Pneumatic Control Systems
What are the advantages and disadvantages of both electrical and pneumatic control systems on ships ?
Electrical Control System
Advantages
No air compressor and associated machinery required.
High efficiency since there is no leakages.
Instantaneous response.
Very little affected by normal temperature variations.
Very accurate.
Electric cables are cheap and easy to lay.
Disadvantages
The system require uninterrupted power supply with no voltage fluctuations.
Standby batteries required in case of power failure.
There is risk of fire due to overheating. Expensive intrinsically safe / explosion proof equipment may be required.
Moisture may cause damage to the system.
Damage readily occurs in the event of a fire.
Pneumatic Control System
Advantages
Not affected by ship’s power supply variations for short term.
No heat generated and hence no risk of fire.
Actuators are cheaper and accurate than electric systems.
Simple control air piping system.
Disadvantages
Require air compressor and associated systems.
A clean, dry and oil free supply of good quality air is essential for satisfactory operation of the system.
Good quality air require driers, filters with drains, etc. which increases maintenance.
May have transmission lags in large systems.
What is meant by ‘fail safe’ and ‘fail set’ in a pneumatic control systems ?
On failure of control air supply, the pneumatic actuator may be arranged to move to a position that allows the plant to continue to operate safely, in other words, fully opened or fully closed. This is known as fail safe. For example, in the case of a jacket cooling water system, on failure of control air, the actuator opens fully to allow jacket water to the cooler without bypass. On the other hand a fuel oil control valve for boiler closes completely on such a failure. This ensures safety of the plant.
In some other systems, control air supply failure locks the position of valve at that time of failure. This is called fail set. The advantage of this system is that the plant gets stable and have time for normal shutdown or can wait for reestablishment of control air supply for some time. Example for such a system is boiler water level control.
Explain two step control action, ratio control and proportional plus integral control with applications on ships ?
Two step control action is the simplest form of control, in that it assumes one or two preset positions, a switch is ON or OFF or in the case of a valve it is OPEN or CLOSE. For example, a pressure switch controlling the starting air compressor is an acceptable form of control since there can be fairly large deviation between measured value and desired value. Another example is hydrophore pumps which cut in and cut off with the hydrophore pressure.
Ratio control is one in which one variable is kept at a fixed ratio to another variable. Ratio control is found in boiler combustion control system where the ratio between the air-fuel must be controlled to ensure good combustion. The air flow is controlled by dampers to match the fuel flow.
In a proportional plus integral controller, the proportional element acts if there is any deviation between the output (measured value) and the desired value, but the action results in a permanent difference between the measured and desired value (called offset). The integral element removes this difference or offset. For more details on the basics of proportional, integral and derivative control, please click on the links below.
Proportional, Integral and Derivative actions basic principles
Application of the proportional-integral control is the water level control system for boiler on ships, which maintains desired water level always without offset.
What is the effect of a leaking proportional bellows or measurement bellows in pneumatic control systems ?
When proportional bellows start leaking, its ability to introduce the negative feedback is reduced. This results in increased gain of the controller and control valve starts moving to the extreme positions causing instability.
When measurement bellows leak, it prevent the flapper from moving towards the nozzle, and the desired or set value is changed, again causing hunting of the system.
Faults in Alarm Monitoring and Control System
The alarm monitoring and control system on the ship may give false alarms and incorrect data printouts. These faults could be:
Localized to a particular area of engine operation or
General to the engine room
When the alarms are specific to a certain area of the engine room it would be suspected that the following faults could be present
Cable fault: If the cable connecting the sensors with the control room were damaged, the resulting short/open circuits could generate false alarms
Control card/panel fault: The alarms could be grouped within a certain alarm or monitoring panel, and there could be a defect within this panel.
External interference: Machinery could be operating in the suspect area, which is poorly screened, and the resulting interference could be transmitted via the instrument cables to the monitoring panels.
When the alarms are general to the whole engine room, then the main supply to the alarm and monitoring panel would be checked for:
Earth faults: A combination of earth faults could affect the earth potential and hence the reading of the instrumentation.
Supply voltage level: The monitoring circuit would use low 24V supply, and this would need to be at the correct level without any voltage ripple present.
