Page 1
Manual
Units for regulation and
monitoring of pressure
type KRT
2002 KRT_manual_11-27-02
Page 2
Units for regulation and monitoring of pressure, type KRT
Pressure controls type KRT
0 5 10 15 20 25 30 bar
Standard
pressure controls
∆
p = 0-0,9 bar
∆
p = 0,1-1 bar
∆
p = 0,1-1,5 bar
∆
p = 0,5-4 bar
∆
p = 0,5-6 bar
∆
p = 1-6 bar
Pressure controls for steam plant
approved by Vd
Pressure controls with
adjustable neutral zone
Differential pressure controls
TÜV
Range Further
p
e
bar
−1 −0 RT 121
0 −0.3 RT 113
0.1 −1.1 RT 112
0.2 −3 RT 110
−0.8 −5 RT 1, 1A
0.2 −6 RT 200
1 −10 RT 116
4 −17 RT 5, 5A
10 −30 RT 117
0.1 −1.1 RT 112 W
0 −2.5 RT 33B, RT 35 W
1 −10 RT 30 AW/AB/AS
1 −10 RT 116W
2 −10 RT 31 W/B/S
5 −25 RT 19 W/B/S
5 −25 RT 32 W/B/S
−0.8 −5 RT 1AL
0.2 −3 RT 110L
0.2 −6 RT 200L
4 −17 RT 5AL
10 −30 RT 117L
−1 −6 RT 266AL
−1 −6 RT 263AL
−1 −9 RT 262AL/A
−1 −18 RT 260AL/A
−1 −36 RT 260A
−1 −36 RT 265A
Type information
See contents page 3
2
Page 3
Pressure controls, type KRT
Contents Page
Survey of types, pressure ranges 2
Introduction 3
Technical data and code nos., all types 4-5
Approvals 5
Technical data and material in contact with medium 6
Dimensions and weight 7
Installation 8
Introduction Type RT pressure controls incorporate a
pressure controlled, single-pole changeover
switch where the contact position depends on the
pressure in the connection port and the set value.
The RT series consists of pressure controls,
differential pressure controls and pressure
controls for neutral zone regulation. These units
are for general use within the industrial and
marine sectors.
The RT series also covers safety pressure
controls for steam boiler plant.
Setting range knob
(Seal cap for tamper proof available)
Page
Function description 9
Function description fail-safe, JJ 10
Pressure control for liquid level control 10
Pressure controls with adjustable neutral zone 11-12
Differential pressure controls 13-14
For installations in which operation is particularly
critical from safety and economic points of view,
the use of fail-safe pressure controls is recommended. The use of gold-plated contacts is also
recommended in such installations, provided
operation involves only a few switching cycles or
low signal currents and voltages.
IP66 enclosure
(units with ext. reset IP 54)
Polyamide cover
(Cover without windows
available)
Stainless steel
bellows
2 x PG 13.5
cable diameter 6
→→
→ 14 mm
→→
SPDT contact system.
Exchangeable. Several types
of contact systems as spare
parts, among other things
gold-plated contacts
Pressure connection G
G ½ A (Vd
J J
J approved)
J J
3
/8 A.
3
Page 4
Pressure controls, type KRT
Technical data and code nos.
RT 113
for manual setting;
cover with windows
RT 116
for tamper proof;
cap and blank cover
When ordering, please state type and code
number.
The type designation for the letters below means:
A: Unit suitable for the medium ammonia
L: Unit with neutral zone
Pressure controls
Regulation Adjustable/ Max. Max.
