MV Indirect Fired Heater

PRODUCTION OPERATORS - IV MANUAL INDIRECT FIRED HEATERS

UNION TEXAS PAKISTAN

BASIC GAS PRODUCTION EQUIPMENT AND PRINCIPLES OF OPERATION

MODULE PO-IV 5

INDIRECT FIRED HEATERS

BP PAKISTAN EXPLORATION & PRODUCTION, INC.

PAGE # [i]

REVISION 0


1 INTRODUCTION..................................................................................................................1 2 HEATER EQUIPMENT DESCRIPTION..........................................................................3 2.1 FIRETUBE........................................................................................................................3 2.2 MAIN BURNER...............................................................................................................3 2.3 PILOT BURNER..............................................................................................................5 2.4 STACK..............................................................................................................................5 2.5 FUEL CONTROL SYSTEM............................................................................................5 2.6 ACCESSORIES................................................................................................................5 2.7 STACK ARRESTOR........................................................................................................9 2.8 FUEL GAS SCRUBBER..................................................................................................9 2.9 FUEL GAS HEATING HEATING COIL...........................................................................................9 COIL..................................................................................... ......9 3 PRINCIPLES PRINCIPLES OF COMBUSTION COMBUSTION AND FIRING ........................................................ ................................................ ..........10 ..10 3.1 COMBUSTION..............................................................................................................10 3.2 THE TRANSFER TRANSFER OF HEAT..........................................................................................11 HEAT.................................................................................... ......11 3.3 THE MOVEMENT OF HEAT.......................................................................................13 3.4 SEQUENCE OF EVENTS.............................................................................................15 3.5 HOT SPOTS...................................................................................................................15 4 HEATER SYSTEM SYSTEM CONTROLS......................................................... CONTROLS................................................................................... .............................17 ...17 4.1 FUEL RATE TO BURNER............................................................................................17 4.2 AIR FLOW TO BURNER..............................................................................................19 5 HEATER PROCESS PROCESS OPERATIONS OPERATIONS ............................................................................. ............................................................ ...................21 ..21 5.1 START-UP......................................................................................................................21 5.2 SHUT DOWN.................................................................................................................21 5.3 ROUTINE OPERATOR CHECKS................................................................................21 6 HEATER SAFETY SYSTEMS..........................................................................................25 6.1 TYPICAL HEATER HEATER SAFETY SYSTEMS......................................................... SYSTEMS................................................................. ...........25 ...25 7 OPERATIONAL OPERATIONAL PROBLEM PROBLEM SOLVING.......................................................... SOLVING.................................................................. ............... .......27 27


PAGE # [ii]

REVISION 0


1 INTRODUCTION..................................................................................................................1 2 HEATER EQUIPMENT DESCRIPTION..........................................................................3 2.1 FIRETUBE........................................................................................................................3 2.2 MAIN BURNER...............................................................................................................3 2.3 PILOT BURNER..............................................................................................................5 2.4 STACK..............................................................................................................................5 2.5 FUEL CONTROL SYSTEM............................................................................................5 2.6 ACCESSORIES................................................................................................................5 2.7 STACK ARRESTOR........................................................................................................9 2.8 FUEL GAS SCRUBBER..................................................................................................9 2.9 FUEL GAS HEATING HEATING COIL...........................................................................................9 COIL..................................................................................... ......9 3 PRINCIPLES PRINCIPLES OF COMBUSTION COMBUSTION AND FIRING ........................................................ ................................................ ..........10 ..10 3.1 COMBUSTION..............................................................................................................10 3.2 THE TRANSFER TRANSFER OF HEAT..........................................................................................11 HEAT.................................................................................... ......11 3.3 THE MOVEMENT OF HEAT.......................................................................................13 3.4 SEQUENCE OF EVENTS.............................................................................................15 3.5 HOT SPOTS...................................................................................................................15 4 HEATER SYSTEM SYSTEM CONTROLS......................................................... CONTROLS................................................................................... .............................17 ...17 4.1 FUEL RATE TO BURNER............................................................................................17 4.2 AIR FLOW TO BURNER..............................................................................................19 5 HEATER PROCESS PROCESS OPERATIONS OPERATIONS ............................................................................. ............................................................ ...................21 ..21 5.1 START-UP......................................................................................................................21 5.2 SHUT DOWN.................................................................................................................21 5.3 ROUTINE OPERATOR CHECKS................................................................................21 6 HEATER SAFETY SYSTEMS..........................................................................................25 6.1 TYPICAL HEATER HEATER SAFETY SYSTEMS......................................................... SYSTEMS................................................................. ...........25 ...25 7 OPERATIONAL OPERATIONAL PROBLEM PROBLEM SOLVING.......................................................... SOLVING.................................................................. ............... .......27 27


PAGE # [ii]

REVISION 0


MODULE PO-IV 5 INDIRECT FIRED HEATERS

LEARNER OBJECTIVES On completion of this training module the learner will be able to: 1.

