Case Study: Inspection and Engineering Review of FRP Vent Stack

February 24, 2021

One of the world’s leading chemical companies turned to UTComp recently for expertise in composite inspection and engineering to assess the condition of an FRP vent stack at a plant in Texas that manufactures chemicals for the agriculture industry. In order to maximize the value and safe performance of the 30-year-old stack, the company plans to add a five-foot extension to the 50-foot column. Working with a local onsite data collection team, UTComp inspected the stack using UltraAnalytix® to inform a detailed mechanical integrity report and determine the stack’s suitability for continued service with the extension. Based on the results, UTComp engineers provided the design specifications for the stack extension.


Over the years, fiber-reinforced plastic (FRP) has emerged as the material of choice for ventilations stacks, chimneys and chimney liners used to vent corrosive gases. Factories, chemical plants and power stations all over the world have switched to FRP stacks, piping, ducts, scrubbers and other applications for long-term, worry-free operations. FRP is corrosion-resistant, able to handle acids, alkalis and bleach solutions, and very high operating temperatures. It’s also lightweight, mechanically strong and versatile, and has been proven across many industries as an economical, low-maintenance alternative to steel and concrete.

The FRP stack evaluated in this case study has been in service for approximately 30 years at a large herbicide manufacturing facility in Texas. Used to vent HCl vapour, the freestanding column is 50 feet tall with an inside diameter of 3 feet at the base and 2 feet at the top. The owners of the plant want to extend the height of the stack by 5 feet and asked UTComp to evaluate its condition and determine whether it is suitable for continued service with the addition. The client also asked UTComp to design the FRP extension and provide the specifications for manufacturing, installation and quality control.

UTComp specializes in engineering innovative solutions to support a full range of FRP and composite materials needs, from engineering to material selection, design, fabrication, installation supervision and ongoing inspection. The UltraAnalytix® inspection system combines ultrasound technology with a patented algorithm for data analysis to ensure the safe performance and maximum lifespan of FRP and other composite material industrial assets. It’s the only proven non-destructive, non-intrusive evaluation method for fast, reliable, non-destructive testing and pro-active service-life forecasting.

All data analysis, load and design calculations for this project were completed in accordance with appropriate building codes and recognized standards of organizations such as the American Society of Civil Engineers (ASCE), American Society of Mechanical Engineers (ASME) and ASTM International (ASTM).

Project Summary

  1. Inspection

In December 2020, UltraAnalytix data was collected by a Texas-based team in accordance with UTComp training guidelines and standard practices. The inspection plan called for a total of 184 UltraAnalytix readings to be taken at 23 condition monitoring locations (CML) along the length of the column. The technicians completed a minimum of 8 readings per CML focused on key components of the structure including:

  • Reinforcing pads (repads) located in areas that support additional stresses and loads, such as manways and inlet nozzles
  • Butt & wrap joints that connect sections of FRP pipe with resin and layers of fiber overwrap
  • Spool sections of bare FRP pipe that aren’t adjacent to joints or supports and therefore aren’t expected to experience additional stresses and loads
  • Protective FRP skirts.

The inspection paid particular attention to the base of the column, where stresses are highest, near the top of the existing stack where the addition was to be attached, and a butt and wrap joint located about 15 feet below the top of the stack, to assess the ability of the stack to resist wind and seismic loads. The plan called for measurements to be taken at points above and below each change in thickness elevation (approximately every 8 feet), as well as readings to detect damage to the corrosion barrier.

The raw UltraAnalytix data was sent to UTComp in Cambridge, Ont., Canada for analysis and reporting.

Using the data provided by the on-site team, UTComp engineers calculated FRP thickness and percentage of design stiffness (PDS), which represents the material’s loss of strength and stiffness over time compared to its as-new condition.

  1. Engineering Report

Following the inspection, UTComp completed a detailed engineering review of the vent stack to assess its current condition and determine whether it is suitable for a 5-foot extension. The review included analyzing the UltraAnalytix data plus the original design specifications, drawings and other information provided by the client.

The review considered:

  • Design wind and seismic loads
  • Adequacy of the existing stack with the extension added
  • Design requirements of the stack addition.

Using conservative assumptions about the equipment’s original/theoretical material properties, the UTComp engineering team calculated the factored resistance and factored loading for the expected loads in accordance with recognized standards including ASCE 52-10 (Design of Fiberglass-Reinforced Plastic Stacks). The anchor bolts were assessed on the basis of the maximum overturning moment – the energy required to cause the stack to collapse or topple. Environmental loads from seismic forces and wind were included in the calculation in accordance with ASCE 7-16 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures).

Trilam software for laminate analysis was also used to determine axial and hoop modulus. Calculations took into account photos taken in 2016 of repairs that appear to have been completed using hand lay-up methods.

By analyzing all the data, the UTComp team was able to determine FRP stress and strain during future intended use. Results were then incorporated into the design for the extension.

  1. Engineering and Design Specifications

As FRP subject matter experts, UTComp was asked by the client to provide engineering services to design the 5-foot stack extension. With results from the review of UltraAnalytix inspection data and original design specifications supplied by the client, UTComp addressed the requirements for manufacturing, installation and quality control of the addition. Design work and drawings are based on structural and load calculations in accordance with ASCE 52-10 and ASCE 7-16 as well as the principles of other standards and codes including ASME RTP-1 (Reinforced Thermoset Plastic Corrosion-Resistant Equipment) and ASTM D3299 (Standard Specification for Filament-Wound Glass-Fiber-Reinforced Thermoset Resin Corrosion-Resistant Tanks).