The following action should be taken to ensure continued safe operation of the vessel if the defects with alarm monitoring and control system were general to the engine room.
The problem with intermittent alarms and incorrect printouts would be that the engineer on duty would be unable to rely on the information given to him by the remote instrumentation and alarm panel. Hence a greater use would need to be taken of the local readings from pressure gauges and thermometers.
This would mean that the UMS operation would be stopped and watch-keeping practices with a manned engine room would be commenced. The watch keepers would be instructed to complete a full manual log of the various operational parameters of pressure, temperature and the various tank levels. This manual log would be taken every watch period of four hours.
The engineering staff would be divided between the various watch duties, ensuring that all watch keepers were certified and had the appropriate duty and rest periods assigned to them.
Any machinery units requiring manual control would have their operation explained to all engineers, and a procedural checklist compiled for the operation of all such machinery.
Troubleshooting Alarm Monitoring and Control System
The possible faults stated earlier would be the earth fault or supply irregularity. The earth fault could be identified by observing the 24V distribution panel that would have an earth detection unit fitted. If an earth fault was indicated on this panel then
A positive earth would be traced by disconnecting the supply fuses in turn to parts of the engine room to identify the problem area
A negative earth would require that each instrument have its earth wire disconnected and left disconnected until the fault is traced. This would mean the loss of many of the instruments within the engine room and could only be carried out when watch keeping duties were active. Each wire would be marked to ensure correct replacement.
The supply irregularity could be identified by using an oscilloscope, which would show both the level of voltage present and if any supply ripple was present. A simple AVO meter would also indicate correct voltage levels. The earth fault would be rectified by locating the cable/insulation defect and then replacing the cable, or repairing the defect by re-insulating the damaged area. The supply irregularity would be rectified by replacing the charging unit of the 24V DC supply, and/or replacing any defective batteries.
Automatic controllers are used onboard ships for the adjustment of one or more parameters in a system. Function of the controller is to maintain the parameter as per desired value (value set by the operator). The parameter could be jacket water temperature (for engine jacket water cooling system), lubricating oil temperature (for engine lube oil system), fuel oil pressure (for boiler fuel oil system), etc. PID controls are commonly used for these applications. Advantage of automatic indicating controller is that it controls as well as indicates the parameter reading. Refer to the diagram and explanation below to understand the basic principle of PID controllers. These controllers are normally located near the variable measuring point so that use of a transmitter (to transmit parameter signals to the controller) is not required.
On-Off or Two Step Action
This is the simplest form of control. Here only two control positions are allowed; on or off. In the case of a valve, it opens or closes by the signal from the controller. Examples for such control system are; water level control for boiler cascade tank, air compressor cut in cut off control, temperature control for fuel oil storage tanks, etc. Let us consider the case of a fuel oil tank heating steam two step controller. It consists of a controller that continuously senses the system parameter (fuel oil temperature). This is called measured value. The desired value or set point (temperature of fuel oil to be maintained) is fixed by the operator. When the temperature of the fuel oil drops below set point, controller sent signal to the steam valve thereby passing steam through coils provided inside the fuel oil tank. Now temperature of fuel oil starts increasing. When the temperature rises to the set point, controller shuts the steam valve. The system response is illustrated down below.
Now as we know there is a thermal inertia in this system. Or in other words the position of temperature sensor and that of steam coil is different. That is why we cannot observe an increase in temperature as soon as the steam valve opens. The same phenomenon explains why temperature is increasing even after the steam valve is shut. Anyway disadvantage of this control is the large deviation from the desired value or set point. Hence the application of such control is limited.
Proportional Action
This is a continuous control action in which the controller output is proportional to the deviation between measured value and desired value. Let us take an example of a simple water lever control as shown in the figure below.
Consider a water tank with an outlet valve, a water supply valve, control lever on a pivot, control strings, and float as shown in the figure. One end of the control lever is connected to the float while other end to the water supply valve. The system is designed in such a way that, when float goes down supply valve is opened more thereby increasing water supply to the tank. Similarly an elevation of the float results in closure of the supply valve. At this point quantity of water supplied and flown out of the tank is same or the system is in equilibrium. Suddenly demand of water increases or the discharge valve is opened more. This result in reduction in water level inside the tank and causes float to lower. As float lowers it raises the control string connected to the water supply valve, thereby increasing the supply of water.
