CEN/TR 16911:2015

SLOVENSKI STANDARD
kSIST-TP FprCEN/TR 16911:2015
01-oktober-2015
7RSORWQLãWHYFL3ULSRURþLOD]DREWRþQHYRGHYLQGXVWULMLLQVLVWHPLKGDOMLQVNHJD
RJUHYDQMDWHUQMLKRYRGHORYDQMH
Heat meters - Recommendations for circulation water in industrial and district heating
systems and their operation
Ta slovenski standard je istoveten z: FprCEN/TR 16911
ICS:
91.140.10 Sistemi centralnega Central heating systems
ogrevanja
kSIST-TP FprCEN/TR 16911:2015 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

kSIST-TP FprCEN/TR 16911:2015
kSIST-TP FprCEN/TR 16911:2015
TECHNICAL REPORT
FINAL DRAFT
FprCEN/TR 16911
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
July 2015
ICS 91.140.10
English Version
Heat meters - Recommendations for circulation water in
industrial and district heating systems and their operation

This draft Technical Report is submitted to CEN members for Technical Committee Approval. It has been drawn up by the Technical
Committee CEN/TC 176.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United
Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to
provide supporting documentation.

Warning : This document is not a Technical Report. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a Technical Report.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2015 CEN All rights of exploitation in any form and by any means reserved Ref. No. FprCEN/TR 16911:2015 E
worldwide for CEN national Members.

kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
Contents Page
European foreword .4
Introduction .5
1 Scope .6
2 Normative references .6
3 Terms and definitions .6
3.1 General .6
3.2 Types of water .7
3.3 Units .8
3.3.1 General .8
3.3.2 Measurands .8
4 Symbols and abbreviations .9
4.1 Chemical terms .9
4.2 Technical terms. 10
5 Water quality . 10
5.1 General . 10
5.2 Effects of the water constituents . 11
5.2.1 Gases . 11
5.2.2 Water-insoluble substances . 12
5.2.3 Water-soluble substances . 12
5.2.4 Oils/greases . 12
6 Systems engineering . 12
6.1 Systems conception . 12
6.1.1 General . 12
6.1.2 Materials . 13
6.1.3 Pressure maintenance and water supply . 14
6.2 Water treatment techniques . 15
6.2.1 General . 15
6.2.2 Filtering . 15
6.2.3 Demineralization . 16
6.2.4 Softening . 16
6.2.5 Degassing . 16
6.2.6 Catalytic and electrochemical oxygen scavenging. 16
7 Production technology . 17
7.1 Standard values for the circulation water . 17
7.2 Low-salt operation . 17
7.3 Salty operation . 17
7.4 Technical aspects related to the operation . 18
7.4.1 General . 18
7.4.2 Filling and supplementary water . 18
7.4.3 Underpressure . 19
7.4.4 Exceptional operating conditions . 20
7.4.5 Direct heating . 20
7.4.6 Indirect heating . 20
7.4.7 Partial evaporation. 20
7.5 Conditioning . 20
7.5.1 General . 20
7.5.2 pH value increase . 21
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
7.5.3 Hardness stabilizing . 22
7.5.4 Oxygen scavenging . 22
7.5.5 Corrosion inhibitors . 23
7.5.6 Water tracing dyes for the circulation water . 24
7.5.7 Antifreezing agents . 24
7.6 Monitoring . 24
7.6.1 General . 24
7.6.2 Assessment criteria . 24
7.6.3 Measurement frequency . 26
7.6.4 Dosing of conditioning agents . 27
7.6.5 Sampling. 28
7.6.6 Measurement procedures . 30
8 Hygienic, toxicological and environmental aspects . 30
8.1 General . 30
8.2 Hygienic and toxicological aspects . 30
8.3 Environmental aspects . 31
Bibliography . 32

kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
European foreword
This document (FprCEN/TR 16911:2015) has been prepared by Technical Committee CEN/TC 176 “Heat
meters”, the secretariat of which is held by SIS.
This document is currently submitted to the Technical Committee Approval.
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
Introduction
This document is based on the German Guideline AGFW FW 510 prepared by the German Heat and Power
Association (AGFW) that represents the state of the art but does not have a normative status has been
reproduced in this Technical Report with the permission of AGFW.
This Technical Report is an informative document that describes a process that may be applied for the
operation of district heating facilities and gives recommendations for the water used in such facilities. The
water quality described in this Technical Report can be used also during testing of heat meters.
