Boiler doc 07 control self acting

The Steam and Condensate Loop 7.1.1
Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7
Module 7.1
Self-acting Temperature Controls
SC-GCM-61CMIssue2©Copyright2005Spirax-SarcoLimited

The Steam and Condensate Loop7.1.2
Self-acting Temperature Controls
What are self-acting temperature controls and how do
they operate?
There are two main forms of self-acting temperature control available on the market: Liquid
filled systems and vapour tension systems.
Self-acting temperature controls are self-powered, without the need for electricity or compressed air.
The control system is a single-piece unit comprising a sensor, capillary tubing and an actuator.
This is then connected to the appropriate control valve, as shown in Figure 7.1.1.
2-port
control valve
Control system
Actuator Capillary tube Sensor
Fig. 7.1.1 Components of a typical self-acting temperature control system
Adjustment knob

The self-acting principle
If a temperature sensitive fluid is heated, it will expand. If it is cooled, it will contract. In the case
of a self-acting temperature control, the temperature sensitive fluid fill in the sensor and capillary
will expand with a rise in temperature (see Figure 7.1.2).
2-port control valve
Packless
gland
bellows
Actuator
Action
Expansion
Capillary
tubing
Heat
Heat
Adjustment
piston
Sensor
Adjustment
Temperature
overload
device
Temperature
sensitive liquid fill
Fig. 7.1.2 Schematic drawing showing the expansive action of the liquid fill when heat is applied to the sensor
Flow
The force created by this expansion (or contraction in the case of less heat being applied to the
sensor) is transferred via the capillary to the actuator, thereby opening or closing the control
valve, and in turn controlling the flow of fluid through the control valve. The hydraulic fluid
remains as a liquid.
There is a linear relationship between the temperature change at the sensor and the amount of
movement at the actuator. Thus, the same amount of movement can be obtained for each equal
unit rise or fall in temperature. This means that a self-acting temperature control system gives
‘proportional control’.
To lower the set temperature
The adjustment knob is turned clockwise to insert the piston further into the sensor. This effectively
reduces the amount of space for the liquid fill, which means that the valve is closed at a lower
temperature. The set temperature will therefore be lower. On control systems with dial-type
adjustments, the same effect will be achieved (typically) by using a screwdriver to turn the
adjustment screw clockwise.
To raise the set temperature
The adjustment knob is turned anticlockwise to decrease the length of the piston inserted in the
sensor. This increases the amount of space for the liquid fill, which means that a higher
temperature will be needed to cause the fill to expand sufficiently to close the control valve. The
set temperature will therefore be higher.
Again, typically for a dial-type adjustment, a screwdriver is used to turn the adjustment screw
anticlockwise.
Protection against high temperatures
In the event of a temperature overrun above the set temperature (possible causes of which
might be a leaking control valve, incorrect adjustment, or a separate additional heat source); a
series of disc springs housed inside the piston will absorb the excess expansion of the fill. This
will prevent the control system from rupturing. When the temperature overrun has ceased, the
disc springs will return to their original position and the control system will function as normal.
Overrun is typically 30°C to 50°C above the set temperature, according to the control type.

A vapour tension system follows a unique pressure/temperature saturation curve for the fluid
contained by the system. All fluids have a relationship between pressure and their boiling
temperature. The result can be plotted by a saturation curve. The saturation curve for water can
be seen in Figure 7.1.4.
Figure 7.1.4 illustrates how a 5°C temperature change at 150°C will cause a 0.65 bar change in
system pressure. At the bottom of the scale, a 5°C temperature change only results in a 0.18 bar
change in system pressure. Thus for the same temperature change, the valve will move a greater
amount at the top end of the temperature range than at the bottom end.
0.65 bar0.18 bar
5°C
5°C
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4
160
150
140
130
120
110
90
80
100
Pressure (bar g)
Temperature(°C)
Fig.7.1.4 Vapour pressure curve for water
Vapour tension systems
A vapour tension control system has a sensing system filled with a mixture of liquid and vapour.
An increase in the sensor temperature boils off a greater proportion of the vapour from the
liquid held within it, increasing the vapour pressure in the sensor and capillary system. This
increase in pressure is transmitted through the capillary to a bellows or diaphragm assembly at
the opposite end (see Figure 7.1.3).
Bellows assembly
Packing gland
Return spring
Adjustment nut
2-port
control
valve
Capillary tubing
Sensor bulb
Fig. 7.1.3 Diagram showing a typical vapour tension temperature control system
Flow

