Huge steam losses in the summer
Huge steam losses in the summer
(OP)
My summer steam losses (low load)are much higher than my winter losses (20% higher) under the same operating pressure but lower load. I thought my losses should be lower since my load is much lower in the summer (from 220,000 pph to 90,000 pph) at the same pressure of 100 psi. Can somebody explain this apparent discrepancy?. My meters operate poorly at lower loads, and there's more rain in the summer that makes my steam to condense faster, apart from these, what other factors could be responsible for these huge summer losses? My boss would kill me if I don't figure this out.
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RE: Huge steam losses in the summer
RE: Huge steam losses in the summer
RE: Huge steam losses in the summer
Even though the heat transfer coefficient at the steam-pipe interface will be somewhat less at reduced velocities, the h.t. rate will not diminish as fast as the residence time is increasing. This is especially true if the steam is saturated and condensing.
You will have more losses at low flows because each pound of steam has more time to transfer heat from the piping.
Rain water in the piping could add to the problem, but you should be able to correlate losses to rainfall in order to decide if you should repair the jacketing on your piping system.
RE: Huge steam losses in the summer
We deliver steam as if your life depends on it.
RE: Huge steam losses in the summer
I would think that the most likely cause is the accuracy of the meter. Are you measuring your losses by difference (i.e. steam raised minus steam used) or can you measure the losses directly via the return condensate? The full scale accuracy of the meter becomes a higher fraction of the production at lower rates.
Any suggestions given can only be guesses without knowing more detail.
BTW, poetix99 is wrong to say you will lose more when the velocity is low because the residence time is longer. The heat transfer mechanism is unaware how long the steam sits there - it simply transfers heat into or out of whatever is in the pipe. It is true that you will lose a higher fraction of each pound in the pipe, but not in absolute terms.
RE: Huge steam losses in the summer
We deliver steam as if your life depends on it.
RE: Huge steam losses in the summer
Are the steam flow meters pressure compensated? If they're not, what pressures are they set for? Without pressure compensation, the meter error between 100 and 150 PSIG will be significant. If your meters are set for 125 PSIG, then at 100 they will read high, and at 150, they will read low.
The theory as to why line losses are higher at low loads vs high load can be debated, but that it happens, cannot. I ran a steam distribution system with excellent, pressure compensated metering, and the line losses were higher in the summer. Not huge in absolute terms - in the order of 7,000#/hr in the summer vs 5,000#/hr in the winter. When viewed as a percentage of steam generated & sold though, it was VERY large, as both of those numbers dropped dramatically in the warm weather, while the line loss went up.
RE: Huge steam losses in the summer
We deliver steam as if your life depends on it.
RE: Huge steam losses in the summer
RE: Huge steam losses in the summer
We deliver steam as if your life depends on it.
RE: Huge steam losses in the summer
I'm not sure how you're calculating the distribution system losses. In the plant I ran, we had meters on each boiler, then one on the main line out of the steam plant, after all of the inplant uses were taken off (DA, intake air heater, etc). We knew how much steam we had made from totalling the steam output from each boiler, and how much had actually been sent to the distribution system. We subtracted the total steam generated from what went out the door, to get the parasitic steam use of the plant. The individual customer meters were totalled at the end of each month, and this value was subtracted from what actually left the plant. The difference between what was sent out and what was billed, was the loss in the distribution system.
RE: Huge steam losses in the summer
Just curious about your statement
"We deliver steam as if your life depends on it".
I hope you are not referring to your self and your boss.!!!
Anyway, your steam pressure and temperature is the same.
I am assuming that you are collecting the condensate. If so, then the following may be logical.
Consider your condensate piping. In winter you generate a lot of steam for the processes, therefore your condensate return from the process will be greater. The condensate return header will be at a higher pressure. Hence DP is less and any open trap will discharge less.
In summer, reverse is the case. Hence DP is greater, therefore your losses through an open trap will be greater.
This should lead you to check trap performance. You should have a vent valve after each trap and a block valve downstream of it. You can check trap performance. Some times you can listen to the noise of trap openning and closing.