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
1 Scope
This Technical Report applies to industrial and district heating supply by means of high-temperature water
heating facilities (flow temperature > 100 °C). This also applies to high-temperature water heating facilities
(flow temperature ≤ 100 °C) that are directly connected to district heating networks. In this Technical Report,
the aforementioned supply variants will, in the following, be referred to as “district heating facilities”.
This document applies without limitations to new facilities. For existing district heating facilities, the application
of this Technical Report is recommended in order to prevent faults due to the chemical composition of the
circulation water that would affect the facilities' safe operability and availability.
NOTE Informative notes in the form of guidance and recommendations are identified correspondingly and set in
italics for better differentiation.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
EN 1717, Protection against pollution of potable water in water installations and general requirements of
devices to prevent pollution by backflow
ISO 11466, Soil quality — Extraction of trace elements soluble in aqua regia
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1 General
3.1.1
district heating
heat (regardless of its origin) which is supplied by means of a transfer medium (mostly hot water or steam)
commercially on the basis of a supply agreement and from the delivery of which no collateral duties arise with
regard to leasing regulations
3.1.2
hot-/warm-water heating plants
hot-/warm-water generating facility in connection with a district heating network
3.1.3
water treatment
measures taken to remove solid particles, water-soluble substances (salts) and gases from the filling-,
supplementary- or circulation water
3.1.4
primary network
district heating network in indirect (e. g. heat exchanger) or direct connection with the heat generator
3.1.5
secondary network
district heating network separated from the primary district heating network by a substation with different
system parameters
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
3.1.6
tertiary network
end-user's domestic installation
3.1.7
heat exchanger with intermediary medium
heat exchanger with a safety system for the indirect heating of drinking water and in which the heating side
and the drinking water side are separated by two walls; the space between the two walls is filled with a
medium
3.1.8
chalk/carbonic acid equilibrium
if calciferous water is heated up, the concentration of bonded calcium hydrogen carbonate decreases with
increasing temperature, and the so called “chalk/carbonic acid equilibrium” shifts from the side of the calcium
hydrogen carbonate through the escaping carbon dioxide towards the side of the calcium carbonate:

Ca (HCO ) ⇔ CaCO ↓+ CO ↑+ H O
3 2 3 2 2
3.1.9
bicarbonate decomposition
after sofenting and in cause of higher temperature, sodium bicarbonate gradually decomposes into at least
sodium hydroxid, water and carbon dioxide (at about 55°C, higher pressure)
2 NaHCO → Na CO + CO ↑ + H O
3 2 3 2 2
Na CO + 2H O → 2 NaOH + H CO
2 3 2 2 3
H CO → CO ↑ + H O
2 3 2 2
3.1.10
boiler scale
conglomerate of low-solubility alkaline earth salts which form at temperatures < 100 °C, mainly CaCO3 and
MgCO3
3.1.11
limescale
conglomerate of low-solubility alkaline earth salts, mainly CaCO3, MgCO3, CaSO4 and CaSiO3
Note 1 to entry: They form either by heat conversion of the alkaline earth salts dissolved in the water (carbonate
hardness) or by overstepping the point of solubility which is also temperature-dependent. Soluble alkaline earth salts are
available as hardness components or neutral salts in drinking water.
3.1.12
heat transfer medium according to Class 4 pursuant to EN 1717
heat transfer medium which contains toxic, very toxic, carcinogenic or radioactive substances
3.1.13
water conditioning
improving certain quality parameters of the circulation water (e.g. increasing the pH value) by means of
conditioning chemicals
3.2 Types of water
3.2.1
untreated water
water available upstream from the treatment plant, regardless of a possible previous treatment outside the
plant
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
3.2.2
soft water
oxygenated water that has been treated by ion exchange to remove earth alkali (the process is called
softening)
3.2.3
demineralized water
oxygenated water that has been treated to remove the major part of dissociated, water-soluble substances
and is characterized by a pH value < 7, a conductance < 20 µS/cm and a silicic acid concentration < 0,5 mg/l
3.2.4
distilled water
deionized water
oxygenated water that has been treated by ion exchange to fully remove all dissociated, water-soluble
substances
3.2.5
filling water
conditioned water with which district heating facilities are initially, partly or re-filled
3.2.6
supplementary water
conditioned water with which temperature-related volume differences and losses due to evaporation and
leakage are compensated
3.2.7
circulation water
water that flows through the heat generator/heat exchanger, the piping network, heat transfer stations and, if
applicable, radiators. The term not only applies to primary networks, but also to water in a secondary network
3.2.8
feedwater
water that is used to feed a steam generator. It consists of supplementary water and condensate water after
full treatment and conditioning
Note 1 to entry: Feedwater is considered as salt-free if its cation conductance is < 0,2 µS/cm and the silicic acid
concentration is < 0,02 mg/l (not to mistake for distilled water!).
