Page
0.0 Cover Sheet 1
1.0 Introduction 2
2.0 Purpose 2
3.0 Heat Transfer Principle 2 – 4
4.0 Determination of Insulation Thickness 3 – 5
5.0 Insulation Type 5 – 6 6.0 Insulation System 6 – 9 7.0 Applicable IS Standards 9 Applicable Revision: Prepared: Date: Checked: Date: Approved: Date: First Edition: R0 Prepared: DNL Date: Checked: AKB Date: Approved: RUD Date:
File Name: C- 33 Server: PUNE: KUMUS 207 VKO: KUMUS 209
1.0 INTRODUCTION:
Heat Transfer is one of the most common unit operations in Process Industry. In an ideal situation one would want to achieve heat balance between the Source and Sink without any loss of heat energy to the atmosphere. Unfortunately the above can not be achieved in absolute terms, even though an attempt can be made to control the Heat Transfer in such a way so as to limit the heat loss to atmosphere to a minimum by employing Insulating Material over the metallic surface exposed to atmosphere. Besides, there could be some other factors (e.g. condensation and subsequent freezing of moisture over the exposed surface) which may necessitate using Insulating Material depending upon the operating conditions of the System under question. The cost considerations do however prevail upon, in deciding the appropriate level of Insulation that would prove to be most effective from an overall angle.
2.0 PURPOSE:
The objectives of providing the Thermal Insulation can be summarized as follows:
• To prevent Heat Loss from hot surface.
• To prevent Heat Gain by the cold surface.
• To prevent condensation (and subsequently ice formation) on cold surface.
• To provide Personnel Protection against accidental contact of human body with hot metallic surface.
3.0 HEAT TRANSFER PRINCIPLE:
The Heat Loss in case of a hot insulated circular pipe takes place due to heat flow through the following 4 steps refer sketch (Figure-1, Exhibit 39.1) below
1. Heat flow Q1 from fluid to inside surface of the metal wall by way of Convection 2. Heat flow Q2 across the metal wall by way of Conduction.
3. Heat flow Q3 across Insulation Layer by way of Conduction
4. Heat flow Q4 from the outer surface of metal wall to the atmosphere, predominantly by way of Convection.
Under steady state condition the rate of Heat Transfer through the above 4 steps will be same.
i.e. Q1 = Q2 = Q3 = Q4
In case of Heat Gain, by a cold insulated pipe the direction of heat flow will be opposite to that of the hot insulated pipe.
Rate of Heat Transfer = K A * ∆t Where K = Thermal Conductivity By Conduction A = Surface Area
∆t = Temperature Gradient per Unit length = ∆t / R Cond R Cond (Thermal Resistivity due to conduction)
= 1/ KA
Rate of Heat Transfer = h A *∆T Where h = Convection Heat Transfer Coefficient By Convection
= ∆t / R Conv R Conv (Thermal Resistivity due toConvection)
= 1/ hA
∆T = Differential temperature
Applying the above basis equations to the Insulated Pipe Cross Section and ignoring the thermal resistivity R Conv and R Cond for heat transfer in Step 1 and 2 (i.e. assuming that Step 1
and Step 2 practically offer no resistance to Heat Flow)
The Heat Loss across the Insulation
Q3 = K * 2πL * ∆t1 (Eq 39.1) Ln (D2/ D1)
Where:
D2 = Outer Diameter of Insulation D1 = Inner Diameter of Insulation L = Length of Insulated Pipe
∆t1 = Temperature differential between inner and outer surface of Insulation. The Heat Loss from Insulation Outer Surface to the Atmosphere
Q4 = h * π D2 L *∆t2 (Eq 39.2) Where:
∆t2 = Temperature Differential t between the Insulation Outer surface and the Atmosphere.
Under steady state condition Q3 = Q4
K * 2πL * ∆t1 = h * π D2 L * ∆t2 Ln (D2/ D1)
Conduction Heat Transfer Coefficient for Insulating Material K is typically 0.02 – 0.04 W/ m. O C Convection Heat transfer Coefficient for Air (natural convection) is typically 15 – 20 W/ m2 .O C
4.0 DETERMINATION OF INSULATION THICKNESS:
The selection of Insulation Thickness is done based on 1 of the following cases for the given fluid and ambient temperature, wet bulb temperature (considered for cold insulation only) and wind speed (accounted for in the convection heat transfer coefficient for heat flow from insulation top surface to the atmosphere).
Case 1:
To maintain the temperature of the outer surface of the Insulation to a specified value from process angle (i.e. for controlling the heat gain) in case of Cold Insulated Piping.
Case 2:
To maintain the temperature of the outer surface of Insulation above wet bulb temperature to avoid condensation and subsequent freezing of the atmospheric moisture in case of Cold Insulated Piping.
Case 3:
To maintain the temperature of the outer surface of Insulation from personnel Protection point of view in case of Hot Insulated Piping. A maximum temperature of 52 O C is considered to be acceptable.
