Pipes that carry hot liquid and gases will lose heat energy to their surroundings unless they are insulated. Insulation materials cost money---but so does the heat that is being wasted by pumping a hot fluid through an uninsulated pipe. The selection of the insulating materials to be used, and the thickness of the insulation to be wrapped around the pipe, should be based on comparing the value of the heat energy being lost with the cost of insulating the pipe.
Measure the diameter of the bare pipe. Take a piece of string and wind it tightly around the pipe one time, then tie it in a knot. With a pair of scissors, trim away the free ends of string close to the knot. Cut the string from around the pipe and keep hold of it. The length of this piece of string is the outer circumference of the pipe. Measure the length of the string, in inches. Divide this number by 3.14159 (the value of "pi"). The answer is the outside diameter of the pipe in inches.
Determine the nominal pipe size (NPS) of the pipe. The outside diameter off the pipe you have measured is a little larger than the NPS of the pipe. For instance, a nominal 1-inch pipe has an outside diameter of 1.315 inches. Go online to the pipe size conversion tables (see first Reference). Look for an outside diameter in inches close to the value you have calculated. The table will show you the NPS that corresponds to the outside diameter of the pipe.
Estimate the temperature difference between the fluid in the pipe and the outside air. Take a thermometer that measures up to at least 180F, and measure the temperature of the water in the pipe. Measure the air temperature around the pipe. For example, the temperature of the hot water coming from the hot water tank in a house is usually between 140 and 160F. The air temperature depends on whether the pipe is inside or outside the house. Use an average figure for the air temperature around the pipe. Subtract the temperature of the air from the temperature of the water in the pipe. This is the temperature difference that will be used to estimate the loss of energy from the bare pipe.
Go online and look at the diagram and table that show heat loss for different sizes of bare copper pipe at different temperatures (see second Reference). For example, if you have a 1-inch pipe and a temperature difference of 99F between the hot fluid and the outside air, the table shows that the heat lost to the air from a bare pipe is 79 Btu per hour per foot of pipe (Btu/hr. ft).
Refer to the diagrams that show heat loss from insulated pipes (see third Reference). Look at the first diagram, which shows heat loss from a 1-inch pipe with 4 inches of insulation. On the bottom axis, find the mark for a 1-inch pipe. The pink line on the diagram shows heat loss for a temperature difference of 100F. Read across to the scale on the left. The heat loss is approximately 2.1 watts per foot (W/ft). Multiply this number by 3.41 to calculate the heat loss as 7.2 Btu/hr. ft.
Repeat this procedure for the next diagram, which shows heat loss from pipe with 3 inches of insulation. For the example of a 1-inch pipe with a temperature difference of 100F, the heat loss is approximately 2.5 W/ft. Multiply by 3.41 to calculate the heat loss as 8.5 Btu/ft. Repeat this procedure for other thicknesses by reading from the other diagrams for of 2.5, 2, 1.5,1 and 0.5 inches of insulation.
Estimate the energy saved for each insulation thickness. For example, if you were to add 4 inches of insulation to a 1-inch bare pipe, instead of losing energy at the rate of 79 Btu/hr. per foot, the pipe would now only be losing heat at a rate of 7.2 Btu/hr. So you would be saving 79 minus 7.2, which gives a saving of 71.8 Btu/hr. per foot of pipe. Since the heat loss from the bare pipe was for a temperature difference of 99F, not 100F, the savings are going to be a little bit more, so you could round up the energy savings to 72 Btu/hr. per foot of pipe.
Repeat this procedure for the other insulation thicknesses. You will then have a table of energy savings for a 1-inch pipe, with a temperature difference of 100F, and for insulation thicknesses between 1/2 and 4 inches. Note that these diagrams are for pipes with fibreglass insulation. Make the necessary adjustments if you use mineral wool or rigid cellular polyurethane insulation. This is explained in the notes above the first table.
The energy savings you have calculated are for 1 foot of pipe. Measure the length of pipe that you want to insulate, and multiply the values calculated above by the length of the pipe in feet to estimate the total energy saved by adding insulation to the pipe. The units will be Btu per hour.