3.2.9
boiler water
water contained in water piping and large-scale water boilers and whose properties differ from those of
feedwater due to densification processes during use
3.3 Units
3.3.1 General
Pursuant to the “Units in Metrology Act”, the below-mentioned water-chemical terms and units apply.
3.3.2 Measurands
3.3.2.1
molar amount
concentration of substances contained in the water is stated in mmol/l or in mg/l
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
3.3.2.2
pH value
index for the acidic, neutral or alkaline reaction of water
Note 1 to entry: At a reference temperature of 25 °C, the pH value scale from 0 to 14 applies. Water is acidic at pH
values < 7, neutral at a pH value = 7, and alkaline at pH values > 7.
3.3.2.3
electrical conductivity
the salt concentration is generally determined by measuring the electrical conductivity which includes all
dissociated elements of the investigated medium, i.e. bases, acids and salts. In water chemistry, the reference
temperature used to measure electrical conductivity is 25 °C, the unit of measurement is µS/cm
3.3.2.4
sum of alkaline earth (hardness)
the former term “hardness” has been replaced by the term “sum of alkaline earth“
Note 1 to entry: The former units for the alkaline earth concentration (°d and mval/l) have been replaced by mmol/l,
mol/m and mg/l. The following applies to the conversion of the units:
1 mmol/l = 1 mol/m3 corresponding to 2 mval/l that will give 56 mg CaO/l
Note 2 to entry: Example of calculation for the conversion of the former units:
3,4 mval/l: 2 = 1,7 mmol/l
Note 3 to entry: Contrary to the concentration indications derived from the term “hardness” (°dH), technical
expressions such as “water softening” and “softened water” remain in usage.
4 Symbols and abbreviations
4.1 Chemical terms
3+
Al aluminium ion
2+
Ca calcium ion
CaCO3 calcium carbonate
CaSiO3 calcium silicate
CaSO4 calcium sulphate
−
Cl chloride ion
CO carbon dioxide
2+
Cu+ / Cu copper(I) ion / copper(II) ion
EDTA ethylenediaminetetraacetic acid or ethylenediaminetetraacetate
Fe iron
Fe 2+ / Fe 3+ iron(II) ion / iron(III) ion
KS4.3 acid capacity up to pH value 4,3
KS8.2 acid capacity up to pH value 8,2
2+
Mg magnesium ion
MgCO3 magnesium carbonate
N nitrogen
NaCl sodium chloride (common salt)
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
NaHCO3 sodium hydrogen carbonate
NaOH sodium hydroxide (caustic soda)
Na3PO4 trisodium phosphate
Na2SO3 sodium sulphite
Na2SO4 sodium sulphate
NTA nitrilotriacetic acid
O oxygen
PO43- orthophosphate ion
S2- sulphide ion
SO32- sulphite ion
SO42- sulphate ion
2+
Zn zinc ion
4.2 Technical terms
EV expansion vessel
DEV diaphragm expansion vessel
MIF magnetic inductive flow measurement
DFR differential pressure regulator
DOC dissolved organic carbon
TOC total organic carbon
5 Water quality
5.1 General
Untreated water may contain insoluble and, especially, soluble substances as well as gases.
Insoluble substances are frequent in surface water, infrequent in groundwater, whereas water from public
supply networks only contains traces of them.
Soluble substances occur in untreated water in the form of inorganic salts (especially calcium-, magnesium-
and sodium salts) and organic substances. The soluble gases are mostly oxygen, nitrogen from the air, and
carbon dioxide.
In district heating facilities, these water constituents can lead to malfunctions and either have to be removed,
or their effects to be limited.
The use of insufficiently treated filling or supplementary water or the inflow of water and/or air into district
heating facilities from the outside can lead to system malfunctions due to deposits and corrosion.
When assessing the cost-effectiveness of protective measures to prevent the diverse types of damages, the
fact that damage may, under certain circumstances, lead to considerable costs that cannot be calculated in
advance has to be taken into account.
