EN 12952-7:2002

SLOVENSKI STANDARD
01-november-2002
Vodocevni kotli in pomožne napeljave - 7. del: Zahteve za opremo kotla
Water-tube boilers and auxiliary installations - Part 7: Requirements for equipment for the
boiler
Wasserrohrkessel und Anlagenkomponenten - Teil 7: Anforderungen an die Ausrüstung
für den Kessel
Chaudieres a tubes d'eau et installations auxiliaires - Partie 7: Exigences pour
l'équipement de la chaudiere
Ta slovenski standard je istoveten z: EN 12952-7:2002
ICS:
27.060.30 Grelniki vode in prenosniki Boilers and heat exchangers
toplote
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 12952-7
NORME EUROPÉENNE
EUROPÄISCHE NORM
May 2002
ICS 27.040
English version
Water-tube boilers and auxiliary installations - Part 7:
Requirements for equipment for the boiler
Chaudières à tubes d'eau et installations auxiliaires - Partie Wasserrohrkessel und Anlagenkomponenten - Teil 7:
7: Exigences pour l'équipement de la chaudière Anforderungen an die Ausrüstung für den Kessel
This European Standard was approved by CEN on 25 March 2002.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2002 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 12952-7:2002 E
worldwide for CEN national Members.

Contents
1 Scope .4
2 Normative references .4
3 Terms and definitions.5
3.1 Types of steam boilers and hot water generators.5
3.6 Classification of pressure generation systems (Hot water generators) .5
4 General requirements for steam boilers and hot water generators .7
4.1 Safeguards against excessive pressure .7
4.2 Heat supply system .7
4.3 Ash removal plants.7
4.4 Flue-gas cleaning plants .7
4.5 Requirements for limiting devices and safety circuits .8
4.6 Cleaning, access and inspection openings .8
4.7 Feed lines and protection against feedwater backflow .12
4.8 Connection of steam boilers or hot water generators .12
5 Special requirements for steam boilers .13
5.1 Requirements for pumps .13
5.2 Isolating and drain devices.14
5.3 Lowest permissible water level (LWL).14
5.4 Water level and flow indicators .15
5.5 Water supply control and protective device against water shortage.16
5.6 Pressure and temperature measuring devices.16
5.7 Marking .16
6 Special requirements for hot water generators.16
6.1 Requirements for hot water generating systems .16
6.2 Requirements for pumps .17
6.3 Shut-off and drain devices.18
6.4 Lowest permissible water level and installation of flow and return line.19
6.5 Water level and flow indicators .19
6.6 Water supply and drain control, protective device against water shortage.20
6.7 Safeguards against minimum pressure .20
6.8 Safety device for temperature control .20
6.9 Pressure and temperature indicating devices.20
6.10 Marking .21
7 Additional requirements for plants without permanent supervision .21
7.1 General.21
7.2 Steam boilers.21
7.3 Hot water generators .22
8 Final assessment of safety devices.24
8.1 Performance of inspection.24
8.2 Equipment on the steam and water side .24
8.3 Firing system equipment .25
8.4 Electrical safety circuit.25
8.5 Operating instructions .25
9 Small plant boilers .25
9.1 Steam boilers.25
9.2 Hot water generators .26
Annex A (normative) Chemical recovery boilers (Black liquor recovery boilers) .28
Annex B (informative) Design examples for hot water generating systems.30
Annex ZA (informative) Clauses of this European Standard addressing essential requirements or other
provisions of the Pressure Equipment Directive.49
Foreword
This document (EN 12952-1:2002) has been prepared by Technical Committee CEN/TC 269 "Shell and water-tube
