EN 12953-10: Water Quality Requirements in Industrial Shell Boilers
Technical analysis of the parameters the standard defines for feedwater and boiler water, and their significance for the integrity and safety of steam generation systems.
Water quality is, alongside design and manufacturing conditions, the single factor that most influences the long-term integrity of a shell boiler. The European standard EN 12953-10 establishes minimum water quality requirements for feedwater and boiler water in this type of equipment, with the fundamental aim of minimising risk to personnel and surrounding installations.
For process engineers, maintenance managers and plant operators running steam generation systems, understanding the framework this standard defines — which parameters it controls, why, and according to what criteria — is an essential element of plant technical management.
1. Normative framework and scope
The standard EN 12953-10:2003 forms part of the EN 12953 series, which collectively regulates the design, manufacture, documentation and operation of shell boilers (also referred to as firetube boilers). Part 10 deals specifically with the quality requirements for feedwater and boiler water.
Its scope covers all shell boilers heated by combustion of one or more fuels or by hot gases, intended for the generation of steam and/or hot water. The standard applies to components between the feedwater inlet and the steam outlet of the generator. The quality of the steam produced is expressly excluded from scope; where specific steam purity requirements apply, additional normative documents are required.
Spanish Royal Decree 2060/2008 of 12 December, approving the Pressure Equipment Regulations, requires operators of steam or hot water boilers to maintain water within the specifications of UNE-EN 12953-10 (shell boilers) or UNE-EN 12952-12 (water-tube boilers). Compliance is therefore a legal obligation for the installation operator.
2. Technical purpose of the standard: damage mechanisms to be prevented
The precipitation of calcium, magnesium and silicate salts onto heat transfer surfaces creates layers of low thermal conductivity. A deposit as thin as 1 mm can increase fuel consumption by around 5–8 % and locally raise the metal wall temperature to values that compromise its integrity.
Dissolved oxygen and free carbon dioxide are the primary corrosive agents. Oxygen corrosion produces localised pitting that can progress until the tube wall is perforated. Inadequate pH promotes various forms of chemical attack on carbon steel.
The presence of total dissolved solids (TDS) at elevated concentrations, or of certain organic substances, can cause foam formation at the water surface. This leads to carry-over of boiler water droplets into the steam (priming), contaminating the steam with salts.
Suspended impurities and precipitates not removed by blowdown can accumulate as sludge in low-velocity water zones, impeding circulation and heat transfer, and promoting under-deposit corrosion.
3. Fundamental distinction: feedwater and boiler water
The standard precisely distinguishes two types of water that have different requirements and are controlled independently.
Feedwater is the water entering the boiler to replace the evaporated volume. It is typically a mixture of recovered condensate and make-up water, having undergone the necessary external pre-treatment processes.
Boiler water is the water present inside the boiler drum during operation. Because feedwater is a continuous source of impurities, boiler water undergoes progressive concentration of these substances. Its admissible parameters are managed through system blowdown.
4. Quality parameters: technical description
Determines the acidic or alkaline character of the water. Moderately alkaline feedwater pH inhibits oxygen corrosion; in boiler water, alkalinity is required to maintain steel passivation.
Expresses the concentration of calcium and magnesium ions, the main scale-forming species. The standard requires extremely low levels in feedwater, which in practice necessitate softening or demineralisation treatment.
Primary corrosive agent. Must be eliminated by combining thermal deaeration with oxygen scavenger dosing. The standard distinguishes limits according to the design pressure of the boiler.
Indirect indicator of total dissolved solids (TDS) concentration. The standard classifies the operating regime according to whether feedwater direct conductivity is above or below 30 µS/cm.
Determined by passing the sample through a strongly acidic cation exchanger. Particularly sensitive to CO₂, chlorides and sulphates, providing a more reliable measure of aggressive anions.
Originates primarily from corrosion of steel pipework in the condensate circuit. Forms deposits on heating surfaces that degrade heat transfer performance.
Originates from corrosion of copper-alloy equipment and pipework in the circuit. Its deposition on steel surfaces can accelerate galvanic corrosion.
Forms calcium and magnesium silicate scale with very low thermal conductivity and high mechanical hardness, difficult to remove without chemical cleaning. Its limit in boiler water varies with operating pressure.
Their presence causes intense foaming and water carry-over with steam. Can promote corrosion by forming films on metal surfaces that alter heat transfer conditions.
Organic substances can thermally decompose under boiler operating conditions, generating carbonic acid and other acidic products that raise acid conductivity and cause corrosion.
5. Boiler water parameters: the role of blowdown
Because boiler water concentrates progressively, quality management requires an active strategy for impurity removal. The fundamental tool for this is blowdown.
