2 Typical Self Acting t.c

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The Steam and Condensate Loop 7.2.1

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

Module 7.2

Typical Self-acting TemperatureControl Valves and Systems S C - G

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o p y r i g h t 2 0 0 5 S p i r a x - S a r c o L

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The Steam and Condensate Loop7.2.2


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 controldesigns 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 worthnoting that this system is limited to 1" (DN25) temperature control valves. This configuration isuseful where the control valve position is more accessible than the sensor position.

Fig. 7.2.2 Adjustment at actuator

Capillary Sensor

Temperaturecontrol valve

Valve actuator

Set temperature knob

Valve actuator

Temperature control valve

Capillary

Sensor


Flow

Flow





Sensor


CapillaryValve actuator

Fig. 7.2.3 Remote adjustment

CapillariesIt should be noted that capillaries of 10 metres or more in length may slightly affect the accuracyof 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.

PocketsPockets (sometimes called thermowells) can be fitted into pipework or vessels. These enable thesensor 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 changequickly, should be filled with an appropriate conducting medium to increase the heat transfer tothe 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 stainlesssteel. Long pockets of up to 1 metre in length are available for special applications and in glassfor corrosive applications. However, these longer pockets are only suitable for use where theadjustment 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 wherethe adjustment is located between the sensor and the temperature control valve actuation. Thistype of system is referred to as remote adjustment, and is helpful when either the control valveor 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 deviceA separate overheat protection system, as shown in Figure 7.2.4, is available to comply withlocal 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 preventingoverheating 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, prisonsand 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 thehigh limit cut-out unit and snaps the isolating valve shut if the pre-set high limit temperature isexceeded.

The fail-safe actuator unit does not drive the control valve directly, but a shuttle mechanism inthe high limit cut-out unit instead. When the temperature is below the set point, the mechanismlies dormant. A certain amount of shuttle travel is allowed for in either direction, to avoid spuriousactivation of the system.

However, when the system temperature rises above the adjustable high limit temperature, theactuator 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, andafter the system has cooled below the set temperature, the high limit cut-out can be manuallyreset, using a small lever. The system can also be connected to an alarm system via an optionalmicroswitch.

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


Flow

High limitcut-out

unit

Fail-safeactuatorunit

StorageCalorifier

Adjustabletemperature

sensor





For heating applications, the high limit valve must be fitted in series with the temperature controlvalve, as shown in Figure 7.2.5. However, in cooling applications, the temperature control valveand high limit valve will both be of the normally-open type and must be fitted in parallel witheach 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 closingoperation 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


Flow

Return

Cold watermake-up

Hot waterstorage calorifier

Condensate

High limittemperature

sensor

High limitcut-out unit

High limitprotection

Fail-safeactuatorunit

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 separatevalve to the temperature control valve. This is preferable because the high limit valve remainsfully open during normal operation and is less likely to harbour dirt under the valve seat. Thehigh limit valve should be line size to reduce pressure drop in normal use, and should be fittedupstream of the self-acting (or other) control valve and as close to it as possible.

Condensate

Separator

Normaltemperaturesensor





Fig. 7.2.6 Typical self-acting 2-port temperature control valves

Typical self-acting 2-port temperature control valves

Normallyopenmediumcapacityvalve

Reverse actinghigher capacity

valve

Normally openlow capacityvalve

Reverse actingmedium capacityvalve

Bellows

balancedvalve

Doubleseatedvalve

Double seatedreverse actingvalve





Self-acting temperature control ancillaries

Manualactuator

Fig. 7.2.8

Manual actuator

Twin sensor adaptorA twin sensor adaptor, Figure 7.2.7, allows one valve to be operated by a control system with theoption 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, asthere is no protection against failure of the valve itself.

Manual actuatorA 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-actingtemperature control system, allowing manual shutdown without interfering with the controlsettings, as shown in Figure 7.2.7

SpacerA spacer (Figure 7.2.9) enables the system to operate at higher temperatures. Each control valveand temperature control system has its own limiting conditions. A spacer, when fitted betweenthe 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 valveitself is able to tolerate such high temperatures.

Spacer

Fig. 7.2.9

Spacer

Twinsensoradaptor

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 applicationswhere 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 andschools, 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 dryingstove 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 accordancewith 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



The Steam and Condensate Loop7 2 12


2 Typical Self Acting t.c

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