1.1 SCOPE
1.2 HOW TO USE THIS MANUAL 1.3 WINTERIZATION METHODS
1.3.1 Operating Techniques 1.3.2 Insulation
1.3.3 Protective Heating
1.3.4 Heated Buildings and Enclosures 1.4 WINTERIZATION PROCEDURE
1.5 DISCIPLINE RESPONSIBILITIES 2.0 INSULATION
2.1 OVERVIEW
2.2 FUNCTIONS AND CHARACTERISTICS 2.3 BASIC TYPES
2.4 GENERAL CATEGORIES AND AVAILABLE FORMS 2.5 PROPERTIES 2.6 PROTECTION OF INSULATION 2.6.1 Weather barrier 2.6.2 Vapor barrier 2.6.3 Condensate barrier 2.6.4 Forms of barriers
2.7 MATERIAL CLASSIFICATION BY TEMPERATURE RANGE 2.8 PRINCIPAL TYPES
2.8.1 Calcium Silicate 2.8.2 Mineral Fiber
2.8.3 Expanded Silica or Perlite 2.8.4 Glass
2.9 REQUIREMENTS
2.10 REFERENCES 3.0 STEAM TRACING
3.1 OVERVIEW
3.2 TYPES OF STEAM TRACING 3.2.1 External Tracing
3.2.2 Cement External Tracing 3.2.3 Jacketed Tracing
3.2.4 Internal Tracing
3.3 GENERAL CONSIDERATIONS 3.3.1 Steam Pressure 3.3.2 Tracer Length 3.3.3 Tracer Pocket Depth 3.4 SYSTEM DETAILS
3.4.1 Ambient Sensing Valves 3.4.2 Temperature Control 3.4.3 Steam Traps
3.4.4 Mechanical Traps 3.4.5 Thermostatic Traps
3.5.1 Advantages 3.5.2 Disadvantages 3.6 REFERENCES 3.7 COMPUTER PROGRAM 4.0 ELECTRIC TRACING 4.1 OVERVIEW
4.2 ADVANTAGES AND DISADVANTAGES OF ELECTRIC TRACING 4.2.1 Advantages
4.2.2 Disadvantages
4.3 TYPES OF ELECTRIC TRACING
4.4 SELF LIMITING PARALLEL RESISTANCE CABLE 4.5 MINERAL INSULATED SERIES RESISTANCE CABLE 4.6 ZONED SERIES RESISTANCE TRACING
4.7 SKIN EFFECT CURRENT TRACING 4.8 TEMPERATURE CONTROL
4.9 REFERENCES 5.0 LIQUID MEDIA SYSTEMS
5.1 LIQUID MEDIA USED AND RANGE OF APPLICATION 5.1.1 Water
5.1.2 Water/Glycol Mixtures 5.1.3 Organic Compounds 5.2 GENERAL SYSTEM DETAILS
5.2.1 Equipment Requirements 5.2.2 Tracing Methods
5.2.3 Design Considerations
5.3 ADVANTAGES AND DISADVANTAGES OF LIQUID MEDIA SYSTEMS 5.4 REFERENCES
6.0 COMPARISON OF HEAT TRACING METHODS 6.1 UTILITY AVAILABILITY 6.2 CLIENT PREFERENCE 6.3 SAFETY 6.4 TEMPERATURE CONTROL 6.5 MAINTENANCE 6.6 ENERGY EFFICIENCY 6.7 RATE OF HEAT UP 6.8 RELIABILITY 6.9 COST 6.10 REFERENCES 7.0 PIPING WINTERIZATION 7.1 OVERVIEW 7.2 OPERATING TECHNIQUES 7.3 PIPING COMPONENTS
7.4 PIPING DESIGN AND INSTALLATION 7.5 INSTRUMENTATION
8.1 COMPRESSORS
8.2 PUMPS
8.3 VESSELS AND COLUMNS
8.4 SHELL AND TUBE HEAT EXCHANGERS 8.5 AIR COOLERS
8.5.1 Air flow control 8.5.2 Concurrent Flow
8.5.3 Non-contained Internal Air Recirculation 8.5.4 Contained Internal Air Recirculation 8.5.5 External Recirculation
8.5.6 Steam Coil 8.6 COOLING TOWERS
8.7 FIRED HEATERS, HRSGs AND GAS/STEAM TURBINES 8.8 TANKAGE
8.9 REFERENCES
9.0 APPENDICES
9.1 APPENDIX I - CALCULATIONS FOR HEAT LOSS IN PIPING 9.1.1 List of Variables
9.1.2 Safety Factor 9.1.3 Heat Loss Types 9.1.4 Sample Calculations
1.0 INTRODUCTION
1.1 SCOPE
This manual is intended to be a comprehensive reference source for process engineers on the subject of winterization and insulation. The manual therefore includes such material as recommended procedures, discipline responsibilities, guidelines and recommended practices for winterizing and insulating equipment, piping and instruments. Both winterization and insulation have been included in one manual to provide a single reference source.
"Winterization" is a general term that applies to all measures taken to prevent either interruption or failure of plant operations as a result of excessive heat loss to the environment. This can cause a number of undesirable effects that may result in operating problems with equipment, piping and instruments. Some examples of these are:
Freezing or congealing of liquids Increased liquid viscosities
Condensation of liquids from vapor streams Crystallization of solids from liquid solutions
Separation of one liquid phase into two liquid phases Formation of ice crystals or hydrates in gas streams
The occurrence of such effects depends on the characteristics of the fluids present and the ambient temperature. As the ambient temperature decreases, more problems of this nature arise, so that the degree and extent of winterization must be increased. It is apparent that a wide variety of fluids can be adversely affected by ambient conditions. The need for winterization, therefore, should be considered for all onsite, utility, and offsite systems. The following sections of this manual discuss in detail winterization needs, and methods to avoid potential problems.
Insulation is applied to equipment and piping for the following purposes: Conserve heat or cold and thus save energy costs
Protect personnel from injury by accidental contact with a very hot or very cold surface
Provide a cleaner plant by avoiding pipe sweating Reduce heat absorption by equipment during a fire Attenuate noise
1.2 HOW TO USE THIS MANUAL
When this manual is being used to help define winterization needs at the start of a project, it is recommended that the procedure in Section 1.3 is followed. This stepwise procedure facilitates making decisions on the type of winterization required and also refers to specific sections in the manual where further information is available.
For quick reference purposes the following is a summary of the contents of this manual by section:
Section 1.3 - Winterization Methods - Aids in establishing the need for
winterizing with heat tracing and insulation or insulation alone.
Section 1.4 - Winterization Checklists - Provides a step by step procedure on
how to approach winterization and insulation design.
Section 1.5 - Discipline Responsibilities - Checklist identifying disciplines with
primary and support responsibility for various tasks.
Section 2.0 - Insulation - An overview of various applications of insulation. Section 3.0 - Steam Tracing - Descriptions of the types of steam tracing,
system details and considerations in its use.