range fixed operating test Pressure
(p
= effektive mechanical pressure pressure connection Type
e
pressure) differential PB p' ISO 228/1
bar bar bar bar
−1 −0 0.09 −0.4 7 8 G
0 −0.3 0.01 −0.05 0.4 0.5 G
0 −0.3 0.01 −0.05 0.4 0.5 G
0.1 −1.1 0.07 −0.16 7 8 G
0.1 −1.1 0.07 7 8 G
0.2 −3 0.08 −0.25 7 8 G
0.2 −3 0.08 7 8 G 3/8A 017-5110 RT 110
−0.8 −5 0.5 −1.6 22 25
−0.8 −5 0.5 22 25
−0.8 −5 0.5 −1.6 22 25 G
−0.8 −5 0.5 22 25 G
−0.8 −5 1.3 −2.4 22 25 G
0.2 −6 0.25 −1.2 22 25 G
0.2 −6 0.25 22 25 G
1 −10 0.3 −1.3 22 25 G
1 −10 0.3 22 25 G
4 −17 1.2 −422 28G
4 −17 1.2 22 28 G
4 −17 1.2 −422 28G
4 −17 1.2 22 28 G
10 −30 1 −442 47G
1)
Supplied wiht ∅ 6/∅ 10 mm weld nipple.
Preferred versions
2)
With seal cap. 3) DNV approved with EPDM diaphragm.
Code no.
3
/
A 017-5215 RT 121
8
3
/
A 017-5196 RT 113
8
3
/
A 017-5073
8
3
/
A 017-5191 017-5193 RT 112
8
3
/
A 017-5192 RT 112
8
3
/
A 017-5291 017-5292 RT 110
8
7
/
-20 UNF 017-5245 RT 1
16
7
/
-20 UNF 017-5246 RT 1
16
3
/
A1) 017-5001 RT 1A
8
3
/
A1) 017-5002 RT 1A
8
3
/
A1) 017-5007 RT 1A
8
3
/
A 017-5237 017-5240 RT 200
8
3
/
A 017-5238 017-5239 RT 200
8
3
/
A 017-5203 017-5200 RT 116
8
3
/
A 017-5204 017-5199 RT 116
8
3
/
A 017-5255 017-5253 RT 5
8
3
/
A 017-50942) RT 5
8
3
/
A1) 017-50462) RT 5A
8
3
/
A1) 017-50472) RT 5A
8
3
/
A 017-5295 017-5296 RT 117
8
3)
RT 113
Pressure controls with adjustable neutral zone
Regulation Max. operating Max. test
range Mechanical Adjustable pressure pressure Pressure
) differential neutral zone PB p' connection Code no. Type
(p
e
RT 262 A
Differential pressure
control
bar bar bar bar bar
−0.8 −5 0.2 0.2 −0.9 22 25 G
0.2 −3 0.08 0.08 −0.2 7 8 G
0.2 −6 0.25 0.25 −0.7 22 25 G
4 −17 0.35 0.35 −1.4 22 25 G
10 -30 1 1 -3.0 42 47 G
1
) Supplied with ∅ 6/∅ 10 mm weld nipple.
3
/
A1) 017L0033 RT 1AL
8
3
/
A 017L0015 RT 110L
8
3
/
A 017L0032 RT 200L
8
3
/
A1) 017L0040 RT 5 AL
8
3
/
A 017L0042 RT 117L
8
Differential pressure controls
Regulation Mechanical Adjustable Operation pressure pressure connection
range differential neutral zone range PB p' ISO 228/1 Code no. Type
∆p
bar bar bar bar bar bar
0 −0.9 0.05 0.05 -0.23 −1 −67 8G
0.1 −1.0 0.05 0.05 -0.23 −1 −67 8G
0.1 −1.5 0.1 0.1 -0.33 −1 −911 13G
0.1 −1.5 0.1 −1 −911 13G
0 −0.3 0.035 −1 −10 11 13 G
0.5 −4 0.3 0.3 -0.9 −1 −18 22 25 G
0.5 −4 0.3 −1 −18 22 25 G
0.5 −6 0.5 −1 −36 42 47 G
1.5 −11 0.5 −1 −31 42 47 G
1 −6 0.5 −1 −36 42 47 G
Max. operating Max. test Pressure
3
/
A1) 017D0081 RT 266AL
8
3
/
A1) 017D0045 RT 263AL
8
3
/
A1) 017D0043 RT 262AL
8
3
/
A1) 017D0025 RT 262A
8
3
/
A1) 017D00272) RT 262A
8
3
/
A1) 017D0048 RT 260AL
8
3
/
A1) 017D0021 RT 260A
8
3
/
A1) 017D0023 RT 260A
8
3
/
A 017D0024 RT 260A
8
3
/
A1) 017D00723) RT 265A
8
4
Page 5
Pressure controls, type KRT
Technical data and code nos. B: Safety units with external reset
The designation letters mean:
A: Units suitable for the medium ammonia.
