Cost of Steam 1

[alka5eltzer]

Hi there all,

I’ve been asked to justify the repair/replacement of lagging on Steam lines going from one part of the site to another. Its external and crosses a bridge. They’re old enough lines and there’s loads of lagging wrecked.
I’ve had a contractor come in and give me costs for replacing the lagging… so I just need to find the ROI.
I’m having a bit of bother finding the Costs of the Steam losses. I’m trying to determine the cost / meter / hr of the losses.

I think I’ve been able to find my Steam losses:
(taken from example 2.14.5 in this page:

http://www.spiraxsarco.com/resource...transfer/steam-consumption-of-plant-items.asp

)

= (3.6*Q*L)/hfg

Total length (L)136.4
w/m (Q) 3699
hfg 2015.1
ms (kg) 901.373113


=(3.6*3699*136.4)/2015.1
=901.37kg/hr of Steam

(Can I work out my cost per meter with this?)

I found w/m below from this program: (

http://www.pipeinsulation.org/pages_v4/download.html

)
= 3699
I found hfg (kJ/kg) from the steam tables (evaporation at 9Bar):

http://www.simetric.co.uk/si_steam.htm

= 2015.1

The program above also can work out the actual cost of the losses per meter but the figures don’t look right

Here’s what I put in to the program in the Energy/cost of energy page (changed units to SI on menu bar):

Pipe = Horizontal
System Units = ASTM C585
Fuel Type = Natural Gas
Heat Content (calorific value of the Gas) = 3.826E+07
Fuel Cost = 0.26$ / m3 (currently £24/MWh of Gas at present)
Efficiency (of boiler) = 81.20%
Process Temp (Stem Temp) = 175.4
Ambient Temp = 9DegC
NPS Pipe Size (Pipe size) = 250mm
Wind Speed = 4.5m/s
Annual Operation = 8765hrs/yr

Soo… you’ll see it spits out that having a bare pipe costs 0.98 ($/m/yr) (don’t think the currency matters) and the W/m = 32430000

Then if I multiply the cost by the meterage = £0.98*136.4m= £133.67 for the whole line / year.
I don’t wanna show this as it doesn’t sound right since the ROI would be massive. Soo… a 136m length of 250mm Steam line at 175Deg C is only costing us £133/yer in lost heat?... doesn’t sound right??

Can you see where I’m going wrong?... maybe I’m going about this the long way.

What does the w/m mean… is this watts/meter?

Sorry this is soo long... thanks in advance for your help.

Alka

 

[gwolf2]

Hi alka5eltzer,

I supply the following very simple calculation based on forced convection around a pipe. It is assumed conservatively that the steam is flowing quite quickly down the pipe and does not drop much in temperature - remember this is only a ball park calc.

heat flow q = h.A.(Tp-Ta)

Pipe area A = pi x 0.25 x 136.4 = 107.1 m^2
Temperature Difference = 175.4 - 9 = 166.4 C

I estimate that the heat transfer coefficient for this size pipe in a 4.5m/2 wind is in the range 5-15 W/m^2.C

q = 10 x 107.1 x 166.4 = 178214 W = 178 kW

Or as watts/m 178.2/136.4 = 1.3 kW/m

I could believe that number for a large diameter pipe. It could be half this or double this depending on the true value of h.

For 1 year, 178 kW x 8765 hours = 1.56 x 10^6 kWh

I don't know about tariffs but using a UK domestic gas price of 12p/kwh

Cost/year = 1.56 x 10^6 x 0.12 = £1872/year

It could be half or double this depending on h and obviously depends on tariff. If someone could come up with a better value of h then that would be good.

gwolf

 

[gwolf2]

Having got a figure of around £2000/year you then have to question whether you are actually able to reap the benefit.

The steam will in fact cool as it goes down the pipe, if it's still hot enough at the end to do the job, and you throw any remaining heat away after then don't bother.

If it is more likely returned to the boiler then it will save you money only if the boiler is thermostatically controlled.

Assuming that this is for space heating, the biggest loss of heat is most probably the walls and ceiling of the building, especially if it is a workshop with an uninsulated roof.

gwolf.

 

[25362]

Some fellow did a calculation on hot piping for the optimal thermal insulation thickness lumping in all managerial factors, when the cost of crude oil was near USD 100/bbl and growing.

He found that at this level of oil prices and independently from other usual economic factors, this thickness would have to reduce the heat loss to less than 3% of the heat loss of an uninsulated pipe.

Of course, personnel protection should be always present as a minimum.

 

[alka5eltzer]

Brilliant gwolf2
You're a star.

Thanks loads for your help.
I found the attached today and it confirms your really simple way of doing it.

Thanks again,

Alka

 

[gwolf2]

The attachment which you have found is the correct way to calculate h.

 

[alka5eltzer]

Hi there gwolf2...

Sorry to question you... but in this line:
"Cost/year = 1.56 x 10^6 x 0.12 = £1872/year"

Is this correct?
Should it be: £187,200

Thanks loads,

Alka

 

[gwolf2]

Wow! Yes, sorry about the mistake. £187,000 seems too much - or is it?

Sanity Check:

178 kW is like a medium sporty car engine running at full bore for one year. The car would be doing about 150 mph and about 10 mpg. 150x365x24=1.314x10^6 miles, 131400 gallons, x 4.5 = 591e3 liters = £581,000/year. Say petrol price is 80% tax so to get back down to non-vehicle tax is roughly 581,000x.2 =£118,000

£187,200 looks in the right ball park for 178 kW.

Looks like it's always worth really good lagging. I had hoped that someone with direct experience would have posted by now.

Sorry again for the cost error. Maybe I'm losing my touch :-(

gwolf