Figure 5-1 is a schematic diagram showing the equipment items required for a liquid medium system. These include an expansion tank, a heater, circulation pumps and a side stream filter. Although this configuration is typical, the specific characteristics of the fluid employed has a bearing on the design criteria and specification of each item. These matters are discussed below on an equipment item basis.
A liquid medium heat tracing method requires several items of equipment, it is advisable that the heat tracing selection procedure is completed as early in the project as is practical. This will ensure that the equipment data sheets are prepared in a timely manner and that space is allocated on the plot plan.
Due to the flammable nature of many of the liquids that may be used, consideration must also be given to the electrical area classification.
a. Expansion Tank
This equipment item provides the surge volume for expansion of the liquid medium on heating. It also acts as the main venting point of the system for air and oxidation products. Usually the expansion tank is sized so that it is 1/4 full when the system is at its minimum ambient temperature and 3/4 full when the system is at its operating temperature. However, its minimum volume is set at twice the expansion volume.
Figure 5-1
The expansion tank should be blanketed with an inert gas such as nitrogen or sweet, dry fuel gas if the liquid medium decomposes in the presence of oxygen. This will occur at a measurable rate at temperatures above 250 o
F (121 o
C) for glycol solutions. The degradation temperature and decomposition products for commercial formulations vary widely. The vendor should be consulted for system design recommendations. The corrosion inhibitors and buffers contained in commercial glycol formulations are intended to minimize corrosion from oxidation products. Such additives are used to increase the operating temperature of glycol/water solutions. For example Dowtherm 4,000 is rated by Dow Chemicals for use up to a temperature of 350 o
F (177 o
C).
A pressurized inert gas blanket may also be required to suppress boiling water when either water or a water/glycol solution is used as the liquid medium, and it is not possible to locate the expansion tank at the high point of the system.
The double "drop leg" arrangement shown on Figure 5-1 facilitates purging air from the system during start-up and ensures that flow to the pump is uninterrupted at all times.
b. Circulation Pumps
For the circulation rates and differential heads encountered in this service, centrifugal pumps will normally be specified. Mechanical specifies the specific requirements of this pump service (e.g., mechanical seal materials and arrangement and lube oil cooling) from data supplied on the pump data sheet provided by Process. Depending on service temperature and pump characteristics there may be the need to provide cooling water for bearings or stuffing box jacket cooling. As this is a critical pump service, the usual practice is to provide one 100 % duty, normally operating pump and one 100 % spare. A minimum flow bypass is recommended for start-up and summer operation.
c. Heater
Heat for liquid media tracing systems may be supplied by a number of means:
A shell and tube heat exchanger using excess low pressure steam as the heating source. This method would be most applicable to a glycol/water system.
Fired heater, either direct or firetube type, with the former preferred.
Waste heat recovery unit, such as that for recovering waste heat from gas turbine exhaust.
Electric heating.
Organic compounds will degrade when subjected to excessively high temperatures. For commercial formulations the degradation products are mainly "low boilers" that are periodically vented. This is a source of fluid loss from the system that must be replaced by makeup. The parameters of the heater design should ensure that the maximum film temperature, recommended by the vendor for the specific fluid, is not exceeded. One approach to avoid this is to maintain relatively high fluid velocities, generally 4-10 ft/sec (1.20 - 3.05 m/s) over the heating surface. System controls and pump design should ensure that the maximum recommended film temperature is not exceeded during turndown operation.
d. Filter
A sidestream filter is not essential but may be included if there is reason to believe that degradation products will form in the system. The vendor of the liquid medium should be consulted on this matter. A capacity of 2-10 % of system circulation rate is sufficient, along with the capability to remove particles 10-20 m in size.
5.2.2 Tracing Methods
Liquid media supply heat to piping, equipment and instruments through the use of tracing and jacketing in a similar manner to steam. Thus the concepts described for steam tracing in sections 3.1.1 through 3.1.5 apply to liquid media also. An obvious difference is that steam condenses in use and the condensate is collected in a condensate return system after it exits the tracing through a steam trap whereas a liquid medium is collected in a return header system that forms a closed loop with the supply system.
5.2.3 Design Considerations
a. Determining System Heat Duty
Making an accurate estimate of the heat duty for liquid media tracing systems is extremely important since it forms the basis for the equipment specifications. The tendency is to underestimate the heat duty, especially during the earlier stages of a project when the demands on the system are not well defined. This would not be of great consequence for steam tracing systems, due to the ready availability of low pressure steam. However, a significant underestimate of heat duty for a liquid medium system will be reflected in operation as a deficiency in equipment performance that may be difficult and costly to rectify. The schedule for a project may require the issue of the equipment specifications before all of the duties on the heat tracing system are adequately quantified or even identified. The process engineer
consumers. Then judgement is used in evaluating the contingency that is to be added, and incorporated into the equipment specifications, to account for growth over the remainder of the project.
The heat duty on the tracing system will be the sum of individual duties from many consumers. The principal components will be from supplying tracing heat to the process piping, equipment and instruments. However the heat tracing system is often utilized as a convenient heat source and may also be used in the following services:
Providing small quantities of low grade process heat Heating buildings
Heating protective instrument boxes, required for cold climate plant locations
b. Distribution System Hydraulics
As stated above, a liquid medium is supplied to and returned from the plant consumers through piping headers. In practice the individual consumers (pipe and instrument tracers, tank heating coils, etc.,) are usually supplied from sub-headers that branch off the main supply header into specific areas or units. There is a corresponding arrangement of sub-headers to collect the heating medium and return it to the main return header. Sizing the piping in this complex hydraulic network of supply and return headers, sub-headers and consumers, presents a difficult process design problem requiring careful analysis. The objective for the process engineer sizing the piping is to ensure that there is reasonably even distribution throughout, so that an adequate supply of heating medium is available to all areas of the plant.
The required calculations can be assisted through the use of a PC hydraulics network program, such as KYPIPE. This program, available in all Fluor Daniel offices, is specifically designed for single phase hydraulic network analysis. It is recommended that, as a minimum, the system model include the main supply and return headers and subheaders. The program will optimize header and subheader size and turndown and off design operations can be evaluated.
The task of system hydraulic design is simplified if the flow of liquid medium through the principal consumers is controlled.
Two possible methods for this are:
Adjusting the tracer block valves located at the return sub-headers to balance the system
Installing at the outlet of each tracer a self contained regulator specifically intended to control liquid medium flow rate by
Note that these control methods are seldom actually used due to the increased operator attention to adjust valves and the high cost of individual regulator valves.
Even with a carefully designed system, it is not always possible to provide an adequate supply of heating medium to remote consumers e.g., long product lines and flare knock-out drums. In such cases it may be necessary to selectively use electric tracing in preference to the liquid medium.
c. Development of Heat Tracing Selection Charts
The normal Fluor Daniel method of determining the number and size of tracers required to maintain a fluid in a pipe above its minimum allowable temperature is by means of a heat loss chart. A standardized chart, for use with steam as the heating fluid, is provided in Piping Engineering Technical Practice 000 250 1601. A similar set of charts is not available for liquid media. When a liquid medium tracing system is selected an equivalent set of charts must be developed by process engineering for the specific fluid. The charts are then used by piping to determine the number and size of tracers required by each traced line. The method used to develop the charts is described in Appendix I.
5.3 ADVANTAGES AND DISADVANTAGES OF LIQUID MEDIA SYSTEMS