If you do not collect condensate, consider this:
The amount of discharge from a trap depends on the pressure immediately after it. You will flash more in winter than in summer because of ambient temperatures. Your back pressure will be greater in winter than in summer. So you will have less DP in winter than in summer. Hence the steam discahrge will be less in winter than in summer.
I would welcome and appreiate comments on this hypothesis.
If this works out, ask your boss for a raise, otherwise don't push your luck!!!!!!!
RE: Huge steam losses in the summer
We deliver steam as if your life depends on it.
RE: Huge steam losses in the summer
Unless there's an obvious problem with other aspects of your system, I'd recommend looking at the metering first. If you don't have accurate metering for what enters the distribution system & what the points of use take, you simply don't have a starting-point for measuring the effectiveness of any other modifications you make.
RE: Huge steam losses in the summer
If, in fact, you are actually losing this much steam, your make up requirements will go up accordingly. So, if your make up flow matches your preceived steam losses, then you you have a genuine problem.
But, if you have the perceived flow losses, and the make up rate does not vary accordingly, then it is metering.
Are you a hospital? Just curious about your signature.
rmw
RE: Huge steam losses in the summer
RE: Huge steam losses in the summer
We deliver steam as if your life depends on it.
RE: Huge steam losses in the summer
First of all, an answer to your query on pressure compensation because an understanding of this is important to what I will propose (below) as a possible reason for your problem.
The pressure of the steam affects the density, and all meters need to "know" the density of the fluid they are measuring in order to convert the measured variable (usually pressure drop) to a mass flowrate. A pressure compensated meter will have an algorithm that will correct the density as the pressure changes. If the pressure increased, the density would increase as well, but if your meter is not compensated more mass will be flowing through the meter than what it "thinks" is flowing.
I think that TBP has hit the nail on the head when he asks "Are the PRV's ahead of the meters?" I could not pick up an answer from you to that question, and you also do not state what the downstream pressure is (after the PRV's).
If the client meters are on the downstream (i.e. low pressure) side of the PRV's then I think I can explain your problem. The problem will be the quality of the steam deteriorating in summer with the lower loads.
For the purposes of argument let us assume that the losses between your production meter and the client meters are constant in absolute terms (i.e. BTU per hour) for summer and winter. In summer you are putting less steam through the lines, so the losses per pound of steam flowing in the line are greater, and a higher proportion of the steam will condense. Therefore, depending on how good your separators and traps are, it is likely that in summer the steam will be wetter than in winter. If your losses in summer are actually higher than in winter then the summer steam will be of an even worse quality.
Let us assume that in winter the steam reaching the PRV's is dry saturated at 100 psig. It may not be perfectly dry, but I have to assume something to do some calcs. 100 psig dry saturated steam has a heat content of 1189.6 BTU/lb. If this is expanded to 20 psig, and no heat is lost, it will become superheated. The temperature will be 302 F compared with the 20 psig saturated temperature of 259 F, and the density will therefore be less than expected - 0.078 lb/ft3 instead of 0.083 lb/ft3.
If in summer the steam quality deteriorates from 100% to 98% the heat content will reduce to 1172.0 BTU/lb on average. If this is expanded to 20 psig some heat is consumed by evaporating the condensate and the temperature will be only 267 F and the density 0.082 lb/ft3.
What do all these numbers mean? It means that in winter you are selling your customers superheated steam at a lower density than expected, and this will cause uncompensated meters to over-read i.e. in winter you are over-charging your customers. So yes, you have a perceived loss in summer, but it is not a physical loss of steam. You would need to compensate your meters for pressure and temperature to overcome the problem, and to re-negotiate the price to the customers to avoid monetary losses in winter. I would just keep quiet and count the extra pennies in winter as a bonus.
Note that the numbers I have chosen are for illustration only, and do not affect the conclusion. As long as the summer steam is wetter (or less superheated) than the winter steam you will be charging your customers less in summer per useful pound of steam and it will appear that you are losing more steam in summer.