Equation 7.1.1
G
)RUFH RQ YDOYH VWHP QHZWRQ

[ 3

π
∆
ò
Where:
d = Diameter of valve orifice (mm)
DP = Differential pressure (bar)
Therefore to move a valve from fully open to fully closed requires a greater temperature change
at the bottom end of the range than at the top. Manufacturers of these types of vapour tension
control systems often suggest that the control be used only at the top end of its range, but this
means that to cover a reasonable temperature span, different fills are used (including water,
methyl alcohol and benzene).
Alternatively, a liquid filled system will give a true linear relationship between temperature
change and valve movement, largely due to liquid being incompressible. The set temperature
can be calibrated in degrees and not simply by a series of numbers. There is no confusion over
adjusting the set temperature; which reduces commissioning time. Also, adjustment, which is
carried out by altering the amount of space available for the liquid fill, can be carried out anywhere
between the control valve and the sensor. This is not so with vapour tension systems, which can
usually only be adjusted at the control valve.
o Vapour tension control valves sometimes leak through the stem. To avoid the extra cost of
having a second bellows sealing mechanism, most manufacturers of vapour tension controls
use a mechanical seal on the valve stem. These tend to be either too loose, causing leaks; or
too tight, causing too much spindle friction and the valve to stick.
o In liquid systems, because the valve movement is truly proportional to temperature change
and the valve seal is frictionless, the temperature control has a very high rangeability and can
control at very light loads.
Liquid self-acting temperature control valves
The valves for use with self-acting temperature control systems can be divided into three groups:
o Normally open two-port valves.
o Normally closed two-port valves.
o Three-port mixing or diverting valves.
Normally open two-port control valves
These valves are for heating applications, which is the most common type of application. They
are held in the open position by a spring. Once the system is in operation, any increase in
temperature, detected by the sensor, will cause the fill to expand and begin to close the valve,
restricting the flow of the heating medium.
Normally closed two-port control valves
These valves are for cooling applications. They are held in the closed position by a spring.
When the system is in operation, any increase in temperature will cause the fill to expand and
begin to open the valve, allowing the cooling medium to flow.
Force required to close a self-acting control valve
The required closing force on the valve plug is the product of the valve orifice area and differential
pressure as shown in Equation 7.1.1. Note that for two-port steam valves, differential pressure
should be taken as the upstream absolute steam pressure; whereas for two-port water valves it
will be the maximum pump gauge pressure minus the pressure loss along the pipe between the
pump and the valve inlet.

Example 7.1.1
Calculate the force required to shut the valve if a steam valve orifice is 20 mm diameter and the
steam pressure is 9 bar g. (The maximum differential pressure is 9 + 1 = 10 bar absolute).
This means that the actuator must provide at least 314 newton to close the control valve against
the upstream steam pressure of 9 bar g.
It can be seen from Example 7.1.1 that the force required to shut the valve increases with the
square of the diameter. There is a limited amount of force available from the actuator, which is
why the maximum pressure against which a valve is able to shut decreases with an increase in
valve size.
This would effectively limit self-acting temperature controls to low pressures in sizes over DN25,
if it were not for a balancing facility. Balancing can be achieved by means of a bellows or a
double seat arrangement.
Bellows balanced valves
In a bellows balanced valve, a balancing bellows with the same effective area as the seat orifice
is used to counteract the forces acting on the valve plug. A small hole down the centre of the
valve stem forms a balance tube, allowing pressure from upstream of the valve plug to be fed to
the bellows housing (see Figure 7.1.5). Similarly, the forces on the valve plug pressurise the
inside of the bellows. The differential pressure across the bellows is therefore the same as the
differential pressure across the valve plug, but since the forces act in opposite directions they
cancel each other out.
The balancing bellows may typically be manufactured from either:
o Phosphor bronze.
o Stainless steel, which permits higher pressures and temperatures.
G
)RUFH RQ YDOYH VWHP [ 3

)RUFH RQ YDOYH VWHP [

π
∆
π
ò
ò
)RUFH RQ YDOYH VWHP 1
Flow
Seat
Balancing bellows
Pressure transfer passageway
(balance tube)
Valve stem
Fluid exits the balance tube
here into the bellows housing
Fig. 7.1.5 Two-port, normally open, bellows balanced valve
Fluid enters the
balance tube here
Valve plug

Double-seated control valves
Double-seated control valves are useful when high capacity flow is required and tight shut-off is
not needed. They can close against higher differential pressures than single seated valves of the
same size. This is because the control valve comprises two valve plugs on a common spindle
with two corresponding seats, as shown in Figure 7.1.6. The forces acting on the two valve plugs
are almost balanced. Although the differential pressure is trying to keep one plug off its seat, it is
pushing the other plug onto its seat.
However, the tolerances necessary to manufacture the component parts of the control valve
make it difficult to achieve a tight shut-off. This is not helped by the lower valve plug and seat
being smaller than its upper counterpart, which enables removal of the whole assembly for
servicing.
Also, although the body and the valve shuttle are the same material, small variations in the
chemistry of the individual parts can result in subtle variations in the coefficients of expansion,
which adversely affects shut-off. A double-seated control valve should not be used as a safety
device with a high limit safeguard.
Valve
plug
Actuator connection
Valve seat
Valve seat
Fig. 7.1.6 Schematic of a double seated (normally closed) self-acting control valve
Flow
Valve
plug

Control valves with internal fixed bleed holes
A normally closed valve will usually require a fixed bleed (Figure 7.1.7) to allow a small amount
of flow through the control valve when it is fully shut. Normally closed self-acting control valves
are sometimes referred to as being reverse acting (RA).
A typical application for this type of valve is to control the flow of cooling water (coolant) for an
industrial engine such as an air compressor (Figure 7.1.8). The control valve, controlling the flow
of coolant through the engine, is upstream of the engine and the temperature sensor registers its
temperature as it leaves the engine.
If the coolant leaving the engine is hotter than the set point, the control valve opens to allow
more coolant through the valve. However, once the water leaving the engine reaches the
required set temperature the valve will shut again. Without a bleedhole, the coolant would no
longer flow and would continue to pick up heat from the engine. Without the downstream
sensor detecting any temperature rise, the engine is likely to overheat.
If the control valve has a fixed diameter bleed hole, enough cooling water can flow through the
valve to allow the downstream sensor to register a representative temperature when the valve is
shut. This feature is essential when the sensor is remote from the application heat source.
A normally closed valve might also have an optional fusible device (see Figure 7.1.7). The device
melts in the event of excess heat, removing the spring tension on the valve plug and opening
the valve to allow the cooling water to enter the system. It is usual with this kind of safety device,
that once the fusible device has melted, it cannot be repaired and must be replaced.
Return spring
Sleeve soldered to
valve spindle
Retaining plug
Actuator connection
Fixed
bleed
Fusible device
Fig. 7.1.7 Normally closed control valve with fixed bleed
Fig. 7.1.8 Engine or compressor cooling system
Cooling water supply
Sensor downstream of engine
RA control valve
with minimum
bleed facility
upstream of
the engine
Hot water off
Stationary engine
Valve seat
Valve plug

