Troubleshooting

Utility Landfill Gas Flare Operation and Trouble Shooting

Introduction

Flares are applied in many industries for safety, environmental and process reasons. In the case of landfill gas flares their use is for any one or a combination of the three reasons, with safety and environmental being the most common.

Landfills produces gas with a nominal characterization of 50% Methane 50% Carbon Dioxide. Although this is a simplified view of landfill gas composition it is sufficient to describe the major safety, environmental and beneficial properties of landfill gas (LFG).

Operation - Before You Start

Confirm Interlocks and Control Element States


    With interlocks confirmed flare operation is permitted.

    The basic sequence is simple:

  1. Pilot Ignition
  2. Pilot Proved - Gas On
  3. Combustion Monitoring

    Sequence Complete!

A detailed sequence is found in the table below
Each column describes the sequence step

Flare Sequence Table

Step 1- Pilot Ignition 2 - Pilot Proved / Gas On 3 - Combustion       Monitoring
a
Pilot Gas On
No, Check   Interlocks
Landfill Gas Blower Starts
(proves via auxiliary switch on contactor) Failed to start, see "Blower Failure"
Pilot Gas is Turned Off
b
Spark Ignition Initiated
No, See "Spark Ignition Diagnostics"
Inlet / Header Valve Open
(proves open via limit switch)
Failed to open, see "Main Gas Valve"
Flame Monitoring Enabled
(UV Flame Sensor) Flame Not Established, See, "UV Flame Loss"
c
Pilot Proves on Temperature
(Gas On Set Point Met)
Not Heating, See "Pilot Gas"
Pilot Attains Pilot Off Temperature
Not Heating, See "Pilot Gas"
Main Burner Proves on Temperature
Temperature Not Made, See, "Low Temperature"
d
Temperature Monitoring Enabled
- -
Flare is in Operation

Flare Operation

After the flare is in operation landfill conditions may change. Changes in gas quality and quantity may lead to operation outside of recommended limits for the flare design.
Operation outside recommended limits can result in unstable operation, regulatory violations and / or equipment damage. Typical flare monitoring prevents continued operation outside safe limits, but some investigation may be needed to reveal the root cause. Common operational issues are presented as ideas to consider during the investigation.

Long Term Consideration

Long term gas quality is affected by the contents of the landfill, moisture distribution, gas extraction rate, gas collection system design and maintenance.

Long term gas production is projected through mathematical models. This data is then used to determine equipment size and life span. In addition to setting flare performance requirements the gas model projections are used as measuring stick to indicate if the care and maintenance of the landfill is on track. Long term gas quality is usually not concern in day to day operation as it is planned into the collection system and flare design

Short Term Considerations

Short term gas quality is impacted by human error, accidental break in the collection system, watered in sumps and gas lines or intentional opening of the collection system for maintenance. Air intrusion dilutes gas heating value. An important aspect of air intrusion is the possibility of creating an explosive gas mixture.

When gas quality and/or volume change significantly an immediate effect on the flare and landfill operation can be realized. A common cause is water in the collection system.

Water in the gas collection system can block gas flow at wells, sumps and headers.

Constant Flow - Manual Valve Throttling
Vacuum on the remaining wells increase and added air intrusion is possible. Overtime the remaining wells cannot keep up with increased demand. Expect reduced volume, heating / methane content and additional air dilutes energy per unit volume.

Constant Vacuum - Blower Speed Throttling
As in the previous case the reduced volume increases header suction. In this case the increased suction pressure is detected and the control system adjust accordingly to restore suction to the operator set point. Suction restored, remaining wells continue without issue and gas quality remains constant, however volume is reduced by the number of wells out of service.

In either control case very low flow to the flare is possible and lead to flame flash back (fire in gas piping).

Short term issues commonly related to water build up and sump failure are ...

Flash Back Protection

Flame Arrestor - To prevent flame travel to the KOP and landfill gas collection system a class D flame arrestor is required (primary protection).

High Temperature Switch - Flame at the flame arrestor will eventually damage the arrestor. To prevent damage from flash back a temperature switch (secondary protection) detects flashback and interlocked to stops gas delivery.

Combustion Monitoring

To confirm combustion at the flare burner two methods of monitoring are common, temperature and UV (ultraviolet light) monitoring.

Temperature - The most commonly experienced byproduct of combustion is heat. Water in the air is heated by combustion at the flare burner. The elevated temperature is used to indicate combustion is taking place. Temperature is measured using a thermocouple or "T/C" installed at the burner.

