INTRODUCTION Fired equipment such as Heaters and Boilers, normally have to comply with either NFPA 85/86 or API 556 for North America and some other countries which apply such standards, or EN 746-2 for Europe including other countries which adopt the European standards, or SANS 329 for South Africa, which was originally the same as EN 746-2. Although SANS 329 specifically deals with the installation equipment and the burner management system (BMS) safety standards required to ensure safe start-up, operation and emergency shutdown (ESD), stable firing control is required to maintain operational safety as well as operational efficiency. To maintain the desired operating load, whether it is the process outlet temperature of a Fired Heater or the steam pressure of a Boiler, the fuel supply has to be controlled together with combustion air in the correct fuel/air ratio to achieve the optimal required heat energy input over varying process loads. A major concern is to maintain a safe and stable flame during start-up and at much reduced operating load, this will to a certain extent depend on the number of burners used in the basic design and if these burners are individually fired or have group firing design on larger heaters where 2 or 3 burners share a common fuel supply block valve. Historically, there have often been problems in starting burners especially when the furnace is cold, with operators having to resort to their own best methods to achieve a successful light-off, such as cracking open hand valves to control burner fuel pressure or venting fuel gas to achieve stable header pressure until a number of burners have been lit. The primary culprit for this is the fuel gas control valve turndown rangeability. This is why it is necessary to apply a short time delay on the fuel gas header pressure trips due to the pressure fluctuations on start-up, refer to the last section on Fuel Gas Pressure Override for further discussion on this subject. The modern burners used today are low Nox burners to comply with the environmental regulations and are hence very efficient with a low turndown ratio of typically 4% or 25:1 on pressure and 20% or 5:1 on heat release, the difference being due to the non-linear combustion curve, see below. Typical Gas Burner Curve Pressure (Bar) From the burner curve giving heat release in GJ/hr, one can determine the required fuel pressure required from minimum firing to maximum rated load per burner at the lower heating value (LHV), which will also vary according to the molecular weight (MW) of the fuel gas. One can also determine the required flow from the equivalent MJ/nm³ of the gas used, again being dependant on the MW. This can present a gas pressure control problem especially during start-up if the fired equipment has Page 1 of 8
many burners as indicated in the following example. If we had a gas burner requiring a minimum flow of say 45nm³/hr and a maximum flow of 250nm³/hr, and we have say 18 burners which are grouped in pairs (9 groups or sets), then we have a required gas flow turndown requirement for start-up of 90/4500 = 2% or 50:1 when lighting the first burner group, and when all burners are lit on minimum firing this gives a turndown of 810/4500 = 18% or 5.5:1. With this example, the gas control valve for start-up of the first burner group therefore requires a turndown or rangeability of 50:1, which may be just manageable with a globe valve but would be better with using a rotary plug, such as the Camflex type or a V ported ball valve which can give a rangeability of up to 200:1. However, one must remember that a control valve has to be selected (sized) to cover the maximum required fuel flow plus about 20%, and in practice may be much more depending on the standard control valve selection Cv sizing steps, so in the example given and assuming that a plus 20% Cv sizing is available, the actual control valve rangeability (selected valve Cv/minimum controlled Cv required, the definition requires maintaining the plug characteristic within 25% at constant DP), for start-up will be 90/5400 = 1.6% or 60:1. If we had individual burner firing instead of pair grouping in above example, the rangeability would be 45/5400 = 0.8% or 120:1. So, the rule with this type of control application using many burners is to first try and group burners to minimise the initial start-up gas flow turndown requirement, which also has a good economic benefit by reducing the number of burner automatic block valves required with associated BMS I/O, including installation and maintenance costs, and second, do not oversize the fuel gas control valve (which has to be sized for the lowest gas MW). The following will discuss the various pressure control options for fuel gas firing specifically in maintaining minimum safe firing conditions for start-up or when operating on much reduced heater load. The above issues do not apply with pilot gas due to low/constant flow, but are in principle also applicable to fuel oil firing, however, the turndown on fuel oil burners is not as high as with gas burners, but also being more complex due to steam atomization requirements. FUEL GAS CONTROL OPTIONS One must consider the required Force Manual Closed (FMC) actions of the fuel gas controller output to the control valves on closure of the main gas safety Double Block (DB) valves and then the reopening. When any ESD action or main isolation block valve is tripped closed, it is always good practice to automatically close the associated control valve in series with the ESD block valve/s, in case the ESD trip valve fails to operate, it also puts the control valve in the correct position for startup. The fuel gas pressure or flow controller FMC feed forward action is usually activated via a 5sec. pulse on closure command of the main fuel gas isolation DB valves. A closed controller output status (0%), or minimum firing interlock limit