Three-port control valves
Most of the control valves used with self-acting control systems are two-port. However, Figure 7.1.9
illustrates a self-acting piston type three-port control valve. The advantage of this type of valve
design allows the same valve to be used for either mixing or diverting water applications; this is
not normally the case with valves requiring electric or pneumatic actuators.
Fig. 7.1.9 Three-port control valve
Port O (Common port)
Port X Port ZSeal Hollow
piston
Valve stem
Actuator connection
Fig. 7.1.10 Typical three-port control valve used in a mixing application
Circulation pump
Load
circuit
Common flow line
Boiler flow line
Boiler Mixing
circuit
Room
being
heated
Load
O
X Z
The most common applications are for water heating, but three-port control valves may also be
used on cooling applications such as air chillers, and on pumped circuits in heating, ventilating
and air conditioning applications.
When a three-port control valve is used as a mixing valve (see Figure 7.1.10), the constant volume
port 'O' is used as the common outlet.
Boiler return line

When a three-port control valve is used as a diverting valve (see Figure 7.1.11), the constant volume
port is used as the common inlet
Self-contained three port control valves
Another type of three-port self-acting control valve contains an integral temperature sensing
device and thus requires no external temperature controller to operate.
It can be used to protect Low Temperature Hot Water (LTHW) boilers from fire tube corrosion
during start-up sequences when the temperature of the secondary return water is low (see
Figure 7.1.12). At start-up, the valve allows cold secondary water to bypass the external system
and flow through the boiler circuit. This allows water in the boiler to heat up quickly, minimising
the condensation of water vapour in the flue gases. As the boiler water heats up, it is slowly
blended with water from the main system, thus maintaining protection while the complete
system is brought slowly up to temperature.
This type of control valve may also be used on cooling systems such as those found on air
compressors (Figure 7.1.13).
Fig. 7.1.11 Typical three-port control valve used in a diverting application
Fig. 7.1.12 Self contained three-port control valve reducing fire tube corrosion
Circulation pump Load circuit
Common flow
line from boiler
Boiler
Diverting
circuit
Room
being
heated
O
X
Z
Load
Load circuit
Mixing valve
Boiler
Common flow line
Bypass line
Circulation
pump
O
X Z
Boiler return line
Return line to boiler
Return line from load

Water cooler
Water coolant circulating pump
Oil cooler
Fig. 7.1.13 Self-contained three-port valves used to control water and oil cooling systems on an air compressor
Oil coolant circulating pump
Air compressor
O
X Z
O
X Z

Questions
1. Name the components of a self-acting temperature control system.
a| Control valve and actuator ¨
b| Control valve, actuator and sensor ¨
c| Control valve, actuator, capillary tube and sensor ¨
d| Control valve, actuator and capillary tube ¨
2. What is the purpose of overtemperature protection within the self-acting
control system?
a| To protect the valve from high temperature steam ¨
b| To protect the liquid fill in the capillary from boiling ¨
c| To protect the control system from irreversible damage ¨
d| To protect the application from overtemperature ¨
3. If the liquid expands with temperature, how can cooling control be achieved?
a| By fitting two control valves in parallel fashion ¨
b| It cannot because expanding liquid can only shut a control valve ¨
c| By using a bellows balanced control valve ¨
d| By using a normally closed control valve that opens with rising temperature ¨
4. Why do larger control valves tend only to close against lower pressures?
a| The control valve orifice is larger and needs a higher force to close ¨
b| The PN rating of larger control valves is less than smaller control valves ¨
c| The actuators are not designed to operate with high pressures ¨
d| The higher forces involved can rupture the capillary tubing ¨
5. Name two solutions which allow larger control valves to operate at high pressures.
a| Large actuators and large sensors ¨
b| Bellows balanced control valves or double-seated control valves ¨
c| It is not possible to allow larger control valves to operate at higher pressures ¨
d| Larger springs or a higher density capillary fluid ¨
6. Why are three-port self-acting control valves used?
a| To mix or divert liquids especially water ¨
b| To dump steam to waste under fault conditions ¨
c| Where cooling applications are required ¨
d| When large valves are required to meet large capacities ¨
1:c,2:c,3:d,4:a,5:b,6:a Answers

Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems
Module 7.2
Typical Self-acting Temperature
Control Valves and Systems

Typical Self-acting Temperature
Control Valves and Systems
Typical self-acting temperature control systems
The required temperature for the system in Figure 7.2.1 is adjusted at the sensor. It is the most
common type of self-acting temperature control configuration, and most other self-acting control
designs are derived from it.
Fig. 7.2.1 Adjustment at sensor
Figure 7.2.2 illustrates a design which is adjusted at the actuator end of the system. It is worth
noting that this system is limited to 1 (DN25) temperature control valves. This configuration is
useful where the control valve position is more accessible than the sensor position.
Fig. 7.2.2 Adjustment at actuator
Capillary Sensor
Temperature
control valve
Valve actuator
Set temperature knob
Valve actuator
Temperature control valve
Capillary
Sensor
Flow
Flow