Advantages

Disadvantages

UV Sensor - Another byproduct of combustion is light, specifically light in the ultraviolet range. During combustion significant UV radiation is present making it a reliable flame detection method. UV sensors are like vacuum tubes where the UV light impinges on a specially coated grid. Photons striking the grid create a pulsing electron flow. The frequency of the pulses indicates the intensity of the UV light. This signal is applied to electronics that energize a relay once the frequency of the pulses is sufficient to indicate combustion.

note: Opening of the collection system increases total volume and dilutes overall methane content by displacing methane with air. This can result in flame loss or blower motor overload.

Blower Protections

Several items can be monitored to prevent blower damage, the first two are considered mandatory.

KOP Liquid Level - Liquid carry over commonly results in blower damage.

Low Gas Flow - Centrifugal blowers depend on the process gas to carry heat away from the blower.

Bearing Temperature - This is an early indication of process or maintenance issue that can lead to bearing failure.

Vibration - Excess vibration results in premature failure of the bearing and surrounding electrical components, wiring and mechanics.
note: Excess vibration also indicates a process problem.

Operational Problems

Following is a list of common problems that lead to flame loss. The list is not a comprehensive trouble shooting guide. It is intended to provide some insight to the miriad issues that can lead to flame loss

Flame Loss - UV

When flame loss is indicated by the UV flame sensor it can be falsely generated by the following events

Flame Loss - Temperature
Low temperature as an indication of flame loss can be caused by some of the following:
  Flame Loss - Equipment Failure

Interlock Trouble Shooting Table

Step

Event

Check 1

Check 2

Check 3

Check 4
1
Start
E-Stop Extended?
Yes, Step 2
No, Check 1
Extend E-stop operator and press reset
Alarm message cleared?
Yes, Step 2
No, See suggestions for: E-Stop\MCR Troubleshooting
-  -
- -
-   -
2
Temperature below "Gas On" set point
Yes, Step 3
No, See suggestions for: Temperature to high to start
Has suffcient time elapsed for cool down?
(typically  <30min.)
Yes, go to Check 1
No, Wait for cool down
Flame at burner?
Yes, see check 3;
No, Go to step 3
Are drains connected to gas system?
Yes, Goto Check 4
No, Check Main Gas Valve
Is gas system under pressure?
Yes, close drain valves and confirm flame extinguishes
No, Check Main Gas Valve
3
Flame sensor indicating flame?
Yes, Go to Check 1
No, Goto next step
Flame Relay indicating error / fault?
Yes, Follow flame sensor manufacturers recommendations
No, Goto to Check 2
Ambient light affecting sensor?
Yes, Reposition sensor to prevent interference
No, Goto to Check 3
Are drains connected to gas system?
Yes, Goto Check 4
No, Check Main Gas Valve
Is gas system under pressure?
Yes, close drain valves and confirm flame extinguishes
No, Check Main Gas Valve
4
Flash back temperature limit exceeded?
Yes, Close all gas and drain valves
Go to Check 1
No, Go to next step
Is temperature elevated at stack?
Yes, determine errant gas path and correct
Go to Check 2
Low gas flow during operation
Yes , See Flashback
No, Go to Check 3
Determine error with Flashback sensor / switch circuit
-   -
5
Inlet Valve Closed
Yes, Next Step
No
Goto Check 1
Visually confirm valve position
Valve is closed, goto Check 2
Valve is open, goto Check 3
Is the valve closed position limit made / closed ?
Yes, go to Check 3
No , confirm limit switch function and wiring
Confirm valve actuator is functional
Valve is electric
Valve is pneumatic
-   -
6
Blower selected for automatic operation?
Yes, Next Step
No, Select a blower for automatic operation
-   -
-   -
-   -
-   -
7
Condensate Knock Out Pot (KOP) high level switch is clear / not tripped
Yes, Next Step
No, Goto Check 1
Does KOP sight glass indicate liquid in tank?
Yes, Check KOP valves and drains
No, Check high level switch, wiring and interlock relay
-   -
-   -
-   -
8
Flare Started
Yes, Done!
No
Goto detailed troubleshooting guide
-   -
-   -
-   -
-   -

Spark Ignition Diagnostics

Ignition Output - Off

Ignition Output- On / Ignition transformer not energized

Ignition transformer energized, no spark

Back to table

Pilot Gas

Pilot not heating

Back to table

Blower Operation

Blower Failure

Back to table

Main Gas Valve Operation

Valve failed to open

Back to table

UV Flame Loss

Did not establish a flame during start sequence

Back to table

Low Temperature

Did not attain minimum temperature during start sequence

Back to table

E-stop \ MCR Troubleshooting

Back to table

Temperature to high to start

Back to table

Main Gas Valve

Back to table

Limit Switch Trouble Shooting

Back to table

Flashback, high gas temperature at flare inlet

Back to table

Limt switch is not indicating closed

Back to table

Electric valve did not close

Back to table

Pneumatic valve did not close

Back to table

High liquid level in KOP

Back to table

High level switch, wiring and interlock relay

Back to table

KOP level trip during operation


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