boilers", the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an identical text or
by endorsement, at the latest by November 2002, and conflicting national standards shall be withdrawn at the latest
by November 2002.
For relationship with EU Directive(s), see informative annex ZA, which is an integral part of this document.
This Part of this European Standard has been prepared under a mandate given to CEN by the European Commis-
sion and the European Free Trade Association and supports essential safety requirements of the Pressure Equip-
ment Directive (PED) [1]. For relationship with Pressure Equipment Directive see informative Annex ZA, which is an
integral Part of this Standard.
The European Standard EN 12952 concerning water-tube boilers and auxiliary installations consists of the following
Parts:
Part 1: General.
Part 2: Materials for pressure parts of boilers and accessories.
Part 3: Design and calculation for pressure parts.
Part 4: In-service boiler life expectancy calculations.
Part 5: Workmanship and construction of pressure parts of the boiler.
Part 6: Inspection during construction; documentation and marking of pressure parts of the boiler.
Part 7: Requirements for equipment for the boiler.
Part 8: Requirements for firing systems for liquid and gaseous fuels for the boiler.
Part 9: Requirements for firing systems for pulverized solid fuels for the boiler.
Part 10: Requirements for safeguards against excessive pressure.
Part 11: Requirements for limiting devices of the boiler and accessories.
Part 12: Requirements for boiler feedwater and boiler water quality.
Part 13: Requirements for flue gas cleaning systems.
Part 14: Requirements for flue gas DENOX-systems.
Part 15: Acceptance tests.
Part 16: Requirements for grate and fluidized bed firing systems for solid fuels for the boiler.
CR 12952-17: Guideline for the involvement of an inspection body independent of the manufacturer.
Although these Parts can be obtained separately, it should be recognized that the parts are inter-dependent. As
such, the design and manufacture of water-tube boilers requires the application of more than one Part in order for
the requirements of the standard to be satisfactorily fulfilled.
NOTE Parts 4 and 15 are not applicable during the design, construction and installation stages.
The Annex A is normative. The Annex B of this European Standard are informative.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following coun-
tries are bound to implement this European Standard: Austria, Belgium, Czech Republic, Denmark, Finland,
France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Spain, Swe-
den, Switzerland and the United Kingdom.
1 Scope
This Part of this European Standard specifies the requirements for equipment for steam boilers and hot water
generators as defined in EN 12952-1, wherein steam or hot water will be generated. Requirements for equipment
for chemical recovery boilers are given in Annex A, and design examples for hot water generating systems are
given in Annex B.
2 Normative references
This European Standard incorporates by dated or undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications are listed hereafter. For
dated references, subsequent amendments to or revisions of any of these publications apply to this European
Standard only when incorporated in it by amendment or revision. For undated references the latest edition of the
publication referred to applies (including amendments).
EN 837-1, Pressure gauges — Part 1: Bourdon tube pressure gauges — Dimensions, metrology, requirements and
testing.
EN 12952-1,
 