Sensor
CapillaryValve actuator
Fig. 7.2.3 Remote adjustment
Capillaries
It should be noted that capillaries of 10 metres or more in length may slightly affect the accuracy
of the control. This is because a larger amount of capillary fluid is subjected to ambient
temperature. When the ambient temperature changes a lot, it can affect the temperature setting.
If long lengths of capillary are run outside, it is recommended they are lagged to minimise this effect.
Pockets
Pockets (sometimes called thermowells) can be fitted into pipework or vessels. These enable the
sensor to be removed easily from the controlled medium without the need to drain the system.
Pockets will tend to slow the response of the system and, where the heat load can change
quickly, should be filled with an appropriate conducting medium to increase the heat transfer to
the sensor.
Pockets fitted to systems which have relatively steady or slow changing load conditions do not
usually need a conducting medium. Pockets are available in mild steel, copper, brass or stainless
steel. Long pockets of up to 1 metre in length are available for special applications and in glass
for corrosive applications. However, these longer pockets are only suitable for use where the
adjustment head is not fitted at the sensor end.
Figure 7.2.3 depicts a third configuration which is similar to the one in Figure 7.2.1 but where
the adjustment is located between the sensor and the temperature control valve actuation. This
type of system is referred to as remote adjustment, and is helpful when either the control valve
or the sensor, or both, are likely to be inaccessible once the control valve has been installed.
Flow
Temperature control valve

Enhancements for self-acting temperature control
systems
Overheat protection by a high limit cut-out device
A separate overheat protection system, as shown in Figure 7.2.4, is available to comply with
local health and safety regulations or to prevent product spoilage. The purpose of the high limit
cut-out device is to shut off the flow of the heating medium in the pipe, thereby preventing
overheating of the process. It was originally developed to prevent overheating in domestic hot
water services (DHWS) which supply general purpose hot water users, such as hospitals, prisons
and schools. However, it is also used for industrial process applications.
The system is driven by a self-acting control system, which releases a compressed spring in the
high limit cut-out unit and snaps the isolating valve shut if the pre-set high limit temperature is
exceeded.
The fail-safe actuator unit does not drive the control valve directly, but a shuttle mechanism in
the high limit cut-out unit instead. When the temperature is below the set point, the mechanism
lies dormant. A certain amount of shuttle travel is allowed for in either direction, to avoid spurious
activation of the system.
However, when the system temperature rises above the adjustable high limit temperature, the
actuator drives the shuttle, displacing the trigger, which then releases the spring in the high limit
cut-out unit. This causes the control valve to snap shut. Once the fault has been rectified, and
after the system has cooled below the set temperature, the high limit cut-out can be manually
reset, using a small lever. The system can also be connected to an alarm system via an optional
microswitch.
The high limit system also has a fail-safe facility. If the capillary is damaged and loses fluid, a
spring beyond the shuttle is released, pushing it the other way. This will also activate the cut-out
and shut the control valve.
The trigger temperature can be adjusted between 0°C and 100°C.
Fig. 7.2.4 High limit cut-out unit with fail-safe control system
Temperature
control valve
Flow
High limit
cut-out
unit
Fail-safe
actuator
unit
Storage
Calorifier
Adjustable
temperature
sensor

For heating applications, the high limit valve must be fitted in series with the temperature control
valve, as shown in Figure 7.2.5. However, in cooling applications, the temperature control valve
and high limit valve will both be of the normally-open type and must be fitted in parallel with
each other, not in series.
The following valves can be used with the high limit system:
o Two-port valves, normally open for heating systems.
o Two-port valves, normally closed for cooling systems.
o Three-port valves.
Valves having a ball shaped plug cannot be used with the cut-out unit. This is because the closing
operation could drive the ball into the seat and damage the valve.
Also, a double seated valve should not be used with this system because it does not have tight
shut-off.
Steam
Temperature
control valve
Flow
Return
Cold water
make-up
Hot water
storage calorifier
Condensate
High limit
temperature
sensor
High limit
cut-out unit
High limit
protection
Fail-
safe
actuator
unit
Fig. 7.2.5 Typical arrangement showing a high limit cut-out on DHWS heat exchanger
The fail-safe actuator unit shown in Figure 7.2.5 is only suitable for use with a high limit cut-out
unit. The systems shown in Figures 7.2.1, 7.2.2 and 7.2.3 can also be used with the cut-out unit
but they will not fail-safe. Figure 7.2.5 shows the high limit cut-out unit attached to a separate
valve to the temperature control valve. This is preferable because the high limit valve remains
fully open during normal operation and is less likely to harbour dirt under the valve seat. The
high limit valve should be line size to reduce pressure drop in normal use, and should be fitted
upstream of the self-acting (or other) control valve and as close to it as possible.
Condensate
Separator
Normal
temperature
sensor

Fig. 7.2.6 Typical self-acting 2-port temperature control valves
Typical self-acting 2-port temperature control valves
Normally
open
medium
capacity
valve
Reverse acting
higher capacity
valve
Normally open
low capacity
valve
Reverse acting
medium capacity
valve
Bellows
balanced
valve
Double
seated
valve
Double seated
reverse acting
valve

Self-acting temperature control ancillaries
Manual
actuator
Fig.7.2.8
Manualactuator
Twin sensor adaptor
A twin sensor adaptor, Figure 7.2.7, allows one valve to be operated by a control system with the
option of having a manual isolation facility.
The adaptor can be used with both 2-port and 3-port control valves. The advantage offered by
the adaptor is that the cost of a separate valve is saved. However, it is not recommended that
temperature control and safeguard high limit protection be provided with a common valve, as
there is no protection against failure of the valve itself.
Manual actuator
A manual adaptor as shown in Figure 7.2.8, is designed to be used with 2-port and 3-port
control valves. It can also be used in conjunction with a twin sensor adaptor and a self-acting
temperature control system, allowing manual shutdown without interfering with the control
settings, as shown in Figure 7.2.7
Spacer
A spacer (Figure 7.2.9) enables the system to operate at higher temperatures. Each control valve
and temperature control system has its own limiting conditions. A spacer, when fitted between
the control system and any 2-port or 3-port control valve (except DN80 and DN100 3-port
valves), enables the system to operate at a maximum of 350°C, providing that the control valve
itself is able to tolerate such high temperatures.
Spacer
Fig.7.2.9
Spacer
Twin
sensor
adaptor
Fig.7.2.7
Twin sensor
adaptor