Case 4:
To maintain the loss of Heat Energy to a specified value from the point of view of limiting the plant operating cost in case of Hot Insulated Piping. A value of 100 Kcal/ hr m2 is generally assumed to be satisfactory from the point of view of rationalizing the Annual Capital Investment vis-à-vis Annual Plant Operating Cost.
The calculation of Insulation Thickness for Cases 1 to 3 is done in following steps
Step 1: Assume an arbitrary Insulation Thickness.
Step 2: Determine Q4 based on the predetermined value of ∆t2 (i.e. differential between the given temperature of outer surface of Insulation and the ambient) and the assumed value of Insulation Thickness in Step 1, as per equation (EQ 39.2)
Step 3: Equate Q3 = Q4
Step 4: For the value of Q3 arrived above, calculate the value of Insulation Thickness as per
equation (Eq 39.1)
Step 5: Based on the calculated value of Insulation Thickness recalculate Q4 Step 6: Repeat Steps 3 to 5 until the value of Q3 and Q4 becomes practically same.
Step 7: Select the Insulation Thickness as calculated in Step 4 corresponding to the steady
state achieved in Step 5.
The calculation of Insulation Thickness for case 1 is done on the similar principle as for case 1 to 3 with a minor variation in the approach, which is as follows
Step 1: same as above
Step 2 and Step 3: Not required since the Heat Loss (i.e. Q3 = Q4) is already specified. Step 4: For the specified value of Q3 = Q4 calculate the value of ∆t1 (i.e. temperature differential between the inner and outer surface of Insulation) based on the assumed value of Insulation Thickness in Step 1.
Step 5: Calculate the temperature of the outer surface of the Insulation based on the inner
surface temperature as fluid temperature and ∆t1 calculated in Step 4 above.
Step 6: Calculate ∆t2 (i.e. differential between the calculated temperature of outer surface of Insulation in Step 5 above and for the given ambient temperature).
Step 7: Calculate Insulation Thickness based on ∆t2 calculated above and the specified value of Q4.
Step 8: Repeat Steps 4 for the calculated value of Insulation Thickness in Step 7 above and
specified value of Q3 = Q4 to arrive at a new value of ∆t1
Step 9: Repeat Steps 5 to 7 to arrive at the new value of Insulation Thickness
Step 10: Repeat Steps 8 and 9 until a steady state condition is achieved (i.e. calculated
Insulation thickness in step 7 becomes constant)
5.0 INSULATION TYPE:
Based on the functional requirements, the Insulation Material is classified into 2 types as below
Hot Insulation:
Insulation used on hot surfaces for the purposes of Heat Conservation or for the purpose of Personal Protection.
Following material are typically used as Hot Insulating Material
Material Thermal Conductivity Admissible
Temperature
(mW/ Cm O C ) Range (O C)
Mineral Wool (unbonded) 0.48 (Note 1) 600 Mineral Wool (bonded) 0.43 (Note-1) 750
Glass Wool 0.43 (Note-1) 450
Calcium Silicate 0.55 500
Cold Insulation:
Insulation Used on cold surface for the purpose of Cold Conservation or for the purpose of avoiding Condensation.
Following Materials are typically used as Cold Insulating Materials
Material Thermal Conductivity Admissible
Temperature
(mW/ Cm O C) Range (O C)
Polyurethane Foam 0.29 (Note-1) -150 to 110 Expanded Polystyrene Foam 0.32 (Note-1) -150 to 80 Expanded Perlite Foam
Notes: 1) Thermal conductivity at 0 O C.
6.0 INSULATION SYSTEM:
Insulating Material:
Normally the Insulating Materials are available in unbonded mats and bonded or foamed preformed Pipe Sections/ Slabs to suit various applications. Polyurethane Foam and Expanded Perlite Foam can also used by in-situ foaming.
Protective Coating:
Normally thermal Insulation is provided with an external covering for protection against entry of water or process fluid, mechanical damage, exposure to fire and ultraviolet degradation (in case of foam material). The protective cover could be in the form of
• Coating (asphalt, polymer or resin)
• Membrane (felt or paper)
• Sheet Material (fabric, metal or plastic)
Vapor Barrier:
Cold Insulation Systems operating at subzero (below 2 O C) are normally provided with Vapor Barrier and sealed at the joints to prevent condensation and vapor transmission. Metallic Foils and Mastic embedded Glass Fabric are commonly employed for this purpose.
Insulation Thickness Selection
Uhde Standard provides the recommended thickness for various pipe sizes for the following Insulation Systems
• Cold Insulated Piping System
• Hot Insulated Piping System
• Personal Protection System
Insulation Material Properties:
Insulation Material in general shall be chemically neutral, rot-proof and free from impurities. In addition following properties are required to be considered in selecting the Insulating Material
• Thermal Conductivity
• Density
• Fire Resistance (to be rated as incombustible)
• Chloride Content
• Sulphur Content
• Moisture Absorption
• Shot Content
• Recovery after Compression
• Heat Resistance
Foam Insulation/ Thermocole
• Thermal conductivity
• Density
• Compression Strength and Hardness
• Water Vapour Permeability
• Auto Ignition
• Fire Resistance
• Heat Resistance
Application:
Following steps are followed while applying Thermal Insulation on Piping/ Equipment Items.