When complying with the standard values, the alkalinization of the water on metallic surfaces furthers the
formation of homogeneous oxidic covering layers which are highly resistant against corrosion. A prerequisite
is, however, that the filling and supplementary water be treated correctly.
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
In district heating facilities, one fundamentally differentiates between low-salt and salty operation, depending
on the quality of the circulation water.
Further plant-specific prescriptions and guidance can be found in the Technical Connection Conditions (TCC).
5.2 Effects of the water constituents
5.2.1 Gases
5.2.1.1 General
Gases enter the circulation water due to the following processes:
— utilization of non-degassed filling and supplementary water;
— air leakage into the system in the event of underpressure (e.g. insufficient pressure maintenance);
— air inclusion during the initial, partial or new filling of the system;
— external water inflow;
— diffusion through permeable components (e.g. diaphragms, plastic pipes, seals).
5.2.1.2 Oxygen
Oxygen (O ) causes unalloyed and low-alloy ferrous materials to corrode. Oxygen inflow therefore has to be
prevented as far as this is technically justifiable.
Damage directly due to corrosion can manifest in the form of perforations in heat generators, pipes and
radiators made of unalloyed or low-alloy ferrous materials. The blinding of sieves, measuring equipment and
filters due to corrosion products is considered as an indirect consequence of corrosion.
5.2.1.3 Nitrogen
Nitrogen (N ) is an inert gas and, as a water constituent, only causes problems when its concentration is so
high that free nitrogen fractions (gas bubbles) form inside the system. Gas bubbles may occur, since the
solubility of gases decreases with increasing temperature and decreasing pressure. Circulation faults,
disturbing noises and erosion of protection layers (erosion corrosion) are the consequences.
Experience has shown that no system malfunctions due to nitrogen bubbles have to expected with nitrogen
contents of < 10 mg N per litre of water at a positive excess pressure of min. 0,5 bar (at the highest point of
the system).
5.2.1.4 Carbon dioxide
If the circulation water is not sufficiently alkalinized, the quantity of water-soluble carbon dioxide (CO )
influences the pH value – i.e. increasing CO cause the pH value to drop. Due to the in-creasing solubility of
iron(II)-hydroxide occurring at decreasing pH values, deposited corrosion products can be partially dissolved
by water having a relatively low pH value (<8). The increased iron(II) ion concentration can lead to an
increased formation of magnetite (Fe O ) in the form of hard, black deposits on the hot side of heat exchanger
3 4
surfaces.
This causes the increase of the overall heat transfer resistance and, thus, the thermal performance to
decrease. In particularly critical cases, this may even lead to overheating which, in turn, may lead to crack
formation.
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
5.2.2 Water-insoluble substances
Insoluble substances cause deposits and blockages and have to be removed from the untreated water by
means of suitable techniques (mud flaps, filters).
5.2.3 Water-soluble substances
5.2.3.1 Hardness components (alkaline earth)
When using unsoftened filling water, especially the alkaline earth ions contained in the water in connection
with the hydrogen carbonate ions lead to the formation of hard deposits, mainly containing calcium carbonate
(limescale, boiler scale). This causes the increase of the overall heat transfer resistance and, thus, the thermal
performance to decrease. In particularly critical cases, this may even lead to overheating which, in turn, may
lead to crack formation in heat generators (e.g. heat exchanger, vessels).
5.2.3.2 Chloride and sulphate
From all the water-soluble anions contained in the water, especially chloride and sulphate, in the presence of
oxygen, further local corrosion (e.g. crevice corrosion) in unalloyed ferrous materials.
Under critical conditions (e.g. concentration under deposits or in crevices), chloride ions can lead to pitting
corrosion or stress-corrosion cracking in non-corroding steels.
In addition, chlorides can cause corrosion in aluminium materials.
5.2.3.3 Hydrogen carbonate
The anion hydrogen carbonate primary react with the cations calcium and magnesium and form hardness-
causing salt (see 5.2.3.1). By means of a softening unit with a weakly acidic cation exchanger calcium- and
magnesium ions will be substituted against sodium ions. This results to sodium bicarbonate which reacts at
higher temperature and raised pressure to sodiumcarbonate. As result of so-called soda decomposition arise,
that means sodiumcarbonate decompose into soda lye and carbon dioxide gas, which escape out of the
system. The formed soda lye result in a selfalkalinization of the circulating water and can cause an increase of
the pH-value up to a value of > 10.