Typical environments and applications
Environments suitable for self-acting temperature controls:
o Any environment where the sophistication of electrical and pneumatic controls is not required.
Especially suited to dirty and hazardous areas.
o Areas remote from any power source.
o For the accurate control of storage or constant load applications, or for variable load applications
where high accuracy is not required.
Industries using self-acting temperature controls:
Foods
o Milling, heater battery temperature control (non-hazardous).
o Abattoirs - washing down etc.
o Manufacture of oils and fats - storage tank heating.
Industrial
o Metal plating - tank heating.
o Tank farms - heating.
o Refineries.
o Industrial washing.
o Steam and condensate systems.
o Laundries.
Heating, ventilation and air conditioning (HVAC)
o Domestic hot water and heating services in nursing homes, hospitals, leisure centres and
schools, prisons and in horticulture for frost protection.
The most commonly encountered applications for self-acting temperature controls:
Boiler houses
o Boiler feedwater conditioning or direct steam injection heating to boiler feedtank.
o Stand-by generator cooling systems.
Non-storage calorifiers
o 2-port temperature control and overheat protection, (steam or water).
o 3-port temperature control and overheat protection (water only).
o 2-port time / temperature control (steam only).
Storage calorifiers
o 2-port temperature or time/temperature control and overheat protection (steam or water).
o 3-port control and overheat protection (water only).
Injection (or bleed-in) systems
o 2-port or 3-port injection system.

Heating systems
o Basic mixing valve and compensating control.
o Zoned compensating controls.
o Basic compensator plus internal zone controls.
o Control of overhead radiant strip or radiant panels.
Warm air systems
o Heater battery control via room sensor, air-off sensor or return air sensor.
o Compensating control on air-input unit.
o Low limit and high limit control.
o Frost protection to a heater battery.
Fuel oil control
o Bulk tank heating coil control.
o Control of line heaters.
o Control of steam tracer lines.
Process control
o Acid pickling tank.
o Plating vat.
o Process liquor boiling tank.
o Brewing plant detergent tank.
o Drying equipment, for example, laundry cabinet or wool hank dryer, chemical plant drying
stove for powder and cake, tannery plant drying oven.
o Continuous or batch process reaction pan.
o Food industry jacketed pan.
Cooling applications
o Diesel engine cooling.
o Rotary vane compressor oil cooler control.
o Hydraulic and lubricating oil coolers.
o Cooling control on cold water to single-stage compressor.
o Closed circuit compressor cooling control.
o Air aftercooler control.
o Air cooler battery control.
o Jacketed vessel water cooling control.
o Degreaser cooling water control.

Special applications
o Control for reducing fireside corrosion and thermal stress in LTHW boilers.
o Hot water cylinder control.
o Temperature limiting.
Applications for the high limit safeguard system
o Preventing temperature overrun on hot water services, or heating calorifiers, in accordance
with many Health and Safety Regulations. Good examples include prisons, hospitals and schools.
An optional BMS/EMS interface to flag high temperature trip is available.

Questions
1. Where is a self-acting temperature control system adjusted?
a| Locally to the control valve ¨
b| Locally to the sensor ¨
c| Remotely, at a point between the control valve and sensor ¨
d| Any of the above ¨
2. Why are sensor pockets sometimes used?
a| To protect the sensor from overheating ¨
b| To allow the sensor to be removed without draining the system ¨
c| To contain any leakage of liquid fill from the sensor ¨
d| To enable small sensors to fit into large diameter pipes ¨
3. How can fail-safe temperature protection be achieved?
a| By fitting two control valves in series ¨
b| By fitting a proprietary spring-loaded actuator and control valve ¨
c| By setting the control system at a lower temperature ¨
d| By fitting a cooling valve in parallel with the heating valve ¨
4. What does a proprietary fail-safe protection device do?
a| It protects the control valve from high operating temperatures ¨
b| It protects the steam system from overpressure ¨
c| It protects the water system from overtemperature ¨
d| It allows one valve to act as a control and high limit valve ¨
5. For what application is a self-acting temperature control system not suitable?
a| An application with slow changes in heat load ¨
b| An application in a hazardous area ¨
c| An application with fast and frequent changes in heat load ¨
d| A warm air system such as a heater battery control ¨
6. What is the purpose of a twin sensor adaptor?
a| To close the control valve under fault conditions ¨
b| To allow two control valves to be operated by one controller ¨
c| To allow one control valve to be operated by two controllers ¨
d| To allow both heating and cooling with one valve ¨
1:d,2:b,3:b,4:c,5:c,6:c Answers

Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications
Module 7.3
Self-acting Pressure Controls
and Applications