W: Units for control purposes.
Pressure controls for steam plant, approved by Vd J
Regulation Adjustable/ Max. Max. test
range fixed operating pressure
(p
-effektive mechanical pressure Pressure Type
e
pressure differential connection
PB p'
bar bar bar bar
S: Safety units with internal reset
Preferred versions
Code no.
For rising pressure, approved acc. to PED
0.1 −1.1 0.07 7 8 G ½A 017-5282 RT 112W
0 −2.5 0.1 7 8 G ½A 017-5280 RT 35W
1 −10 0.8 22 25 G ½A 017-5187 RT 30AW
1 −10 0.4 22 25 G ½A 017-5188 RT 30AB
1 −10 0.4 22 25 G ½A 017-5189 RT 30AS
5 −25 1.2 42 47 G ½A 017-5181 RT 19W
5 −25 1 42 47 G ½A 017-5182 RT 19B
5 −25 1 42 47 G ½A 017-5183 RT 19S
For falling pressure
0 −2.5 0.1 7 8 G ½A 017-5262 RT 33B
2 −10 0.3 −122 25 G ½A017-5267 RT 31W
2 −10 0.3 22 25 G ½A 017-5268 RT 31B
2 −10 0.3 22 25 G ½A 017-5269 RT 31S
5 −25 0.8 −342 47 G ½A017-5247 R T 32W
5 −25 0.4 42 47 G ½A 017-5248 RT 32B
Pressure control for low pressure steam plant (pressure monitoring)
0.1 −1.1 0.07 −0.16 7 7 G ½A 017-5184 RT 112
Approvals
RT 1 RT1A RT 1AL RT 5 RT 30AW RT 31W RT 33B RT 110 RT 112 RT 113 RT 116 RT 117L RT 260A
RT 5A RT 30AB RT 31B RT 35W RT 117 RT 200L RT 262A
RT 121 RT 30AS RT 31S RT112W RT 200 RT 265A
xx x x x x x x x x x x x
xxx xxx FGermanischer Lloyd, Germany
x x x x x x x x x x x x x Registro Italiano Navale, Italy
xx x x x x x PPolski Rejestr Statków, Poland
x x x x x x x x x x x x x RMRS, Russian Maritime Register of Shipping,
x x x x x x x Nippon Kaiji Kyokai, Japan
Note: In addition we refer to the certificates, the copies of which can be
ordered from Danfoss.
RT 19W RT 32W RT116W RT 260AL Approvals
RT 19B RT 32B RT 262AL
RT 19S RT 32S RT 263AL
xxx x
x x x x Det Norske Veritas, Norway
x x x x Lloyds Register of Shipping, UK
x x x x x Bureau Veritas, France
RT 266AL
DEMKO, Denmark. CE marked acc. to EN 60947-4/-5
J
Vd TÜV, Germany
All RT are
• CE marked in accordance with EN 60947-4/-5 for sale in Europe
• Further, the RT 19, RT 30, RT 35, and RT 112 series is CE marked in accordance with PED 97/23/EC, category IV, safety
equipment.
5
Page 6
Pressure controls, type KRT
Technical data
Designation RT pressure controls
Ambient temperature In general −50 to 70°C
Vd J-appoved −40 to +70°C
Contact system
Single-pole changeover switch (SPDT)
Contact load Alternating current:
AC-1: 10A, 400 V Fig. 6
AC-3: 4A, 400 V
AC-15: 3A, 400 V
Contact material: Direct current:
AgCdO DC-13: 12 W, 230 V
(see fig. 6)
Special contact systems Optional
Cable entry 2 PG 13.5 for 6 - 14 mm diameter cables
Enclosure IP66 acc. to IEC 529 and DIN 40050. Units supplied with external reset
IP54. The thermostat housing is made of bakelite acc. to DIN 53470.
Cover is made of polyamide.
