Steam trap /vacuum system question

Chgo399

We operate a low pressure boiler on a vacuum system with perimeter radiation. I’m having a hard time understanding something. How is the steam able to pass through a radiator while the steam trap/vacuum side is in the closed position? With the steam trap closed is the radiator itself in a deep enough vacuum from start up to pull the hot steam through until it condenses, falls, hits the trap, and the trap opens? Any insight will be greatly appreciated.

ethicalpaul

I'll try it despite some ignorance of two-pipe systems and some confusion with your question.

At the start of the heat cycle, the rad is full of air. It gets pushed out past the trap. Once the steam reaches the trap it closes, right? I think this is where your question comes in.

The steam in the rad gives up its heat and condenses to water, collapsing to 1/1700 its former volume. This creates a lower pressure area that allows more steam to flow in and that cycle repeats.

In fact, that cycle repeats as the radiator is filling with steam, little by little. The radiator doesn't fill all at once.

Sound right everyone?

JohnNY

Yes. The trap is open to vent air in its static state. Then it closes when energized by the heat of the steam and condensate.

Jamie Hall

The steam never does pass through the radiator -- it condenses in the radiator to a tiny fraction of its original volume, and in the process heats the radiator. That tiny fraction is the condensate, which pools in the bottom and, when enough collects, opens the trap, zips away, and the trap closes.

Jamie Hall

plus see my comment on your other thread on the same subject.

Chgo399

Thank you all for the comments. To clarify JohnNY, the static state you’re referring to is the trap being open when the heating system is completely off?

Jamie Hall

Chgo399 said:

Thank you all for the comments. To clarify JohnNY, the static state you’re referring to is the trap being open when the heating system is completely off?

Yes

Chgo399

Thank you for taking the time to reply. If I understand this correctly then the radiator lives in a constant vacuum even when the steam trap is in the closed position and steam is entering the radiator? It’s somewhat of a hard concept for me to wrap my mind around the radiator being in a vacuum while the steam trap is in the closed position since the vacuum comes from the steam trap being open.

Chgo399

We have a low pressure steam boiler that usually operates around 11 or 12 pounds. We have been contemplating whether or not we need that much steam when we have around 5 inches of mercury being created on the boiler piping at the boiler. I’d like to start decreasing the boiler is operating pressure to see if it makes any difference in our ability to heat the building. Also, it seems like the vacuum at the boiler increases when the boiler pressure is lower.

gerry gill

11 or 12 pounds????

Chgo399

PSI

acwagner

What are you heating with this system?

Chgo399

Perimeter radiators in a 500,000 square/ft high rise.

acwagner

Well, the Empire State building famously operates at something like 2psi, so you should be able to operate at much lower pressure then you are now.

Is the vacuum created using a pump or is this natural vacuum from the collapsing steam?

Chgo399

The vacuum is created using two vacuum pumps. I’ve read that at the Empire State Building. I’d love to know more about their operation.

Jamie Hall

11 to 12 psi???? That is a very unusual pressure for heating, unless the heat is being provided by large air handlers using steam coils.

Since normal system vents and traps valves are generally rated for a maximum pressure of 15 psi, and for operation at less than 5 psi, I'm going to assume that you have a system which is not designed, piped, or operated in what is regarded as the common way.

Do you, for example, have F&T traps and require the high pressure to raise condensate from the traps to returns?

To understand how normal steam systems -- including the Empire State Building -- work, I strongly recommend that you get a copy of The Lost Art of Steam Heating, available from this site or, I believe, Amazon and review it thoroughly. To help understand your particular system, you might see if @The Steam Whisperer (Formerly Boilerpro) is available still. Otherwise, you may have to find a mechanical engineer experienced in steam heating systems -- and they are kind of rare.

EBEBRATT-Ed

In you case the vacuum starts at the trap outlets and the vacuum in your system aids in returning condensate to the boiler feed tank and then on to the boiler.

Most vacuum return systems use smaller than normal return piping because vacuum is helping the condensate return.

The vacuum does not have much effect on the steam side of the system

AMservices

> @Chgo399 said:
> The vacuum is created using two vacuum pumps. I’ve read that at the Empire State Building. I’d love to know more about their operation.

