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What Is Trunnion Support in Piping Engineering?
What Is Trunnion Support in Piping Engineering?
A trunnion support, also called a dummy leg, is a short length of pipe or solid bar welded perpendicularly to the parent pipe that transfers the pipe’s weight to an underlying structural steel member or concrete foundation. The trunnion extends downward from the pipe and its lower end rests on, or is welded to, a base plate or secondary structural support. Trunnion supports are classified as primary pipe supports because they are welded directly to the pipe itself rather than clamping around it.
Why Trunnion Supports Are Used
Standard pipe shoes rest underneath the pipe on a beam. They work well where the pipe sits close to structural steel. Where the pipe must be elevated well above its supporting structure, or where a pipe runs vertically and requires support at a defined point, a trunnion is the preferred solution. The trunnion provides a direct structural connection between the pipe and the support structure without requiring the pipe to be directly in contact with the steel beam. Welding the trunnion to the pipe also eliminates the clamp and fastener assembly that standard shoe supports require, simplifying both fabrication and inspection.
Trunnion versus Pipe Shoe
A pipe shoe is a structural plate or channel section welded to the underside of a horizontal pipe. The pipe rests on a beam through the shoe. A trunnion is a pipe section welded to the side or underside of the parent pipe and supports the pipe from a lower elevation. Trunnions are preferred when thermal movement is large, when the pipe must be elevated above the structural steel level, or when the pipe runs vertically and requires lateral stability from two opposing trunnion arms. Trunnions can also be welded to elbows to support horizontal lines where the pipe changes direction and no other support attachment point is available.
Applications in Piping Engineering
Vertical Pipe Support
Vertical pipe runs cannot be supported from underneath in the same way as horizontal runs. Two trunnion arms, one on each side of the vertical pipe at the same elevation, rest on a horizontal structural beam and provide the vertical support. This arrangement resists the pipe’s downward weight load and provides lateral stability in both the pipe axis direction and the transverse direction. The trunnion arms must be spaced far enough apart that the pipe can be bolted or guided to the structural member with clearance for any lateral thermal displacement that the vertical run undergoes during operation.
High-Temperature Insulated Piping and Slide Plates
On high-temperature piping, the trunnion tip is fitted with a slide plate of PTFE, graphite, or lubricated steel that allows the pipe to move axially along the supporting beam as it expands thermally. Without a slide plate, the friction between the trunnion tip and the beam would generate significant axial load in the pipe at every support point. Slide plates reduce this friction force to an acceptably low level. For extreme-temperature or high-load applications, engineers specify low-friction bearings or rockers in place of simple slide plates to achieve the required freedom of axial thermal movement.
Reinforcing Pad for High-Load Applications
When the pipe load is large relative to the pipe wall thickness, or when the weld junction stress between the trunnion and the parent pipe would otherwise exceed acceptable limits, a reinforcing pad is welded around the trunnion at the parent pipe junction. The reinforcing pad is a curved plate that conforms to the outer surface of the parent pipe and distributes the concentrated load from the trunnion weld over a larger area of the pipe wall. Alternatively, a pipe saddle beneath the full circumference of the parent pipe provides even broader load distribution for very heavy trunnion loads on large-diameter thin-wall pipe.
Cryogenic Piping and Thermal Isolation
Trunnion supports for cryogenic piping such as LNG transfer lines require a thermal break between the cold pipe and the structural steel. Without thermal isolation, cold pipe temperatures conduct through the trunnion into the structural beam, causing the beam to cool, contract, and develop thermal stress. Additionally, moisture condenses and ice forms on the cold steel, accelerating corrosion. Insulated trunnion designs incorporate a block of high-performance insulating material between the trunnion base and the structural steel to prevent this heat path. The insulating block must be mechanically strong enough to transmit the full pipe load through the insulation to the steel below.
Weld Junction Stress Analysis
The intersection of the trunnion pipe with the parent pipe is the most structurally critical location in the trunnion assembly. Pipe loads applied at the trunnion tip create a bending moment and shear force at this junction. The stress intensification factor at the weld junction amplifies the calculated nominal stress, and the resulting local stress must be checked against the allowable stress for the piping code and material at the operating temperature. Engineers use the Kellogg method or finite element analysis to calculate junction stresses. Keeping the trunnion height as short as practical reduces the bending moment at the weld junction and improves the load capacity of the assembly. API 570 inspection programmes should include periodic examination of trunnion-to-pipe welds for fatigue cracking in high-load or high-cycling applications.
