steam pressure reducing from 180psi to 80psi 1

[semfoss]

At our plant we use our boiler as a heat transfering energy. my problem is we max out our boiler when we have a full load and i have an idea of fixing it if it works. we used to go to 150psi on our boiler which heated our railcars fairly well over a 24 hour period but we decided to try new things to reduce the coast of natural gas its been out of the roof! so we went to 85 psi on our boiler our railcars dont heat as fast but our gas usage has been great! my idea is if we crank our boiler up to 180psi and use a steam pressure reducer to 80psi. our boiler will easily put out 80psi without using alot of fuel and keep its pressure at 180 saving gas and still having our temp in the steam correct? basicly creating a super heated steam. so will we save money and heat our railcars faster? or will this back fire?

 

[Unotec]

If I am understanding it right, you are planning to use the boiler at 180psi as a, basically, heat reserve by holding the pressure and only letting out 80psi. Am I correct? The heat draw would be the same as it is mass based but now you'll have superheated steam. Am I reading your proposal correctly?

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[semfoss]

thats correct my think is take a air compressor if its at 150psi and open the valve wide open it drains the tank and the pump is trying to keep it at 150 but if you crack the valve it takes longer and the pump dosent work hard to keep the pressure.

 

[jistre]

Hmm.

I don't know if you'll get the benefits you're thinking of. No matter where you set your boiler, you're typically going to be generating saturated steam for your plant if you don't have a superheater in the loop. When you take the pressure from 180 psig to 85 psig in the boiler, you definitely will see a big decrease in your fuel reduction due to the massive reduction in the amount of energy you're pumping into the steam.

However, if you generate at 180 psig and then use a regulator to take the pressure down, you have to remember that for the system to be steady, every pound of steam coming out of the system after the regulator has to be accounted for by adding one pound of feedwater to replace it, which the boiler will heat up until it is 180 psig steam. That one pound leaving at 85 psig should have almost the same amount of energy in it (minus the losses due to inefficiencies in the system) that the 180 psig steam generated by the boiler has. In short, I believe you're going to get 85 psig superheated steam out of the system and the boiler will take just as much fuel to run because it's still generating 180 psig steam for use.

 

[jistre]

I just saw that my pressures weren't exactly what you gave us. My mistake. It's the idea that's important, though. The gist is that you have to look at how many pounds of steam an hour you use, the state of the feedwater, and the set point of the boiler to determine energy usage.

The boiler doesn't care what you do to the steam after it leaves. It just delivers the required mass flowrate of steam at the pressure it's set at.

 

[semfoss]

sounds like ill be gaining more heat in the steam but the fuel might go up in the process to keep it at 180? now concidering we have to heat the railcars longer because we have the boiler running at 80 psi do you think our fuel consumption might be lower if i run with this idea? my reasoning is the railcars will heat up faster and not have to keep the boiler running longer to get these railcars up to heat if my therory is correct we could heat fast and than completly turn off the boiler we only use the boiler to get the heat in the railcars hot enough to pump out the oil than turn the boiler off. maybe no matter how i look at it ill be using the same amount off fuel either in a fast burst or having to run the boiler longer. now the question is which way should i go just so many veriables its hard to calculate plus the faster we get these railcars off the more switches we get in which is a big part also.

 

[jistre]

Okay, I think I see what you're getting at now. I misunderstood the original question a bit I think. You WANT to generate steam at 180 psi and have superheated steam at 85 for use and are wondering if it'll save money in your particular situation. This doesn't seem to have as neat a theoretical answer as I originally thought.

<begin rambling. This is not meant to be serious.>
Sure, steam with more energy in it will generally be able to heat quicker than steam with less energy, but to heat the rail car from point A to point B always takes the same amount of energy, so the fuel costs should be the same. However, you have inefficiencies in the system that change according to the setpoints of the system. You also have throughput of the railcars to worry about... and so on and so forth.
<end rambling>

You can see how you might be in the situation where you can end up mired in questions, equations, and theory when one inexpensive test is really all you need if you can do it SAFELY (remember, steam holds HUGE amounts of energy and if it gets half a chance, it'll happily maim or kill). Could you take the opportunity to test each of your different operating conditions and generate datasets for each condition? Then, you can chart energy usage and productivity so you can compare how your plant reacts to the different conditions to pick the best.

With this problem, we can sit here all day and talk about the theory, but I think you've just got so much going on trying to modify an existing plant that spending a few weeks performing a conclusive test is going to be much better than trying to pin down all the places where the system will deviate from theory.

 

[semfoss]

thank you for your insite like you said so many things to consider and to keep in mined. like they say for every upside there a down side, and the question is what over comes more good than bad or visa versa. what i think i might do is start some smaller tests and trials before forking over thousands of dollars to see somthing fail. I agree with everything you said the main goal is to save money here, with this in mined i may be wishing to many things and figuring realisticly what works and what dosent. with all said and done do you have any isightful ideas even if it means a new boiler! any ideas would be very aprecitive.

 

[itdepends]

Don't forget that the heat transfer coefficient for superheated steam is much lower than saturated steam (in short- saturated steam at say 150degC will heat up something faster than superheated steam at 150degC if the heat transfer area is at all limited).

It also depends on what happens to the condensate your generating once you pass the steam into the rail car heating system- is it held at the same pressure as the supply steam and returned to the boiler? or blown down to atmospheric pressure?

 

[davefitz]

If the end purpose is to heat railcars, the same amount of heat is required regardless of the steam's pressure or superheat. You can only reduce the natural gas fuel cost by improving the efficiency of the process, either by improving the boiler efficiency or by reducing the amount of waste heat that escapes the railcar heating process.

I think you would need to study the railcar heating process and aim at minimzing the amount of heat lost from that process, perhaps there is a steam vent leak to atmosphere.

 

[semfoss]

davefitz-
impossible and this process is a dead end. we cant do a whole lot more on the railcars persay, so im looking into other options.
and when the steam passes through the railcars it hits a thermal dynamic steam trap and dumps into our condensate line to a satalite tank than goes to our main condinsate tank. which both condinsate tanks are opened to the atmosphere. our boiler is just not big enough to handle the load of 15 railcars thats why i was thinking of reducing the pressure but i think thats a lost cause.

 

[itdepends]

Assuming zero heat losses from the rail cars and boiler- the most efficient method would be to produce steam at a low pressure (just hot enough to heat up the rail cars to the temp you need)and take longer to do the actual heat up.

This minimises the energy that you have to put into the steam which would otherwise be lost once the steam condenses and is blown off through the steam trap (I'm not familiar with a "thermal dynamic steam trap"- so I'm just assuming that once the steam has condensed- the condy will be blown down through the trap).

Now obviously because you ARE going to lose heat through the boiler and the rail cars- your going to want to run a higher temperature to heat the rail cars up faster. The actual temp you want to run at will be a trade off between faster heating (and higher energy losses due to condy blow down) vs faster heat up time and reduced thermal losses.

Looks like you're already part way to that optimisation process because you've noticed a decrease in gas consumption when you first dropped your boiler operating pressure.

 

[BenThayer]

to answer your last question, it will backfire in that it will accoplish your goal.

running at 85 psig will always be less expensive from a utility viewpoint.

your boiler should have lower temperatures in the exhaust and all your steam leaks and bad traps will not dump as much steam as well.

either accept the lower heating rates or pay the higher prices.

and for the railcars, are you using wild, live steam for internal coils or are the internal coils trapped? the rail car coils should be equipped with traps.