Section 4.0 - Electric Tracing - Descriptions of the types of electric tracing,
system details and considerations in its use.
Section 5.0 - Liquid Media Systems - Description of the use of liquid media
heat tracing.
Section 6.0 - Comparison of Heat Tracing Methods - Comparison of steam,
electric, and liquid media tracing methods, including cost, technical and operability considerations.
Section 7.0 - Piping Winterization - Describes piping winterization design
issues.
Section 8.0 - Equipment Winterization - Describes equipment winterization
considerations.
Section 9.0 - Appendices - Outlines calculation methods for heat loss from
piping and equipment. 1.3 WINTERIZATION METHODS
There are four principal methods of winterization:
Heated buildings
Each of these methods will be described so that the selection process provided in Section 1.4 can be better understood and thus used to full advantage.
1.3.1 Operating Techniques
This is winterization that does not utilize heat tracing or insulation but depends on operating procedures that have been incorporated in the plant design. As this is the most economical winterization method, due to the high cost of installing and operating heat tracing, it should be considered first. When feasible to do so, it may be worthwhile making the modifications to a system that would allow operating techniques to replace heat tracing. This method is usually vigorously opposed by operations personnel, since it puts an added burden on their time. It may be necessary to convince a client's project personnel that significant money can be saved by not taking an overly conservative approach. To avoid the risk of operating errors, careful consideration should be given to start-up, shutdown and alternate operating modes during the design process. Details of the techniques used should be brought to the operators attention through notes on the P&ID and descriptions in the operating manual. If there is any uncertainty in the use of this approach, then heat tracing should be installed, since the consequences of freezing can be quite costly, and even hazardous.
The principle on which this approach to winterization is based is that of avoiding stagnant or low flow conditions, or liquid accumulation. This is accomplished by such means as bypasses around equipment or suitably located drain valves. In general, a flowing liquid will not freeze during normal operating conditions. For further details of this method see Section 7.0 of this manual.
1.3.2 Insulation
In certain cases, insulation alone, without heat tracing, can provide adequate winterization. Omitting the heat tracing has substantial economic benefits by reducing the capital and maintenance costs and eliminating energy costs. This method would typically be used to protect continuously flowing streams after a shutdown or during a low flow condition. The insulation helps to conserve heat within the system and thus ensure that the plant operators have sufficient time to take the necessary follow up action; such as, restart the plant, drain lines or displace heavy oil fractions with flushing oil. Insulation alone will not ultimately prevent a stagnant fluid in a line from freezing or congealing.
Local experience will often indicate if this approach is feasible. However, the heat loss calculation methods provided in Appendix I can be used to estimate the time available before fluids in lines freeze or congeal. This can then be compared to the expected duration of a low flow or flow interruption condition. Lines of small diameter should be given careful consideration when applying this method as they cool quickly due to a high ratio of surface area to volume.
The heat loss calculation would normally be based on the insulation thickness specified in the project insulation specification. However, in certain instances where the calculation indicates that heat tracing would be necessary, consideration should be given to increasing the insulation thickness. The process engineer should discuss this possibility with Piping and Electrical before an economic comparison is made.
A convenient method of winterization is to use common insulation for adjacent warm and cold lines. This can only be done if there is assurance that the warm line will always remain warm, as in the case of steam and water lines at a utility station.
1.3.3 Protective Heating
This method of winterization involves the addition of heat to piping, equipment and instruments to compensate for heat lost to the environment. The quantity of heat provided must be sufficient to maintain the fluid at a temperature that is high enough to avoid the types of operating problems previously described. Heat is supplied by providing tracing or jacketing for piping, instruments and some equipment, and internal coils for vessels and tanks. Heat tracing and jacketing are expensive forms of winterization due to substantial installation, operation and maintenance costs. Thus, alternative techniques should be used wherever feasible to minimize these costs.
There are several types of heat tracing systems available:
- External tube or pipe through which a hot fluid, such as steam, a glycol/water mixture or heating oil flows
- Internal tube or pipe, also referred to as gut tracing, through which a hot fluid flows
- Electric heat tracing
A detailed description of various methods of heat tracing can be found in Sections 3.0, 4.0, and 5.0 and a comparison is provided in Section 6.0 of this manual. The details contained in these sections, used in conjunction with the procedure outlined in Section 1.4, will assist in selecting the optimum heat tracing method for a process facility. Protective heating methods applicable to specific types of equipment are described in Section 8.0.
1.3.4 Heated Buildings and Enclosures
Process plants located in geographical regions that have severe winter conditions usually have a portion, and in some cases all of the facilities, housed inside heated buildings and enclosures. The items of equipment most frequently housed include analyzers, process compressors, large process pumps and utility systems equipment. A housing serves the dual purpose of providing a comfortable working environment for operations and maintenance personnel
material such as Charpy impact tested steels. When equipment and piping containing flammable or toxic materials are enclosed, consideration must be given to providing adequate ventilation. In the case of flammable materials, it may be necessary to compare the cost of upgrading the electrical hazard area classification inside the building to Division I with the cost of maintaining a Division II classification and providing additional makeup air.
1.4 WINTERIZATION PROCEDURE
Below are the recommended steps for establishing the winterization needs of a process facility and the optimum heat tracing method. This procedure should be carried out as early in a project as is practical. By doing so, adverse impacts of the selected heat tracing method on the utility systems will be avoided. In addition, full advantage can be taken of heat integration and waste heat recovery opportunities, e.g. recovery of waste heat from the convection section of a fired heater. Plot space, if required for heat tracing system equipment, can be allocated on the plot plan from the start.
Step 1 - Obtain Climatic Conditions
The extent and degree of winterization required by a process facility is dependent upon the ambient temperatures that prevail at the plant site and the characteristics of the process fluids. In general, the lower the ambient temperature , the greater the extent and degree of winterization that will be required. Winterization requirements are primarily based on a representative minimum value of the ambient temperature termed the low ambient design temperature. This temperature can be used to determine the extent of heat tracing required by categorizing the plant site within a certain temperature zone (see below for zone definitions). The wind also affects winterization requirements by increasing piping and equipment heat losses. Both the low ambient design temperature and the wind velocity taken together determine the extent of tracing required. The values of the low ambient design temperature and the associated wind velocity should be provided by the client, and may be available in the project design basis documents. If this is the case then proceed to Step 2.
If these data have not been provided then they must be determined from climatic data approved by the client for the plant site. The following data are required for the coldest month of the year at the plant site:
- Daily mean and low temperatures.
Length of time the average temperature is below 30 o
F (- 1.1 o
C). This is required if the plant site is in a Zone I location.
- Wind velocity.
Judgement will have to be used in deciding how many years of data are required. Often times, data for the exact plant site are not available, and weather data for adjacent locations must be extrapolated. When the data has been compiled, further judgement will be required in selecting a value of the mean temperature that will be designated as the low ambient design temperature. This temperature will determine the zone for the plant site and will be used in heat loss calculations for tracing selection. It must therefore
be selected low enough so the plant is adequately winterized, without being so low as to significantly increase winterizing costs.