Materials in contact with the medium
Material Part W. no. DIN 1 1A 5 5A 110 112 113 116 117 121 200/ 260A 262A/ 260AL 265A
Stainless steel 18/8 Bellows 1.4301 17440 x x x x x x x x x x x x x x x
Stainless steel 17/7 Spring 1.4568 17224 x x x x x
Brass Housing 2.0402 17660 x x x x x x x
Brass Bellows ring 2.0321 17660 x x x x x x x
Free-cutting steel Flare connect. 1.0718 1651 x
Deep-drawn steel
(nick.-plated surface)
Non-alli. carbon steel Weld connect.
C20 for connection
Aluminium Gasket 3.0255 1712 x x x x x x x x
Case hardening steel Weld connect.
C15 Bellows connet.
Stainless steel
NBR-rubber Diaphragm x
Deep-drawn steel Diaphragm
(surface housing with
DIN 50961 weld conn. welded
Fe/Zn 5C) connector
Spring thread Spring 1.1250 17223 x
Housing 1.0338 1623 x x x x x x x x
1.0402 1652 x x x x x x x
1.0401 1652
Spring guide
+ screw
1.4305 17440
1.0338 1623 x
RT RT RT RT RT RT RT RT RT RT RT RT RT RT RT RT
200L 262AL 266AL
263AL/
Materials in contact with the medium. Vd-
Material Part W.no. DIN 19W 30AW 31W 32W 33B 112 W 112 116W
Stainless steel 18/8 Bellows 1.4301 17440 x x x x x x x x
Stainless steel 17/7 Orifice 1.4305 17440 x x
Steel C15 Connector 1.0401 1652 x x
Deep-drawn steel + Ni Bellows ring 1.0338 1623 x x x x x x
Stainless steel 17/7 Bellows spring 1.4568 17224 x x
Stainless steel Ring 1.4305 17440 x
Deep-drawn steel + Ni Housing 1.0338 1623 x x x x x x
Stainless steel Bellows connect. 1.4305 17440 x
Stainless, weldable
free-cutting steel
Deep-drawn steel + Sn Spring guide 1.0338 1623 x
Brass Housing 2.0402 17660 xx
Brass Bellows ring 2.0321 17660 xx
Connector 1.4301 17440 x x x x x
J
approved controls
RT RT RT RT RT RT RT RT
B,S AB, AS B, S B, S 35W
6
Page 7
Pressure controls, type KRT
Dimensions and weight
RT 5, 110, 112,
116, 117, 200
Special version with tamper
proof cap and blank cover
RT 113 RT 5, 110, 112,
RT 1
RT 1A, 1AL
116, 117,117L,
121, 200, 200L
RT 262A,
RT 260A, 260AL 262AL, 263 AL RT 5 RT 5A, 5AL
: External reset knob
only for RT...B
RT 30 AW, -B, -S L=225
RT 19 W, -B, -S L=228
RT 33 B, 35W L=221
RT 112 W L=210
RT 116 W L=235
RT 31 W-B,-S L=212
RT 32 W-B, -S L=212
Weight: approx. 1 kg
7
Page 8
Pressure controls, type KRT
Installation
RT units have two mounting holes which become
accessible when the front cover is removed. Units
fitted with switch 017-0181*) must be installed with
the setting knob upwards. When installing differential
pressure controls, the low pressure side (marked
LP) must be installed upwards. The other pressure
controls in the RT series can be installed in any
position, expect that on plant subjected to severe
vibrations it is advantageous to have the screwed
cable entry downwards.
*) Contact system with snap-action contact.
See spare parts and accessories, page 28
Fig. 1. Positioning of unit
5. Setting knob
9. Range scale
19. Differential setting
disc
Pressure connection
When fitting or removing pressure lines, the spanner
flats on the pressure connection should be used to
apply counter-torque.
Steam plant
To protect the pressure element against temperature
in excess to the maximum temperature of the
medium 150°C (RT 113 90°C), the insertion of
water-filled loop is recommended.
Water systems
Water in the pressure element is not harmful, but if
frost is likely to occur a water-filled pressure element
may burst. To prevent this happening, the pressure
control can be allowed to operate on an air cushion.