Descr Describe ibe the the appli applica catio tion n and and funct functio ion n of the the fou fourr indir indirect ect fired fired hea heate ters rs use used d of the BP. BP. gas production installations at Colarchi and Turk.

2.

State State the the fun functi ction on of of the the five five majo majorr items items of con constr struc uctio tion n and and acce accesso ssorie ries s in a typi typical cal indirect fired heater.

3.

Desc Descri ribe be and and dra draw w the the basi basic c cons constr truc ucti tion on of of an ind indir irec ectt fire fired d heat heater er..

4.

Expl Explai ain n the the thre three e basi basic c prin princi cipl ples es of combu combust stio ion n and and heat heater er firi firing ng as app applied lied to general gas field heater operations.

5.

Ident Identify ify a firet firetub ube e and and heate heaterr shell shell hot hot spo spot, t, stat state e the the caus causes es and and desc describ ribe e what what acti action on should be taken to prevent a dangerous situation.

6.

State State the two major major contro controll sys system tems s use used d in in firi firing ng an indire indirect ct fired fired heate heater. r.

7.

Desc Descri ribe be how how the the fuel fuel gas gas rat rate e to to the the burn burner er is gove govern rned ed..

8.

Draw Draw a typ typic ical al heat heater er cont contro roll sys syste tem m and and expl explai ain n how how it work works. s.

9.

Deter Determin mine e by calcu calculat latio ion n the the heat heat tran transfe sferr rate rate of of an indire indirect ct fire fire heate heaterr in Btu/hr Btu/hr..

10.

State State three three reaso reasons ns for for adjust adjusting ing the the air air flow flow to the burner burner in in an indirect indirect fired fired heater. heater.

11.

Describe the basic procedure for setting the primary and secondary air on an indirect fired heater to achieve the correct flame pattern and efficiency.


PAGE # [iii]

REVISION 0


12.

Sketch three simple diagrams to show the effect of fuel: air ratio on burner flame patterns.

13.

Describe the safe operating procedure to be employed if a flame failure occurs on an indirect fired heater.

14.

Describe the correct and safe heater start—up procedure in sequence.

15.

Describe the correct and safe heater shut down sequence.

16.

Itemize eight routine operator checks on an indirect fired heater.

17.

Draw and describe the five most common heater safety systems and devices.

18.

Identify the causes and explain the problem solving actions to be taken when a heater will not supply its rated heat output.

19.

Identify the causes and explain the problem solving actions to be taken when a firetube fails in an indirect fired heater.


PAGE # [iv]

REVISION 0


1 INTRODUCTION The indirect fired heater is one in which the liquid being heated is outside of the tube, and the fire is inside the tube. This type of heater is used at BP. field installations in the following applications:

Golarchi

-

Condensate heater treater. Glycol dehydration regenerator reboiler.

Turk

-

Condensate stabilizer reboiler heater. GlycoI dehydration regenerator reboiler.

Indirect fired heaters are usually used in remote production areas where steam or other forms of process heat is not available. One of the advantages of this type of heater is that they require little or no electrical power for operation.


PAGE # [1]

REVISION 0



PAGE # [2]

REVISION 0


2 HEATER EQUIPMENT DESCRIPTION An indirect fired heater is a pipe called a firetube with a burner inside it. Figure 5.01 shows the construction details of a heater with two firetubes and two burner assemblies. The complete unit is immersed in the liquid that it is- heating, e.g. glycol, condensate.