Benefits of Thermal Shock Analysis
Quantifying Wall Loss and Predicting Retirement
Systematic thickness measurement at consistent TMLs over successive inspection cycles provides a directly measured corrosion rate rather than one estimated from published data or process chemistry. This measured rate is more accurate and more defensible than estimated rates for the specific operating conditions of the pipe in service. It allows the engineer to predict, with quantifiable confidence, when the pipe wall will reach the minimum required thickness and to plan replacement or repair before that date rather than in response to a failure.
Supporting Risk-Based Inspection Optimisation
Risk-based inspection programmes use remaining life data from thickness measurements to prioritise inspection resources. Circuits with short remaining life and high-consequence service receive frequent, intensive measurement campaigns. Circuits with long remaining life and low-consequence service receive extended intervals between measurements. This prioritisation reduces the total inspection cost while improving the safety performance of the inspection programme. Without thickness measurement data, risk-based inspection cannot function because it has no quantitative basis for calculating the probability of failure due to wall thinning.
Enabling Fitness for Service Assessment
When thickness measurements reveal that a specific location has thinned below the minimum required thickness calculated from the pressure design formula, a fitness for service assessment under API 579 determines whether the component can continue to operate safely with its reduced wall. Fitness for service assessments of localised thinning require the measured minimum thickness, the extent of the thinning area from the scan map, and the calculated required thickness at the design conditions. Without accurate thickness measurement data, fitness for service assessment cannot be performed and the engineer must default to immediate repair or replacement.
Limitations to Consider
Single-Point Measurement and Pitting
A single ultrasonic thickness reading at a TML represents the wall thickness at exactly one point on the pipe surface. It can miss localised pitting or crevice corrosion that occurs between measurement points. A pipe with a calculated average corrosion rate well within acceptable limits can have localised pits that are already at or below the minimum required thickness between the designated TML points. Close-grid scanning at locations known to be susceptible to pitting, rather than single-point measurements, provides more reliable minimum thickness data for such services.
Surface Condition and Measurement Accuracy
Ultrasonic thickness measurement accuracy depends on adequate acoustic coupling between the transducer and the pipe surface. Scale, pitting, internal deposits, and rough outer surfaces all degrade coupling and reduce measurement accuracy. The inspector must clean the measurement point before applying couplant and verify the reading stability before recording. On severely corroded or pitted surfaces, multiple readings at slightly different locations within the TML area and recording the minimum value is better practice than accepting a single reading. Additionally, the acoustic velocity through the pipe material must be correct for the measurement to be accurate.
Access and Insulation Removal
Conventional ultrasonic contact measurement requires direct access to the metal pipe surface. On insulated piping, insulation must be removed before measurement and reinstated afterwards. This adds significant cost and time to the inspection, particularly when scaffolding is required for access to elevated pipe. Pulsed eddy current and long range ultrasonic testing reduce but do not eliminate the need for insulation removal by screening large areas quickly and focusing the detailed investigation on suspect locations only.
Thickness Measurement FAQ
What is thickness measurement in piping engineering? Thickness measurement is the non-destructive evaluation of the current pipe or vessel wall thickness at defined locations to detect material loss from corrosion and erosion. Inspectors use ultrasonic thickness gauges, pulsed eddy current equipment, or other techniques to measure the wall thickness and compare it to the minimum required thickness calculated from the pressure design formula. API 570 requires systematic thickness measurement at designated thickness measurement locations to calculate corrosion rates, determine remaining life, and set inspection intervals.
How is thickness measurement used to calculate the corrosion rate? The corrosion rate is calculated by dividing the difference between two successive thickness measurements at the same TML by the time interval between those measurements. API 570 requires engineers to calculate both the long-term corrosion rate, based on the full measurement history from the original installation thickness, and the short-term corrosion rate, based on the two most recent readings. The more conservative of the two rates is used in the remaining life calculation. An unexpectedly high short-term rate indicates a change in corrosion conditions that requires investigation before the next inspection interval is set.
What is the difference between nominal wall thickness and minimum required thickness? Nominal wall thickness is the published design thickness of the pipe as designated by its schedule or weight class. Minimum required thickness is the calculated wall thickness below which the pipe can no longer safely contain its design pressure, calculated from the ASME B31.3 pressure design formula using the design pressure, pipe diameter, material allowable stress, mill tolerance, and corrosion allowance. The nominal thickness is always greater than the minimum required thickness to account for manufacturing tolerance, the specified corrosion allowance, and to provide a safety margin. When measured thickness approaches the minimum required thickness, the component is approaching retirement.
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