Energy conservation in steam traced winterization applications was not a concern until the price of energy rose rapidly in the 1970's. It was common practice to turn on the winterizing steam in the fall before the first freeze and let it run continuously through the winter until it was shut off in the spring. Steam was consumed with little regard for the ambient temperature or maintenance temperature of the process. Control valves are now available which will supply steam in response to the ambient temperature and the flow rate of steam can be regulated in response to the process temperature. Specification of these valves should be considered in any winterization application where the ambient temperature is above freezing during part of the winter.
Table 3-1
OVERSIZED INSULATION REQUIREMENTS (1)(2)
3/8" O.D Tubing or Electric Tracing 1/2" O.D. Tubing 3/4" O.D. Tubing or 1/2" NPS Pipe Pipe Size NPS One Tracer NPS Two Tracers NPS One Tracer NPS Two Tracers NPS One Tracer NPS Two Tracers NPS WITHOUT TRANSFER CEMENT
1/2 1 1 - 1/4 1 - 1/4 1 - 1/4 2 2 3/4 1 - 1/4 1 - 1/4 1 - 1/4 1 - 1/2 2 2 1 1 - 1/2 1 - 1/2 1 - 1/2 2 2 2 - 1/2 1 - 1/2 2 2 2 - 1/2 2 - 1/2 2 - 1/2 3 2 2 - 1/2 2 - 1/2 3 3 3 3 2 - 1/2 3 3 3 3 - 1/2 3 - 1/2 3 - 1/2 3 3 - 1/2 3 - 1/2 3 - 1/2 4 4 4 4 5 5 5 5 5 5 6 7 7 7 7 7 7 8 9 9 9 9 9 9 10 11 11 11 11 11 11 12 14 14 14 14 14 14 14 15 15 15 15 15 15 16 17 17 17 17 17 17 18 19 19 19 19 19 19 20 21 21 21 21 21 21 22 23 23 23 23 23 23 24 25 25 25 25 25 25
WITH HEAT TRANSFER CEMENT AND CHANNELS
1 - 1/2 2 - 1/2 2 - 1/2 2 3 - 1/2 3 3 4 4 4 4 5 5 4 5 5 5 5 6 6 6 8 8 8 8 8 8 8 10 10 10 10 10 10 10 12 12 12 12 12 12 12 14 14 14 14 14 14 14 16 16 16 16 16 16 16 18 18 18 18 18 20 18 20 20 20 20 20 22 20 22 22 22 22 22 24 24 26 26 26 26 26 26 Notes:
(1) Double traced lines calculated on 90 o
spacing.
3.4.1 Ambient Sensing Valves
Ambient sensing valves can be installed on individual tracers or on steam sub-headers which supply a number of tracers. The valve is actuated by changes in the ambient temperature, so it can be set to supply steam only when it is required. Care must be taken in installing such a valve to ensure that the valve senses the true ambient temperature, and not a locally elevated temperature due to heat from nearby process equipment.
One manufacturer of an ambient sensing valve is Ogontz Controls Co. Their valves are self-actuating and can be ordered set to open at temperatures from 35 oF to 255 oF (1.7 oC to 123.9 oC) in sizes from 1/2" to 2", with steam capacities
up to 2,400 lb/hr (1,090 kg/hr).
The amount of steam which can be saved by using an ambient sensing valve can be readily estimated from climatic data by calculating the fraction of the winterization period during which the ambient temperature is above the required maintenance temperature. Economic payout of the control valve can then be calculated.
3.4.2 Temperature Control
Steam tracing for winterization is designed to maintain a specified minimum process temperature at the low ambient design temperature. The design includes a safety factor to ensure the system will work. The net result is that during most of the winter, the process temperature will be higher than the required maintenance temperature. This wastes energy and,in the case of some process fluids, may cause undesirable effects such as increased corrosion rates or decomposition of thermally sensitive compounds.
The process temperature can be controlled by using a temperature control valve to regulate the flow of steam to the tracer. Each tracer must be individually controlled if uniform temperatures are required and even in this case, the process temperature will very over a ± 20 o
F (11 o
C) range. This is due in part to varying steam temperature over the length of the tracer.