Let's think about the operation and break down the different parts.
You have the boiler where your producing steam.
You have supply piping that carries steam from boiler to radiators.
You have the radiators where we want all the steam to condense.
You have steam traps preventing steam from entering the condensate return piping.
At end of return piping, you have vacuum pumps.
The vacuum pumps purpose is to mechanical suck some of the air out of all the piping and radiators, that extends all the way back to the boiler, and effectively lower the pressure drop through the system.

When system is in a cold start, the boiler come on, vacuum pump comes on and before steam is made, the needle on your compound gauge starts bending towards the Hg scale. steam is filling the supply piping, pushing remaining air in front of it through the system.
Steam enters the radiators and starts heading for the low pressure made by the vacuum pump on the return side, but is stopped by the steam trap that forces it to stay in the radiator and condense.
When the condensate collects against the trap, it cools the element and the trap open to let condensate return to the boiler.
As the steam condenses, a natural vacuum takes place that pulls more steam into the radiator.

Now imagine that all the radiators are full and all the traps are closed.
The vacuum pump makes 10Hg and turns off.
The radiators are maintaining their own natural vacuum and as long as the boiler is on, the system stays filled.

When the boiler turns off, steam production stops, all the steam in the system starts to condense and a natural vacuum takes over.

If you watch the gauge, you'll notice that the needle never gets to 30 Hg. Might get to 10-15 Hg, but then as everything is cooling down, the needle starts back towards 0. All the air has refilled the system though any crack or packing nut it can.
Its okay because you have a vacuum pump to make it all happen again on the next heat cycle.

If you are caring for this system, crank it down. Check your pressure trolls and test your gauge.
Don't need 11-12 psi to steam heat Any building.

Chgo399

Jamie...I will be purchasing that book. The steam traps are thermostatic traps and don’t operate off of pressure.

AMServices...this response has probably helped me understand my system more than anything. The part I was missing is that I was unaware that what steam condenses it creates a natural vacuum.

We will be making some adjustments to the boiler pressure in the near future to see how low of a psi we can sufficiently satisfy the building.

retiredguy

The necessary steam operating pressure for a building is usually set by the heating engineer that designs the system. Most of the steam heating systems that I worked on for school buildings were designed for a 3-5 psig boiler set point. The design system pressure dictated the supply and return pipe sizes, the size of the steam traps, amount of radiation installed and to some extent the size of the condensate feed system. Even though most of the systems that I worked on used a steam supply pressure of 3-5 psig, the actual system steam pressure could have been anything the heating engineer wanted. If the afore mentioned Empire State building was engineered to operate at 2 psig then that dictates the steam pressure setting of the steam boiler. I rarely saw a steam boiler used for heating standing radiators or fan coils, convectors and the like operating above a 5 psig. Many of the systems that I worked on were very old and in need of repair or upgrading . If for example the system was designed to use a vacuum return unit and it did not work or would not pull a vacuum, the obvious choice was to raise the steam pressure above the design point to help with the return of condensate. Most places are "penny wise and pound foolish" so they think that they will save money by not repairing the vacuum return unit, traps, or any other installed equipment. They will sometimes install a system trap at the condensate tank inlet to fix the problem of bad steam traps. That is a "mortal Sin" and should never be done. Most of the heating engineers that were versed in steam systems that I worked with wanted an interlock installed on the steam boiler or piping to keep the vacuum return unit off until steam was present in the system. That way, only the return piping saw the vacuum. So for you @Chgo399, the pressure setting of 11-12 psig is questionable. Someone who is not well versed in steam heating systems may have raised the set point to overcome a problem in the system. If you don't have an interlock and the boiler, piping and radiation are all in a vacuum, that will not greatly affect the operation of the system and when the hot condensate hits the thermostatic traps they will close allowing the system to run normally. The steam continues to enter the radiation and as some of it condenses to water allows more steam to enter.

Chgo399

@retiredguy I think you are absolutely correct in saying that someone may have increased the boiler pressure over the years to overcome another problem (possibly a vacuum issue). We do not use an interlock on the boiler or vacuum pumps, but are careful to never run the vacuum pumps dry. I appreciate you taking the time, as well as others here, to help me better understand my system.

Pumpguy

One thing that affects the performance of vacuum pumps is the temperature of the returning condensate.

High steam pressure frequently results in high condensate temperature, and high condensate temperature results in reduced performance and even damage to the vacuum pump.

Typically I recommend vacuum pumps not be operated with condensate temperature exceeding 175*F. and suggest 140*F. as a realistic temperature for vacuum pump operation.

Attached are files from my website with details.