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ASME CRTD-103-2014 — Consensus on Best Tube Sampling Practices for Boilers & Nonnuclear Steam Generators
📌 ASME CRTD-103-2014 is an ASME research report prepared by the Tube Sampling Task Group of the Water Technology Subcommittee of the ASME Research and Technology Committee on Water and Steam in Thermal Systems. It represents industry consensus on the best practices for collecting and analyzing tube samples from boilers and nonnuclear steam generators, including waterwall tubes, superheater tubes, and other heat-absorbing surfaces. Because personnel turnover at both industrial steam plants and power plants has left many facilities unfamiliar with standard practices for tube sample collection and analysis, this document was written to provide a single, consensus-based reference covering the complete process: routine sampling for cleanliness assessment, sampling after tube failures, and the fundamentals of sample removal, identification, and shipping.
💡 The report is a companion to earlier ASME consensus documents on feedwater and boiler water chemistry control, lay-up of boilers and turbines, avoiding steam purity problems, and water and steam chemistry control in combined-cycle and cogeneration plants. Together they form the ASME library of water-chemistry operating guidance for thermal systems.
1. Overview and Scope 📋
The body of the report is organized into three main parts: (1) routine tube sampling for cleanliness assessments, (2) tube sampling following tube failures, and (3) tube sample removal, identification, and shipping basics. As a general rule, routine tube sampling is used to assess the condition and cleanliness of tubes that are filled with steam/water mixtures during service. The primary objectives are to measure the internal deposit weight per unit area — referred to in this document as the deposit weight (also called deposit weight density, deposit density, deposit loading, or specific deposit weight) — and to determine the deposit composition. This information is used to decide whether a chemical cleaning is required and to assess the performance of the feedwater and boiler water treatment programs.
The report explicitly identifies exclusions: waste heat boilers or process coolers of shell-and-tube design are usually excluded from routine sampling because sample collection is often impractical owing to steam generator geometry or materials of construction; steam-cooled tubes generally are not sampled for routine cleanliness assessments because they are sampled instead to assess tube condition and remaining life, which is beyond the scope of this document; and fire tube boilers generally are not sampled for deposit weight assessments — the normal practice is to obtain a tube sample after a tube failure to determine the failure mechanism.
⚠️ The consensus sampling recommendations in this report are primarily directed at boilers operating at or above 600 psig (about 4.1 MPa). For lower-pressure units, or units subject to rapidly fluctuating loads, sampling plans should be established by a risk assessment of potential failure.
2. Sampling Frequency and Evaluation Objectives 🔍
The consensus recommendation is that tube sampling be performed every one to five years and after any major boiler (or HRSG) water chemistry upset or tube failure. The goal over time is to accumulate enough data to define the trend in deposit accumulation. For power boilers and heat recovery steam generators, the report cites EPRI guidance and other references that may be useful in developing a site-specific sampling plan, and it suggests contacting the equipment supplier for recommendations regarding sampling frequency for the specific unit.
Besides deposit weight and composition, another objective of routine tube sampling is to perform dimensional measurements — tracking changes in wall thickness and the depths of any pits or depressions — which feed directly into remaining-life assessments. Standard methods for deposit removal and deposit weight measurement are cited from the references; boiler manufacturers and EPRI have also published suggested maximum acceptable deposit weights and guidance for determining when cleaning is needed.
3. Sample Location: Heat-Flux-Zone Selection by Firing Design 🔥
Selecting the right sample location is the single most important factor for sample representativeness. Table 1 of the report suggests tube sample locations in reference to the top set of burners commonly in service, because normally the upper burners are the last burners placed in service and the area of high heat flux is tied to burner operation. The table below summarizes the recommended sampling logic for several basic firing designs:
| Firing Design | Recommended Sample Location | Basis for Selection |
|---|---|---|
| Single Wall Fired | Target (opposite) wall in the high-heat-flux zone, just above the top burners normally in service | Flame is relatively short and wide, putting more heat in the side/target walls; low-NOx burners alter the heat distribution |
| Opposed Wall Fired | Both side walls, referenced to the top set of commonly used burners | Flames from opposite walls meet in the furnace center |
| Tangential Fired with Division Wall | Selected according to flame rotation and division-wall arrangement | Rotating flame creates a complex heat-flux distribution |
| Black Liquor Recovery Boiler | Determined by the special geometry and combustion characteristics of recovery boilers | Black liquor combustion differs fundamentally from conventional fuels |
| Spiral Wall Boiler | Just above the entrance to the second-pass wall | Second pass is straight tubing forming a wall; sample just above the wall entrance |
Angle-wall-fired boilers (Figure 6 of the report) are not included in Table 1 because the preferred sample location is unit specific; they are discussed in the text instead. The preferred horizontal position of tube samples can vary with flame shape, alignment, and heat input. For boilers not consistently operated with all burners in service, the high-heat-flux area may shift; at low loads the flame shortens and the horizontal position of the samples may need to move closer to the burners.