Media resistance
See table of materials in contact with the medium. If
seawater is involved, diaphragm pressure controls
types KPS 43, 45 and 47 are recommended.
Pulsations
The pressure control must be connected in such a
way that the pressure element is affected by
pulsations as little as possible. A damping coil can
be inserted (see "Accessories"). With strongly
pulsating media, diaphragm pressure controls types
KPS 43, 45 and 47 can be advantageous.
Einstellung
Die Bereichseinstellung wird unter gleichzeitigem
Ablesen der Skala (9) mit dem Einstellknopf (5)
vorgenommen. Bei Druckschaltern mit Deckknopf
ist für die Einstellung ein Werkzeug erforderlich. Für
Geräte mit fester Differenz ist der Unterschied
zwischen dem Ein- und Ausschaltdruck gegeben.
Bei Geräten mit einstellbarer Differenz ist der
Deckel zu entfernen. Die Differenzrolle (19) wird
nach dem Diagramm eingestellt.
Fig. 2
Wahl der Differenz
Um einen zweckmäßigen automatischen Betrieb
einer Anlage zu erreichen, ist eine angemessene
Differenz erforderlich. Bei einer zu kleinen Differenz
ergeben sich kurze Betriebszeiten mit dem Risiko
der Pendelung. Bei einer zu großen Differenz treten
erhebliche Druckschwankungen auf.
Differenzskalenwerte sind anleitend.
Fig. 3. Obtainable differential disc scale
8
Page 9
5
Pressure controls, type KRT
Function
a. RT 19, RT 30, and pressure controls with max.
reset
When the pressure exceeds the set range value,
contacts 1-4 make and contact 1-2 brake. The
contacts changeover to their initial position when the
pressure falls to the range value minus the differential (see fig. 4).
− − −− − −
− − −
Scale setting
− − −− − −
Mechanical differential
Fig. 4. Contact function, setting for rising pressure
b. All other RT pressure controls
When the pressure falls to the set range value,
contacts 1-2 make and contacts 1-4 brake. The
contacts changeover to their original position when
the pressure again rises to the set range plus the
differential (see fig. 5).
I. Alarm for rising pressure given at the set range
value.
II. Alarm for falling pressure given at the set range
value minus the differential.
Units with max. reset can only be reset at a pressure
corresponding to the set range value minus the
differential, or a lower pressure.
I. Alarm for falling pressure given at the set range
value.
II. Alarm for rising pressure given at the set range
value plus the differential.
Units with min. reset can only be reset at a pressure
corresponding to the set range value plus the
differential.
− − −− − −
− − −
Mechanical differential
Scale setting
− − −− − −
234
Fig. 5. Contact funktion, setting for falling pressure
Example 1
An extra cooling water pump must start if the cooling
water pressure falls below 6 bar , and must stop
when the pressure exceeds 7 bar .
Choose an RT 116 with a range of 1-10 bar and an
adjustable differential of 0.2-1.3 bar .
The start pressure of 6 bar must be set on the range
scale. The differential must be set as the difference
between the stop pressure (7 bar) and the start
pressure (6 bar) = 1 bar. According to fig. 3, the
differential setting disc must be set on 8.
Example 2
The burner on a steam boiler must cut out when the
pressure exceeds 17 bar. Automatic restart must not
occur.
Choose an RT 19B with external reset. If extra
safety is demanded, an RT 19S with internal max.
reset can be used.
The range is 5-25 bar and the differential is fixed at
approx. 1 bar. The range scale must be set at 17
bar. After cut-out of the burner, manual reset is
possible only when the pressure had fallen to the
setting of 17 bar minus the differential: in this case,
16 bar and below.
Example 3
The min. permissible lubricating oil pressure for a
gear is 3 bar. Reset must not be possible until the
reason for oil pressure failure has been investigated.
Choose an RT 200 with min. reset.
The range value must be set while reading the
range scale. Manual reset is possible only when the
pressure has reached 3.2 bar (the differential is
fixed at 0.2 bar or higher).