When the above design ambient conditions have been discussed with and approved by the client proceed to Step 2.
Step 2
Determine the Temperature Zone and the Appropriate Check Lists to Complete as Explained in Table 1-1, below.
Table 1-1 is a tabular representation of the temperature zones defined in Piping Master Specification 000 250 50300. This specification defines three temperature zones (Zones I, II, and III) with Zone I itself subdivided into two separate zones. For purposes of clarification, Zone I will be subdivided into Zones 1A and 1B in this manual.
Table 1-1
TEMPERATURE CONDITION ZONE CHECK LIST
TO COMPLETE Low ambient design temperature of the area
above 30 oF (- 1.1 oC)(a).
Zone IA 1
Low ambient design temperature below 30 oF
(- 1.1 oC), but for periods not exceeding 24
hours in duration.
Zone IB 1, 2
Low ambient design temperature below 30 o
F (- 1.1 o
C) for periods longer than 24 hours in duration, but not below 0 o
F (- 17.8 o
C).
Zone II 1, 2
Lowest average ambient temperature below 0 oF (- 17.8 oC)
Zone III 1, 2
Notes:
(a) Where daily temperature changes bring the minimum temperature below 30 o
F (- 1.1 o
C) for more than four hours (e.g. desert areas), classification as Zone IB should be evaluated.
Step 3 - Complete the Appropriate Winterization Checklists
Use the winterization checklists in Tables 1-2 and 1-3 and the methods outlined in Section 1.3 to determine winterization requirements.
If heat tracing is required, go to Step 4. If insulation only is required, go to Step 11.
Step 4
Make preliminary determination of which tracing method is technically most appropriate for the application.
This step is intended to be a preliminary assessment of heat tracing methods to determine which are acceptable for process or other technical reasons. It is intended to be a preparation for discussions with the client in step 5. The issues that need to be considered in this matter are covered in Sections 3.0, 4.0, 5.0 and 6.0 of this manual. Step 5
Discuss and Agree on Heat Tracing Method with Client
The client should be involved in the selection of the heat tracing method and may in fact instruct which one is to be used. However, the client may need to solicit Fluor Daniel's advice on the matter. Through discussion of the technical issues alone it may be possible to reach agreement on a heat tracing method. If this is the case then continue as follows:
Electric tracing selected. Go to Step 7. Steam tracing selected. Go to Step 8.
Liquid medium system selected. Go to Step 10.
If the client is not in a position to make this selection, or there are no overriding reasons to choose a particular system, then it will be necessary for Fluor Daniel to undertake a study in this matter. In general, the selection of the most cost effective heat tracing system requires careful analysis of the process constraints, ambient conditions, economics and system reliability. A study of this type is the responsibility of Piping. However, Process and Electrical must provide Piping with the necessary technical support.
Step 6
The Piping group performs an economic analysis with Process and Electrical input to determine which tracing method is the most economical
When Piping has completed the draft study report it should be reviewed and its recommendations agreed to by Process and Electrical. The report is then submitted to the client for review and approval. When the recommendation has been approved by the client, continue the procedure sequence at step 7, 8 or 10, depending on the heat tracing method selected.
Step 7
Provide Electrical with the temperature at which each line must be maintained. See Section 4.0 for further information on electric tracing.
Electrical will determine the heat tracing power requirements. Go to Step 12 (End of procedure).
Step 8
Establish the pressure and temperature level(s) of the tracing steam and the pressure of the steam condensate return header, or obtain this data from the client as applicable. See Section 3.0 for further information on steam tracing.
Step 9
Estimate the expected steam rate for each equipment item or line being steam traced. The heat losses from steam traced piping are determined by Piping through the use of the heat loss chart provided in Piping Technical Practice 000-250-1601. The heat losses so determined are submitted to Process for calculation of the steam rates.
Similarly, equipment heat losses and any other duties are converted to equivalent steam rates.
Go to Step 12 (End of procedure). Step 10
Establish the design parameters for the liquid medium system. (Section 5.0) Specify the liquid medium supply and return temperatures.
Prepare a set of piping heat loss and tracer sizing charts for the specific liquid medium, for use by Piping.
Determine the system heat duty from piping and equipment heat losses and any other duties.
Calculate the liquid medium circulation rate. Go to Step 12 (End of procedure).
Step 11
Establish the functions of the insulation (Section 2.0). Step 12
Table 1-2
WINTERIZATION CHECK LIST 1 - ZONES IA, IB, II, III
CONDITION SEE NOTES
Low ambient design temperature is below the freezing point or pour point of any fluid.
a,b Viscosity of any fluid at the low ambient design
temperature is high enough to significantly decrease the flow below normal value at normal operating pressure differentials.
a,b,h
Pumps handling fluids that can congeal or freeze at the low ambient design temperature.
a,b Pressure reduction results in hydrate or ice formation
from moisture containing gas.
a,l Cooling to the low ambient design temperature will
result in undesirable separation.
a,b Stagnant or intermittently used pipe sections can
contain liquids that freeze above the low ambient design temperature.
a,b
Condensation causes the formation of corrosive compounds
a Equipment contains fluids that would congeal or
freeze at the low ambient design temperature
a,b,e Low ambient design temperature is below the dew
point of the gas in any compressor or blower suction.
a,j Compressors handling propane and heavier
hydrocarbon gases.
a,j Process includes streams containing both ammonia
and hydrogen sulfide together.
a,c
Process involves use of chlorine. a,d
Wax deposits or viscous fluids can plug safety valve inlets.
a,g Plugging of closed drain headers by wax deposits,
congealed viscous fluids, or auto refrigerated releases, such as flashing propane, to the system.
a
Maximum operation temperature of any pipe or equipment item is above 140 oF (60 oC).
f Pipe or equipment items handling heat sensitive
fluids.
a,k Operating temperature above 300 o
F (149 o
C). f,i
Process involves steam at a pressure above 75 psig (0.517 MPag).
i
Table 1-2 (Continued)
WINTERIZATION CHECK LIST 1 - ZONES IA, IB, II, III Notes:
(a) Winterization should be evaluated. The choices are described below (in order of increasing cost):
Special operating procedures (see Section 7.0 for piping and Section 8.0 for equipment).
Insulation alone (see Section 1.3 for guidelines)
Insulation and heat tracing (see Section 1.4 for guidelines)
(b) Systems containing liquids that can freeze or congeal at the low ambient design temperature may have provisions for venting and draining, blowing out with air, or flushing with light stock. These methods are more economical than heat tracing. However, they must be properly documented on P&IDs and in operating procedures to eliminate maintenance and downtime risks.
(c) Electric tracing should not be used for freeze protection of lines containing ammonia and hydrogen sulfide. It will not be sufficiently hot to prevent the formation of solid ammonium sulfide.