💡 Sample representativeness should always take priority over sampling convenience. It is far better to erect scaffolding to reach a genuine high-heat-flux zone than to take an easily accessible but unrepresentative sample, which will systematically underestimate deposit accumulation.
4. Deposit Analyses and Multiple Tube Samples 🔬
Routine analyses typically cover deposit weight per unit area and deposit elemental composition (iron, copper, silica, phosphate, calcium, magnesium, and others), with metallographic examination where tube condition needs verification. The report discusses multiple tube samples: taking several tubes at several locations on a single unit provides a much more reliable picture of deposit distribution across the boiler and avoids the chance findings of a single specimen.
5. Sampling After Tube Failures and Sample Handling 🔧
Tube sampling following tube failures serves a different purpose from routine sampling — it supports failure mechanism analysis. Original tube sections around the failure must be preserved, and the fracture surface and internal deposits must not be damaged during removal or transport. Fire tube boilers, which are not routinely sampled, should be sampled after a tube failure to determine the failure mechanism. After removal, every sample must be clearly identified: the report defines a Tube Sample Identification nomenclature requiring unit, location, elevation, and orientation (fireside/backside) information, so samples cannot be confused. Packaging and shipment requirements are also specified to protect the samples and preserve traceability.
🚨 Sample identification errors are among the most costly mistakes in failure analysis — a mislabeled sample can send an entire investigation to the wrong conclusion. Verify labels and the accompanying record sheet at every step: cutting, packaging, and shipment.
6. Engineering Practice Essentials 🛠️
- 📅 Build a sampling plan: schedule tube sampling into overhauls every one to five years, and add immediate samples after a tube failure or a major chemistry upset.
- 📍 Location first: choose high-heat-flux zones based on firing design and burner arrangement; consult the boiler manufacturer when in doubt.
- ⚖️ Establish a baseline: the first sampling campaign on a unit with no sampling history becomes the initial deposit survey against which future trends are compared.
- 📦 Traceability chain: maintain a complete record from cutting → identification → packaging → shipment → laboratory receipt.
- 🧪 Link to cleaning decisions: compare deposit weight and composition against EPRI and manufacturer limits to decide whether chemical cleaning is required.
7. Frequently Asked Questions ❓
❓ Q: Why does routine cleanliness sampling target tubes filled with steam/water mixtures?
A: These tubes accumulate deposits most rapidly and therefore need the closest monitoring. Steam-cooled tubes are sampled for different purposes — assessing tube condition and remaining life — which fall outside the routine scope of this document.
A: These tubes accumulate deposits most rapidly and therefore need the closest monitoring. Steam-cooled tubes are sampled for different purposes — assessing tube condition and remaining life — which fall outside the routine scope of this document.
❓ Q: How often should tube samples be taken?
A: The consensus recommendation is every one to five years, plus immediately after any major water chemistry upset or tube failure. For utility boilers and HRSGs, refine the plan using EPRI guidance and supplier recommendations.
A: The consensus recommendation is every one to five years, plus immediately after any major water chemistry upset or tube failure. For utility boilers and HRSGs, refine the plan using EPRI guidance and supplier recommendations.
❓ Q: Do fire tube boilers require routine tube sampling?
A: Generally no routine deposit-weight sampling is performed. The normal practice is to obtain a tube sample after a tube failure to determine the failure mechanism.
A: Generally no routine deposit-weight sampling is performed. The normal practice is to obtain a tube sample after a tube failure to determine the failure mechanism.
🔍 Q: What should be done before shipping samples to the laboratory?
A: Identify each sample completely per the report’s nomenclature (unit, location, elevation, orientation), package it to prevent transport damage, and keep the accompanying record sheet matched one-to-one with the sample tags.
A: Identify each sample completely per the report’s nomenclature (unit, location, elevation, orientation), package it to prevent transport damage, and keep the accompanying record sheet matched one-to-one with the sample tags.