Engineering and design specifications covered the selection of the materials to be used (including the type of glass, resin and curing system) as well as outlining the following steps in the modification of the existing stack:

  • Manufacturing the stack addition at a manufacturer’s shop
  • Removing existing cladding and insulation at the top of the stack
  • Cutting off the top 152 mm (6 inches) of the existing stack
  • Preparing the FRP surfaces on the existing stack and the addition for bonding
  • Installing seal bond over the cut line at the top of the existing stack
  • Attaching the addition to the top of the existing stack. 

Key findings, discussion and analysis

Some damage was detected to the corrosion barrier in the inner 5 mm of the FRP laminate. This likely indicates that heat-related damage had occurred to the resin — the matrix that holds the glass fibers together, strengthens and protects the FRP structure from abrasion, corrosion, impact and other environmental factors. However, the damage was not sufficient to have any impact on the structural properties of the laminate. Normal practice is to use resin with Heat Distortion Temperature 20 degrees C (36 F) higher than operating temperature. Heat Transfer Calculations completed for the stack extension review resulted in a stack wall temperature of 166 C (330 F) for design temperature of 177 C (350 F).

  1. Mechanical Integrity

The drawing below illustrates components of the HCl vent stack and the 23 condition monitoring locations where UltraAnalytix readings were taken. Based on the data collected, UTComp calculated FRP thickness, PDS and remaining service life (RSL).

The tables below show results from analysis of the CML inspections for each component type:

Butt & Wrap Joints

A portion of butt and wrap joints is usually exposed directly to the chemicals contained in the ducting system. Any loss of thickness in the duct in the joint should also be expected to occur in the portion of the joint that has been exposed.

Table 1: Butt & Wrap Joints


A spool is a section of the bare stack shell that is not adjacent to joints or supports and therefore is not expected to experience additional stresses and loads.

Table 2: Spool results by CML


Reinforcing pads are used to provide additional strength to parts of the structure that experience additional stresses and forces such as the manway, hot and cold gas inlet nozzles.

Table 3: Repad results by CML

Other / FRP Skirts

Readings were also taken of protective laminate skirts located between the anchor plate and rod, upper rod and insulation clip, and skirt vent above the lower foundation plate.

Table 4: Other / Skirts results

  1. Stack Extension

In addition to the UltraAnalytix inspection data summarized above, UTComp required additional information from the client to complete the review and design of the proposed stack extension including:

  • Size of the FRP asset
  • Required operating pressures and temperatures
  • Fluids or substances to be contained by the vessel / stack
  • Codes and standards that must be met by operating equipment
  • Environmental loads for the installation including wind, rain, snow, earthquake, explosion, etc.
  • Any other owner-specified limits on safety factors or allowable stresses.

The following parameters were used in the calculation:

Asset Properties:

  • Shell inside diameter at base: 914 mm (3 ft)
  • Shell inside diameter at top: 609 mm (2 ft)
  • Total height: 16.761 m (50 ft)
  • Sidewall thickness: per drawing ID 3076 Rev 1
  • Process fluid: HCl fumes.

UTComp System Data  

The table below summarizes PDS calculations for each subsection of the stack.

Table 5: UTComp data

Design Conditions

  • Operating temperature: 433 K (320 F)
  • Design temperature: 450 K (350 F)
  • Operating pressure: 6.895 kPag (1 psig)
  • Design pressure: 6.895 kPag (1 psig)
  • Nominal flow rate: 2.434 m3/s (5157 ft3/min)
  • Thickness of fiberglass insulation: 50.8 mm (2 in)
  • Location: Texas
  • Risk category: III
  • Site class: D
  • Basic wind speed: 62.5 m/s (140 mph)
  • Ground motion parameter 0.2s: 0.0742
  • Ground motion parameter 1-2s: 0.0395
  • Seismic design category: A.

Trilam Results

Trilam analysis was used to calculate theoretical as-new material parameters for the vessel, assuming hand lay-up with Derakane 470 resin at 300 F.

  • Theoretical As-New Laminate Values
    • Tensile Modulus (hoop and axial): 9.25 GPa
    • Flexural Modulus (hoop and axial): 7.82 GPa
    • In-Plane Shear Modulus: 1.55 GPa
    • Out-of-Plane Shear Modulus: 0.69 GPa
    • Poisson’s ratio: 0.16
  • Load and Resistance Factor Results

Table 6: Load and resistance at base

Table 7: Load and resistance at new tip

  • Anchor Bolts
    • Maximum anchor bolt stress resulting from environmental loads: 71 MPa
    • Maximum allowable stress in anchor bolts: 124 MPa


Based on the analysis and review of all the information, UTComp concluded that the FRP stack is suitable for continued service until the next recommended UltraAnalytix inspection in 2023. The review also concluded:

  • Stresses in the FRP at the design conditions comply with all the required standards
  • Factored resistance exceeds the factored loading for all load cases in accordance with ASCE 52-10
  • The stack is structurally suitable for the addition of a 5-foot extension at the top.