Self-acting Pressure Controls and
Applications
Why reduce steam pressure?
The main reason for reducing steam pressure is rather fundamental. Every item of steam using
equipment has a maximum allowable working pressure (MAWP). If this is lower than the steam
supply pressure, a pressure reducing valve must be employed to limit the supply pressure to the
MAWP. In the event that the pressure reducing valve should fail, a safety valve must also be
incorporated into the system.
This is not, however, the only occasion when a pressure reducing valve can be used to advantage.
Most steam boilers are designed to work at relatively high pressures and should not be run at
lower pressures, since wet steam is likely to be produced. For this reason, it is usually more
economic in the long term to produce and distribute steam at a higher pressure, and reduce
pressure upstream of any items of plant designed to operate at a lower pressure.
This type of arrangement has the added advantage that relatively smaller distribution mains can
be used due to the relatively small volume occupied by steam at high pressure.
Since the temperature of saturated steam is closely related to its pressure, control of pressure
can be a simple but effective method of providing accurate temperature control. This fact is
used to good effect on applications such as sterilisers and contact dryers where the control of
surface temperature is difficult to achieve using temperature sensors.
Plant operating at low steam pressure:
o Can tend to reduce the amount of steam produced by the boiler due to the higher enthalpy
of evaporation in lower pressure steam.
o Will reduce the loss of flash steam produced from open vents on condensate collecting tanks.
Most pressure reducing valves currently available can be divided into the following two main groups:
o Direct acting valves.
o Pilot-operated valves.
Direct acting valves
Smaller capacity direct acting pressure reducing valves (Figure 7.3.1)
Method of operation
On start-up and with the adjustment spring relaxed, upstream pressure, aided by a return spring,
holds the valve head against the seat in the closed position. Rotating the handwheel in a clockwise
direction causes a downward movement, which compresses the control spring and extends the
bellows to set the downstream pressure.
This downward movement is transmitted via a pushrod, which causes the main valve to open.
Steam then passes through the open valve into the downstream pipework and surrounds the bellows.
As downstream pressure increases, it acts through the bellows to counteract the adjustment
spring force, and closes the main valve when the set pressure is reached. The valve plug modulates
in an attempt to achieve constant pressure.
In order to close the valve, there must be a build-up of pressure around the bellows. This requires
an increase in downstream pressure above the set pressure in proportion to the steam flow.
The downstream pressure will increase as the load falls and will be highest when the valve is
closed. This change in pressure relative to a change in load means that the downstream pressure
will only equal the set pressure at one load. The actual downstream pressure compared to the
set point is the proportional offset; it will increase relative to the load, and this is sometimes
referred to as ‘droop’.

The total pressure available to close the valve consists of the downstream pressure acting on the
underside of the bellows plus the inlet pressure acting on the underside of the main valve itself
and the small force produced by the return spring. The control spring force must therefore be
larger than the reduced pressure and inlet pressure and return spring for the downstream pressure
to be set.
Any variation in the inlet pressure will alter the force it produces on the main valve and so affect
the downstream pressure.
This type of pressure reducing valve has two main drawbacks in that:
1. It suffers from proportional offset as the steam flow changes
2. It has relatively low capacity.
It is nevertheless perfectly adequate for a substantial range of simple applications where accurate
control is not essential and where steam flow is fairly small and reasonably constant.
Fig. 7.3.1 Small capacity direct acting pressure reducing valve
Adjustment handwheel
Adjustment spring
(control spring)
Bellows
Return spring
Flow
Valve and seat

Larger capacity direct acting pressure reducing valves (Figure 7.3.2)
Larger capacity direct acting pressure reducing valves are also available for use on larger capacity
plant, or on steam distribution mains. They differ slightly to the smaller capacity valves in that
the actuator force is provided by pressure acting against a flexible diaphragm inside the actuator
rather than a bellows.
As these are not pilot-operated, they will incur a change in downstream pressure as the steam
flow changes, and this should be taken into careful consideration when selecting and sizing the
valve.
This type of valve is installed with the actuator below the pipe when used with steam, and has a
water seal pot to stop high steam temperatures from reaching and damaging the actuator’s
flexible diaphragm, which is commonly made out of neoprene. A typical installation for the
reduction of steam mains pressure is shown in Figure 7.3.3.
Pressure sensing connection
Fig. 7.3.2 Large capacity direct acting pressure reducing valve
Pressure reducing valve
Adjustment nut
Actuator
Spring
Fig. 7.3.3 Typical steam pressure reducing station for a large capacity direct acting pressure reducing valve
Flow
Separator
Stop valve
Strainer
Pressure
reducing
valve
Safety valve
Stop valve
WS4
water seal pot
1 m minimum
Steam
Condensate