9
Page 10
Pressure controls, type KRT
Function description
for KRT units
TÜV
approval
Fail-safe function for falling pressure
Fig. 5a shows a cross-section of a bellows element
for the RT 32W with fail-safe function for falling
pressure. On rising pressure the contact arm is
actuated to break the connection between terminals
1 and 2.
On falling pressure the contact arm is actuated to
break the connection between terminals 1 and 4.
If a defect occurs in the bellows the setting spring
actuates the contact arm to break the connection
between terminals 1 and 4, as in the case of falling
pressure. This will occur irrespective of the pressure
on the bellows.
Fail-safe function for rising pressure
Fig. 5b shows a cross-section through a bellows
element for the RT 30W with fail-safe for rising
pressure. On rising pressure the contact arm is
actuated to break the connection between terminals
1 and 2 . If a defect occurs in the inner bellows the
pressure is led to the outer bellows. The outer
bellows has an area three times as large as the
inner bellows. The connection between terminals 1
and 2 becomes broken.
If a defect occurs in the outer bellows, there will be
atmospheric pressure in the gap between the two
bellows. This actuates the contact system to break
the connection between terminals 1 and 2. The
important factor with the double bellows design is
the vacuum between the two bellows, and that in
case of bellows break, no media will leak into the
environment.
Fig. 5a
Fig. 5b
Pressure controls for liquid
level control RT 113
The RT 113 pressure control can be used to control
the liquid level in open tanks. Fig. 6 shows in
principle, four different types of installation.
1. With air bell (see "Accessories")
For control purpose, the air bell should be installed
20 to 40 mm below the lowest liquid level. In
addition, the tube between the RT 113 and the air
bell must be absolutely airtight. If only an indication
is required, the bell can be placed 100 mm below
the max. level. The RT 113 must be set at 0 cm wg
and the differential disc on 1.
2. Connection to the side of the tank with the RT 113
above the liquid level
The horizontal tube A must have a certain length in
relation to the vertical tube B in order to ensure
reliable control. The length ofA can be found from
fig. 7, using B and the range setting pressure C.
3. Connection to the side of the tank with the RT 113
below the liquid level
Where possible, this form of connection should be
used. If an air-absorbing liquid like oil is involved, it
is preferable to 1 and 2. The resulting range setting
is the distance from the liquid surface to the centre
of the diaphragm housing.
4. Connection in the tank with the RT 113 above the
liquid level
This method is for use with air-absorbing liquids
where connection type 3 is not possible. The
shortest horizontal tube length is determined as
described in 2. A shut-off valve is installed between
the oil tank and water reservoir shown so that
impurities can be drained from the water reservoir
through a bottom drain plug. Fresh water can then
be poured into the reservoir through a filling
connector in its top.
Height from tank connector to
pressure control
10
Fig. 6
Min. horizontal tube length
Fig. 7
Page 11
Pressure controls with adjustable neutral zone, type KRT-L
Application RT-L pressure controls are fitted with a switch with
an adjustable neutral zone. This enables the units to
be used for floating control. The terminology
involved is explained below.
Floating control
A form of discontinuous control where the correcting
element (e.g. valve, damper, or similar) moves
towards one extreme position at a rate independent
of the magnitude of the error when the error exceeds
a definit positive value, and towards the opposite
extreme position when the error exceeds a definite
negative value.
Hunting
Periodic variations of the controlled variable from the
fixed reference.
Neutral zone
The interval in the controlled variable in which the
correcting element does not respond (see fig. 13).
The contact system in neutral zone units cannot be
exchanged, as the contact system adjustment is
adjusted to the other parts of the unit.
The required neutral zone can be found in the
diagram for the unit concerned. The position at
which the neutral zone disc (40) must be set can be
Setting of neutral zone
Fig. 8
The range is set using the setting knob (5) fig. 8
while reading the range scale (9). The pressure set
is the break pressure for contacts 1-4 (see fig. 13).
read from the lower scale in the diagram. The
function can be seen in fig. 13.
Fig. 9 Fig. 10 Fig. 11 Fig. 12
Range setting
Inlet pressure
Differential (mechanical
differential) corresponds to
the least neutral zone
a: The neutral zone.Inlet pressure
may vary within this interval without
resulting in a make function 1-2 or
1-4.
setting.