(d) If the ambient temperature can drop below 55 oF (12.8 oC), evaporators in
chlorine service shall be housed in a heated, forced ventilated building.
(e) Venting and draining capability should be provided when the equipment is not in service.
(f) Insulation for personnel protection is required by OSHA for equipment or piping operating at or above 140 o
F (60 o
C).
(g) Process engineering must recommend to the client some form of winterization, up to and including the PSV as a minimum, to avoid a potential safety hazard. (h) Start-up and shutdown conditions should be taken into consideration in this
evaluation.
(i) Consideration should be given to insulating valves and flanges.
(j) Compressor suction lines are typically heat traced downstream of the suction separator.
Table 1-3
WINTERIZATION CHECK LIST 2 - ZONES 1B, II, III
CONDITION SEE NOTES ZONE IB ZONE II ZONE III Underground water systems including
sewers.
a a
Hydrants, monitors, spray, or deluge systems.
b b b
Safety showers and eyewash stations located outside
c c c
Seal legs, flare stack water seal or seal drums.
d d d
Cone roof tanks. e e e
Utility air systems. f f f
Hydrocarbon and water being separated in a vessel.
g g
Fluids at or below their pour point at low ambient design temperature.
h h h
Aboveground stagnant water systems.
i i i,j
Aboveground flowing water systems. i i,j
Fired heaters or boilers. k
Gas turbines. k
Notes:
(a) Underground water systems should be installed at least 1 ft. (0.305 M) below the frost line. The above frost line sections should be winterized by heat tracing. Designing the above frost line portions of the system to drain after each use is also acceptable. Give special attention to the underground connections adjacent to the aboveground sections of these systems. Winterization may be required even though they are below the frost line.
(b) Fire hydrants connected to underground piping shall be self-draining. Monitors and hose reels connected to aboveground piping and underground piping above the frost line shall be provided with self-draining or manually operated drain valves. Spray and deluge systems shall be drained through manually operated valves located at the main operating valve.
Table 1-3 (Continued)
WINTERIZATION CHECK LIST 2 - ZONES 1B, II, III
(c) The preferred location for safety showers and eyewash stations is inside. When an outside location is required, a location next to a building wall is preferred, with the valve installed inside to prevent freezing but operable from the shower or eyewash station. If this is not practical, the exposed water line shall be either self-draining or heat traced with self-limiting electric tracing. Steam tracing shall not be used for safety showers and eyewash stations as the water in the supply line may become overheated beyond safe exposure limits for personnel when not in use. See Section 8.1 for further details.
(d) Freezing of the water seals in these items shall be prevented by heat tracing and insulation or continuous steam injection.
(e) Pressure/vacuum vents shall be provided with non-freezing features.
(f) Provide drain valves at piping low points. Utility air for use in Zones II and III should be dried so that its dew point is below the low ambient design temperature.
(g) The vessel shall be heat traced up to the high water level. (h) Heat tracing should be provided to keep water at 60 o
F (15.6 o
C) and other fluids 50 F degrees (27.8 C degrees) above their pour point1
. (i) Provide winterization.
(j) Block valves at branch take-offs shall be protected by one of the methods below: Locate the valve in an insulated valve box
Locate the valve in a heated valve box Heat trace and insulate the valve. (k) Air inlets will be winterized.
1.5 DISCIPLINE RESPONSIBILITIES
Different disciplines (Process, Piping, Electrical, and Control Systems) are responsible for different aspects of the work associated with winterization and insulation. Table 1-4 identifies the responsible party for different activities. The letter P in this table refers to the discipline primarily responsible for the activity, the letter A refers to disciplines providing assistance, and the letter R refers to disciplines reviewing and providing input.
Table 1-4
DISCIPLINE RESPONSIBILITIES WINTERIZATION/INSULATION
RESPONSIBLE DISCIPLINE ACTIVITY DESCRIPTION
PROCESS PIPING ELECTRICAL
OTHER AS SPECIFIED Establish low ambient design
temperature and design wind velocity.
P Civil &
Project Establish preferred method of
heat tracing(a).
Determine if excess steam is available.
P
Establish data required for comparison of steam/ electrical/liquid media tracing (e.g. cost of steam)
A P A
Establish any special process requirements or constraints that will influence selection of heat tracing methods.
P
Perform economic
comparison of various heat tracing methods.
P
Issue final design selection criteria for type of heat tracing.
R P
Establish equipment and piping requiring insulation/heat tracing. Specify type of insulation (e.g. heat conservation, personnel protection) or heat tracing required (e.g. steam, electric) and identify the same on P&IDs.
P A Civil &
Project
Establish alternate methods to be used for winterization (other than insulation and/or tracing)
Table 1-4 (Continued) DISCIPLINE RESPONSIBILITIES
WINTERIZATION/INSULATION
RESPONSIBLE DISCIPLINE ACTIVITY DESCRIPTION
PROCESS PIPING ELECTRICAL
OTHER AS SPECIFIED Identify which lines and
equipment require heat tracing for heat conservation. Show same on P&IDS.
P
Define type of insulation to be used in different services(b).
A P
Prepare economic insulation thickness tables for various types of insulation(b).
A P
Define any items that require different insulation thickness (as compared to that shown on tables) due to process reasons and show the same on P&IDs.
P A
Define operating temperatures in piping and equipment requiring heat conservation or personnel protection.
P
Define minimum operating temperature to be maintained in piping and equipment requiring winterization. Mark on line lists.
P
Select size and number of tracers for steam and liquid media traced pipes.(c)
A/P P/A
Heat loss calculations for electrical traced pipes and equipment.
P
Heat loss calculations for storage tanks and vessels.
P Complete form (E-459) to
define insulation and
winterization requirements for equipment.
Table 1-4 Continued) DISCIPLINE RESPONSIBILITIES
WINTERIZATION/INSULATION
RESPONSIBLE DISCIPLINE ACTIVITY DESCRIPTION
PROCESS PIPING ELECTRICAL
OTHER AS SPECIFIED Define need for acoustic
insulation and acoustic insulation thickness.
Mechanical
Determine which instrument lines need to be heat traced or insulated.
A Control
Systems Prepare input for operating
manual and mark specific instructions on P&IDs concerning winterization operating procedures.
P
Notes:
(a) The selection is frequently governed by client preference. Hence, the subject should be discussed with client before performing a detailed evaluation.
(b) Assistance from vendors and use of their computer programs may be useful. Client preferences should be considered.
(c) On some projects or at some office locations, Process is responsible for the number and size of tracers.
2.0 INSULATION
2.1 OVERVIEW
The process engineer is responsible for establishing the need for insulation and providing the necessary information as required by the piping insulation group for development of the narrative specification for insulation. The process engineer decides and indicates the insulation requirements on equipment data sheets, P&IDs, line lists and equipment list by using defined insulation codes (e.g., IH-insulate for heat conservation, IS-insulate for personnel protection, etc.).