  


  1: General.
EN 12952-6,
 

  


  6: Inspection during construction; documentation
and marking of pressure parts of the boiler.
EN 12952-8,
 

  


  8: Requirements for firing systems for liquid and
gaseous fuels for the boiler.
prEN 12952-9,
 

  


  9: Requirements for firing systems for pul-
verized solid fuels for the boiler.
prEN 12952-10,
 

  


  10: Requirements for safeguards against ex-
cessive pressure.
prEN 12952-11,
 

  


  11: Requirements for limiting devices of the
boiler and accessories.
prEN 12952-12,
 

  


  12: Requirements for feedwater and boiler
water quality.
prEN 12952-13,
 

  


  13: Requirements for flue gas cleaning sys-
tems.
prEN 12952-14,
 

  


  14:Requirements for flue gas DE-
NOX-systems.
prEN 12952-16,
 

  


  16: Requirements for fluidized bed and grate
firing systems for solid fuels for the boiler.
prEN 50156-1, Electrical equipment for furnaces and ancillary equipment — Part 1: Requirements for application
design and installation.
3 Terms and definitions
For the purposes of this Part of this European Standard, the following terms and definitions apply in addition to
those given in EN 12952-1, prEN 12952-8 and prEN 12952-9.
3.1
Types of steam boilers and hot water generators
3.1.1
natural circulation steam boilers and hot water generators
steam boilers and hot water generators in which the water to be evaporated/heated circulates due to the differ-
ences in density (see Figure B.3-1)
3.1.2
forced or assisted circulation steam boilers and hot water generators
steam boilers and hot water generators in which the water to be evaporated/heated is circulated by means of
pumps (see Figure B.3-2)
3.1.3
once-through steam boilers and hot water generators
steam boilers, with or without separating vessels, where the water flow is determined by the feed pump, and the
water is evaporated completely or in a major portion during one single passage
Hot water generators, where the water flow is effected by the circulating pump of the heating system and is heated
during one single passage, e.g. once-through hot water generator where there is no contact between hot and cold
water in the drum (two-way drum) see Figure B.3-3 once-through hot water generator with header distributing water
from below, see Figures B.3-4 and B.4-9
3.1.4
waste heat steam boilers and hot water generators
generators utilizing heat recovered from outside sources, e.g. gas turbines, blast furnaces
3.2
limits of steam boilers and hot water generators
boundaries of the steam and water spaces located between the shut-off devices of the steam boilers and hot water
generators in the inlet, outlet, pressure retaining, overflow, and drain lines. The bodies of the shut-off devices are
considered to be within these limits
3.3
steam boilers and hot water generating plant
plant consists of the water-tube boiler and its equipment as defined in EN 12952-1
3.4
heat supply system
assembly of components in which the energy of the fuel (including electrical and waste-heat energy) is supplied to
the steam boilers and hot water generator
3.5
Classification of pressure generation systems (Hot water generators)
3.5.1
internally pressurised systems
systems where the pressure is generated by the saturation pressure corresponding to the flow temperature (see
Figures B.3-1, B.3-2, B.3-3, B.3-4, and Figure B.4-9)
3.5.2
externally pressurised systems
systems where the pressure is generated by such methods as gas cushion or pressure pumps (see Figures B.4-1
to B.4-8, B.4-10 to B.4-13)
3.6
expansion system (vessel and tank)
3.6.1
Expansion vessels
pressurized container to compensate temperature-dependent volume changes of the water
3.6.2
Expansion tanks
not pressurized container to compensate temperature-dependent volume changes of the water (see EN 12953-6).
3.7
sinking time
time during which the water level will sink from the lowest permissible water level (LWL) to the highest point of the
heated surface (HHS) (see 4.5.2)
NOTE For hot water generators applicable only in the case of internal pressure generation; see Figures B.3-1, B.3-2,
B.3-3, B.3-4, and B.3-9.
3.8
maximum continuous rating (MCR)
maximum continuous steam output that can be generated during continuous operation taking the specified steam
condition into consideration (see EN 12953-6)
3.9
allowable heat output
maximum heat output (water mass flow times the difference between outlet and inlet enthalpy) that can be gener-
ated during continuous operation and at which hot water generators may be operated
3.10
maximum allowable pressure
maximum pressure for which equipment is designed, as specified by the manufacturer, and at a location specified
by the manufacturer
NOTE This may be the location of connection of protective and/or limiting devices or the top of equipment or, if not appli-
cable, any point specified.
3.11
allowable flow temperature