2.1

FIRETUBE

Most firetubes take the shape of a U-tube or hairpin. They can be mounted on a flange on the vessel in which it is inserted, and can be easily removed for repair or replacement. The firetube usually has brackets on the bottom or side to prevent it from touching the side or bottom of the vessel. Because the tube is totally immersed in liquid, the temperature of the tube will be approximately the same as the temperature of the liquid, even though the temperature of the combustion gases inside the tube may be over 3000 0F. If the tube were to touch the vessel, a hot spot could develop that may distort or melt the firetube and the vessel. If the liquid that surrounds the firetube is glycol or condensate a liner is usually installed inside the tube which acts as a protection against metal failure. The liner is made of a heat resistant metal such as monel or stainless steel. It functions to prevent the flame from striking the tube wall and causing it to overheat and fail, If the fire contacts the liner, it will eventually melt and require replacing, but the firetube will not be damaged if the liner is replaced quickly.

2.2

MAIN BURNER

The main burner in an indirect fired heater is designed to produce a long narrow flame pattern. It is centered in the tube. Examples of typical gas field burners are shown in Figures 5.03 and 5.04.

THE PARTS AND FUNCTIONS OF A BURNER 1. Fuel Orifice The orifice is a hole in the end of the fuel supply line. The hole is precision drilled so that an excessive amount of fuel cannot flow into the burner. The precise size of the hole will depend upon the allowable fuel rate, and also the heating or calor-ific value of the fuel. 2. Mixing Chamber The mixing chamber is designed so that fuel and air mix thoroughly before combustion. 3. Burner Tip The tip of the burner is precision drilled with holes that are sized and located so that the flame will travel in the middle of the firetube and not touch the walls. BP PAKISTAN EXPLORATION & PRODUCTION, INC.

PAGE # [3]

REVISION 0



PAGE # [4]

REVISION 0


4.

Primary Air Control Most of the air used in burning the fuel gas comes from the primary air control opening. The air is sucked into the burner as the fuel gas velocity increases when it flows through the fuel orifice.

5.

Secondary Air Control This device has the same function as the primary air control, and supplies additional combustion air to the burner. Smaller burners often use only primary air and have no secondary air control.

2.3

PILOT BURNER

The pilot burner is similar in construction to the main burner, but it is much smaller, It really serves as a safety device to ensure that the main burner stays alight. The pilot burner has no secondary air control, and on small heaters it often has a fixed primary air opening with no adjustment.

2.4

STACK

The stack is a section of pipe that fits over the outlet end of the firetube. Its height will vary from 6 to 20 ft., depending upon the location and duty. High stacks are used when leaking hydrocarbons may be present.

2.5

FUEL CONTROL SYSTEM

A diaphragm actuated control valve is used to regulate the flow of fuel gas to the burner. The control valve is actuated by a temperature controller which has its primary detecting element immersed in the liquid that surrounds the firetube, see Figure 5.05. The control system also includes a pressure reducing regulator valve on the supply fuel gas line to reduce the pressure to the maximum design pressure of the burner. Excessive fuel gas pressure can cause the burner to over—fire and burn out the firetube. Also included in this control system is a smaller line with a pressure reducing regulator valve to supply fuel gas to the pilot burner. This regulator valve is set to hold the correct flame pattern on the pilot burner. Shut down devices are also included in this system of control and will be described in detail in a separate section of this training module.

2.6

ACCESSORIES

Flame Ignition System. Larger heaters are equipped with a spark plug and high voltage coil that will light the pilot burner either manually or automatically. The system operates on 120/240 volts, but can be used with batteries.


PAGE # [5]

REVISION 0



PAGE # [6]

REVISION 0



PAGE # [7]

REVISION 0



PAGE # [8]

REVISION 0


Flame arrestor. The flame arrestor prevents a back—fire in the heater. Fire could come out of the burner end of the firetube for many reasons. The most dangerous situation would be caused by the ignition of hydrocarbon liquid or gas that was escaping from a leak near the heater, If the leaking liquid or gas were to ignite, the fire would travel back (flash—back) to. the source of the leak and may result in a major vessel or equipment fire or explosion. The flame arrestor is constructed of thin aluminum sheet that is wound in a spiral coil. It is designed so that if gas or vapour outside the unit enters and passes through the coil and ignites inside the burner, the flame will cool and extinguish as it moves back from the inside to the outside of the coil. The flame arrestor functions as a type of heat exchanger in which ambient air temperature cools a burning gas below its combustion temperature. The flame arrestor covers the entire end of the burner assembly. Both primary and secondary air flow through the flame arrestor to the burner.