With the above information provided by Process, the piping insulation group will then be responsible for insulation material selection. Process will also supply the economic and design criteria (utility values, incremental capital payback, ambient conditions, etc.) so that Piping can determine the optimum thickness of insulation for a given operating temperature, and develop insulation thickness charts for each specific application. This section outlines the fundamentals of insulation which a process engineer should know. Topics on insulation which fall within the job scope of the piping insulation group are briefly covered along with subjects discussed in engineering practices and specifications.
2.2 FUNCTIONS AND CHARACTERISTICS
The basic functions that insulation may perform in a process plant are listed below: Heat conservation for hot services
Heat conservation for cold services Fire protection
Maintain a specified temperature for process reasons Retard a change in temperature
Prevent vapor condensation on either the inner or outer surface Noise attenuation
An ideal insulation material should have the following characteristics: Low material and installation cost
Low thermal conductivity over a wide range of temperatures Non-flammable and nontoxic
Neutral pH (pH = 7)
Chloride ion content below 10 ppm to prevent attack on austenitic stainless steel surfaces
Vermin proof
Non-hygroscopic and resistant to water
Available in preformed sections, slabs and blanket form for ease of installation Asbestos free
Compatible with the chemical being processed
Reproducible physical properties to reduce batch to batch variation Corrosion resistance
2.3 BASIC TYPES
The five basic types of thermal insulation from which other types of insulation evolve are: Flake insulation - is composed of small particles or flakes which finely divide the air space. These flakes may or may not be bonded together.
Fibrous insulation - is composed of small diameter fibers which finely divide the air space. These fibers may be organic or inorganic and may or may not be bonded together.
Granular insulation - is composed of small nodules which contain voids or hollow spaces. It is not considered a true cellular material since gas can be transferred between individual spaces.
Cellular insulation - is composed of small individual cells sealed from each other. Reflective insulation - is composed of parallel thin sheets, or foil, of high thermal reflectance and spaced to reflect radiant heat back toward its source. The spacing is also designed to provide restricted air (or gas) space that reduces heat transfer by convection and conduction.
2.4 GENERAL CATEGORIES AND AVAILABLE FORMS
Thermal insulators are materials or combination of materials which have air or gas filled pores or void pockets that slow down heat transfer. The most commonly available materials fall in different categories:
Fibrous or Cellular (mineral) - alumina, asbestos(a)
, glass, perlite, rock, silica, slag, or vermiculite.
Fibrous or Cellular (organic) - cane, cotton, wood, and wood bark (cork).
Cellular organic plastics - elastomer, polystyrene, polyisocyanate, polyisocyanurate, and polyvinyl acetate.
Cements
Heat-reflecting metals (reflective) - aluminum, nickel, stainless-steel. Mass insulations come in various forms, sizes, shapes, and thickness. The available forms and their application are:
Blanket (Felt and Batt) - used where it is supported by other members; such as in cavity wall, wrapping or lining of ducts, and where compressibility and expansibility are necessary. It is frequently used in combination with rigid insulation to provide a thermal and mechanical cushion.
Block and Board - used to insulate large diameter pipe, top heads of vertical vessels, and walkways on top of storage tanks.
Cements - used to insulate small fittings on contoured equipment , and in combination with rigid insulations, to fill voids, and sometimes level coat over block or curved segments.
Loose Fill - used for filling of cavities or tight enclosures. Foil and Sheet - used to enclose and separate air spaces.
Formed/Foamed-In-Place - used to fill cavities, but the walls of the cavity must be sufficiently strong to withstand forming pressure, which is 1.5-5 psig (10.3 _ 34.5 kPag).
Semi-Rigid, or Flexible - used in walls, ceilings, and other areas where it obtains support from other members, where mechanical abuse is light, or where its flexibility is desired to conform to curvatures.
Rigid - used where structural strength is needed and where it is exposed to mechanical abuses e.g., cold storage walls and ceilings, hot or cold piping, or exposed ducts, or where the insulation must be at least partly self supporting. Preformed - used to insulate pipe fittings and piping, especially of smaller diameter.
2.5 PROPERTIES
A knowledge of the properties required by thermal insulation is necessary for proper material selection and the prevention of insulation failure. Although this is not a process engineer's job, the information below will give a better understanding why some insulation materials are not suitable in some applications. Unfortunately, not all property information is readily available from manufacturers. Shown in Table 2-1 are the types and properties of insulating materials commonly used in the chemical process industry. Note that properties of insulation materials differ slightly according to their physical form.
Table 2-1
PROPERTIES OF INSULATION MATERIALS
Material Description Service Temperature Average Density, Max. (lb/ft3 ) Average Compressive Strength, Min. (psi)
Average Water Vapor Permeability Max. (Perm-Inch) Manufacturer References Min. (o F) Max. (o F) Calcium silicate Hydrous calcium silicate
with mineral fibers
amb. 1,200 14 60 @ 5 % N/A John-Mansville "Thermo-12"
Owens-Corning "Kaylo 10" Pabco "Super Caltemp" Mineral Wool Mineral fiber bonded with
heat resistant binder
amb. 1,200 12 -- N/A Eagle-Picher "200" Forty-Eight "4800" Fiber Glass Resin bonded fibrous
glass wool
- 120 450 4 -- 75 Owen-Corning
"Fibeglas 25" Johns-Manville Cellular Glass Foamed glass to form a
rigid material with hermetically - sealed cells - 450 800 9.5 75 0.005 Pittsburgh-Corning "Foam-Glass" Polyurethane (Isocyanurate Foam) Poly-isocyanate reacted with poly-hydroxy compounds. Expanded with fluorocarbon blowing agent to rigid cellular form
- 250 250 2.5 17 @ 10 % 3.0 CPR "Trymer 210" Owens-Corning
Insulating and Finishing Cement
Mineral fiber blended with hydraulic-setting binders and filters
amb. 1,200 42 N/A N/A Forty-Eight "Quik-Set" John-Mansville
Table 2-1 (Continued)
PROPERTIES OF INSULATION MATERIALS
Material Description Service Temperature Average Density, Max. (lb/ft3) Average Compressive Strength, Min. (psi)
Average Water Vapor Permeability Max. (Perm-Inch) Manufacturer References Min. (oF) Max. (oF)
Flashing Compound High temperature asphalt sealant for caulking application.
- 75 300 ≈ 75 N/A 0.001 Childers "Stalastic CP-79" Foster "30-45
Mastic Weathercoat (Breathing Type)
Breathing type water based elastomeric coating
- 40 180 ≈ 78 N/A 0.13 Childers "Vi-Cryl CP-10" Pittsburgh-Corning "Pittcote 400" Vapor Barrier (Foil-to-Mylar) Laminated polyester-aluminum-polyester flexible film (Emissivity 0.4)
- 100 302 N/A 0 Alumiseal "Zero Perm" Metal Jacketing Flat or corrugated aluminum
(3003 H-14 or 5005 H-14) with factory applied poly- kraft vapor barrier (Emissivity 12)
N/A N/A N/A N/A N/A Childers Premetco Metal Clad
Reinforcing Fabric 5 by 5 open weave fiber cloth N/A N/A N/A N/A N/A Childers "Chil-glass 5" Mitered fitting cement Sodium silicate based
adhesive
40 850 ≈ 100 N/A N/A Childers "Fibrous Adhesive CP-97"
Vapor Barrier and Waterproofing Coating
Elastomeric polymer based coating. Set to a flexible uniform monolithic film.