The standard provides for the dosing of chemical conditioning agents into boiler water with the aim of: maintaining pH within the prescribed range, controlling suspended sludge, inhibiting scale formation and removing residual oxygen traces. The type and dose of these agents must be determined by a water treatment specialist.
The standard sets boiler water pH ranges significantly more alkaline than those for feedwater. This alkalinity is necessary for steel passivation, but must be controlled: certain alkalinity levels are not permissible for boilers operating above 20 bar, since an excess of free sodium hydroxide can cause caustic cracking.
6. Feedwater conductivity classification
One of the structural features of EN 12953-10 is that boiler water parameters are presented in two distinct regimes, depending on whether feedwater direct conductivity is above or below 30 µS/cm:
- Feedwater direct conductivity ≤ 30 µS/cm: corresponds to the low-TDS regime, characteristic of installations using demineralisation or reverse osmosis as pre-treatment.
- Feedwater direct conductivity > 30 µS/cm: corresponds to the high-TDS regime, common in installations using only ion-exchange softening as pre-treatment.
7. Silica: a pressure-dependent constraint
Unlike most parameters, the maximum permissible silica limit in boiler water varies continuously with operating pressure, following the curve shown in Figure 5-2 of the standard. As operating pressure increases, the relative volatility of silica — its tendency to pass into steam as orthosilicic or metasilicic acid — also increases.
Heat exchangers using steam as the heating medium may be affected by steam quality if silica or other salts are carried over. Furthermore, in configurations where process water from the heat exchanger returns as condensate to the boiler feedwater system, management of condensate contamination is critical to maintaining feedwater quality within the limits prescribed by EN 12953-10.
8. Make-up water for hot water boilers
The standard also includes specific provisions for the make-up water of hot water boilers operating in closed circuits. In these systems, requirements focus primarily on initial filling and the composition of water in service, with particular attention to pH and the absence of corrosive agents.
The standard establishes differentiated pH ranges for circulating water in hot water boilers depending on whether the circuit uses only ferrous metals or also includes non-ferrous metals (copper, brass, bronze).
9. Monitoring and analysis requirements
Control frequency and method
The standard prescribes that relevant parameters — pH, direct conductivity, acid conductivity, hardness and oxygen or oxygen scavenger — must be verified continuously and/or periodically. The use of reliable continuous analysers allows the frequency of manual checks to be reduced, but does not eliminate them.
Sampling points
Samples must be taken at representative points in the system. The standard identifies typical sampling points as: feedwater at the inlet valve, boiler water from a downcomer or continuous blowdown line, make-up water downstream of the treatment equipment, and condensate at the condenser outlet.
Reference analytical methods
| Parameter | Reference analytical standard |
|---|---|
| Acid capacity (alkalinity) | EN ISO 9963-1 |
| Conductivity | ISO 7888 |
| Copper | ISO 8288 |
| Iron | ISO 6332 |
| Dissolved oxygen | ISO 5814 |
| pH | ISO 10523 |
| Phosphates | ISO 6878-1 |
| Potassium / Sodium | ISO 9964-2 / ISO 9964-1 |
| Total organic carbon (TOC) | ISO 8245 |
| Total hardness (Ca + Mg) | ISO 6059 |
10. Operating conditions requiring specific consideration
- The presence of heated crevices or heated phase interfaces, which can locally concentrate solutes.
- Operation at pressures significantly below the design pressure, which may alter heat transfer conditions.
- The use of materials other than carbon steel — such as stainless steel — which exhibit different corrosion mechanisms.
- The presence of organic substances in the water, whose composition and behaviour under operating conditions are difficult to predict.
- Applications where steam or hot water is intended for the food, pharmaceutical or turbine-feed industries.
The standard expressly states that the prescribed values apply to continuous operation. During start-up, shutdown or significant operational changes, some parameters may transiently deviate from normal values for a brief period. When deviations occur during continuous operation, they may be attributable to inadequate make-up water treatment, insufficient conditioner dosing, condensate contamination or active corrosion processes in parts of the installation.
11. Operator responsibilities and obligations
Compliance with water quality requirements is not merely good operational practice: within the Spanish regulatory framework, it constitutes a legal obligation on the installation operator. Royal Decree 2060/2008 places on the user the responsibility for maintaining water within specifications, and for carrying out — directly or through third parties — the requisite analyses.
12. Concluding remarks
EN 12953-10 defines a rigorous technical framework grounded in the known damage mechanisms that water quality can induce in a shell boiler. Its proper application requires, in practice, the coordinated action of three parties: the boiler manufacturer, the water treatment specialist, and the operator — who bears ultimate responsibility for running the installation within the prescribed limits.
The standard is not, in any sense, a directly applicable recipe: it is a framework of minimum requirements that must be interpreted and adapted to each specific installation.