maximum allowable temperature at which the hot water generator may be operated
3.12
controls
devices used for holding the variable to be controlled (e.g. water level, pressure, temperature) at a specified value
(set point)
3.13
limiters
device that, on reaching a fixed value (e.g. pressure, temperature, flow, water level) is used to interrupt and lock
out the energy supply, and requires manual unlocking before restart (see EN 12953-6)
3.14
fail-safe
functional unit is fail-safe if it possesses the capability of remaining in a safe condition in the event of certain faults
occurring (e.g. internal faults)
3.15
self-monitoring
regular and automatic determination that all chosen components of a safety device are capable of functioning as
required
3.16
redundancy
provision of more than one device or system so that in the event on a fault, the necessary facilities will still be pro-
vided
3.17
independent
ability to function as required without interference from other equipment
3.18
reliability
ability to perform a required function under specified conditions and for a given period of time without failing
3.19
lock-out
isolation of the energy supply which requires a manual intervention to reinstate
3.20
unit test
individual testing of any unit(s) of the safety device
3.21
functional test
testing of the safety device to ensure it performs its intended function (see EN 12953-6)
3.22
permanent supervision
supervision during which the boiler operator stays close to the boiler, boiler room, operating platform or control
room so that he has sufficient time to intervene in any dangerous condition
4 General requirements for steam boilers and hot water generators
4.1 Safeguards against excessive pressure
Each steam boiler and hot water generator shall be equipped with safeguards against excessive pressure in accor-
dance with prEN 12952-10.
4.2 Heat supply system
4.2.1 For the heat supply of steam boilers and hot water generators the following Parts of this European Stan-
dard shall apply:
 EN 12952-8 for firing systems for liquid and gaseous fuels;
 prEN 12952-9 or prEN 12952-16 for solid fuels.
4.2.2 The heat supply shall be adapted to the allowable heat output as well as to the intended mode of operation.
4.3 Ash removal plants
Ash removal plant shall be in accordance with prEN 12952-9 or prEN 12952-16.
4.4 Flue-gas cleaning plants
Flue-gas cleaning plants shall be in accordance with prEN 12952-13 and prEN 12952-14.
4.5 Requirements for limiting devices and safety circuits
4.5.1 All limiters and their installation shall be in accordance with prEN 12952-11. The electrical safety circuits
shall correspond to prEN 50156-1 (see safety levels given in Table 4.6.1 and Annex B).
4.5.2 Functional testing of limiters shall be possible at any time for any operating condition, e.g. by simulation
where appropriate. If any part of the functional test involves delaying the cut-off and lock-out of the heat supply,
then the time delay shall not exceed a value specified by the manufacturer. For boilers fitted with water level limiter,
the time delay shall not exceed the time for the water level to fall from the lowest controlled water level to LWL at
MCR:
a) for steam boilers in the case of complete loss of feedwater supply and at maximum steam output, i.e.
V
t =
Q ⋅ v
St St
b) for hot water generators in the case of interrupted feeding and recirculation and at the allowable heat output,
i.e.
V
t =
Q ⋅ v
HW HW
where
Q is the equivalent steam generation calculated according to the allowable heat output, in kg/s;
HW
Q is the maximum continuous rating, in kg/s;
St
t is the sinking time, in s;
3;
V is the water volume between HHS and LWL, in m
v is the specific water volume, m /kg;
HW
v is the specific water volume at saturated steam temperature, in m /kg.
St
4.5.3 The result of each limiter check shall be clearly recognizable to the boiler operator, e.g. by the lighting of a
signal.
4.6 Cleaning, access and inspection openings
4.6.1 Steam boilers and hot water generators shall be provided with openings to permit cleaning and inspection
of the boiler interior. Sufficient space shall be provided to permit access to the steam and water spaces and the
combustion chamber as well.
4.6.2 Water or steam-containing components with an inside diameter of more than 1 200 mm and components
with a diameter of more than 800 mm and a length of 2 000 mm shall be so designed as to permit internal inspec-
tion. Internals shall be of such a design as not to obstruct inspection of the component walls, or it shall be possible
to remove them.
4.6.3 Headers and similar components shall be provided with adequate access for inspection of their inner sur-
face as follows:
 for inside diameters in excess of 80 mm where mud deposits can accumulate;
 for inside diameters in excess of 150 mm for any other cases.