- 40 190 ≈ 80 N/A 0.001 Childers "Encacel X" Vapor Barrier
Joint Sealant and Flashing Compound
Flexible elastomeric vapor barrier sealant
Table 2-2
THERMAL CONDUCTIVITIES OF INSULATION MATERIALS(1)(2) Btu in/hr ft2o F Mean Operator Temperature Degree F Calsil Mineral Wool Fiber Glass Cellular Glass Ceramic Fiber Polyurethane Insulation Cement - 300 - - - 0.190 - - -- 200 - - - 0.232 - - -- 100 - - 0.190 0.270 - 0.150 -- 50 0.380 0.280 0.200 0.280 - 0.170 -0 0.380 0.280 0.250 0.310 - 0.180 -100 0.380 0.280 0.250 0.360 - 0.180 -200 0.410 0.340 0.310 0.420 0.250 - 0.90 300 0.440 0.410 0.400 0.490 0.300 - 0.95 400 0.480 0.485 0.500 0.590 0.350 - 1.00 500 0.530 0.560 0.610 0.710 0.400 - 1.05 600 0.580 0.617 - 0.720 0.450 - 1.10 700 0.650 0.675 - 0.730 0.515 - _ -800 0.710 0.675 - 0.740 0.580 - -900 0.780 0.775 - 0.780 0.665 - -1,000 0.860 0.775 - 0.860 0.750 - -1,100 0.930 0.775 - 0.930 0.875 - -1,200 1.040 0.775 - 0.040 1.000 - -1,300 - - - - 1.125 - -1,400 - - - - 1.250 - -1,500 - - - - 1.350 - -1,600 - - - - 1.450 - -Notes:
(1) Values from FDI Insulation Optimization computer program (2) To convert to W/m oK multiply above values by 0.144
Listed below are the properties for insulation, some of which apply to insulation in the wet-to-dry state.
Abrasion resistance
Alkalinity or acidity - the tendency of a material to have a basic or acidic reaction. A knowledge of this is important to avoid corrosion of the material being covered.
Breaking load - the property of a material which indicates its strength in flexure when loaded as a simple supported beam with a constantly increasing concentrated load at its center.
Capillarity - the capability of a material to absorb liquid.
Chemical reactivity - the property of a material which measures its tendency to chemically combine (or react) with other material which may come into contact with or be absorbed by it. Such reaction may cause a fire hazard, and in addition to holding combustible liquids, the intermixing may change the combustibility of the chemical by lowering their flash, fire, and self-ignition points.
Coefficient of expansion Combustibility
Compressive strength
Corrosion to substrates - the property of a material which indicates its chemical effect on metals. Of particular importance is the stress-corrosion effect of chlorides on austenitic stainless-steel.
Density - the weight of a unit volume. This is used to calculate loadings.
Dimensional stability - the property of a material which indicates its ability to retain its size or shape after aging, cutting, or being subjected to temperature or moisture.
Flexural strength - the property of a material which measures its ability to resist bending (flexing) without breaking.
Hardness
Hygroscopicity - the property of a material which measures its ability to absorb and retain water, in either the liquid or vapor state from the ambient air.
Incidence of cracking - the property of a material which indicates its tendency to crack when applied to hot surfaces.
Resistance to chemicals - the property of a material which indicates its ability to resist decomposition by various acids, caustics, and solvents.
Shear strength - the property of a material which indicates its ability to resist cleavage.
Shrinkage Specific heat Temperature limits
Temperature rise - some insulating materials will undergo an internal exothermic reaction when heated above a certain temperature.
Tensile strength Thermal conductivity Thermal diffusivity
Thermal shock resistance - the ability of a material to resist rapid temperature changes without physical failure. This property is important when insulations are used in cyclic operations and for fire protection.
Vibration resistance - the property of a material which indicates its ability to resist mechanical vibration without wearing away, settling or dusting off.
Warpage - the change in dimension of one surface of insulation as compared to that of another surface due to difference in temperature of the two surfaces. Water adsorption - the property of a material which measures the amount of water it will adsorb when submerged in water.
Adhesion-wet - the ability of a wet material to adhere to a surface without sliding or falling off.
Adhesion-dry - the ability of a material to bond to the surface to which it is applied and remain in service.
Shrinkage-wet to dry - the measurement of changes in volumetric and linear dimensions which occurs in the drying of insulating cements and mastics.
Expansion-wet to cured - the measurement of changes in volumetric dimensions of a poured, or foamed in place material.
Coverage-wet - the property of a material which measures the amount of material necessary to cover a given area to obtain a specific dried or cured thickness.
Compaction or settling - the property of blankets or batts which measures their change in density and thickness resulting from loading or vibration.
Recovery of thickness - measurement of recovery of thickness after compression. This is a significant property of insulation used for cushion blankets and expansion joints.
Resistance to air movement - the property which indicates the ability of a blanket-type material to resist erosion by air currents over its surface.
2.6 PROTECTION OF INSULATION
Thermal insulation requires protection from mechanical damage, vapor passage, fire or chemical attack. Protection can be provided in the form of metal jacketing, mastic coating or a combination of both depending upon the application, service, and economic requirements. The different types of protection for thermal insulation are:
2.6.1 Weather barrier - a material installed on the outer surface of thermal insulation to protect the insulation from the weather, solar radiation, atmospheric contamination and mechanical damage.
2.6.2 Vapor barrier - a material installed on the high vapor pressure side of insulation which retards the passage of moisture to the lower vapor pressure side.
2.6.3 Condensate barrier - a material normally used as an inner lining for the metal jacket of an insulation to prevent the formation of alkaline condensate that tends to form on the inner surface on the metal jacket.
2.6.4 Forms of barriers - All insulation has a covering in the form of metal or plastic jacketing or mastic coatings. The former is commonly used since it is more efficient, gives more protection, and is more aesthetic. Sometimes plastic jacketing is used because of its chemical resistance. Listed below are the two forms of weather-vapor barriers:
a. Metal jacketing - Some examples are aluminum, galvanized steel and stainless steel. The advantages of aluminum are low initial cost and easy workability. Its disadvantages are low chemical resistance and low mechanical strength. Stainless steel has the opposite properties from aluminum and is widely used. Its disadvantages are high initial cost and susceptibility to stress corrosion cracking in the presence of chloride ions.
b. Mastic coatings - This protective coating is usually applied on irregular surfaces (e.g., pipe fittings and valves) by trowel and spraying and is not used in high traffic areas or where it can be subjected to mechanical abuse. These are made of resins or asphaltic material with inorganic filler.