5.2.3.4 Organic substances
Insoluble and soluble organic substances – analytically determined as TOC or DOC – can both affect the
water treatment techniques and further microbiological reactions in the circulation water.
5.2.4 Oils/greases
The contamination of circulation water by oils or greases – e.g. due to the inflow of operating fluids or due to
valves, pipes, heating surfaces, etc. that have been treated with a temporary corrosion protection and with
processing aids – can cause massive malfunctions. As a film or coating on heated surfaces, oils and greases
hamper heat transfer and can, alone or in connection with other substances, cause malfunctions of the
regulation and safety devices. Oils and greases are nutrients for microorganisms and therefore increase the
probability of microbiologically influenced corrosion processes.
6 Systems engineering
6.1 Systems conception
6.1.1 General
For reasons related to corrosion, district heating facilities have to be designed and operated in such a way that
the inflow of air is, as far as possible, prevented and that water losses are minimized.
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
System extensions and alterations may only be carried out in consideration of the existing district heating
facility.
6.1.2 Materials
Water treatment and conditioning play a considerable role with regard to the selection of materials and the
possibilities of combining them.
Taking the standard values mentioned in 7.1 into account, unalloyed ferrous materials, non-corroding steels
and copper may be used – alone or combined.
With austenitic steels, the risk of alkali- and chloride-induced stress-corrosion cracking (see 7.4.7 and 7.6.2)
shall be taken into account. This does not apply to unalloyed steels.
NOTE 1 Selection of materials currently used for steel pipes:
— P235 TR1 (EN 10217–1)
— P235 TR2 (EN 10217–1)
— P235 GH (EN 10216–2, EN 10217–2, EN 10217–5)
— P355 N (EN 10216–3, EN 10217–3)
— E215+N (EN 10305–1)
— E195+N (EN 10305–3)
— 1.4301 (EN 10088)
— 1.4404 (EN 10088)
— 1.4571 (EN 10088)
Aluminium and/or aluminium alloys shall not be used in direct contact with the circulation water, since alkaline-
induced corrosion may otherwise occur. They may be used in domestic systems, but only if they are
connected indirectly to the district heating network.
Due to the material-related temperature limitation, the use of plastic piping is limited. Furthermore, the oxygen
diffusion rate of this material shall be taken into account. It shall be ensured that the other components are not
damaged by corrosion processes due to oxygen inflow. If necessary, the system has to be divided.
NOTE 2 Selection of materials currently used for plastic pipes:
— PE-Xa (EN ISO 15875-2)
— PE-Xc (EN ISO 15875-2)
— PB-1 (EN ISO 15876-2)
Materials containing copper may corrode due to the formation of copper sulphide.
With components consisting of a copper/zinc alloy (brass) and which have not been specially treated, damage
due to stress-corrosion cracking may occur. A prerequisite is the presence of ammonium ions in connection
with mechanical tensile stress.
NOTE 3 Materials currently used for valves in the network:
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
— cast steel
— cast iron / spheroidal graphite cast iron
6.1.3 Pressure maintenance and water supply
Pressure maintenance and water supply should be guaranteed mainly for the following reasons:
— ensuring the operating pressure level and the pressure at rest level;
— compensating volume and pressure variations due to temperature variations, and
— compensating water losses in operation and triggering a corresponding fill-up (water supply).
The appropriate selection and dimensioning as well as the correct integration of the pressure maintenance
and of the circulation pumps in district heating facilities have a decisive influence on the inflow of gases,
especially oxygen and nitrogen. Attention should be paid to ensuring a positive excess pressure at all times at
the high point of the system (recommendation: 0,5 bar positive ex-cess pressure).
Generally, both internal (formation of a steam pressure cushion by evaporation inside the heat generator) and
external pressure maintenance (PM) are possible – here, only the static and dynamic types of external
pressure maintenance will be dealt with in detail.
The expansion vessel (EV) is available with or without a diaphragm, and as a closed or open system.