Fig.13
11
Page 12
Pressure controls with adjustable neutral zone, type KRT-L
Example
Together with a VLT® static frequency converter, RT
200L neutral zone pressure controls can be used for
the infinite control of a pump in, for example, a
pressure boosting plant.
In this case, the pump must be up and downregulated at 32 and 25 m wg.
The RT 200L must be set using the setting knob (5)
fig. 8 page 11 at 3.5 bar (35 m wg) minus the fixed
differential of 0.2 bar.
The range setting is 3.5 - 0.2 = 3.3 bar.
The neutral zone, 35 - 32 = 3 m wg, corresponding
to 0.3 bar, must be set on the neutral zone disc (40)
fig. 8 page 11. According to the diagram fig. 12 the
disc setting is 1 or just over. A more accurate setting
can be obtained by using the test setup shown in
fig. 14.
Pressure
gauge
Signal lamp for
falling and rising
Compressed
air
pressure
Fig. 14
Test setup for setting the pressure control
12
Page 13
Differential pressure control, type KRT
Application
Setting
5. Setting disc
9. Range scale
Control and monitoring of pressure differentials
A differential pressure control is a pressure controlled switch that cuts in and cuts out the current
dependent on the pressure differential between the
counteracting bellows elements and the set scale
value. This unit is also available with an adjustable
neutral zone (like the RT-L which is described on
page 11.)
The setting disc (5) becomes accessible when the
front cover is removed. The differential pressure is
set by turning the disc with a screwdriver while
reading the scale (9).
For differential pressure controls with a changeover
contact system, the contact differential is given as
the differential pressure controls have a fixed
differential.
In units with an adjustable neutral zone, the neutral
zone disc must also be set. See diagram in fig. 16.
Fig. 16
Fig. 15
Note:
When installing, the low
pressure connection (LP)
must always be upwards
13
Page 14
6
Differential pressure controls, type KRT
Function
a. Units with changeover switch (SPDT)
If the differential pressure falls below the set v alue,
contacts 1-2 make and contacts 1-4 break.
Contacts 1-2 break again and contacts 1-4 make
when the differential pressure has risen to the set
range value plus the fixed contact diff erential.
Scale setting
Mechanical differential
−−−
2345
b. Units with adjustable neutral zone (SPDTNP)
If the differential pressure rises above the set value
plus the differential, contacts 1-4 make.
If the pressure falls by the amount of the diff erential
(which is fixed in this unit), contacts 1-4 break.
If the pressure falls to the neutral zone minus the
differential, contacts 1-2 make. When the differential
pressure rises again by an amount corresponding to
the differential, contacts 1-2 break again.
I. Contacts make when differential pressure falls
below the range scale setting.
II. Contacts make when pressure rises above the
range scale setting plus the fixed mechanical
differential.
The contact function can be summed up as follows:
I. Setting disc set for falling differential pressure .
II. Neutral zone disc set for rising differential
pressure.
Range setting
Inlet pressure
Mechanical differential
Differential (mechanical differential)
corresponds to the least neutral zone
setting.
a: The neutral zone and inlet differential
pressure may vary within this interval
without resulting in a make function 1-2
or 1-4.
Fig. 18. Contact function, setting of neutral zone
Example 1
When the differential pressure exceeds 1.3 bar, a
filter needs cleaning. The static pressure over the
filter is 10 bar.
According to the ordering table on page 4, the
choice is an RT 260A (the RT 262A has a max.
operating pressure on the low pressure side (LP) of
6 bar and is therefore not suitable for this
application).
Setting: Since a signal is required f or rising differential pressure, the setting becomes 1.3-0.3 bar = 1.0
bar.
Example 2
The speed of a circulation pump must be controlled
to give a constant differential pressure of 10 m wg in
a heating plant. The static plant pressure is 4 bar.
The choice is an RT 262AL.
The differential disc (5) fig. 15 page 13, must be set
at 1 bar (10 m wg) minus the fixed differential of 0.1
bar, i.e. 0.9 bar. The neutral zone disc is factory-set
(marked in red).
14
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