2.7 MATERIAL CLASSIFICATION BY TEMPERATURE RANGE
Insulation materials are normally classified according to the temperatures for which they are suitable for. Operating above their temperature limit can cause structural damage by crystallizing, melting, or burning of the insulating material. Some insulating materials can withstand operation beyond their maximum temperature limit but tend to be more costly because of the increase in thickness due to higher thermal conductivities. It is important to look at the whole system and make sure that the insulation chosen can withstand the full range of operating and upset temperatures. There are several insulating materials available within each temperature range and the selection among the various materials generally depends on their properties and cost.
The cryogenic temperature range is considered to be - 150 oF (- 101 oC) down to
absolute zero - 459.4 oF (- 273 oC). Cryogenic insulation is mostly applied for the liquid
separation of gases, e.g., nitrogen and oxygen. There are basically two types-one type is composed of powders in a partial vacuum and the other multilayered, spaced reflective sheets in a partial vacuum.
For "low" temperature applications [- 150 o
F to 212 o
F (- 101 o
C to 100 o
C)], the major problem is the permeability of the insulating material to water vapor. The best choice is foam glass because of its low water permeability. Fiberglass can also be used above freezing temperature, although massive failure can occur when deeply permeated water vapor freezes. Some cryogenic insulations are used at the lower end of this range. The most commonly encountered temperature range in process industries is the intermediate temperature range [ambient up to 600 o
F (316 o
C)]. In this application, calcium silicate is usually selected because of its resistance to mechanical abuse.
For high temperature applications [600 to 1,600 oF (316 oC to 871 oC], mineral wool and
calcium silicate are the predominantly used insulation. Denser materials such as perlite and diatomaceous earth are used as a filler when they are used at a higher temperature. Mineral wool tends to have lower installation cost than calcium silicate and shipping damage is minimal. Refractory and reflective insulations can be applied at the higher end of this range.
Insulations used at temperatures above 1,600 oF (871 oC) are called refractory. The
predominantly used refractories are ceramic refractory, reflective insulation, and inexpensive materials such as firebricks. Alumina beads are poured where there is an irregular surface.
2.8 PRINCIPAL TYPES
The principal types of thermal insulations for the intermediate temperature application are listed:
2.8.1 Calcium Silicate - A mixture of lime and silica reinforced with organic and inorganic fibers and molded into the shape desired. Temperature range covered is 100 o
F to 1,500 o
F (38 o
C to 816 o
C). Flexural strength is good and compressive strength is high. It meets specifications for prevention of stress corrosion cracking of austenitic stainless steel. It is available in both half-round and quarter-round segments for pipes, as well as in the form of blocks.
2.8.2 Mineral Fiber - For this material, rock and slag fibers are bonded together with a heat resistant binder. The upper temperature limit is up to 1,200 oF (649 oC)
(generally about as high as for calcium silicate). The material has practically neutral pH. Its compressive strength is much less than for calcium silicate. It is available in both rigid molded form for piping or vessel use, and as a flexible blanket for irregular surfaces.
2.8.3 Expanded Silica or Perlite - An inert siliceous volcanic rock with some combined water. The rock is expanded by heating it above 1,600 oF (871 oC), where the
water vaporizes expanding the rock volume and creating a structure of minute air cells surrounded by vitrified product. Added binders hold it together and resist moisture penetration, and inorganic fibers reinforce the structure. The material has low shrinkage. Water absorption is low. The chloride content is low enough to eliminate its causing stress-corrosion stress cracking in austenitic stainless steel.
2.8.4 Glass - In various forms, glass products are used from the cryogenic level to around 1,200 o
F (649 o
C) , with some specialty, high-purity products good up to 1,800 o
F (982 o
C). Among the forms available are flexible fiber blankets without binder, glass fiber with an organic binder, [generally a thermosetting resin good up to 1,000 o
F (538 o
C) or higher], semi-rigid boards [for up to 850 o
F (454 o
C)] molded sections for pipe and foam or cellular foam, used for low temperature insulation applications. Glass has low moisture absorption and can be easily dried. Upon drying, strength and thermal qualities fully recover. Its low chloride content keeps it from promoting stainless-steel stress-corrosion cracking.
2.9 REQUIREMENTS
The basic requirements for insulating equipment and piping are determined by the process engineer at the early stages of a project. P&IDs are stamped by Process with process design and operating condition information. This includes insulation or heat tracing requirements for each line. The piping insulation group will select the material and determine the optimum thickness. Piping will determine the size and the number of tracers required. The insulation and tracing nomenclature listed below is also shown on the standard Flowsheet Legend and Symbology drawing.
lH - Insulate for heat conservation. IC - Insulate for cold conservation.
IS - Insulate for personnel protection. This instructs Piping to provide safety insulation (usually thinner than for heat conservation) wherever operators could come in contact with hot surfaces during their normal activities.
IA - Insulate for noise suppression. ST - Steam traced.
ST(W) - Steam traced for winterization. These tracers may be installed on a different header, so they can be isolated with a single valve during warm weather.
STJ - Steam jacketed pipe.
STT - Steam traced with Thermon, or equivalent heat transfer cement.
STS - Steam traced with a spacer to reduce the impact of high tracing temperature.
ET - Electric traced. 2.10 REFERENCES
1. Miner, J., Insulation Design: Using the New Computer Programs, Hydrocarbon Processing, July 1980.
2. Harrison, M. R. and Pelanne, C. H., Cost-Effective Thermal Insulation, Chemical Engineering, December 19,1977.
3. Martin, R. B., Guide to Better Insulation, Chemical Engineering, May 12,1975. 4. Turner, W. C., Criteria for Installing Insulation Systems in Petrochemical Plants, 5. Marks, J. B., Holton, K.D., Insulation Practices: Protection of Thermal Insulation, 6. Abramovitz, J. L., Economic Pipe Insulation for Cold Systems, Chemical
Engineering, October 25,1976.
7. Ganapathy, V., Sizing Piping Insulation, Chemical Engineering, November 21, 1977.
8. Cordero, R., The Cost of Missing Pipe Insulation, Chemical Engineering, February 14,1977.
9. Irwin, W., Avoid Common Mistakes When Insulating Piping, Hydrocarbon Processing, October 1991.
10. Szeto, E., Prevent Insulation from Punking, Hydrocarbon Processing, October 1991.
3.0 STEAM TRACING
3.1 OVERVIEW
Steam is commonly used for winterization in refineries and chemical plants. Generally, this is because steam is readily available, is safe for use in all area classifications and plant personnel are familiar with the details of steam tracer installation and maintenance. Typically, the pressure level of the steam used in winterization will be in the range of 15 - 200 psig (0.103 - 1.379 MPag). This corresponds to a saturated steam temperature range of 260 - 385 o
F (127 - 196 o
C). The pressure level used in any given application will depend on the cost of steam, plant safety requirements, the amount of heat which must be provided, and the available pressure levels in the plant.