Fig.7.3.4 Pilot-operatedpressurereducingvalve
A pilot-operated pressure reducing valve works by balancing the downstream pressure via a
pressure sensing pipe against a pressure adjustment control spring. This moves a pilot valve to
modulate a control pressure. The control pressure transmitted via the pilot valve is proportional
to the pilot valve opening, and is directed, via the control pipe to the underside of the main
valve diaphragm. The diaphragm moves the pushrod and the main valve in proportion to the
movement of the pilot valve. Although the downstream pressure and pilot valve position are
proportional (as in the direct acting valve), the mechanical advantage given by the ratio of the
areas of the main diaphragm to the pilot diaphragm offers accuracy with small proportional
offset.
Under stable load conditions, the pressure under the pilot diaphragm balances the force set on
the adjustment spring. This settles the pilot valve, allowing a constant pressure under the main
diaphragm. This ensures that the main valve is also settled, giving a stable downstream pressure.
When downstream pressure rises, the pressure under the pilot diaphragm is greater than the
force created by the adjustment spring and the pilot diaphragm moves up. This closes the pilot
valve and interrupts the transmission of steam pressure to the underside of the main diaphragm.
The top of the main diaphragm is subjected to downstream pressure at all times and, as there is
now more pressure above the main diaphragm than below, the main diaphragm moves down
pushing the steam underneath into the downstream pipework via the control pipe and surplus
pressure orifice. The pressure either side of the main diaphragm is balanced, and a small excess
force created by the main valve return spring closes the main valve.
Any variations in load or pressure will immediately be sensed on the pilot diaphragm, which will
act to adjust the position of the main valve accordingly, ensuring a constant downstream pressure.
The pilot-operated design offers a number of advantages over the direct acting valve. Only a
very small amount of steam has to flow through the pilot valve to pressurise the main diaphragm
chamber and fully open the main valve. Thus only very small changes in control pressure are
necessary to produce large changes in flow. The fall in downstream pressure relative to changes
in steam flow is therefore small, typically less than three hundredths of a bar (3 kPa; 0.5 psi) from
fully open to fully closed.
Adjustment spring
Pilot diaphragm
Pressure sensing pipe
Pilot valve
Main valve return spring
Main valve and pushrod
Surplus pressure orifice
Main diaphragm
Pilot pressure directed to
underside of diaphragm
by control pipe
High pressure
Low pressure
Control pressure
Pilot-operated valves
Where accurate control of pressure or a large flow capacity is required, a pilot-operated pressure
reducing valve can be used. Such a valve is shown schematically in Figure 7.3.4. A pilot-operated
pressure reducing valve will usually be smaller than a direct acting valve of the same capacity.

Although any rise in upstream pressure will apply an increased closing force on the main valve,
the same rise in pressure will act on the underside of the main diaphragm and will balance the
effect. The result is a valve which gives close control of downstream pressure regardless of
variations on the upstream side.
In some types of pilot-operated valve, a piston replaces the main diaphragm. This can be
advantageous in bigger valves, which would require very large size main diaphragms. However,
problems with the piston sticking in its cylinder are common, particularly in smaller valves.
It is important for a strainer and separator to be installed immediately prior to any pilot-operated
control valve, as clean dry steam will prolong its service life.
Selection and installation of pressure reducing valves
The first essential is to select the best type of valve for a given application.
Small loads where accurate control is not vital should be met by using simple direct acting
valves. In all other cases, the pilot-operated valve is the best choice, particularly if there are
periods of no demand when the downstream pressure must not be allowed to rise.
Oversizing should be avoided with all types of control valve and this is equally true of reducing
valves. A valve plug working close to its seat when passing wet steam can suffer wiredrawing and
premature erosion. In addition, any small movement of the oversized valve plug will produce a
relatively large change in the flow through the valve, making it more difficult for the valve to
control accurately.
A smaller, correctly sized reducing valve will be less prone to wear and will provide more accurate
control. Where it is necessary to make big reductions in pressure or to cope with wide fluctuations
in load, it may be preferable to use two or more valves in series or in parallel.
Although reliability and accuracy depend on correct selection and sizing, pressure reducing
valves also depend on correct installation. Figure 7.3.5 illustrates an ideal arrangement for the
installation of a pilot-operated pressure reducing valve.
Fig. 7.3.5 Typical steam pressure reducing valve station
Many reducing valve problems are caused by the presence of moisture or dirt. A steam separator
and strainer with fine mesh screen, if fitted before the valve, will help to prevent such problems.
The strainer is fitted on its side to prevent the body filling with water and to ensure that the full
area of the screen is effective. Large isolation valves will also benefit from being installed on
their side for the same reason.
All upstream and downstream pipework and fittings must be adequately sized to ensure that the
only appreciable pressure drop occurs across the reducing valve itself. If the isolating valves are
the same size as the reducing valve connections, they will incur a larger pressure drop than if
they are sized to match the correctly sized, larger diameters of the upstream and downstream
pipework.
High pressure
steam flow
Condensate
Separator
Strainer
Isolating
valve
Pressure reducing
valve
Safety
valve
Isolating
valve
Low
pressure

If the downstream pipework or any connected plant is incapable of withstanding the maximum
possible upstream pressure, then a safety valve or relief valve must be fitted on the downstream
side. This valve should be set at, or below, the maximum allowable working pressure of the
equipment, but with a sufficient margin above its normal operating pressure. It must be capable
of handling the full volume of steam that could pass through the fully open reducing valve, at the
maximum possible upstream pressure.
Pilot operation also allows the reducing valve to be relatively compact compared to other valves
of similar capacity and accuracy, and allows a variety of control options, such as on-off operation,
dual pressure control, pressure and temperature control, pressure reducing and surplussing control,
and remote manual adjustment. These variations can be seen in Figure 7.3.6.
Direct acting and pilot-operated control valves can be used to control either upstream or
downstream pressures. Pressure maintaining valves (and surplussing valves) sense upstream
pressure, while pressure reducing valves sense downstream pressure.
Fig.7.3.6 Fourcomplementaryversionsofpilot-operatedpressurereducingvalve
Withtemperaturecontrol Withdualpressurecontrol
Withon-offcontrolBasicpilot-operatedpressurereducingvalve
A solenoid valve which interrupts
the signal to the main diaphragm
Switchable pilot valves
to change the control pressure