2.7

STACK ARRESTOR

In some heaters, the temperature of the exhaust combustion gases leaving the stack is high enough to ignite fuel that may have leaked near to the stack. This stack arrestor has the same function and construction as the flame arrestor described above.

2.8

FUEL GAS SCRUBBER

This is a small vertical separator type vessel used to remove and drain liqiuds from the fuel gas supply to the heater. Liquids are either manually or automatically drained to prevent them entering the burner and causing an internal or external fire hazard. Larger heaters may have a separator with conventional level controllers to drain liquid, and high level shut down devices.

2.9

FUEL GAS HEATING COIL

When condensation is likely in the fuel gas supply line a small section of piping is installed in the liquid that surrounds the firetube so that the fuel gas will be sufficiently heated to vaporize any liquids that are in the fuel gas. All these accessories are -illustrated in Figure 5.06.


PAGE # [9]

REVISION 0


3 PRINCIPLES OF COMBUSTION AND FIRING 3.1

COMBUSTION

Natural gas is usually composed bf 90 - 95% methane (CH 4, or C1), the remainder is ethane (C2H6, or C2) and other hydrocarbons. For this module of training and for ease of explanation, assume that natural gas used as fuel gas is 100% methane. When methane gas burns, a chemical reaction takes place between the gas and the oxygen in the air: CH4 1 part methane

+

202 4 parts oxygen

CO2

1 part carbon dioxide When the above reaction occurs, heat is released:

2H20

+

2 parts water

1 cubic foot of methane gas burns, to produce 1000 Btu of heat. The oxygen which combines with the methane gas comes from the air. The basic composition of air is 20% oxygen and 80% nitrogen. To be able to burn 1 volume of gas, 10 volumes of air must be present, if less than 10 volumes of air are supplied the gas will not completely burn. When gas and air are burned, the heat of the reaction must go somewhere. It heats the products of combustion, CO 2 and water (the water is in the form of steam), to about 3500 0F. If less than 10 volumes of air for 1 volume of fuel gas are present, not all of the gas will burn. If more than 10 volumes of air are supplied, the gas will completely burn, but some of the heat of the reaction will be used to heat the excess air that is present. For example, if 11 volumes of air are supplied, the temperature of the combustion products will be only 3225 0F. The 1% excess air has absorbed approximately 8% of the heat. Or, 8% of the fuel is wasted in heating the excess air. To burn without excess air, the gas and air must be thoroughly mixed before burning occurs. It is practically impossible to mix gas and air so that 1 part of gas is surrounded by 10 parts of air. Some will have 9 parts, and some will have 11 parts. Therefore, in practical firing operations it is always necessary to have a little excess air supplied to the burner to completely burn all the fuel gas. Usually, an air to gas ratio of 12:1 is set as a standard for most indirect fired heaters to ensure that all of the gas burns. It is more efficient and safer to fire with an excess of air and burn all the gas than have a shortage of air and not burn all the gas. More heat will be produced when all the fuel gas burns. A shortage of air not only produces less heat but also produces a deadly toxic gas called carbon monoxide (CO), causing a safety hazard to personnel.


PAGE # [10]

REVISION 0


When the correct volumes of gas and air are mixed in the burner, a blue coloured flame will be seen. The flame may have streaks of yellow or red colour, this is an indication of incomplete combustion. The fuel gas and air were not correctly mixed before ignition and must he adusted by the operator to correct the flame colour to blue. This adjustment is explained in Section 5.04.

3.2

THE TRANSFER OF HEAT

Basically an indirect fired heater is a heat exchanger in which the heat from the hot products of combustion (CO 2 and steam) inside the firetube is transferred to the liquid outside the firetube. The temperature of the combustion gases inside the firetube will vary from 3000 0F at the burner to 900 0F as the gases enter the stack. The formula for calculating the transfer of heat that takes place in a heat exchanger is: Q = U x A x ΔT where: Q = Heat transferred, Btu per hour. U = Exchanger coefficient. A = Tube surface area, square feet. ΔT = Average temperature difference between the hot gases inside the tube and the liquid outside the tube. In heater equipment and operations, the coefficient and ΔT are combined in a single term called flux rate. The flux rate is the amount of heat that passes across each square foot of surface area of the firetube. The heat transfer formula used for heaters is: Q = A x Fl where: Q = Heat transferred, Btu per hour. A = Area of firetube, square feet, sq.ft. Fl = Flux rate. The formula can be transposed to determine the area of the firetube: A = Q FI EXAMPLE: A firetube is installed in a condensate heater treater on a gas production facility. The heater firetube is a 16 inch pipe and has a total length of 9.8 feet. The flux rate is 10 000 Btu/sq.ft. What is the heat transfer rate of the heater?