Inspection access shall be designed as head, hand or inspection holes where manual inspection methods are to be
used or, as inspection nozzles where inspection devices are used e.g. endoscopes.
4.6.4 Sizes of openings in water and steam spaces shall be:
a) Manholes shall not be smaller than 320 mm · 420 mm or have an inside diameter of at least 420 mm. The
nozzle or ring height shall not exceed 300 mm, and 350 mm in the case of a tapered design. For structural or
design reasons, the openings of manholes may be reduced to 300 mm · 400 mm clear width or to 400 mm in-
side diameter. In such cases, the nozzle or ring height shall not exceed 150 mm, or 175 mm in the case of a
tapered design. For minimum number of openings see table 4.6-1;
b) Head holes shall not be smaller than 220 mm · 320 mm or have an inside diameter of at least 320 mm. The
nozzle or ring height shall not exceed 100 mm, or 120 mm in the case of a tapered design. For minimum num-
ber of openings see table 4.6-1;
c) Hand holes shall not be smaller than 100 mm · 150 mm or have an inside diameter of at least 120 mm. The
nozzle or ring height shall not exceed 65 mm, or 95 mm in the case of a tapered design. For minimum number
of openings see table 4.6-1;
d) Inspection holes shall have a diameter of 50 mm. However, they shall be provided only where a hand hole is
not possible for design reasons. For minimum number of openings see Table 4.6-1;
e) Inspection nozzles shall have an inside diameter of approximately 50 mm. They may be arranged in axial
and/or radial direction as shown in Figure 4.6-1. Radial inspection nozzles may be omitted, if connecting tubes
can be used in lieu of such nozzles.
Table 4.6-1 — Selection for type and minimum number of openings
Inside
length
Type and minimum number of openings required
diameter,
mm
mm
— By agreement as part of the design approval process£ 300
£ 1 200 2 inspection holes at the front faces
1 hand hole in the mid-third of cylindrical length or at one front face£ 1 500
> 300
£ 450
At least 2 hand holes, one each either near header ends or in front faces,
> 1 500 in which case the distance between the hand holes shall not exceed
2 000 mm
1 hand hole in the mid-third of cylindrical length£ 1 500
1 head hole in the mid-third of cylindrical length
> 1 500 2 hand holes, one each either near header ends or in front faces, in
£ 3 000 which case the distance between the hand holes shall not exceed
> 450 2 000 mm
£ 800
The number of the inspection holes shall be increased accordingly. In the
cylinder, the greatest distance between head holes shall not exceed
> 3 000 3 000 mm and between hand holes shall not exceed 2 000 mm. One
hand hole each shall be provided near the header ends or in the front
faces.
1 head hole in the mid-third of cylindrical length or
£ 2 000
> 800
2 hand holes near the header ends or in the front faces
£ 1 200
> 2 000 1 manhole
> 1 200 — 1 manhole
Figure 4.6-1 — Examples of arrangements of inspection nozzles
4.6.5 The following shall apply with regard to the size of openings on boiler plant spaces which are to be in-
spected and contain neither water nor steam:
a) Access openings for inspection by personnel using auxiliary or personal protective equipment shall have an
inside diameter of at least 600 mm. For design reasons the minimum size of access openings may be reduced
to an inside diameter of 500 mm in which case the nozzle or ring height shall not exceed 250 mm;
b) Access openings for inspection without use of auxiliary or personal protective equipment shall have a clear
width of at least 320 mm · 420 mm. For design reasons the minimum size of access openings may be reduced
to a clear width of 300 mm · 400 mm in which case the nozzle or ring height shall not exceed 150 mm, and
175 mm in the case of a tapered design;
c) Inspection openings shall have a width of 100 mm · 150 mm or an inside diameter of 120 mm.
4.6.6 Access and inspection doors of the type in which the internal pressure forces the door against a flat gasket
shall have a minimum gasket bearing width of 15 mm for manholes and headholes. For handholes the gasket
bearing width may be reduced to 10 mm. The total clearance between the door frame and the spigot or recess of
such doors shall not exceed 3 mm, i.e. 1,5 mm all round, and the spigot depth shall be sufficient to trap the gasket.
The selection of the gasket material combined with the design of such doors shall be adequate to provide an
effective and safe sealing assembly.
4.7 Feed lines and protection against feedwater backflow
4.7.1 Each feed line leading to the steam boiler or hot water generator shall be equipped with a protection device
against backflow and a shut-off device. If the shut-off device and the protection device against backflow are not
installed in direct connection to each other, pressure relieving shall be possible for the intermediate piping section.