3.2 TYPES OF STEAM TRACING
Several different types of steam tracing systems are used to transfer heat from the steam to the process line including:
External tracer
External tracer with heat transfer cement Jacketed pipe
Internal tracer Integral tracer 3.2.1 External Tracing
External steam tracing consists of tubing containing the steam which is run along the outside of the process line. Oversized insulation covers the pipe and tubing. Depending on the required heat load, multiple tracers may be required. The tracers are generally run parallel to the pipe, but in some cases may be wrapped in a spiral around the pipe. Spiral-wrapped tracing should be avoided if possible since installation costs will be higher than for parallel tracing and draining of condensate to steam traps is more difficult.
For steam pressures below 175 - 200 psig (1.206 - 1.379 MPag) and
temperatures of less than 400 o
F (204 o
C), copper tracer tubing may be used. At higher pressures and temperatures exceeding 400 o
F (204 o
C) or where there is a sour gas atmosphere or where steam may be contaminated with ammonia, copper tubing should not be used. Stainless steel tubing should be used for steam temperatures above 400 o
F (204 o
C) provided that insulation is carefully selected. Calcium silicate as insulating material usually contains chloride ions which corrodes stainless-steel. This type of insulation, when used for stainless steel, should contain sodium silicate to neutralize the chlorides or be a special low chloride type.
The minimum size of tubing which should be used is 3/8" O.D. tubing. Larger tubing is preferable because there is less chance of plugging a tracer and longer tubing runs can be used. For ease of installation, smaller tubing is used to wrap around smaller sizes of equipment and instruments (e.g., pump casing,
Heat transfer from the tracer to the pipe is accomplished primarily by convection and radiation. No credit should be taken for conductive heat transfer between the tracer and pipe, because there is very little actual contact. As a result, bare tracers have a limited capacity for heat transfer. Enhancing the conductive heat transfer by welding the heat tracer to the pipe is not a good practice due to differential expansion and expensive installation (Figure 3-1).
3.2.2 Cemented External Tracing
The amount of heat which a tracer can deliver can be greatly increased if heat transfer cement is applied to the tracer. Heat transfer cement forms a highly conductive path for heat between the tracer and pipe. A single cemented tracer can usually replace two or three bare tracers, so fewer tracers can be used or faster heat-up times are possible. Installation costs for a single cemented tracer will be higher than for a single bare tracer, but a savings can be achieved if the cemented tracer replaces two or more bare tracers. Various manufacturers such as Thermon sell preformed cement strips and steel channels which allow simple installation of cemented tracers. High labor costs are incurred if the heat transfer cement is hand troweled (Figure 3-2).
3.2.3 Jacketed Tracing
Steam jacketing consists of a product pipe placed concentrically within a larger steam pipe. Steam flowing in the annulus heats the process line. This type of system is expensive and is only used when a high rate of heat transfer is needed and a highly uniform wall temperature is required. A typical application of steam jacketing is on liquid sulfur lines in a Claus plant that require a steam temperature range of 250 o F to 320 o F (121 o C to 160 o C) (Figure 3-3). 3.2.4 Internal Tracing
Internal tracing consists of a small diameter pipe or tube passing through the process line. This form of tracing provides excellent heat transfer and can allow rapid start-up of lines in intermittent services. Disadvantages of this type of tracing are that only straight runs of pipe that are free of valves can be traced, allowance for differential thermal expansion is required, the line can't be pigged or cleaned with a rotary brush, cross contamination is possible, and special alloys may be required for the tracer to prevent corrosion. Internal tracing is applicable only if leakage of heating fluid into the product can be tolerated (Figure 3-4).
Figure 3-1
Figure 3-3
Figure 3-4 INTERNAL TRACER
3.3 GENERAL CONSIDERATIONS
Two major concerns in the design of a steam tracing system are the steam pressure to be used and the maximum length of each tracer.
3.3.1 Steam Pressure
Fluor Daniel Master Specification 000-250-50300 sets out some guidelines for steam pressures in tracing service which are summarized in the following table:
Winterization Zone
Steam pressure, psig (MPag)
Minimum Maximum
IA & IB 15 (0.103) 150 (1.034)
II 25 (0.172) 150 (1.034)
III 60 (0.414) 200 (1.379)
In most cases, a client will already have selected tracing steam pressures based on their existing installations.
Low pressure steam is inexpensive in most plants, but there are some offsetting factors which must be considered. Low pressure steam requires the use of larger headers and tracers to minimize pressure drop in the steam system. The tracers are generally shorter so there are more of them, and more traps. Capital and maintenance costs will be higher for a low pressure steam tracing system. If 15 psig (0.103 MPag) steam is used, the condensate will be at too low a pressure for return in a closed system unless a condensate return tank and pump are provided. If not, it must be discharged to the plant sewer system. This is not recommended in freezing climates because of the possibility that ice slicks will form at grade. If the condensate is collected, the minimum usable steam pressure is 25 psig (0.172 MPag).
3.3.2 Tracer Length
Maximum tracer lengths are selected so that the pressure drop is 10 % of the inlet pressure psig (MPag) or 10 psig (0.069 MPag) whichever is greater. This will be a function of the tracer size, steam pressure, and steam rates. The following table, adapted from Fluor Daniel Master Specification 000-250-50300 sets limits on the maximum allowable tracer length for bare tracing.
MAXIMUM TRACER LENGTH, (FEET) (m)
Steam Pressure, psig (MPag)
Tracer Size (O.D.)
3/8" - 1/2" 3/4" - 1" 15 - 25 (0.103 - 0.172) 200 ft (61 m) 300 ft (91 m) 50 - 200 (0.345 - 1.379) 200 ft (61 m) 400 ft (122 m)
Tracer lengths should not exceed these limits even if the pressure drop is still within the guidelines. Because cemented tracers have higher heat transfer rates than bare tracers, the maximum tracer length will be lower than that shown in the above table. The cement vendor should recommend the maximum tracer length. 3.3.3 Tracer Pocket Depth
Tracer pocket depth is the distance the tracer rises in the direction of flow from a low point to a high point.
The total pocket depth is the sum of all risers of the tracer. The maximum allowable total pocket depth shall be equal to 40 % of steam gage pressure in psig (MPag) expressed in feet (mm).
3.3.4 Insulation of Steam Traced Lines
For externally traced piping the internal diameter of the insulation material must be increased over the same sized non-traced piping by 0.5" - 1.0" (12 - 25 mm) to provide space for the steam tracer tubing. For tracers applied with heat transfer cement the increase must be 1.0" - 2.0" (25 - 50 mm). Calculation of heat loss must consider this larger outside surface of the insulation and steam consumption must reflect the higher loads. Increase of heat losses of 12 - 15 % can be expected.
The oversized insulation requirements of the various tracing options are shown in Table 3-1.
3.4 SYSTEM DETAILS
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.