PAGE # [11]

REVISION 0



PAGE # [12]

REVISION 0


Heat transfer rate Q Area of firetube

Heat transfer rate Q

= = =

Area A 3.1 3.1

x Flux x Fl x x

=

40.5 sq.ft.

=

40.5 x 10 000

=

405 000 Btu/hr.

diameter l6in 12

x x

length 9.8

EXAMPLE: The flux rate in-a glycol reboiler heater is 7500 Btu/sq.ft. The area of the firetube is 50 sq.ft. Determine the heat transfer rate in Btu/hr. Flux rate Fire tube area Heat transfer rate

7500 50 sq.ft. Area x A x 50 x Q

Btu/sq.ft. Flux Fl 7500

= 375000 Btu/hr.

In the indirect fired heater, when the flux rate increases, the temperature of the firetube increases. At a flux rate of 6000 Btu/sq.ft., the temperature of the firetube will be a few degrees above that of the liquid. Figure 5.07 (above) shows a heater tube with a flux rate of 7500 Btu/sq.ft., the temperature of the firetube is 30 0F higher than the average liquid temperature. Figure 5.07 (below) shows a heater tube with a flux rate of 12000 Btu/sq.ft., the temperature of this firetube is 63 0F above the average liquid temperature.

3.3

THE MOVEMENT OF HEAT

The temperature of the firetube is approximately the same temperature as the liquid that surrounds it, even though the combustion gas inside the tube may be 2732 0F higher than the liquid temperature. The reason for this is that the heat flows into the liquid much more quickly than it flows out of the combustion gases. The liquid is much denser than the combustion gases, so heat flows much faster in and through Figure 5.08 shows the movement or flow path of heat from the combustion gases inside the firetube to the liquid outside.


PAGE # [13]

REVISION 0



PAGE # [14]

REVISION 0


3.4

SEQUENCE OF EVENTS

a.

Heat moves from the hot gases inside the tube to the firetube inner wall. Because the gas is not very dense, heat does not travel very fast.

b.

Heat moves through the metal firetube from inside to outside. Because metal is very dense, heat moves rapidly through the metal tube wall into the liquid outside.

c.

Heat from the outside wall of the firetube moves into the liquid. Heat does not move through the liquid as fast as it moves through the metal tube, but it moves much faster through the dense liquid than it does through the combustion gases inside the fire tube.

3.5

HOT SPOTS

Because heat moves rapidly through the firetube, its temperature will be about the same as that of the liquid. However, if scale deposits form on the outside of the firetube, the flow of heat through the metal tube will be slowed or reduced, and the tube skin temperature will increase. A heavy build—up of scale usually results in a temperature rise of the firetube to the degree that the tube melts, and has to be replaced. This condition is known as a hot spot. Also, if the top of the firetube becomes exposed, i.e. uncovered by liquid, the temperature of the tube instantaneously rises to the same temperature inside the tube. The tube will melt and fail causing a very dangerous situation of fire. The firetube must never be allowed to be uncovered by the liquid. Refer to Figure 5.09.


PAGE # [15]

REVISION 0



PAGE # [16]

REVISION 0


4 HEATER SYSTEM CONTROLS There are two major control systems on an indirect fired heater: a

fuel rate to the main burner(s),

b

air flow to the main burner(s).

4.1

FUEL RATE TO BURNER

The fuel rate to the burner is governed by two factors: a

fuel gas pressure,

b

percentage that the fuel control valve (TV) is open.