Summary of pressure reducing valves
A valve that senses and controls the downstream pressure is often referred to as a ‘let-down’
valve or ‘pressure reducing valve’ (PRV). Such valves can be used to maintain constant steam
pressure onto a control valve, a steam flowmeter, or directly onto a process.
Pressure reducing valves are selected on capacity and type of application.
Table 7.3.1 Typical characteristics for different types of pressure reducing valve
Directacting
Pilot-operated
Bellowsoperated Diaphragmoperated
Small capacity Very large capacity Large capacity
Compact Relatively large Compact for capacity
Low cost Robust Extremely accurate
Steady load Steady load Varying loads
Coarse control Coarse control Fine control
Pressure maintaining valves
Some applications require that upstream pressure is sensed and controlled and this type of valve
is often referred to as a ‘Pressure Maintaining Valve’ or ‘PMV’. Pressure maintaining valves are
also known as surplussing valves or spill valves in certain applications.
An example of a PMV application would be where steam generation plant is undersized, and
yet steam flow is critical to the process. If steam demand is greater than the boiler output, or
suddenly rises when the boiler burner is off, the boiler pressure will drop; progressively wetter
steam will be supplied to the plant and the boiler operation may be jeopardised. If the boiler
can operate at its design pressure, optimum steam quality will be maintained.
This can be achieved by fitting PMVs on each non-critical application (perhaps heating plant or
domestic hot water plant), thereby introducing a controlled diversity to the plant. These will
then progressively shut down if upstream pressure falls, giving priority to essential services.
Should all supplies be considered essential, a variety of options are available, each of which has
a different cost implication.
The cheapest solution might be to fit a PMV in the boiler steam outlet, (see PMV 1 in Figure 7.3.7).
This will maintain a minimum steam pressure in the boiler, regulate maximum flow from the
boiler and, in so doing, retain good quality steam to the plant.
If it is possible to shut off non-essential equipment during times of peak loading, PMVs can be
installed in distribution lines or branch lines supplying these areas of the plant. When the steam
boiler becomes overloaded, the non-essential supplies are gradually shut down by PMV 2 allowing
the boiler to maintain steam flow to the ‘essential’ plant at the proper pressure.
Fig. 7.3.7 Alternative positions for PMVs
Boiler
Separator PMV 1
PMV 2
Non essential line
Essential line
Drain pocket
and trap set

It should be recognised that a PMV will not always cure the problems caused by insufficient
boiler capacity. Sometimes, when there is little plant diversity, only one real alternative is available,
which is to increase the generating capacity by adding another boiler.
However, there are occasions when the cheaper alternative of a steam accumulator is possible.
This allows excess boiler energy to be stored during periods of low load. When the boiler is
overloaded, the accumulator augments the boiler output by allowing a controlled release of
steam to the plant (see Figure 7.3.8).
In Figure 7.3.8, the boiler is designed to generate steam at 10 bar g, which is distributed at both
10 bar g and 5 bar g to the rest of the plant.
PRV 1 is a pressure reducing valve, and is sized to pass the boiler capacity minus the high
pressure steam load.
Fig.7.3.8 Typicalboilerandaccumulatorarrangement
For sizing purposes, the capacity of the pressure reducing valve PRV 2 should equal the maximum
discharge rate and time for which the accumulator has been designed to operate, whilst the
differential pressure for design purposes should be the difference between the minimum
operating accumulator pressure and the LP (Low pressure) distribution pressure. In this example,
PRV 2 would probably be set to open at about 4.8 bar g.
PMV is a pressure maintaining valve whose size is determined by the recharging time required
by the accumulator and the available surplus boiler capacity during recharging. When recharging,
the pressure drop across the PMV is likely to be relatively small, so the PMV is likely to be quite
large, typically the same size as the line in which it is installed. The PMV is usually set to operate
just below the boiler maximum pressure setting.
When the total plant load is within the boiler capacity, PRV 2 is shut and the boiler supplies the
LP steam load through PRV 1 which is set to control slightly higher than PRV2. Any excess steam
available in the boiler will cause the boiler pressure to rise above the PMV set point, and the
PMV will open to recharge the accumulator. Recharging will continue until the accumulator
pressure equals the boiler pressure, or until the plant load is such that the boiler pressure again
drops below the PMV set point.
Should the LP steam load continue to increase, causing the LP pressure to drop below PRV 2 set
point, PRV 2 will open to provide steam from the accumulator, in turn supplementing the steam
flowing through PRV 1.
There is more than one way in which to design an accumulator installation; each will depend
upon the circumstances involved, and will have a cost implication. The subject of accumulators
is discussed in more detail in Module 3.22 ‘Steam accumulators’.
Boiler
PMV PRV 2
Low
pressure
(LP) steam
5 bar g
High pressure
(HP) steam
10 bar g
PRV 1
Accumulator

Pressure surplussing valves
The ability to sense upstream pressure may be used to release surplus pressure from a steam
system in a controlled and safe manner. The surplussing valve is essentially the same as a PMV,
opening when an increase in upstream pressure is sensed. The surplussing valve is sometimes
referred to as a ‘dump’ valve when releasing steam to atmosphere.
A ‘surplussing valve’ is often used to control the maximum pressure in a flash recovery system.
Should the demand for flash steam be less than the available supply, the flash pressure will rise
and the surplussing valve will open to release any excess steam to atmosphere. The surplussing
valve will be set to operate at a pressure below the safety valve setting.
Important: Whilst this allows the controlled release of steam to atmosphere, it does not replace
the need for a safety valve, should the plant conditions require it.
In Figure 7.3.9 the PRV replenishes any shortfall of flash steam generated by the high pressure
(HP) condensate, and the surplussing valve releases any excess flash steam to either a condenser
or to atmosphere.
The safety valve is sized on the full capacity of the PRV plus the capacity of the steam traps and
any other source feeding into the flash vessel.
Fig. 7.3.9 Typical surplussing valve on a flash vessel application
Steam
make-up PRV Surplussing valve
Excess steam
to atmosphere
LP steam
to plant
LP condensate
HP condensate
Flash
vessel
Safety
valve

Boiler doc 07 control self acting

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