The critical role of commissioning

Martin Cooke, Technical Director at EOGB Energy Products Ltd, looks at the steps that need to be taken to ensure a successful commissioning process.

 

Commissioning is an extremely important and necessary part of any plant installation process to ensure it has been correctly installed to industry standards as per IGEM/UP/4 Edition 3 and IGEM/UP/10 Edition 4. Additional guidance should also be taken from BS 6644, BS 6230 and BS 6880, BS EN 676 and as stated by the manufacturers of the plant equipment.

A commissioning engineer’s role is extensive and varied as they must have a thorough knowledge of combustion principles, mechanical functions, electrical controls, current regulations and legislation.

Commissioning is an area of work that should not be looked upon as an unnecessary process that can simply be done by the installer at the point of installation. However, sadly this is often the case, especially in recent tough economic times when companies are looking to reduce costs and a final commissioning can be perceived as a cost that can be avoided.

Often the client may be under the impression that the installation has been fully commissioned and adjusted to ensure maximum possible efficiency and not simply ‘turned on’ and handed over.

 

Plant inspection

After any required health and safety site inductions have taken place and the client has issued the commissioning engineer with the correct permits to work, it’s important to have an initial meeting with the person who will be in charge of the plants’ operation, or the engineer in charge of the installation who will be handing over to the client upon completion.

The commissioning engineer should be informed of:

• What process the plant is being used for
• What is expected from the operation
• How the plant is controlled
• If the temperature is critical and, if so, to what point

The commissioning engineer also needs to have the following documentation made available to them:

• Drawings of the plant layout, including flue and ventilation design
• All manufacturer’s instructions
• Gas supply testing and purging certificates as per IGE-UP-1 or 1A Edition 2
• Electrical schematic drawings

 

Visual inspection

Before any commissioning can commence it is vital that the plant is visually inspected by the commissioning engineer and preferably the person who will have overall responsibility of the plants’ operation. At this point the following questions should be addressed:

• Are boilers or plant machinery and/or equipment installed correctly, safely and to manufacturer’s instructions?
• Is the area safe to have this installation installed?
• Is the gas supply correct?
• Is the flue correctly designed and installed?
• Are the necessary electrical supplies correct?
• Is the equipment installed suitable for the application intended?

 

Pre-commissioning checks

During any initial checks prior to attempting to run the plant, the equipment (in this case the boiler and burner) needs to be inspected to ensure that the installation is correct.

Initially, the boiler must be checked against the manufacturer’s instructions to ensure that:

• The electrical connections to the boiler are correctly fitted and that the boiler can be electrically isolated
• The burner is correctly installed (particular attention should be paid to the burners insertion depth into the boiler – if in doubt the boiler and burner manufacturers should be consulted)

The gas supply to the plant should be designed and installed to IGEM/UP/2 Edition 2 and should also be inspected to ensure that:

• Testing and purging has been carried out and documented
• The correct materials have been used
• The pipework is correctly sized
• The correct methods of jointing have been used (e.g all joints must be welded over 50mm)
• Emergency control valves have been fitted and are usable and the control levers fall to the ‘off’ position
• There is an emergency control valve fitted at point of entry into the plant room
• Any regulators required are fitted in the correct location
• Purge points have been suitably capped

 

Flue inspection

The flue design and installation should be reviewed and guidance sought from IGEM/UP/10, BS6644:2011 and BS5854. The commissioning engineer is required to visually check that:

• The flue is jointed securely and suitably secure
• The flue material is suitable
• Any flue draught stabilisers have balance weights fitted or present for commissioning
• Any condensate drains fitted are terminated correctly
• Flue bends and horizontal runs have been kept to a minimum
• The flue terminates correctly
• A flue flow test has been performed and observed along the whole flue length

 

Ventilation requirements and electrical checks

Natural draught or mechanical ventilation provisions need to ensure the following:

• They are suitably sized for the max KW heat input of the plant
• They have suitably designed vents to ensure unrestricted air flow
• If mechanical and flue extract fans are used, care must be taken when measuring air flow that the plant room is not being put into a negative state

Finally, the following pre-commissioning electrical tests are required:

• Earth Continuity Check
• Polarity Check
• Resistance to Earth Check
• Three Phase Motor checked for voltage and correct rotation.

 

Dry and live run procedures

In order to check the safe operation of the installation, a dry run procedure must be performed. This ensures that the burners start up and lockout sequences are operating correctly before introducing gas.

Once the preliminary safety checks have been completed, the live run procedure can be carried out. After a successful ignition, the controls can be checked for successful shutdown when enable is removed.

The combustion process can then begin. Guidance from the burner manufacturer can be sought if required, but all stages must be checked thoroughly. It’s extremely important to monitor O2, CO2, CO and flue gas temperature carefully with a calibrated combustion analyser to achieve good combustion. The low fire flue gas temperatures also need to be observed and the flue gases and a true gas rate of each appliance checked accordingly.

 

Combustion Calculations

Stoichiometric calculations are often discussed but realistically cannot be achieved in practice as a reasonable percentage of excess air is required to maintain complete combustion.

For example, with natural gas the equation for a simple hydrocarbon fuel combustion process is written as the following:

C_x H_y + (x +  ) (O₂ + 3.76N₂)   xCO₂ +  H₂O + 3.76 (_X +  )N₂

If we know that the chemical elements that make up methane is CH₄ (one carbon and four hydrogen), by replacing x and y with the correct values we can calculate the following:

C₁H₄ +(1 + ) (O₂ + 3.76N₂)₁CO₂ +  H₂O + 3.76 (1 +  )N₂

Therefore the chemical formula for complete combustion is based on the following equation:

Reactant  CH₄ + 2(O₂+3.76N₂) = CO₂ + 2H₂O

Products: CO2 + 2H₂O + 3.76N₂

By looking at the chemical reaction, to burn 1m³ of methane requires 2m³ of oxygen, and the products of this combustion are 1m³ of Carbon Dioxide and 2m³ of water. It is important to remember that air in the atmosphere contains 21% Oxygen, so to burn 1m³ of natural gas you need 10m³ of air.

As discussed, a percentage of excess air is then added on to the combustion process to maintain a safe combustion.

Excess air is often calculated on a combustion analyser, but can also be easily calculated if the oxygen levels recorded in the combustion analysis are known and the oxygen content in air is 20.9%.

 

Excess air calculation

A typical example would be to add 5% O2 to the combustion process, and the calculation below indicates the percentage of excess air within the combustion process.

Both too little and too much excess air can encourage carbon monoxide to be produced. High excess air content can also encourage higher heat losses out of the flue, so it’s important to be aware of excess air levels during boiler commissioning.

Once the plant has been thoroughly checked and all safety interlocks proved, it can then be handed over to the site and all necessary operative training provided, including advice on maintenance schedules discussed.