Overheating is a potential problem with steam tracing because the temperature of the steam is often much higher than the required winterization temperature. Even with bare tracers, there may be local hot spots in the pipe wall which may cause a degradation of process fluids or increases in stress corrosion in piping that contains acids or caustic. If this poses a problem, insulating spacers can be placed between the tracer and pipe to reduce the rate of heat transfer. Overheating is most likely to occur in dead legs or stagnant lines. Impulse lines and sample lines are another place where overheating may occur. Use of electric tracing should be considered if overheating appears to be a concern. 3.4.3 Steam Traps
Each tracer terminates with a steam trap. The steam trap prevents the passage of live steam and releases condensate and noncondensable gases. Control
Systems is responsible for specifying the steam trap. Process must indicate what the expected condensate load will be. Desirable features for a steam trap are:
Resistance to damage due to freezing Operation on light condensate loads Intermittent abrupt discharge
Listed below are the three general types of steam traps and the principle of operation on how steam and condensate are separated:
Mechanical traps-by density difference Thermostatic traps-by temperature difference Thermodynamic traps-by energy difference 3.4.4 Mechanical Traps
There are many designs for mechanical traps (Figure 3-5). All depend on the fact that the trap's internal mechanism will float on water. The three basic mechanical steam traps are: inverted bucket, open bucket and float.
The inverted bucket (Figure 3-5a) design is superior to the open bucket (Figure 3-5c) type (which has no air handling capacity) and is the most commonly used mechanical trap. It is similar to the open bucket trap, but the bucket is inverted and open at the bottom. The valve linkage mechanism attached to the top of the bucket permits the valve to open and close as the bucket moves up and down. Bucket traps should always be primed to prevent loss of steam on initial startup.
The float trap (Figure 3-5b) is used commonly as a continuous drainer of condensate. The lever "A" (Figure 3-5b) is connected to the valve arm "B" by means of a toggle link "C". The levers are arranged to give a small movement to the valve for a large movement to the float when the trap is nearly empty; however, the higher the float rises the more rapidly the valve opens. For a given pressure, each float level corresponds to a valve opening and discharge flow. Mechanical traps should be installed at least 12 inches below the bottom of the steam header to allow sufficient static head for operation of the float or bucket trap without exchanger flooding. These traps also require a vent for elimination of noncondensible gases. This vent should be piped to a point on the heat exchanger where there is little chance for blockage with condensate.
3.4.5 Thermostatic Traps
Thermostatic traps (Figure 3-6) are activated by the temperature of the fluid reaching the trap. The balanced-bellows trap (Figure 3-6a) consists of a flexible bellows containing water. The variation of the vapor pressure within the closed bellows due to the condensate temperature, opens and closes the outlet orifice. For this reason, the condensate collected in the trap must be slightly sub-cooled to open the outlet. Because of this requirement for sub-cooling, thermostatic traps should be installed uninsulated and it is best to leave at least 24 inches (0.61 mm) of uninsulated inlet piping ahead of the trap.
Sometimes a thermostatic trap is combined with the float trap. These combination traps (Figure 3-6b) are intended for low pressure applications where large volumes of air must be eliminated. The float controls condensate discharge and the thermostatic element controls elimination.
3.4.6 Thermodynamic Disk Traps
The most commonly used trap today is the thermodynamic disk trap (Figure 3-7). It uses the higher kinetic energy of steam over water to close the outlet with static pressure. When condensate enters the trap, it simply lifts the disk and flows through the trap outlet. If steam reaches the trap, it flows with a higher velocity, which builds up sufficient pressure above the trap disk to press it closed. This cycle is repeated as steam continues to condense. A small air vent is provided for relief of non-condensible gases.
The main advantage of the of the disk trap are its small size and low first cost. It is widely used for light load applications such as steam line drips and steam tracers. This trap always expels a little steam so that the condensate line temperature stays above freezing.
3.5 ADVANTAGES AND DISADVANTAGES OF STEAM TRACING