A single once-through steam boiler in which the heat supply is cut off automatically in the case of feedwater failure,
requires neither a shut-off device nor protection device against backflow if the feed pump is of the positive
displacement type where it can be ensured that no dangerous backflow can occur.
4.7.2 Except for once-through boilers, the feed line shall be connected to the steam boiler or hot water generator
in such a way that, in the case of leakage of the protection device against backflow, the steam boiler cannot be
drained to below the highest point of the downcomers.
4.7.3 Feed pumps shall be capable of isolation from common suction or pressure lines.
4.7.4 If there is a possibility of backflow through a feed pump, the suction line between the valve and the pump
shall be designed for full pressure or appropriate devices shall be installed to exclude the danger from closing the
isolating valve in the suction line.
4.8 Connection of steam boilers or hot water generators
Where a steam boiler or hot water generating plant has several steam boilers or hot water generators connected by
common lines and the shut-off devices in the steam or hot water and feed lines remain permanently connected with
these lines during inspection of the steam boilers or hot water generators, two shut-off devices with intermediate
venting shall be installed in each line which shall be lockable in the closed position and be protected against undue
actuation.
5 Special requirements for steam boilers
5.1 Requirements for pumps
5.1.1 Number of feedwater pumps
5.1.1.1 One feed pump shall suffice if the following requirements are met:
a) In the case of failure of the feed pump power source the heat supply shall be cut off automatically;
b) The steam boiler shall be equipped with an oil or gas firing system or the design of the heat supply system
shall ensure that, upon cut-off of the heat supply, no unacceptable evaporation of the water stored in the
steam boiler occurs due to the heat accumulated in the furnace and in the boiler passes. This requirement
shall be deemed to have been met, if it is proved that, after cut-off of the firing system from full-load
steady-state condition, the flue-gas temperature at the highest point of the heating surface (HHS) falls to a
value below 400 °C before the water level has sunk from the lowest permissible water level (LWL) to 50 mm
above the highest point of the heating surface (HHS).
5.1.1.2 Steam boiler that do not meet the requirements of 5.1.1.1 shall be equipped with at least two feed
pumps.
5.1.2 Capacity of feedwater pumps
5.1.2.1 The feed pump capacity shall correspond at least to 1,25 times the allowable steam output of all steam
boilers. For safety reasons 1,15 times of maximum continuous rating is enough. For availability and difference in
service conditions a greater margin can be necessary.
For once-through steam boilers a feed pump capacity of at least equal to 1,0 times the maximum continuous rating
can suffice.
Where boiler water is constantly blown down in volumes exceeding 5 % of the allowable steam output, the feed
pump capacity shall be increased by the corresponding percentage , e.g., if the blow down is 8 % of the allowable
steam output, the feed pump capacity shall be increased by 8 %.
5.1.2.2 The feed pumps shall be capable of supplying to the steam boiler both the feedwater quantities at
maximum allowable pressure as specified in 5.1.2.1 and the feedwater quantity corresponding to the allowable
steam output at 1,1 times the allowable working pressure.
If it is proved that the pressure relief devices are capable of discharging the steam generated in the case of excess
of the allowable working pressure by less than 10 %, a correspondingly lower factor than 1,1 can be used in the
calculation.
5.1.2.3 If at least two feed pumps are required according to 5.1.1.2, the following shall apply:
a) In the case of failure of the feed pump with the highest capacity, the remaining feed pumps shall meet the re-
quirements of 5.1.2.1 and 5.1.2.2;
b) Two independent power sources shall be available. By exception, steam operation of all feed pumps from only
one steam main shall be permissible. The feed pumps shall be connected to the power sources in such a way
that in the case of failure of one power source the remaining feed pumps shall meet the requirements of
5.1.2.1 and 5.1.2.2. Where the pumps are electrically driven one line per feed pump drive can suffice if the line
can be switched over to the bus bars;
c) In the case of steam boiler plant of a unitized arrangement, it shall be permitted to rate the second feed pump
for 0,5 times the allowable steam output if it is ensured that in the case of failure of the feed pumps rated for
the allowable steam output, the heat supply system is, at the same time, automatically adjusted to the reduced