switch (optional) on the fuel gas control valve must be within this limit to enable the opening (Reset) of the main fuel gas isolation DB valves, this then ensures a safe minimum gas header firing pressure (once the first burner block valve is opened), this interlock is applicable to all fuel gas control options shown below. On opening of the fuel gas isolation DB valves, this action can also force the fuel gas controller into automatic control with the safe minimum firing Set Point (SP), this would be recommended with Option 2 on split range control. Option 1 - Single Control Valve Using a single control valve as shown in Figure 1, has the lowest installed cost and is applicable where the overall control valve required rangeability for lighting the first burner or group of burners is above 2% or 50:1 (could go lower depending on selected valve type and size), even so, one may have to crack open the first 1 or 2 burner manual hand valves until the gas header pressure has stabilised. A minimum controller output limit is required in Automatic only, to prevent the gas pressure control valve closing fully so as to maintain a minimum safe burner gas pressure. As an option, this can also be achieved by using a fuel gas minimum safe pressure constraint override high signal selector as shown (FY), and is used when the fuel gas is on flow control which is preferred, because if one burner is removed from duty during operation, the others will immediately Page 2 of 8
compensate to maintain same desired gas flow (heat load), as opposed to pressure control which will maintain the same burner pressure until increased by the outlet temperature falling. Also, as with Option 2 on split range control, the control valve can be fully closed (FMC) on any trip action. Figure 1 Option 2 - Split Range Control Valves This configuration shown in Figure 2 is used where the overall control valve rangeability required is below 2% or 50:1, i.e. on larger heaters using multiple burners or burner groups, this is the preferred design for high turndown applications and normally incorporates the flow control scheme as shown, which can be used for fuel/air ratio combustion control and more complex feed forward load control. This option also includes a minimum and maximum firing fuel gas header pressure override constraint, which can also be used to start-up the heater on pressure control only if required, and then switch the FIC into automatic for flow control once the heater is at higher load. These pressure override constraints are required to prevent the flow controller fully closing or fully opening the control valve under certain process upset conditions or by the number of burners in operation, so minimise probable fuel gas pressure trips. To enable this function, two pressure controllers are required which their control set points (SP) at 4-5% before the pressure trip settings. PIC A set point is set at just above the safe minimum low pressure for burner firing and should the FIC try to close the control valve below the minimum safe firing pressure, PIC A will take over control via the high signal selector FY A and the switch (SW) set in the S(1) position by the Set/Reset Latch being set to logic 1. PIC B set point is set at just below the maximum safe firing pressure and should the FIC attempt to open the control valve beyond this maximum safe firing pressure, PIC B will take over control via the low signal selector FY B and the switch (SW) set in the R(0) position by the Set/Reset Latch being reset to logic 0. It is only necessary to have this maximum function on the large control valve FV B as the smaller valve FV A will already be fully open. In order not to confuse the operator, PIC B is not normally displayed on the DCS. Once either extreme constraint limit is reached, an information alarm is displayed on the DCS. To enable smooth bumpless changeover between the three controllers, the output signals to the control valves are feed back to controllers to prevent integral action wind-up. The smaller Cv control valve (FV-A) will be used for start-up on minimum firing pressure SP (or on much reduced load), and when fully open, the larger control valve (FV-B) will start to open providing the capacity for increased firing to full load. The Cv ratio between the small and large valve will typically be about 5:1 or 4:1, i.e. the small valve having 20-25% capacity of the larger valve (do not forget to subtract the A valve flow capacity from the B valve for sizing B ). The controller output from 4mA (0%) to 8mA (25%) will then fully open the smaller valve and the larger valve will then start to open above 8mA. In practice the controllers will have 2 outputs providing 4mA to 8mA converted to an external output of 4mA to 20mA to fully stroke the small valve (FV-A), and 8mA to Page 3 of 8
20mA is then converted to an external output of 4mA to 20mA to fully stroke the larger valve (FV-B). Refer to section on DCS Graphics Display for more operator information with this option. Figure 2 Option 3 - Control Valve with Self Regulating Valve This configuration is shown in Figure 3, which is basically the same principle as Option 2, but with a self regulating pressure control valve (PCV) replacing the smaller split range control valve. The installed cost is lower than Option 2, the PCV is set to cover the start-up minimum firing gas pressure and providing the flow capacity to cover all burner minimum firing flow requirements. Once the control valve takes over increasing gas pressure above that set on PCV, the PCV will close, so the control valve in this case requires to be sized for full burner flow. Figure 3 Page 4 of 8
