1-self-acting t.c.

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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 S C - G

C M - 6 1

C M

I s s u e 2

© C

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

i m i t e d



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: Liquidfilled 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-portcontrol 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 principleIf a temperature sensitive fluid is heated, it will expand. If it is cooled, it will contract. In the caseof a self-acting temperature control, the temperature sensitive fluid fill in the sensor and capillarywill expand with a rise in temperature (see Figure 7.1.2).

2-port control valve

Packlessgland

bellows

Actuator

Action

Expansion

CapillarytubingHeat

Heat

Adjustmentpiston

Sensor

Adjustment

Temperatureoverload

device

Temperaturesensitive 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 thesensor) is transferred via the capillary to the actuator, thereby opening or closing the controlvalve, and in turn controlling the flow of fluid through the control valve. The hydraulic fluidremains 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 equalunit rise or fall in temperature. This means that a self-acting temperature control system givesproportional control.

To lower the set temperatureThe adjustment knob is turned clockwise to insert the piston further into the sensor. This effectivelyreduces the amount of space for the liquid fill, which means that the valve is closed at a lowertemperature. The set temperature will therefore be lower. On control systems with dial-typeadjustments, the same effect will be achieved (typically) by using a screwdriver to turn theadjustment screw clockwise.

To raise the set temperatureThe adjustment knob is turned anticlockwise to decrease the length of the piston inserted in thesensor. This increases the amount of space for the liquid fill, which means that a highertemperature will be needed to cause the fill to expand sufficiently to close the control valve. Theset temperature will therefore be higher.

Again, typically for a dial-type adjustment, a screwdriver is used to turn the adjustment screwanticlockwise.

Protection against high temperaturesIn the event of a temperature overrun above the set temperature (possible causes of whichmight 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. Thiswill prevent the control system from rupturing. When the temperature overrun has ceased, thedisc 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 fluidcontained by the system. All fluids have a relationship between pressure and their boilingtemperature. The result can be plotted by a saturation curve. The saturation curve for water canbe 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 insystem pressure. At the bottom of the scale, a 5°C temperature change only results in a 0.18 barchange in system pressure. Thus for the same temperature change, the valve will move a greateramount 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)

T e m p e r a t u r e ( ° 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 theliquid held within it, increasing the vapour pressure in the sensor and capillary system. Thisincrease 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-portcontrolvalve

Capillary tubing

Sensor bulb

Fig. 7.1.3 Diagram showing a typical vapour tension temperature control system

Flow





Equation 7.1.1G

)RUFHRQYDOYHVWHPQHZWRQ [ 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 changeat the bottom end of the range than at the top. Manufacturers of these types of vapour tensioncontrol systems often suggest that the control be used only at the top end of its range, but thismeans 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 temperaturecan be calibrated in degrees and not simply by a series of numbers. There is no confusion overadjusting the set temperature; which reduces commissioning time. Also, adjustment, which iscarried out by altering the amount of space available for the liquid fill, can be carried out anywherebetween the control valve and the sensor. This is not so with vapour tension systems, which canusually 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 controlsuse a mechanical seal on the valve stem. These tend to be either too loose, causing leaks; ortoo tight, causing too much spindle friction and the valve to stick.

o

In liquid systems, because the valve movement is truly proportional to temperature changeand the valve seal is frictionless, the temperature control has a very high rangeability and cancontrol 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 valvesThese valves are for heating applications, which is the most common type of application. Theyare held in the open position by a spring. Once the system is in operation, any increase intemperature, 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 valvesThese 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 andbegin 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 differentialpressure as shown in Equation 7.1.1. Note that for two-port steam valves, differential pressureshould 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 thepump and the valve inlet.





Example 7.1.1Calculate the force required to shut the valve if a steam valve orifice is 20 mm diameter and thesteam 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 thesquare of the diameter. There is a limited amount of force available from the actuator, which iswhy the maximum pressure against which a valve is able to shut decreases with an increase invalve 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 adouble seat arrangement.

