Page 63 - Boiler plant and distribution system optimization manual

48 Boiler Plant and Distribution System Optimization Manual

ed with steam systems, such as steam trap $20,000. Flash steam losses are much less at 72,000
flash losses, steam trap malfunctions [often Btu per hour, worth about $3,000 a year.
as high as 40%], packing gland leakage, wa-
ter hammer damage and high temperature
stresses caused by rapid temperature chang- FLASH STEAM LOSSES
es.
FROM INSULATION LOSSES
3. Because of the higher heat content, piping
sizes are smaller saving on construction There is another significant loss resulting
costs. from the insulation losses. When steam in the
4. On the down side, pumps are required to cir- piping loses energy through the pipe wall, it
culate the hot water throughout the system. condenses then goes through the steam trap as
water. In this case of the 600 psi steam, the water
temperature will drop from 489F to 212F across
INSULATION LOSSES the trap.
The Btu content of the water at 600 psi is 475
All sections of hot pipe, valve bodies, unions, Btu per pound before the trap and is only 180
flanges and most steam traps should be insulated. Btu/lb on the low pressure side of the trap. At
Insulation will pay for itself quickly, less than one atmospheric pressure, 970 Btus/lb are required to
half year, as well as prevent the overheating of form a pound of steam. This results in about 30.
working spaces and the elimination of hot and 4% flash steam.
dangerous surfaces. With an average surface loss of 200 Btu per
Steam users should be fully aware of the lineal ft for 10,000 ft [3050 meters] of piping, in-
need to insulate hot surfaces but it takes constant sulation losses from the 600 psi system will form
vigilance to keep steam systems in good condi- 2,743 pounds of water per hour. One pound of
tion. water forms for each 729.1 Btu lost from the pipe.
In our example we have 10,000 ft [3,050 So, with the 30.4% flash rate, 834 pounds of steam
meters] of well insulated piping which has an av- per hour will be produced. This steam will have
erage heat loss of 200 Btus per lineal ft per hour, a heat value of 1120 Btu/lb. If this flash steam is
this works out to 2 million Btus per hour which is lost through atmospheric vents it will cost $39,522
1.9% of the system load costing $82,344 a year. a year.
Our system also includes 10,000 feet [3,050
meters] of condensate return piping plus a num-
ber of tanks and pumps. The criteria for surface BARE PIPING LOSSES
temperature of insulated piping us generally
agreed to be about 140F [60C], so in the case of If the 600 psi [40 Bar] piping system were not
condensate return piping insulation is often not insulated but bare, it would lose about 3,400 Btu
used because it is uneconomical and the surface per lineal foot rather than 200 Btu per ft, a factor
temperature will not normally exceed 140 F [60C]. of 17 greater. In the 175 psi [12 Bar] system the
Under these conditions, surface losses will be value for bare pipe is 1200 Btu per lineal ft per
about 60 Btu per lineal ft per hour. For our ex- hour compared to 120 Btu for economical insu-
ample, this loss will be 0.6 million Btus an hour lation. The increase of losses from bare pipe is a
costing 24,700 a year. factor of 10 greater than economical insulation.
In the 4,000 ft [1,220 meter] system at 175 psi, The North American Insulation Manufactur-
because of lower temperatures and Btu values, ers Association (NAIMA) has developed software
the losses are less; 120 Btu per lineal ft. The insu- applications to calculate heat losses from bare
lation losses in this section are about half a mil- pipe and for various combinations of insulation.
lion Btus per hour with an annual cost of about This software solves these problems rather handi-