The fuel gas supply pressure regulator valve PRV-1 usually requires very little attention after it has been set, unless there is an erratic pressure variation upstream of the valve, or liquids enter the line. Most pressure regulator valves have a small orifice through which the fuel gas flows. A piece of scale or dirt can block or plug this orifice and restrict the flow of fuel gas. A strainer is fitted in the line upstream of the pressure regulator valve to trap any debris. The fuel control valve (TV) is a diaphragm actuated valve, and changes in instrument air pressure on the diaphragm cause the valve to open or close. Pressure to the diaphragm comes from a temperature controller that detects the temperature in the liquid being heated, and signals the fuel valve to open or close as required to maintain the liquid temperature at the set point on the controller. If the liquid temperature starts to decrease, the temperature controller will signal the fuel control valve to open, .allowing more fuel gas to the main burner. The volume of liquid in the vessel is large, and it will take a few minutes to heat the liquid to the set point on the controller. There is usually a lag in response of the temperature controller so that the liquid temperature rises to the set point before the controller begins to close the fuel valve, and the liquid temperature increases above the set point. The reverse control action is also true where the fuel valve does not open quickly enough when the liquid temperature decreases. The effect of the delay in response is to produce a cycle in the fuel rate and the liquid temperature. This cycling effect can be minimized by adjusting the controller so that it has a slow response to temperature changes, and therefore makes small changes in the pressure signal to the fuel control valve. This stable type of operation will result in a relatively constant fuel rate and flame pattern. If the action of the temperature controller is set for a quick response, the effect will be to fully open or close the fuel control valve, causing maximum and minimum firing. There is a possibility of burning out the firetube with the burner firing at full rate. Burners should be fired at partial rates to prevent damage to the firetube. It is good operating practice to stabilize the firing rate as much as possible and avoid full firing even for short periods. BP PAKISTAN EXPLORATION & PRODUCTION, INC.

PAGE # [17]

REVISION 0



PAGE # [18]

REVISION 0


4.2

AIR FLOW TO BURNER

Air flow to the burner is adjusted for three major reasons: 1. 2. 3.

To provide a stable flame pattern so that a flame—out, caused by a change in fuel gas flow or from a gust of wind will not occur. To keep the flame in the centre of the firetube so that the flame does not touch the wall of the tube and cause it to burn out. To conserve fuel. An excess of air, that is, more air than is required to burn the fuel, will cool the combustion gases and reduce the heat transferred by the heater. The burner will require more fuel gas and fire harder to heat the excess air.

The flame pattern in a firetube should be a long narrow flame that does not touch the tube. The flame pattern is set by adjusting the primary air flow rate. Primary air also supplies the air for combustion during the first half of the flame travel down the firetube. Secondary air is supplied for combustion in the last half of flame travel and comes from the secondary air ports. An excess of primary air results in the flame moving away from the burner, see Figure 5.11. A shortage of primary air will appear as an irregular yellow flame that usually extends far down the firetube. EXAMPLE PROCEDURE FOR SETTING THE PRIMARY AND SECONDARY AIR 1.

Set the secondary air register to the half open position.

2.

Adjust the primary air register to give a long narrow flame that is not more than ½ inch away from the burner tip. The first half of the flame should now be blue with small yellow tips. Adjust the secondary air register to remove most of the yellow colour out of the last half of the flame. Correct adjustment of the secondary air register often leaves the plate only 10 to 30% open. If any excess air is present, most of it will come from the secondary air ports. Trim the combustion air to the burner by adjusting the secondary air once the primary air has been set. Repeat steps 2 and 3 until a good flame pattern is established with 15 - 20% excess air. Close and open the burner fuel valve several times to ensure that the flame is stable at low and high fuel rates, and also that no flame-out occurs.

3.

4. 5.

WARNING If a flame-out occurs, quickly close the fuel ball valve and allow the heater to stand for at least ten minutes before re—opening the valve. A momentary flame-out can leave fuel in the burner and the firetube, and an explosion can result if the burner is relit. Allow ten minutes for a safe air purge. BP PAKISTAN EXPLORATION & PRODUCTION, INC.

PAGE # [19]

REVISION 0


An inspection plate or ‘peep—sight’ located near to the burner assembly box allows the operator to check the flame pattern in the heater. When opening this inspection plate, scale or dirt is dislodged and enters the firetube, causing the flame to show a yellow colour. This gives the same appearance as that of insufficient air. Allow sufficient time for this effect to disappear when using the inspection plate, and checking flame patterns. When adjusting the flame pattern, or making any operational changes to the heater, it is important to do one thing at a time, and allow time for the effect to be seen before attempting something else. If many changes or adjustments are made at the same time, the correct adjustment will be offset with an incorrect adjustment. Therefore, it is very difficult to determine which is correct and which is not correct. Smaller indirect fired heaters have burners that are rated at 1 000 000 Btu/hr or less and do not have a secondary air adjustment fitted. Therefore, only the primary air register requires adjustment by the operator. As the process of combustion occurs in the firetube, the hot combustion gases expand, and become less dense. This reduction in combustion gas density decreases the pressure inside the firetube creating an area of low pressure, this allows air from outside the heater to be sucked into the firetube. The air that enters the firetube will cool the combustion gases and require the heater to fire harder in order to heat the air. The burner box end of the heater is tightly sealed to prevent additional air from entering the firetube and wasting fuel.