The following parameters should considered when selecting the type of pressure maintenance:
a) The open, elevated EV without diaphragm is no longer used today due to the risk of an oxygen inflow into
the system. The oxygen inflow is particularly high when hot water flows through the EV. In the case of
existing facilities, appropriate measures should be taken (e.g. by-pass pipe, the integration of the safety
flow pipe, pump design) to prevent hot water from flowing through the diaphragm.
b) Besides a slower oxygen inflow, an open EV with a diaphragm has the advantage of building a system-
internal inert gas cushion beneath the diaphragm.
c) In the case of a closed EV without a diaphragm, pressure maintenance is only possible if a stable steam
or inert gas cushion is always ensured.
d) In the case of a closed EV with a diaphragm, exclusively inert gas should be used to fill up the gas
headspace (e.g. N ), because membranes made of elastomers are permeable to gas. This also applies to
the separation of the water space from the headspace by means of membranes.
e) In order to protect the diaphragms from high thermal stress > 70 °C and to minimize the oxygen inflow in
the event of volume variations, it makes sense to use buffer vessels (upstream vessel).
f) In the case of pump pressure maintenance in connection with open EVs, with or without diaphragm, the
pressure-control pump used to minimize the oxygen inflow shall be operated intermittently. Under certain
conditions, it may make sense to include a degassing system in the return of the system when using
open EVs. If the pump is in constant operation, the overflow pipe should not lead into the open EV, but
directly to the suction side of the pump.
g) Compressor pressure maintenance is not recommended – also not in the case of systems equipped with
a diaphragm – due to the increased oxygen inflow.
Under chemical considerations related to corrosion, it has turned out that diaphragm EVs that are pressurized
with inert gas offer the best possible safety with regard to the oxygen inflow. Table 1 shows a short overview
of the essential features of the different pressure maintenance systems.
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
Table 1 — Overview of pressure maintenance (PM) procedures
Type Static pressure maintenance Dynamic pressure maintenance
Design open EV closed EV diaphragm pump PM compressor PM
EV
Headspace/ air gas mixture, inert gas, gas mixture, air
environment usually steam usually N usually
steam/air
Diaphragm in w/o with w/o with mandatory w/o with w/o with
the EV
6.2 Water treatment techniques
6.2.1 General
In district heating, two water-chemical modes of operation are common:
— low-salt operation, and
— salty operation.
These are characterized by the standard values according to Table 2.
The water treatment techniques described below may refer to the supplementary water and/or to a partial flow
of the circulation water.
The quality of the circulation water can be adulterated by external water inflow, gas inflow or corrosion
processes, and modified by means of conditioning. With the aid of a partial flow treatment plant integrated into
the bypass (filtering, degassing and ion exchange), the suspended and dis-solved substances contained in
the water can be removed.
The dimensioning of partial flow treatment facilities for the circulation water should be such that 1 to 3 % of the
circulation water volume are treated by the partial flow treatment plant daily. If this facility is also to treat the
supplementary water volume, this should taken into account for the designing and dimensioning of the facility.
6.2.2 Filtering
When removing water-insoluble substances, various mechanical processes are used in order to prevent
deposits and malfunctions in downstream components.
For fine-particle substances,
— candle cartridge filters,
— bag filters, or
— pre-coated candle cartridge filters
can be used.
For larger dirt particles, mud flaps/filters are used.
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
6.2.3 Demineralization
The salts (cations and anions) dissolved in the water are removed using ion exchanging processes in order to
reduce electrical conductance. For this purpose, strongly acidic cation exchangers are normally used in
connection with strongly alkaline anion exchangers.
To a large extent, demineralization can be attained with the aid of diaphragms (in general using reverse
osmosis). Under the influence of pressure, the ionogenic substances are separated from the water by semi-
permeable diaphragms. The water is generally softened upstream in order to protect the diaphragm.
When using the above mentioned processes in partial flow operation, temperature limits have to be observed.
6.2.4 Softening
By using cation exchangers that can be regenerated with common salt (NaCl), the hardness components
(calcium and magnesium ions) are exchanged for sodium ions. The water is thus free of hardness
components and can no longer cause deposits and limescale. This type of water treatment is commonly used
in salty operation.
6.2.5 Degassing
To remove the part of solved gases (such as O , N and CO ) naturally present in water, the methods of
2 2 2
thermal degassing and vacuum degassing have established themselves in practice.
Also the method of diaphragm degassing can be applied under certain conditions.
Atmospheric degassing procedures basically only remove nitrogen. The standard value required in 6.1 for
oxygen cannot be achieved using this method.
6.2.6 Catalytic and electrochemical oxygen scavenging
Contrary to the procedures of thermal degassing and vacuum degassing, in which all gases present in water
are removed, the procedures described below only deal with the removal of oxygen.