Anyone else have any comments on this theory?
RE: Huge steam losses in the summer
Steam is typically not sold by BTU, but rather by the lb, normally $X.xx per 1,000 lb. This translates roughly into 1 million BTU.
The calculation for pressure compensation is based on the differences in specific volumes at varying pressures. The correction factor is: the specific volume at the design pressure divided by the specific volume at the actual pressure. Take the square root of this number, and that's the correction factor that needs to be applied to the meter reading. If the actual pressures are lower than the meter was set for, then the meter will read high, and vice versa. Pressure compensating incorporates this calculation automatically into the instantaneous display, and the totalizer.
Pressure comp is important, but the biggie is turndown. You may find that you need to install parallel meters for summer and winter loads for some customers, because the big winter load requires a meter so big, that lets a large portion of the summer load sneak past, without reading it.
RE: Huge steam losses in the summer
I did not mean that the superheat would be a process problem - only that it would affect the density and the measured flowrate if the meter was not compensated for temperature.
What sort of meters allow "a large portion of the load to sneak past, without reading it"? I am not an instrument engineer, but I would expect an orifice, annubar or vortex meter to be linear (after square root extraction where necessary) to within 0.5% over a 3:1 turndown ratio. Any intrument guys able to comment?
RE: Huge steam losses in the summer
I am not now a controls engineer now, but I once was. Unfortunately that was a long time ago when the only way to get an accurate flowmeter that would maintain its accuracy through out a real life range of turndown, was to use a pressure compensated ring balance Hagan Flow Meter.
I can't picture those mercury filled cadillacs being PC right now.
I have to rely on the controls guys now to put the best flow metering in, and in that regard, I would suggest to TBP that you post a thread in the controls forum asking specifically for advice on a pressure compensated flow meter for your steam service and turndown range.
Katmar, your general question in the previous thread is a valid controls forum question as well, and I would love to see the answers you would get there in case those guys aren't reading this thread.
rmw
RE: Huge steam losses in the summer
The pressure compensation is often not a huge factor, but when you've got money changing hands, based on a steam meter reading, it had better be dead accurate. Little old ladies in the customer's accounts payable depts will challenge you to a knife fight in the alley over what they precieve to be $10 descrepency in the steam bill. Ask me how I know that... Put the pressure compensation in, because it's the cheapest thing to do. You'll spend more time and resources in meetings explaining WHY you didn't do it, then the installation cost. If you're downsizing meters (and that's very likely), install a pressure gauge at the pressure transmitter for the compensation. If you don't, you'll get blamed for every problem in the building, because "this new steam meter is too small..." With the gauge in place, you can walk the complaining party a few feet downstream of the meter, and show him that there's lots of pressure available. Any problems he's suffering are NOT related to the meter size. Again, ask me how I know that...
RE: Huge steam losses in the summer
But what TBP says, makes a point. Particularly with Orifice meters, 3:1 is a critical turndown ratio and accuracy may be erratic at the lower range. It may seem, initially, that you are measuring at the lower range of 90000 pph(well in the ratio of 3:1) but there are fair chances that it may be 70000 pph or so and the meter is measuring 90000 pph. In this region the ordinate(accuracy) forms an asymptote with the meter curve.
RE: Huge steam losses in the summer
One of the things I learned from working in the district steam business for over 10 years, is that most existing steam meter installations are inaccurate, to say the least. The most accurate ways to go (5 or 6 years ago, anyway) are pressure compensated vortex shedders, or, when possible, drum style condensate meters. If you really want to see just how crappy most steam meters are, meter the condensate that has already been metered as steam. That's a real eye-opener. You can easily have a 20% error. The outfit I worked for operated a number of district heating operations, and a lot of people spent a considerable amount of time and effort to determine how to accurately meter steam. The end result, was properly sized vortex shedders with pressure compensation.
Steam metering is very much a "go big, or stay home" situation. Install the pressure compensation.
RE: Huge steam losses in the summer
We deliver steam as if your life depends on it.
RE: Huge steam losses in the summer
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