• Insulation Supports in the form of Ring, Lugs are welded to the Vertical Vessels and Tanks as per UN 2000–06 Part 1 Page 1 (for Hot insulation) and as per UN 2000-06 Part 1 Page 2 (for Cold Insulation).
• Horizontal Vessels will not require Insulation Supports
• In case of Cold Insulated Vessels the Insulation will extend up to 5 times the insulation thickness where there are protrusions (e.g. skirts/ leg supports etc.). Supports and Brackets in case of hot insulated Equipment are normally not insulated.
• The materials forming part of Insulation System (e.g. Cement, Coating, Fabric etc.) shall be free from Asbestos except mill board used for avoiding metal to metal contact.
• The Carbon Steel and Low Alloy Steel surface to be insulated shall be painted (for corrosion protection) with paint system as per Painting Specification recommended for the service.
• The Insulation Work shall commence only after the Hydro Test on Equipment/ Piping is completed and the items handed over for Insulation.
• Generally the Insulation shall be applied over the entire metal surface including flanges and stiffening rings etc. except over the parts (e.g. Gland Plate for valve gland packing etc.) which require frequent dismantling for maintenance purpose.
• As far as possible and practical the voids due to the profile of the external surface of any item (e.g. Valve body) will be filled with loose insulating material.
• In case of Cold Insulation the cladding shall be done without using self-tapping screws to avoid rapture of the Vapour Barrier. This however does not apply to in-situ foaming.
• Wherever applicable the joints between the Vapour Barrier and steel surface/ cladding are sealed to avoid ingress of moisture.
• In case of Insulation Thickness in excess of 75 mm it is recommended that the Insulation will be applied in multiple plies.
• Insulating Material used in the Process Plants in which Nitric Acid or Ammonium Nitrate are produced, shall not contain organic binding materials (e.g. Phenolic Resins).
• In the Process Plants with likely hood of formation of volatile flammable vapours, only insulation material with a closed surface (e.g. Foam Glass) shall be used.
• In case of application of Insulation in multiple plies, the seams shall be staggered.
• Insulation material on vertical or nearly vertical surfaces shall be prevented from sliding by means of suitable supports and tie wires or banding.
• Closely spaced lines (small bore)or Tubing may be insulated in a common envelop (up to 6 lines)
• In case of insulation of Heat Traced lines it is recommended to place a thermal shield (metallic foil) between insulation material and the tracing/process pipe for better heat transfer and for preventing the insulation from penetrating between the tracer and the process pipe.
• Vapour Barrier Foils in case of Cold Insulation shall be overlapped (approximately 50 mm) at the joints.
• Installation of Insulation Material is done in following steps:
Spacers:
• The purpose of providing Spacers is to enable the cladding to retain its shape and concentricity with respect to the surface to be insulated
• The spacers are required only for mineral fiber mats or for in-situ foaming
• The spacers are fabricated as per the details given in UN 5001-02 (Part-1)
• The spacers are positioned (fixed) at the required spacing on metallic/ plastic surface as per the details given in UN 5001-02 (Part –1)
• In case of Vertical equipment the spacers are attached to the vessels by means of Insulation Clips as per UN 2000-06 (Part-1 and Part-2)
Insulation Material:
• The Insulation Material in case of Mineral Fiber Mats, is fixed to the cylindrical surface by means of metal wire tied in a helical manner around the cylindrical surface.
• The Insulation Material in case of Mineral Fiber Preformed Shell or Slabs is glued to the metal surface or held together with closely butted joints.
• Insulation Material in case of preformed foam shells and slabs are held in position by gluing the end seams. In case of multiple layers the seams shall be staggered with respect to each other.
• In case of in-situ Foaming the foam is generated within the cavity formed between the metal surface to be insulated and the external cladding.
Depending upon the contour of the surface to be insulated, it may be necessary to fill the cavities and voids by means of loose mineral fibers or with the same type of foam
Cladding:
• Standard Sheet Metal (Galvanized) shall be used as Cladding Material. Aluminum Sheet can be used as alternative material (except Caustic Chlorine Plants)
• Metal Banding or self-tapping screws may be used for attaching the Cladding. Suitable Turn Buckles or Snap Catches may be used connecting the ends of Banding
• Cladding Joints shall be sealed by elastomeric sealing tape.
• The Cladding Joints are made by crimping or foalding.
7.0 APPLICABLE IS STANDARDS:
Glass Wool IS 3677 / IS 3690
Rock Wool IS 8183/ IS 9842
Polyurethane Foam IS 12436
Expanded Polystyrene IS 4671
Determination of Thermal Conductivity IS 3346