As shown in the figure above, a new equilibrium is attained by the system. Once again supply and demand of water is same. But we can see a drop in water level from the desired value. This deviation from the measured value and desired value is known as offset. It is an inherent property of the proportional control. Offset can be reduced but cannot be eliminated in such systems. The amount by which the input signal value must change to move the correcting unit between its extreme positions is known as proportional band. This concept can be made clear from the figures shown above. Move the position of the pivot towards right (closer towards the float). Now we can see that a small change in water level causes an amplified effect in opening or closing the supply valve. In other words the system becomes more sensitive. Also here the offset is reduced comparing to the scenario before. Similarly, moving the pivot towards the left cause minor changes for the supply valve even though float lowers or rises drastically. Here sensitivity of the system is less. It is clear that offset can be reduced to minimum when sensitivity is higher. But this results in hunting of the system. Hunting means excessive fluctuation of the measured value around desired value. System response to proportional action is shown below.
So far we have discussed proportional action with reference to a simple float and valve system. Same can be explained using a flapper-nozzle mechanism too. In fact such an example is much closer to the actual control system.
Referring to the diagram above, as the measured value deviates from set point, flapper moves closer to the nozzle. This results in an increase in output air pressure. This increase in output air pressure changes the controlled condition (parameter to be controlled), say by closing or opening a valve. At the same time the increased output air pressure act on the feedback bellows also. This negative feedback pushes the flapper away from the nozzle, thereby reducing output air pressure. Now the system is in equilibrium. Remember that an offset is inevitable here. Moving the nozzle away from the feedback bellows increase sensitivity and hunting of the system. Also moving nozzle towards feedback bellows reduces hunting and increases offset. So position of the nozzle (or Proportional Band) can be adjusted carefully to obtain a stable system with minimal offset and hunting.
Integral or Reset Action
Integral action or reset action is used in conjunction with proportional action, to remove offset from the system. Here controller output varies at a rate proportional to the deviation between measured and desired value. Refer to the figure below.
Here in addition to the feedback bellows, we have integral action bellows also. Consider the integral action valve is open. Now as the flapper moves closer to the nozzle because of a deviation, output air pressure increases, the same air pressure acts on feedback bellows to move the flapper away from nozzle to an equilibrium position. Now there is an offset. Again the same control air pressure acting on the integral bellows moves the flapper towards the nozzle to increase control air pressure. Hence a new equilibrium is attained with no offset. Note that all these actions take place simultaneously. When the integral action valve is fully open, integral action will be too fast that result in hunting. When the valve is crack open, there is no hunting but it takes long time to remove offset. In other words time required to remove the offset (Reset Time) can be adjusted by opening or closing integral action valve. Closure of the integral action valve means no integral action. Reset time to be set in such a way that system operates fast to remove offset and minimal hunting. All pressure control systems onboard ships are normally proportional-integral (PI) controls.
Derivative or Rate Action
Derivative action is utilized along with proportional and integral actions. This is applied in systems where time delay between changes in measured value and their correction is long. Example is temperature control for jacket water system. Here controller output is proportional to the rate of change of deviation. Refer to the figure below.
Here a derivative action valve is introduced as shown in the figure. Closing this valve any amount would introduce derivative action in the system. Consider the derivative action valve is closed 70%. When flapper moves towards nozzle because of a deviation, controller output pressure increases, the same pressure acts on the feedback bellows through the derivative action valve. As the valve is only little open, it takes time to move the flapper away from the nozzle by negative feedback, thereby allowing control air output pressure to be higher for long time, which again allow more time for corrective action. Also derivative action can be varied by adjusting opening or closing derivative action valve. When derivative action valve is fully open, there is no derivative action.
Troubles
Causes
Counter-measures
[PID CONTROLLERS]
Output air pressure of pid controllers do not increase or decrease in spite of the change of detection input.
When it does not increase.
Supply air pressure is abnormal.
Orifice of pilot relay is clogged.
Leakage at the air piping at output side (due to cracking), or leakage at soldered joint or damage of diaphragm of control valve.
