Huge steam losses in the summer 2

[onuigbo]

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.

We deliver steam as if your life depends on it.

 

[CRG]

Are you generating superheated steam? Does your metering system account for the heat loss from superheated steam to saturated steam?

 

[TBP]

If rain has a measureable effect on the system losses, that would tend to indicate that there are insulation issues that should be addressed. What kind of steam meters are you using? Are they correctly sized, and pressure compensated?

 

[poetix99]

If the steam flow rate is reduced from 220 to 90, the "residence time" of the steam in the piping has increased by a factor of ~(22/9) = 2.4

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.

 

[onuigbo]

I am generating saturated steam at 100 t0 150 psi.

We deliver steam as if your life depends on it.

 

[katmar]

Are talking of losses in absolute terms or relative terms? If you are losing 5% of your 220,000 pph in winter you will lose 11,000 pph. If you lost the same absolute amount in summer it would look much worse because 11,000 is 12% of your production. But I'm sure this is too obvious and it is not your problem.

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.

 

[onuigbo]

Some of our meters are shunt flow and some are condensate meters, neither is very good at low flows. we are currently shifting towards vortex flow meters

We deliver steam as if your life depends on it.

 

[TBP]

Drum type condensate meters have an infinite turndown. What kind of condensate meters are you using?

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.

 

[onuigbo]

If it is true that-according to POETIX99 - that more of the steam will evaporate at lower velocities than at higher velocities at the same pressure, then good insulation may alleviate this problem. If pressure compensation does not solve the problem according to the last thread, then how can we explain why there are higher steam losses at low flows than during winter months? This brings me to the question again.

We deliver steam as if your life depends on it.

 

[TBP]

Do you run different steam pressures between summer and winter? I know many plants that do, and they often claim fabulous boiler eff improvements at low pressures. Most often what they're seeing (and not realizing it), is that their steam meters are simply giving them erroneously high readings, since the vast majority of steam meters aren't pressure compensated.

 

[onuigbo]

No we run the pressure for both winter and summer loads, of course, we have PRV's at customer sites that reduce the pressure as required. I am almost certain that our meters are not pressure compensated. The steam losses I am speaking of are line losses before the metering station. The meters may be faulty but that's another source of loss, my problem is that the controls in the boiler plant register huge drop in the load at low flows, at the same pressure.

We deliver steam as if your life depends on it.

 

[TBP]

Are the PRV's ahead of the meters? If so, this can help with the lack of pressure compensation, as the meter will see a pretty constant steam pressure. The downside is, that the lower steam pressure at the meter means a much larger meter body than if it was installed on the high pressure side of the PRV. This is often in the scale of a 2" meter upstream of the PRV, where downstream might require 4" or 6".

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.

 

[aberta]

onuigbo:
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!!!!!!!

 

[onuigbo]

Thanks Aberta, your thread is the most compelling hypothesis so far. By back pressure I suppose you mean the pressure right after the trap orifice, and not the critical pressure or the throat pressure at the orifice. The back pressure is usually less than the critical pressure, which in the case of saturated steam is about half the operating pressure. I am just wondering, will pressure compensated metering make a difference to this hypothesis? because some earlier responses mentioned something about using pressure compensated meters. I am only concerned with line losses by radiation, before the steam gets to the meters.

We deliver steam as if your life depends on it.

 

[TBP]

Steam that enters the distribution system, but doesn't get metered at the point of use, will show up as "line losses", regardless of the cause. Excessive line loss can be due to trap issues, insulation problems, leaks, poor metering, etc. If your meters don't have sufficient turndown, they'll lose accuracy at low loads, and some steam will be flowing through the meter, without being recorded. If the system pressure drifts high in the summer, then steam flow meters without pressure compensation will read low. In my experience, oversizing and lack of pressure compensation account for about 90 - 95% of steam metering problems. Most industrial & institutional steam metering is so inaccurate as to be essentially worthless, providing not much more than an "educated guess".

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.

 

[rmw]

What is your ratio of make up to condensate return. If TBD is right, and I think he is, since if you have older flow meters, they probably have a 2:1 turndown, meaning that they are worthless at less than 50% flow.

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

 

[25362]

When you say losses in summertime are 20% higher, do you mean higher than the total losses in winter, or higher than the relative losses in winter ? Thanks.

 

[onuigbo]

Thanks TBP for your analysis, first we don't return our condensate and that is a major source of loss, second, can somebody explain to me why pressure compensation could reduce this problem, just curious, since most of the threads have mentioned it without stating the relative benefit. No, we are not a hospital but we deliver steam to all the hospitals in the city and all the food processing industries in the metro area.

We deliver steam as if your life depends on it.

 

[katmar]

Hi Onuigbo,

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?

 

[TBP]

I've never really found superheat after most PRVs to be an issue, in practical terms. If any amount of superheat reaches heat transfer equipment, like heat exchangers, or coils, they just won't work properly. Superheat is sensible heat, and you have to get past that to get to the latent heat, which is where all the action is. A heat exchanger that is getting any amount of superheated steam will act as if it's airbound. In most industrial or institutional applications, the little bit of superheat you get downstream of a PRV station is gone within a few feet of pipe. The usual end result is just very good quality steam.

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.