with 100 psig steam from a central plant. Upon entering the building, the steam passes through a PRV which reduces it to 15 psig. The condensate in this building is under vacuum, and then the condensate is pumped back to the central plant.

During part of the year the pressure in the central plant will be reduced to 15 psig. During those times, I will open the bypass line around the PRV since my incoming pressure is already 15 psig.

My question is will this have any affect on the vacuum return? I have been told by someone that I may have a problem because the steam line from the central plant will now be "exposed" to vacuum. Before, I had the PRV as some sort of protection against this. I need some convincing that this will be a problem. It seems to me that the steam line is only "exposed" to vacuum during the short periods when the traps are open. This building by the way has every type of trap you can think of, but everything works fine now. I need reassurance that it will continue to work if I open the PRV bypass when the boiler is at 15 psig.

 

[katmar]

I cannot see that the vacuum can have any bad effect on the system. However, one potential problem that does jump out at me is whether you will be able to get sufficient steam to flow through the steam main. Have you checked that the main will carry the required mass of steam at the reduced pressure (and presumably with a reduced pressure drop available)?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BronYrAur]

katmar,

thanks for the concern, but the line sizes are OK. The load is greatly reduced during the time when low pressure will be used.

 

[SAK9]

I am guessing that you are using the steam in the AHUs for building heating.If that is the case, a condesate return under vacuum is not good for efficient heat transfer with in the AHU heating coil.A heating coil under vacuum( caused by rapid condesation rate than the steam flow rate)reduces the steam flow through the coil reducing its output.

I have installed vacuum breakers on the coil to get around this problem.

To answer your question,the vacuum condition will exist only in the heat exchanger or heating coil whatever the case may be.The steam supply line will always be at around 1psi or more(depending on how much pressure drop is there on the line).So the boiler will not be affected by it.

 

[SAK9]

OOps! the last sentence needs correction, 15 psi or more

 

[lilliput1]

I recommend that you do not bypass the steam PRV but instead require the central plant to operate just above the minimum pressure that your PRV can produce adequate 15 psig steam. Someone may operate it at high pressure with your building PRV bypassed and ruin equipment not protected with relief valve or waste losts of steam at popping relief valves.

 

[TBP]

SAK9 - There are buildings with steam heating systems that were specifically designed to have the condensate return to a receiver via vacuum pump.

BronYrAur - Check the drip traps on the steam mains. They may be undersized for the much lower operating pressure being considered.

Also be aware that running steam boilers at much below their designed operating pressure normally results in very poor quality steam. Typically, the carry-over is spectacular.

My experience with plants like yours dropping the steam pressure is that it's a lot of work/headaches for not much in the way of payback. Good luck.

 

[ReedSprung]

I do consulting and training in this area. First of all, your boilers are in no danger of being "exposed" to the deadly vacuum of your return system ;-) There is a reason you have a bypass for the PRV. There is absolutely no need to be concerned about this. The PRV protects the low-pressure system and vacuum system from being exposed to the higher pressure from the boilers, not the other way around. If you are running 15 psig in the boilers then you have simply eliminated the need for the PRV.

If you are still unsure, think of it this way; most vacuum return systems are installed on low-pressure heating systems that do not even have PRVs. Effectively, their bypasses are always open.

Also, the advice regarding steam traps and avoiding low-pressure in your high-pressure system may be good advice depending upon the circumstances.

What type of boilers do you have? Are they water-tube or fire-tube?

Concerns are generally related to the boilers themselves. At times, baffles in the steam drums need to be removed when reducing the operating pressure. Of course, this presents a problem if you still plan to operate at the higher pressure during portions of the year because these baffles are generally cut out with a torch.

Sometimes an economizer will also need to be bypassed which reduces savings achieved by operating at a lower pressure.

Many times you will need to reduce the maximum firing rate to as little as 1/3 of the rated maximum. This may be needed due to circulation concerns within the boiler. To stay efficient you may need to have your burners adjusted to provide efficient operation down in this range.

A fire tube boiler does not usually have a problem with some of these things because they have a more basic design than a water tube boiler does. A water tube boiler is designed with much more detail in mind and the flow and circulation throughout the boiler is looked at very closely during the design process. When you change the operating pressure you change the circulation pattern within the boiler which could result in hot spots that could damage portions of the boiler.

If your boiler is a water tube you should contact the manufacturer and get a recommendation in writing before using your high-pressure (section I, ASME) boiler for 15 psig service.

If your steam flow is very low during the times you are running at 15 psig then your main steam line traps may be OK. If your steam flow is not low then your traps are probably under-sized. You should check the traps for proper operation and capacity during the highest load conditions when you are running at 15 psig. A hint of having trap problems would be water hammer (shaking and rattling pipes). Do you notice this when running at 15 psig?




Reed Sprung
Sprung Services

http://www.sprungservices.com

 

[ReedSprung]

Also, if you have steam flow meters installed they will generally not be accurate when running at a drastically lower pressure than intended.

Reed Sprung
Sprung Services

http://www.sprungservices.com

 

[BronYrAur]

Reed,

Thank you for the valuable information. The low pressure steam will actually be supplied by seperate low pressure boilers. Steam traps have been paralleled, and mains will be large enough because the load is reduced during the summer. It's funny that you mention meters, because we do want to meter steam going to one building. I have been talking to meter reps to see what they can do with dual pressures.

Thanks again.

 

[sterl]

You might want to look at stack temperatures and excess air rates depending on fuel and etc. At these low temps and low fire rates, you may be making a lot of condensation....And if there is any chemistry to the fuel???