Inferior connection or disconnection of link systems inside the controller.
Leakage of various piping or tubes inside the controller.
Damage of element at measuring part.
Adjust it into normal conditions.
Press the cleaning push button.
Adjust it into normal conditions.
Adjust it into normal conditions.
Replace the tube
Replace the element.
When it does not decrease.
Nozzle at the flapper is clogged.
Clogging of piping from pilot relay to piping from down of tube
Loosening of pilot relay orifice
Inferior connection or disconnection of link mechanism inside the controller.
Leakage of various piping or tubes inside the controller.
Clean it with a wire of less than 0.4mm
Adjust it into normal conditions.
Retighten it
Adjust it into normal conditions.
Replace the tube
[CONTROL VALVE]
Control valve life does not change due to the increase or decrease of output pressure of controller.
Sticking of valve guide and valve stem.
When positioner is provided sticking of positioner pilot.
Disconnection of diaphragm stem and positioner.
Inspection by disassembly.When foreign matters are caught into the parts, remove them and repair any possible damage.
Repair the damage caused by sticking using fine mesh grinding paper or file.
Adjust it into normal conditions.
[FILTER REGULATOR]
Pressure does not increase even when setting knob is turned to increase pressure. Or pressure increase is abnormal or pressure does not go down.
When pressure does not increase.
Damage of diaphragm.
Damage of setting spring.
Sticking of pilot valve.
Leakage at air piping at secondary side.
Replace the parts.
Replace the parts.
Repair the damage caused by sticking using fine mesh grinding paper or file.
Adjust it into normal conditions.
When pressure does not come down.
Sticking of pilot valve.
Clogging of exhaust hole of the cover.
Repair the damage caused by sticking using fine mesh grinding paper or file.
Clean it.
When the controller is installed properly, perform the following maintenance periodically according to a preset program. Proper maintenance of PID controllers extends their useful life.
Remove drain deposited in the air supply tube by loosening the drain plug at the bottom of the filter regulator.
Be care to maintain the supply air pressure always at 140kPa. When the performance of filter regulator is unstable, repair or replace it.
According to the conditions of supply air, inspect the orifice holes of nozzle or pilot relay.
Periodically press the orifice cleaning push button to clean the orifice.
When it is necessary to take out the orifice out of pilot relay, disassemble it according to the following procedures.
Stop air supply to the controller.
Carefully remove the entire orifice at the left end of the pilot relay by a spanner.
After taking off the orifice, clean the surface with thinner, etc. if oil or grease is sticking to it. Dry the disassembled orifice and then reassemble it.
When it is necessary to disassemble the clogged nozzle, follow the procedures below.
Carefully disassemble nozzle cleaning plug of proportional dial by a driver.
Clean the nozzle with a wire having thickness of less than 0.4mm.
Clean dusts, etc. deposited on the flapper surface (to which the nozzle is contacting).
PID Controller
Setting Knob
Set the setting pointer (red) to the desired graduation on the scale by setting knob.
Proportional Band Control Dial
As we have discussed before, shifting position of the nozzle with respect to flapper changes proportional band. This is achieved by turning proportional band dial. The smaller the value on the dial (i.e., Proportional band becomes narrower), it becomes sensitive. But if it becomes too sharp, hunting occurs. When the value on the dial becomes too dull, deviation of measured value from set value, in the case of load change, becomes excessive.
Reset Time Dial
Reset time dial actually controls opening and closing of integral action valve. Reducing reset time means opening the valve while increasing means closing the valve. When reset time is shortened, the time required for balance point is shortened. But when it becomes too short, stability is lost and it apt to cause hunting. When reset time is made too long, it consumes too long time before balance is established at the set value.
Rate Time Dial (This dial is not given in the case of proportional + integral action)
Rate time dial varies opening and closing of derivative action valve. It is used for the process which involves much time lag (delay). When rate time is made too long it causes hunting. Whereas if it is too short, satisfactory effect is not obtained.
Change-over of Direct-Reverse Action
Direction of operation of control valve is determined by the nature of the process. It is determined by whether the control valve is a direct operation or a reverse. For example, in the case of the figure shown above, it acts in reverse. To change the set up into direct action (i.e., the action where increase of input causes increase in output), turn the proportional dial to the right for ¾ turns.