steam output. If water is blown down, the quantity of blow down water shall be fully considered when rating the
second feed pump;
d) For natural and assisted circulation steam boilers, the stand-by pump shall have been put into operation within
the sinking time if the operating pump fails, or if the water level has sunk below the lowest water level (see
4.5.2);
e) In the case of once-through steam generators, the stand-by pump shall have been put into operation, within a
time interval to be specified by the manufacturer, if the operating pump fails, or if the flow through the steam
boiler has fallen to below the minimum flow required, thus ensuring that an inadmissible excess of the tempe-
rature is avoided.
5.1.2.4 No specific margins need to be met by feed pumps regarding the volume flow and the pump head if
two water level limiters cut off and lock out the heat supply in the event of the water level falling below the prede-
termined lowest permissible water level, and upon cut-off of heat supply, the heat accumulated in the furnace and
in the boiler passes does not lead to an unacceptable evaporation of the water contained in the steam boiler.
5.1.3 Circulating pumps
5.1.3.1 For forced circulation steam boilers one circulating pump shall suffice if:
a) in the case of failure of the power source for the circulating pump, the heat supply is cut off and the require-
ments of 5.1.1.1 b) are met;
b) the steam boiler is only heated with gases not exceeding a temperature of 400 °C or if it is proved that higher
temperatures can be withstood without danger.
5.1.3.2 Forced circulation steam boilers not corresponding to 5.1.3.1 shall be equipped with at least two circu-
lating pumps. A common standby circulating pump can suffice for several forced circulation steam boilers of one
steam boiler plant, if it can be connected to each steam boiler.
5.1.3.3 An alarm system shall become effective in the case of failure of one circulating pump and whenever
the flow rate in the circulation system has fallen to a specified minimum value.
5.1.3.4 For the power sources of the circulating pumps 5.1.2.3 b) shall apply.
5.2 Isolating and drain devices
5.2.1 Each steam boiler shall be provided with sufficient devices by which it can be positively isolated from the
system. Except in the case of unitized arrangements such devices shall be located as close as possible to the
steam boiler.
Where duplicate isolating devices are installed in series, but not adjacent to each other, the intervening tubework
shall be provided with means for venting and condensate removal.
5.2.2 Steam boilers shall be equipped with drains. Such drains and the nozzles thereof shall be protected
against the effects of combustion gases. Self-closing blowdown devices shall be capable of being locked manually
in the closed position unless a further shut-off device is installed in the line.
5.2.3 Automatic devices by which the water level may be lowered to below LWL (see 5.3) shall only be used if
the following requirements are met.
The steam boiler shall be equipped with oil or gas firing system or the design of the heat supply system shall
ensure that no unacceptable evaporation of the water inside the steam boiler occurs due to the heat accumulated
in the boiler passes upon cut-off of the heat supply.
5.3 Lowest permissible water level (LWL)
5.3.1 For all steam boilers except once-through steam boilers, the lowest permissible water level (LWL) shall be
determined and shall be permanently marked on the boiler.
5.3.2 The lowest permissible water level (LWL) shall be at least 150 mm above:
 the uppermost heated point of the drum and;
 the highest connection of the downcomers (top edge) to the boiler drums.
In the case of two-drum boilers with an evaporator tube bundle between them, the outer tubes shall not be
considered as downcomers.
5.3.3 If 5.1.1.1b) applies the water level indicating device shall be so arranged as to make the value "50 mm
above HHS" visible.
5.4 Water level and flow indicators
5.4.1 All steam boilers, except once-through steam boilers, shall be provided with at least two devices (e.g. glass
water gauge) which make the water level to be maintained during operation clearly visible from the boiler control
room. One glass water gauge may be replaced by:
a) either two remote water level indicators;
b) or a water level controller or limiter indicating the water level at least indirectly. An indication of the limit values
for safety-relevant water level is sufficient.
A glass water gauge located at the water level controller or limiter shall be considered a direct-reading water level
indicator.
In case a) the glass water gauge need not be in the field of vision of the boiler operator responsible for the
feedwater.
5.4.2 The connecting tubes between the steam boiler and the local water level indicators shall have an inside