Bellows balanced valvesIn a bellows balanced valve, a balancing bellows with the same effective area as the seat orificeis used to counteract the forces acting on the valve plug. A small hole down the centre of thevalve stem forms a balance tube, allowing pressure from upstream of the valve plug to be fed tothe bellows housing (see Figure 7.1.5). Similarly, the forces on the valve plug pressurise theinside of the bellows. The differential pressure across the bellows is therefore the same as thedifferential pressure across the valve plug, but since the forces act in opposite directions theycancel 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)RUFHRQYDOYHVWHP [ 3

)RUFHRQYDOYHVWHP [

π∆

π

ò

ò

)RUFHRQYDOYHVWHP 1

Flow

Seat

Balancing bellows

Pressure transfer passageway(balance tube)

Valve stem

Fluid exits the balance tubehere into the bellows housing

Fig. 7.1.5 Two-port, normally open, bellows balanced valve

Fluid enters thebalance tube here

Valve plug





Double-seated control valvesDouble-seated control valves are useful when high capacity flow is required and tight shut-off isnot needed. They can close against higher differential pressures than single seated valves of thesame size. This is because the control valve comprises two valve plugs on a common spindlewith two corresponding seats, as shown in Figure 7.1.6. The forces acting on the two valve plugsare 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 valvemake 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 forservicing.

Also, although the body and the valve shuttle are the same material, small variations in thechemistry 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 safetydevice with a high limit safeguard.

Valveplug

Actuator connection

Valve seat

Valve seat

Fig. 7.1.6 Schematic of a double seated (normally closed) self-acting control valve

FlowValveplug





Control valves with internal fixed bleed holesA 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 valvesare 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 anindustrial engine such as an air compressor (Figure 7.1.8). The control valve, controlling the flowof coolant through the engine, is upstream of the engine and the temperature sensor registers itstemperature as it leaves the engine.

If the coolant leaving the engine is hotter than the set point, the control valve opens to allowmore coolant through the valve. However, once the water leaving the engine reaches therequired set temperature the valve will shut again. Without a bleedhole, the coolant would nolonger flow and would continue to pick up heat from the engine. Without the downstreamsensor 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 thevalve to allow the downstream sensor to register a representative temperature when the valve isshut. 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 devicemelts in the event of excess heat, removing the spring tension on the valve plug and openingthe 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 tovalve spindle

Retaining plug

Actuator connection

Fixedbleed

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 valvewith minimum

bleed facilityupstream ofthe engine

Hot water off

Stationary engine

Valve seat

Valve plug





Three-port control valvesMost of the control valves used with self-acting control systems are two-port. However, Figure 7.1.9illustrates a self-acting piston type three-port control valve. The advantage of this type of valvedesign allows the same valve to be used for either mixing or diverting water applications; this isnot 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 Hollowpiston

Valve stem

Actuator connection

Fig. 7.1.10 Typical three-port control valve used in a mixing application

Circulation pump

Loadcircuit

Common flow line

Boiler flow line

Boiler Mixingcircuit

Roombeingheated

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, ventilatingand air conditioning applications.

When a three-port control valve is used as a mixing valve (see Figure 7.1.10), the constant volumeport '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 volumeport is used as the common inlet

Self-contained three port control valvesAnother type of three-port self-acting control valve contains an integral temperature sensingdevice and thus requires no external temperature controller to operate.

It can be used to protect Low Temperature Hot Water (LTHW) boilers from fire tube corrosionduring start-up sequences when the temperature of the secondary return water is low (seeFigure 7.1.12). At start-up, the valve allows cold secondary water to bypass the external systemand flow through the boiler circuit. This allows water in the boiler to heat up quickly, minimisingthe 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 completesystem is brought slowly up to temperature.

This type of control valve may also be used on cooling systems such as those found on aircompressors (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 flowline from boiler

Boiler

Divertingcircuit

Roombeingheated

O

X

ZLoad

Load circuit

Mixing valve

Boiler

Common flow line

Bypass line

Circulationpump

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



The Steam and Condensate Loop7 1 12


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

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