Option 4 - Control Valve with Restriction Orifice This configuration is shown in Figure 4, and is similar to Option 3, but with a control valve bypass restriction orifice (RO) for maintaining minimum flow requirements should the control valve be fully closed or fail closed under normal operation. This has the lowest installed cost, however, it is only practical to use with a reasonable steady supply gas pressure and also when using a fixed number of burners at the specified flow at minimum firing pressure. This can create a problem during startup as the gas header pressure will initially be at the gas supply pressure until a number of burners are lit, and thus will not reach the minimum firing pressure until most of the burners are lit, thus is not normally used on installations with many burners. In practice during start-up, the RO downstream isolation hand valve may be cracked open to control downstream pressure and gradually opened fully as more burners are lit. Figure 4 It should be remembered that with all the above options, the pilot gas has to be lit for any associated fuel gas burner block valve to be opened, and is interlocked such that if the pilot gas flame fails the associated fuel gas burner block valve will also close. Where a number of burners are grouped for cost savings (usually to a maximum of 3 due to common burner block valve failure), in the application of grouping burners (which will also apply to the pilot gas), it should be remembered that if the group automatic block valve trips or fails, then all associated fuel burners will be extinguished, this needs to be considered should the remaining burners be required to pick-up and maintain the full load, and may be a practical constraint in the number of burners used in a group. Should a single burner pilot flame fail in a group of burners, it is not necessary to trip the common group block valve, provided that the gas pressure is within the trip settings, as it will most probably be the failure of a pilot flame ionization rod or electronics and not the actual loss of the pilot flame. DCS GRAPHICS DISPLAY It is important that the fuel gas controls and burner operation have good ergonomic operator graphic displays, specifically for start-up or when on much reduced load, see Figure 5 for a typical DCS control group display which is shown for the split-range Option 2. The trip transmitter indications with trip settings from the BMS PLC (normally transferred via the BMS safety PLC redundant software communications link), must be displayed adjacent to their respective process DCS indicators or controllers, both should be of the same range so that a transmitter signal comparison discrepancy fault alarm can be raised. The BMS trip pressure transmitter indication which may be a voted input is required as a back-up and check display for the operator should the DCS transmitter Page 5 of 8
fail, where the operator would then in this case, control the process fuel gas pressure with the controller switched into Manual and observing the BMS trip transmitter pressure indication. Also, having this BMS trip indicator display on the DCS is also useful when performing trip transmitter testing, where voting is used, e.g. 1oo2 or 2oo3, these can be displayed on a 2 or 3 point DCS indicator. It is often required due to the low burner pressure turndown during start-up or when on low load, to have a lower trip transmitter range for the low gas pressure trip, e.g. 0 to 1bar, than for the high gas pressure trip transmitter which may be 0 to 3bar. The low gas pressure trip which may be set at 0.06bar, would only be 2% of a 0 to 3bar range, which is deemed too close to zero, and using a 0 to 1bar transmitter raises this trip setting to 6% of range, which is more manageable. Displaying this low range trip pressure is also useful during start-up on minimum firing, where the burner pressure required is very low and gives a better pressure measurement resolution than the normal DCS pressure controller range of 0 to 3bar. During initial start-up, the gas pressure controller will normally be on Manual control (except possibly for the split range Option 2) and as more burners are lit, the controller output is gradually increased to maintain the gas header minimum firing pressure until all or most burners are lit, and may then be switched into Automatic to slowly open the control valve to maintain the required gas header firing pressure. When all burners are lit and increased firing is required, the pressure controller is then switching to Remote TIC SP to raise the firing load as the process flow is increased, or as with Option 2, the flow controller is switched into Automatic and switched to Remote TIC SP. The main advantage with the first 3 Options is that as more burners are lit, the minimum gas pressure will be self regulating. With Option 2 using a split range control valve configuration, due to the opening of the smaller (Cv) valve A first, it is useful to include separate fuel gas control valve output indications for valve A and valve B, which also helps in test stroking of the individual valves as shown in Figure 5. Figure 5 Typical DCS Group Display for Fuel Gas Control (Split-Range) Figure 6 shows a typical dynamic fired heater burner graphic display, where the control room operator can readily see the status of the operating burners. The control room operator is in close communication with the field operator at the local control panel (LCP) during start-up, the LCP contains the same burner status information plus the block valve operating pushbuttons, including the block valve Leak Test (Valve Proving System), and Purge Cycle controls. Other graphics will display the BMS detailed start-up sequence with trip interlocks and the heater combustion control loops together with the process variables. The burners on large heaters require to be started in a specified sequence in order to ensure even heat distribution for thermal expansion. Burner heat release calculations and heater efficiency curves also help the operator to maintain optimum Page 6 of 8