PAGE # [20]

REVISION 0


5 HEATER PROCESS OPERATIONS 5.1 1. 2. 3. 4 5. 6. 7. 8. 9. 10.

11.

5.2

START-UP Fill the vessel in which the heater is installed, ensure that the level of liquid is above the top of the firetube, usually 4 inches. Close the ball valves in the fuel gas lines to the main burner and the pilot burner. Open the valve in the fuel gas supply line upstream of pressure regulator valve PRV1. Set the fuel gas pressure regulator valve PRV-1 according to manufacturers instructions. Pressure setting is usually 15 — 30 p.s.i.g. Drain the fuel gas scrubber, ensure no liquids in the fuel gas supply line. Ensure the firetube is purged. Set the pilot gas pressure regulator valve PRV-2 according to manufacturers’ instructions. Pressure setting for pilot gas is usually 4 p.s.i.g. Open the ball valve in the pilot gas line and light the pilot burner. Put the temperature controller and control valve on the main burner in service. The control valve, TV, should open. Slowly open the main burner ball valve, the main burner will light from the pilot burner. Check the flame pattern and adjust the primary and secondary air to achieve a stable flame pattern. As the temperature of the liquid approaches the control set point, check to see that the temperature controller closes and adjusts the fuel gas control valve.

SHUT DOWN

1.

Close the ball valve in the fuel gas supply line upstream of the pressure regulator valve PRV-1. Allow the fuel gas in the line downstream of PRV-1 to depressure and burn out.

2.

When the flame is extinguished, close the valves in the fuel lines to the main burner and the pilot burner.

5.3 1.

ROUTINE OPERATOR CHECKS Check that the temperature controller is controlling the fuel control valve so that the temperature is within 5 0F of the set point.

2.

Monitor the level of liquid in the vessel to see that it does not decrease below the top of the firetube.


PAGE # [21]

REVISION 0



PAGE # [22]

REVISION 0


3.

Observe the flame pattern and adjust the air if necessary.

4.

Check the fuel gas system for leaks.

5.

Drain the fuel gas supply to the burners.

6.

If a flame arrestor is installed, obstruction from dust, rags, debris, etc.

visually

7.

Test each shut down it functions correctly.

three

B.

Blow—down the fuel gas strainer to remove any trapped solid particles.

scrubber,

device

ensure

at


no

liquids

inspect

month

in

it

the

for

intervals

PAGE # [23]

fuel

plugging

to

see

gas

or

that

REVISION 0



PAGE # [24]

REVISION 0


6 HEATER SAFETY SYSTEMS There are many types of safety systems attached to indirect fired heaters, the selection of which type depends upon potential hazard in the exact field location and operation. Safety systems are provided to protect equipment and personnel. Safety systems can operate electrically or pneumatically and each heater manufacturer will select his own preferred type. It is almost impossible to show all combinations of safety systems which are available, therefore only five of the most common are described in this text.

6.1

TYPICAL HEATER SAFETY SYSTEMS

Refer to Figure 5.13. 1. Low Level Shut Down (LSLL) The level detecting device, a switch, is located near the top of the firetube. This level switch will activate if the level falls to the point that some of the tube will be uncovered by the liquid in the vessel. The firetube will burn up and melt in a few minutes if it is not completely covered by the liquid~. The low level switch shuts down the fuel control valve, or a special shutdown valve, in the fuel gas supply to the main burner. The heater trips. 2.

High Liquid Temperature Shut Down (TSHH) A temperature sensing device measures the temperature of the liquid in the heater. This device is a switch that will activate when the liquid temperature increases 10 0F above the normal operating temperature. For example, the normal operating temperature in a glycol reboiler is 400 0F. The high temperature switch will be set at 4100F. At the set temperature the switch activates and shuts off the fuel gas to the burner. After the liquid temperature returns to normal, the high temperature switch deactivates, and the heater can be returned to normal operation.

3.

