Boiler doc 07 control self acting

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Boiler doc 07 control self acting

  1. 1. 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
  2. 2. The Steam and Condensate Loop7.1.2 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 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
  3. 3. The Steam and Condensate Loop 7.1.3 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 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.
  4. 4. The Steam and Condensate Loop7.1.4 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 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
  5. 5. The Steam and Condensate Loop 7.1.5 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 Equation 7.1.1 G )RUFH RQ YDOYH VWHP QHZWRQ
  6. 6. [ 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.
  7. 7. The Steam and Condensate Loop7.1.6 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 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
  8. 8. )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
  9. 9. The Steam and Condensate Loop 7.1.7 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 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
  10. 10. The Steam and Condensate Loop7.1.8 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 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
  11. 11. The Steam and Condensate Loop 7.1.9 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 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
  12. 12. The Steam and Condensate Loop7.1.10 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 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
  13. 13. The Steam and Condensate Loop 7.1.11 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 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
  14. 14. The Steam and Condensate Loop7.1.12 Self-acting Temperature Controls Module 7.1Control Hardware: Self-acting ActuationBlock 7 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
  15. 15. 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 Temperature Control Valves and Systems SC-GCM-62CMIssue2©Copyright2005Spirax-SarcoLimited
  16. 16. The Steam and Condensate Loop7.2.2 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical 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 Set temperature knob Flow Flow
  17. 17. The Steam and Condensate Loop 7.2.3 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems Sensor Set temperature knob 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
  18. 18. The Steam and Condensate Loop7.2.4 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems 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
  19. 19. The Steam and Condensate Loop 7.2.5 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems 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
  20. 20. The Steam and Condensate Loop7.2.6 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems 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
  21. 21. The Steam and Condensate Loop 7.2.7 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems 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
  22. 22. The Steam and Condensate Loop7.2.8 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems 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.
  23. 23. The Steam and Condensate Loop 7.2.9 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems 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.
  24. 24. The Steam and Condensate Loop7.2.10 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems 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.
  25. 25. The Steam and Condensate Loop 7.2.11 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems 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
  26. 26. The Steam and Condensate Loop7.2.12 Control Hardware: Self-acting ActuationBlock 7 Module 7.2Typical Self-acting Temperature Control Valves and Systems
  27. 27. The Steam and Condensate Loop 7.3.1 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications Module 7.3 Self-acting Pressure Controls and Applications SC-GCM-63CMIssue2©Copyright2005Spirax-SarcoLimited
  28. 28. The Steam and Condensate Loop7.3.2 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-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’.
  29. 29. The Steam and Condensate Loop 7.3.3 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications 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
  30. 30. The Steam and Condensate Loop7.3.4 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications 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
  31. 31. The Steam and Condensate Loop 7.3.5 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications 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.
  32. 32. The Steam and Condensate Loop7.3.6 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications 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
  33. 33. The Steam and Condensate Loop 7.3.7 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications 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
  34. 34. The Steam and Condensate Loop7.3.8 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications 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
  35. 35. The Steam and Condensate Loop 7.3.9 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications 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
  36. 36. The Steam and Condensate Loop7.3.10 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications 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
  37. 37. The Steam and Condensate Loop 7.3.11 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications Questions 1. In a self-acting pressure control system, which of the following is proportional to the control valve opening? a| The deviation of the downstream pressure from the set point ¨ b| The difference between upstream and downstream pressure ¨ c| The difference between upstream pressure and the set point ¨ d| The spring force ¨ 2. What is ‘proportional offset’? a| The rise in downstream pressure as flow increases through the control valve ¨ b| The fall in downstream pressure as flow decreases through the control valve ¨ c| The difference between the set point and actual downstream pressure ¨ d| The rise in upstream pressure when the control valve shuts ¨ 3. Name an advantage that a pilot-operated pressure reducing valve has over a direct acting pressure reducing valve? a| It is usually smaller for the same capacity ¨ b| It has a much lower proportional offset ¨ c| It is more accurate over large changes in load ¨ d| All of the above ¨ 4. What is the basic difference between a PRV and a PMV? a| A PRV reduces pressure and a PMV increases pressure ¨ b| As downstream pressure drops, a PRV will close and a PMV will open ¨ c| As the sensed pressure drops, a PRV will open and a PMV will close ¨ d| As upstream pressure drops, a PRV will close and a PMV will open ¨ 5. What can a PMV be used for? a| To reduce non-essential loads, maintaining steam distribution pressure ¨ b| To maintain boiler pressure under overload conditions ¨ c| To exhaust surplus steam from a flash steam system ¨ d| All of the above ¨ 6. Which of the following can a PMV not be used as? a| A safety valve ¨ b| A pressure maintaining valve ¨ c| A pressure surplussing valve ¨ d| A pressure dump valve ¨ 1:a,2:c,3:d,4:c,5:d,6:a Answers
  38. 38. The Steam and Condensate Loop7.3.12 Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications

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