The procedure of catalytic oxygen scavenging is implemented by converting into water the soluble oxygen
present in water with added hydrogen using the catalytic effect of certain noble metals (e.g. palladium).
When considering the electrochemical procedures of oxygen scavenging, one has to differentiate between the
galvanic procedure without external current, and the electrolytic procedure with external current.
Galvanic procedures are preferably performed in arrangements where the sacrificial anodes, which are
normally used to protect the cathodes, are made of magnesium or zinc and are mounted in a steel vessel in
order for them to be metallically conductive.
Electrolytic procedures are preferably performed in arrangements where anodes made of aluminium,
magnesium, zinc or iron are insulated to be led through the walls of the vessel, and are loaded with a dc
voltage acting between the vessel and the anode.
The iron-, zinc- or magnesium hydroxides, respectively created react in an alkalescent manner and, thus, lead
to a (desired) pH value increase. The aluminium hydroxide occurring due to the electrolysis of aluminium
anodes leads to the formation of protective layers on ferrous materials. The slurry caused by this process has
to be regularly removed from the vessel.
The efficience of an electrochemical procedure, which is characterized by the oxygen removal rate (preferably
expressed in g/h), mainly depends on the cathodic surface (size of the vessel) and on the exposure time of
the water in the vessel.
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
7 Production technology
7.1 Standard values for the circulation water
The standard values states in Table 2 apply to the circulation water to ensure the safe and economical
continuous operation of district heating facilities.
In the start-up mode and in the event of an incident, deviations from the standard values may occur. These
deviations shall be corrected. If potable drinking water is directly heated, the hygienic aspects mentioned in
Clause 8 should be observed.
Table 2 — Standard values for the circulation water of directly or indirectly heated systems
Property Unit Low-salt Salty
a µS/cm 10 – 30 > 30 – 100 ≥ 100 – 1500
Electrical conductivity at 25 °C
b clear, free of suspended substances
Apperance
c 9,0 – 10,0 9,0 – 10,5 9,0 – 10,5
pH value at 25 °C
Oxygen mg/l < 0,1 < 0,05 < 0,02
Sum of alkaline earth mmol/l < 0,02 < 0,02 < 0,02
(°dH) (<0,1) (<0,1) (<0,1)
(hardness)
The following aspects have to be taken into account:
a
Electrical conductivity: at low conductivities, faulty measurements may occur in the case of flow measurements according to
the MID principle. In addition, at conductivities < 20 µS/cm, the function of water level electrodes is no longer guaranteed.
b
Appearance: Water becoming turbid due to the presence of gas bubbles immediately after the sample of circulation water has
been taken suggests the possibility of malfunctions in the operating facility.
c
pH value: A deviation from these values is permitted in facilities that are heated indirectly. For more details, see Section 7.5.2.
7.2 Low-salt operation
In circulation water, the lower the electrical conductivity, the smaller the risk of corrosion due to oxidation. An
oxygen concentration of 0,05 mg/l or 0,1 mg/l can therefore be tolerated under low-salt operation conditions.
The prerequisite for this is that the electrical conductivity has to be limited to < 100 µS/cm. Hence,
demineralized water should be used as filling and supplementary water.
Additionally during low salty operation conditions the danger of a microbial growing in the district heating
facilities is slight since the nutriments for growing have been removed.
7.3 Salty operation
District heating facilities may be operated with salty circulation water if the inflow of oxygen (<0,02 mg/l) and
other gases can be practically precluded.
As long as the standard values for the oxygen concentration, the pH value and the electrical conductivity are
sure to be complied with in continuous operation, it is not necessary to use oxygen scavengers and/or
corrosion inhibitors.
kSIST-TP FprCEN/TR 16911:2015
FprCEN/TR 16911:2015 (E)
Conditioning with oxygen scavengers or corrosion inhibitors can make sense, for example, to reduce
corrosion probability in the event of external water inflow or of oxygenated supplementary water.
In addition, attention should be paid to maintaining the circulation water soft.
7.4 Technical aspects related to the operation
7.4.1 General
Essential influence factors for flawless operation in practice have turned out to be:
— the constructional design of the pressure maintenance system, including the expansion vessels and their
integration into the system;
— the tightness of the potable drinking water boiler in the customer's facilities in the case of a direct
connection;
— the quantities of supplementary water for water loss.
The following influence factors, which will be described in more detail in the next sub-sections, are responsible
for gas infl
...