diameter of at least 20 mm. If the water level indicators are connected by means of common connecting lines or if
the water-side connecting tubes are longer than 750 mm, the latter shall have an inside diameter of at least 40 mm.
Connecting tubes on the steam side shall be designed so that condensate does not accumulate. Water-side con-
necting tubes shall always be arranged horizontally to the water level indicators.
However, when reverse measuring method is used, impulse tubes to transmitters on the steam and water side shall
be full of condensate and the transmitters shall be below the water level. Impulse tubes to transmitters can have a
smaller inside diameter.
5.4.3 Water level indicators shall permit isolation from the steam boiler for blowdown. Where cocks are used, the
open position shall be indicated.
5.4.4 During operation the water level shall always be visible. The lower limit of the indicating range of a glass
gauge shall be at least 30 mm below the lowest permissible water level (LWL) (also see 5.1.1.1 b)).
5.4.5 On each glass gauge the lowest permissible water level (LWL) shall be permanently and clearly marked by
the letters LWL at the height of the level mark in accordance with to 5.3.1.
5.4.6 All glass gauges shall be provided with an internal self-closing safety device. Cylindrical glass gauges shall
be provided with a safety device for protection against damage by bursting glass.
5.4.7 Blowdown systems on water level indicators, controllers and limiters shall be so installed as to prevent ac-
cidents. Any blowdown operation shall be detectable.
5.4.8 Once-through steam boilers shall be equipped with at least one low-flow indicator device which will indicate
water shortage to the coil. Indirect measuring methods may be used.
5.4.9 Forced circulation steam boilers shall be equipped with at least one low-flow indicator device in the circu-
lating line.
5.5 Water supply control and protective device against water shortage
5.5.1 All steam boilers, except once-through boilers, shall be equipped with a water level limiter. The water level
limiter shall respond, at the latest, when the level drops to the lowest permissible water level (LWL).
Where required, the low water condition may be tested by simulation.
For the protection against water shortage, once-through boilers shall be provided with a limiter (flow or temperature
limiters) in order to prevent the boiler walls from becoming overheated.
5.5.2 The connecting lines of external water level controllers and limiters shall be in accordance with 5.4.2. The
shut-off devices in the connecting lines of limiters shall only permit operation of the heat supply system in the open
position (interlocking). The design of these devices shall permit functional testing, by simulation, where appropriate,
for all operating conditions.
5.6 Pressure and temperature measuring devices
5.6.1 Each steam boiler shall be provided with at least one pressure gauge with a direct connection to the steam
space. The inside diameter of the connecting pipework shall be not less than 8 mm. The effects of blockage from
corrosion products should be taken into account. The connecting line shall be provided with a siphon seal and in-
corporate a facility to connect a test pressure gauge in the vicinity of the pressure gauge. It shall always be possi-
ble for the boiler operator to read the pressure gauge.
Pressure gauges shall comply with the requirements of EN 837-1.
5.6.2 The pressure gauge shall indicate the gauge pressure, in bar. The maximum allowable pressure shall be
indicated by a fixed and readily visible red mark on the pressure gauge. The pressure gauge shall be so installed
that it is protected against heat.
5.6.3 Temperature measuring instruments shall be installed at the outlet of the first stage superheater and at the
inlet and outlet (superheater/reheater) of the following stages.
5.7 Marking
Each steam boiler, isolatable superheater, isolatable economizer, and reheater shall be marked in accordance with
EN 12952-6 and include the maximum continuous rating, if applicable.
6 Special requirements for hot water generators
6.1 Requirements for hot water generating systems
6.1.1 The heated water is normally used in a closed cycle.
6.1.2 Filling, make-up and recirculation water shall meet the requirements of prEN 12952-12. The water drained
from the plant from time to time to adapt to volume changes, shall also comply with prEN 12952-12, when being
returned to the system.
6.1.3 If required, provision shall be made to ensure the temperature of the water returned to the hot water gen-
erator will not fall below a value to be determined by the manufacturer. This requirement shall not be applicable
during start-up and shutdown.
6.1.4 Pressure generation units shall be desig
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