Optimum Adjustment of the Controller
The optimum adjustment of the controller can be obtained by the following steps.
Set the reset time dial at max. (20 min.) and when there is a rate time dial, set it at minimum value (0.05min).
Starting from 250%, gradually reduce the proportional dial from 250% to 10%, while checking the result of adjustment. It causes hunting when it comes to a certain proportional zone. The optimum value is at 2-4 times of such proportional zone.
Gradually reduce the reset time by turning reset time dial. When it comes less than the marginal value, it causes hunting. So reverse it slightly from such point and then fix it.
Fix the rate time dial at about ¼ of the ultimate reset time or about ½ of the time delay of the process and control device. In this case, proportional zone can be made slightly smaller.
Proportional (P) Control Action
When measured value becomes higher than the set value (deviation), the upper end of proportional lever shifts to the right. Thus the flapper approaches to the nozzle and the back pressure of the nozzle, i.e., the pressure charged upon the pilot relay, increases. Consequently valve in pilot relay open and supply pressure flows into control side to increase the pressure. At the same time, this pressure is charged upon proportional bellows and lifts up proportional lever and thus flapper is detached from the nozzle and control pressure is set in proportion to such deflection. All of the above actions occur simultaneously in the actual operation. When both pointers overlaps (deviation is zero) control pressure becomes 60 kPa (20-100kPa).
Proportional-Integral (PI) Action
Assuming that the controller is acting properly and measured value and set value are in equilibrium, (or deviation is zero), and the same pressure as control pressure is sealed in the proportional bellows and reset bellows. If measured value becomes too high as in the above case, P action immediately takes place and control pressure increases. Thus control pressure flows into the reset bellows through reset throttle valve.
As the pressure inside reset bellows increases, proportional lever comes down and flapper approaches to the nozzle and back pressure increases. Consequently pilot relay valve opens to increase the control pressure and the increasing pressure inside proportional bellows lifts up the proportional lever and causes the nozzle to detach from the nozzle again. This resetting effect continues until control pressure increases to such extend that the control valve opening enables the reversion of the measured value to the set value (until deviation becomes zero). Pressure of proportional bellows and reset bellows thus balances and the original balance condition is established.
Proportional-Integral-Derivative (PID) Action
Rate throttle valve and bellows chamber are connected in parallel between the pilot relay and proportional bellows. In the aforesaid P and PI action, the inside pressure of proportional bellows is in proportion to the amount of deviation. Therefore, when the measured value changes, control pressure flows in or out with the speed corresponding to such changes, so that the pressure inside the proportional bellows will synchronize with the change of measured value. Since the pressure reduction taking place as it passes through the rate throttle valve is in proportion to the speed of fluctuation of measured value, pilot relay output, i.e., control pressure also become larger or smaller than the internal pressure of proportional bellows to the extent of the differential pressure at the throttle valve.
Rate bellows chamber is provided to transmit control pressure to the proportional bellows utilizing the volume change of rate bellows caused by its elasticity and to give stability to the system.
Therefore, when rate action is utilized, control valve opening can be adjusted more quickly and it certainly gives convenience especially to the process where time lag is great.
This controller can be applied to all possible fields of process control such as pressure, differential pressure, temperature, liquid level, flow rate, viscosity, etc. when used in combination with diaphragm control valves at the operating end. It is a pneumatic controller which can automatically regulate various process conditions at the optimum level.
It is constructed compactly and weights light, incorporating least possible number of parts. Its control, handling and maintenance is therefore simple and yet it has considerable durability. It is an instrument most suitable not only for fixing on graphic panel but also for local control, having usage in versatile fields.
Construction and Specification
Construction
Specifications
Control air pressure (output): 20-100kPa
Supply air pressure: 140kPa
Proportional band: 10-250%
Reset time: 20-0.1 Min.(std.)
Rate time: 10-0.05 Min.
Both direct and reverse actions are available
Accuracy: Within 1% of full scale
Method of fixing: Panel mount or Wall mount type
Casing: Drip-proof and dust proof type
Mass: 5-6 kg
Air consumption, Normally 1 Nl/min, Maximum 30 Nl/min