High or Low Fuel Gas Pressure Shut Down (PSHH/PSLL) This system is usually used on large heaters only. It protects against fluctuations in fuel gas supply pressure. These pressure fluctuations can cause the burner to overfire on high pressure, and possibly cause an explosion in the firetube on low pressure. This system shuts off the fuel to the heater.

4.

Flame Out Shut Down (BSLL) A flame out or flame failure condition occurs when both the main burner and the pilot burner go out. This system uses an ultraviolet sensing device which operates on the intensity of light and will activate to shut down the heater in the event of both burners failing. This system shuts off the fuel supply to prevent fuel from entering the unfired heater. The heater must be allowed to purge before recommencing the start-up procedure.


PAGE # [25]

REVISION 0


5.

High Stack Temperature Shut Down (TSHH) This safety system is used in glycol reboilers, and hydrocarbon service where the liquid that surrounds the firetube is inflammable. This system is used to indicate a leaking firetube and will activate to shut down the heater.

Most safety systems use a solenoid or 3 way type valve device to actuate the shut down valve, that is to make the SDV close and therefore isolate the fuel gas from the heater. When the detecting device and switch activate, the 3 way valve will move to the vent position and dump the supply instrument air from the SDV diaphragm. The SDV is a fail close valve, so at zero pressure the valve will close. Each safety system should be operated and tested at three month intervals, any faulty devices should be repaired promptly. Most heater fires would not have happened if the protective safety systems had been checked and in working order.


PAGE # [26]

REVISION 0


7 OPERATIONAL PROBLEM SOLVING The most frequent and common operating problems that occur on indirect fired heaters involve the failure of accessories or shut down systems. The best method of reducing problems on heater operation is by careful and conscious prevention maintenance planning techniques. Every instrument should be checked for correct operation each month. Listed below are three basic operational duties that will help to prevent problems occuring: 1. Instrument air lines should be blown out frequently. 2. The fuel gas scrubber should be drained of liquid regularly. 3. Flame arrestors -should be removed and cleaned at six month intervals. The following procedures for problem solving on operational indirect fired heaters are given as a guideline and will apply to most oil/gas heaters. PROBLEM:

1.

2.

3.

HEATER WILL NOT SUPPLY ITS RATED HEAT OUTPUT CAUSE Low fuel pressure.

gas

Incorrect air setting on burner.

Suspect fuel gas flow rate.

PROCEDURE Check pressure setting of fuel gas supply pressure regulator valve. i) Check f lame p attern and colour. ii) Check for excess air to burner. Compare rate with design.

ACTION Increase to design setting, if low. a. b.

a.

— — —


PAGE # [27]

Adjust air, if necessary. Decrease secondary air if in excess of 20%. If rate is near or above design, the heater may be supplying the design heat, but the process requirements have exceeded the design rate. b. If fuel rate is below design, check for accumulation of dirt in: fuel control valve, burner fuel orifice, holes in burner tip.

REVISION 0


CAUSE Flame arrestor is blocked, no air flow to burner.

4.

5.

PROCEDURE Inspect flame arrestor.

ACTION Clean if dirty. Inspect and clean.

Firetube is coated Check stack temp. This Inspect and clean. with carbon on inside, will increase as carbon or or has scale on scale deposits accumulate. outside. A temp. of more than 12000F indicates a fouled fire tube.

6.

Calorific value of fuel gas has changed.

Check calorific value of If the calorific value of the fuel gas changes, a new fuel gas. burner design may be required. Consult burner manufacturer.

7.

Burner is blocked or damaged

nspect burner for a. blockage, soot, or-damage b. to holes in burner tip.

PROBLEM:

Clean and repair. Renew.

FREQUENT FLAME FAILURE OR FLAME-OUT

CAUSE 1.

Excess shortage primary air.

2.

Fuel fluctuates.

PROCEDURE or of

pressure

Adjust burner.

i) Check fuel gas pressure downstream of pressure regulator valve. ii) Check action of fuel control valve.

ACTION primary air

to

a. If pressure varies more than 2 p.s.i.g. repair regulator valve. b. If control valve action is abrupt, investigate: leaking diaphragm, - actuator spring too tight -


PAGE # [28]

REVISION 0


or loose, - controller set for fast response (snap action) rather than slow response,


PAGE # [29]

REVISION 0



PAGE # [30]

REVISION 0

MV Indirect Fired Heater

Recommend Documents