performance. A Manual hardwired ESD pushbutton will also be installed on the control room operator s desk. Figure 6 Typical Fired Heater Burner Graphics FUEL GAS PRESSURE OVERRIDE Fuel gas pressure trip override has been touched upon in the main sections above; the following is a more detailed review of the problem and solution. It has been fully described in the introduction about the problems during start-up due to controlling low fuel gas pressure when lighting the first few burners and the turn-down or rangeability of the fuel gas control valve. The real issue which causes most operator problems during start-up is the tripping of the fuel gas block valves due to fuel gas header instability and reaching the high or low fuel gas pressure trip setting due to fluctuations in the fuel gas header while trying to light the first few burners. If the fuel gas control valve does not have the turndown rangeability to light the first few burners, then the fuel gas header may reach the fuel gas supply pressure. A number of solutions can minimise this problem, one is to open the fuel gas header vent valve (normally a 1 ) to a safe location until a few burners are lit, but this wastes fuel and may be in contravention of the environmental regulations. The other option is to override the fuel gas pressure trip which is mostly used today. The override can be manually initiated, but this has the disadvantage of the operator remembering to remove the override as soon as possible, Page 7 of 8
although a jog reminder alarm can be used, so the preferred method is to automatically initiate and remove the override via the BMS logic at the optimum time, this action then becomes transparent to the operator during the critical start-up period. There is also a permanent short time delay on the gas pressure trips of 3-4 seconds for normal operating transients, this should not be confused with the trip start-up operational override. It should be noted that a trip override in all cases is applied once a trip or shutdown has occurred, plus 30 seconds to allow for trip action and alarm display/logging, and it is only the override timer initiation that starts with opening of valves as described below. This has to be the case or one would not be able to Reset a trip group for startup, as all trip inputs have to be in the non-trip alarm state (healthy) or overridden, the operator is continually informed of the trip override status. Initialisation of the fuel gas pressure trip overrides (normally both high and low), can be performed when the main fuel gas isolation DB valves are opened, this then sets a short override timer of say 2 minutes in which 1 or 2 burner block valves must be opened to create a gas firing flow to keep the gas pressure within its lower and upper trip settings. Prior to opening of the main fuel gas isolation DB valves, the fuel gas control valve must be set to minimum firing position to provide a gas flow restriction to limit the fuel gas header pressurisation rate. This minimum firing gas control valve position is normally interlocked with the main fuel gas isolation DB valves, and once the DB valves are opened the interlock is removed. However, this can still put some urgency and stress on the operator to get the first burner block valve opened in time before the gas pressure trip override is automatically removed. A preferred alternative is to only apply the fuel gas pressure trip override once the first burner block valve is opened thus moving it one operating sequence step, and then apply a shorter trip override time of 20-30 seconds. In this case, in order that the fuel gas header is not sitting at a high pressure for too long a period, a timer is set between opening of the main fuel gas isolation DB valves and opening of the first burner block valve, this time is set at 5-10 minutes, so although there is not as much urgency to open the first burner block valve, the operator must still act within a reasonable safety time. The first few burner block valves should be opened as quickly as possible while checking for a good burner flame to create a gas flow to stabilise the gas header pressure, bearing in mind that there is a fixed time delay of 15-20 seconds between successive fuel gas burner light-offs. These preliminary timer settings are re-adjusted during heater commissioning to optimise operator reaction time during burner start-up. This then raises a further problem of opening an automatic burner block valve with a possible high gas header pressure which causes difficulty in lighting the burner due to fuel gas high velocity and could also blow out the pilot. This is minimised with the local operator being present during start-up and there are 2 options available, the first is to crack open the first few burner manual hand valves to say ±15% prior to opening the associated automatic block valves to restrict gas flow due to the additional pressure drop, until the gas header is at normal safe firing pressure. The second and preferred option is to limit the opening speed of the burner automatic block valve to ±20 seconds by a pneumatic restriction in the opening direction when energising the block valve trip solenoid, the required trip action closure time is not affected. This has the advantage of no additional local operator requirements during start-up and also minimises gas header pressure bumps when opening any burner at any time during operation, especially when firing burner groups. In addition to above, the gas header should always be depressurised to lower than 5kPa on a fuel gas shutdown, either by burning off the gas into the furnace (preferred for normal shutdown), or by header venting (emergency shutdown). This then increases the time for the gas header to pressurise via the minimum firing position of the fuel gas control valve when the main fuel gas isolation DB valves are opened, thus providing more time for the operator to light the first few burners. End of Technical Paper Page 8 of 8